Good Welding Practice - Removing Heat Tint
During welding, it is important that high quality welding wire is used and that the user has sufficient knowledge of the process to achieve a good result. Once the actual welding is completed, it is important to check for heat tint.
Heat tint occurs when the natural oxide layer on the surface of stainless steel thickens due to heat during welding. Chromium is drawn to the surface of the metal making it richer while the layers underneath become less so. The colours formed are similar to temper colours and are created by a light-interference effect. Optical path-length differences are the cause of this effect as light is reflected between the oxide/metal interface and the surface of the oxide film, resulting in the characteristic array of heat tint colours. These colours vary in shade dependent on the thickness of the oxide layer.
If stainless steel has been chosen for its resistance to oxidation and/or heat, removal is not such an important process as the heat tinting will simply blend with localised weld tinting. If it is to be used in a corrosive or low-temperature environment, stainless steel should have any heat tint removed as a reduction of the chromium beneath the surface of the metal will negatively effect its corrosion-resistant property. Where stainless steel comes into contact with drinking water, the guidelines are clear. The Drinking Water Inspectorate Code of Practice (Operational Guidelines and Code of Practice for Stainless Steel Products in Drinking Water Supply) states that, 'In order to achieve the optimum corrosion performance of stainless steel, welded joints, crevice features, contamination and at least all weld heat tints deeper in colour than a pale yellow shall be removed by mechanical dressing followed by acid pickling of the joint.'
It is recommended that, to ensure full corrosion resistance, weld heat tint is always removed. This helps to avoid unnecessary future problems, particularly in fabrication weld areas. It is good practice to treat all stainless steel applications as if they were to be used for drinking water, removing heat tint and allowing the metal to perform to its optimum level.
Our next article will look at the finishing and cleaning techniques used in removing heat tint from stainless steel. Please check back soon or, to be kept updated with all the latest news and information from BS Stainless, subscribe to our regular newsletter.
You can contact the BS Stainless team directly for information on our range of stainless steel strip coil, products, stainless steel sheet, duplexes, welding wire and more by calling 01772 337555 or by email at email@example.com.