Effects of heat treatment, pre-strain and magnetic field on the formation of alpha ' martensite in Fe-25.5Ni-4Cr and 304L steels

The effects of solution heat treatment temperature, cooling rate from this temperature, pre-strain and re-heating on alpha' martensite formation were examined in Fe-25.5Ni-4Cr steel and 304L steels. The distribution of the po tential energy of nucleation sites for oil martensite was also estimated us ing results obtained in magnetic fields of up to 20.70 MA/m. In Fe-25.5Ni-4 Cr steel, solution heat treatment temperature and cooling rate from this te mperature had almost no effect on the amount of alpha' martensite formed in magnetic fields over about 8 MA/m. In contrast, the amount of alpha' marte nsite formed in magnetic fields lower than about 8 MA/m increased as soluti on heat treatment temperature increased and as cooling rate decreased. This cooling rate effect in Fe-25.5Ni-4Cr steel is opposite to that in 304L ste el, and the reason for this remains to be clarified. In Fe-25.5Ni-4Cr steel , re-heating almost did not affect the amount of alpha' martensite. From th is result, it was expected that internal strain or lattice defects generate d by water-quenching almost do not act as nucleation sites in this steel. I n Fe-25.5Ni-4Cr steel, pre-strain of up to about 10% suppressed the formati on of alpha' martensite and pre-strain over 10% enhanced the formation of a lpha' martensite. Such a change of the amount of alpha' martensite with pre -strain was reduced as the magnetic field increased. In Fe-25.5Ni-4Cr steel , it was predicted that specimens that were solution heat treated at higher temperatures would have two peaks on the curve of the potential energy dis tribution of nucleation sites and that the small peak with a high potential energy would be easily annihilated by a small pre-strain. However this pea k was not thought to be annihilated by re-heating. On the other hand, the l arge peak was expected to increase at larger pre-strain. This enhancing eff ect of pre-strain was not observed in 304L steel even in high magnetic fiel ds. This supported the view that pre-strain suppresses the formation of alp ha' martensite not through work hardening but through the annihilation of n ucleation sites. In contrast, in Fe-25.5Ni-4Cr steel, some strain concentra tion induced by a large pre-strain was expected to generate nucleation site s. As mentioned above, clear differences were revealed in the effects of he at treatments and pre-strain on the formation of alpha' martensite between Fe-25.5Ni-Cr steel and 304L steel. This shows that the nature of the nuclea tion sites and the procedure for the formation of alpha' martensite differ between these two kinds of steels.