Effect of cooling modes after austenitization on stabilities of microstructure and hardness of P92 steel aged at 650℃

Abstract

The stabilities of microstructures and hardness of P92 steel samples austenitized at 1060℃ for 1h followed by air cooling (AC)/slow cooling (SC) and tempered at 760℃ for 2h and then aged at 650℃ for 1000h/3000h were studied by SEM-SE/BSE,EDS and multiphase separation technology (MPST). The results show that the microstructures of P92 steel aged samples consist of martensite and precipitations (M23C6 and Laves phase). The cooling modes after austenitization have an obvious effect on microstructures and hardness of the samples aged at 650℃. Compared with the AC samples, the total volume fraction of the precipitations/Vickers hardness of the SC sample is higher/lower and the volume fraction of the Laves/M23C6 phase is remarkably higher/lower. Moreover, the Laves phase particles of SC sample is much more coarsened. The hardness of the SC sample decreases after ageing treatment at 650℃ for 1000h/3000h, while that of the AC sample is not obviously varied. In addition, the stabilities of microstructure and hardness of P92 steel aged at 650℃ are related to the stability of the austenite which in return affects the stability of the martensite transformed during cooling. Therefore, the P92 pipes cooled from austenitization should dissipate heat as soon as possible in the manufacturing scene.

Keywords

• P92 steel
• austenitization
• cooling mode
• ageing treatment
• microstructure
• microhardness