Pressure Vessel Stress-Relief as an Important Factor PWHT, on the other hand, does not affect the fatigue performance of the joint and may be performed for other purposes, such as dimensional control, increasing fracture performance, or preventing hydrogen cracking. If tensile residual stress of, say, 100 MPa persists after PWHT, practically all applied fatigue cycles (assuming stress ranges less than 100 MPa) still result in totally tensile stresses at the weld toe and are wholly destructive.īecause the whole applied stress cycle is almost still tensile, fatigue design regulations see no improvement in fatigue life and need the exact design methodology for as-welded and PWHT joints. It does not, however, decrease these levels to zero, and even in a very well-regulated thermal cycle, ultimate residual stress levels are unlikely to be much lower than 30% of the material yield strength. PWHT (post-weld heat treatment) decreases the tensile residual stresses in a joint. Post-weld Heat Treatment in Pressure Vessels
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