“Postweld Heat Treatment (PWHT): any heat treatment subsequent to welding” This is a short definition of PWHT in ASME SEC IX. Although many think that PWHT is a good practice for any weldments, recent studies have shown vice versa. In this article a brief review of ASME STP-PT-033 report on degradation of toughness due to PWHT, will be discussed.
This report is a natural follow-up to three major studies sponsored by the ASME.
It should be noted that early publications identified a PWHT as a highly desirable treatment for weldments. As will be evident from the review that follows and the summary of key observations, this recommendation is suitable for some steels but unsuitable for others.
Some engineers have taken the position that a PWHT should be applied to all/most weldments. These views may reflect “poor teaching” by the academic community.
The Reheat Cracking is a potential issue with low alloys steel, the 300 series SS and super alloys such as Alloy 800H
2. Pros and Cons of PWHT:
- PWHT reduce the residual stresses which is important in controlling distortion , residual stresses also prevents the risk of SCC (Stress Corrosion Cracking). Hardness is sometimes used as an index of susceptibility to SCC. Sometimes the critical level of 20 Rockwell C is satisfactory.
- PWHT have the desirable effect of out gassing hydrogen , but in certain steels, it may introduce reheat cracking and/or reduction in notch toughness .
- The PWHT practice cannot only introduce cracks that may go undetected but also degrade notch toughness thereby reducing the tolerance for cracks.
3. Reheat Cracking
The undesirable effects of introducing reheat cracks is of special concern because the nature of these micro cracks is that they may not be detected by routine nondestructive testing such as radiography. Moreover, the reheat cracks are typically found in the coarse-grained region of the heat affected zone (HAZ) of a weldment. Reheat cracking from the bismuth addition are a form of liquid metal embrittlement. There are certain steels that are prone to reheat cracking:
Some equations have been developed to quantify the effect of specific alloying elements on reheat cracking:
Reheat Cracking Index=%Cr + 3.3 (%Mo) +8.1 (%V)-2
If the Index is above 0, reheat cracking may be observed. Note the powerful effect of vanadium on the susceptibility of steels to reheat cracking. Copper is also known to promote reheat cracking.
4. How to Reduce the Propensity for Reheat Cracking
- Controlled weld bead size thereby limiting grain coarsening,
- Keeping hydrogen levels below the threshold values for delayed cracking
- Quality control that avoids stress raising details of weldments
- Good inspection practices
- Practices that reduce the level of residual stresses
More and detailed information, could be find in the ASME STP-PT-033