Heavy Metal, Part 6 - Things To Look For In Welding Material Selection, Mill Scale
Heavy Metal. Things to look for in welding material selection. Part 6
Mill scale, which is essentially a coating that forms on the surface of the metal primarily in the form of an oxide during the manufacturing process as the red hot metal cools. Though there can be other coatings and finishes which will be discussed later. It is a potential contaminant that is usually removed just before welding in most situations. Mill scale does help inhibit rust to an extent, but when welded can leave the weld with porosity and contamination. In fact it must be removed when TIG welding or it will leave a dull gray and usually porous weld. The weld arc will tend to spark and snap while TIG welding if any mill scale is present. In fact that’s an often reported problem when TIG welding that some people mistake for an issue with the welder or torch. Often inexperienced welders will think that they’ve removed the mill scale, and all they’ve done is polished the scale to a bright sheen. Flap discs are often used to help remove mill scale but they tend to polish the mill scale rather than remove it. Hard grinding discs will usually cut right through to the bright, unoxidized steel below with moderate effort. It takes a keen eye to recognize when it is has been completely removed but a grinder will nearly always do the trick. On particularly porous metal surfaces, like those that have been extruded such as angle iron, a bit more work may be necessary before attempting to weld it with a TIG welder, but a hard disk will usually make quick work of stubborn mill scale.
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