Quenching and Tempering Heat Treatment Process of 45# Steel and 40Cr Steel
Quenching and tempering is a dual heat treatment of quenching and high temperature tempering, and its purpose is to make the workpiece have good comprehensive mechanical properties.
There are two kinds of quenched and tempered steel: carbon quenched and tempered steel and alloy quenched and tempered steel. Whether carbon steel or alloy steel, its carbon content is strictly controlled. If the carbon content is too high, the strength of quenched and tempered workpiece is high, but the toughness is not enough. For example, the carbon content is too low, the toughness is improved and the strength is not enough. In order to obtain good comprehensive properties of quenched and tempered parts, the carbon content is generally controlled at 0.30~0.50%.
When quenching and tempering, the workpiece is required to be quenched in full section, so that the microstructure of the workpiece is mainly fine needle-shaped quenched martensite. Homogeneous tempered sorbite structure was obtained by high temperature tempering. It is impossible for small factories to carry out metallographic analysis on each furnace, and generally only do hardness test, that is to say, the hardness after quenching must reach the quenching hardness of materials, and the hardness after tempering should be checked according to the requirements of drawings.
The operation of workpiece quenching and tempering treatment must be carried out in strict accordance with the process documents. We just put forward some views on how to implement the process in the operation process.
Quenching and tempering of 1 and 45 steel
45 # steel is a kind of 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 that the workpieces with low hardenability, large cross-section size and high requirements are not suitable for use.
The quenching temperature of No.45 steel is A3+(30~50)℃, which is generally the upper limit in practical operation. Higher quenching temperature can accelerate the heating speed of workpiece, reduce surface oxidation and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is needed. If the actual charging capacity is large, it is necessary to extend the holding time appropriately. Otherwise, uneven heating may lead to insufficient hardness. However, if the holding time is too long, there will be unfavorable factors such as coarse grain size and serious oxidation decarbonization, which will affect the quenching quality. We think that the heating and holding time should be extended by 1/5 if the charge is greater than the amount specified in the process documents.
Because of the low hardenability of 45 # steel, 10% salt solution with fast cooling rate should be adopted. After the workpiece enters the water, it should be quenched, not chilled. If the workpiece is cooled in salt water, it may break. This is because when the workpiece is cooled to about 180℃, austenite rapidly transforms into martensite, resulting in excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature range, the method of slow cooling should be adopted. Because the outlet water temperature is difficult to grasp, it must be operated by experience. When the jitter of the workpiece in water stops, the outlet water can be cooled by air (it would be better if it could be cooled by oil). In addition, when the workpiece enters the water, it should be moving rather than static, and it should move regularly according to the geometry of the workpiece. Static cooling medium and static workpiece lead to uneven hardness and stress, which leads to large deformation and even cracking of the workpiece.
The hardness of quenched and tempered parts of 45 steel should reach HRC56~59, and the possibility of large section is low, but it cannot be lower than HRC48. Otherwise, it shows that the workpiece is not completely quenched, and sorbite or even ferrite structure may appear in the structure, which remains in the matrix after tempering, and the purpose of quenching and tempering cannot be achieved.
The high temperature tempering of 45 # steel after quenching is usually at 560~600℃, and the hardness requirement is HRC22~34. Because the purpose of tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawing requires hardness, the tempering temperature should be adjusted according to the drawing requirements to ensure hardness. If some shaft parts require high strength and hardness; However, some gears and shaft parts with keyways have to be milled and inserted after quenching and tempering, so the hardness requirement is low. As for the tempering holding time, it depends on the hardness requirements and the size of the workpiece. We think that the hardness after tempering depends on tempering temperature and has little to do with tempering time, but it must be tempered. Generally, the tempering and heat preservation time of workpieces is always more than one hour.
Quenching and tempering treatment of 2.40Cr steel
Chromium can improve the hardenability, strength and tempering stability of steel, and has excellent mechanical properties. Chromium steel should be used for quenched and tempered workpieces with large or important sections. But Cr steel has the second temper brittleness.
The quenching and tempering of 40Cr workpiece are specified by various parameters and process cards. Our experience in practical operation is as follows:
(40Cr workpiece should be oil-cooled after quenching. 40Cr steel has good hardenability, can be cooled and quenched in oil, and the deformation and cracking tendency of the workpiece are small. However, in the case of tight oil supply, small enterprises can quench workpieces with uncomplicated shapes in water, and no cracks are found, but operators should strictly control the import and export temperatures by experience.
(The hardness of 40Cr workpiece is still high after quenching and tempering, and the second tempering temperature should be increased by 20~50℃, otherwise the hardness will be difficult to decrease.
(3) After tempering at high temperature, the 40Cr workpiece with complex shape is cooled by oil neutralization and simple water in order to avoid the influence of brittleness of secondary tempering. After tempering and rapid cooling, if necessary, stress relief treatment should be carried out on the workpiece.
The level of operators is an important factor affecting the quality of quenched and tempered workpieces. At the same time, there are many reasons such as equipment, materials, pretreatment and so on. We believe that:
(1) The transfer speed of the workpiece from the heating furnace to the cooling tank is slow, and the water inlet temperature of the workpiece has dropped below the Ar3 critical point, resulting in partial decomposition and partial quenching of the workpiece, which can not meet the hardness requirements. Therefore, the cooling liquid of small parts should pay attention to the speed, and the cooling time of large parts should be mastered.
(2) The furnace charging quantity of the workpiece should be reasonable, and it is advisable to use 1~2 layers. Workpieces overlap each other, resulting in uneven heating and uneven hardness.
(3) The arrangement of workpieces entering the water should keep a certain distance. If it is too dense, the vapor film near the workpiece will be blocked and the hardness near the workpiece will be low.
(4) Open the furnace for quenching, not quenching at one breath. Depending on the temperature drop, the furnace should be closed halfway to raise the temperature again to make the hardness of the workpiece consistent after quenching.
(5) Pay attention to the temperature of the coolant. If the temperature of 10% brine is higher than 60℃, it cannot be used. The coolant must not contain impurities such as oil and mud, otherwise the hardness will be insufficient or uneven.
(6) If the unprocessed blank is quenched and tempered, the hardness will be uneven. If you want to obtain good quenching and tempering quality, the blank should be coarsened and the bar should be forged.
(7) Strictly control the quality, the hardness after quenching is low 1~3 units, and the tempering temperature can be adjusted to meet the hardness requirements. However, after quenching, the hardness of the workpiece is too low, and some even only have HRC25~35, so it must be quenched again. It must not be tempered only at medium and low temperature to meet the requirements of the drawings, otherwise it will lose the significance of tempering and may cause serious consequences.