Investigation on gradual degradation of mechanical property and microstructure in 9% Cr heat-resistant steels via interrupted creep test

High-temperature strength and microstructure stability in heat-resistant steels are the major concerns for their practical applications. A comprehensive understanding of the microstructure evolution and mechanical property degradation during creep exposure is of significant importance for safety operations. In this work, a series of interrupted creep tests of 9% Cr steels were conducted at 873 K and 160 MPa exposed to different creep durations. The gradual degradation of mechanical properties and microstructure evolution such as dislocation density, martensite lath, precipitates and grain boundary characteristics were studied. It was found that the high-temperature yield strength and micro-hardness significantly decreased from the initial state to 1000 h duration then kept relatively stable to 1500 h duration, and gradually declined until rupture. The degradation of mechanical properties in primary creep stage was mainly attributed to decline in dislocation density and coarsening of martensite lath. Coarsening of martensite lath, as well as precipitation strengthening played a significant role in secondary creep stage. In tertiary creep stage, synergistic effect of coarsening of martensite lath and precipitates and decline of dislocation density resulted in accelerated degradation of mechanical properties. The precipitation of Laves phase in early stage provided additional strengthening effect and compensated for the strength loss caused by the coarsening of martensite lath, contributing to the stability of mechanical properties from 1000 to 1500 h. Moreover, the growth of Laves phase induced insignificant stain concentration and ductility deterioration. Quantitative relationships between microstructure and mechanical properties were established and well-validated by experimental results.