The carbon content in steel is the main "key" in hardening steel by heating and rapid cooling - hardening. It is the carbon content that determines the maximum level of hardness, which can be achieved in steel. The influence of the carbon content on the attainable hardness is shown in the figure. 1.
Maximum hardness in steel
Maximum hardness only requires 0,60 %% carbon, which may seem strange. From the state diagram of iron-cementite, it would be logical to assume, that hardness should increase, until the carbon content reaches 0,77 %.but, as we see in the picture 1, significant increase in hardness after carbon content 0,60 % not visible. Despite this oddity, these data are perfectly reliable and easily reproduced on very thin carbon steel specimens.
This data in the figure 1 so accurate, which is often used for the inverse problem, namely, the rapid determination of the carbon content in an unknown steel. To do this, cut off thin samples of this steel, heated to a temperature above the transformation temperature in steel, they are quenched in water and the carbon content in steel is estimated by the level of hardness.
If the hardness level in the sample is about 50 HRC, then the carbon content in steel is about 0,20 %, with a hardness of about 60 HRC – 0,40 %. Obviously, that this method does not work with hardness higher than 60 HRC, since the hardness there is already almost independent of the carbon content.
For normal practice of heat treatment of steels, it must be remembered, that the data in the figure 1 based on heat treatment of very thin specimens, which cooled from their austenitizing temperature to room temperature within a few seconds to form martensite throughout their thickness. Therefore ideal conditions, shown in the figure 1, rarely seen in practice.
Effect of product size on steel hardness
Under the conditions of industrial practice, the hardness values of steel, indicated in the figure 1 rarely achieved. The most important factor, which affects the maximum hardness, achieved by thermal hardening of steel, is the mass of the metal, which is hardened. From small samples, heat is "extracted" very quickly and the critical cooling rate for 100 %-formation of martensite is exceeded significantly. The critical cooling rate is the rate of cooling, which prevents the formation of non-martensitic austenite decomposition products in the tsel. With an increase in the size of the section of the product, it becomes difficult to “extract” heat from it quickly enough, to avoid the formation of non-martensitic structures in the steel.