The behaviour of advanced quenched and tempered steels during arc welding and thermal cutting

Abstract

Quenched & tempered (Q&T) wear-resistant plate steels with martensitic microstructures have been in use for many years in the mining, defence, and construction industry due to their excellent mechanical properties (up to 1700 MPa of tensile strength and \u3e10% elongation to failure). These mechanical properties are achieved by utilisation of up to 0.4 wt.% Carbon (C), \u3c1.5 wt.% Manganese (Mn), microalloying with Molybdenum (Mo), Chromium (Cr), Nickle (Ni), Titanium (Ti), and sometimes Boron (B), and a combination of carefully designed thermomechanical processing schedule and post rolling heat treatment. In the last 10 years addition of \u3c1.5 % Ti was shown to provide superior wear resistance at a moderate C content. Improvement in the wear resistance was achieved via the formation of TiC hard particles embedded in the tempered martensite matrix. Moderation of the C content in Ti-alloyed steels allowed to obtain steels with relatively low hardness, high toughness, and enhanced weldability (due to the low carbon equivalent of the steel composition). A combination of moderate hardness and high toughness positively influenced the wear resistance. Fabrication of tools and equipment from the Q&T steels is carried out using conventional fusion arc welding and thermal cutting with oxy-fuel or plasma jet. The main problem, in this case, is the formation of an edge microstructure highly susceptible to cold cracking or hydrogen-induced cracking (HIC), which results in deterioration of mechanical properties, making steel unsuitable for the required application. In the case of Ti-alloyed steels, the heat input associated with thermal cutting and welding alters the TiC particle size distribution, in addition to the tempering of the martensitic microstructure, occurring in conventional Q&T steels. However, fabrication parameters may be controlled to avoid catastrophic microstructure deterioration and product failure. Generally, a type of welding process, environment, alloy composition, joint geometry, and size are the main causes of cracking after cutting and welding. Cracking susceptibility increases as the weld metal hydrogen content, material strength, and thickness increase. Cold cracking will occur if three conditions are satisfied: susceptible microstructure; type and magnitude of residual stresses; and importantly, the level of diffusible hydrogen that enters the weld pool. Cold cracking can be avoided through the selection of controlled heat input (depends upon current, voltage, and travel speed of welding) and preheating temperature

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