Stress Relief of Steels

When manufacturing metal parts, there will be a certain amount of stress induced into the part. Stress can be caused by welding, machining, cutting, drilling, grinding and any other process that moves metal. The stress can sometimes cause noticeable distortion during the manufacturing process. Residual stress in a part can be detrimental in service, cracking and general instability can result, but to combat the problem, we use steel stress relieving.

Stress relieving is applied to both ferrous and non-ferrous alloys.

Carbon steels may be stress relieved by holding a temperature at 1100 to 1250°F (600 to 675°C) for 1 hour per inch (25 mm) of thickness. Stress relieving offers several benefits. For example, when a component with high residual stresses is machined, the material tends to move during the metal removal operation as the stresses are redistributed. After stress relieving, however, greater dimensional stability is maintained during machining, providing for increased dimensional reliability.

Temperature, time and time at temperature are key process variables are showed in Table 1 for many alloys. In general, heating steel to a temperature of about 165°F (75°C) below the transformation temperature (Ac1) for an hour (or until the entire part reaches the temperature), will allow for removal of most internal stresses. Typical temperature ranges are:

For many alloy steels, little or no stress relief occurs at temperatures less than approximately 500°F (260°C), while approximately 90% of the stress is relieved by 1000°F (540°C). The maximum temperature for stress relief is limited to 55°F (30°C) below the tempering temperature used after quenching from the hardening process. After removing from the furnace or oven, the wire must be cooled in still air. If cooled in any other manner, stresses are reintroduced into the part.

Table 1: Oven Stress Relief Temperatures and Times for Wire

As can be seen in Table 1, Stainless steels may be stress relieved too. There are several stress relief treatments.

• Stress redistribution at 290 to 425°C (550 to 800°F), which is below the sensitization range. When stainless steel sheet and bar are cold reduced greater than about 30% and subsequently heated to 290 - 425°C (550 - 800°F), there is a significant redistribution of peak stresses and an increase in both tensile and yield strength. Stress redistribution heat treatments at 290 - 425°C (550 - 800°F) will reduce movement in later machining operations and are occasionally used to increase strength. Since stress redistribution treatments are made at temperatures below 425°C (800°F), carbide precipitation and sensitization to intergranular attack (IGA) are not a problem for the higher carbon grades.

• Stress relief at 425 to 595°C (800 to 1100°F) is normally adequate to minimize distortion that would otherwise exceed dimensional tolerances after machining. Only the low carbon "L" grades or the stabilized 321 and 347 grades should be used in weldments to be stress relieved above 425°C (800°F) as the higher carbon grades are sensitized to IGA when heated above about 425°C (800°F).

• Stress relief at 815 to 870°C (1500 to 1600°F) is occasionally needed when a fully stress relieved assembly is required. Only the low carbon "L" grades, 321 and 347 should be used in assemblies to be heat treated in this range. Even though the low carbon and stabilized grades are used, it is best to test for susceptibility to IGA per ASTM A262 to be certain there was no sensitization during stress relief treating in this temperature range. Thermal stabilization treatments at 900°C (1650°F) minimum for 1 to 10 hours are occasionally employed for assemblies that are to be used in the 400 to 900°C (750 to 1650°F) temperature range. Thermal stabilization is intended to agglomerate the carbides, thereby preventing further precipitation and intergranular attack (IGA). As with 815 to 870°C (1500 to 1600°F) stress relief, it is best to test for susceptibility to IGA per ASTM A262.

Industries that use steel stress relieving

References

1. Tool Steel, Accessed 06-2016

2. Stress Relieving, Normalising and annealing, CHTA Contract Heat Treating Association, 1996;

3. Heat Treating-Stress Relieving, Accessed 06-2016;