About welding methods for different stainless steels
Austenitic stainless steels
Austenitic stainless steels generally have good weldability compared to many other metals. Since they do not harden when cooled, they exhibit good toughness and ductility and do not require preheating or post-heat treatment before or after welding. However, in certain circumstances, cracks may occur in the weld (or filler metal) or heat-affected zone (HAZ).
Solidification cracking of the weld metal is more likely to occur in fully austenitic structures, which are more susceptible to cracking than structures containing small amounts of ferrite. Fully austenitic grades include grades 310, 320, and 330. However, since the most commonly used austenitic stainless steels actually contain small amounts of ferrite, this problem is actually not as serious as it seems. For example, 316 alloy contains 3% to 10% ferrite. Fermomic 50 (XM-19, UNS S20910, 1.3964, Nitronic 50), Fermomic 60 (UNS S21800, Nitronic 60) and Alloy 254 (UNS S31254, 1.4547, 254SMO, 6Mo) also contain small amounts of ferrite. These small amounts of ferrite microstructure are able to dissolve impurities that can cause intergranular cracking or low melting point segregation formation. These are associated with the presence of phosphorus or sulfur, which are considered stray elements because they are not intentionally added but carried over from the starting scrap, raw materials and processes.
Carbon in austenitic stainless steels can cause intergranular corrosion in the weld metal or HAZ after welding. Chromium carbides form on the grain boundaries of austenitic stainless steels in the temperature range of 550-900°C. This means that the area surrounding the carbides is now lower in chromium because the diffusion of chromium in the parent metal is very slow. These areas with lower chromium content are therefore less resistant to corrosion and any corrosion is most likely to start here. This phenomenon can be caused by the temperatures experienced during welding and is known as sensitization.
Reducing the carbon content will reduce the likelihood of post-weld sensitization. For this reason, many standard grades are available in versions with significantly reduced carbon content, such as 316L alloy (C < 0.03%) compared to 316 alloy (C < 0.08%).
Stabilized grades, such as 316Ti alloy, use titanium additions to improve performance at elevated temperatures. This also reduces sensitization, as any carbon in the metal will prefer to bond with titanium rather than chromium.
Finally, if austenitic stainless steels are exposed for extended periods of time between 550-900°C, detrimental sigma phase may form from small amounts of ferrite.
Duplex and Super Duplex Stainless Steels
As with the most common austenitic stainless steels, the presence of some ferrite in the microstructure can help limit the likelihood of hot cracking during welding. Given that duplex and super duplex stainless steels contain nearly equal proportions of austenite and ferrite, this is certainly not a problem. Duplex steels are therefore easy to weld, but the welding process must be qualified and controlled to avoid the formation of undesirable microstructures.
The main problem with duplex stainless steels is their tendency to form a sigma phase microstructure from the transformation of ferrite. This transformation occurs over a range of different temperatures and times and is best demonstrated by a TTT (temperature-time-transformation) diagram. Sigma is a non-magnetic intermetallic phase rich in iron and chromium. The area surrounding the sigma phase has a lower chromium content and therefore has much lower corrosion resistance than other areas. In addition, the transformation of ferrite to the sigma phase can lead to voids, which can lead to the appearance of cracks and a significant decrease in mechanical strength, especially impact toughness. Therefore, if duplex and super duplex stainless steels are exposed to higher temperatures, their excellent corrosion resistance and mechanical properties will be completely offset.
Ferralium 255 (UNS S32550, F61, 1.4507) is less likely to form sigma phase 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 phase, welding conditions must be controlled to limit the time at temperature. As the TTT diagram shows, sigma phase can form in relatively short times at around 800-900°C. Due to the relatively large size of the parent metal compared to the weld area, the heat generated by welding is usually dissipated fairly quickly. Longer times at lower temperatures can also result in the same microstructural transformation. Therefore, for multi-pass welds, it is important to limit the welding temperature. This can be achieved by reducing the welding heat input, providing some cooling, or pausing between weld passes.
Another major challenge in welding duplex and super duplex stainless steels is maintaining a balanced austenite:ferrite microstructure. In the weld metal area, nitrogen is often lost. Since nitrogen is an austenite stabilizer, the loss of nitrogen in the weld area encourages a greater proportion of ferrite, resulting in a loss of mechanical and corrosion properties. This can be overcome by selecting an over-alloyed filler metal, i.e. containing a higher percentage of nickel (another austenite stabilizer) or by using nitrogen as the shielding gas itself so that the weld metal absorbs a small amount of nitrogen.
