A Comprehensive Guide: Mastering TIG Welding of Stainless Steel

TIG welding stainless steel can be a rewarding experience when approached with the right techniques. In this comprehensive guide, we will walk you through the challenges posed by stainless steel and the different types of stainless steel. As well as the step-by-step process of setting up and executing high-quality TIG welds on this versatile material.

Understanding the Challenges:

Stainless steel is an alloy of iron, carbon, nickel, and chromium. It can present unique challenges due to its heat retention properties. The melted joint and the heat-affected zone (“HAZ”) do not dissipate heat into the rest of the welded part and air as efficiently as mild steel. The risk of carbide precipitation, which can compromise corrosion resistance, is a critical factor in stainless steel welding. If stainless steel gets heated enough to undergo a carbide precipitation process, it will lose corrosion resistance. This renders the material useless and severely impacts its load-bearing capabilities.

Reading the Stainless Steel Welds And Preventing Carbide Precipitation

Luckily for us welders, stainless steel provides a much-needed feedback. Judging by the weld colour, you can quickly tell if the weld and HAZ have undergone the carbide precipitation.

• Straw-coloured, bright yellow welds are safe and of high-quality
• Light blue coloured welds are likely safe
• Dark blue and purple welds have a higher chance of carbide precipitation
• Black welds and HAZ have experienced carbide precipitation and should be reworked

To prevent carbide precipitation, you “just” need to keep the stainless steel joint below extreme temperatures, weld fast, and provide appropriate shielding gas coverage. This is easier said than done, and we’ll explain the necessary strategies in the remainder of the article, but let’s first quickly touch on stainless steel types.

Different Stainless Steel Types:

We will delve into the various types of stainless steel. Each type comes with its own characteristics and weldability factors, making it crucial to choose the right stainless steel for your welding project.

Stainless steel types are:

• Austenitic – 300-series, mainly 304 and 316. It has excellent weldability and is the most widely used stainless steel type.
• Ferritic – 400-series, mainly 430 and 434. It’s magnetic and has lower nickel content. Ferritic stainless is a bit more challenging to weld than austenitic.
• Martensitic – Also 400-series, mainly 420. It has high tensile strength, but it’s less often used.
• Duplex – Family of stainless steels made by mixing austenitic and ferritic stainless steel. It’s often used in pipeline applications because it’s very strong and resistant to corrosion.

TIG Setup for Stainless Steel Welding:

Setting up for TIG welding stainless steel involves considerations such as shielding gas, polarity, tungsten electrode selection, tungsten tip preparation, filler wire types, and amperage output. Following these guidelines ensures optimal welding conditions and prevents issues like overheating.

Shielding Gas

Using 100% argon for the shielding gas works best in almost all situations. However, adding helium to argon is sometimes needed to improve weld penetration and puddle fluidity. The more helium you add, the higher the heat input. So, unless you know exactly what you are doing, it’s best to stick to straight argon to TIG weld stainless steel.

Polarity

To TIG weld stainless steel, use a DC machine with electrode negative polarity (DCEN). That means you should attach the TIG torch to the negative polarity of the welder and the ground clamp to the positive polarity.

Tungsten Electrode

You can use every tungsten type for stainless steel except pure tungsten, which is meant for aluminium. Red, thoriated tungsten is a very popular choice, but it’s radioactive. If you prefer more safety, consider blue, lanthanated tungsten electrodes.

Tungsten Tip Preparation

Don’t make a taper of more than 2.5 times the electrode diameter. You’ll achieve good penetration, thin beads, and a narrow HAZ by staying below this taper.

The table below shows what TIG filler wire to buy for the stainless steel alloys. The most common TIG filler wires are E308/E308L, but you should always strive to use a matching filler material as the welded stainless alloy.

The filler wire should be thinner than the welded material. If it’s thicker, you’ll need more heat to melt the wire than the base metal. As a result, you’ll overheat the stainless steel before melting the filler material.

Amperage Output

Since stainless steel retains heat well, it requires about 10-20% lower heat input than mild steel. The general rule of thumb for mild steel is that 1 amp of power welds about .001” of steel thickness. So, 200A can weld between 3/16” and 1/4″. But, these rules underestimate most welding machines; still, it’s a good starting point. Whatever your stainless steel joint thickness is, convert it to amps and lower the amperage output by about 10-20%.

This may take some trial and error depending on your travel speed, technique, and machine capabilities.

Pre-Cleaning Stainless Steel:

Cleanliness is paramount when working with stainless steel. The welding joint must be free from oils, grease, and contaminants to achieve a sound weld. Tips on pre-cleaning stainless steel, including removing plastic peel coats and using dedicated tools can be found in our other article, Metal Surface Preparation for Welding Excellence. 

Techniques for TIG Welding Stainless Steel:

Gas purging, copper/aluminium backing, pre and post-shielding gas flow, heat management, travel speed and torch angle techniques are discussed in detail. These techniques, including foot pedal control, pulsed TIG, managing puddle size, travel speed, and torch angle, provide a comprehensive understanding of the intricacies involved in TIG welding stainless steel.

Gas Purging

You must have heard about gas purging before. But the question is, do you need to gas purge the stainless steel joint?

If you are welding stainless steel pipes and need excellent weld quality from inside the tube, you must purge the pipe. In most cases, welding stainless pipes requires purging. But, it’s often also necessary to purge the back side of the joint when welding two stainless steel plates side by side with a butt weld configuration.

If you don’t purge the pipes, the oxygen from the inside will weaken the weld. The welded joint is protected from the argon shielding gas coming from the TIG torch cup, but the underside of the weld joint is not protected from the oxygen found inside the pipe.

So, purging the oxygen by releasing the argon gas inside the pipe forces the oxygen outside and leaves a protective argon atmosphere inside the pipe. That way, you are shielding the welding joint from outside and inside.

To purge a pipe, all you have to do is tape both pipe ends shut, poke a hole in one end and stick an argon hose with a diffuser inside, poke another spot on the opposite end for oxygen to be purged, and release the argon gas. This is an elementary DIY purging method. There are more advanced gas purging systems out of this article’s scope.

Important note: Use halogen-free, heat-resistant aluminium tape to close pipe endings! Most regular tapes contain halogens like chlorine. If these compounds heat up, they can be absorbed into the welded joint and cause weak, brittle welds.

Copper/Aluminum Backing

If you don’t want to achieve maximum stainless steel weld quality but to create a solid, good enough joint, you can substitute purging with backing. This is especially useful when welding stainless steel plates.

Sometimes it’s just not necessary to achieve a perfect weld from underneath the joint. Purging the joint from beneath takes time, effort, and shielding gas, which is costly.

Instead, you can place a copper or aluminium backing beneath the joint line and clamp it firmly so there is a minimal gap between the backing and stainless steel joint. This is not perfect, so you won’t achieve a clean, silvery weld from beneath the joint. But, in most cases, this will suffice.

The copper and aluminium won’t melt as you weld over them because they conduct heat away rapidly, unlike stainless steel. As a result, these materials require more amperage to melt than stainless steel. But, to be sure, use thicker backing parts than the welded stainless steel.

Pre and Post Shielding Gas Flow

It’s a good practice to provide pre-welding shielding gas flow to thoroughly drench the joint before the arc starts melting the base metal. Preflow prevents contamination by pushing atmospheric contaminants out from the joint and creates a safe, argon-rich environment. About one second should be enough for most stainless steel welding jobs. Just make sure the TIG cup is right above the joint.

Post-welding shielding gas flow is even more critical. It prevents oxygen from damaging the weld at the ending phase. You cannot just complete a weld and remove the TIG torch. The super hot stainless steel would oxidize as it comes in contact with air. It’s necessary to keep the shielding gas over the end point of the weld for about one second for every 10A of welded current.

Heat Management

Managing heat when welding stainless steel is crucial. So, let’s discuss all techniques you can use to prevent carbide precipitation from overheating the stainless.

• Starting Amps

A low amp start is helpful when welding stainless steel because you’ll give the material more time to heat up slowly. If you input too much heat immediately after starting the arc, there are high chances that you’ll warp the material. Plus, since stainless retains heat too well, you can avoid concentrating heat during the first second or two until you get moving along the joint line.

You should set your starting amps depending on the material thickness. But anywhere between 5A and 20A is a good starting point. You’ll quickly learn how to set this setting with some experience. So, practising on a scrap piece of stainless steel is a good idea.

• Foot Pedal Control

The TIG foot pedal will be your best friend when welding stainless steel. Since this material retains heat, it’ll get hotter as you weld. This means that you may need to back off the amperage as you go along the joint.

Foot pedal control lets you set the lowest and highest amperage output. The highest amperage is when you press the pedal all the way, like giving the full throttle to a car. But, when you back off the pedal, the TIG welder will output less amperage. As a result, you can modify heat input in real-time as necessary.

But, foot pedal control adds the challenge to TIG welding in general. When performing this welding method, you’ll control the TIG torch with one hand and the filler wire with the other. So, managing the foot pedal simultaneously makes things challenging.

• Pulse TIG

Pulsed TIG is another excellent way to control heat input. You can use pulsed welding with or without the foot pedal control. So, you can double down on heat control or just use pulsed TIG on its own if your welder supports this feature.

This function also has a low and high amperage output set, just like using a foot pedal. But, the machine automatically alternates between high and low amperage many times per second. You can pulse as low as 1 pulse per second (“PPS”) or as high as 200 PPS.

Pulsing also lets you tailor the penetration, weld width, and resulting bead aesthetics.

SA Welding offers the following machines with pulsed TIG support:

• TIG325X AC/DC TIG Welder With Foot Pedal
• PRIMEWELD TIG225X

TIG325X AC/DC TIG Welder With Foot Pedal

PRIMEWELD TIG225X

• Puddle Size

Try to keep the puddle size equal to the thickness of the base metal. If the puddle becomes larger, you are unnecessarily melting away the material and inputting too much heat. Back off the foot pedal and increase travel speed to resolve this issue.

At the end of the weld, you’ll need to back off the TIG pedal and add filler metal to chill the puddle and avoid crater cracking. Once the arc is turned off, remember to keep the post-flow active, so don’t remove the TIG cup away from the weld.

Travel Speed

You need to move quickly when TIG welding the stainless steel. Otherwise, you’ll concentrate too much heat. As a result, the weld and the HAZ will undergo carbide precipitation, leading to intergranular corrosion and cracking.

If you are uncomfortable with high travel speeds, you can use backing chill bars from aluminium or copper, as we discussed earlier. These two metals conduct heat away rapidly, so they’ll take on themselves a part of the heat you input into the stainless steel. Still, this will only help you so much. You must move the torch quickly to TIG weld stainless steel.

Torch Angle

Keep the torch at about a 70-degree angle to the welded joint and a filler metal at about 10 to 25-degree angle to the joint. That’s a typical setup. But, you may have to adjust angles depending on the joint type and accessibility.

Final Thoughts:

Mastering TIG welding of stainless steel requires attention to detail and practice. By adhering to the guidelines outlined in this guide, you can enhance your skills and produce high-quality welds on stainless steel, paving the way for successful projects ranging from exhaust systems to interior details. Remember, practice on scrap metal is key to gaining confidence before tackling valuable items.

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