casting design 03 - heat treatment

Many metal castings meet the specified property requirements in their as-cast form, but there are times when even the cleverest use of the casting process and alloy will leave a part lack of the requirements. In these instances, thermal treatment can change the mechanical properties to meet those critical needs.

Heat treatment consists of heating and cooling that enhances a material’s strength, consistency or ductility. The type of heat treatment required by a casting depends on the alloy and the final properties required.

Aluminum Treats

Ferrous and nonferrous castings receive different types of heat treatment. Aluminum castings are heat treated using different combinations of operations, called tempers (Table 1). Heat treatment of aluminum castings can result in homogenization, stress relief, and improved stability, machinability and mechanical properties.
The thermal processing involves three basic processes—solution, quenching and aging. During solution, elements that will later cause age hardening are dissolved, undissolved constituents become spheroids, and the microstructure of the casting is homogenized. Homogenization distributes the alloying and impurity elements of a casting throughout its matrix, so the casting’s properties will be more uniform.

Rapid cooling, or quenching cycles, retain the dissolved elements in the solution. Rapid quenching increases the response to age hardening, but it also creates residual stresses and distortion. Dissolved elements that are trapped in the solution during quenching eventually precipitate slowly at room temperature. After a time at room temperature, some alloys will harden appreciably. Aging can be accelerated by heating castings to intermediate temperatures in a process called artificial aging. Increased time at age temperature or aging at a greater temperature further evolves the precipitate structure, and hardness increases to a peak hardness condition. After the peak is hit, further aging, or overaging, reduces the hardness.

Aging also affects ductility. During overaging, a loss of hardening mechanisms permits extensive deformation to occur before fracture and ductility increases. Annealing, which is extreme overaging, maximizes ductility.

Although each alloy and temper have recommended solution, quench and age times, these cycles are often customized to meet specific requirements for strength and ductility. One way that designers inadvertently bump up their part cost is by overspecifying heat treatment.

“Sometimes a customer will ask for too high of mechanical properties,” said Dick Kneip, sales manager for Stahl Specialty Co., Kingsville, Mo., which performs heat-treating at its aluminum permanent mold casting facility. “For instance, they’ll try to get steel properties out of aluminum, and that’s just not possible. Other times, an engineer will specify T-6 heat treatment when T-5 will be just fine.”

The cost of heat treating varies by temper because certain cycles take longer than others. Discuss your desired properties with the heat treater beforehand. A less costly temper may be adequate to fit your needs.

Hot Iron

Many iron casting applications benefit from heat treating, as well, particularly when the casting shape is complex and features interconnected thick and thin sections. Iron castings can be prone to retain residual stresses and structural variations after cooling when they feature more complex shapes. The structural variation can cause distortion and non-uniform mechanical properties. To reduce this, iron castings can go through annealing, stress relieving or normalizing thermal processing.
Annealing softens the cast iron by slow-cooling the austenitic matrix, creating a ferritic microstructure. It can relieve residual stresses if the slow cooling is continued to a low enough temperature.

There are three types of annealing: high, medium and low (subcritical). In high-temperature annealing, a casting is heated above the critical range to a temperature at which primary carbides and free cementite decompose to ferrite and graphite. If the casting is cooled slowly to below the critical range, a ferritic structure is formed and minimum hardness is obtained.

Medium-temperature annealing is used if massive carbides are absent. In this process, a casting is heated to just above the critical range, then slow-cooled.

Low temperature annealing, or ferritizing, heats a casting to just below the critical range, followed by slow-cooling. This is meant to convert pearlitic carbides, in the absence of free cementite, to ferrite and graphite by a gradual diffusion process, rather than by transformation.

Stress relieving is used to relieve stress in the subcritical stage to minimize distortion. Slow-cooling sand castings in the mold can rid them of residual stresses, but could result in a casting that is too soft. Selecting the proper stress-relieving temperature-time cycle is a compromise between the two.

In normalizing treatments, ferrous alloys are heated to a temperature above the transformation range and air-cooled to room temperature, creating a pearlitic microstructure. This gives the casting a higher hardness and strength than is obtained in the as-cast or annealed condition.

Further enhancement of strength properties can be achieved through a fourth iron heat treatment, austempering, in which a part is heated to a temperature range of 1,550-1,700F, cooled rapidly to 450-750F and then held at the austempering temperature to produce ausferrite. Austempering makes parts stronger and tougher than tempered martensitic structures obtained with conventional cast iron heat treatment.

 Aluminum Heat Treatment Tempers

In order to improve dimensional stability and corrosion resistance and enhance strength and ductility, aluminum castings can be thermally processed by a series of heat treating and cooling cycles involving solution, quenching and aging. Combinations of these processes are called tempers, which are outlined here.

T-4: Solution treat and age naturally to a substantially stable condition. Natural aging may continue slowly, particularly at elevated service temperatures, so structural stability may not be satisfactory.

T-6: Solution treat and age artificially. In castings, T6 commonly describes optimum strength and ductility.

T-61: Solution treat, quench and age artificially for maximum hardness and strength. This variant of T6 yields additional strength and stability but at reduced ductility.

T-7: Solution treat, quench and artificially overage or stabilize. This temper improves ductility, thermal stability and resistance to stress corrosion cracking.

T-71: Solution treat, quench and artificially overage to a substantially stable condition. This temper further increases thermal stability and resistance to stress corrosion cracking and reduces strength.

T-5: Age only. Stress relief or stabilization treatment. Cool from casting temperature and artificially age or stabilize (without prior solution treatment). Frequently, the as-cast condition provides acceptable mechanical properties but is accompanied by microstructural instability or undesirable residual stresses. Perhaps the possibility of in-service growth is the only constraint against using a casting in the as-cast state. In each case, the T5 temper is appropriate.

Annealing: Castings that have low strength requirements but require high dimensional stability are annealed. Annealing also substantially reduces residual stress, a need in die castings. Annealing is a severe stabilization treatment and an elevated temperature variant of the T5 temper. Softening occurs because annealing depletes the matrix of solutes, and the precipitates formed are too large to provide hardening.

 Iron Heat Treatments

Ferrous heat treatments are used to reduce residual stress and non-uniform mechanical properties and improve strength and toughness.

Annealing: The austenitic matrix of cast iron is slowly cooled through its critical temperature range, softening the material. This creates a ferritic microstructure. If the slow cooling is continued to a low enough temperature, residual stresses in the casting can be relieved.

Stress-Relieving: Used to relieve stress in the subcritical stage, minimizing distortion. Selecting the proper stress-relieving temperature-time cycle is a compromise that reduces residual stress while allowing the desired mechanical properties to be maintained in the casting.

Normalizing: Used to obtain a higher hardness and strength. Ferrous alloys are heated to a suitable temperature above the transformation range and then air-cooled to room temperature, thus creating a pearlitic structure.

Austempering: Castings are heated to produce an austenitic matrix, quenched rapidly to avoid the formation of ferrite and pearlite and held at the austempering temperature to produce ausferrite. This produces castings that are stronger and tougher than conventionally heat treated iron castings