Suppressing type IV failure via modification of heat affected zone microstructures using high boron content in 9Cr heat resistant steel welded joints

Creep rupture strength at 923 K and microstructural evolution of welded joints have been investigated for high boron-low nitrogen-9Cr heat resistant steels developed at the National Institute for Materials Science ( Japan). Welded joints were prepared from plates containing 47 180 ppm boron using gas tungsten arc welding and Inconel type filler metal, and showed superior creep properties to those of welded joints of conventional high chromium steels such as P92 and P122. No type IV failure was observed in the boron steel welded joints. A large grained microstructure was observed in the heat affected zone heated to Ac-3 (Ac-3 HAZ) during welding, whereas the grains are refined at the same location in conventional steel welded joints. The simulated Ac-3 HAZ structures of the boron steels have a creep life almost equal to that of the base metal. Large grained HAZ microstructures and stabilisation of M23C6 precipitates are probable reasons for suppression of type IV failure and improved creep resistance of the boron steel welded joints.