Question:
We are looking into the stress relief of large 4-inch-thick (100 mm) plate made of 316L stainless steel. Can you advise us on the proper stress-relief temperature? Our experience suggests 1100-1150ºF (590-620ºC) might be better than what we find in the literature. Please comment.

Answer:
Your question on the stress relief of 316L plate is a good one. While the 450-750ºF (230-400ºC) stress-relief temperatures are recommended for dimensional stability (Table 2), there is evidence to suggest that they may not be as effective for plate as using higher temperature, that is 1100-1150ºF (590-620ºC) that you mention. In either case, a slow cooling rate is the key.

A. Stress-Relief Objectives
1. Provide dimensional (or shape) stability. The risk of distortion can be reduced during forming or machining operations by stress relief.
2. Removal of residual (tensile stresses). The risk of stress corrosion cracking (SCC) is minimized by stress relief.
3. Reduce the stress state in cold-worked structures so as to improve corrosion resistance.

B. Important Raw-Material Considerations
1. Annealed condition (from the mill)
2. Cleanliness (inclusions, trace elements)
3. Grain size

C. General Considerations[1]
1. Low-temperature, that is, 450-750ºF (230-400ºC) stress relief results in modest reduction of internal stress and is especially useful when austenitic stainless steels have been cold worked to develop high strength since it will also increase the proportional limit and (compressive) yield strength.
2. High-temperature, that is, 1100-1150ºF (590-620ºC) stress relief significantly reduces residual stresses that might otherwise lead to SCC or dimensional instability in service.

D. Stress-Relief Time (Table 1)

E. Stress-relief temperatures (Table 2)

Key:
1 = 1925-2050ºF (1050-1120ºC), slow cool
2 = 1650ºF (900ºC), slow cool
3 = 1925-2050ºF (1050-1120ºC), fast cool
4 = 440-900ºF (225-480ºC), slow cool (approximately 4 hours per inch on cross section)

Notes:
Treatment 2 is also intended to reduce the risk of “knife-line” attack in stabilized grades (due to the solution of titanium or niobium carbides at higher annealing temperatures).

F. Special Notes
1. The low-carbon grades (304L or 316L) or the stabilized grades (321 or 347) should not be at risk from corrosion sensitization during stress relief. Grades such as 304 or 316 are, therefore, processing in the temperature range of 900-1600ºF (480-870ºC) should be avoided. Slow cooling through this range can also be problematic.
2. A full solution-anneal stress-relief heat treatment will retransform any martensite formed back to austenite. Slow cooling is the key as uneven cooling leads to higher stress states.
3. 316L contains molybdenum and is susceptible to sigma-phase formation as a result of long exposures in the range of 1200-1600ºF (650-870ºC).