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 2 Fig. 3 — Light untempered (left) and tempered (right) plate; martensite next to retained austenite[5]. Fig. 5 — EBSDmaps showing the effects of strain on retained austenite (a) 0%/6.3% RA; (b) 5%/6.1%RA; (c) 10%/5.4%RA; (d) 20% 3.9% RA[6]; and (e) pipe 4.8% RA. Fig. 4 — Schematic of the magneto-inductive measuring method(6). ite darker as shown in Fig. 3a. After additional tempering at 150°C, the martensite appears darker than the retained austenite due to the carbide precipitation[5]. Magnetic. As illustrated in Fig. 4, the sample is magnetized to saturation and the saturation polarization is measured. The difference between measured and theoretical saturation, the retained austenite content, can be calculated: VRA = 100Vol.% −VM %. Electron backscatter diffraction (EBSD). EBSD is a destructive technique that measures small volumes. The sample is (monochromatic) scanned in a scanning electron microscope and moved. The results are somewhat similar to XRD with much higher efforts on preparation, operator, and equipment. An example of retained austenite detected by EBSD is shown in Fig. 5. X-ray diffraction (XRD), illustrated in Fig. 6, is considered the most accurate method of determining the amount of retained austenite in steels. It is a nondestructive analytical technique used to identify and quantify phases in a material. Every crystalline phase produces a characteristic diffraction pattern (e.g., fingerprint). The volume fraction can be determined by the x-ray diffraction since x-ray diffraction intensities are directly proportional to the volume of the phase considered. The most severe shortcoming with this method is the problem of calibration due to orientation, because all materials have a preferred orientation 12 (a) (b) (a) (b) (c) (d) (e) Austenite Ferrite
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