Forging heat treatment

Knowledge of forging heat treatment - conditioning

I. definition:

Tempering is a double heat treatment of quenching and tempering at high temperature.

Tempering treatment: the heat treatment method of high temperature tempering after quenching is called tempering treatment. High tempering refers to tempering between 500-650℃. Tempering can make the properties of steel, material to a great extent of adjustment, its strength, plasticity and toughness are better, with good comprehensive mechanical properties. Tempered sostenite was obtained after tempering treatment. Tempered sorbite is formed when martensite is tempered. It can only be distinguished by 500~600 times larger under an optical metallographic microscope. It is a composite structure with carbides (including cementite) spheres distributed within the ferritic matrix. It is also a tempering structure of martensite, a mixture of ferrite and granular carbides. At this point, the ferrite is basically free of carbon supersaturation, and the carbides are also stable carbides. Normal temperature is a kind of equilibrium structure.

Two, conditioning application example: 45 steel conditioning

Tempering is often used in medium - carbon (low - alloy) structural steels as well as in low - alloy cast steels. In a word, the structural parts with high mechanical requirements should be tempered. (hardening and tempering; Thermal refining) one of the metal heat treatment process. Tempering at high temperature after quenching is called tempering treatment. The purpose is to make the steel with high toughness and enough strength, with comprehensive excellent mechanical properties. Such as vertical shaft, lead screw, gear, etc. Generally speaking, it is carried out after the parts are processed, and also can carry out mechanical processing after the coarse blank is tempered.

Tempering of 45 steel

45 steel is medium carbon structure steel, cold and hot processing performance is good, mechanical performance is good, and the price is low, the source is wide, so widely used. Its biggest weakness is the hardenability is low, the size of the section is large relatively high requirements of the workpiece should not be used. 45 steel quenching temperature in A3+(30~50 ℃), in the actual operation, is generally the upper limit. High quenching temperature can make the workpiece heating speed, surface oxidation reduction, and improve the efficiency. In order to make the austenitic uniformity of the workpiece, a sufficient holding time is required. If the actual load is large, it is necessary to extend the holding time. Otherwise, the phenomenon of insufficient hardness caused by uneven heating may occur. But holding time is too long, also can also appear grain thick, oxidation decarburization serious defect, affect quenching quality. In our opinion, if the loading capacity is greater than the requirements in the process documents, the heating and holding time shall be extended by 1/5.

Because of the low permeability of 45 steel, 10% brine solution with high cooling rate should be used. After the workpiece enters water, it should be quenched, but not cold. If the workpiece is cooled in brine, it may crack the workpiece. This is because when the workpiece is cooled to about 180℃, austenite is rapidly transformed into martensite, resulting in excessive tissue stress. Therefore, when the quenched workpiece is rapidly cooled to the temperature region, the method of slow cooling should be adopted. As the water temperature is difficult to grasp, must rely on the verified operation, when the water workpiece shaking stop, water air cooling (such as oil cooling is better). In addition, the workpiece into the water appropriate to move not static, should be in accordance with the workpiece geometry, regular movement. Static cooling medium plus static workpiece, resulting in uneven hardness, stress and deformation of the workpiece, and even cracking. After quenching, the hardness of 45 steel quenched parts should reach HRC56~59, the possibility of large section is lower, but not lower than HRC48, otherwise, it means that the workpiece has not been completely quenched, and there may be sostenitic or even ferrite structure in the structure, through tempering, still retained in the matrix, unable to achieve the purpose of quality control.

45 steel after quenching tempering, heating temperature is usually 560~600℃, hardness to be HRC22~34. Because the purpose of conditioning is to obtain comprehensive mechanical properties, so the hardness range is relatively wide. However, if the drawing has hardness requirements, the tempering temperature shall be adjusted according to the requirements of the drawing to ensure the hardness. If some shaft parts require high strength, high hardness requirements; And some gear, with a keyway shaft parts, because after conditioning milling, insert processing, hardness requirements are lower. About the tempering holding time, depending on the hardness requirements and the size of the workpiece, we believe that the tempering hardness depends on the tempering temperature, and the tempering time is not important, but must be back through, generally the tempering holding time of the workpiece is always more than one hour.