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

Abstract

Abstract The HP steel has been commonly used in structural components that work at high temperatures, up to approximately 1150 ºC, with 927 °C being a typical working temperature. In this work, the mechanical properties of ASTM A297 / A297M-19 HP steel modified with niobium were characterized at as-cast microstructural conditions and after aging at 927 ºC for 1000 hours through mechanical tests of hardness, tensile, creep, fatigue, and metallographic analyzes. It was observed that the heat treatment of aging with the time of 1000 hours at 927 ºC provided the precipitation of secondary carbides (Cr23C6) in the austenitic matrix of the alloy and the presence of G-phase. Thus, the heat treatment of aging caused intense precipitation in the interdendritic space, where the chromium carbides coalesced and became coarser. The hardness results showed an increase of hardness by approximately 17% in the aged. In the tensile tests, it was observed that the aging increased the value of the mechanical resistance parameters and decreased the total elongation to approximately 48% when compared to the values obtained in the as-cast condition. The results of the Charpy impact tests on specimens with V-notches, performed at a temperature of 25 ºC, presented values lower than the values obtained at a temperature of 927 ºC in all conditions tested. In the creep tests, for the as-cast material, small variations of the Norton Diagram's stress exponent (n) were observed in the temperature range of 871 to 1066 ºC. It was noted that at the highest temperature of 1093 ºC, the stress exponent showed a sharp decrease due to the transition into dislocation-climb controlled mechanism zone. The fatigue crack propagation tests showed that the crack propagation rate da/dN, for each condition of the material, is higher at 927 ºC than at 25 ºC since the material presented better ductility at high temperatures. In creep crack growth tests, it was observed that the lower initial stress intensity factor (Ki) applied provided a decrease in the crack growth rate in the second stage when compared to the higher Ki value. The experimental results raised in the development of this work provided values of the parameters of monotonic mechanical strengths, nucleation, and crack propagation by fatigue and creep, which are of fundamental importance for the characterization, from the perspective of time-dependent fracture mechanics, the ability of the ASTM A297 / A297M-19 HP modified with niobium to resist nucleation and crack propagation by fatigue and creep mechanisms at elevated temperatures.