FRACTURE TOUGHNESS OF SIMULATED HAZ IN HSLA STEEL WELDS

摘要

In the present work, the influence of simulated HAZ microstructures on fracture toughness is investigated by means of tests on precracked specimens at different temperatures. Micromechanisms involved in cleavage fracture are studied using axisymetrically notched tensile tests and acoustic emission and subsequent SEM and EBSD analyses. Fracture properties are strongly affected by both the bainitic microstructure and Martensite-Austenite (M-A) constituents. Depending on stress triaxiality and temperature, microvoids at the M-A/ matrix interface and microcracks in the M-A constituents are found to initiate cleavage fracture. This first step may be critical at very low temperature (<150°C). At higher temperature, subsequent step in the cleavage process is controlled either by propagation of the M-A sized microcrack to the surrounding matrix and/or by crack progression across the matrix surmounting high angle boundaries. The implications of these fracture micro-mechanisms on fracture toughness modelling are discussed in terms of statistical "weakest link" model with two barriers for cleavage propagation.