Hardness carbon steel depends on many different factors, including carbon content, number and type of other elements in the alloy, as well as features of the technology for creating steel. Low carbon steels, which contain from 0,05 to 0,25 % carbon, they are usually the softest. They can be hardened using the process, which are called cementation or carburizing.
High carbon steels, which contain carbon up to 2 %, are usually the hardest, their final hardness depends on the mode of their heat treatment. Quenching can quadruple the hardness of carbon steel, and subsequent tempering of steel reduces its hardness to a given level.
Carbon steels are an alloy, which is mainly composed of iron and carbon. Other elements may also be present, but usually in very small quantities. for instance, elements such as silicon and copper are present in carbon steels in an amount not exceeding 0,6 %. Some carbon steels can contain up to 1,6 % elements such as manganese.
Higher carbon is harder than steel
The main factor, which determines the hardness of carbon steel is the carbon content of the steel. Low carbon steels are usually soft, whereas high carbon steels can be very hard and brittle. The physical properties of carbon steels can be changed by various treatments and, thereby, increase or decrease hardness.
Cementation of low carbon steel
Although mild steel is relatively soft, through process, which are called cementation, it can be made much harder. This heat treatment process literally forces the steel to absorb carbon from the solid, liquid or gaseous medium rich in carbon. Usually carbon is only absorbed by the surface layer of steel. This gives a very hard surface layer to the part., what is useful, eg, for wear resistance. The core of the part remains low-carbon and therefore ductile and tough. This is highly beneficial for reliability and brittle fracture resistance for the part as a whole.
Solid carburizing steel
For solid carburizing, the parts are placed in a box with a carburetor. Carburetor is a carbonizing agent, usually charcoal with various additives. At a temperature 900-950 º oxygen in the air combines with the carbon of charcoal to form carbon monoxide. On contact with iron, carbon monoxide decomposes into carbon dioxide and atomic carbon, which is absorbed by the surface of the part. Solid carburizing is used in small batch and one-off production, eg, in rural smithies.
Gas carburizing steel
In mass production, the main process is gas carburizing.. It is carried out in stationary or continuously operating (methodical) ovens. Cementing gases are carbon monoxide and gaseous hydrocarbons - methane, ethane, propane, butane, and also just natural gas.
Quenching and Tempering High Carbon Steel
High-carbon steels usually have high hardness by themselves.. However, the hardening process can make them much harder., although they become more fragile. Therefore, quenching is almost always combined with tempering.. As a result of tempering, the hardness of the steel decreases, and the plasticity increases.
After annealing, normalizing or tempering carbon steel is composed of ferrite, free and plate, and carbide inclusions (cementite). Ferrite has low strength and high ductility. Cementite has a very high hardness (near 800 NV) and virtually zero ductility. With a small amount of cementite inclusions, plastic deformation develops relatively easily and the hardness of the steel is therefore low.
When steel is hardened, martensite. The high hardness of martensite is explained by the fact, that its elementary crystal cells are distorted. This makes plastic deformation and shear more difficult.. The more carbon in steel, the greater the distortion of the tetragonal lattice of martensite and the higher its hardness. So, martensite in steel, containing 0,1 % carbon, has a hardness of about 30 HRC, a pri 0,7 % carbon - 64 HRC.
Effect of tempering on steel hardness
Typically, the final heat treatment of steel is tempering.. Tempering gives the steel product its final properties. Changes in hardness during tempering occurs due to changes in the structure of the steel structure. When heated above 100 ºС hardness decreases due to coarsening of carbide particles and depletion of carbon in solid alpha solution.
The general trend is, that the hardness decreases with increasing tempering temperature, as well as other strength indicators (tensile strength and yield strength), while the plasticity indices (elongation and contraction) increase.