The phenomenon of embrittlement in austenitic stainless steel welds exposed to high temperature is accelerated by the presence of delta ferrite.
To avoid solidification cracking in austenitic stainless steel welds (see What is hot cracking/solidification cracking?), the composition of the filler material must be optimised to ensure that there is some delta ferrite present in the weld metal (typically >3%).
However, delta ferrite transforms to intermetallic phases, notably sigma phase, faster than austenite either during high temperature service or during postweld heat treatment (PWHT). Sigma phase is an intermetallic with an approximate chemical formula FeCr and, as with most intermetallics, it is very brittle and hence has a deleterious effect upon mechanical properties. It has been shown that, for a variety of iron-chromium-nickel alloys, Charpy toughness drops off exponentially with increasing sigma phase content.[1] The more delta ferrite a nominally austenitic stainless steel has, the more susceptible it will be to sigma phase formation. To avoid significant embrittlement it is typically desirable to limit the delta ferrite content in the original microstructure to below 10%.
Weld metal containing delta ferrite will also be susceptible to '475°C embrittlement'. This phenomenon is observed when stainless steels are heated into the range 400 - 550°C (although the effect is most pronounced at 475°C) and a dramatic fall in toughness is observed after extended exposure. This is due to the formation of chromium-rich domains and precipitates within the iron-rich matrix by spinodal decomposition of ferrite at these temperatures. This effect becomes more pronounced as chromium content increases. However, 475°C embrittlement is not generally significant for materials with a ferrite number less than 14FN.
Reference
- Peckner D and Burnstein M: 'Handbook of stainless steels'. Publ. McGraw-Hill, New York, 1977. ISBN 007049147X