Consideration on Tempering and PWHT Temperatures of C-Mn and Low Alloy Steels Used for the Fabrication of Pressure Vessels: Smart Tuning of Heat Treatment Parameters

Abstract

For many years, process licensors and/or end-users have frequently specified that the tempering temperature of C-Mn alloys and low alloy steels (i.e. Cr-Mo, Mn-Mo-Ni alloys) should be greater than the post-weld heat treatments (PWHT). Most of the time, tempering temperature is then required as much as 30°C (54°F) above the PWHT temperature, making it very difficult for steelmakers to be able to supply compliant materials, especially for heavy wall components. Application of rules in the applicable codes often leads steelmakers to request for deviations in cases where they become not compatible with material capabilities. This report is intended to illustrate the combined effect of tempering and PWHT on materials properties and to provide recommendations on how to tune smart the tempering treatment with the aim of proposing the most efficient complete heat treatment sequence. Data provided within this paper for C-Mn steels and low alloy grades (Cr-Mo and Mn-Mo-Ni alloys) prove that tempering can be performed at temperatures below, at or above one of PWHT without any adverse effect. Data from actual mill production records show that stringent material specifications can be met by steelmakers when they are allowed to tune smartly the heat treatment parameters (tempering temperature) in accordance with applicable construction codes. The data also demonstrate that limiting the tempering temperature in the lower range of allowed temperatures may be beneficial to customers as it gives more safety margins for fabrication and maintenance (i.e. potential repairs/modifications) of pressure vessels. It permits either to consider more cycles for PWHT or to perform PWHT at higher temperatures or for longer durations, while on the opposite, current trend imposing high tempering temperatures limits flexibility.