Precipitation in 18 wt% Ni maraging steel of grade 350

The evolution of precipitates in maraging steel of grade 350 was studied us ing the complementary techniques of small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). These investigations revealed that ageing the steel at 703 K involved a rhombohedral distortion of the supers aturated b.c.c. martensite accompanied by the appearance of diffuse omega-l ike structures. This was followed by the appearance of well-defined omega p articles containing chemical order. At the ageing temperature of 783 K, Ni- 3(Ti, Mo) precipitates were the first to appear with a growth exponent of 1 /3. The values of the Pored exponent obtained from the SAXS profiles indica ted that the omega particles, formed below 723 K, had diffuse interfaces up to an ageing time of 48 h. On the other hand, Ni-3(Ti, Mo) precipitates, f ormed above 723 K, developed sharp interfaces in just about an hour. Also, the steel exhibited scaling in phase separation both at 703 and 783 K, but only during the early stages. Through this study it was established that, a t temperatures of ageing less than 723 K, evolution of omega particles take s place through the collapse of the unstable b.c.c. lattice and, at tempera tures above 723 K, precipitation of A(3)B type of phases through the mechan ism of clustering and ordering of atomic species. Sharp interfaces develop rather quickly when the mechanism of precipitation involves development and amplification of a concentration wave alone as in the nucleation of Ni-3(T i, Mo) at 783 K than when an interplay of both the displacement and concent ration waves is required as in the evolution of omega at 703 K. These resul ts indicate towards the possibility of existence of two separate time-tempe rature-transformation (TTT) curves, one for the evolution of omega-phase an d another for nucleation and growth of Ni-3(Ti, Mo). (C) 2000 Acta Metallur gica Inc. Published by Elsevier Science Ltd. All rights reserved.