Optical and electron microscopy have been employed to study structural changes in an FeNiMn martensite (α′) during heating. The reverse transformation (α′ → γs) occurs between 500 and 600°C and appears to be divided into three classes. The first is a long-range diffusional transformation which occurs by nucleation and growth. This type of reversion is observed on slow heating (5°C s-1). The formation of secondary austenite (γs) is observed at the lath interfaces. The second is a short-range diffusional transformation observed on heating at intermediate rates (approximately 50°C s-1) which results in a refined, equiaxed austenite and resembles a massive transformation. The third is a diffusionless transformation observed on fast heating (100°C s-1) and appears to be martensitic in nature.
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