Effects of Nitrogen Content and Tempering Treatment on Creep Rupture Properties of 9Cr-1Mo-V-Nb Steel with Simulated HAZ Heating

Creep rupture properties of 9Cr-1Mo-V-Nb steel welded joints change with chemical compositions and heat treatments. Therefore, simulated HAZ heating below or over Ac1 temperature of 830°C was conducted in order to investigate microstructural changes of welded joints of the steel. Effects of nitrogen contents and tempering treatments on hardness and creep rupture properties were discussed. The results are summarized as follows.(1) Hardness and creep rupture strength of the steel heated at 850°C ( simulated HAZ heating) decreased significantly after post-weld heating at 740°C for 5 h. It was confirmed that recovery and softening of martensitic structure were accelerated by partially γ-transformed heating just over Ac1 temperature.(2) On the above simulated HAZ heating, finely dispersed Vanadium Nitride (VN), which increased in amount by addition of nitrogen, was observed. These precipitates were confirmed to retard recovery, and to improve creep rupture strength effectively.(3) In case of relatively low temperature tempering at 620°C, excess free carbon and nitrogen precipitated as fine VN and M23C6 during simulated HAZ heating and post-weld heat treatment. These precipitates were confirmed to improve creep rupture strength.