Local PWHT in ASME Codes

Local PWHT in ASME Codes BAHER ELSHEIKH INTRODUCTION ASME B31, Pressure Piping codes, historically have different requirements of preheat and PWHT until the recent years especially from 2014 editions when some of the requirements changed as an attempt to close this gap and achieve consistency between the different code sections. For example, in edition 2014 of ASME B31.3, the mandatory PWHT for welds in P. No.1 materials eliminated. In 2018, The new standard ASME B31P-2017 (Standard Heat Treatment for Fabrication Processes) released to govern the requirements of the heat treatment (preheat, PWHT and Post forming heat treatment) for all ASME B31 code sections. So far this standard is not referenced in the most recent ASME B31 codes as it is just released in 2108. It is expected to have this change in the next editions of the ASME B31 codes by referring to the new standard ASME B31P in the applications of the heat treatments. The below graph indicated the definition and extend of each of the heat treatment terms: Soak Band (SB), Heated Band (HB) and Gradient Control Band (GCB). These terms are clearly identified in the ASME B31P and previously in the existing editions of ASME B31 codes and also in WRC 452 (Recommended practice for local heating of welds in pressure vessels) and in AWS D10.10. This is not the case in ASME Boiler and Pressure Vessel Code Sec. VIII Div.1 which includes only the soak band while there is no specific requirements for the HB or GCB. Although the code requires the portion of the vessel outside the sank band to be protected so that the temperature gradient is not harmful. But the absence of clear requirement led to the misunderstanding and improper applications of the local PWHT in some cases. This is very critical especially in the thick vessels where the application the heating limited only to the soak band which is not sufficient to have uniform heat at the desired temperature values across the vessel wall. In such cases, the weld and HAZ might have high stress values even after the application of the PWHT which made it susceptible to the degradation and may be failure. -1- Baher Elsheikh Local PWHT in ASME Codes The above photo is for a 90 mm thickness P11 (1.25 Cr- 0.5 Mo) vessel during the fabrication local PWHT process of the closure weld. The white band is the insulation applied which covers only the soak band while the other imaginary dark red band is the extent of the GCB as it should be per the requirements of BS5500 as identified in WRC 452. This weld cracked after few years of operations due to the improper application of the PWHT. REFERNCES: [1] ASME B31.3 – 2016 [2] ASME B31.1 – 2016 [3] ASME BPVC SEC. VIII DIV.1 – 2017 [4] ASME B31P- 2107 [5] WRC 452 – June 2000 Author: Baher Elsheikh, Mechanical Engineer Linkedin Profile : https://www.linkedin.com/in/baher-elsheikh-6687a63b/ -2- Baher Elsheikh