Effect of the Thermal History on Macrostructure and Microstructure Development in High-Strength Steel Welds

Abstract

The present work addresses the microstructural and macrostructural development in multipass welded joints. It focuses on multiple thermal cycles induced by successive deposition of welding passes. The local welding-related thermal history was related in detail to the evolution of austenite grains during the manufacturing of two high-strength low alloy steel welds. The analytical Rosenthal thermal model was used to identify the thermal cycles experienced within typical weld metal regions. Selected heat cycles were applied to laboratory specimens, taken from the same weld metal, to investigate microstructural evolution during the welding process. Heat cycle experiments, involving full austenitization, showed the persistence of columnar zones resulting from a memory effect of the prior austenite grains during the reverse transformation. Intercritical heat cycles led to white etching, softer regions with high fractions of retained austenite. They also showed that the memory of austenite grains was actually stored in elongated retained austenite particles that remained after complete welding. This memory effect vanished under high peak temperatures (typically, 130 °C higher than Ac3); this was linked to a competition between growth and merging of elongated, intragranular retained austenite particles, and growth of equiaxed, intergranular austenite particles. Finally, a low peak temperature promoted refined, harder final microstructures.