Abstract
Abstract
In this study two different heat treatments were conducted on an X 37 Cr Mo V 5‐1 hot‐work tool steel, resulting either in a tempered fully martensitic matrix or a matrix almost consisting of tempered bainite. Short‐term creep tests were performed at a high stress level of 800 MPa and at temperatures in the range from 450 °C to 500 °C. Creep specimens consisting of a tempered fully martensitic microstructure exhibited a three times longer creep‐to‐rupture time, than those consisting of a tempered almost bainitic microstructure. Microstructural investigations of creep specimens were performed by transmission electron microscopy. Results of these investigations revealed that due to a lower cooling rate, which is necessary to form bainite, the tempered bainitic microstructure consists of large former bainitic plates, whereas tempered martensite shows fine former martensitic laths. Tempered bainite also exhibits a higher number density of large M3C, M7C3 and MC carbides than tempered martensite. Small M2C carbides appear in both microstructures in the same quantity, however, nanometer‐sized MC carbides could only be found in tempered martensite. Thus poor short‐term creep behavior of the tempered almost bainitic microstructure can be explained by the lesser amount of strengthening relevant precipitates, a smaller size‐effect due to distance of bainitic interfaces as well as lower solid solution hardening.