Non-metallic inclusions
As non-metallic inclusions are precipitates referred to in the material resulting from the manufacture and internal defects of a material can be attributed. The term non-metallic inclusion stems from the fact that the precipitates formed are mostly oxides , sulfides or nitrides of metals, but these no longer have any metallic properties (e.g. deformability, magnetizability). Non-metallic inclusions affect the properties of the material, mostly negatively.
origin
In the production of steel, a little of the refractory materials always dissolve due to the high temperatures in the blast furnace or the other components during the production process and is in the molten steel. Since the refractory materials mostly consist of aluminum oxide with admixtures of other oxides (e.g. MgO , CaO , ZrO ), the inclusions formed in the material when the melt cools down are also oxides, so-called exogenous inclusions.
Another source of non-metallic inclusions is so-called deoxidation . In the secondary metallurgy process, deoxidizing agents are added to the steel melt (e.g. scrap aluminum or silicon). By melting the aluminum or silicon , it can react with the dissolved oxygen in the steel melt to form aluminum oxide or silicon oxide and thus remove the oxygen from the melt in order to prevent the formation of z. B. to reduce bubbles or cavities. The oxide formed has a lower density and can float in the steel melt and thus form slag . Nevertheless, parts of it remain in the melt and are cast with it. These are endogenous inclusions that are formed in the material itself.
Another way of creating non-metallic inclusions is the reaction of sulfur with metals or alloy elements in the melt. The sulphides precipitating during cooling are also distributed in the material. In this case, however, it is largely possible to control the number of sulfides by controlling the sulfur content.
Effects of inclusions
Non-metallic inclusions are usually disadvantageous for the material behavior. Depending on the mechanical loading of the component made from a material with inclusions, the service life, especially with dynamic loading, can decrease to a small fraction of the expected service life for a component made from a purer material. Particular attention is therefore paid to the degree of purity. B. in the manufacture of roller bearings , placed with highly loaded shafts or materials for fine and ultra-fine wires. Particularly in the case of safety components, the failure of which can pose a risk to life and limb, there is a particularly high demand for a good degree of purity, i.e. H. a small amount of non-metallic inclusions.
Occasionally, a high proportion of non-metallic inclusions is also beneficial. A very high content of sulfur and thus sulfur-containing inclusions (sulphides) in free- cutting steels means that the chip breaks early during machining and thus a better result and more economical work is achieved.
The proportion of non-metallic inclusions is determined with tests to determine the degree of purity .