Chemical elements used in steel and what they affect?
Steel as a material is widely used in every area of life, from construction, through the construction of machines, medicine, tools, and the kitchen itself. It is not difficult to notice that the steels of which the knives are made diametrically differ from those of, for example, forks. Where does this difference come from?
Steel consists mainly of iron, which is a kind of "base", to which other elements are added in the production process to improve its properties, for example increasing hardness or corrosion resistance. It is the chemical composition (type and content of elements) that determines the properties of the material - its intended use.
One of the most important additives is Coal. Its content in tool steels varies from 0.3% to even 3% and it is mainly responsible for the microstructure, and therefore the hardness of the blade after hardening. This hardness increases with increasing carbon content to about 0.8%. Above this, undissolved carbides remain in the microstructure after quenching, which additionally increases the wear resistance. This is why high-carbon steels keep sharpness so well, even though their hardness often does not differ from less alloyed grades. Unfortunately, the high carbon content also brings disadvantages such as low impact strength and more difficult sharpening, which means that such grades have specific applications and are selected by experienced users.
The second, very important element is Chromium, which is added for two purposes. First, it significantly improves hardenability. It means that during hardening the steel can be cooled more slowly and the obtained blade hardness will be comparable. This reduces the chances of bending and breaking the blade throughout the process. The second goal is to improve corrosion resistance. Chromium creates a tight oxide layer on the surface of the blade, which slows down, and from above 12%, it almost stops corrosion in a humid environment. Chromium also dissolves in carbides and hardens them, further increasing wear resistance. Unfortunately, in this case, it no longer forms a protective layer on the surface and rust may appear on some high-carbon steels. In order to prevent this from happening, the chromium content is increased to 16-17% as, for example, in R-2 steel. It is a myth that nickel is added to increase corrosion resistance. In tool steels, it is sometimes added to slightly improve hardenability and toughness, however, it is disproportionately expensive to the obtained properties. Most often it gets into steel during production, from scrap metal as an unwanted contamination. An additional contraindication to the use of nickel is the increase in the phenomenon of allergy to this element in an increasing part of the population.
Other elements frequently appearing in chemical composition are Tungsten and Molybdenum. As they have a high affinity for carbon, they form very hard carbides of the M7C3 and M6C type, significantly increasing the abrasion resistance, and it is for this purpose that they are found in steel. Sometimes molybdenum is added in amounts up to 0.5% in less alloyed steels to increase the hardenability and protect against brittleness of the second type of tempering, i.e. drop in toughness during tempering.
Vanadium added in amounts up to 0.3% prevents overheating of the steel (excessive growth of austenite grains) during forging and heating for hardening. In larger amounts, it is added for the same purpose as tungsten and molybdenum - it forms hard MC type carbides that increase wear resistance.
In high-speed steels with a very rich chemical composition, such as ZDP-189 steel - tungsten, vanadium and molybdenum have an additional function. Due to a different heat treatment, some of them dissolve to separate out in the form of small, evenly spaced carbides, strengthening the steel. This effect is called secondary hardness and it is helped by cobalt added in amounts up to 15%, increasing the dispersion of carbides and their nucleation rate. This effect, it is possible to obtain a hardness of 66-68 HRC and excellent abrasion resistance.
Elements such as Phosphorus, Hydrogen, Oxygen, Nitrogen and Sulfur remain in the steel after the manufacturing process and are impurities that deteriorate the properties of the steel, therefore it is important that there is as little of them as possible. The next elements that I will discuss are mainly added to prevent the negative effects of these pollutants. Manganese is sometimes added to increase hardenability, while its main function is to bind sulfur into manganese sulphides (MnS). Compounds of this type are plastic and during forging will not cause cracking of the steel (the effect of hot brittleness), unlike iron sulphides (FeS), which can be formed in steel without the addition of manganese. Aluminum (up to 0.06%) and silicon (up to 0.3%) are added to the steel to deoxidize it, while silicon in the content of 1-2% delays the processes taking place during tempering and thus indirectly prevents brittleness of the first type of tempering.
In summary, the chemical composition of the steel selected by engineers significantly affects the final properties of the steel, so it is important to choose the right knife grade. The most important from the user's point of view is the content of carbon and chromium, because it is mainly them that determines how the tool will perform in the kitchen. Elements such as Tungsten, Vanadium, Molybdenum and Cobalt are added to extend the "life" of the cutting edge for the most demanding customers.