An in situ high-temperature x-ray diffraction study of phase transformations in maraging 300 steel

Abstract

An in situ high-temperature X-ray diffraction (HTXRD) study in maraging 300 steel was carried out to study the martensite to austenite transformation and effect of time of exposure in the austenite reversion below austenite start temperature. Solution annealed materials were subjected to controlled heating-holding cycles. The first sample was heated at a rate of 10 ºC/min from room temperature to 800 ºC, showing that the microstructure is completely martensitic (α’110) until 600 ºC. From 650 ºC until 800 ºC, the microstructure is gradually changing from martensitic to austenitic, showed by the increasing peaks of γ111 and reducing peaks of α’110. At 800 ºC the microstructure is completely austenitic (γ111). Another sample was heated at 10 ºC/min from room temperature to 600 ºC and held for 4 hours. At 600 ºC, at 0 h time of exposure, only a martensitic peak was observed. An austenite peak can be observed after some time of exposure at this temperature. The volume fraction of austenite increased with increasing time of exposure at 600 ºC, reaching 50/50 volume fraction after 4 hours of exposure. XRD diffraction patterns for the same sample that was held for 4 hours at 600 ºC and then cooled down in air to room temperature showed the same intensity of austenite and martensitic peaks found in situ at 600 ºC for 4 hours (retained austenite), with the volume fraction of 50/50 of austenite and martensite phases. The HTXRD technique can be used to identify and quantify martensite to austenite transformation and austenite retention.