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 1 10 The deformation of austenite above a temperature called Md (higher than Ms), lowers Ms temperature resulting in increase of retained austenite. This untransformed austenite is called retained austenite. For example, steels with carbon less than 0.25%, when quenched to room temperature have little retained austenite, because room temperature is below the Mf at such carbon contents in steels. WHY MEASURE RETAINED AUSTENITE? Retained austenite is the result of an unfinished martensite transformation after quenching of high carbon steel after austenitizing. This results in: Lower hardness: Austenite being a softer phase, if RA% is high, one will observe soft spots. The soft spots are detrimental in applications such as cutting tools where wear resistance is one of the most important factors affecting the service performance and life. Volume change: The transformation of austenite to martensite is accompanied by increase in volume. Therefore, retained austenite, when it transforms to martensite during the service life of the component, would affect the dimensional stability of the component. This is detrimental in applications such as bearings and gauges where dimensional stability is extremely important. Premature failure: The fraction of austenite retained (untransformed) at the end of the hardening process is likely to transform to martensite during subsequent tempering or when the hardened part undergoes stress and strain in service. This newly transformed martensite will remain untempered and cause brittleness, which can cause premature failure of the component in service. Due to this reason, retained austenite is detrimental in applications such as tools and dies, where high impact loading is an essential service condition. Increased fatigue resistance: Finely dispersed retained austenite resists the propagation of fatigue cracks and improves rolling contact fatigue (RCF) stress. Therefore, some amount of retained austenite is considered beneficial in certain applications. One such application is bearings working with contaminated lubricants, such as railway bearings. In some cases, bearing components made of through hardening steels like SAE 52100 are carbonitrided because the carbonitriding process gives higher surface hardness and increases wear resistance and it also promotes retained austenite[2]. MEASURING RETAINED AUSTENITE The volume fractions of phases in materials are typically evaluated by optical microscopy, magnetic analysis, and x-ray diffractometry. Among these methods, the x-ray diffraction method is one of the most efficient means[3]. The accuracy of the techniques used to identify and measure austenite in steel decreases significantly with decreasing amounts of austenite. As RA fractions become small, the morphology makes measurement difficult, for instance, thin RA films between martensite plates or laths[4]. The most common measurement techniques are: visually with a microscope, magnetic (martensite is ferro magnetic and austenite is paramagnetic), electron backscatter diffraction (EBSD), and x-ray diffraction (XRD) measuring different lattice structure of the martensite (BCT) and the retained austenite (FCC). Microscope. Measuring retained austenitewith a light microscope is performed on a metallographic prepared (cut, ground, high polished) and etched microsample typically at 500x magnification. The proper segmentation of the sample out of the part or gear is very crucial to receive a representative quantity of the measured area. The mechanical force and heat introduction during segmentation must be minimized to avoid the transformation of austenite. The removal of surface layer during grinding and polishing of the microsample may also change the evaluation plane. The right etchant must be selected for the material (3 to 5% Nitric acid in alcohol is commonly used) and the etching time and intensity can vary the contrast of the structure, in a way, that untempered (tetragonal) martensite plates cannot be differentiated from the residual unetched retained austenite. Figure 3 shows plate martensite next to retained austenite in a 1.31% C, 0.18% Si, 0.2% Mn steel after 950°C austenitizing and water quenching. In the as-quenched condition, the martensite is light and the retained austenFig. 2 — Retained austenite as a function of carbon content for plain carbon steels quenched to room temperature[1]. 11
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