Transformation twins in the weld HAZ of a low-carbon high-strength microalloyed steel

Transformation twins were found in the weld heat-affected zone (HAZ) of a low-carbon microalloyed steel. The effect of weld thermal cycle and homogenisation of austenite with respect to dissolution of microalloy carbides on twinning is examined by applying a range of heat inputs as well as conducting simulated heat treatments with short and long annealing periods. Although twinning is more frequent in the martensitic coarse-grained HAZ when low values of heat input are applied, twinning can occur in the ferritic fine-grained HAZ when the peak temperature is not very high and cooling rates are low. It is also found that the annealing period during high-temperature holding of austenite does not significantly affect the volume fraction of twins, suggesting that inhomogeneities in the austenite cannot be a major cause of twinning. Weld effects, especially thermal stresses, are therefore concluded to be a major reason for twinning in the HAZ.