Welding stainless steel: with what and how
Stainless steel is welded by TIG, by a semi-automatic welder in shielding gas or with electrodes. For 12Х18Н10Т the welding electrodes are selected for the grade of the base metal and the thickness.
It is not the method itself that decides, but three things: a filler of the same class, reliable protection of the weld from air and restoration of the passive film after cooling. A mistake in any of the three points gives a weld that rusts earlier than the base metal.
Below are the methods, the consumables, the gas and how to preserve corrosion resistance.
Welding methods
The method is chosen by the thickness, the position of the weld and the appearance requirements. Thin metal and critical joints are more conveniently welded by TIG: it gives control over the pool and a neat weld. Thick metal goes faster with a semi-automatic welder. Assembly and repair in the field are often done with electrodes, because no gas cylinder is needed.
| Method | Designation | Where it is appropriate | What to watch |
|---|---|---|---|
| Manual TIG | TIG | thin sheet, pipes, critical welds | filler, cleanliness of the edge, root purging |
| Semi-automatic | MIG | medium thickness and above, production welds | wire for stainless steel, gas composition |
| Manual arc | MMA | assembly, repair, thick metal | electrode grade, slag cleaning |
| Automatic submerged | SAW | long straight welds, thick metal | flux for stainless steel, sequence of welds |
| Welding with purging | — | pipes and vessels where the weld root matters | protection of the reverse side from oxidation |
Welding of thin-walled pipes stands separately: here not only the outer weld matters but also the backing bead. If the weld root has oxidised from the inside, the pipe loses resistance exactly where nothing can be cleaned afterwards. That is why pipes and vessels are welded with shielding gas purged into the cavity.
Electrodes and wire
The filler must correspond to the base metal. For 304 an austenitic filler with low carbon is taken, for 316L one with molybdenum, for 321 and 12Х18Н10Т one stabilised with titanium or niobium. Stainless steel must not be welded with a filler for carbon steel: the weld will get a different composition, lose resistance and become the weak link of the structure.
| Base metal | What the filler should contain | Why |
|---|---|---|
| 304, 08Х18Н10 | low carbon, austenitic class | against intergranular corrosion |
| 316L, 03Х17Н14М2 | molybdenum and low carbon | resistance to chlorides |
| 321, 12Х18Н10Т | a stabilising addition | operation under heating, weld resistance |
| 430, 12Х17 | austenitic filler | ferrite tends to grain growth, the weld is more ductile |
| Dissimilar joint | a compromise austenitic filler | compatibility of different grades |
Specific grades of electrodes and wire are stated in the welding procedure, not chosen "by the colour of the packaging". Before work, check whether the consumables have documents, and store them separately from consumables for carbon steel: an electrode accidentally taken in the wrong grade becomes noticeable only after the weld starts to corrode.
Shielding gas
Stainless steel does not forgive poor protection of the pool. For TIG, argon is taken; for thick metal, mixtures with helium; for a semi-automatic welder, mixtures based on argon with a small addition of an active gas. Pure carbon dioxide is not suitable for stainless steel: it carburises the weld, and carbon is exactly what one tries to avoid in stainless steel.
The gas flow is set so that the zone around the pool is covered, but the flow does not create turbulence. A draught in the workshop, open gates and a running fan blow the protection away, and then pores and oxides remain in the weld. For pipes and closed cavities, purging is supplied from the reverse side: argon displaces the air, and the weld root stays bright.
How not to lose corrosion resistance
The corrosion resistance of a weld is not an automatic consequence of the grade. Heating changes the structure of the metal, and the air leaves scale and temper colours on the surface, under which chromium is bound and does not work. That is why after welding the weld and the heat-affected zone are cleaned, and then pickled or passivated, to bring back the oxide film.
What helps preserve resistance:
- do not overheat: short welds, pauses for cooling, cooling where possible;
- use low-carbon or stabilised grades for critical assemblies;
- do not weld stainless steel with a filler and tools from carbon steel;
- clean off scale, spatter and temper colours, not just the visible bead;
- do not leave marks on the surface from a grinding disc that has been used on carbon steel;
- check penetration and the absence of oxidation in the weld root on pipes and vessels.
A separate mistake is welding wet or contaminated metal. Moisture, oil and dirt give pores, and pores retain moisture and start corrosion from inside the weld. The edge is cleaned and degreased before welding, not "warmed up with the dirt on it".
The overall picture of the operations adjacent to welding is in the article processing of stainless steel.
Frequently asked questions
What is stainless steel welded with
By TIG, by a semi-automatic welder in shielding gas or with electrodes. The choice depends on the thickness, the position of the weld and the appearance requirements. For thin metal and pipes, TIG is more often taken.
Can stainless steel be welded with a semi-automatic welder
It can, if the wire is made of stainless steel and the gas is selected for the task. Ordinary wire and pure carbon dioxide are not suitable: the weld will get a different composition and lose resistance.
Which electrodes for welding 12Х18Н10Т
The filler is selected for the grade of the base metal — for 12Х18Н10Т an austenitic class stabilised with titanium or niobium is needed. The specific electrode grade is stated in the welding procedure. Electrodes for carbon steel do not work here.
What gas is needed for welding stainless steel
For TIG — argon, for large thicknesses — mixtures with helium. For a semi-automatic welder, mixtures based on argon are taken. Pure carbon dioxide carburises the weld and is not used for stainless steel.
Why a weld rusts
Because of scale and a chromium-depleted layer that remain after heating, or because of an unsuitable filler. The weld is cleaned and pickled, and critical assemblies are welded with low-carbon grades.
What is purging and when is it needed
It is the supply of shielding gas from the reverse side of the weld. It is needed for pipes and vessels where it matters that the weld root does not oxidise. Without purging, resistance is lost from the inside, where it is impossible to clean the surface.
Is it necessary to pickle the weld after welding
For food, damp and aggressive media — yes. Pickling and passivation bring back the oxide film and remove traces of heating. For dry non-critical structures, mechanical cleaning is enough.
What to do next
For a welded assembly, take rolled steel with a clear grade and an even edge: then the weld will be predictable. For a welded assembly you need rolled steel with a predictable composition: the GCX exchange has four common grades — AISI 201, 304, 316L and 430 — in the form of pipe, sheet, round bar and angle. An order fixes the grade, size, surface and shipment week.