Carbon dissolves in the lattice of iron to form solid solution, carbon dissolves into α – iron in the solid solution called ferrite, dissolved into γ – iron in the solid solution called austenite. Ferrite and austenite both have good plasticity. When the carbon in the iron-carbon alloy can not be dissolved into the ferrite or austenite, the remaining out of the carbon will form a compound with iron – iron carbide (Fe 3C) The crystal organization of this compound is called carburite, which is extremely hard, plasticity is almost zero.
From the iron-carbon equilibrium state diagram, which reflects the relationship between the organization of steels and the carbon content of steels and the temperature of steels, it can be seen that when the carbon content is exactly equal to 0.77%, which corresponds to an alloy in which carburite (iron carbide) accounts for about 12% and ferrite for about 88%, the phase transformation of the alloy is achieved at constant temperature. That is, in this specific proportion of carburite and ferrite, when the phase change occurs, if it disappears both disappear at the same time (when heated), if it appears then both appear at the same time, in this point this organization is similar to the phase change of pure metals. For this reason, people treat this two-phase organization composed of a specific ratio as a kind of organization, and name it pearlite, and this kind of steel is called eutectic steel. That is, the steel with exactly 0.77% carbon content is called eutectic steels, and its organization is pearlite
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 or specimen in tension, when the stress exceeds the elastic limit, even if the stress no longer increases, and the steel or specimen still continues to undergo significant plastic deformation, called the phenomenon of yield, and the yield phenomenon when the minimum stress value is the yield point. Set Ps for the yield point s at the external force, Fo for the sample area, the yield point σ s = Ps/Fo (MPa), MPa is called MPa equal to N (Newton) / mm2, (MPa = 106Pa, Pa: Pascal = N/m2 )
The yield point of some metallic materials is extremely inconspicuous and difficult to measure, so in order to measure the yielding characteristics of the material, the provisions produce permanent residual plastic deformation equal to a certain value (generally 0.2% of the original length) when the stress, known as the conditional yield strength or simply yield strength σ 0.2.
Material in the tensile process, from the beginning to the maximum stress value reached when fracture occurs. It indicates the size of the steel’s ability to resist fracture. The corresponding tensile strength and compressive strength, bending strength, etc. Let Pb be the maximum tensile force reached before the material is pulled, Fo is the cross-sectional area of the sample, the tensile strength σ b = Pb/Fo (MPa). 4.
After the material is pulled, the plastic elongation of the length of the original sample length of the percentage called elongation or elongation.
The yield point of steel (yield strength) and the ratio of tensile strength, called the yield strength ratio. The greater the yield strength ratio, the higher the reliability of structural parts, generally 0.6-0.65 for carbon steel, 0.65-0.75 for low-alloy structural steel, 0.84-0.86 for alloy structural steel.
Hardness indicates the ability of a material to resist the pressure of hard objects into its surface. It is one of the important performance indicators of metal materials. Generally the higher the hardness, the better the wear resistance. Commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.
With a certain load (generally 3000kg) to a certain size (diameter is generally 10mm) of hardened steel ball pressed into the surface of the material, maintained for a period of time, to load, the ratio of the load and its indentation area, that is, the Brinell hardness value (HB), the unit of kilograms of force /mm2 (N/mm2).
When HB>450 or the specimen is too small, the Brinell hardness test can not be used and Rockwell hardness measurement is used instead. It is a diamond cone with a top angle of 120 ° or a steel ball with a diameter of 1.59, 3.18 mm, pressed into the surface of the material under a certain load, and the depth of the indentation to find out the hardness of the material. Depending on the hardness of the test material, there are three different scales to express.
HRA: is the hardness obtained by using a 60kg load and a diamond cone indenter, and is used for extremely hard materials (e.g. carbide).
HRB: is the hardness obtained with a 100 kg load and a 1.58 mm diameter hardened steel ball, and is used for materials of lower hardness (e.g. annealed steel, cast iron, etc.).
HRC: is the hardness obtained by using 150kg load and diamond cone indenter, used for very hard materials (such as hardened steel, etc.).
The Vickers hardness (HV) is obtained by dividing the surface area of the indentation by the load value with a load of 120 kg or less and a diamond square cone indenter with a top angle of 136° into the surface of the material.
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.
