Today, several hundred different steel grades exist, which differ according to the many different compositions of their alloy elements. Depending on the planned place of use, steel grades with different properties must be chosen. Apart from steels that are not susceptible to rust, among other things, steels that are not susceptible to acid or are salt water resistant are also required. In places with extreme heat, steel grades have been developed that do not lose their strength even when heated to high temperatures.
Unalloyed steels is the name given to all iron-carbon alloys, which have a carbon content of 0.05 up to maximum 2 % and apart from iron only contain the natural iron accompanying elements such as sulphur, manganese, phosphorous. However, during melting the proportion of the individual elements can be adjusted, for example, by adding lime to the smelt.
But at certain temperatures, unwanted elements evaporate and form a slag with line, which can then be removed. Depending on the composition and concentration of the accompanying auxiliary elements in steel, the properties such as hardness and tensile strength can be increased or reduced.
Unalloyed steels are frequently used to make tools and are also used in mechanical engineering. However, at higher temperatures, unalloyed steels lose their hardness. For this reason, alloyed steel grades must be used for this area of use.
Alloyed steel grades
Alloyed steel grades are differentiated into low and high-alloyed steels. High-alloyed steels are those grades that contain more than 5% of a specific alloying element.
Low-alloyed steel grades are identified by the analysis of their contents. The elements that they contain and their concentrations are given. However, the concentration must be given a whole numbers. This means that elements with a concentration less than one percent have to be multiplied by a certain factor.
The factor for carbon is 100, for chromium and nickel it is 4 and rarer elements such as molybdenum and vanadium are multiplied by 10.
We have a steel with 0.15% carbon, 1.0 % chromium and 0.4 % molybdenum
Theoretically, the steel grade should be 0.15 CrMo 1-0.4 – but this is not allowed.
Multiplied by the factors the correct designation is 15 CrMo 4-4
In principle, the steel grades for high-alloyed steels have the same structure. As the concentration of the individual elements is over 5% there is no need to multiply them with individual factors. The only exception is carbon, which continues to be multiplied by the factor 100. The prefix X is used in order to distinguish high-alloyed steels immediately from other steel grades.
The correct steel grade is X10 CrNi 18-8 for a steel
with 0.10 % carbon, 18% chromium and 8% nickel.