An introduction to fabrication
The welding of stainless steels29th April 2020
Stainless steels are considered to have good weldability compared to many other metals and can be successfully welded by a number of different techniques with the correct settings and conditions.
Austenitic stainless steels
Generally speaking, austenitic stainless steels are insensitive to cracking after welding. As they are non-hardenable on cooling, they exhibit good toughness and ductility, whilst there is no need for pre- or post-weld heat treatment. However, under select circumstances cracking in either the weld (or filler) metal or heat affected zone (HAZ) can 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 grades. However, as the most widely used austenitic stainless steels actually contain a small amount of ferrite, then this is actually less of an issue than it first appears! For instance, Alloy 316 will contain 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 small quantity of ferrite microstructure present is able to dissolve impurities that could lead to the formation of inter-dendritic cracks or low melting temperature segregates. These are related to the presence of phosphorus or sulphur, considered tramp elements as they are not deliberately added but instead are picked up from the starting scrap, raw materials and process.
The presence of 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 of austenitic stainless steels in the temperature range 550–900°C. This means that the areas surrounding the carbides are now lower in chromium content, as chromium diffusion within the parent metal is very slow. These areas of lower chromium content are therefore less resistant to corrosion, and any corrosion that occurs is most likely to initiate here. This phenomenon can be caused by the temperatures experienced during welding and is known as sensitisation.
Having a lower carbon content will reduce the likelihood of sensitisation after welding. Therefore, many standard grades are available with significantly lower carbon content, such as Alloy 316L (C < 0.03%) compared with Alloy 316 (C < 0.08%).
Stabilised grades such as Alloy 316Ti use additions of titanium to improve properties at elevated temperatures. This also reduces sensitisation as any carbon present in the metal will preferably combine with the titanium rather than the chromium.
Finally, if austenitic stainless steels are exposed between 550-900degC for extended periods then it is possible to form the deleterious sigma phase from the small amounts of ferrite present. This mechanism is covered below for duplex stainless steels.
Duplex and super duplex stainless steels
As with the most common austenitic stainless steels, the presence of some ferrite in the microstructure can help to limit the likelihood of hot cracking during welding. Given that duplex and super duplex stainless steels have almost equal proportions of austenite and ferrite then this certainly isn’t an issue. Therefore, duplex steels are readily weldable, but the welding procedure must be qualified and controlled in order to avoid creating undesirable microstructures.
The main issue with duplex stainless steels is their propensity to form sigma phase microstructure from the transformation of ferrite. This transformation happens across a range of different temperatures and times, as best demonstrated in a TTT (temperature-time – transformation) chart. Sigma is a non-magnetic intermetallic phase, rich in iron and chromium. Areas around the sigma phase will be lower in chromium content, and therefore of far lower corrosion resistance. In addition, the transformation of ferrite to sigma may result in voids which lead to the presence of cracks and a significant drop in mechanical strength and particularly impact toughness. Therefore, the excellent corrosion resistance and mechanical properties of duplex and super duplex stainless steels are completely negated if exposed to 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) grades.
To avoid the formation of sigma, weld conditions must be controlled to limit the time at temperature. As shown by the TTT diagram, relative shorter periods of time at or around 800-900degC can form sigma phase. Due to the relatively large size of the parent metal compared to the weld area, then the heat of welding is usually dissipated quite quickly. Longer periods of time at lower temperatures can ultimately lead to the same microstructural transformation. Therefore, for multi-pass welds, it is important to limit the weld temperature. This can be achieved by reducing the weld heat input, providing a degree of cooling or pausing between passes.
The other main challenge with the welding of duplex and super duplex stainless steels is maintaining the balanced austenite: ferrite microstructure. In the weld metal area, there would typically be a loss of nitrogen. As nitrogen is an austenite stabiliser, the loss of nitrogen from the weld area encourages a greater proportion of ferrite resulting in a loss of mechanical and corrosion properties. This can be overcome by selecting a filler metal which is over-alloyed i.e. with a greater percentage of nickel (another austenite stabiliser) or using nitrogen as the shielding gas itself so that the weld metal picks up a small amount of nitrogen.
Further details on the welding of Ferralium 255 can be found on the relevant product page here.