Stainless steel is a popular material choice for the fabrication industry. It offers strength, durability and renowned resistance to corrosion (hence the term “stainless” or no rust). However, stainless steel welding does pose a few challenges, particularly compared to mild steel. Welding stainless steel with a conventional process (like MIG, TIG, SAW or PAW) can be challenging, and considering the expense of stainless steel, any mistakes and reworks can be costly.
Choosing the right welding process is essential in making stainless steel welding easy. Unfortunately, when dealing with conventional welding options, there really is no perfect solution. Fabricators must weigh up a variety of factors, ranging from productivity, to the cost of filler metal and even the skill of the operator.
What is Stainless Steel?
Stainless steel is a generic term used to describe an entire family of corrosion resistant alloy steels that contain over 10.5% chromium.
The most common stainless steels are graded into either chromium-nickel versions (300, or austenitic) or straight chromium versions (400, or ferritic and martensitic). The straight grades and carbon steel share low coefficients of linear expansion, which is a measure of how a material expands and contracts under pressure and temperature. Straight grades have a lower melting point than carbon steel but melt at a higher point than chromium-nickel grades of stainless. Both straight and chromium-nickel have low thermal conductivity and higher electrical resistance than carbon steel
The Benefits of Welding with Stainless Steel
Stainless steel is held in high regard due to its ability to withstand corrosion. Given the chromium content in stainless steel, a naturally occurring chromium-rich oxide film forms on the surface of stainless steels. This very thin, inert film adheres tightly to the surface of the metal, giving it an extremely protective coating in the face of a wide array of corrosive substances and environments. This film is also able to rapidly repair itself, so long as oxygen is present.
Stainless steel can also withstand both hot and cold temperatures without losing its strength. Another key advantage is the fact that it repels bacterial growth, which is why it is such a popular choice for the medical and food industries.
Its durability and anti-corrosive properties, make it the go-to option for petrochemical and piping applications, aerospace and energy industries, as well as tanks for the transport of corrosive chemicals.
The Challenges of Stainless Steel Welding
The alloying elements within stainless steel provide its thermal conductivity, or heat “insulation”. However, this can make arc welding processes like MIG and TIG a bit of a challenge. The insulation means the heat from the arc can become concentrated in the weld pool. This leads to a range of problems, including oxidation, warping and burn-through, and makes process and filler metal choices critical to weld quality.
Another common problem is discolouration, or ‘sugaring’. Discolouration is a sign that the gas shielding during welding was inadequate, and potentially chrome has been pulled out of the stainless steel, which affects its corrosion-resistance properties. Sugaring is a worst case condition and usually requires rework, for both quality and aesthetic reasons, and the filler metals for stainless steel welding are more expensive than those required for carbon steel.
Choosing the Right Procedure for Welding Stainless Steel
Take a look at the processes below to see how they stack up when it comes to providing quality welds for stainless steel:
This wire feed welding process delivers fair bead appearance and efficiency, making it useful when productivity is in order. Development in the equipment and filler metal have made the process a little easier, but it is still not appropriate for welders that are new to stainless steel. The cost of shielding gas also has to be considered, as it is a necessary element to minimize spatter and protect the weld and Heat Affected Zones (HAZ).
Flux-Cored Arc Welding (FCAW)
Is more productive than MIG but does add time to cost and clean-up due to slag and spatter generation. The filler metals required are also the highest cost per pound, thanks to the expensive alloying elements required for the flux.
Low spatter level makes submerged arc welding a popular choice for stainless steel welding, and it is particularly suited to large applications and thick materials. It doesn’t require as much skill as other processes, but it can only be used in the flat position. The flux creates a slag layer that must be chipped or ground away, however, and this can be very time consuming.
This procedure requires a high degree of skill, and productivity is usually low. It produces no spatter, and it has a moderate cost because it is a slow and complicated process.
So, What’s the Best Process for Stainless Steel Welding?
The easiest method for stainless steel welding is K-TIG. An optimised and much more efficient version of TIG, K-TIG is the product of an extensive, scientific study into the gas-tungsten-arc process and was developed by the Australian Government's Commonwealth Science and Industrial Research Organisation (CSIRO). The K-TIG process features many innovations in heat removal, process efficiency, weld pool stabilisation and arc characteristics.
It is ideally suited to stainless steel fabrication, and it has already become the process of choice for international manufacturers.
It offers 8 x the penetration of conventional TIG and can perform single pass welding on stainless steel materials of up to 13 mm. It also requires no edge bevelling and moves at up to 100 x the speed of conventional TIG. On top of this increased penetration and speed, it also reduces gas consumption by 90% and wire consumption by 90% to 100%.
High energy density in the welding arc opens the keyhole, which allows for full penetration, high speed welding. The K-TIG torch also creates a plasma jet out of a high current arc which penetrates the material and creates high surface tension on the underside.
The weld pool remains exceptionally stable throughout the weld, thanks to the geometry of the keyhole, which allows the exit of arc gases and the minimisation of surface energy. The tension created also prevents any molten metal from falling from the root face.
Using K-TIG with Stainless Steel
K-TIG can be used for all stainless steel welding applications for materials between thicknesses of 3 and 13 mm. As long as the material is within this range, full penetration, single pass butt welds are possible with no need for edge bevelling or a gap.
It can be used for longitudinal and circumferential welding in both 1G and 2G positions. It is especially suited for welding stainless steel tanks and pressure vessels, and many fabricators are switching from PAW to avoid the issues with closing out the keyhole that are common with this far more complicated process.
Other Benefits of Stainless Steel Welding
By switching to K-TIG, you'll open your workshop and fabrication efforts up to all the following benefits:
Speed: K-TIG can weld stainless steel at up to 1,0000 mm/m, while still providing single pass, full penetration and high quality welds. That means you increase your productivity.
Penetration: Full penetration saves enormous amounts of time, and because K-TIG doesn’t need any edge bevelling, you’ll save time and money on not having to use expensive weld filler wire and costly V- or J- groove preparations that are required when using TIG or MIG welding processes.
Limit Shrinkage and Distortion: One of the difficulties of welding stainless steel is dealing with its tendency to distort. The full penetration and single pass welding achievable with KTIG mean distortion is drastically reduced. This makes K-TIG perfect for pipe spooling, and other projects where multiple joints are needed.
Simplicity: The process is incredibly simple, and operators can typically be trained sufficiently in under 3 hours. There is no need to balance gas and electrical flow manually, as everything is controlled automatically by the sophisticated K-TIG 1000 Evolve Controller.
K-TIG worked with a US-manufacturer of high-pressure cylinders, general-purpose tanks and transport vessels. K-TIG provided a savings assessment for 6,350 mm (250 inch) circumferential and 1,800 mm (72 inch) longitudinal joints, and the results were incredible.
H3: Circumferential Joints 6,350mm (250 inch) length joints in Stainless Steel 304 – 316
At 10 Ga (3.5mm): K-TIG took just 6 minutes, compared to 61 minutes for TIG/MIG
At 7 Ga (4.7mm): K-TIG took just 10 minutes, compared to 80 minutes for TIG/MIG
At 6 mm (1/4 inch): K-TIG took just 12 minutes, compared to 99 minutes for TIG/MIG
At 8 mm (5/16 inch): K-TIG took just 16 minutes, compared to 99 minutes for TIG/MIG
At 9.5 mm (3/8 inch): K-TIG took just 21 minutes, compared to 112 minutes for TIG/MIG
H3: Longitudinal Joints (1,800mm (72 inch) length joints in Stainless Steel 304 – 316)
At 10 Ga (3.5mm): K-TIG took just 1.8 minutes, compared to 18 minutes for TIG/MIG
At 7 Ga (4.7mm): K-TIG took just 2.8 minutes, compared to 23 minutes for TIG/MIG
At 6 mm (1/4 inch): K-TIG took just 4.5 minutes, compared to 12 minutes for TIG/MIG
At 8 mm (5/16 inch): K-TIG took just 4.5 minutes, compared to 29 minutes for TIG/MIG
At 9.5 mm (3/8 inch): K-TIG took just 6 minutes, compared to 34 minutes for TIG/MIG
Bilfinger also achieved an amazing reduction of 92% on costs after switching to K-TIG.
Contact the team at K-TIG for more information on making the switch!