GRAIN GROWTH IN 18 Ni CO-FREE MARAGING STEEL

The grain growth behaviour in a 18Ni-Co-free (T-250) vis-a-vis 18Ni-8Co-5Mo (M-250) type maraging steels have been investigated. The study has indicated that these steels follow classical growth model (delta)D= Kt^n , where (delta)D is the increase in grain size. A ʹtime lagʹ has been observed in both the steel for grain growth to start at lower annealing temperatures. This has been explained on the basis of unique shear type (alpha) ->(gamma) transformation in these steels. The activation energy for growth has been found to be in the range of 316-339 kJ.mole^-1 which indicates that the overall growth process is controlled by self diffusion in (gamma)-iron. A comparison of the grain growth behaviour in the two steels show that although these steels have, in general, identical growth pattern, they differ in actual ʹlag timeʹ for growth to start and increase in grain size in identical heat treatment condition. The ʹtime lagʹ in case of 18Ni-8Co-5Mo maraging steel is 0.38 h against 0.17 h in 18Ni-Co-free steel, at 1173 K. Also, the grain size of M-250 steel at a particular time and temperature of annealing is always finer than T-250 steel. This increased resistance of grain growth in M-250 steel has been attributed to the presence of cobalt in the steel.