Transmission electron microscopy evaluation of solid-state upset welds in Type 304L stainless steel

摘要

Transmission electron microscopy (TEM) was used to characterize the microstructures at and near the weld interface in upset welded Type 304L stainless steel test samples. Two sample configurations were examined in this study; upset welded cylinders prepared using a commercial resistance welder and cylindrical shaped samples welded in a Gleeble 1500 thermomechanical simulation device. The Gleeble samples evaluated were welded at 800 C, 900 C and 1,200 C with a 0.5 cm weld upset. The base microstructure of the samples varied with weld temperature. The lower temperature specimens contained a large free-dislocation density and distinct dislocation cells. The higher temperature specimens contained well-developed subgrains and a much lower free-dislocation density. The microstructure of the upset welded samples most closely resembled the 1,200 C Gleeble sample. No distinct bond line was observed by TEM in any of the specimens, i.e., diffusion and grain growth occurred across all weld interfaces. However, weld interfaces in both specimen configurations were characterized by the presence of 50--300 nm diameter particles spaced between 300 and 1,300 nm apart. Through the use of electron diffraction analysis and X-ray microanalysis two precipitate types were identified in both specimen configurations. A crystalline phase very similar to Mn(sub 1.5)Cr(sub 1.5)O(sub 4) and an amorphous phase enriched mainly in Si and Al were observed. Surface oxides and/or internal impurities may be sources for these precipitates. Future work will include a controlled study designed to determine the origin of the interface precipitates.