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, the composition of the filler material must be optimized to ensure that there is some delta ferrite present in the weld metal (typically >3%). However, delta ferrite transforms into intermetallic phases, notably the sigma phase, faster than austenite either during high-temperature service or during post-weld 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 … Continue reading Austenite Stainless Steel at high-temperature (embrittlement)

22%Cr duplex stainless steels are not suitable for elevated temperature unprotected seawater applications because they suffer from the crevice and pitting corrosion at even moderate temperatures. Super duplex stainless steels present superior corrosion resistance to duplex stainless steels in seawater applications, and provided weld quality is maintained, pitting should not occur in seawater up to about 40°C. The operational limit stipulated in the Norsok Standard (M-001, Rev.2, November 1997) is one of the most conservative for unprotected super duplex service and is more restrictive than most operators’ own requirements. The maximum operating temperature is set at 15°C and the maximum … Continue reading Duplex piping and seawater