Stress Relief Heat Treatment
The most common form of PWHT is a “stress relief” or “tempering” heat treatment. This heat treatment involves heating the material to a temperature high enough to significantly relax residual stresses from welding, but low enough to avoid metallurgical phase transformation. Heat treatment at this temperature accomplishes the following functions:
• Reduces residual welding and forming stresses. This is the main benefit of any PWHT. Reducing residual stresses improves resistance to corrosion and all forms of stress corrosion cracking. It allows for dimensional stability during machining, and improves mechanical properties like impact toughness and ductility. See Figure 600-23 for an example of how various PWHT temperatures and holding times affect residual stress levels in weldments. Note that PWHT temperature is much more important than holding time.
• Softens (or “tempers”) hard weld metal HAZs. This only applies to hardenable carbon and alloy steels. Austenitic stainless steels are not thermally hardenable. See Figure 600-24 for the effect of various PWHT temperatures and holding times on weld metal hardness. Note again that PWHT temperature is much more important than holding time.
• Outgasses hydrogen from the weld metal. Outgassing helps prevent delayed hydrogen cracking of hardenable carbon and alloy steel weldments when cooled to ambient temperature.
The need for “stress relief” depends on several factors. Some of the applications where stress relief should be specified are summarized below. (See Section 100 of the Welding Manual, the Corrosion Prevention and Metallurgy Manual, or consult a materials engineer for additional guidance on when to specify PWHT.)
1. All carbon steel welds greater than 1.5-inches thick should be stress relieved to reduce residual stresses and to improve impact properties (ASME Code requirement).
2. All C -½ Mo welds greater than 5/8-inches thick should be stress relieved to reduce weld metal and HAZ hardness and improve impact properties (ASME Code requirement).
3. All chrome-moly alloy steel welds should be stress relieved to reduce weld metal and HAZ hardness and improve impact properties (ASME Code requirement in some cases).
4. Many austenitic stainless steel welds and cold formed parts should be stress relieved for resistance to “chloride stress corrosion cracking.” Some general guidelines are presented below. Consult a materials engineer for additional guidance.
– Regular carbon grades (Types 304 and 316) of austenitic stainless steels should not be stress relieved. This avoids loss of corrosion resistance due to excessive sensitization at temperatures between 800°F and 1500°F.
– Stress relief should be considered for low carbon grades (Types 304L and 316L) and stabilized grades (Types 321 and 347) of austenitic stainless steels for insulated vessels which operate continuously or intermittently above 150°F. (Even “low-chloride” types of insulation will concentrate chlorides.)
– Stress relief should also be considered for low carbon and stabilized grades of austenitic stainless steels for uninsulated vessels which operate continuously or intermittently above 150°F if they are in “high-chloride” environments. (Examples of some high chloride environments are: coastal locations and off-shore platforms with salt spray, locations with salt water fire spray systems, and process streams with high chloride contents.)
5. Carbon steel and austenitic stainless steel welds and cold formed parts should be stress relieved for resistance to “caustic stress corrosion cracking” (also called “caustic embrittlement”) if above 140°F for concentrations from 1 wt% to 30 wt% caustic, and if above 110°F for concentrations greater than 30 wt%.
6. Carbon steel welds and cold formed parts should be stress relieved in the following services for resistance to various types of stress corrosion cracking (SCC):
– Amines (DEA, MEA, MDEA, DIPA, etc.) at any temperature, for resistance to “amine SCC”
– Concentrated anhydrous ammonia at any temperature, for resistance to “ammonia SCC”
– Potassium carbonate at any temperature, for resistance to “carbonate SCC”
– Ammonium nitrate and calcium nitrate above 110°F, for resistance to “nitrate SCC”
– Hydrofluoric acid at any temperature, for resistance to “hydrogen embrittlement cracking”
– FCC Fractionator overhead systems, for resistance to “carbonate SCC.” (Stress relief temperature should be increased to 1150 — 1250°F for this service.)
– Sour (wet H2S) services, for resistance to “sulfide SCC” (also called “hydrogen embrittlement cracking”).
7. Carbon steel welds in specific severe sour services where “hydrogen blistering” and “hydrogen induced cracking” (HIC) have previously occurred (referred to as “HIC” services) should be stress relieved. Special “HIC resistant steels” should also be used. (See Specification PVM-MS-4750 for additional details.)
8. Carbon steel welds and cold formed parts in boiler feedwater deaerators which use steam should be stress relieved to avoid “corrosion fatigue.”
Minimum recommended stress relief temperatures and holding times are summarized in Figure 600-25 for common pressure vessel materials. (Note that these recommendations exceed the ASME Code minimum values for some materials.) See Sections 100, 300, and Appendix A of the Welding Manual for other materials or for additional information on stress relief heat treatment.
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