Piet van der Horst

Piet van der Horst

In 1970 Piet made welding his trade en since then he never stopped learning about that trade. By now he is well past his retirement age, but not welding is still not an option for him. It is not just work, it is a passion.

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Gases for welding processes

Inert and active gases

Classification according to the standard In NEN-EN-ISO 14175 standard, gases are divided into main and subgroups. In this article we discuss four main groups of

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The mill-scale on steel and welding

Mill-scale; how is it formed, what are its characteristics and more importantly, how can you remove it? Piet van der Horst explaines it all in this article.

During the warm, or rather hot rolling of steel, oxides are formed on the surface of the material for a variety of reasons. As the process progresses, more and more of these oxides will form. These oxides are very hard and brittle and are pressed into the material during the rolling process. At the end of the rolling process, a blue-black hard brittle layer, the mill-scale, forms on the material.

Characteristics of mill-scale

There is no metallurgical connection between the mill-scale and the underlying material. This means that the oxides are pressed into the underlying material and are not connected, as in the case of welding, for example. As a result, this brittle mill-scale will break and release from the material when bending it, for instance. In any case, the mill-scale becomes detached from the underlying material over time. This has to be taken into account during the finishing process, such as painting or coating. Also, the mill-scale has very poor electrical and thermal conductivity. 

Depending on the rolling process, a mill-scale can have different thicknesses and different compositions. The most common, with their melting temperature, are:

  • 1377 degrees Celsius – Fe=0
  • 1565 degrees Celsius – Fe203
  • 1597 degrees Celsius – Fe304

The melting temperature of the underlying steel is 1538 degrees Celsius.

Removing a mill-scale

There are several ways to remove a mill-scale: Firstly, iIt can be done by grinding, shot blasting or chemical removal. Material with a mill-scale is also deliberately allowed to rust (oxidise). By oxidizing, the mill-scale dissolves more or less and can easily be removed by sanding. Another way of removing mill-scale is “drumming”. This is mainly used for small sheet material. Because the sheets, in the rotating drum, always fall on top of each other, the mill-scale breaks. Breaking the mill-scale is actually a side effect of drumming. The original intention is to remove the rough edges that were created during the cutting process.

The mill-scale is not always removed. Sometimes sheet metal with a mill scale is used in design projects because of its beautiful blue-black colour.

Like water and fire

Mill-scale on steel and welding are like water and fire. This is not the best combination. There are two main reasons for this:

  1. The higher melting temperature of the mill-scale
  2. The poor heat and current conductivity

In the most common mill-scale, the melting temperature is slightly higher than that of the steel and additional arc energy is required to melt the mill-scale. However, this is not the biggest problem. The very poor heat and current conductivity ensures that the arc spot remains too small and that the weld cannot flow nicely. This results in  a convex weld. The weld also contains a lot of silicates, because the mill-scale contains a lot of impurities. 

The above is especially the case when welding MAG with solid wires. When welding with, for example, rutile or metal powder cored wires, the problem is less likely to occur, because the arc spread for this type of wire is bigger. However, these wires do not completely solve the problem. The same applies to pulse MIG welding with solid wires. Also with this process you have more arc spread (read: a higher arc tension) which makes the weld flow slightly better. Again, the weld remains too convex and there are many silicates on the weld. In all cases it remains necessary to remove the mill scale before welding. 

Does removing the mill scale have only advantages? No, there is also a disadvantage: the welding spatters stick to the cleaned material and are therefore more difficult to remove. In order to prevent the adhesion of spatters, water-based anti-spatter sprays are available on the market. CAUTION: never use oil-based anti-spatter sprays for this purpose!

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