MICROSTRUCTURAL CHANGES IN HSLA-100 STEEL THERMALLY CYCLED TO SIMULATE THE HEAT-AFFECTED ZONE DURING WELDING

The microstructural changes that occur in a commercial HSLA-100 steel thermally cycled to simulate weld heat affected zone (HAZ) behavior we re systematically investigated primarily by transmission electron micr oscopy (TEM). Eight different weld thermal cycles, with peak temperatu res representative of four HAZ regions (the tempered region, the inter critical region, the fine-grained austenitized region, and the coarse- grained austenitized region) and cooling rates characteristic of high heat input (cooling rate (CR) 5 degrees C/s) and low heat input (CR 60 degrees C/s) welding were simulated in a heating/quenching dilatomete r. The as-received base plate consisted of heavily tempered lath marte nsite, acicular ferrite, and retained austenite matrix phases with pre cipitates of copper, niobium-carbonitride, and cementite. The microstr uctural changes in both the matrix and precipitate phases due to therm al cycling were examined by TEM and correlated with the results of (1) conventional optical microscopy, (2) prior austenite grain size measu rements, (3) microhardness testing, and (4) dilatometric analysis. Man y of the thermal cycles resulted in dramatic changes in both the micro structures and the properties due to the synergistic interaction betwe en the simulated position in the HAZ and the heat input. Some of these microstructures deviate substantially from those predicted from publi shed continuous cooling transformation (CCT) curves. The final microst ructure was predominantly dependent upon peak temperature (i.e., posit ion within the HAZ), although the cooling rate (i.e., heat input) stro ngly affected the microstructures of the simulated intercritical and f ine-grained austenitized regions.