Microstructure stability and mechanical properties of extremely low carbon deposited metal of 9Cr steels

An extremely low carbon deposited metal of 9Cr ferritic/martensitic (F/M) steel was independently designed and investigated on its mechanical properties and microstructure evolutions after post-weld heat treatment (PWHT) and thermal aging at 550 °C for 500, 1000, 3000, and 10,000 h. The results indicate that only the VN phase is present in the PWHT state, while many large-sized Cr 2 N and Laves phases (a semi-coherent precipitate, which has an orientation relationship of ( 1 ‾ 1 ‾ 0 ) α //( 0002 ) L , [ 00 1 ‾ ] α //[ 10 1 ‾ 0 ] L with martensite) precipitate after thermal aging. The Laves phase does not reach precipitation equilibrium during the aging process up to 10,000 h, while Cr 2 N reaches precipitation equilibrium after 1000 h aging. The coarsening rate of the Laves phase is maximum during aging 500–3000 h, which is relatively higher than that of Cr 2 N. There are two nucleation modes of the Laves phase: independent nucleation and dependent nucleation at the vicinity of Cr 2 N, the latter forms the granular Laves phase earlier. The results of mechanical properties tests show that the yield strength and elongation have little change during thermal aging. However, the impact absorbed energy decreases significantly after thermal aging. This is due to a large increase in the content of precipitates distributed on HAGBs , which leads to a dramatic reduction in the ability of the deposited metal to resist crack propagation .