Sensitization Behavior of Type 308 Stainless Steel Weld Metals after Postweld Heat Treatment and Low-Temperature Aging and Its Relation to Microstructure

摘要

Type 308 weld metal overlaid on a low-alloy steel by a submerged arc welding (SAW) process using wide strip electrodes can become sensitized by not only postweld heat treatments (PWHTs) at around 600 deg C but also low-temperature aging (LTA) at 500 deg C for 24 hours after PWHT. A slow strain-rate test (SSRT) in oxygenated, high-temperature pure water revealed that type 308 SAW weld metals after PWHT were susceptible to intergranular stress corrosion cracking (IGSCC) when sensitized such that the intergranular corrosion (IGC) rate in the modified ASTM A262E test exceeds 1 mu m/h. The effect of microstructure (ferrite phase distribution) on the sensitization behavior was evaluated using a microstructural parameter N_L~(alpha-gamma). Though mostly useful, the average N_L~(alpha-gamma) was not always accurate in distinguishing IGC-susceptible, SAW overlay weld metals. However, local N_L~(alpha-gamma) newly introduced to deal with a rather large fluctuation in ferrite distribution within SAW overlay weld metals, was partly able to explain the IGC susceptibility of a theoretically resistant heat with a high average N_L~(alpha-gamma). The beneficial effect of surface remelting by plasma arc on the IGC resistance was described.