Post Weld Heat Treatment and Formation of Untempered Martensite in 410 Steel Welds

Martensitic grades of stainless steels are primarily used in applications requiring increased hardenability while maintaining adequate corrosion resistance against halide stress corrosion cracking (SCC), polyphonic acid stress corrosion cracking, oxidation, and sulfur attack at elevated temperatures. Martensitic stainless steels are extensively used in the oil and gas industries where presence of sulfur is elevated in working conditions. A study on the unexpected high hardness and loss of toughness in Type 410 steel weldments experienced in industry has been carried out at the Welding Engineering Laboratory of the Ohio State University. A hypothesis of exceeding the A1 temperature during PWHT in the ASME recommended temperature range was evaluated using thermodynamic software Thermo-Calc™. It was fond that the A1 temperature in Type 410 steel base and weld metal can vary in a temperature range of 686°C to 850°C. A fairly good correlation between the A1 temperatures predicted with Thermo-Calc™ and experimentally validated with Single Sensor Differential Thermal Analysis (SS-DTA) has been found. Based off these results, the PWHT range of 760 to 800°C suggested by ASME B31.3 is likely to be too high for avoiding formation of untempered martensite in the weld metal and heat affected zone of Type 410 steel welds.