Tinder layer

Tinder refers to a highly flammable material that catches sparks and thus ignites a fire. In metalworking, tinder refers to the corrosion that occurs on a metal surface due to exposure to oxygen.

In metallurgy, "burn-off" refers to the loss of metal due to the action of oxygen. This can occur through burning, gasification, slagging, or spattering.

In metalworking, material loss occurs on metal surfaces when metal is exposed to oxygen and other oxidizing gases at high temperatures, such as during gas welding. This results, for example, in the formation of small iron oxide flakes that break off the workpiece during iron forging.

In metallurgy, the terms “scale” and “scaling” refer to the similarity of the combustion process that occurs when tinder is smoldered to start a fire. The melting of metal alloys also takes place at high temperatures. This results in the loss of individual elements through combustion. Chemical reactions in this process form slag in the molten metal bath. During the tempering of forged parts in the die-forging process, a scale layer forms. This is the oxide layer on iron or iron alloys that forms during rolling, forging, and hardening processes at higher temperatures through reaction with oxygen.

The amount of scale formed during oxidation must be taken into account when adding alloying elements. This is because, depending on the amount of raw material and the type of alloy being formed, hot forging processes can produce up to three percent by weight of scale relative to the initial weight. However, the correct amount of raw material is required to achieve the desired chemical composition of the molten metal. This composition determines the properties of the product (such as a cast billet or a casting).

The structure of the scale is always similar. Scale is characterized by three layered phases:

1. The outermost layer, hematite, represents the highest-valent iron oxide (oxide layer) in the scale.
2. The middle phase consists of magnetite and wüstite.
3. This innermost phase is also the most unstable and has the lowest iron valence. It is mechanically removed from the forged piece during descaling.