Mechanical softening of tempered martensitic steel resulting from long-term aging and creep

Abstract

The mechanical softening of tempered martensitic steel has been characterised as a result of long-term aging and creep. Normalised and tempered steels were heat-treated at 700°C for 4000 h at each predetermined aging time. Creep tests were conducted at the same temperature as the aging test under a constant stress of 60 MPa. The microstructure showed significant changes in precipitate size, precipitate number, dislocation density and martensite lath width with aging and creep. Moreover, the microstructural changes of the creep damage were more significant than with aging, indicating that stress has an important effect on micromechanical degradation. The hardness and creep damage decreased with respect to the micromechanical degradation. The strength degradation of long-term aging and creep specimens were discussed with the support of microscopic analyses and mechanical tests.

Keywords:

• Tempered martensitic steel
• Aging
• Creep
• Degradation