Heat treating iron castings: Part 1.

Many metal castings are designed with relatively uniform wall thickness and section size and therefore experience little significant residual stress and structural variation after solidification. However, iron castings can be prone to retain residual stresses and structural variations after cooling, especially when they have a complex shape with many interconnected thick and thin sections. This variation can cause distortion and non-uniform mechanical properties. Dimensional instability is most likely to become evident during machining. Annealing, stress relieving and normalizing are thermal treatments that can reduce the residual stress and non-uniform mechanical properties likely to be found in certain iron casting geometries.

Annealing Treatments

Annealing is a thermal treatment process that softens cast iron by slow-cooling the austenitic matrix (the phase where the iron is composed of a solid solution of cementite) through its critical temperature range. This creates a ferritic microstructure. Annealing can relieve residual stresses in castings if the slow cooling is continued to a low enough temperature.

There are three types of annealing for iron castings, high, medium and low (sub-critical). In high-temperature annealing, a casting is heated above the critical range to a temperature (1,650-1,750F/900-950C) at which both primary carbides and free cementite decompose to ferrite and graphite. A ferritic structure is produced and minimum hardness is obtained if the casting is cooled slowly to below the critical range. This slow cooling often is referred to as "furnace cooled."

In the absence of massive carbides or if minute quantities of well-dispersed carbides are present, medium-temperature annealing (also known as full annealing) is used. In this process, a casting is heated to just above the critical range (1,500-1,650F/ 820-900C), depending mainly on the silicon content, then slow-cooled.

Low-temperature (or sub-critical) annealing, also known as ferritizing, is accomplished by heating to just below the critical range, followed by slow cooling. The purpose is 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 Treatments

Stress relieving is a heat treatment used to relieve stress in the subcritical stage and thus minimize distortion. Sand castings slowly cooled in the mold are largely free of residual stresses. However, this slow cooling could result in a casting that is too soft and has mechanical properties that are out of specification.

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. Figure 1 shows the percentage of stress that is relieved at various temperatures during a one-hour treatment for a gray iron.

[FIGURE 1 OMITTED]

Normalizing Treatments

A normalizing heat treatment typically is applied to iron castings in order to obtain a higher hardness and strength than is obtained in the as-cast or annealed condition. In normalizing treatments, ferrous alloys are heated to a suitable temperature above the transformation range and then air-cooled to room temperature, thus creating a pearlitic structure.

The heating rate for normalizing is not important. The casting can be cooled in still air or with large fans. However, large differences in temperature in a complex casting below 1,000F (540C) can cause distortion or, possibly, cracking. Because the carbon diffusion rate in iron is rapid at normalizing temperatures, the required holding time is the time needed to obtain a uniform temperature throughout the casting.

Typical holding temperatures are about 100F (40C) above the critical temperature range as shown in Table 1. The determining factor is the silicon content, since increasing silicon content increases the critical temperature range.

The Iron Casting Engineering Handbook (ICEH) contains more information on iron heat treatment.

Table 1. Normalizing Temperature Range and Silicon Range
(in weight percentage) For Each Iron Family

                               Normalizing        Silicon Range,
Iron Family                Temperature, [F (C)]      (wt. %)

Malleable Iron                 1,475-1,525
                                (800-830)            1.2-1.9

High Strength Gray Iron        1,500-1,600
                                (810-870)            1.0-3.0

Lower Strength Gray Iron       1,550-1,650
                                (840-900)            1.5-3.0

Ductile Iron                   1,600-1,650
                                (870-900)            1.8-3.0