Microstructural damage and residual mechanical properties in helium-bearing gas metal Arc weldments |
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Authors: | S H Goods N Y C Yang |
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Affiliation: | (1) Engineering Materials Division, Sandia National Laboratories, 94551 Livermore, CA |
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Abstract: | The influence of entrapped helium on microstructural damage and residual mechanical properties subsequent to applying low-penetration
gas metal arc (GMA) weld overlays was examined for an AISI Type 304 stainless steel. Two helium levels were examined: 22.5
and 85.0 atomic parts per million (appm) He. Detailed scanning electron microscopy (SEM) revealed the presence of intergranular
cracks in the weld heat-affected zone (HAZ). The crack surfaces exhibited a dimple structure that was characteristic of a
gas bubble embrittled material. Transmission electron microscopy (TEM) revealed that the size and spacing of the grain boundary
helium gas bubbles remained virtually unchanged (relative to that established by the charging and aging procedure) at distances
greater than 1 mm from the fusion line. Within this first millimeter, the diameter of the bubbles increased rapidly, and the
bubble spacing increased to the characteristic spacing of the dimples that decorated weld-induced cracks. Mechanical testing
revealed a loss in strain-to-fracture and ultimate tensile strength (UTS) at the higher helium level. While the majority of
the fracture occurred in a transgranular, ductile manner, some deformation-induced intergranular cracking was observed. This
cracking occurred over a very narrow region localized to the HAZ of the weldment. At the lower helium level, ductility and
strength were unaffected compared to helium-free specimens. |
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