Post-weld Heat Treatment in Pressure Vessels

October 26, 2022

PWHT (post-weld heat treatment) decreases the tensile residual stresses in a joint. 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. 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.

Because 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. 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.

Pressure Vessel Stress-Relief as an Important Factor

Post-weld heat treatment, often known as PWHT, is occasionally done to welded fabrications with the notion that it will give superior fatigue performance. This is sometimes the case. Although there are other valid reasons for PWHT, such as ensuring that dimensional stability is maintained during service or providing a reduced risk of brittle fracture, PWHT will rarely provide any benefit in terms of fatigue strength. Other valid reasons for PWHT include ensuring that dimensional stability is maintained during service; or providing a reduced risk of brittle fracture.

Under tensile loading, there is some evidence to suggest that the slope of the S–N curve for stress-relieved joints may be slightly higher than for as-welded specimens and that the endurance limit may be increased. In addition, the slope of the S–N curve may be slightly higher for stress-relieving joints than for as-welded specimens. Recent research on as–welded and stress–relieving joints that were then exposed to a variety of positive stress ratios showed the possibility of a rise in the fatigue limit but failed to show any change in either the slope or location of the S–N curve.

Stress-Relief in Large Pressure Vessels

Despite this, it is essential to issue a word of caution concerning the efficacy of stress alleviation, particularly in the case of significant fabrications. As mentioned earlier, the measurements taken before and after the stress alleviation in the experiments have demonstrated that there might still be quite large amounts of residual stress in welded joints even after full PWHT has been applied. Because long-range residual stresses can occur within the structure as a whole in addition to residual stresses that arise as a result of restraint within a joint, stress relief of large structures is subject to further doubt. This is because residual stresses can arise from restraint within a joint. As a consequence, extreme caution is always advised when contemplating the potential advantages of PWHT.

Other Techniques Considered

Other methods of lowering residual stresses may be specified, either used while welding or after the manufacturing process has been completed. Among these are utilising an interrun peening process, adopting a balanced welding method, and reducing vibratory stress. These methods may lower residual stress levels in some situations; nevertheless, it is highly improbable that they will positively affect fatigue performance. Vibratory stress alleviation can cause fatigue damage to the structure; therefore, it should only be utilised with the utmost discretion.

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