Tackling Stainless: A guide to stainless steel welding

By Greg Holster

I get so many people asking me what is the best way to weld stainless steel. There are many different versions of the best way. Hopefully this article will give you Sheddies a few helpful hints on welding stainless. For the sake of simplicity, I will stick to the common 300 series stainless steels. One of the things I like about welding stainless is that the welding machinery is generally the same amperage and has the same material thickness capability as mild steel. Within reason, you could say they were cousins. 

However, there are a few differences with some of the physical and chemical properties of stainless steel compared to mild steel and this can affect your welding variables. And, no, I am not going to give you a chemistry lesson. 

Austenitic stainless steels are the most common types that we come across in day-to-day life. Marine fittings are normally 316L; the likes of kitchen benches, shower trays, etc, are often 304L. You will notice when purchasing MIG wire, TIG rod and arc-welding electrodes that the wires have an “L” designator, indicating that the material or consumable has a carbon level that falls in the low end of the carbon range. You will find that most consumables also have an “Si” designation which means that the consumable has a higher silicon content, which provides more weld pool fluidity and controllability. 

1: Stainless steel 316L MIG wire 2: Stainless steel MIG weld using argon

Austenitic stainless steels are the most common types that we come across in day-to-day life. Marine fittings are normally 316L; the likes of kitchen benches, shower trays, etc, are often 304L. You will notice when purchasing MIG wire, TIG rod and arc-welding electrodes that the wires have an “L” designator, indicating that the material or consumable has a carbon level that falls in the low end of the carbon range. You will find that most consumables also have an “Si” designation which means that the consumable has a higher silicon content, which provides more weld pool fluidity and controllability. 

The coefficient of thermal expansion for these austenitic 300 series stainless steels is about 50 percent greater than that of mild steel. This translates into a recipe for more distortion than you may find when doing mild steel work. So my advice to minimise warping, in particular on lighter-gauge sheet metals, is to try welding at the lowest current or level that you possibly can. However, not too low, as you still need good fusion between the consumable being used and the parent stainless steel. 

If you’re using MIG, don’t be afraid to move quickly. Faster travel speeds means less heat input. The higher silicon in stainless wires helps the weld ow at these faster welding speeds. Practise on scrap pieces and you will be pleasantly surprised. Stainless steel also has lower thermal conductivity, which means that heat created from the welding process does not dissipate from the welded area as quickly. Whichever process you choose, this also means that the welding will not require as much amperage to achieve good fusion because the heat is not leaking away from the welding area as rapidly. A heat sink attached to the weldment can be a good idea.

Stainless steel’s lower electrical conductivity means that your wire feed speed will be at a similar rate as that when welding mild steel. You will notice that the weld pool appears to produce a lower current level with stainless steel welding than with mild steel. Something to mention regarding the lower conductivity of stainless is that maintaining a consistent contact tip/tungsten electrode to material distance is more important when welding stainless steel. This is because inconsistent variations can result in a greater degree of change in welding current levels. I like to keep a short arc no matter what process is being used and a steady hand is also important. 

Welding mild steel to stainless is an easy and painless procedure. Use 309LSi as it has a higher percentage of chrome and the carbon content is also higher.
And let’s not forget stainless steel’s corrosion resistance, which means it doesn’t rust like steel does. This is possible because stainless has higher levels of alloys in the base metal, mainly chromium and nickel. These higher levels of goodies in stainless also decrease the weld-ability and you can have a wrinkled or ropey weld profile settings and techniques are out of sync. 

MIG welding
As far as wire feed (amps) and voltage settings go, most good, single-phase MIG welders should have a recommended settings chart on the door of the machine or in the instruction book. If for some reason it does not have settings for stainless steel, then use as a starting point the recommended settings for mild steel. From here you can adjust the settings to ne-tune the MIG welding arc. You may need to run a slightly higher wire feed speed rate than you would with mild steel. A higher voltage setting may be needed. This will help wet out the weld pool a bit more, giving a flatter face and better shape at the edge of the weld bead, especially when using part-helium mixtures in the shielding gas. Don’t be afraid to move faster than you would with normal mild-steel MIG welding. Feeding stainless wire needs no special options; no fancy liners, just normal “V” groove rollers. I like to use 0.8 mm wire, usually 316LSi, as this will cover 304, 308 and 316 applications. Use only a forehand or push technique, not backhand or dragging. Pushing will give a nice at mitre- fillet weld. 

Often on thin stainless sheet, short circuit or dip transfer mode will be your best machine setting. This will transfer the least amount of heat into the piece you are welding. Dip transfer has a much lower heat input compared to arc welding or TIG welding. While dip transfer has a lower heat input and can reduce the chances of burning through on thin sheet, it can also produce a cold, rounded-looking weld bead. 

Let’s clarify what is meant by dip transfer in MIG. Let’s assume your machine is 250 amps single phase. As you set your machine from its lowest setting to maximum, there are three changes the arc will go through. The first is short- circuit metal transfer, or dip transfer. The second is globular transfer and the third is spray arc. These three transfers describe the manner in which metal is transferred from the MIG wire to the weld pool on the parent metal. In dip transfer, you hear a fast “crackling” sound, often likened to the sound of bacon frying. At low amps, dip transfer is a stiff arc ideal for thinner materials. Globular transfer is when the droplets of weld burn off and drop into the weld pool. It makes an uneven, splattering sound, with the odd little hiss here and there. Globular transfer is not often used in industry. It’s more like the rough ride from dip until you get to spray. Spray transfer is exactly what it sounds like—a nice hot, hissing sound as the hot metal transfers in a steady stream. 

The factors that affect the molten weld metals are the welding current/wire feed speed, voltage, wire size, arc length, power supply (transformer or inverter) and the type of shielding gas being used. Using stainless 0.8 mm 316LSi wire, dip transfer will occur up to about 150 amps. Between 150 to 180 amps we have globular transfer and above this we have spray transfer. For 0.9 mm add about 10 percent on top of these amperages. 

Shielding gases

If you want to do things the right way, using a recommended stainless steel shielding gas is extremely important. I like to use Stainshield Universal which has 55 percent helium, 1.5 percent CO2 and the balance is argon. There is a 2 percent CO2, 98 percent argon mix as well which is also very popular. These gas mixtures are made to maximise penetration, sidewall fusion and break up the surface tension in the weld pool. I get asked quite often if you can use pure argon, instead of paying for another cylinder that may not be used up completely. 

The metallurgist’s answer to that is: no. You will get a hot, globular, unstable weld pool, with below average sidewall and toe of the weld fusion. The weld will be rounded and ropey looking. The Sheddie’s answer to this question is what service is your project/weldment doing? Corrosive resistance will be fine, strength average and the look—depending on how well you can weld—may be a bit below average. But that’s what grinders and flap discs are for. I have used straight argon to do the odd little job on MIG and given it a good grind afterwards. 

So can you weld stainless with standard mild steel Argoshield mixes? Again the metallurgist’s answer is no. But you can weld with these gases. The problem is the CO2 in these mixtures is far too high. It can cause cracking and the carbon content will rise. It forms chromium oxides and the corrosion resistance goes out the window. But once again, what purpose will this item serve? The problem here is the weld will look perfect but it won’t show the metallurgical deficiencies. Many muffler/exhaust shops still weld stainless exhaust pipes with CO2 and mild steel wire. This is because everything is bracketed or mounted. The mufflers themselves are all MIG welded or TIG welded with the correct gas and consumables. 

There is a stainless flux-cored wire that uses CO2 as its shielding gas but this is for heavier sections of stainless and the chemistry in the flux counteracts the problems that CO2 can cause.
The Stainshield Stainless mixes with between 25-55 percent helium are quite good for mild-steel MIG welding, especially the Universal which has 55 percent helium. Although it may be a bit more expensive, if you want to do a lot of stainless and mild steel, consider this as an option. 

TIG welding stainless
When you want to start welding stainless steel, there are a few points of the TIG process that are very important to maintain stainless steel’s corrosion resistance and strength. Just like MIG welding, stainless distortion is a problem so minimising heat input is a priority. Choosing the correct TIG filler rod is also similar to MIG. Some people actually use MIG wire as a TIG filler. When welding 300 series stainless, keep to the simple fillers—I would suggest 316LSi. Joint preparation is more important when welding stainless steel than when welding mild steel, mostly because of distortion. Bracing, strong backs or heat sinks are all good options, so try to make room for one of these options when choosing a joint prep. A clean, dry and oil-free set up is also vital. 

I won’t get into the different types of TIG machines but what you need is a DC power source. High frequency start is a bit of a luxury and I don’t mind using scratch start or lift start. TIG welding stainless uses argon gas. A negative electrode, positive earth should be the terminal set up with argon gas set at around 6-10 litres per minute. Use a thoriated tungsten electrode ground to a point. The size of tungsten is dependent on the amps being used and material thickness. I use a 2.4 mm for everything from 1 mm to 10 mm. TIG torch cup sizes range from 6 mm diameter to 15 mm. Larger sizes give the operator a wider envelope of shielding gas but can make seeing the end of the tungsten more dif cult. The answer to that would be to use a gas lens set up. A gas lens streams the argon gas through the nozzle more like a beam or channel. This means that the tungsten can be protruding up to 20 mm or more if you turn the argon ow up. This is awesome for teaching welding as it makes seeing and controlling the weld pool so much easier. 

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The thing about the TIG welding process on any material is you are welding the lowest manual welding deposition rate and normally at very slow welding speeds. This can make welding stainless steel much harder than normal mild steel. Learning and improving your TIG welding technique will enable you to weld faster, while still retaining the same weld quality. This has an effect on heat input and therefore results in less distortion. By the way, when good results are achieved, for goodness’ sake record the settings. 

Thoroughly clean the area to be welded. Cleanliness cannot be stressed enough. It’s not like MIG or stick where you might get away with welding over a bit of oil or unclean metal. If you try to TIG weld over oily, greasy, paint or heavy oxide, you will contaminate the tungsten electrode and the weld pool and upset the weld chemistry. 

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Make sure that the weld joint area and also the filler rods are as clean as possible. This is an important preparation. Remove all oil, grease, paint, rust and dirt. Contaminants such as these may result in arc instability, contaminated welds and/or contaminated tungsten. Clamping: Clamping when welding and tacking stainless may be needed if the work piece cannot be held during tacking or welding. Remember, you need both hands for TIG welding. 

Tack welds: On thin materials, it is a good idea to make short 2-3 mm tack welds close together, as the heat will distort the joint as it is being welded. Good welds: So what constitutes a good weld? Look for good penetration into the base material, not too much penetration or oxides on the underside, a nice at bead profile, or a nice at mitre fillet if doing a fillet weld. Good bead width and good fusion at the edges of the weld are also important. Beware of under cut. This is where the toes or edges of the weld have burnt grooves into the base material. If this is the case, you have probably welded too slowly, at too hot a temperature, starved the weld pool of filler rod or a mixture of all of these variables. The amount of filler rod is something the operator will decide on depending on the profile needed. Practise on an offcut first. Keep the tungsten as close as possible to the weld pool—this will give you good penetration and aid in reducing heat input. Remember to let the argon ow until the tungsten has stopped glowing. If you have post ow, set it at 10 seconds or more. An oxidised tungsten makes for a dirty start or tack. 

Oxidisation and discolouration of the weld area can really spoil a nice weld. If sanding or polishing to clean up welding beads, make sure you use iron-free abrasives or you will quickly see the rust spots forming. You will get oxidisation on the back side of thin sheet. This chromium oxide should be cleaned off, as it is a given it will rust or oxidise. Back purging will stop this happening and there are also tapes and fluxes available, such as solar ux. 

Passivating paste or pickling paste is a good alternative for cleaning up welds, especially if you want to show them off. But be prepared for criticism, as everybody becomes a welding expert when they find out you’ve done it yourself. Depending on the type of finish you require, most fabricators will want to sand and polish back the welds. My choice was a 41⁄2 inch (115 mm) Hitachi angle grinder complete with iron-free cutting, grinding, sanding and polishing discs. My little angle grinder turns out to be as important as the welding part of some of my projects. 

Every good welding professional, if they are honest, will tell you that they were more proficient with the angle grinder or the scrap bin long before they were proficient with good welding techniques. Nothing is different in my shed, my trusty angle grinder sometimes gets a good workout, mostly by my choice. The reality is that no matter how magnificent the weld looks, or which process used to do the welding, if you have good penetration and the weld is going be ground ush anyway, who cares how pretty or ugly the original weld looked. Just make sure you have looked after the chemistry in the weld, or it will bite you in the arse when the corrosion starts. But I am a realist. I might not quite be a Rembrandt with a welder but I am certainly a Michelangelo with a 41⁄2 inch (115 mm) grinder and a ap disc. 

Arc welding
Arc welding stainless is a breeze, as long as you don’t want to weld anything really thin, or weld out of position. Much of it comes down to joint configuration— maybe a butt joint becomes a lap joint, or a backing bar can be used. I have welded tube 1.5 mm wall thickness and have sleeved the joint.
Some of the advantages: 
• Lower skill level required.
• You can buy the 2.5 mm 316L arc rods in small rod packs. I always keep a few on hand.
• No shielding gases required. Single-handed operation; two if you shake like I do.
• Normally low spatter when welding with small-gauge stainless electrodes.
• Can be used outside.
• Tacks are flat. Stainless rods run really smoothly and at lower amps than mild steel rods.
• I like the fact that I can turn the machine on, stick a rod in and weld.
• No tungstens to grind, no gas cylinders, good basic “in the shed” making re with an arc rod. 

The disadvantages: 
• You need a good t-up and gaps can create easy burn-through. Once you have burnt that hole, it just keeps growing. 
• Arc rods can be difficult to use out of position. Learners will not nd it easy to weld tube or sheet with a 1.5 mm thickness or less. 
Practise first—get the amps right with smooth movement and you’ll have no problems. 

Safety
Be aware the fumes have chrome and other nasties wafting past your nose, so keep that in mind. Also the refection off bright stainless can give you a real burning arc tan—not a good look. If you are stick welding, be aware of the slag, it's glassy. As it cools, it will “ping” off without warning. Get a hot sliver of that in your eye and you won’t be very happy. Practising makes welding fun and will give you a real sense of achievement. Don’t throw away those old stainless sinks and shower trays—build yourself that smoker, wood- red spa heater, an artistic stainless garden sculpture or barbecue hood...