12%Cr Ni-Mo alloyed low carbon martensitic stainless steel weld metals have been investigated in this work from metallurgical structure and related property development aspects, keeping in view the application aims of extremely high strength, good ductility, and adequate SSCC resistance.

410 NiMo martensitics are often called to satisfy certain critical and often restrictive technical requirements such as conformance to both AWS A5.4 and NACE MR-0175 simultaneously. The designer has the unenvious task of marrying the Minm. tensile of 110KSI to a Minm. elongation of 15% as specied by AWS and joining this to the NACE requirement of a Maxm. Hardness of RC 22, which almost crowds the tensile range. This poses great challenges to the metallurgist as well as the fabricator.

It was found from the present work that mechanical properties and microstructures are closely related in the 410NiMo welds and derive largely from the kinetics of austenite and martensite transformations. The extent to which martensitic transfomartion takes place on cooling and the degree of stability of austenite at interpass temperatures are critical for successful achievement of weldments with high strength, adequate ductility, and at the same time a restricted hardness level to satisfy NACE requirements.

It was observed that it is necessary to design the alloys to certain level of Ms temp. and carry out martensitic welding process, so that austenite breakdown can be sufficiently retarded at the subcritical range, however without making the austenite too stable . The design results in a fully martensitic structure on cooling.It was noticed that presence of excessive retained austenite could be detrimental to ductility and hardness of the weld metal. Even a second tempering may not be able to mitigate the problem.

Low carbon content had a decided advantage in ensuring ductility of the welds. Microstructure was another determining factor. With optimum design of the weld metal and right fabrication procedures, 410 NiMo weldments can achieve the high strength toughness and SSCC resistance at the same time.

It was however seen that some of the heat treatment procedures could be time intensive and uneconomic.Hence it is suggested that further work may be taken up towards alloy design, may be with cobalt addition, and also fitness for purpose studies at higher hardness levels.