Welding stainless steel: methods, fillers and passivation

Stainless steel is welded by TIG or MIG under an argon shield, with a low-carbon filler matched to the grade: 308L for 304, 316L for 316 and 309L for dissimilar joints. The challenges are distortion from stainless steel’s high thermal expansion, sensitisation that can rob the weld of corrosion resistance, and oxidation that must be kept off the back of the joint and cleaned off the front. Get the filler, the shielding and the post-weld cleaning right and a stainless weld is as corrosion-resistant as the parent metal. This guide covers each step.

Which welding process to use

Stainless steel welds well by several arc processes. TIG welding, also called GTAW, gives the cleanest, most controlled result and is the default for thin sheet, tube and visible work. MIG welding, or GMAW, is faster and suits thicker sections and production runs. Manual metal arc, using coated stick electrodes, is used on site and for heavier fabrication. All three keep the molten pool shielded from air by an inert or specially formulated gas or flux, because oxygen is the enemy of a sound stainless weld.

Choosing the right filler

The filler must match or slightly over-match the parent grade, and low-carbon fillers are the standard choice.

The L in each filler means low carbon, below 0.03 percent, which suppresses the chromium carbide precipitation that causes sensitisation. Using 316L rather than 308L on a 316 joint keeps the molybdenum and so the chloride resistance in the weld itself, which matters in marine and chemical work. For a dissimilar joint, 309L carries enough extra alloy to stay sound despite dilution from the less-alloyed side.

Shielding gas

For TIG welding of stainless, pure argon is the standard shielding gas, giving a stable, clean arc and good control of the weld pool. MIG welding of stainless typically uses argon with a small addition of carbon dioxide or oxygen to stabilise the arc and improve wetting. The shielding gas protects the molten and cooling weld from the air, and any lapse lets oxygen reach the hot metal and spoil both the appearance and the corrosion resistance of the joint.

Controlling distortion and heat input

Stainless steel expands about half again as much as carbon steel when heated and conducts heat away far more slowly, so it distorts more and holds heat in the joint. The defences are low, controlled heat input, short weld runs, clamping and jigging, and skip or back-step welding to spread the heat. Keeping the interpass temperature down, letting the work cool between passes, also limits distortion and reduces the time the metal spends in the sensitising range. Planning the weld sequence is as important as the welding itself.

Sensitisation and weld decay

The main metallurgical risk is sensitisation. Holding stainless in the range of roughly 450 to 850 degrees Celsius lets chromium carbides form at the grain boundaries and deplete the chromium beside them, so that band loses its corrosion resistance and can corrode later, a failure called weld decay. The proven defence is low-carbon L grades and fillers, which keep carbon too low for the carbides to form, backed by controlled heat input. Stabilised grades such as 316Ti resist it too. Our guide to why stainless steel rusts explains the mechanism.

Back-purging and sugaring

When the back of a weld, such as the inside of a pipe, is left open to air, the hot underside oxidises into a rough, dark, granular layer known as sugaring, which destroys corrosion resistance exactly where a pipe carries fluid. The defence is back-purging: filling the reverse side with argon before and during welding to drive the oxygen down to around 0.2 to 0.3 percent. On critical pipework this is essential, and the purge is maintained until the weld has cooled enough not to oxidise. A properly purged root is bright and clean rather than black and sugared.

Heat tint and why it must be removed

Even a well-shielded weld leaves a band of oxidised colour beside it, the heat tint, ranging from straw to blue. That tint is a chromium-depleted, weakened layer, and it corrodes more readily than the surrounding steel, so on any corrosion-sensitive work it is removed rather than left for looks alone. Removing it also strips any embedded iron and surface damage from fabrication, returning the joint to full corrosion resistance. Leaving heat tint in place is one of the most common reasons a stainless weld later rusts.

Pickling and passivation after welding

Restoring corrosion resistance after welding is a two-step chemical clean. Pickling, with a paste or bath of nitric and hydrofluoric acid, removes the heat tint, the embedded iron and the chromium-depleted layer beneath. Passivation, in a nitric or increasingly a citric acid solution, then removes any remaining free iron and lets a uniform chromium oxide film reform across the surface. Citric acid is gaining ground because it is safer to handle. Together, pickling and passivation return a stainless weld to the corrosion resistance of the parent metal.

Welding stainless to carbon steel

Joining stainless to carbon steel is routine but needs care. The filler is 309L, whose extra alloy content copes with the dilution from the carbon steel side and keeps the weld sound. Bear in mind that the finished joint mixes two metals with different corrosion behaviour, so in a wet environment the carbon steel side still needs protection, and the dissimilar couple should be considered for galvanic effects. The same galvanic thinking applies to fasteners, as our guide to joining aluminium profiles discusses.

Preparing stainless before welding

A sound stainless weld starts before the arc is struck. The joint and the surrounding surface are degreased, because oil and marker ink burn into the weld and cause porosity and carbon pick-up. Any oxide or contamination is removed with a dedicated stainless brush or flap disc, never one that has touched carbon steel, since embedded iron particles would rust and seed corrosion later. Edges are prepared square and the fit-up kept tight, so the filler is not asked to bridge large gaps. Clean gloves and a clean filler rod matter too, because a fingerprint or a dirty rod end shows up as a defect in a stainless weld.

Weldability across the stainless families

Not every stainless grade welds the same way. The austenitic grades, 304, 316 and their L versions, are the most weldable, which is why most stainless fabrication is austenitic. Ferritic grades such as 430 are weldable but prone to grain growth and reduced toughness in the weld, so they need care and are often kept to lighter work. Martensitic grades such as 420 are hardenable and can crack on cooling, usually needing pre-heat and post-weld heat treatment. Duplex grades such as 2205 weld well but demand controlled heat input to keep the balance of their two phases, using a matching 2209 filler. Knowing the family tells you what to expect at the arc.

Stainless steel from ULAMEX

ULAMEX has supplied weldable stainless steel since 1988, in the grades that fabricators specify.

Fabricating in stainless? Tell ULAMEX the grade, the forms and the environment and the desk will advise on grade and filler and return a non-binding quotation. See our guides to 304 and 316 stainless steel and duplex 2205, and our supply capabilities, or contact the export desk at [email protected] or +48 504 424 761.

Frequently asked questions