Quenching and tempering treatment: the heat treatment method of high temperature tempering after quenching is called quenching and tempering treatment. High temperature tempering refers to tempering between 500-650°C. Quenching and tempering can make the performance and material of steel be adjusted to a large extent, and its strength, plasticity and toughness are good, and it has good comprehensive mechanical properties.
After quenching and tempering, tempered sorbite is obtained. Tempered sorbite is formed during tempering of martensite. It can be distinguished only by magnifying 500~600 times under an optical metallographic microscope. It is a ferrite matrix with carbides (including Cementite) the composite structure of spherulites. It is also a tempered structure of martensite, a mixture of ferrite and granular carbide. At this time, the ferrite has basically no carbon supersaturation, and the carbides are also stable carbides. It is a balanced organization at room temperature.
Quenched and tempered steel has two categories: carbon quenched and tempered steel and alloy quenched and tempered steel. Regardless of whether it is carbon steel or alloy steel, its carbon content is controlled strictly. If the carbon content is too high, the strength of the workpiece after quenching and tempering is high, but the toughness is not enough. If the carbon content is too low, the toughness will increase and the strength will be insufficient. In order to obtain good overall performance of quenched and tempered parts, the carbon content is generally controlled at 0.30~0.50%.
When quenching and tempering, the entire section of the workpiece is required to be quenched, so that the workpiece can obtain a microstructure dominated by fine needle-shaped quenched martensite. Through high temperature tempering, a microstructure dominated by uniform tempered sorbite is obtained. It is impossible for small factories to conduct metallographic analysis for each furnace, and generally only perform hardness testing. This means that the hardness after quenching must reach the quenching hardness of the material, and the hardness after tempering is checked according to the requirements of the drawing.
Quenching And Tempering Of 45 Steel
45 steel is a medium carbon structural steel with good cold and hot workability, good mechanical properties, low price and wide sources, so it is widely used. Its biggest weakness is its low hardenability, large cross-sectional dimensions and high-demand workpieces.
The quenching temperature of 45 steel is A3+(30~50)℃. In actual operation, the upper limit is generally taken. A higher quenching temperature can speed up the heating of the workpiece, reduce surface oxidation, and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is required. If the actual amount of furnace installed is large, the holding time needs to be extended appropriately. Otherwise, there may be insufficient hardness due to uneven heating. However, if the holding time is too long, coarse grains and serious oxidative decarburization will also occur, which will affect the quenching quality. We believe that if the furnace load is greater than the provisions of the process documents, the heating and holding time needs to be extended by 1/5.
Because 45 steel has low hardenability, a 10% saline solution with a high cooling rate should be used. After the workpiece enters the water, it should be quenched, but not cold. If the workpiece is cooled in salt water, it may crack the workpiece. This is caused by the rapid transformation of austenite into martensite when the workpiece is cooled to about 180°C. Caused by excessive tissue stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature region, a slow cooling method should be adopted. Since the temperature of the outlet water is difficult to control, it must be operated by experience. When the shaking of the workpiece in the water stops, the outlet can be air cooled (oil cooling is better). In addition, the workpiece should be moved but not static when it enters water, and should be moved regularly according to the geometry of the workpiece. The static cooling medium and the static workpiece result in uneven hardness, uneven stress and large deformation or even cracking of the workpiece.
The hardness of the 45 steel quenched and tempered parts after quenching should reach HRC56~59. The possibility of a large cross section is lower, but it cannot be lower than HRC48. Otherwise, it means that the workpiece has not been completely quenched, and sorbite or even ferrite may appear in the structure. Organization, this kind of organization is still retained in the matrix through tempering, failing to achieve the purpose of quenching and tempering.
For high temperature tempering of 45 steel after quenching, the heating temperature is usually 560~600℃, and the hardness requirement is HRC22~34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawings have hardness requirements, the tempering temperature must be adjusted according to the drawings to ensure the hardness. For example, some shaft parts require high strength, so the hardness requirements are high; and some gears and shaft parts with keyways need to be milled and inserted after quenching and tempering, so the hardness requirements are lower. Regarding the tempering and holding time, it depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature, and has little to do with the tempering time, but it must be back through. Generally, the tempering and holding time of the workpiece is always More than one hour.