Mechanical properties and microstructural characterization of simulated heat-affected zones in 10 wt pct Ni steel

The effects of welding thermal cycles on the mechanical properties of a high strength, high ballistic resistance 10 wt pct Ni steel were studied. Welding simulations were performed using a Gleeble 3500 thermal-mechanical simulator to replicate microstructures observed in the heat-affected zone of this steel, which is a transformation induced plasticity or TRIP steel. The microstructural influences on austenite content, strength, and toughness were determined using a variety of characterization techniques including XRD, SEM, EBSD, and STEM/EDS. The results demonstrate that with increasing peak temperature of the welding thermal cycle, the amount of austenite present in the microstructure decreases. However, the toughness results do not directly correlate with the austenite contents. Poor toughness is observed in the intercritical heat-affected zone as a result of brittle high carbon martensite in this region. This region represents the greatest challenge in terms of maintaining high ballistic resistance of welds of 10 wt pct Ni steel. These results are significant in that the toughness of this steel is not solely based on the austenite content, which would be expected given it is a TRIP steel, but instead is a function of other microstructural influences.