Atom probe field ion microscopy of type 308 CRE stainless steel welds

Additions of controlled residual elements (CRE), such as titanium, boron and phosphorous, to commercial 308 type stainless steel welds have been found to improve the creep-rupture properties. In both conventional and CRE-type stainless steel welds, during high temperature service, various microstructural changes take place such as the formation of M23C6 carbide networks along the austenite(γ)-ferrite (δ) interface, the formation of sigma (σ) phase, and the dissolution of 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 modify the austenite-M23C6 carbide interface, the distribution of boron was measured with an atom probe field ion microscope. Atom probe analysis revealed significant boron and carbon enrichments along the ferrite-austenite boundary in the as-welded state. The ratio of the carbon-to-boron concentration at the interface was found to be ∼4.