Remelting

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Making good even better

Maybe not everything was better in the past, but a lot was easier. In the early years of steel production, for example, a lot of fresh air was simply blown into the liquid pig iron, the so-called refining, in order to reduce the high carbon content of the pig iron through the resulting oxidation and to remove some of the impurities from the metal. Fast, simple and already quite effective.

With steels of this quality, however, mankind would not have been able to convert the force of the tides into electrical energy, carry out medical operations in the micrometer range, or place a robot on Mars. They weren't pure enough for that. But luckily it can be done even better:

Pig iron is still refined today - technical oxygen is now used for this; however, so-called remelting processes have also been used since the 1970s. With their help, the material properties of the steel can be improved many times over compared to the quality achieved by the basic manufacturing process.

We would like to explain the two most important ones to you in this article.

Steel for the highest standards

When we talk about remelting, then we specifically mean the electro-slag remelting process (short: ESR) and the vacuum arc remelting (short: VAR).
Both have in common that a melt that has already been cast - i.e. steel that has already been liquefied and treated and is therefore ready for use for many purposes - is further refined. The aim of remelting is, both in the ESR and in the VAR, to further increase the macroscopic and microscopic degree of purity of the material and to further improve its material properties. The procedure in both procedures, however, is very different.

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A bit like making coffee: electro-slag remelting

Roughly speaking, the ESR works like your coffee machine at home. The only difference is that in metallurgy the coffee machine is a water-cooled mold (i.e. a reusable mold for pouring metals), the coffee powder is a cylindrical block made of raw steel and the filter is a slag bath made of lime and clay. By the way, it gets much hotter too.

In the mold, the lower end of the crude steel dips into the slag and at the same time serves as an electrode for an electrical circuit. The mold itself forms the counter-electrode, the slag acts as an electrical resistor.

Now an arc is struck. Due to its high resistance, the slag heats up strongly and the tip of the steel cylinder dipping into it begins to melt. That is why we also speak of a self-consuming electrode.

The steel droplets wander through the slag filter and are freed from impurities such as slag inclusions, refractory particles, casting powder, steel pests and undesirable accompanying elements - entirely according to the principle of filtration. The slag continues to rise slowly until the entire crude steel electrode has been melted.

The filtered steel collects at the bottom of the mold and can solidify there again. This re-solidification process happens very evenly, so that the result is not only a particularly pure material, but also a material that has an extremely homogeneous structure.

The power of the empty space: vacuum arc remelting

At first glance, the VAR is very similar to the ESR: Here, too, a pig iron electrode is inserted into a mold and melted off by means of an electric arc. The steel gathers again on the cooled bottom of the mold to form a remelting block in a uniform solidification. However, there is one crucial difference to the ESR.

Because the name already suggests: This time the remelting takes place in an approximate (pressure of 0.001 mbar) vacuum. Why? Because the proportion of unwanted gases in crude steel can be reduced so significantly.

According to the laws of physics, the solubility of gases depends on the ambient pressure. The lower the pressure, the less gas a liquid can absorb. The molten steel releases a large part of the bound gases; these are then derived from the mold. One therefore speaks of degassing. Since oxygen is one of the pearlescent substances, deoxidation takes place at the same time, which further increases the quality of the steel.

The resulting remelting block is therefore almost free of undesirable impurities and oxidic inclusions.

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The advantages of remelting

What does it mean for the end product if the content of non-metallic inclusions in the material is reduced? The benefits are as follows:

  • Greater homogeneity, i.e. chemical and mechanical uniformity across the cross-section
  • Improved toughness
  • Greater corrosion resistance
  • Greater resistance to cracks

Overall, the steel becomes significantly more resilient and less sensitive. This makes it suitable for numerous areas of application, ranging from energy technology to aerospace and medical technology. Wherever steel of particular purity is required, there is no avoiding remelting.

Due to the high technical effort and the associated costs, however, only small batches of the ultra-pure material can be produced. You will therefore not find a mundane kitchen knife made of remelted steel. A patient's new hip joint and similar implants, on the other hand, are almost certainly ES- or VA- remelted.

Special steel for extreme purposes

If you have any questions about our manufacturing or finishing processes or would like to know more about the special steels we produce, please contact us directly. Our chat function is available to you during our business hours for any matter.

We are also happy to take your order for your needs - whether your material has been remelted or not. In any case, we look forward to hearing from you.