Transient creep behavior of a novel tempered martensite ferritic steel G115

The transient creep deformation behavior and dislocation evolution in G115 steels were investigated in the temperature range 625-675 °C for applied stresses of 120-220 MPa. The transient creep curves showed that creep rate decreased with increases in time and creep strain. In this study, a novel method was proposed to determine the transient creep strain and the transient creep time. The strain decreased with increase in applied stress between 625 °C and 675 °C. A phenomenological constitutive equation was used to characterize the transient creep curves of G115 steels. The results showed that the constitutive equation for the G115 steel had a high precision. An internal stress was introduced to study the interactions of dislocations and precipitates, and a mechanism-based equation for transient creep in G115 steels was derived. The dependences of normalized strain and transient creep time on applied stress and temperature were analyzed in detail to better understand the transient creep deformation mechanism. It was concluded that dislocation annihilation at the grain boundaries was the dominant rate-controlling mechanism in the transient creep deformation of G115 steels. It was observed that the dislocation structures became more complex and no obvious textural features occurred after the transient creep deformation. The calculated dislocation density decreased at 650 °C relative to the initial value, which was mainly attributed to the annihilation processes occurring at the grain boundaries and the subsequent formation of subgrain boundaries.