PHASE-STABILITY AND ATOM-PROBE FIELD-ION MICROSCOPY OF TYPE-308 CRE STAINLESS-STEEL WELD METAL

Improvement in high-temperature creep-rupture properties of type 308 s tainless steel welds due to the controlled addition of boron is relate d to microstructural evolution during welding and thermal phase stabil ity at creep service temperatures. The microstructure of boron-contain ing type 308 austenitic stainless steel welds, in the as-welded state, consisted of 8 to 10 pct ferrite in an austenite matrix. Atom probe f ield ion microscopy studies revealed segregation of boron and carbon t o ferrite-austenite boundaries in the as-welded state; the segregation level was less than one monolayer coverage. On aging at 923 K for 100 hours, M(23)C(6) carbides precipitated at ferrite-austenite boundarie s. On further aging at 923 K for 1000 hours, the ferrite transformed i nto sigma phase. Similar microstructural evolution was observed in a t ype 308 stainless steel weld without boron addition. The volume fracti ons of M(23)C(6) carbides were identical in boron-containing and boron -free welds. Atom probe results from the welds with boron addition in the aged condition showed that the boron dissolved in the M(23)C(6) ca rbides. However, lattice parameter analysis showed no apparent differe nce in the extracted carbides from the welds with and without boron. C reep property improvement due to boron addition could not be related t o any change in the volume fraction of carbides. However, the results suggest that the incorporation of boron into M(23)C(6) carbides may re duce the tendency for cavity formation along the M(23)C(6) carbide-aus tenite boundaries and hence improve the resistance to creep fracture. The observed microstructural evolution in welds is consistent with the rmodynamic calculations by THERMOCALC software.