Welding Stainless Steel: Heat Tint, Distortion and Clean Processing
Knowledge · Welding in contract manufacturing
Why is stainless steel corrosion-resistant, and what does welding do to that?
Austenitic grades such as 1.4301 (304) or 1.4404/1.4571 (316 types) contain at least 10.5 % chromium, which forms an invisible, self-healing chromium-oxide passive layer in air. Welding heat destroys this layer locally and produces heat tint: oxide colours from straw yellow to blue beside the seam. Beneath those oxides the layer is chromium-depleted, and that is exactly where corrosion starts later if the tint is not removed.
How is heat tint removed correctly?
Three proven routes. Pickling (paste or bath) dissolves the oxides chemically and is the most thorough method; after rinsing, the passive layer rebuilds. Electrochemical cleaning works precisely along the seam without large-scale acid handling. Grinding and brushing with tools reserved exclusively for stainless works mechanically but demands care. In TIG work, good purging (shielding gas on the seam back side) limits heat tint from the start, especially on pipes and vessels.
What is cross-contamination and how do you prevent it?
Rust film appears when carbon-steel particles land on stainless surfaces via grinding discs, brushes, workbenches or flying sparks from the neighbouring bay. The embedded particles rust and look like corrosion of the stainless itself. The remedy is organisational: separate tools and separate work zones, ideally a dedicated stainless area. For buyers, asking about black-white separation is a simple and telling supplier check.
Why does stainless distort more than structural steel?
Two physical reasons: austenitic stainless expands about a third more (roughly 16 µm/(m·K)) and conducts heat only about a third as well. Heat stays in the seam zone and shrinkage forces grow. In practice, low-energy processes help (TIG with adapted sequence, hand-held laser on thin sheet), smaller throat thicknesses and a planned sequence; fundamentals in the article on welding distortion.
Materials and processes at a glance
| Material | Typical use | Preferred processes |
|---|---|---|
| 1.4301 (304) | frames, panelling, general plant building | TIG, MAG with low-CO₂ gas, laser |
| 1.4404 / 1.4571 (316 types) | chemically loaded environments, food industry | TIG, laser, MAG for thick sections |
| Duplex grades | highly loaded corrosive applications | TIG/MAG with controlled heat input |
A practical tip: specify the seam finish in the order (pickled, electrochemically cleaned, ground, passivated on request). “Clean seam” is not a verifiable requirement; “pickled and passivated” is.
Frequently asked questions
Must heat tint always be removed?
In corrosive environments yes, because the passive layer beneath the oxides is chromium-depleted and the seam corrodes there first. Only for dry indoor applications can it be waived by agreement.
What is the difference between pickling and passivating?
Pickling chemically removes oxides and chromium-depleted zones, creating the basis for an intact surface. Passivating then accelerates the rebuild of the protective chromium-oxide layer; in practice it often follows pickling directly.
Can stainless be welded to structural steel?
Yes, such black-white joints are everyday work with suitable filler (309 types). They require experience in filler selection and heat control.
Why is stainless processed separately from carbon steel?
Because carbon-steel particles cause rust film on stainless. Separate tools, brushes and work zones are the only reliable protection and a good sign of a careful workshop.