Duplex and super duplex belong to a family of stainless steels that are characterised by a combination of easy fabrication, high strength and excellent corrosion resistance. This group of materials is most similar to ferritic and carbon steel, but it has some physical properties in line with those found in austenitic stainless steel.
The chromium, tungsten, nitrogen and molybdenum content in duplex and super duplex create the resistance to crevice corrosion and chloride pitting that makes these materials so popular. These steels also have resistance to chloride stress corrosion which is significantly greater than austenitic grades, without sacrificing on durability or ductility.
The nature of duplex and super duplex makes it a common choice for the pipelines and pressure vessels in the petrochemical industry, as well as pipework systems, such as risers and manifolds in the oil and gas industry.
The resistance to corrosion and oxidisation isn't matched by an ability to cope with high temperatures. Brittle phases form within the ferrite at low temperatures compared to other materials. This has a drastic effect on the toughness of the duplex and super duplex, so the service temperature of all grades of these materials is restricted to as low as 250°C for super duplex steels and below 315°C for other codes.
Welding Duplex and Super Duplex Stainless Steels
Duplex and super duplex stainless steels have excellent resistance to hot cracking, so unlike austenitic stainless steel, the major welding problems occur within the heat affected zone (HAZ) and not the base metal
The most common problems within the HAZ are losses of toughness, post-weld cracking and decreased corrosion resistance. To avoid this, it’s essential that duplex welding procedures focus on limiting the amount of time at ‘red hot’ temperature, instead of trying to manage heat input over one pass.
Like all welding procedures, the key to success is in the preparation. Here are the key areas and duplex stainless steel welding best practices to focus on.
The Area: A specific area should be used when welding duplex and super duplex, and specific brushes and tools should also be used.
Joint Preparation: Always use a slightly larger joint angle and root gap than you would use for stainless steel. A slight loss in austenite will occur during welding, and you need to account for this. If using a TIG process for welding duplex stainless steel, nitrogen should be added in small quantities to back purging and shielding gases to stabilise the austenite.
Preheating: No preheating is required, although a hot air blower should be used to remove any condensation – especially when welding in the morning.
Heat Input: Special attention should always be centred on both heat input and the interpass temperature. If the heat input is too low, rapid cooling can affect the 50-50 composition and create more ferrite which reduces corrosion resistance. However, too much heat can cause embrittlement through σ-phases, χ-phases and other phases.
Pre-Weld Cleaning: This is essential. The welding groove and joints need to be frequently cleaned, and you must use Acetone to remove all grease, oil and dirt. Any rough edges should be ground away.
Tack Welding: Tack welding needs to be precise, or oxidation and shrinkage can occur in the weld groove. Spacers should also be included in the welding joints
Arc: If using a conventional arc welding process, like SAW, TIG and MIG, the technique of creating and extinguishing the arc is essential. This is difficult, so only highly-skilled welders should be used to perform duplex and super duplex welds with these processes.
Common Problems When Welding Duplex
The most common problem when welding duplex is not getting the heat input and interpass temperatures right. As mentioned above, heat has a dramatic effect on duplex, as a low heat input or a high interpass temperature can disrupt the delicate balance between ferrite and austenite, and corrosion resistance and toughness are affected.
This is another reason only to use highly-skilled welders with conventional arc processes, because the weld involves a near constant check and balancing of heat input and interpass temperatures.
This delicate balance means quality checks are essential. Welding duplex stainless steels with MIG, TIG, SAW and even PAW is incredibly challenging, and without proper impact toughness, corrosion resistance and ferrite/austenite mix tests, it's impossible to deliver a reliable weld qualification.
Duplex and Super Duplex Welding with K-TIG
K-TIG is an optimised version of TIG that uses a keyhole to revolutionise the welding process. It can perform both circumferential and longitudinal welds on plate, spooling, vessel, tank and other materials. The best part is, you can forget some of the information listed above because K-TIG simplifies the entire process.
K-TIG macro of duplex on a corner butt joint with cover pass
Firstly, no root gap is required, so all you need is a square butt joint. Also, the procedure can weld pipe diameters down to 3 inches, while performing single pass, full penetration welds on duplex and super alloys up to 13 mm.
It also removes the problem of controlling heat input and igniting the arc. The arc structure and the keyhole are created simultaneously and are maintained automatically by the K-TIG 1000 Evolve Controller. Additionally, the system can be completely automated, so there is no need to control heat input and electrical flow throughout the weld.
The set-up is incredibly easy and, provided that the root face (weld land) is within the recommended range, double-u, double-j, double-v, v-prep, j-prep, u-prep, single bevel and single-j preparations can all be easily-welded.
Other Benefits of K-TIG Welding
If you’re still not convinced, take a look at what else you can expect from a switch to K-TIG:
- Reduce Consumables Consumption: No filler material is required, and there is no electrode consumption within the weld pool. You'll slash your consumables consumption and your eco-footprint in the process.
- High-Quality Welds: By delivering a consistent process, free of speed variation, you can deliver high-quality welds on every job.
- Safer Workshop: K-TIG operates with lower energy consumption and produces fewer fumes. Arc efficiency is improved, and K-TIG operators are usually removed from the arc zone, as it can be controlled remotely. That means you can create a safer workshop and safer jobs for your welders.
More information on how K-TIG could improve your productivity and quality when welding duplex and super duplex can be found in our video library.