FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 2 6 0 Retained austenite is that fraction of austenite which remains untransformed at the end of the hardening process. Retained austenite is considered detrimental or undesirable in most cases but there are certain applications where some amount of retained austenite is considered as desirable. Retained austenite strongly influences the properties of steel. The fatigue life, ductility, toughness, hardness, yield strength, and machinability all depend on austenite content. Accurate measurement of the volume percent of retained austenite is of critical importance to the optimization of heat treatment procedures. Austenite, being an unstable phase at room temperature, will transform to martensite during use, causing brittleness and an increase in volume potentially leading to failure of critical components. However, the accurate measurement in manufactured steels remains a challenge as commonly used visual metallurgical sample investigations are subjective and mostly provide a very false reading, magnetic measurements need part specific calibration, and electron back scattering (EBSD) measurements require expensive equipment, intensive sample preparations, and long measurement times. New developments in x-ray equipment provide measurements in minutes and can also compensate for the influences of carbides in high carbon steels or texture orientations in rolled sheet metals. RETAINED AUSTENITE CREATION Hardening of steels requires heating to an austenitic phase and quenching to room temperature to produce a hard martensitic phase. Austenite is an FCC phase that is stable above a temperature of 727°C. Due to incomplete transformation during quenching some austenite is retained at room temperature. Retained austenite can dramatically decrease the mechanical properties of the steel. Properties such as fatigue strength, toughness, hardness, yield strength, and machinability can be influenced by retained austenite. Austenite can transform in service as a result of thermal cycles, plastic deformation, or shock. Shot peening, TECHNIQUES FOR DETERMINING RETAINED AUSTENITE Accurate measurement of retained austenite levels is important in the development and control of a heat treatment process. Thomas Wingens* Wingens LLC, Sewickley, Pennsylvania 9 for example, will transform the austenite on the surface of gear teeth. Exposure to extreme cold renders the austenite increasingly unstable as the temperature diminishes. The transformation of austenite to ferrite involves a nominal 4% volume increase. A linear dimensional increase on the order of the cube root of that would lead to seizure and excessive interference in precision gearing and bearings. Accurate measurement of the retained austenite levels is important in the development and control of a heat treatment process. Austenite transforms to martensite between the Ms and Mf temperatures However, this transformation never goes to completion for carbon contents higher than 0.25 wt%, i.e., 100%martensite (Fig. 1). The Ms and Mf temperatures are lowered by most alloying elements and an increasing austenitizing temperature but mostly by increasing the carbon content as seen in Fig. 2. Higher austenitizing temperature brings more carbon and alloying elements into solution in austenite. Also, this increased temperature results in more thermal stresses on quenching, which oppose martensitic transformation, which is to say, both factors increase retained austenite. *Member of ASM International Fig. 1 — Martensite start (Ms) and finish (Mf) temperatures as a function of carbon content for plain carbon steels[1]. 10
RkJQdWJsaXNoZXIy MTYyMzk3NQ==