Transmission electron microscopy (TEM) together with scanning electron microscopy (SEM) and optical microscopy have been employed to analyze the microstructures that develop in a copper-containing, low-carbon (0.04% by weight) HSLA-100 alloy. Specifically, the martensitic microstructures that develop in the simulated, coarse-grained heat-affected zones (HAZs) have been examined and compared with those that develop in the base plate during conventional quenching from the austenitic phase field. It has been shown that the lath martensitic packet size is increased dramatically in the HAZ as compared with the base plate. In addition, considerably more retained austenite is found in the HAZ. No evidence for the so-called granular bainite microconstituent has been found in either material. However, for cooling rates somewhat less than that experienced in the coarse-grained HAZ, a microconstituent that we term granular ferrite has been documented.
Finally, the effect of tempering on the martensite in the base plate has been examined. As soon as they are observable, the copper (Cu) precipitates can be identified as face-centered cubic (fcc) ε-Cu.