Study of creep and creep crack growth fatigue of aging ASTM A297/A297M-19 HP steel modified with niobium

The ASTM A297/A297M-19 HP steel is a commonly used material in high-temperature structural components. In this study, researchers examined the effects of niobium modification on the mechanical properties of HP steel. The tests included hardness, tensile, creep, fatigue, and metallographic analysis. The heat treatment of aging at 927 ºC for 1000 h resulted in the precipitation of secondary carbides and G-phase presence. The aging caused intense precipitation in the interdendritic space, where the chromium carbides coalesced and became coarser. The aged specimens showed an increase in hardness by approximately 17%. The tensile tests showed an increase in mechanical resistance parameters and a decrease in total elongation. The Charpy impact tests presented lower values at 927 ºC. In the creep tests, the stress exponent showed a sharp decrease at the highest temperature of 1093 ºC. The fatigue crack propagation rate was higher at 927 ºC than at 25 ºC due to the material’s better ductility at high temperatures. In creep crack growth tests, a decrease in the crack growth rate was observed in the second stage. These experimental results are important for understanding the ability of the modified HP steel to withstand fatigue and creep mechanisms at elevated temperatures from a time-dependent fracture mechanics perspective.