Mechanical Properties and Creep Behavior of MX-Type Precipitates Strengthened Heat Resistant Martensite Steel

There has been a push in the past few decades to increase the operating temperature of steam generators to the ultra-supercritical (USC) regime. This requires that creep-resistant alloys can operate at 650-700 degrees C for 30 years. P91 and P92 steels are commercially applied in USC steam generator applications. However, these steels fail due to the coarsening of M23C6 and Laves phases during long-term service. Therefore, it is significant to restrict the formation of easily coarsened precipitates. In this study, a martensitic heat-resistant steel strengthened by single MX precipitates is designed using the Thermo-Calc software, as Fe-0.03C-10Cr-0.2Zr-0.3V. The yield strength, tensile strength, and elongation at room temperature are 266 MPa, 413 MPa, and 38%, respectively. The high-temperature hardnesses of specimens aged at 700 degrees C for 1, 10, 100, and 1000 h were tested at 700 degrees C after normalizing treatment, which illustrates that the high-temperature hardness of the specimens remains stable with increased aging time. In addition, TEM was used to characterize the precipitates in the heat-resistant steel aged for different times. It is found that with the increase of aging time (1-1000 h), the average size of the precipitates increases from 10.8 to 17.8 nm. The composition of MX precipitates in the specimens aged for 1000 h is Zr0.46Nb0.14C0.4 and the volume fraction is 0.29%. According to the creep test results, the threshold stresses at 650 and 700 degrees C are 54.5 and 28.4 MPa, respectively.

Automated summary

In recent decades, there has been a push to increase the operating temperature of steam generators to the ultra-supercritical regime. However, creep-resistant alloys face the challenge of coarsening of M23C6 and Laves phases during long-term service. Therefore, restricting the formation of easily coarsened precipitates is significant.