Steel Metal

Work hardening is used extensively to produce strain-hardened tempers of the non-heat-treatable alloys. The severely cold worked or full-hard condition (H18 temper) is usually obtained with cold work equal to about 75% reduction in area. The H19 temper identifies products with substantially higher strengths and greater reductions in area. The H16, H14, and H12 tempers are obtained with lesser amounts of cold working, and they represent three-quarter-hard, half-hard, and quarter-hard conditions, respectively.

A combination of strain hardening and partial annealing is used to produce the H28, H26, H24, and H22 series of tempers; the products are strain hardened more than is required to achieve the desired properties and then are reduced in strength by partial annealing.

A series of strain-hardened and stabilized tempers - H38, H36, H34, and H32 - are employed for aluminum-magnesium alloys. In the strain-hardened condition, these alloys tend to age soften at room temperature. Therefore, they are usually heated at a low temperature to complete the age-softening process and to provide stable mechanical properties and improved working characteristics.

Products hardened by cold working can be restored to the O temper, a soft, ductile condition, by annealing. Annealing eliminates strain hardening, as well as the changes in structure that are the result of cold working.

The distorted, dislocated structure resulting from cold working of aluminum is less stable than the strain-free, annealed state, to which it tends to revert. In zone-refined aluminum, this reversion may take place at room temperature. Lower-purity aluminum and commercial aluminum alloys undergo these structural changes only with annealing at elevated temperatures. Accompanying the structural reversion are changes in the various properties affected by cold working. These changes occur in several stages, according to temperature or time, and have led to the concept of different annealing mechanisms or processes. The first of these, occurring at the lowest temperatures and shortest times of annealing, is known as the recovery process.

Recovery

Structural changes occurring during the recovery of polygonization and subgrain formation has been obtained by x-ray diffraction and confirmed with the electron microscope. The electron micrographs may show the change in structure that accompanies advanced recovery. The reduction in the number of dislocations is greatest at the center of the grain fragments, producing a subgrain structure with networks or groups of dislocations at the subgrain boundaries. With increasing time and temperature of heating, polygonization becomes more nearly perfect and the subgrain size gradually increases. In this stage, many of the subgrains appear to have boundaries that are free of dislocation tangles and concentrations.

The decrease in dislocation density caused by recovery-type annealing produces a decrease in strength and other property changes. The effects on the tensile properties of 1100 alloy are shown in Fig. 1. At temperatures through 450°F (230°C), softening is by a recovery mechanism. It is characterized by an initial rapid decrease in strength and a slow, asymptotic approach to a strength that is lower, the higher the temperature.

Fig. 1. Isothermal annealing curves for 1100-H18 sheet.

Recovery annealing is also accompanied by changes in other properties of cold worked aluminum. Generally, some property change can be detected at temperatures as low as 200 to 250°F (90°C to 120°C); the change increases in magnitude with increasing temperature. Complete recovery from the effects of cold working is obtained only with recrystallization.

Recrystallization

Recrystallization is characterized by the gradual formation and appearance of a microscopically resolvable grain structure. The new structure is largely strain-free. There are few if any dislocations within the grains and no concentrations at the grain boundaries. Recrystallization occurs with longer times or higher heating temperatures than do the recovery effects described in the preceding section, although some overlapping of the two processes is usual.

Recrystallization depends upon time and temperature. This relationship can be expressed by a rate equation of the type:

     1/t = ke^-a/T

where t is time, T is the absolute temperature, e is the base of natural logarithms, and k and a are constants.

The constant a is frequently replaced by Q/R, where R is the gas constant and Q is an energy term, similar to an activation energy. Aluminum alloys generally show good agreement with this time-temperature relationship except when secondary reactions interfere, such as the solution or precipitation of intermetallic phases at annealing temperatures.

Composition also influences the recrystallization process. This is particularly true when various elements are added to extreme purity aluminum; almost any added impurity or alloying element will raise the recrystallization temperature substantially. For commercial-purity aluminum and commercial alloys, however, normal variations in composition have little effect on recrystallization behavior. Extensively cold worked commercial alloys usually can be recrystallized by heating for several hours at 650 to 775°F (340 to 410°C).

Grain size is also strongly affected by composition. Generally, common alloying elements and impurities such as Cu, Fe, Mg, and Mn decrease grain size. The effects of elements of limited solubility, such as Cr, Fe, and Mn, are influenced by the compounds they form with each other and with other elements, and by their distribution in the structure.

The recovery process is not accompanied by any significant change in preferred orientation or texture of the deformed metal. However, the new grains formed by recrystallization frequently develop in orientations that differ from the principal components of the deformation texture. This re-orientation has been extensively studied in rolled sheet and varies considerably with the past history and the composition of the alloy.

Recrystallization produces further changes in the properties of the deformed and recovered metal. These continue until annealing and recrystallization are complete. The properties then are those of the original, unstrained metal, except as they are changed by differences in grain size and preferred orientation. In heat treatable alloys, annealing also may be accompanied by precipitation and changes in solute concentration.

Recrystallization is also accompanied by a further decrease in stored energy, as measured calorimetrically, as well as by complete elimination of residual stresses.

Grain Growth After Recrystallization

Heating after recrystallization may produce grain coarsening. This can take one of several forms. The grain size may increase by a gradual and uniform coarsening of the microstructure. This is usually identified as "normal" grain growth.

It proceeds by the gradual elimination of small grains with unfavorable shapes or orientations relative to their immediate neighbors. This occurs readily in high-purity aluminum and its solid solution alloys, and can lead to relatively large, average grain sizes. Such grain growth is promoted by small recrystallized grains, high temperatures, and extensive heating. Some grain coarsening of this type also occurs in commercial aluminum alloys, but it is greatly restricted by finely divided impurity phases and by intermetallic compounds of elements, such as manganese and chromium that slows down the process pin the grain boundaries, and prevent further movement. Generally, these grains grow only at very high temperatures and may attain diameters of several inches.

Apparently, the normal growth-inhibiting effects of elements such as iron, manganese, and chromium are lost or modified at high temperatures, through solution or through changes in particle size and shape. Because of the high temperatures, the few grains that first lose or overcome these restraints grow rapidly and consume other potential growth centers, and in this manner, a few grains of very large size are formed.

In most alloys, high temperatures alone are not the only cause for "giant grains": A small primary grain size and well-developed annealing texture are other factors that promote this form of grain growth.

Titanium and titanium alloys are heat treated in order to:

• Reduce residual stresses developed during fabrication (stress relieving)
• Produce an optimum combination of ductility, machinability, and dimensional and structural stability (annealing)
• Increase strength (solution treating and aging)
• Optimize special properties such as fracture toughness, fatigue strength, and high-temperature creep strength.

Various types of annealing treatments (single, duplex, (beta), and recrystallization annealing, for example), and solution treating and aging treatments, are imposed to achieve selected mechanical properties. Stress relieving and annealing may be employed to prevent preferential chemical attack in some corrosive environments, to prevent distortion (a stabilization treatment) and to condition the metal for subsequent forming and fabricating operations.

Alloy Types and Response to Heat Treatment

The response of titanium and titanium alloys to heat treatment depends on the composition of the metal and the effects of alloying elements on the α-β crystal transformation of titanium. In addition, not all heat treating cycles are applicable to all titanium alloys, because the various alloys are designed for different purposes.

• Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr and Ti-6Al-2Sn-4Zr-6Mo are designed for strength in heavy sections.
• Alloys Ti- 6Al-2Sn-4Zr-2Mo and Ti-6Al-5Zr-0.5Mo-0.2Si for creep resistance.
• Alloys Ti-6Al-2Nb-1 Ta-1Mo and Ti-6Al-4V, for resistance to stress corrosion in aqueous salt solutions and for high fracture toughness.
• Alloys Ti-5Al-2.5Sn and Ti-2.5Cu for weldability; and
• Ti-6Al-6V-2Sn, Ti-6Al-4V and Ti-10V-2Fe-3Al for high strength at low-to-moderate temperatures.

Effects of Alloying Elements on α-β Transformation. Unalloyed titanium is allotropic. Its close-packed hexagonal structure (α phase) changes to a body-centered cubic, structure (β-phase) at 885°C (1625°F), and this structure persists at temperatures up to the melting point.

With respect to their effects on the allotropic transformation, alloying elements in titanium are classified as α stabilizers or β stabilizers. Alpha stabilizers, such as oxygen and aluminum, raise the α-to-β transformation temperature. Nitrogen and carbon are also stabilizers, but these elements usually are not added intentionally in alloy formulation. Beta stabilizers, such as manganese, chromium, iron, molybdenum, vanadium, and niobium, lower the α-to-β transformation temperature and, depending on the amount added, may result in the retention of some β phase at room temperature.

Alloy Types. Based on the types and amounts of alloying elements they contain, titanium alloys are classified as α, near-α, α-β, or β alloys. The response of these alloy types to heat treatment is briefly described below.

Alpha and near-alpha titanium alloys can be stress relieved and annealed, but high strength cannot be developed in these alloys by any type of heat treatment (such as aging after a solution beta treatment and quenching).

The commercial β alloys are, in reality, metastable β alloys. When these alloys are exposed to selected elevated temperatures, the retained β phase decomposes and strengthening occurs. For β alloys, stress-relieving and aging treatments can be combined, and annealing and solution treating may be identical operations.

Alpha-beta alloys are two-phase alloys and, as the name suggests, comprise both α and β phases at room temperature. These are the most common and the most versatile of the three types of titanium alloys.

Oxygen and iron levels have significant effects on mechanical properties after heat treatment. It should be realized that:

• Oxygen and iron must be near specified maximums to meet strength levels in certain commercially pure grades
• Oxygen must be near a specified maximum to meet strength levels in solution treated and aged Ti-6Al-4 V
• Oxygen levels must be kept as low as possible to optimize fracture toughness. However, the oxygen level must be high enough to meet tensile strength requirements
• Iron content must be kept as low as possible to optimize creep and stress-rupture properties. Most creep-resistant alloys require iron levels at or below 0.05wt%.

Stress Relieving

Titanium and titanium alloys can be stress relieved without adversely affecting strength or ductility.

Stress-relieving treatments decrease the undesirable residual stresses that result from first, nonuniform hot forging or deformation from cold forming and straightening, second, asymmetric machining of plate or forgings, and, third, welding and cooling of castings. The removal of such stresses helps maintain shape stability and eliminates unfavorable conditions, such as the loss of compressive yield strength commonly known as the Bauschinger effect.

When symmetrical shapes are machined in the annealed condition using moderate cuts and uniform stock removal, stress relieving may not be required. Compressor disks made of Ti-6Al-4V has been machined satisfactorily in this manner, conforming with dimensional requirements. In contrast, thin rings made of the same alloy could be machined at a higher production rate to more stringent dimensions by stress relieving 2 h at 540°C (1000°F) between, rough and final machining. Separate stress relieving may be omitted when the manufacturing sequence can be adjusted to use annealing or hardening as the stress-relieving process. For example, forging stresses may be relieved by annealing prior to machining.

Annealing

The annealing of titanium and titanium alloys serves primarily to increase fracture toughness, ductility at room temperature, dimensional and thermal stability, and creep resistance. Many titanium alloys are placed in service in the annealed state. Because improvement in one or more properties is generally obtained at the expense of some other property, the annealing cycle should be selected according to the objective of the treatment.

Common annealing treatments are:

• Mill annealing
• Duplex annealing
• Recrystallization annealing
• Beta annealing

Mill annealing is a general-purpose treatment given to all mill products. It is not a full anneal and may leave traces of cold or warm working in the microstructures of heavily worked products, particularly sheet.

Duplex annealing alters the shapes, sizes, and distributions of phases to those required for improved creep resistance or fracture toughness. In the duplex anneal of the Corona 5 alloy, for example, the first anneal is near the β transus to globularize the deformed α and to minimize its volume fraction. This is followed by a second, lower-temperature anneal to precipitate new lenticular (acicular) α between the globular α particles. This formation of acicular α is associated with improvements in creep strength and fracture toughness.

Recrystallization annealing and β annealing are used to improve fracture toughness. In recrystallization annealing, the alloy is heated into the upper end of the α-β range, held for a time, and then cooled very slowly. In recent years, recrystallization annealing has replaced β annealing for fracture critical airframe components.

β (Beta) Annealing. Like recrystallization annealing, β annealing improves fracture toughness. Beta annealing is done at temperatures above the β transus of the alloy being annealed. To prevent excessive grain growth, the temperature for β annealing should be only slightly higher than the β transus. Annealing times are dependent on section thickness and should be sufficient for complete transformation. Time at temperature after transformation should be held to a minimum to control β grain growth. Larger sections should be fan cooled or water quenched to prevent the formation of a phase at the β grain boundaries.

Straightening, sizing, and flattening of titanium alloys are often necessary in order to meet dimensional requirements. The straightening of bar to close tolerances and the flattening of sheet present major problems for titanium producers and fabricators.

Unlike aluminum alloys, titanium alloys are not easily straightened when cold because the high yield strength and modulus of elasticity of these alloys result in significant springback. Therefore, titanium alloys are straightened primarily by creep straightening and/or hot straightening (hand or die), with the former being considerably more prevalent than the latter.

Straightening, sizing, and flattening may be combined with annealing by the use of appropriate fixtures. The parts, in bulk or in fixtures, may be charged directly into a furnace operating at the annealing temperature. At annealing temperatures many titanium alloys have a creep resistance low enough to permit straightening during annealing.

Creep straightening may be readily accomplished during the annealing and/or aging processes of most titanium alloys. However, if the annealing/aging temperature is below about 540 to 650°C (1000 to 1200°F), depending on the alloy, the times required to accomplish the desired creep straightening can be extended. Creep straightening is accomplished with rudimentary or sophisticated fixtures and loading systems, depending on part complexity and the degree of straightening required.

Creep flattening consists of heating titanium sheet between two clean, flat sheets of steel in a furnace containing an oxidizing or inert atmosphere. Vacuum creep flattening is used to produce stress-free flat plate for subsequent machining. The plate is placed on a large, flat ceramic bed that has integral electric heating elements. Insulation is placed on top of the plate, and a plastic sheet is sealed to the frame.

Stability. In α-β titanium alloys, thermal stability is a function of β-phase transformations. During cooling from the annealing temperature, β may transform and, under certain conditions and in β alloys, may form a brittle intermediate phase known as ω.

A stabilization annealing treatment is designed to produce a stable β phase capable of resisting further transformation when exposed to elevated temperatures in service. Alpha-beta alloys that are lean in β, such as Ti-6Al-4V, can be air cooled from the annealing temperature without impairing their stability. To obtain maximum creep resistance and stability in the near-α alloys Ti-8Al-1 Mo-1 V and Ti-6Al-2Sn-4Zr-2Mo, a duplex annealing treatment is employed. This treatment begins with solution annealing at a temperature high in the α-β range, usually 25 to 55°C (50 to 100°F) below the β transus for Ti-8Al-1Mo-1Vand 15 to 25°C (25 to 50°F) below the α-β transus for Ti-6Al-2Sn-4Zr-2Mo.

Solution Treating and Aging

A wide range of strength levels can be obtained in α-β or β alloys by solution treating and aging. With the exception of the unique Ti-2.5Cu alloy (which relies on strengthening from the classic age-hardening reaction of Ti2Cu precipitation similar to the formation of Guinier-Preston zones in aluminum alloys), the origin of heat-treating responses of titanium alloys lies in the instability of the high-temperature β phase at lower temperatures.

Heating an α-β alloy to the solution-treating temperature produces a higher ratio of β phase. This partitioning of phases is maintained by quenching; on subsequent aging, decomposition of the unstable β phase occurs, providing high strength. Commercial β alloys generally supplied in the solution-treated condition, and need only to be aged.

After being cleaned, titanium components should be loaded into fixtures or racks that will permit free access to the heating and quenching media. Thick and thin components of the same alloy may be solution treated together, but the time at temperature is determined by the thickest section. Time/temperature combinations for solution treating are given in Table 1. A load may be charged directly into a furnace operating at the solution-treating temperature. Although preheating is not essential, it may be used to minimize the distortion of complex parts.

Table 1. Recommended solution and aging treatments for titanium alloy

Alloy	Solution temperature [°C]	Solution time [h]	Cooling rate	Aging temperature [°C]	Aging time [h]
α or near-α alloys
Ti-8Al-1Mo-1V	980-1010	1	Oil or water	565-595
Ti-2.5Cu (IMI 230)	795-815	0,5-1	Air or water	390-410	8-24 (step 1)
				465-485	8 (step2)
Ti-6Al-2Sn-4Zr-2Mo	955-980	1	Air	595	8
Ti-6Al-5Zr-0.5Mo-0.2Si (IMI 685)	1040-1060	0,5-1	Oil	540-560	24
Ti-5.5Al-3.5Sn-3Zr-1Nb-0.3Mo-0.3Si (IMI 829)	1040-1060	0,5-1	Air or oil	615-635	2
Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.3Si (IMI 834)	1020	2	Oil	625	2
α-β alloys
Ti-6Al-4V	955-970	1	Water	480-595	4-8
	955-970	1	Water	705-760	2-4
Ti-6al-6V-2Sn (Cu+Fe)	885-910	1	Water	480-595	4-8
Ti-6Al-2Sn-4Zr-6Mo	845-890	1	Air	580-605	4-8
Ti-4Al-4Mo-2Sn-0.5Si (IMI 550)	890-910	0.5-1	Air	490-510	24
Ti-4Al-4Mo-4Sn-0.5Si (IMI 551)	890-910	0.5-1	Air	490-510	24
Ti-5Al-2Sn-2Zr-4Mo-4Cr	845-870	1	Air	580-605	4-8
Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si	870-925	1	Water	480-595	4-8
β or near-β alloys
Ti-13V-11Cr-3Al	775-800	1/4-1	Air or water	425-480	4-100
Ti-11.5Mo-6Zr-4.5Sn (Beta III)	690-790	1/8-1	Air or water	480-595	8-32
Ti-3Al-8V-6Cr-4Mo-4Zr (Beta C)	815-925	1	Water	455-540	8-24
Ti-10V-2Fe-3Al	760-780	1	Water	495-525	8
Ti-15V-3Al-3Cr-3Sn	790-815	1/4	Air	510-595	8-24

Solution treating of titanium alloys generally involves heating to temperatures either slightly above or slightly below the β transus temperature. The solution-treating temperature selected depends on the alloy type and practical considerations briefly described below.

β (Beta) alloys are normally obtained from producers in the solution-treated condition. If reheating is required, soak times should be only as long as necessary to obtain complete solutioning. Solution-treating temperatures for β alloys are above the β transus; because no second phase is present, grain growth can proceed rapidly.

α-β (Alpha-beta) alloys. Selection of a solution-treatment temperature for α-β alloys is based on the combination of mechanical properties desired after aging. A change in the solution-treating temperature of α-β alloys alters the amounts of β phase and consequently changes the response to aging.

To obtain high strength with adequate ductility, it is necessary to solution treat at a temperature high in the α-β field, normally 25 to 85°C (50 to 150°F) below the β transus of the alloy. If high fracture toughness or improved resistance to stress corrosion is required, β annealing or β solution treating may be desirable. However, heat treating α-β alloys in the β range causes a significant loss in ductility. These alloys are usually solution heat treated below the β transus to obtain an optimum balance of ductility, fracture toughness, creep, and stress rupture properties.

Magnesium alloys usually are heat treated either to improve mechanical properties or as means of conditioning for specific fabricating operations. The type of heat treatment selected depends on alloy composition and form (cast or wrought), and on anticipated service conditions.

Solution heat treatment improves strength and results in maximum toughness and shock resistance. Precipitation heat treatment subsequent to solution treatment gives maximum hardness and yield strength, but with some sacrifice of toughness. As applied to castings, artificial aging without prior solution treatment or annealing is a stress-relieving treatment that also somewhat increases tensile properties. Annealing of wrought products lowers tensile properties considerably and increases ductility, thereby facilitating some types of fabrication. Modifications of these basic treatments have been developed for specific alloys, to obtain the most desirable combinations of properties.

The basic temper designations for magnesium alloys, the same as those applied to aluminum alloys, are used throughout this article to indicate the various types of heat treatment.

The mechanical properties of most magnesium casting alloys can be improved by heat treatment. Casting alloys can be grouped into six general classes of commercial importance on the basis of composition, as follows:

    * Magnesium-aluminum-manganese
    * Magnesium-aluminum-zinc
    * Magnesium-zinc-zirconium
    * Magnesium-rare earth metal-zinc-zirconium
    * Magnesium-rare earth metal-silver-zirconium, with or without thorium
    * Magnesium-thorium-zirconium, with or without zinc.

In most wrought alloys, maximum mechanical properties are developed through strain hardening, and these alloys generally are either used without subsequent heat treatment or merely aged to a T5 temper. Occasionally, however, solution treatment, or a combination of solution treatment with strain hardening and artificial aging, will substantially improve mechanical properties.

Wrought alloys that can be strengthened by heat treatment are grouped into four general classes according to composition:

    * Magnesium-aluminum-zinc
    * Magnesium-thorium-zirconium
    * Magnesium-thorium-manganese
    * Magnesium-zinc-zirconium.

Types of Heat Treatment
Annealing. Wrought magnesium alloys in various conditions of strain hardening or temper can be annealed by being heated at 290 to 455°C (550 to 850°F), depending on alloy, for one or more hours. This procedure usually will provide a product with the maximum anneal that is practical.

Because most forming operations on magnesium are done at elevated temperature, the need for fully annealed wrought material is less than with many other metals.

Stress Relieving of Wrought Alloys. Stress relieving is used to remove or reduce residual stresses induced in wrought magnesium products by cold and hot working, shaping and forming, straightening, and welding.

When extrusions are welded to hard rolled sheet, the lower stress-relieving temperature and the longer time should be used to minimize distortion-for example, 150°C (300°F) for 60 min, rather than 260°C (500°F) for 15 min.

Stress Relieving of Castings. The precision machining of castings to close dimensional limits, the necessity of avoiding warp age and distortion, and the desirability of preventing stress-corrosion cracking in welded magnesium-aluminum casting alloys make it mandatory that cast components be substantially free from residual stresses. Although magnesium castings do not normally contain high residual stresses, the low modulus of elasticity of magnesium alloys means that comparatively low stresses can produce appreciable elastic strains.

Residual stresses may arise from contraction due to mold restraint during solidification, from no uniform cooling after heat treatment, or from quenching. Machining operations also can result in residual stress and require intermediate stress relieving prior to final machining.

Solution Treating and Aging. In solution treating of magnesium-aluminum-zinc alloys, parts should be loaded into the furnace at approximately 260°C (500°F) and then raised to the appropriate solution-treating temperature slowly, to avoid fusion of eutectic compounds and resultant formation of voids. The time required to bring the load from 260°C to the solution-treating temperature is determined by the size of the load and by the composition, size, weight and section thickness of the parts, but 2 h is a typical time.

During aging, magnesium alloy parts should be loaded into the furnace at the treatment temperature, held for the appropriate period of time, and then cooled in still air. There is a choice of artificial aging treatments for some alloys; results are closely similar for the alternative treatments given.

Reheat Treating. Under normal circumstances, when mechanical properties are within expected ranges and the prescribed best treatment has been carried out, reheat treating is seldom necessary. However, if the microstructures of heat treated castings indicate too high a compound rating or if the castings have been aged excessively by slow cooling after solution treating, reheat treating is called for. Most magnesium alloys can be reheating treated with little danger of germination.

Effects of Major Variables
Casting size and section thickness, relation of casting size to volume capacity of the furnace, and arrangement of castings in the furnace are mechanical considerations that can affect heat treating schedules for all metals.

Section Size and Heating Time. There is no general rule for estimating time of heating per unit of thickness for magnesium alloys. However, because of the high thermal conductivity of these alloys, combined with their low specific heat per unit volume, parts reach soaking temperature quite rapidly. The usual procedure is to load the furnace and then begin the soaking period when the loaded furnace reaches the desired temperature.

In the heat treating of magnesium alloy castings with thick sections a good rule is to double the time at the solution treating temperature. For example, the usual solution treatment for AZ63A castings is 12 h at about 385°C (725°F), whereas 25 h at about 385°C is suggested for castings with section thickness greater than 50 mm.

Similarly, the suggested solution-treating schedule for preventing excessive grain growth in AZ92A castings is 6 h at about 405°C, 2 h at about 350°C and 10 h at about 405°C; but for castings with sections more than 50 mm thick, it is recommended that the last soak at 405°C be extended from 10 h to 19 h. The best way to determine whether or not additional solution treating time is required is to cut a section through the thickest portion of a scrap casting and examine the center of the section microscopically: if heat treatment is complete, this examination will reveal a low compound rating.

Protective Atmospheres. Although magnesium alloys can be best treated in air, protective atmospheres are almost always used for solution treating. Government specification for heat treating of magnesium castings requires a protective atmosphere for solution treating above 400°C (750°F). Protective atmospheres serve the dual purpose of preventing surface oxidation (which, if severe, can decrease strength) and of preventing active burning should the furnace exceed proper temperature.

The two gases normally used are sulfur dioxide and carbon dioxide. Inert gases also may be used; however, in most instances, these gases are not practical because of higher cost. Sulfur dioxide is available bottled, while carbon dioxide may be obtained either bottled or as the product of recirculated combustion gases from a gas-fired furnace. A concentration of 0.7% (0.5% min) sulfur dioxide will prevent active burning to a temperature of 565°C (1050°F), provided that melting of the alloy has not occurred. Carbon dioxide in a concentration of 3% will prevent active burning to 510°C (950°F), and a carbon dioxide concentration of 5% will provide protection to about 540°C (1000°F).

Equipment and Processing
In solution treating and artificial aging of magnesium alloys, it is standard practice to use an electrically heated or gas-fired furnace equipped with a high-velocity fan or comparable means for circulating the atmosphere and promoting uniformity of temperature. However, because the atmosphere for solution treating sometimes contains sulfur dioxide, only furnaces that are gastight and that provide an inlet for introducing protective atmosphere are suitable.

Quenching Media. Magnesium alloy products normally are quenched in air following solution treatment. Still air usually is sufficient; forced-air cooling is recommended for dense loads or for parts that have very thick sections.

Dimensional Stability
In normal service up to approximately 95°C (200°F), all magnesium casting alloys exhibit good dimensional stability and can be considered free from additional dimensional changes.

Some cast magnesium-aluminum-manganese and magnesium-aluminum-zinc alloys in certain tempers exhibit slight permanent growth after relatively long exposure to temperatures exceeding 95°C. This growth, although slight, can give rise to problems.

In contrast to the growth characteristics of the magnesium-aluminum-zinc alloys are those of the magnesium alloys containing thorium, rare earth metals and zirconium as major alloying elements. These alloys normally are used in the T5 or T6 temper, and they shrink, rather than grow, on exposure to elevated temperatures.

Lead is normally considered to be unresponsive to heat treatment. Yet, some means of strengthening lead and lead alloys may be required for certain applications. Lead alloys for battery components, for example, can benefit from improved creep resistance in order to retain dimensional tolerances for the full service life. Battery grids also require improved hardness to withstand industrial handling.

The absolute melting point of lead is 327.4°C (621.3°F). Therefore, in applications in which lead is used, recovery and recrystallization processes and creep properties have great significance. Attempts to strengthen the metal by reducing the grain size or by cold working (strain hardening) have proved unsuccessful. Lead-tin alloys, for example, may recrystailize immediately and completely at room temperature. Lead-silver alloys respond in the same manner within two weeks.

Transformations that are induced in steel by heat treatment do not occur in lead alloys, and strengthening by ordering phenomena, such as in the formation of lattice superstructures, has no practical significance.

Despite these obstacles, however, attempts to strengthen lead have had some success.

Solid-Solution Hardening

In solid-solution hardening of lead alloys, the rate of increase in hardness generally improves as the difference between the atomic radius of the solute and the atomic radius of lead increases.

Specifically, in one study of possible binary lead alloys it was found that the following elements, in the order listed, provided successively greater amounts of solid-solution hardening: thallium, bismuth, tin, cadmium, antimony, lithium, arsenic, calcium, zinc, copper, and barium.

Unfortunately, these elements have successively decreasing solid-solution solubilities, and therefore the most potent solutes have the most limited solid-solution hardening effects. Within the midrange of this series, however, are elements that, when alloyed with lead, produce useful strengthening.

A useful level of strengthening normally requires solute additions in excess of the room-temperature solubility limit. In most lead alloys, homogenization and rapid cooling result in a breakdown of the supersaturated solution during storage. Although this breakdown produces coarse structures in certain alloys (lead-tin alloys, for example), it produces fine structures in others (such as lead-antimony alloys). In alloys of the lead-tin system, the initial hardening produced by alloying is quickly followed by softening as the coarse structure is formed.

At suitable solute concentrations in lead-antimony alloys, the structure may remain single phase with hardening by Guinier-Preston (GP) zones formed during aging. At higher concentrations, and in certain other systems, aging may produce precipitation hardening as discrete second-phase particles are formed.

Alloys that exhibit precipitation hardening typically are less susceptible to over aging and therefore are more stable with time than alloys hardened by GP zones. Lead-calcium and lead-strontium alloys have been observed to age harden through discontinuous precipitation of a second phase Pb-Ca and Pb-Sr in lead-strontium alloys as grain boundaries move through the structure.

Solution Treating and Aging

Adding sufficient quantities of antimony to produce hypoeutectic lead-antimony alloys can attain useful strengthening of lead. Small amounts of arsenic have particularly strong effects on the age-hardening response of such alloys, and solution treating and rapid quenching prior to aging enhance these effects.

Hardness Stability. For most of the two-year period, the solution-treated specimens were harder than the quench-east specimens. Other investigations have also shown that alloys cooled slowly after casting are always softer than quenched alloys. The alloys with 2 and 4% Sb harden comparatively slowly, and the alloy containing 6% Sb appears to undergo optimum hardening.

Application. Because of the detrimental effect of antimony on charge retention, the effort to reduce antimony contents of the positive plates in lead-acid storage batteries has led to the trend of replacing eutectic alloys with a Pb-6Sb-0.15As alloy. Battery grids made of this arsenical alloy will age harden slowly after casting and air-cooling. However, storing grids for several days constitutes unproductive use of floor space and results in undesirable interruptions in manufacturing sequences.

Although large-scale solution treatment of battery grids might be difficult to justify economically or to achieve without some distortion, quenching of grids cast from arsenical lead-antimony alloys offers an attractive alternative method of effecting improvements in strength. The suitability of quenched grids can be assessed by comparing with the hardness level that battery grids require in order to withstand industrial handling (about 18 HV, the hardness of the eutectic alloy). The alloy containing 2% Sb clearly does not respond sufficiently to be considered as a possible alternative. The 4% Sb alloy, however, attains a hardness of 18 HV after 30 min, and the alloys that contain 6, 8, and 10% Sb could be handled almost immediately.

Dispersion Hardening

Another mechanism for strengthening of lead alloys involves elements that have low solubilities in solid lead, such as copper and nickel. Alloys that contain these elements can be processed so that no homogenization results; most of the strengthening that occurs is developed through dispersion hardening, with some solid-solution hardening taking place as a secondary effect.

The resulting structure is more stable than those developed by other hardening processes. Dispersion strengthening also has been achieved through powder metallurgy methods in which lead oxide, alumina, or similar materials are dispersed in pure lead

In heat treating of tin-rich alloys, it is difficult to secure an effective and permanent degree of hardening. Tin melts at 232°C (505 K), and therefore room temperature (about 295 K) is well over one-half the absolute melting point. It follows that high-temperature behavior such as recrystallization and recovery can occur in fairly short times, even at room temperature. Tin is also an unusual metal because it can work soften under certain conditions, and so heat treating can be used in these cases to restore some of the original hardness and strength.

Heat treating of tin-rich alloys has been practiced for bearing alloys, pewter ware and organ pipe alloys. Some of the principles underlying these applications will be reviewed first.

Binary Alloys

Tin-antimony, tin-bismuth, tin-lead, and tin-silver alloys can all be temper hardened by solution treatment and aging. However, only the tin-antimony alloys can be permanently strengthened by heat treatment; all other tin-rich binary alloys will gradually soften at room temperature.

The greatest improvement obtainable in binary tin-antimony alloys occurs in the alloy that contains 9% Sb; a hardness of 21 HB and a tensile strength of 51 MPa (7.4 ksi) can be increased to 26 HB and 65 MPa (9.4 ksi). This alloy is tempered for 48 h at 100°C (212°F) after being quenched from 225°C (435°F). During this tempering treatment, ductility decreases from 20 to 10% elongation.

Ternary Alloys

Permanent effects produced by heat treatment also carry over into ternary alloys of tin, antimony, and cadmium. This was discovered during an early investigation of the strength and hardness of ternary alloys containing up to 43% Cd and 14% Sb using chill-cast specimens.

It was found that the strengthening effect of cadmium in the terminal solution tin phase (alpha) is much greater than that of antimony. In this study, the maximum stable values obtained in alloys containing 7 to 9% Sb and 5 to 7% Cd were as follows: tensile strength 108 MPa (15.7 ksi), elongation 15%, and hardness 35 HB. The presence of the sigma phase (principally SbSn) as primary cuboids had no effect on strength or hardness, but the presence of primary epsilon (CdSb) destroyed the useful mechanical properties.

Therefore, alloys containing cadmium generally use compositions that restrict the formation of the primary (CdSb) epsilon phase. The maximum combination of strength, ductility, and hardness is obtained in alloys that have finely dispersed precipitates of the sigma and epsilon phases in an alpha matrix, or finely dispersed epsilon in a matrix of alpha with a eutectoid of (α + γ). These structures are typically achieved by quenching or rapid cooling from elevated temperatures to avoid precipitation of primary sigma and epsilon.

Additional heat-treatment studies have been directed to a group of cold-workable tin-rich alloys containing 3 to 8% Cd and 1 to 9% Sb. Two forms of hardening were observed on quenching of these alloys from 185 to 200°C (365 to 390°F). One form results from the change in solubility of antimony in tin or in the beta phase. The other, which produces more intensive hardening, is analogous to hardening of binary cadmium-tin alloys by quenching and depends on suppression of eutectoid decomposition of the beta phase. Permanent improvement results in the first instance. Therefore, a maximum tensile strength of 101 MPa (14.6 ksi) was achieved in a Sn-3Cd-7Sb alloy that was quenched from 190°C (375°F) and then aged for either 24 h at 100°C or 18 months at room temperature.

Further studies have been carried out on tin-base alloys containing 7 to 10% Sb and 0 to 3% Cd in an effort to locate a bearing alloy that would be suitable at mildly elevated temperatures. In this composition range, it was found that alloys containing 0.5 to 2% Cd (but not 3%) can be strengthened considerably by quenching and tempering.

Optimum properties (tensile strength 92 Mpa) were obtained in a Sn-9Sb-1.5Cd alloy quenched from 220°C (430°F) and then aged for 1000 h at 140°C. This alloy consists of finely divided sigma and epsilon phases in a matrix of alpha.

Pewter

Many pewter articles are manufactured from sheet prepared by cold reduction of cast bars or slabs. Tin-rich pewter alloys containing antimony and copper will work harden during sheet-rolling operations that involve small percentage reductions (20%). If left standing at room temperature, the alloy will recrysiallize and soften until it has reverted to the hardness of the original cast bar or slab. On the other hand, if large reductions (such as 90%) are made and the crystals are heavily worked, the alloy will work soften. Then, as crystals increase in size, hardness increases slightly, but never to the level of the original cast material.

The hardness values of spun pewter ware, or of other articles that have been manufactured by mechanically working the metal, can be restored by heat treatment at temperatures from 110 to 150°C. A tin alloy containing 6% Sb and 2% Cu hardens to 90% of the hardness of the as-cast material after annealing for 1 h at 200 °C. Longer annealing times at lower temperatures have smaller but similar effects on the recovery from work softening.

Copper alloys that are hardened through heat treatment are divided into two general types: those that are softened by high-temperature quenching and hardened by lower-temperature treatments, and those that are hardened by quenching from high temperatures through martensitic-type reactions.

Alloys that harden during low-to-intermediate temperature treatments following solution quenching include precipitation hardening, spinodal-hardening and order-hardening types. Quench-hardening alloys comprise aluminum bronzes, nickel-aluminum bronzes, and a few copper-zinc alloys. Quench-hardened alloys normally are tempered to improve toughness and ductility and reduce hardness in a manner similar to that for alloy steels.

Low-Temperature-Hardening Alloys

For purposes of comparison, Table 1 lists examples of the various types of low-temperature-hardening alloys, as well as typical heat treatments and attainable property levels for these alloys.

Table 1. Heat treatment of low-temperature-hardening alloys

Alloy	Solution -Treating Temperature (a)	Ageing Treatment Temperature, Time		Hardness
	°C	°C	H
Precipitation hardening
C15000	980	500-550	3	30 HRB
C17000, C17200, C17300	760-800	300-350	1-3	35-44 HRC
C17500, C17600	900-950	455-490	1-4	95-98 HRC
C18000 (b), C81540	900-930	425-540	2-3	92-96 HRB
C18200, C18400, C18500, C81500	980-1000	425-500	2-4	68 HRB
C94700	775-800	305-325	5	180 HB
C99400	885	482	1	170 HB
Spinodal hardening
C71900	900-950	425-760	1-2	86 HRC
C72800	815-845	350-360	4	32 HRC

(a)	Solution treating is followed by water quenching.
(b)	Alloy C18000 (81540) must be double aged, typically 3 h at 540°C followed by 3 h at 425°C.

Precipitation Hardening Alloys

Most copper alloys of the precipitation hardening type find use in electrical and heat-conduction applications. The heat treatment must therefore be designed to develop the necessary mechanical strength and electrical conductivity. The resulting hardness and strength depend on both the effectiveness of the solution quench and the control of the precipitation (ageing) treatment ("age hardening" or "ageing" is used in heat-treating practice as substitutes for the terms "precipitation" or "spinodal hardening").

Copper alloys are hardened by elevated temperature treatment rather than ambient temperature ageing as in the case of some aluminum alloys. Electrical conductivity increases continuously with time until some maximum is reached, normally in the fully precipitated condition. The optimum condition generally preferred results from a precipitation treatment of temperature and duration just beyond those that correspond to the hardness-ageing peak. Cold working prior to precipitation ageing tends to improve the heat-treated hardness.

In the case of lower-strength wrought alloys such as C18200 (Cu-Cr) and C15000 (Cu-Zr), some heat-treated hardness may be sacrificed to attain increased conductivity, with final hardness and strength being enhanced by cold working. Two precipitation treatments are necessary in order to develop maximum electrical conductivity and hardness in alloy C18000 (Cu-Ni-Si-Cr) because of two distinct precipitation mechanisms.

When precipitation hardening is performed at the mill, further treatment following fabrication of parts is not required. However, it may be desirable to stress relieve parts to remove stresses induced during fabrication, particularly for highly formed cantilever-type springs and intricate machined shapes that require maximum resistance to relaxation at moderately elevated temperatures.

Transformation Hardening

Transformation hardening strengthens certain alloys by inducing a phase change to a harder and stronger phase. Two-phase aluminum bronzes and some manganese bronzes are given quench-and-temper treatments to increase strength without unduly sacrificing ductility.

These alloys are hardened by cooling rapidly from a high temperature to produce a martensitic type of structure, and then are tempered at a lower temperature to stabilize the structure and partly restore ductility and toughness.

Two-Phase Aluminum Bronzes. Binary copper-aluminum alloys have two stable phases at room temperature when the aluminum content is 9.5 to 16%. When other elements (most notably about 1 to 5% iron) are added, the corresponding aluminum content for two-phase alloys is 8 to 14%. Quenching and tempering can strengthen any of the two-phase alloys. At temperatures of 815 to 1010°C, the two room-temperature phases transform to beta in the same manner that alpha plus Fe3C in steel transforms to austenite. Rapid quenching produces a hard, brittle structure due to formation of metastable, ordered, close-packed hexagonal beta, which is referred to as martensitic beta. Both oil and water quenching are used commercially.

Tempering for 2 h at 595 to 650°C causes reprecipitation of fine alpha in a tempered beta-martensite structure, reducing hardness while increasing ductility and toughness.

Nickel-aluminum bronzes, although more complex, respond to quench-and-temper treatments in a similar manner. Nickel-bearing alloys such as C95500 and C63000 quench to a higher hardness and are more susceptible to quench cracking in heavy and/or complex sections, making oil quenching desirable.

Cast two-phase aluminum bronzes often are normalized by heating to 815°C, furnace cooling to about 550°C and then cooling in air to room temperature. This treatment produces uniform hardness and improves machinability.

Spinodal-Hardening Alloys

Alloys that harden by spinodal decomposition are hardened by a treatment similar to that used for precipitation hardening alloys. The soft and ductile spinodal structure is generated by a high-temperature solution treatment followed by quenching. The material can be cold worked or formed in this condition. A lower-temperature spinodal-decomposition treatment, commonly referred to as ageing, is then used to increase the hardness and strength of the alloy.

Spinodal-hardening alloys are basically copper-nickel alloys with chromium or tin additions. The hardening mechanism is related to a miscibility gap in the solid solution and does not result in precipitation. The spinodal-hardening mechanism results in chemical segregation of the alpha crystal matrix on a very fine scale, and requires the electron microscope to discern the metallographic effects. Since no crystallographic changes take place, spinodal-hardening alloys retain excellent dimensional stability during hardening.

Order-Hardening Alloys

Certain alloys, generally those that are nearly saturated with an alloying element dissolved in the alpha phase, will undergo an ordering reaction when highly cold worked material is annealed at a relatively low temperature. Alloys C61500, C63800, C68800 and C69000 are examples of copper alloys that exhibit this behaviour. Strengthening is attributed to short-range ordering of the solute atoms within the copper matrix, which greatly impedes the motion of dislocations through the crystals.

The low-temperature order-annealing treatment also acts as a stress-relieving treatment, which raises yield strength by reducing stress concentrations in the lattice at the focuses of dislocation pileups. As a result, order-annealed alloys exhibit improved stress-relaxation characteristics.

Order annealing is done for relatively short times at relatively low temperatures, generally in the range from 150 to 400°C. Because of the low temperature, no special protective atmosphere is required. Order hardening is frequently done after the final fabrication step to take full advantage of the stress-relieving aspect of the treatment, especially where resistance to stress relaxation is desired.

Quench Hardening and Tempering

Quench hardening and tempering is used primarily for aluminum bronze and nickel aluminum bronze alloys, and occasionally for some cast manganese bronze alloys with zinc equivalents of 37 to 41%. Aluminum bronzes with 9 to 11.5% Al, and nickel-aluminum bronzes with 8.5 to 11.5% Al, respond in a practical way to quench hardening by a martensitic type reaction. Alloys higher in aluminum content generally are too susceptible to quench cracking, whereas those with lower aluminum contents do not contain enough high-temperature beta phase to respond to quench treatments.

Nickel and nickel alloys may be subjected to one or more of five principal types of heat treatment, depending on chemical composition, fabrication requirements and intended service. These methods include:

• Annealing.A heat treatment designed to produce a recrystallized grain structure and softening in work-hardened alloys. Annealing usually requires temperatures between 705 and 1205^oC, depending on alloy composition and degree of work hardening.
• Stress relieving. A heat treatment used to remove or reduce stresses in work-hardened non-age-hardenable alloys without producing a recrystalized grain structure. Stress-relieving temperatures for nickel and nickel alloys from 425 to 870^oC, depending on alloy composition and degree of work hardening.
• Stress equalizing. A low-temperature heat treatment used to balance stresses in cold worked material without an appreciable decrease in the mechanical strength produced by cold working.
• Solution treating. A high-temperature heat treatment designed to put age-hardening constituents and carbides into solid solution. Normally applied to age-hardenable materials before the aging treatment.
• Age hardening (precipitation hardening). A treatment performed at intermediate temperatures (425 to 870^oC) on certain alloys in order to develop maximum strength by precipitation of a dispersed phase throughout the matrix.

Annealing

As applied to nickel and nickel alloys, annealing consists of heating the metal at a predetermined temperature for a definite time and then slowly or rapidly cooling it, to produce a change in mechanical properties - usually a complete softening as a result of recrystalization.

Nickel and nickel alloys that have been hardened by cold working operations, such as rolling, deep drawing, spinning or severe bending, require softening before cold working can be continued. The thermal treatment that will produce this condition is known as annealing, or soft annealing.

The differences in chemical composition among nickel and nickel alloys necessitate modifications in annealing temperatures as well as in furnace atmospheres. The precipitation-hardening alloys must be cooled rapidly after annealing if maximum softness is desired.

Three soft-annealing methods in general commercial use - open, closed and salt bath annealing - are described bellow (Table 2.).

Open annealing is used most often. The material to be annealed is heated at the selected temperature and protected from oxidation by the products of combustion in a fuel-heated furnace, or by a reducing gas introduced into an electric furnace. Temperature control is critical because the annealing period is short.

Closed (box) annealing requires more time than open annealing because of the lower temperatures used. Temperature control is less critical than in open annealing. In most instances, the weight of the container exceeds that of the work; consequently, the amount of fuel required, heating time and costs are greater than in open annealing.

Table 1. Nickel and nickel alloys

Material Composition Ni Fe Cu Cr Mo Nickel 200 99.5 0.15 0.05 - - Nickel 201 99.5 0.15 0.05 - - Monel 400 66.0 1.35 31.5 - - Monel R-405 66.0 1.35 31.5 - - Monel K-500 65.0 1.00 29.5 - - Inconel 600 76.0 7.20 0.10 15.8 - Inconel 601 60.5 14.1 - 23.0 - Inconel 617 54.0 - - 22.0 9.0 Inconel 625 61.0 2.5 - 21.5 9.0 Inconel 718 52.5 18.0 0.10 19.0 3.0 Inconel X-750 73.0 6.75 0.05 15.0 - Hastelloy B 64.0 5.0 - - 28.0 Hastelloy C 56.0 5.5 - 15.5 16.0 Hastelloy X 48.0 18.5 - 22.0 9.0

Table 2. Soft-annealing methods for nickel and nickel alloys

Material Open annealingoC Closed annealingoC Stress relievingoC Nickel 200 815 to 925 705 to 760 480 to 705 Nickel 201 760 to 870 705 to 760 480 to 705 Monel 400 870 to 980 760 to 815 540 to 565 Monel R-405 870 to 980 760 to 815 - Monel K-500 870 to 1040 Not applicable - Inconel 600 925 to 1040 925 to 980 760 to 870 Inconel 601 1095 to 1175 1095 to 1175 - Inconel 617 1120 to 1175 1120 to 1175 - Inconel 625 980 to 1150 980 to 1150 - Inconel 718 955 to 980 Not applicable - Inconel X-750 1095 to 1150 Not applicable - Hastelloy B 1095 to 1185 - 1095 to 1185 Hastelloy C 1215 - 1215 Hastelloy X 1175 1175 -

Salt bath annealing is used for special work with small parts. Inorganic salts, such as chlorides and carbonates of sodium, potassium and barium, which are relatively stable at temperatures considerably above their respective melting points, are fused in large metallic or refractory containers at temperatures up to about 700^oC. At higher temperatures, heat-resisting Fe-Ni-Cr alloy pots or refractory containers should be used. Excessive fuming of the bath is an indication of its maximum usable temperature.

The material to be annealed is placed in molten salts and absorbs heat rapidly. After being annealed, the work metal is quenched in water to free it from particles of the salt mixture. The annealed material will not be bright and may be flash pickled to achieve a bright surface.

Bright Annealing. The temperatures required for soft annealing of nickel and nickel alloys are sufficiently high to cause slight surface oxidation unless the materials are heated in vacuum or in a furnace provided with a reducing atmosphere. Nickel 200, Monel 400 and similar alloys will remain bright and free from discoloration when heated and cooled in a reducing atmosphere. However, nickel alloys containing chromium, titanium and aluminum will form a thin oxide film. Even if oxidation is not important, the furnace atmosphere must be suitably sulfur-free and not strongly oxidizing.

The protective atmosphere most commonly used in heating nickel and nickel alloys is that provided by controlling the ratio between the fuel and air supplied to burners firing directly into the furnace. A desirable reducing condition may be obtained by using a slight excess of fuel so that the products of combustion contain at least 2% carbon monoxide plus hydrogen (preferably 4%) with no more than 0.05% uncombined oxygen.

Another method of maintaining desired conditions of furnace atmosphere is to introduce a prepared atmosphere into the heating and cooling chambers. This can be added to the products of combustion in a direct-fired furnace; however, introduction of prepared atmospheres is more commonly practiced with indirectly heated equipment.

Prepared atmospheres suitable for use with nickel and nickel alloys include: dried hydrogen, dried nitrogen, dissociated ammonia, and cracked or partially reacted natural gas.

Dead-Soft Annealing. When the nickel alloys are annealed at higher temperatures and for longer periods, a condition commonly described as "dead-soft" is obtained, and hardness numbers will result that are 10 to 20% lower than those of the "soft" condition. This cannot be accomplished without increasing the grain size of the metal. Therefore, this treatment should be used only for those few applications in which grain size is of little importance.

Torch Annealing. Some large equipment is hardened locally by fabricating operations. If the available annealing furnace is too small to hold the work piece, the hardened sections can be annealed with the flames of oil or acetylene torches adjusted so as to be highly reducing.

The work should be warmed gently at first, with sweeping motions of the torch, and should not be brought to the annealing temperature until sufficient preheating has been done to prevent cracking as a result of sudden release of stress. (Note: Torch annealing is a poor method for general use, because it provides irregular and insufficient annealing and produces heavily oxidized surfaces.)

Among the more important process-control factors in annealing nickel and nickel alloys are selection of suitably sulfur-free for heating, control of furnace temperature, effects of prior cold work and of cooling rates, control of grain size, control of protective atmospheres, and protection from contamination by foreign material.

Age hardening

Age-hardening practices for several nickel alloys are summarized in the Table 3. In general nickel alloys are soft when quenched from temperatures ranging from 790 to 1220^oC, however, they may be hardened by holding at 480 to 870^oC or above and then furnace or air-cooling. Quenching is not a prerequisite to aging; the alloys can be hardened from the hot worked and cold worked conditions, as well as from the soft condition.

Table 3. Age-hardening practices for nickel and nickel alloys

Alloy Solution treated Temperature Cooling method Age hardening Monel K-500 980 oC WQ Heat to 595oC, hold 16h; furnace cool to 540oC, hold 6h; furnace cool to 480oC, hold 8h; air-cool Inconel 718 980 oC AC Heat to 720oC, hold 8h; furnace cool to 620oC, hold until furnace time for entire age-hardening cycle equals 18h; air cool Inconel X-750 1150 oC AC Heat to 845oC, hold 24h; air cool; reheat to 705oC, hold 20h; air cool 980 oC AC Heat to 730oC, hold 8h; furnace cool to 620oC, hold until furnace time for entire age-hardening cycle equals 18h; air cool Hastelloy X 1175 oC AC Heat to 760oC, hold 3h; air cool; reheat to 595oC, hold 3h; air cool

Hardening Techniques. Nickel alloys usually are hardened in sealed boxes placed inside a furnace, although small horizontal or vertical furnaces without boxes may be used also. The box or furnace should hold the parts loosely packed, yet afford a minimum of excess space. Electric furnaces provide the optimum temperature uniformity of ± 6°C and the freedom from contamination required for this work. Gas-heated furnaces, particularly those of the radiant-tube type, can be made to give satisfactory results. It is difficult to obtain good results from oil heating, even with the muffle furnaces. All lubricants should be removed from the work before hardening.

Because of the long time of aging and the difficulty of excluding air from the box or furnace, truly bright hardening cannot be accomplished commercially. For semibright hardening, dry hydrogen or cracked and dried ammonia should be used. When bright or semibright hardening is not required, other atmospheres may be used, such as nitrogen, cracked natural gas free of sulfur, cracked city gas, cracked hydrocarbons, or a generated gas. The use of sulfur-free gases is necessary to avoid embrittlement.

Salt baths are used occasionally for small parts. The hardened material is never bright, and must be fresh pickled to restore the natural color. Inorganic salts are used, such as chlorides and carbonates of sodium or potassium, which are relatively stable at temperatures considerably above their respective melting points. It is extremely important that the salts be free of all traces of sulfur, so that the work does not become embrittled.

Heat treating processes for aluminum are precision processes. They must be carried out in furnaces properly designed and built to provide the thermal conditions required, and adequately equipped with control instruments to insure the desired continuity and uniformity of temperature-time cycles. To insure the final desired characteristics, process details must be established and controlled carefully for each type of product.

The general types of heat treatments applied to aluminum and its alloys are:

• Preheating or homogenizing, to reduce chemical segregation of cast structures and to improve their workability
  
• Annealing, to soften strain-hardened (work-hardened) and heat treated alloy structures, to relieve stresses, and to stabilize properties and dimensions
  
• Solution heat treatments, to effect solid solution of alloying constituents and improve mechanical properties
  
• Precipitation heat treatments, to provide hardening by precipitation of constituents from solid solution.

INGOT PREHEATING TREATMENTS (HOMOGENIZING)

The initial thermal operation applied to ingots prior to hot working is referred to as "ingot preheating", which has one or more purposes depending upon the alloy, product, and fabricating process involved. One of the principal objectives is improved workability. The microstructure of most alloys in the as-cast condition is quite heterogeneous. This is true for alloys that form solid solutions under equilibrium conditions, and even for relatively dilute alloys

ANNEALING

The distorted, dislocated structure resulting from cold working of aluminum is less stable than the strain-free, annealed state, to which it tends to revert. Lower-purity aluminum and commercial aluminum alloys undergo these structural changes only with annealing at elevated temperatures. Accompanying the structural reversion are changes in the various properties affected by cold working. These changes occur in several stages, according to temperature or time, and have led to the concept of different annealing mechanisms or processes.

Recovery. The reduction in the number of dislocations is greatest at the center of the grain fragments, producing a subgrain structure with networks or groups of dislocations at the subgrain boundaries. With increasing time and temperature of heating, polygonization becomes more nearly perfect and the subgrain size gradually increases. In this stage, many of the subgrains appear to have boundaries that are free of dislocation tangles and concentrations.

Recovery annealing is also accompanied by changes in other properties of cold worked aluminum. Complete recovery from the effects of cold working is obtained only with recrystallization.

Recrystallization is characterized by the gradual formation and appearance of a microscopically resolvable grain structure. The new structure is largely strain-free-there are few if any dislocations within the grains and no concentrations at the grain boundaries.

Grain Growth After Recrystallization. Heating after recrystallization may produce grain coarsening. This can take one of several forms.

PRECIPITATION HARDENING

General Principles of Precipitation Hardening. The heat treatable alloys contain amounts of soluble alloying elements that exceed the equilibrium solid solubility limit at room and moderately higher temperatures. The amount present may be less or more than the maximum that is soluble at the eutectic temperature.

Nature of Precipitates and Sources of Hardening. Intensive research during the past forty years has resulted in a progressive accumulation of knowledge concerning the atomic and crystallographic structural changes that occur in supersaturated solid solutions during precipitation and the mechanisms through which the structures form and alter alloy properties. In most precipitation-hardenable systems, a complex sequence of time-dependent and temperature-dependent changes is involved.

Kinetics of Solution and Precipitation. The relative rates at which solution and precipitation reactions occur with different solutes depend upon the respective diffusion rates, in addition to solubilities and alloy contents. Bulk diffusion coefficients for several of the commercially important alloying elements in aluminum were determined by various experimental methods.

Nucleation. The formation of zones can occur in an essentially continuous crystal lattice by a process of homogeneous nucleation. Recent investigations provide evidence that a critical vacancy concentration is required for this process and that a nucleation model involving vacancy-solute atom clusters is consistent with certain effects of solution temperature and quenching rate.

The nucleation of a new phase is greatly influenced by the existence of discontinuities in the lattice. Since in polycrystalline alloys grain boundaries, subgrain boundaries, dislocations, and interphase boundaries are locations of greater disorder and higher energy than the solid-solution matrix, they are preferred sites for nucleation of precipitates.

Quenching

Quenching is in many ways the most critical step in the sequence of heat treating operations. The objective of quenching is to preserve as nearly intact as possible the solid solution formed at the solution heat treating temperature, by rapidly cooling to some lower temperature, usually near room temperature.

Critical Temperature Range. The fundamentals involved in quenching precipitation-hardenable alloys are based on nucleation theory applied to diffusion-controlled solid state reactions. The effects of temperature on the kinetics of isothermal precipitation depend principally upon degree of supersaturation and rate of diffusion.

Quenching Medium. Water is not only the most widely used quenching medium but also the most effective. It is apparent that in immersion quenching, cooling rates can be reduced by increasing water temperature. Conditions that increase the stability of a vapor film around the part decrease the cooling rate; various additions to water that lower surface tension have the same effect.

Aging at Room Temperature (Natural Aging)

Most of the heat treatable alloys exhibit age hardening at room temperature after quenching, the rate and extent of such hardening varying from one alloy to another. No discernible microstructural changes accompany the room-temperature aging, since the hardening effects are attributable solely to the formation of zone structure within the solid solution.

Since the alloys are softer and more ductile immediately after quenching than after aging, straightening or forming operations may be performed more readily in the freshly quenched condition.

Precipitation Heat Treating (Artificial Aging)

The effects of precipitation on mechanical properties are greatly accelerated, and usually accentuated, by reheating the quenched material to about 100 to 200^oC. The effects are not entirely attributable to a changed reaction rate; as mentioned previously, the structural changes occurring at the elevated temperatures differ in fundamental ways from those occurring at room temperature. These differences are reflected in the mechanical characteristics and some physical properties. A characteristic feature of elevated-temperature aging effects on tensile properties is that the increase in yield strength is more pronounced than the increase in tensile strength. Also ductility, as measured by percentage elongation, decreases. Thus, an alloy in the T6 temper has higher strength but lower ductility than the same alloy in the T4 temper.

Precipitation Heat Treating Without Prior SoIution Heat Treatment

Certain alloys that are relatively insensitive to cooling rate during quenching can be either air cooled or water quenched directly from a final hot working operation. In either condition, these alloys will respond strongly to precipitation heat treatment.

Precipitation Heat Treating Cast Products

The mechanical properties of permanent mold, sand, and plaster castings of most alloys are greatly improved by solution heat treating, quenching, and precipitation heat treating, using practices analogous to those employed for wrought products.

Heat treating in its broadest sense, refers to any of the heating and cooling operations are performed for the purpose of changing the mechanical properties, the metallurgical structure, or the residual stress state of a metal product.

When the term is applied to aluminum alloys, however, its use frequently is restricted to the specific operations employed to increase strength and hardness of the precipitation-hardenable wrought and cast alloys. These usually are referred to as the "heat-treatable" alloys to distinguish them from those alloys in which no significant strengthening can be achieved by heating and cooling. The latter, generally referred to as "non heat-treatable" alloys depend primarily on cold work to increase strength. Heating to decrease strength and increase ductility (annealing) is used with alloys of both types; metallurgical reactions may vary with type of alloy and with degree of softening desired.

One essential attribute of a precipitation-hardening alloy system is a temperature-dependent equilibrium solid solubility characterized by increasing solubility with increasing temperature. The mayor aluminum alloy systems with precipitation hardening include:

1. Aluminum-copper systems with strengthening from CuAl₂
2. Aluminum-copper-magnesium systems (magnesium intensifies precipitation)
3. Aluminum-magnesium-silicon systems with strengthening from Mg₂Si
4. Aluminum-zinc-magnesium systems with strengthening from MgZn₂
5. Aluminum-zinc-magnesium-copper systems

   The general requirement for precipitation strengthening of supersaturated solid solutions involves the formation of finely dispersed precipitates during aging heat treatment (which may include either natural aging or artificial aging). The aging must be accomplished not only below the equilibrium solvus temperature, but below a metastable miscibility gap called the Guinier-Preston (GP) zone solvus line.

   The commercial heat-treatable alloys are, with few exceptions, based on ternary or quaternary systems with respect to the solutes involved in developing strength by precipitation. Commercial alloys whose strength and hardness can be significantly increased by heat treatment include 2xxx, 6xxx, and 7xxx series wrought alloys and 2xx.0, 3xx.0 and 7xx.0 series casting alloys.

   Some of these contain only copper, or copper and silicon as the primary strengthening alloy addition. Most of the heat-treatable alloys, however, contain combinations of magnesium with one or more of the elements, copper, silicon and zinc. Characteristically, even small amounts of magnesium in concert with these elements accelerate and accentuate precipitation hardening, while alloys in the 6xxx series contain silicon and magnesium approximately in the proportions required for formulation of magnesium silicide (Mg₂Si). Although not as strong as most 2xxx and 7xxx alloys, 6xxx alloys have good formability, weldability, machinability, and corrosion resistance, with medium strength.

   In the heat-treatable wrought alloys, with some notable exceptions (2024, 2219, and 7178), such solute elements are present in amounts that are within the limits of mutual solid solubility at temperatures below the eutectic temperature (lowest melting temperature).

   In contrast, some of the casting alloys of the 2xx.0 series and all of the 3xx.0 series alloys contain amounts of soluble elements that far exceed solid-solubility limits. In these alloys, the phase formed by combination of the excess soluble elements with the aluminum will never be dissolved, although the shapes of the undissolved particles may be changed by partial solution.

   Heat treatment to increase strength of aluminum alloys is a three-step process:

   • Solution heat treatment: dissolution of soluble phases
   • Quenching: development of supersaturation
   • Age hardening: precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment).

   Solution Heat Treating

   To take advantage of the precipitation hardening reaction, it is necessary first to produce a solid solution. The process by which this is accomplished is called solution heat treating, and its objective is to take into solid solution the maximum practical amounts of the soluble hardening elements in the alloy. The process consists of soaking the alloy at a temperature sufficiently high and for a time long enough to achieve a nearly homogeneous solid solution.

   Precipitation Heat Treating without Prior Solution Heat Treatment

   Certain alloys that are relatively insensitive to cooling rate during quenching can be either air cooled or water quenched directly from a final hot working operation. In either condition, these alloys respond strongly to precipitation heat treatment. This practice is widely used in producing thin extruded shapes of alloys 6061, 6063, 6463 and 7005.

   Upon precipitation heat treating after quenching at the extrusion press, these alloys develop strengths nearly equal to those obtained by adding a separate solution heat treating operation. Changes in properties occurring during the precipitation treatment follow the principles outlined in the discussion of solution heat-treated alloys.

   Quenching

   Quenching is in many ways the most critical step in the sequence of heat-treating operations. The objective of quenching is to preserve the solid solution formed at the solution heat-treating temperature, by rapidly cooling to some lower temperature, usually near room temperature.

   In most instances, to avoid those types of precipitation that are detrimental to mechanical properties or to corrosion resistance, the solid solution formed during solution heat treatment must be quenched rapidly enough (and without interruption) to produce supersaturated solution at room temperature - the optimum condition for precipitation hardening.

   The resistance to stress-corrosion cracking of certain copper-free aluminum-zinc-magnesium alloys, however, is improved by slow quenching. Most frequently, parts are quenched by immersion in cold water, or in continuous heat treating of sheet, plate, or extrusions in primary fabricating mills, by progressive flooding or high-velocity spraying with cold water.

   Age hardening

   After solution treatment and quenching hardening is achieved either at room temperature (natural aging) or with a precipitation heat treatment (artificial aging). In some alloys, sufficient precipitation occurs in a few days at room temperature to yield stable products with properties that are adequate for many applications. These alloys sometimes are precipitation heat treated to provide increased strength and hardness in wrought or cast products. Other alloys with slow precipitations reactions at room temperature are always precipitation heat treated before being used.

   In some alloys, notably those of the 2xxx series, cold working or freshly quenched material greatly increases its response to later precipitation heat treatment.

   Natural Aging. The more highly alloyed members of the 6xxx wrought series, the copper-containing alloys of the 7xxx group, and all of the 2xxx alloys are almost always solution heat treated and quenched. For some of these alloys, particularly the 2xxx alloys, the precipitation hardening that results from natural aging alone produces useful tempers (T3 and T4 types) that are characterized by high ratios of tensile to yield strength and high fracture toughness and resistance to fatigue. For the alloys that are used in these tempers, the relatively high supersaturation of atoms and vacancies retained by rapid quenching causes rapid formation of GP zones, and strength increases rapidly, attaining nearly maximum stable values in four or five days. Tensile-property specifications for products in T3- and T4-type tempers are based on a nominal natural aging time of four days. In alloys for which T3- or T4-type tempers are standard, the changes that occur in further natural aging are of relatively minor magnitude, and products of these combinations of alloy and temper are regarded as essentially stable after about one week.

   In contrast to the relatively stable condition reached in a few days by 2xxx alloys that are used in T3- or T4-type tempers, the 6xxx alloys and to an even greater degree the 7xxx alloys are considerably less stable at room temperature and continue to exhibit significant changes in mechanical properties for many years.

   Precipitation heat treatments generally are low-temperature, long-term processes. Temperatures range from 115 to 190°C; times vary from 5 to 48 h.

   Choice of time-temperature cycles for precipitation heat treatment should receive careful consideration. Larger particles of precipitate result from longer times and higher temperatures; however, the larger particles must, of necessity, be fewer in number with greater distances between them.

   The objective is to select the cycle that produces optimum precipitate size and distribution pattern. Unfortunately, the cycle required to maximize one property, such as tensile strength, is usually different from that required to maximize others, such as yield strength and corrosion resistance. Consequently, the cycles used represent compromises that provide the best combinations of properties.

   Production of material in T5- through T7-type tempers necessitates precipitation heat treating at elevated temperatures (artificial aging).

   Differences in type, volume fraction, size, and distribution of the precipitated particles govern properties as well as the changes observed with time and temperature, and these are all affected by the initial state of the structure. The initial structure may vary in wrought products from unrecrystallized to recrystallized and may exhibit only modest strain from quenching or additional strain from cold working after solution heat treatment. These conditions, as well as the time and temperature of precipitation heat treatment, affect the final structure and the resulting mechanical properties.

   Precipitation heat treatment following solution heat treatment and quenching produces T6- and T7-type tempers. Alloys in T6-type tempers generally have the highest strengths practical without sacrifice of the minimum levels of other properties and characteristics found by experience to be satisfactory and useful for engineering applications. Alloys in T7 tempers are overaged, which means that some degree of strength has been sacrificed or "traded off" to improve one or more other characteristics. Strength may be sacrificed to improve dimensional stability, particularly in products intended for service at elevated temperatures, or to lower residual stresses in order to reduce warpage or distortion in machining. T7-type tempers frequently are specified for cast or forged engine parts. Precipitation heat-treating temperatures used to produce these tempers generally are higher than those used to produce T6-type tempers in the same alloys.

   Two important groups of T7-type tempers — the T73 and T76 types — have been developed for the wrought alloys of the 7xxx series, which contain more than about 1.25% copper. These tempers are intended to improve resistance to exfoliation corrosion and stress-corrosion cracking, but as a result of overaging, they also increase fracture toughness and, under some conditions, reduce rates of fatigue-crack propagation.

Copper and copper alloys may be heat treated for several purposes, described in this article.

Homogenizing

Homogenizing is applied to dissolve and absorb segregation and coring found in some cast and hot worked materials, chiefly those containing tin and nickel.

Diffusion and homogenization are slower and more difficult in tin bronzes, silicon bronzes and copper nickels than in most other copper alloys. Therefore, these alloys usually are subjected to prolonged homogenizing treatments before hot or cold working operations.

The high-tin phosphor bronzes (above 8% Sn) are noted for extreme segregation. Although these alloys sometimes are hot worked, usual practice is to roll them cold, making it necessary to first diffuse the brittle segregated tin phase, thereby increasing strength and ductility and decreasing hardness before rolling. These objectives are accomplished by homogenizing at about 760^oC.

Annealing

Softening or annealing of cold worked metal is accomplished by heating to a temperature that causes recrystallization and, if maximum softening is desired, by heating well above the recrystallization temperature to cause grain growth. Method of heating, furnace design, furnace atmosphere, and shape of work piece are important, because they affect uniformity of results, finish, and cost of annealing.

For copper and brass mill alloys, grain size is the standard means of evaluating a recrystallizing anneal. Because many interreacting variables influence the annealing process, it is difficult to predict a specific combination of time and temperature that will always produce a given grain size in a given metal.

Several copper alloys have been developed in which the grain size is stabilized by the presence of a finely distributed second phase. Examples include copper-iron alloys such as C19200, C19400 and C19500, and aluminum-containing brasses and bronzes such as C61500, C63800, C68800 and C69000. These alloys will maintain an extremely fine grain size at temperatures well beyond their recrystallization temperature, up to the temperature where the second phase finally dissolves or coarsens, which allows grain growth to proceed.

Generally, two annealed tempers are available: light anneal, which is performed at a temperature slightly above the recrystallization temperature, and soft anneal, which is performed several hundred degrees higher, at a temperature just below the point at which rapid grain growth begins.

When annealing copper that contains oxygen, the hydrogen in the atmosphere must be kept to a minimum to avoid embrittlement. For temperatures lower than about 480^oC, hydrogen preferably should not exceed 1%.

Stress Relieving

Stress relieving is aimed to reduce or eliminate residual stress, thereby reducing the likelihood that the part will fail by cracking or corrosion fatigue in service. Parts are stress-relieved at temperatures below the normal annealing range that do not cause recrystallization and consequent softening of the metal.

Residual stresses contribute to this type of failure, which is frequently seen in brasses containing 15% zinc or more. Even higher-copper alloys such as aluminum bronzes and silicon bronzes may crack under critical combinations of stress and specific corroding, and all copper alloys are susceptible to more rapid corrosion attack when in the stressed condition.

Stressed phosphor bronzes and copper nickels have comparatively slight tendencies toward stress-corrosion cracking; these alloys are more susceptible to fire cracking, which is cracking caused when stressed metal is heated too rapidly to the annealing temperature. Slow heating provides a measure of stress relief and minimizes non-uniform temperature distributions, which lead to thermal stress.

Using a high stress-relieving temperature for a short time is generally considered best for keeping processing time and cost to a practical minimum, even though there is usually some sacrifice in mechanical properties. Using a lower temperature for a longer time will provide complete stress relief with no decrease in mechanical properties. Actually, the hardness and strength of severely cold worked alloys will increase slightly when low stress-relieving temperatures are used.

An additional benefit of a thermal stress relieving is dimensional stability of cold-formed parts. Also, it is often advisable to stress relieve welded or cold formed structures. For these structures, stress-relieving temperature is 85 to 110^oC above that used for mill products of the same alloy.

Precipitation Hardening

High strength in most copper alloys is achieved by cold working. Solution treating and precipitation hardening is applied to strengthen special types of copper alloys above the levels ordinarily obtained by cold working.

Examples of precipitation hardening copper alloys include the beryllium coppers, some of which also contain nickel, cobalt or chromium; the copper-chromium alloys; the copper-zirconium alloys; the copper-nickel-silicon alloys and the copper-nickel-phosphorus alloys.

All precipitation-hardening copper alloys have similar metallurgical characteristics: they can be solution treated to a soft condition by quenching from a high temperature, and then subsequently precipitation hardened by aging at a moderate temperature for a time usually not exceeding 3 h.

The main advantages of these alloys are:

• Customer fabrication is easily performed in the soft, solution-annealed condition.
• The precipitation-hardening heat treatment performed by the fabricator is relatively simple. It is carried out at moderate temperatures, usually in air. Controlled cooling is not needed, and time of treatment is not of critical importance.
• Different combinations of properties - including strength, hardness, ductility, conductivity, impact resistance and inelasticity - can be obtained by varying hardening times and temperatures. The particular requirements of the application determine the type of hardening treatment.

Age-hardenable alloys are furnished in the solution-treated condition, in the solution treated and cold worked condition or in the age-hardened condition.

Beryllium Coppers. Wrought beryllium coppers, C17000, C17200 AND C17500, can develop wide ranges of mechanical properties, depending on solution treating and aging conditions, on the amount of cold work imparted to the alloy and on whether the alloy is cold worked after solution treating and before aging or is cold worked after aging.

Copper-Nickel-Phosphorus Alloys. Alloys containing about 1% nickel and about 0.25% phosphorus, typified by C19000, are used for a wide variety of small parts requiring, high strength, such as springs, clips, electrical connectors and fasteners. C19000 is solution treated at 700 to 800^oC. If the metal must be softened between cold working steps prior to aging, it may be satisfactorily annealed at temperatures as low as 620^oC. Rapid cooling from the annealing temperature is not necessary. For aging, the material is held at 425 to 475^oC for 1 to 3 h.

Chromium coppers. Chromium coppers containing about 1% Cr, such as C18200, C18400 and C18500, are solution treated at 950 to 1010^oC and rapidly quenched. Solution treating usually is done in molten salt, but may be done in a controlled-atmosphere furnace to prevent surface scaling and internal oxidation. Solution treated chromium copper is aged at 400 to 500^oC for several hours to produce the desired mechanical and physical properties. A typical aging cycle is 455^oC for 4 h or more.

Zirconium Copper. Zirconium copper C15000 (99.8Cu-0.2Zr) is solution treated at 900 to 925^oC, then quenched in water. Time at the solution treating temperature should be minimized to limit grain growth and possible internal oxidation by reaction of zirconium with the furnace atmosphere. Because solution and diffusion of the zirconium occur rapidly at the solution treating temperature, holding at temperature is not required. Aging is done at 500 to 550^oC (930 to 1020^oF) for 1 to 4 h. If the material has been cold worked, following solution treating, aging temperature may be reduced to 375 to 475^oC.

Alpha Aluminum Bronzes. The structure and consequent heat treatability of aluminum bronze varies greatly with composition. Single-phase (alpha) aluminum bronzes, which contain only copper and aluminum (up to about 10% Al), can be strengthened only by cold working. They can be softened by annealing at 425 to 760^oC.