Magnetic measurements of the reverse martensite to austenite transformation in a rolled austenitic stainless steel

The reverse (α′-γ) transformation in 304 stainless steel (SS) has been studied by magnetic measurements. Specimens rolled 15 to 55% reduction in thickness were annealed at various temperatures and times. After annealing at temperatures between 300–;600°C for 5 min the saturation magnetization values increased when compared to the saturation magnetization values after rolling. Specimens rolled to 40 to 55% reduction after annealing at 500°C showed the highest saturation magnetization. Saturation magnetization sharply decreases at annealing temperature above 625°C which indicates the start of reverse (α′-γ) transformation. The decrease in saturation magnetization is rapid for annealing time from 5 to 40 min, whereas, the decrease in saturation magnetization is relatively low for annealing time above 40 min. The hardness values after reverse (α′-γ) transformation at temperatures between 300–;600°C is slightly greater attributed to the increase in α′ martensite and above this temperature the hardness dropped substantially as a result of recovery and recrystallization. The results show that there is a decrease in coercive force at temperatures between 300–;500°C and may be due to an increase in α′ martensite phase. A further decrease in coercive force at temperature between 500 and 625°C may be attributed to the sweeping out of some dislocation from the martensite phase. This is followed by a sharp increase in coercive force at temperature up to 800°C and is attributed to a shape magnetic anisotropy effect. At temperatures between 800–;900°C a rapid decreased in coercive force occurs. At temperatures between 900–;1100°C the decrease in coercive force is not so sharp dominant. The decrease in coercive force above 800°C corresponds to softening of the stainless steel due to recrystallization. From the changes in the values of saturation magnetization the As temperature is estimated to be between 625–;650°C, and the Af to be between 900–;950°C.