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PWHT EXEMPTION REQUIREMENTS IN ASME CODES In order to guess the technical background on PWHT exemption, PWHT exemption requirements in several ASME codes are reviewed. ASME codes suggested that PWHT is not mandatory when the nominal material thickness is not more than the thickness in Table 1.   There was some variation between codes as shown in Table 1, and the reason for this has been not known. There are the general trends that the permitted thicknesses without PWHT is increased with increasing preheat temperature together with reductions in maximum carbon level, and the permitted thickness is decreased with increasing alloy content of steels. Most manufacturers are trying to avoid PWHT based on the Table 1 because there is an economic incentive to avoid PWHT.  REFERENCES   [1] Pingsha Dong and J.K.Hong, "Residual stress relief in post-weld heat treatment", 2008 ASME Pressure Vessels and Piping Conference, PVP2008-61210.  [2] ASME Section III, NX, "Rules for Construct