Characterization of Microstructure and Mechanical Property Evolutions of 42CrMo Steel Served at Elevated Temperatures

42CrMo steels are widely used for many general-purpose components served under thermal environment, owing to their relatively high strengths and low costs. The evolutions of microstructure and mechanical property of 42CrMo steel served at 425, 475, 500, 525, 550 and 575 for 500 h were reported in this paper, aiming to reveal the phase transformation and mechanical property evolution characteristics of the steel served at elevated temperatures. The microstructural investigation and phase composition measurement were performed using thermodynamic calculations, optical microscopy and scanning electron microscopy techniques. The extent of spheroidization for the microstructure was measured through the average aspect ratio of cementite phase. Mechanical properties were characterized through tensile strength, micro-hardness measurements and fracture morphology observation for the steel samples. Results showed that the microstructure consisting of ferrite and lamellar cementite was relatively stable below 500 °C. As the temperature increased from 500 to 575 °C, the spheroidization, dissolution of cementite and the softening of the ferrite matrix combined contributed to the degradation of the mechanical properties. The evolution rules of Cr and Si elements segregated in phases during these temperatures were revealed. A method of aspect ratio measurements (δ = 0.322–0.584) for cementite particles was proposed to assess the microstructure evolution and strength degradation (σb = 650–963 MPa) of the steel. Based on the above, the variation of property for the steel served at elevated temperatures can be naturally divided into three stages, including no change of cementite stage (T < 500 °C), spheroidization of cementite stage (500 °C ≤ T ≤ 550 °C) and dissolution of cementite stage (T > 500 °C). Additionally, the ultimate service temperature range of this type of steel was discussed.