Microstructure and mechanical properties evolutions of 9Cr heat resistant steel deposited metals with different Si contents during 550 °C aging process

Abstract

The stability of microstructure and mechanical properties of 9Cr heat resistant steel under high temperature and long term service conditions has been a hot issue of great interest. In the present study, the relationships between microstructure and mechanical properties evolutions of 9Cr heat resistant deposited metals with different Si contents (0.25 wt.% and 1.07 wt.%) at 550 °C aging for different times were investigated systematically. The results indicate that the degradation of mechanical properties during aging is mainly attributed to the coarsening of precipitates, transformation of precipitate types, decreasing of dislocation density and coarsening of martensite laths, and these factors are related to the Si content in the deposited metals and the aging time. It is worth mentioned that the sub-grain boundaries pinned by M23C6 forming after post weld heat treatment (750 °C/2.5 h) stabilize the tensile strength during aging. The recrystallization occurring after post weld heat treatment and high temperature aging is beneficial for the impact toughness but not for the strength. During aging process, the precipitate transformation follows the following sequence: M23C6→M6X→Laves phase. Moreover, the increase of Si content promotes the nucleation and coarsening of M6X, and then deteriorates the mechanical properties of the deposited metals.

Keywords

• Silicon content
• Tensile strength
• Impact toughness
• High temperature aging
• Post weld heat treatment
• Deposited metal