The welding of stainless steels

Q: What is the main welding challenge for duplex and super duplex stainless steels?

Duplex and super duplex stainless steels have almost equal proportions of austenite and ferrite, which helps limit hot cracking, but their main issue is forming sigma phase from transformed ferrite at certain temperatures and times. Sigma is a non-magnetic, chromium-rich phase that leaves surrounding areas less corrosion resistant and can cause voids and cracks.

Q: How can welders avoid sigma phase formation in super duplex stainless steel welds?

Weld conditions must limit time at temperature, since relatively short periods at or around 800-900°C can form sigma phase. Reducing heat input, providing some cooling, or pausing between passes on multi-pass welds all help. An over-alloyed filler metal with higher nickel, or nitrogen shielding gas, also helps maintain the balanced austenite-ferrite microstructure.

Stainless steels have good weldability compared with many other metals, and can be successfully welded by a number of techniques given the correct settings and conditions.

Austenitic stainless steels

Austenitic stainless steels are generally insensitive to cracking after welding. They are non-hardenable on cooling, keep good toughness and ductility, and need no pre- or post-weld heat treatment. Under certain circumstances, cracking in the weld (or filler) metal or the heat affected zone (HAZ) can still occur.

Weld metal solidification cracking is more likely in fully austenitic structures, which are more crack sensitive than those containing a small amount of ferrite. Fully austenitic grades include 310, 320 and 330. However, the most widely used austenitic stainless steels contain a small amount of ferrite, so this is less of an issue than it first appears. Alloy 316, for instance, contains between 3% and 10% ferrite. Fermonic^® 50 (XM-19, UNS S20910, 1.3964, Nitronic^® 50), Fermonic^® 60 (UNS S21800, Nitronic 60) and Alloy 254 (UNS S31254, 1.4547, 254SMO, 6Mo) similarly contain a small proportion of ferrite. This ferrite dissolves impurities that could otherwise form inter-dendritic cracks or low-melting-temperature segregates, which are linked to phosphorus or sulphur picked up as tramp elements from scrap, raw materials and process.

Carbon in austenitic stainless steels can lead to intergranular corrosion in the weld metal or HAZ after welding. Chromium carbides form at the grain boundaries in the range 550-900°C. The areas around the carbides are then lower in chromium, because chromium diffusion in the parent metal is very slow, and these lower-chromium areas are less corrosion resistant, so any corrosion is most likely to initiate there. Caused by the temperatures of welding, this is known as sensitisation.

A lower carbon content reduces the likelihood of sensitisation after welding. Many standard grades are therefore available with significantly lower carbon, such as Alloy 316L (C < 0.03%) compared with Alloy 316 (C < 0.08%).

Stabilised grades such as Alloy 316Ti use titanium additions to improve properties at elevated temperatures. This also reduces sensitisation, as any carbon present preferentially combines with titanium rather than chromium.

Finally, if austenitic stainless steels are held between 550-900degC for extended periods, the small amounts of ferrite present can form the deleterious sigma phase. This mechanism is covered below for duplex stainless steels.

Duplex and super duplex stainless steels

As with common austenitic grades, some ferrite in the microstructure helps limit hot cracking during welding. Duplex and super duplex stainless steels have almost equal proportions of austenite and ferrite, so this is not an issue. They are readily weldable, but the procedure must be qualified and controlled to avoid creating undesirable microstructures.

The main issue is their tendency to form sigma phase from the transformation of ferrite. This happens across a range of temperatures and times, as shown in a TTT (temperature-time-transformation) chart. Sigma is a non-magnetic intermetallic phase, rich in iron and chromium. Areas around it are lower in chromium and far less corrosion resistant. The transformation of ferrite to sigma can also leave voids that lead to cracks and a significant drop in strength, particularly impact toughness. The excellent corrosion resistance and mechanical properties of duplex and super duplex stainless steels are completely negated if the metal is exposed to these higher temperatures.

The TTT chart suggests that Ferralium^® 255 (UNS S32550, F61, 1.4507) is slightly less likely to form sigma than S32760 (F55, 1.4501, Zeron 100^®), S32750 (F53, 1.4410, SAF2507) or S32205 (F51, 1.4462, duplex 2205).

To avoid sigma, weld conditions must limit time at temperature. As the TTT diagram shows, relatively short periods at or around 800-900degC can form sigma. Because the parent metal is large relative to the weld area, the heat of welding usually dissipates quickly. Longer periods at lower temperatures can cause the same transformation. For multi-pass welds it is therefore important to limit the weld temperature, by reducing heat input, providing some cooling, or pausing between passes.

The other main challenge is maintaining the balanced austenite:ferrite microstructure. The weld metal area typically loses nitrogen, and because nitrogen is an austenite stabiliser its loss encourages more ferrite, reducing mechanical and corrosion properties. This can be overcome by using an over-alloyed filler metal with a higher percentage of nickel (another austenite stabiliser), or by using nitrogen as the shielding gas so the weld picks up a small amount of nitrogen.

See the Ferralium^® 255 product page for welding detail specific to that grade.

We supply austenitic, duplex and super duplex stainless steels, carrying a complete stock range of these high-performance grades in solid bar from ½" (12.7mm) to 16" (406.4mm) diameter.

Frequently asked questions

Why are austenitic stainless steels generally easy to weld?

Austenitic stainless steels are generally insensitive to cracking after welding. They are non-hardenable on cooling, keep good toughness and ductility, and need no pre- or post-weld heat treatment. A small amount of ferrite, as found in Alloy 316 (3-10%) and Fermonic 50/60, helps dissolve impurities that could otherwise cause cracking.

What causes sensitisation when welding austenitic stainless steels?

Carbon in austenitic stainless steels can lead to intergranular corrosion after welding, because chromium carbides form at grain boundaries in the range 550-900°C, leaving the surrounding areas lower in chromium and less corrosion resistant. Lower-carbon grades such as Alloy 316L, or stabilised grades such as Alloy 316Ti, reduce this risk.

What is the main welding challenge for duplex and super duplex stainless steels?

Duplex and super duplex stainless steels have almost equal proportions of austenite and ferrite, which helps limit hot cracking, but their main issue is forming sigma phase from transformed ferrite at certain temperatures and times. Sigma is a non-magnetic, chromium-rich phase that leaves surrounding areas less corrosion resistant and can cause voids and cracks.

How can welders avoid sigma phase formation in super duplex stainless steel welds?

Weld conditions must limit time at temperature, since relatively short periods at or around 800-900°C can form sigma phase. Reducing heat input, providing some cooling, or pausing between passes on multi-pass welds all help. An over-alloyed filler metal with higher nickel, or nitrogen shielding gas, also helps maintain the balanced austenite-ferrite microstructure.

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