Austenitic stainless steels have in general excellent weldability. They do not normally require post-weld heat treatment. Most standard austenitic stainless steels are designed to solidify initially as delta ferrite, which has high solubility of sulphur and phosphorus, and transforms to austenite upon further cooling. The final weld metal structure normally contains a few percent delta ferrite, which is a sign of a sound weld. Recommended filler metals often have an adjusted composition to yield 3-10% delta ferrite to ensure crack-free welds. For a fully austenitic weld metal, impurities can be concentrated to the grain boundaries, resulting in low-melting phases and susceptibility to hot cracking. Consequently, fully austenitic stainless steels, that are more sensitive to hot cracking, should be welded with controlled heat input and with minimum dilution from the parent metal. The interpass temperature, for the same reason, should not exceed 150°C. The level of heat input for most common austenitic grades could be up to about 2.5 kJ/mm. If the welding should be carried out on stabilized or fully austenitic grades, somewhat lower levels may be needed to avoid solidification cracks (≤1.5 kJ/mm).

Austenitic weldment. Weld metal (left), HAZ (right).

Austenitic steels have about 50% higher thermal expansion compared to ferritic and duplex steels. This means that larger deformation and higher shrinkage stresses may be a result from welding.

The classic problem of weld decay due to precipitation of chromium carbides in grain boundaries of austenitic stainless steels is rare today, as the carbon contents in modern steels and fillers are held sufficiently low to avoid this phenomenon. High alloy austenitic stainless steels may show precipitation of intermetallic phases in the weld metal and heat-affected zone ( HAZ). Smaller amounts of precipitates do not usually affect weldment properties, e.g. corrosion resistance. However, it is advisable to weld with moderate heat input and the lowest possible dilution of the parent metal. For high alloy austenitic grades, pitting corrosion resistance can be reduced due to microsegregation, primarily of molybdenum, during solidification. Filler metals are thus, in most cases, over-alloyed with chromium, nickel and molybdenum to enhance the corrosion resistance.

For some applications metastable austenitic steels are used in cold-rolled condition (temper rolled) to very high strength levels. Welding will naturally have a softening effect in the weld zone and this fact should be taken into consideration at the design stage.