Mechanical property change and precipitate evolution during long-term aging of 1.25Cr-0.5Mo steel

The effect of long-term aging at 550 °C on the hardness variation and precipitation evolution of a 1.25Cr-0.5Mo steel with ferrite/pearlite structure was investigated. Also, creep-rupture test of the steel was conducted at 550 °C. The hardness value generally decreased with increasing aging time due to the spheroidization of cementite (M₃C). Interestingly, the hardness value was almost maintained at the aging time ranging from 1,000 to 5,000 h most likely due to the formation of fine needle-like M₂C precipitates in ferrite regions. After 5,000 h of aging, the hardness value again decreased with the decrease in the amount of M₃C. X-ray diffraction and transmission electron microscopy indicate that the amounts of M₂C and M₇C₃ increases instead of M₃C and some of M₇C₃ precipitates nucleated at M₃C particles, growing at the expense of M₃C around ferrite/pearlite interfaces. On the whole, the simulated precipitation kinetics of the steel using the MatCalc software well described the precipitation sequence. The creep-rupture strength of the steel drastically decreased especially after 1,600 h of rupture time, although it generally decreased with increasing rupture time. The accelerated dissolution of M₃C under creep stress/strain was responsible for the drastic decrease in creep-rupture strength.