Most of the commonly used structural steels contain less than 0.5% carbon, because the carbon content is less than 0.77%, so the amount of carburite in the organization is less than 12%, so the ferrite to remove part of the carburite to form pearlite, there will be excess, so the organization of this steel is ferrite + pearlite. The less the carbon content, the smaller the proportion of pearlite in the steels tissue, the lower the strength of steels, but the better the plasticity, this kind of steel is collectively called sub-eutectoid steels.
The carbon content of tool steel is often more than 0.77%, and the proportion of carburite in the organization of this steel is more than 12%, so in addition to the formation of pearlite with ferrite, there is excess carburite, so the organization of this steel is pearlite + carburite. This kind of steel is collectively called over-eutectoid steel.
Steel is heated to a certain temperature and held for a period of time, and then it is slowly cooled, called annealing. Tempering of steel is a heat treatment method in which the steel is heated to the temperature at which the phase change or partial phase change occurs, and then slowly cooled after holding. The purpose of annealing is to eliminate organizational defects, improve the organization so that the composition of uniformity and refinement of grain, improve the mechanical properties of steel, reduce residual stress; at the same time can reduce hardness, improve plasticity and toughness, improve cutting performance. So annealing is not only to eliminate and improve the organization defects and internal stresses left by the previous process, but also to prepare for the subsequent process, so annealing is a semi-finished product heat treatment, also known as pre-heat treatment.
Normalizing is to heat the steels above the critical temperature, so that all the steel is transformed into uniform austenite, and then the heat treatment method of natural cooling in the air. It can eliminate over-eutectoid steels mesh carburizing body, for sub-eutectoid steel normalizing can refine the lattice, improve the overall mechanical properties, the requirements of the parts are not high with normalizing instead of annealing process is more economical.
Quenching is the steels heated to above the critical temperature, holding for a period of time, and then quickly put into the quenching agent, so that its temperature is suddenly reduced to a rate greater than the critical cooling rate of rapid cooling, and obtain a martensite-based unbalanced organization of the heat treatment method. Quenching can increase the strength and hardness of steel, but to reduce its plasticity. The quenching agents commonly used in quenching are: water, oil, alkaline water and salt solutions.
The steel has been quenched and reheated to a certain temperature, and then cooled by a certain method called tempering. Its purpose is to eliminate the internal stresses generated by quenching, reduce hardness and brittleness, in order to obtain the desired mechanical properties. Tempering is divided into high-temperature tempering, tempering and low-temperature tempering three types. Tempering is mostly used in conjunction with quenching and normalizing.
(1) surface quenching: is the surface of the steels parts by rapid heating to above the critical temperature, but the heat has not yet had time to transfer to the heart before rapid cooling, so that the surface layer is quenched in martensitic organization, while the heart does not undergo phase transformation, which achieves the purpose of surface hardening while the heart remains unchanged. Applicable to medium carbon steel.
(2) Chemical heat treatment: refers to the atoms of chemical elements, with the ability of atomic diffusion at high temperatures, it is infiltrated into the surface layer of the workpiece to change the chemical composition and structure of the surface layer of the workpiece, so as to achieve the surface layer of steel with specific requirements of the organization and performance of a heat treatment process. According to the different types of infiltrated elements, chemical heat treatment can be divided into four kinds of carburizing, nitriding, cyanidation and metalizing method.
