Stainless steel welding

304 and 316L need low-heat input and, on thin sheet, a backing gas to protect the root. Heat tint must be removed by pickling or mechanical cleaning before service; brushing with a carbon steel wire brush defeats the purpose. 316L is specified where chlorides are present because 304 will pit.

The commercial consequence is that stainless is priced on its own material line, not blended into a steel rate. Material grade, thickness, tolerance and finish are quoted separately from conversion, so a change of specification is visible in the price.

Ganesh quotes to the drawing first and to ISO 2768 general tolerances where the drawing is silent. For stainless welding, the practical starting points are in the table below. They are starting points, not a guarantee: the part geometry, thickness and fixturing decide what can actually be held, and the quotation states the tolerance offered.

Welding fixtures are designed and built in the Ganesh tool room, which is why assembly geometry is controlled by steel, not by a welder's eye. Stainless must be cut with high-pressure nitrogen to keep the edge oxide-free; that is the difference between a weldable edge and one that has to be pickled. The route is quoted as one sequence so the customer does not have to coordinate a cutter, a former, a welder and a coater.

Welding is the wrong route where the part must remain fully hygienic without post-weld treatment, or where the design should have been a single formed part instead of a fabricated one.

A manufacturability review before quotation is the cheapest place to find this out. Send the drawing, material, finish and annual quantity and the review comes back in writing.

• Hygienic frames
• Railway fittings
• Enclosure bodies
• Architectural assemblies

Weld when the assembly cannot be formed as one part or must be shipped knocked down. Robotic MIG pays back on repeat assemblies where the fixture holds the joint in the same place every cycle; below a few hundred identical assemblies a year, a fixture and manual MIG are usually more economic. Do not weld where the customer needs a flat, unmarked panel or where the assembly must be disassembled for service. For stainless the practical constraint is contamination: carbon steel tooling, brushes and blast media transfer free iron, so stainless work is kept on dedicated tooling and its own handling. Stainless raises the stakes on the decision: tool wear, gas cost and slower forming mean the volume needed to justify a tool is higher than for CRCA, and the wrong early choice is expensive to reverse once a die has been cut.

State the weld symbol or size, whether welds are structural or sealing, the finish requirement on the weld (dressed or as-welded), and the dimensional tolerance on the assembled frame rather than on the individual parts. Say whether the assembly is coated after welding, because that changes the access and the masking plan.

Inspection covers weld size and length, visual defects, distortion against the assembly drawing and, where specified, penetration or leak testing. Fixtures are maintained in the tool room. Weld and dimensional records are kept with the batch, and material is traceable to mill certificates.

Cost splits into fixture amortisation, fit-up, weld time, cleaning and inspection. Fit-up is usually the largest labour element: a part that locates itself in the fixture takes a fraction of the time of one that has to be measured and clamped. Distortion correction, if it is needed, is pure rework cost. On stainless steel, material is quoted as its own line at the prevailing rate for the grade and thickness, so a change of specification is visible rather than hidden in a blended rate.