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

The reverse (alpha'-gamma) 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 - 600degreesC 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 500degreesC showed the highest saturation magnetization. Saturation magnetization sharply decreases at annealing temperature above 625degreesC which indicates the start of reverse (alpha-gamma) 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 (alpha'-gamma) transformation at temperatures between 300 - 600degreesC is slightly greater attributed to the increase in alpha' 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 - 500degreesC and may be due to an increase in alpha' martensite phase. A further decrease in coercive force at temperature between 500 and 625degreesC 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 800degreesC and is attributed to a shape magnetic anisotropy effect. At temperatures between 800 - 900degreesC a rapid decreased in coercive force occurs. At temperatures between 900 - 1100degreesC the decrease in coercive force is not so sharp dominant. The decrease in coercive force above 800degreesC 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 - 650degreesC, and the A(f) to be between 900 - 950degreesC. (C) 2004 Kluwer Academic Publishers.