Microstructural changes in HSLA-100 steel thermally cycled to simulate the heat-affected zone during welding

Abstract

The microstructural changes that occur in a commercial HSLA-100 steel thermally cycled to simulate weld heat affected zone (HAZ) behavior were systematically investigated primarily by transmission electron microscopy (TEM). Eight different weld thermal cycles, with peak temperatures representative of four HAZ regions (the tempered region, the intercritical region, the fine-grained austenitized region, and the coarse-grained austenitized region) and cooling rates characteristic of high heat input (cooling rate (CR) = 5 °C/s) and low heat input (CR = 60 °C/s) welding were simulated in a heating/quenching dilatometer. The as-received base plate consisted of heavily tempered lath martensite, acicular ferrite, and retained austenite matrix phases with precipitates of copper, niobiumcarbonitride, and cementite. The microstructural changes in both the matrix and precipitate phases due to thermal cycling were examined by TEM and correlated with the results of (1) conventional optical microscopy, (2) prior austenite grain size measurements, (3) microhardness testing, and (4) dilatometric analysis. Many of the thermal cycles resulted in dramatic changes in both the microstructures and the properties due to the synergistic interaction between the simulated position in the HAZ and the heat input. Some of these microstructures deviate substantially from those predicted from published continuous cooling transformation (CCT) curves. The final microstructure was predominantly dependent upon peak temperature(i.e., position within the HAZ), although the cooling rate(i.e., heat input) strongly affected the microstructures of the simulated intercritical and finegrained austenitized regions.