Different thermomechanical processing routes lead to ultrafine grain size steels. Warm rolling and annealing within the intercritical or subcritical temperature ranges is one of the industrial routes for producing ultrafine grains.
A ferritic steel, during welding, is partially austenitized within the Heat Affected Zone, which is subjected to a range of different temperatures, and consequently different hardness values can arise. Softening can occur in these regions of coarse grains and/or fine grains.
The objective of this work was to investigate the influence of the welding cycle on the HAZ of an ultrafine grained (1-4 μm) Nb-Ti steel produced via warm rolling at 700^oC, with three passes of 0.22 true strain thickness reduction and air cooling followed by annealing at 550^oC or 800^oC for 5, 60 and 180 minutes and air cooling to room temperature. The HAZ was produced using a GMAW process with an air-balanced, 20%CO₂ shielding gas composition.
In structural steels when the temperature reaches the range 650°C-1100°C HAZ softening can occur. This corresponds to the subcritical and austenitic range, and hardness values of the parent metal are consequently reduced by more that 25HV₅₀₀. This extent of this reduction in hardness is dependent on several variables including chemical composition, processing route and welding process.
The present work shows the influence of the thermomechanical processing route on the hardness and the amount of MA (Martensite–Austenite constituent) present in the HAZ for a Nb-Ti steel. For 550^oC annealing there was an average hardness reduction around 27HV₅₀₀, however for 800^oC annealing there was an average hardening of 36HV₅₀₀, indicating that for similar welding conditions it is possible to control the subsequent softening within the HAZ, during welding, through the appropriate choice of a specific thermomechanical processing route.