| Abstract Scope |
Medium-carbon Cr-Mo-V ultrahigh-strength steels (UHSS) can have complex carbide populations that affect the continuous cooling transformation (CCT) behavior across the heat-affected zone (HAZ) when welded or additively manufactured. This presents challenges when predicting as-welded or as-built UHSS component properties. The objective of this work is to examine the effect of complex pre-existing carbide populations on the CCT behavior of Gleeble-simulated coarse-grain HAZ (CGHAZ), fine-grain HAZ (FGHAZ), and intercritical HAZ (ICHAZ) regions. The on-cooling microstructural transformations were measured by dilatometry across a range of cooling times relevant to welding and wire-arc directed energy deposition (WA-DED). The varied dissolution and evolution of carbides from HAZ thermal cycling were characterized by transmission electron microscopy (TEM) techniques and transmission Kikuchi diffraction (TKD). The dissolution and evolution of carbides were related to the CCT behavior, microstructure, and hardness of each HAZ region. In the CGHAZ, the pre-existing carbides were completely dissolved, producing a homogeneous austenite with high hardenability that transformed to martensite. There was little variation in transformation temperature and hardness across the tested cooling times. In the FGHAZ, the pre-existing carbides partially dissolved and evolved with cooling time. Increased dissolution occurred at longer cooling times, enriching the austenite in substitutional alloying additions, depressing the transformation temperature from 416 °C to 351 °C at the longest cooling time. This formed mixed martensite-bainite microstructures in the FGHAZ. The ICHAZ showed little dissolution of the pre-existing carbides, transforming to bainite at higher temperatures with increasing cooling time due to the solute-lean austenite that formed during HAZ thermal cycling. These results show that the degree of dissolution of pre-existing carbide populations is related to HAZ peak temperature and cooling time. This significantly affects the on-cooling transformation, microstructure, and final mechanical properties across the HAZ in UHSS components. |