ATOM-PROBE FIELD-ION MICROSCOPY OF TYPE-308 CRE STAINLESS-STEEL WELDS

Additions of controlled residual elements (CRE), such as titanium, bor on and phosphorous, to commercial 308 type stainless steel welds have been found to improve the creep-rupture properties. In both convention al and CRE-type stainless steel welds, during high temperature service , various microstructural changes take place such as the formation of M(23)C(6) carbide networks along the austenite(gamma)-ferrite (delta) interface, the formation of sigma (sigma) phase, and the dissolution o f ferrite. Boron-containing CRE welds exhibit similar microstructural evolution as that of conventional types 308 stainless steel welds. As boron may segregate to the ferrite-austenite interface and thereby mod ify the austenite-M(23)C(6) carbide interface, the distribution of bor on was measured with an atom probe field ion microscope. Atom probe an alysis revealed significant boron and carbon enrichments along the fer rite-austenite boundary in the as-welded state. The ratio of the carbo n-to-boron concentration at the interface was found to be similar to 4 .