Microstructural investigations on Type IV cracking in a high Cr steel

In the present study, microstructural changes that lead to Type IV cracking in a high Cr ferritic steel (11Cr-0.5Mo-2WCuVNb) weld joint has been investigated. Microstructure of the heat affected zone (HAZ) of a weld joint made by gas tungsten arc welding (GTAW) process was systematically studied in the as-welded condition, after different post weld heat treatments (PWHT) and after creep test. In addition, HAZ microstructures were simulated both by heat treatment and by using a weld simulator and creep tests were conducted using specimens with simulated HAZ microstructures. Results showed that, undissolved precipitates present in the fine grained HAZ (FGHAZ) and intercritial HAZ (ICHAZ) accelerate the tempering of these zones during PWHT and microstructural deterioration during creep. Creep tests of HAZ simulated specimens indicated that the creep was minimum for the specimens in which peak temperature of simulation was close to Ac-3, Results also suggested that differences in creep properties observed between an actual weld joint and that of a simulated Ac-3, microstructure could be due to mechanical constraints present in the weld joint in which a narrow zone of lower creep strength (FGHAZ) is sandwiched between zones of higher creep strength (coarse grained HAZ (CGHAZ) and weld metal on one side and base metal on the other side).