Effect of heat treatment on the stress corrosion cracking (scc) susceptibiliy of the 13Cr martensitic stainless steel for steam turbine blade

Abstract

Two different constant loads have been used to evaluate the effect of heat treatment on susceptibility to stress corrosion cracking (SCC) of 13 Cr martensitic stainless steel in 3.5% NaCl. The steel specimens were austenitized at 1050°C for 1 hour followed by oil quench to obtain martensite structure. Then, specimens were tempered at 400, 500, 600, 650 and 700°C for 1 hour. To investigate the SCC susceptibility, SCC tests were carried out at two different constant loads of 40% and 80% of ultimate tensile strength (UTS) in accordance with the method developed by Nishimura et.al. After the SCC tests, scanning electron microscope (SEM) observations of the fracture surface on the fractured specimens were conducted to investigate fracture mode. The experimental results showed that the 13Cr martensitic stainless steel was highly susceptible to SCC in a solution 3.5% NaCl at high constant load (80% of UTS) in almost tempered conditions. The least susceptible steel was the tempered one at 700°C. At lower constant load (40% of UTS), the highly susceptible steels were the tempered ones at 400 and 500°C. Steels which were tempered at high tempering temperature were not failed for 240 hours of SCC testing. The mechanism responsible for this susceptibility is thought to be secondary phenomenon where carbide precipitation of M7C or M23C6 occurs. The fracture surfaces exhibited intergranular cracks and dimple rupture fracture for steels tempered at 400, 600 and 650°C and transgranular cracks and cleavage fracture for steels tempered at 500°C.