Major role of delta-Fe in in Austenitic Stainless steel weld is to avoid hot cracking or solidification cracking. Minimum 5% delta-Fe is required in weld to avoid hot cracking. And, if delta-Fe content is increased then tendency of embrittlement of the weld increases, which means lower impact values of weld.
Following explanation is taken from technical paper:
LOW TEMPERATURE THERMAL AGEING EMBRITTLEMENT INAUSTENITIC STAINLESS STEEL WELD, P.K.Singh, V.Bhasin, R.K. Singh, Raghuvir Singh,G.Das
The phenomenon of hot cracking or solidification cracking is of concern in austenitic stainless steel welds. The solidification cracking results from the segregation of low melting point liquid along the grain boundaries during last stage of solidification. If sufficient stresses are generated before the final solidification, boundaries may separate to form a crack. It has been known that presence of retained ferrite in the austenitic stainless steel weld effectively prevents hot cracking. The higher solubility of impurities in ferrite than austenite results in less segregation of low melting impurities, which helps in preventing hot cracking. Delta ferrite has lower thermal coefficient of expansion (a), which helps in reduction of thermal stresses. ASME Boiler and Pressure vessel code calls for minimum of 5% d-ferrite (or 5 FN) in austenitic stainless steel weld to avoid solidification cracking.
The stainless steel weld being a duplex microstructural region containing 5-15% delta (d) ferrite in addition to parent austenite has varieties of microstructural features and defects. The lower limit of the ferrite is specified to minimize the risk of hot cracking during solidification and the upper limit is specified with respect to thermal aging embrittlement (LTE) during fabrication or service. Thermal aging at temperatures above 550°C results in the transformation of d-ferrite to sigma phase whereas aging at temperature below 550°C results in formation of silicide (G) phase and spinoidal decomposition. These products resulting from the phase transformation cause hardening and embrittlement of the component. The spinodal decomposition is reportedly related to the decomposition of ferrite into iron rich (a) and chromium rich (a’) phases and is primarily due to the miscibility gap exist in the Fe-Cr phase diagram. Such transformation of ferrite is further influenced by prolong heating time at temperatures <500oC and may result into even more brittle phase such as G phase, a complex silicide of nickel and silicon rich phase. The embrittlement is manifested as modification in the Charpy V-Notch (CVN) impact properties namely reduction in upper shelf energy (USE) or increase in ductile-brittle transition temperature (DBTT).
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