As per SABP-A-022(Source :- SABP-A-022)
When stainless steel piping or components are manufactured they are normally subjected to a final high temperature solution heat treatment (typically at 1050ºC) followed by water quenching. This leaves the material in the solution annealed condition which is the optimum for corrosion resistance. This heat treatment gives a poor quality high temperature oxide film on the surface which is quite thick, porous and cracked. During manufacturing, this oxide film is normally removed by pickling and passivating the surface in oxidizing acids. The oxide film which is formed at low temperature, either in the passivating solution or just by exposure to air, is thin and dense, giving good protection to the stainless steel. The as-delivered piping is therefore in an optimum condition, with a solution heat treated microstructure and a well-passivated surface oxide finish.When welding is carried out, the metal close to the weld is reheated to high temperatures, from the melting point at the fusion line to lower temperatures through the HAZ. An unprotected HAZ which is exposed to air during welding will therefore show a high level of oxidation. Characteristically, it would be black in color and in the worst condition may be visibly thick and porous. This oxide is far more easily broken down than the original passivated surface and there is a zone of metal under this oxide which is lower in chromium content relative to the bulk metal.Therefore, the HAZ has to be protected from re-oxidising at high temperature during the welding process. This is done by shielding around the weld using an inert gas to exclude air, thus preventing oxidation whilst the weld metal and HAZ is hot. Once the weld is cooler, the HAZ will re-oxidise slightly at lower temperatures but this oxide film is a more protective film than the high temperature oxide film. This 'heat tint' oxide formed at the HAZ at lower temperatures is so thin that it forms interference colors, forming characteristic blue or rainbow colors. AWS D18.2:1999 addresses factors affecting weld discoloration inside a 316L austenitic stainless steel tube. It presents a color guide (Figure 2) relating degree of discoloration to oxygen content in the backing shielding gas. The heat tint oxide can be identified by a number corresponding to the oxygen level in the shielding gas. A straw-to-yellow heat tint (chart no. 3) is considered desirable but a blue tint up to chart no.6 is acceptable for standard applications.
Best regards,
Nambirajan
On Thu, 13 Nov, 2025, 12:52 pm Mahmood Alhinai, <mahmood.m.alhinai@gmail.com> wrote:
Dear Experts,--Kindly, refer attached photos and clarify what could be the reason of block spots appeared throughout DSS welding surface.the welding was carried out as per the approved WPS and approved welder, the DSS filler wire was approved (Bohler / ER2594 / ER2209). Pipe material: A790 UNS 31803Welding process: GTAWThanks & Regards,Mahmood
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