Most welding problems on stainless steel are decided before the torch is lit. The mill certificate fixes three things a welder cannot change: how much carbon is in solution, whether the grade carries a stabilising addition, and whether the structure is austenitic, ferritic or duplex. Filler, preheat, interpass temperature and post-weld treatment follow from those three facts.

Carbon content decides whether the grade can be welded as delivered

Plain grade 304 stainless steel carries carbon up to 0.07 to 0.08 percent; the L version carries 0.03 maximum. The 316 and 316L page states the difference in one line: 316L has a 0.03 maximum carbon and is good for welding, whereas 316 sits at a mid-range carbon level.

On the same page, 304 is listed at 600 N/mm² tensile and 210 N/mm² yield, 304L at 530 and 200, with elongation of 60 percent against 50. For a welded part that will not be stress relieved, the lower carbon and retained ductility usually outweigh the lost strength, as the 304 and 304L seamless pipe specification shows in imperial units. Carbon also explains why Alloy 310S exists: it is 310 with lower carbon, specifically to minimise carbide precipitation during welding.

Stabilised grades: 321 and 347 bind the carbon before it can migrate

Where service temperature will drive carbon to the grain boundaries, the answer is a stronger carbide former. Titanium does that job in 321 and 321H sheet and plate, at five times the carbon plus nitrogen and capped at 0.75 percent. Niobium does it in 347 and 347H seamless pipe, at 10 times carbon for 347 and 8 times carbon for 347H, whose carbon range runs 0.04 to 0.10 percent. The niobium addition suppresses grain boundary chromium carbide precipitation.

Both grades weld by the common fusion and resistance processes, but 321 work hardens and may need annealing after severe forming, so tight bends should be assessed before welding. The 347H specification names flattening and flaring tests among its delivery checks. Those two tests expose a brittle weld or heat-affected zone, so a certificate showing them is worth more to a fabricator than tensile figures alone: 347H is quoted at 515 MPa tensile, 205 MPa yield and 35 percent elongation, 321 at 500 to 700 MPa tensile. Tubular forms appear in the A213 and SA312 347 listing, in 310 and 310S pipe, and across the general stainless steel pipe range.

Distortion is arithmetic, not technique

Austenitic stainless bows when welded, for reasons that sit in the physical properties rather than the welder's hands. Grade 304 has a mean coefficient of thermal expansion of 17.2 by 10 to the minus 6 per kelvin and a thermal conductivity of 16.2 W per metre kelvin, against roughly 11 to 12 for the ferritic 409 grade. The austenitic group expands far more than carbon steel and conducts heat away at about a third of the rate, so heat stays where it was put and the joint pulls as it cools. The response is procedural: low heat input with more passes, balanced sequence around the neutral axis, and backstep on long seams. 310S is the extreme case at 14.4 rising to 17.5 by 649 °C, which is why furnace furniture welded from it is designed with generous allowances.

Precipitation-hardening grades add a step that cannot be skipped. The 17-4PH data sheet is direct: solution anneal at 1020 to 1050 °C with rapid cooling, then age within a very short time after welding, because without post-weld heat treatment the values in the weld seam and heat-affected zone can differ greatly from the parent.

Duplex: heat input has to be in a window, not merely low

Duplex grades behave differently: a two-phase structure has to survive the weld. Alloy 2205 is 22 percent chromium, 3 percent molybdenum, 5 to 6 percent nickel with nitrogen in UNS S31803 or S32205, and its yield strength is about twice that of the austenitic grades, so a thinner wall carries the same load. It suits minus 50 to plus 600 °F; outside that range the alloy needs restrictions, particularly for welded structures. The heat input window is narrow both ways: too much gives sigma phase time to embrittle the joint, too little leaves excess ferrite at the same cost to toughness. Nitrogen in the shielding gas holds the balance at the root, with matching consumables given in the S31803 seamless pipe listing.

Super duplex 2507 pushes the chemistry further, at 25 percent chromium, 4 percent molybdenum and 7 percent nickel with nitrogen between 0.24 and 0.32, limited to below 600 °F or 316 °C because exposure at temperature reduces toughness and corrosion resistance. Its consumables are ER2594 and E2594-16 or E2553T-1, with welded tube and pipe covered by A789 and A790. The most demanding version, super duplex S32760 or F55, carries a pitting resistance equivalent of 40 or above, a proof stress of 550 N/mm² minimum and a tensile of 750 N/mm² minimum, with Charpy impact values of 80 joules minimum at ambient and 45 joules average at minus 46 °C. That is why heat input matters: an overheated joint can pass a visual and a pressure test and still fail on toughness.

Ferritic and martensitic grades: choose the filler deliberately

Grade 409 is stabilised with titanium at six times the carbon up to 0.75 percent, quoted at 450 MPa tensile and 240 MPa yield with 25 percent elongation. Free-machining martensitic grades are a planned compromise. The 416HT sheet specification states poor weldability outright, then gives the procedure for unavoidable cases: 410 low-hydrogen electrodes, preheat to 200 to 300 °C, then annealing or stress relief at 650 to 675 °C. The sulphur that delivers 85 percent of free-cutting carbon steel's machinability is the same element that makes the grade hard to weld. On the ferritic 430 coil and strip range the same logic applies, and an austenitic filler such as 308L or 309L puts ductile weld metal into a joint whose heat-affected zone has little of its own.

Cleanliness and post-weld cleaning

Contamination is the first hazard: the 316 page warns that copper and zinc must not become contaminants at the weld site, because they can create cracking, and carbon steel brushes, grinding discs and slings dragged across a shop floor are the usual carriers. Oxide removal is second. Welding leaves a heat tint that darkens from straw through blue to black as peak temperature and dwell time increase, and that tint is a chromium-depleted layer, not a decorative stain. On components exposed to chlorides or hygienic service it is removed by pickling paste or mechanical means, then passivated back to bright metal. On tube welded from one side the equivalent step is back purging, because an oxidised internal bead cannot be cleaned afterwards.

The short version: read the carbon figure and the stabiliser, match the filler to the structure, control expansion and heat input according to the family, and treat cleaning as part of the joint. Grades discussed here sit in the stainless steel catalogue alongside the 316 and 316L stainless coil range, and where the work is better subcontracted the plate welding service covers fabrication on supplied material.