In heat treating, rapid cooling/quenching alone or in combination with pre-existing internal stresses can result in unwanted distortion, brittle fracture and stress corrosion cracking if near welds in certain grades of metal.
How Do We Perform a Stress-Relief Operation?Stress-relief operations are typically done by subjecting the parts to a temperature approximately 40-75ºC (105-165ºF) below the A1 transformation temperature - about 727ºC (1340ºF) for steel. Stress relief is typically performed for carbon steel at approximately 500-650ºC (930-1200ºF). The elimination of stress is not instantaneous (that is, the process is a function of both temperature and time). To achieve the maximum benefit, some time at temperature (typically one hour per 25 mm of cross-sectional area once the part has reached temperature) is required. This removes more than 90% of the internal stresses. Stress relief on alloy steels is often done at (slightly) higher temperatures. After removal from the furnace or oven, the parts are air cooled in still air. Rapid cooling will only serve to reintroduce stress and is the most common mistake made in stress-relief operations.
For tool steels, the process is similar. It is common to perform a stress-relief operation in the temperature range of 500-550°C (925–1025°F) allowing the parts to slowly cool to room temperature before subsequent operations.
For stainless steels, the situation is more complex. Stress relief is done in the range of 290-425°C (550-800°C), which is below the sensitization range. The operation is typically broken down into several cases.
1. When stainless steel sheet and bar experience a reduction in area in the neighborhood of 30%, there is a peak in internal stress coupled with an increase in both tensile and yield strength. Stress relief will help reduce movement in subsequent machining operations. Since it is performed at temperatures below 425°C (800°F), carbide precipitation and sensitization to intergranular attack (IGA) are not a problem even in the higher carbon grades.
2. After machining, stress relief at 425-595°C (800-1100°F) is normally adequate to reduce stress and minimize distortion that would otherwise exceed dimensional tolerances after machining. If weldments are involved, only the low-carbon "L" grades or the stabilized 321 and 347 grades above 425°C (800°F) as the higher carbon grades are sensitized to IGA when heated above about 425C (800F).
3. A completed assembly subject to stress relief at 815-870°C (1500-1600°F) is occasionally needed.
- Only the low carbon "L" grades, in addition to 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 (Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels) to be certain there was no sensitization as a result of stress relief in this temperature range.
- Thermal stabilization at 900°C (1650°F) minimum for one to 10 hours is occasionally needed for assemblies that are to be used in the 400-900°C (750-1650°F) temperature range. The process is intended to agglomerate the carbides, thereby preventing further precipitation and intergranular attack. Again, it is recommended to test for susceptibility to IGA per ASTM A262.
Applications of Stress ReliefBicycle frames are one example of a product that benefits from stress relief. Bicycle tubing is subjected to a great deal of cold work, and it has been documented that internal stresses lead to reduced fatigue life for a steel frame only when the combined external load stresses plus internal stresses rise above the critical stress level (also referred to as the "fatigue threshold"). For steels, this is about one half of the yield strength. Loads causing stresses below this level do not cause fatigue cycles.
Another example is molds and dies subjected to a stress-relief operation to ensure consistent performance and aid in machine distortion control, long-term shape stabilization and reduce premature cracking in service.
To Be Technically Correct
The proper heat-treatment term is “stress relief,” and the phrase “stress relieving” while often used is incorrect.