How much heat tint is acceptable on welded stainless parts, and when must it be removed?

What does heat tint look like, and how do you tell it from rust or dirt?

Heat tint is an even band of interference colors that follows the weld or the hottest zone, shading from pale straw at the outer edge through gold and brown to blue nearest the heat, and it does not wipe off with solvent. Rust is orange-brown and blotchy, sits in spots or streaks that ignore the weld line, and often lifts off with a mild cleaner.

The Euro Inox handbook on pickling and passivating stainless steel describes heat tint as the thickening of the naturally occurring transparent oxide layer, with colors similar to the temper colors seen on other steels after heat treatment, ranging from pale straw hues to dark blue (Euro Inox, Pickling and Passivating Stainless Steel, 2007). It notes that tint is often seen in the heat-affected areas of welded fabrications even where good gas shielding practice was used, because other parameters such as welding speed also affect how much color forms. With continued heating the same film becomes a dull gray oxide scale.

Where the color sits tells you a lot. A narrow band on the torch side of a fillet weld is the most common pattern. Color on the inside of a welded tube or on the back face of a joint means the root side was not protected by a backing gas. An even straw or bronze color over a whole part points to heat treatment, stress relief or brazing in air rather than welding. A tinted streak on a ground or polished face, away from any weld, usually means the grinding or polishing pressure overheated the surface. Record which of these patterns you see before deciding anything else.

How much heat tint is acceptable, and who sets the limit?

The limit comes from the code or the buyer’s specification for that service, not from the welder: hygienic food and dairy tube welds are commonly judged against the numbered AWS D18.2 color chart, bioprocessing tube against the ASME BPE color charts, and most other wetted or outdoor surfaces against a simple rule that all visible tint is removed. A dry, indoor, hidden face is the main place where some tint can be accepted, and only when the drawing says so.

The American Welding Society describes AWS D18.2 as a laminated sheet with a color photograph showing degrees of coloration on the inside of an austenitic stainless steel tube with increasing oxygen in the backing gas, suitable as a specifying tool and visual examination guide (AWS D18.2:2020). A 2024 water-utility guidance note summarizes the chart as running from level 1, at about 10 ppm oxygen in the backing gas, to level 10, at about 25,000 ppm, with level 2 near 25 ppm and level 3 near 50 ppm (Watercare, 2024). Higher numbers mean darker color and more oxygen, so a specification states the highest level it accepts.

Pharmaceutical work has moved to its own charts. According to the ISPE article, the ASME BPE standard referenced the AWS D18.2 chart in its 2002 to 2009 editions and, from the 2012 edition, published its own color charts for mechanically polished and electropolished 316L tube, focused on lower oxygen levels; the acceptance wording for the weld bead and the heat-affected zone has also changed between editions (ISPE, 2024). Always name the edition. Outside these hygienic codes, Euro Inox calls removal of all visible heat tint good practice, including on hidden faces of welds that will be exposed to the service environment.

How is heat tint removed, and which method fits the part?

Pickling with an acid paste, gel, spray or immersion bath is the standard fix because it dissolves both the oxide and the chromium-depleted layer beneath it. Electropolishing does the same electrochemically, and mechanical removal works only if it goes deep enough and is followed by an acid decontamination.

Euro Inox lists acidic brush-on pastes or gels, spray pickling, immersion tank pickling and electrochemical cleaning as the removal routes, each after careful degreasing, and warns that nitric acid treatments alone cannot be relied on to remove enough metal, so a combination such as grinding or abrading followed by nitric acid decontamination may be needed (Euro Inox, 2007). It also warns that excessive contact time with pickling products can pit the surface. ASTM A380 is the practice that covers descaling and pickling of stainless parts and fabrications, and ASTM B912 states that electropolishing removes heat tint and oxide scale.

Mechanical removal deserves a caution. A 2024 water-utility guidance note points out that a nonwoven abrasive pad or stainless wire brush removes the superficial discoloration but that chemical pickling is still needed to fully restore corrosion resistance (Watercare, 2024). A carbon steel brush or a grinding disc used on carbon steel swaps one problem for another by embedding free iron; the passivation guide covers how that contamination is removed and tested.

Prevention reduces how much removal is needed. On the root side, an inert backing gas with the oxygen level measured before the weld starts is the main control. Watercare’s pipe-welding note, for example, requires argon or nitrogen backing gas to bring oxygen in the bore within a specified 25 to 50 ppm and does not accept flux-backed (TGX) filler wire as a substitute for purging (Watercare, 2024). If a lot keeps arriving with dark root tint, ask the supplier for the purge method and the oxygen reading taken before welding. On the torch side, adequate shielding, a trailing shield on long seams, and modest heat input narrow the tinted band. For parts that are heat-treated after welding or machining, specify a vacuum or protective atmosphere, or plan a descaling step after heat treatment. None of these removes the need for a clear acceptance criterion, but they reduce how much rework the criterion triggers.

Who does what on a Saola order for welded stainless parts?

Machining, and welding where an assembly calls for it, are done at our lead CNC plant, T.K Viet Nam Precision Company Limited. Pickling, passivation and electropolishing are not on our published finishing menu, so name them on the RFQ and the quote will state whether a partner plant we manage can do them, where it is, and the resulting country of origin.

The lead plant has run since 2017 and carries machining centers and CNC lathes, with EDM, welding and grinding and polishing alongside, so parts are cut on the CNC milling and CNC turning lines and welded there when the drawing requires it. Mechanical blending of a weld can be quoted there when your drawing permits mechanical removal on that surface. Chemical treatments are a different matter. The surface finishing page lists the finishes partner plants apply today, and acid pickling, passivation and electropolishing are not among them. Partner plants are not named, they are not necessarily in Vietnam, and the production site and country of origin are stated on each quote.

What helps most at the RFQ stage is the information in the previous section: which surfaces may carry tint, the chart and edition if one applies, the removal method you accept and the features to protect. The parts are inspected against that written criterion before they ship, so a criterion that exists only in someone’s head cannot be met reliably. ISO 9001 certification is in progress, not held. Send the drawing and the heat tint note through the quote form.