Measurement of hardness and volume fraction of retained austenite
Macro- and micro-hardness of test pieces were measured using a Vickers hardness tester with the loads of 294.2N (30kgf) and 0.98N (0.1kgf) respectively. The measurement
was done at more than five locations in random, and the average value was adopted. The volume fraction of retained austenite (V£) was obtained by X-ray diffraction using a special goniometer with an automatic rotating and swinging sample stage.11,12) The diffraction peaks used for ferrite (¡) or martensite (M) calculations were (200)¡ and (220)¡ planes and those for austenite (£) were (220)£ and (311)£ planes. Vilella’s reagent was used to reveal the microstructure. Observation of the microstructure using OM and SEM was introduced to stimulate discussion of the experimental results