What Is Mill Scale In Welding?

Mill scale in welding has become an important factor for welders over the many years. It is the layer of rust that forms on the surface of steel when it is exposed to oxygen, and it can significantly affect the welding process.

It is estimated that mill scale can account for as much as 20% of the overall cost related to welding and metal work. Therefore, a thorough understanding of what mill scale is, how to handle it and how to prevent it, is essential to quality welding and metal working. In this article, we will explore what mill scale is and how to best handle it in order to maximize productivity and quality in welding.

What is Mill Scale in Welding?

Mill scale, referred to as ‘blue scale’, is a by-product formed while welding. It is a thick layer that develops on the surface of the metal due to the high temperatures involved in the welding process. Mill scale appears bluish-black in color and has a rough texture.

How Does Mill Scale Form?

Mill scale is formed when the metal is exposed to high heat, i.

e. >700 degree Celsius. During the welding process, the metal is subjected to very high heat that promotes oxidation.

The oxidation results in the formation of a layer of mill scale that can be up to 5 mm thick.

Effect of Mill Scale on Weld Quality

Mill scale can have an adverse effect on the quality of welds.

If the mill scale is not removed before welding, it can lead to the formation of cracks in the weld. The cracks are more likely to form when welding materials of different thicknesses.

Moreover, mill scale also causes distortion in the weld by reducing its ductility. This can also lead to cracking in the weld.

Removing Mill Scale Before Welding

The best option to reduce or eliminate the risk of these problems is to remove the mill scale before welding. This can be done by chipping or grinding the metal surface to get rid of the mill scale. Another way to remove the mill scale is through the process of pickling, where the surface of the metal is exposed to an acid bath.


Mill scale is a bluish-black layer that forms on the surface of metal due to high temperatures during the welding process. This layer can interfere with the quality of welds, leading to cracking and distortion. Removing mill scale before welding is the best way to reduce or eliminate these problems. Citations: https://www. blog. rotometals. com/what-is-mill-scale/ https://www. mfg. com/resources/what-is-mill-scale https://www. mig-welding. co. uk/cracking-welds. html

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