Effect of laser reheat post-treatment on the microstructural characteristics of laser-cladded ultra-high strength steel

Abstract

Repair of defence-grade ultra-high strength steels such as 300M which are damaged or worn during service is a very challenging problem. More often than not, laser cladding repair of such steels results in the formation of untempered martensite which adversely affects the mechanical properties of the components. On the other hand, bulk heat treatment of the repaired component is an expensive post-treatment operation and may also lead to other deleterious effects. In this study, laser reheat post-treatment (LPHT) is proposed as a solution for localised heat treatment for such repair applications. The microstructural and microhardness characteristics of both the single track and multi-track cladded structures were investigated in the as-deposited and LPHT conditions. The LPHT treatment induced tempering of the martensite resulting in a decrease in the hardness to within a range between 500 and 590 HV. Furthermore, the M23C6 carbides present in the as-deposited state dissolved in the martensite matrix upon laser reheating, ensuing an increase in the volume of retained austenite. In multi-track cladding, the untempered martensite was subjected to partial tempering due to the overlapping track deposition. An additional laser reheat pass further tempered the cladded layer resulting in a reduction of the hard untempered martensite with a uniform distribution of tempered martensite. From this study it can be inferred that LPHT treatment can be effectively employed for localised control of the microstructure in the laser clad repaired parts by tuning the laser reheat parameters.