Abstract

Microstructure development is examined for a specialized spot weld that is used as a solid-state closure process for stainless steel tubing, referred to as pinch welding. In order to elucidate the microstructural evolution of the weld, a series of test welds were made at nominal conditions using both tubes, used in test articles and production like components. These pinch welds normally terminate after twelve cycles of a 60 Hz AC weld process. In this study, tubes with different thermal processing history were welded from one to twelve cycles and the microstructure and weld variables after each individual weld cycle number were characterized using radiography and optical metallography. Two etchants were used that highlighted different microstructural features. The study revealed that: (1) this type pinch weld is largely complete after about six cycles of 60 Hz AC current, half the weld time utilized; (2) the resistance, deformation, and closure length approach “steady state” conditions after six cycles; and (3) both oxalic and nitric acid electrolytic etchants are useful for highlighting specific microstructural attributes of type 304L stainless steel. Finally, two distinct microstructural regions can be identified for these welds: the edge of the weld which is driven by concentrated deformation, recrystallization and grain growth and the center region which is more typical of forge welding and micro-asperity breakdown followed by diffusion and grain-growth. The bond line of annealed tubes exhibits fewer indications and less contamination than that of the partially annealed and as-received cold worked tubes.

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