Steel pipe knowledge: 45 # steel pipe and 40Cr steel quenched and tempered heat treatment process quenching and tempering is a double heat treatment of high temperature tempering, its purpose is to make the workpiece has a good overall mechanical properties.
Quenched and tempered steel has two types of carbon quenched and tempered steel and alloy quenched and tempered steel. Whether it is carbon steel or alloy steel, its carbon content is controlled more strictly. If the carbon content is too high, the strength of the workpiece after tempering is high, but the toughness is not enough. If the carbon content is too low, the toughness increases and the strength is insufficient. In order to obtain a good overall performance of tempered parts, the general carbon content is controlled at 0.30-0.50%.
Quenching and quenching requires that the entire cross-section of the workpiece be hardened, so that the microstructure of the workpiece will be mainly martensite quenched by fine needles. By tempering at high temperature, a microstructure with uniform tempered sorbite is obtained. It is not possible for a small plant to perform metallurgical analysis for every furnace. Generally, it is only used for hardness testing. That is to say, 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 figure.
The operation of the workpiece tempering process must be executed strictly in accordance with the process documentation. We only give some ideas on how to implement the process during the operation.
The No. 45 steel pipe No. 45 is a medium carbon structural steel. Its hot and cold processing properties are good, its mechanical properties are good, and its low price and wide source make it widely used. Its greatest weakness is its low hardenability, large cross-section dimensions and relatively high workpiece requirements.
The quenching temperature of 45th steel pipe is A3+(30~50) °C. In actual operation, the upper limit is generally taken. The high quenching temperature can accelerate the heating of the workpiece, reduce the surface oxidation, and improve the work efficiency. In order to homogenize the austenite of the workpiece, a sufficient holding time is required. If the actual loading capacity is large, it is necessary to extend the holding time appropriately. Otherwise, hardness may be insufficient due to uneven heating. However, if the holding time is too long, there will also be coarse grains, serious defects in oxidative decarburization, and quenching quality. We believe that if the furnace charge is greater than the process documentation, the heating and holding time needs to be extended by 1/5.
Because of the low hardenability of No. 45 steel pipes, a 10% brine solution with a high cooling rate should be used. After the workpiece enters the water, it should be hardened, but it is not cold. If the workpiece is completely cold in the brine, it may crack the workpiece. This is because when the workpiece is cooled to about 180°C, the austenite is rapidly transformed into martensite. Excessive tissue stress. Therefore, when the quenched workpiece is cooled to the temperature range, slow cooling method should be adopted. Because the temperature of the outlet water is difficult to grasp, it must be operated by experience. When the shaking of the workpiece in the water stops, the outlet water can be cooled (if oil cooling is better). In addition, the workpiece should not be moved to the water should be static, should be in accordance with the geometry of the workpiece, making regular movements. Static cooling media plus stationary workpieces result in non-uniform hardness, non-uniform stress, large deformations, and even cracking.
The quenched hardness of 45# tempered steel quenched and tempered parts should reach HRC56~59, and the possibility of 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 and even ferrite may appear in the structure. Body tissue, which survives through tempering, remains in the matrix and does not achieve the goal of conditioning.
The 45-gauge steel pipe is quenched at high temperature and tempered. The heating temperature is usually 560-600°C, 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 has hardness requirements, it is necessary to adjust the tempering temperature according to the drawing requirements to ensure the hardness. For example, some shaft parts require high strength and high hardness requirements; while 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 holding time, depending on the hardness requirements and the size of the workpiece, we believe that the hardness after tempering depends on the tempering temperature, which is not related to the tempering time, but it must be reversed. The general workpiece tempering holding time is always More than one hour.
2. The quenched and tempered Cr of 40Cr steel pipe can increase the hardenability of the steel, improve the strength and tempering stability of the steel, and have excellent mechanical properties. For large or important quenched and tempered workpieces, Cr steel should be used. However, Cr steel has the second temper brittleness.
40Cr quenching and tempering of workpiece quenching and tempering, various parameters of the process card are stipulated, we experience in the actual operation is:
(A) 40Cr parts should be used after oil quenching, 40Cr steel hardenability is better, in the oil cooling can be hardened, and the workpiece deformation, cracking tendency is small. However, in the case of tight oil supply, small enterprises can quench the water in workpieces with uncomplicated shapes and find no cracks. The operator must strictly control the temperature of the incoming and outgoing waters based on experience.
(2) The hardness of 40Cr parts after quenching and tempering is still high, and the second tempering temperature will increase by 20~50°C. Otherwise, the hardness will be reduced.
(C) 40Cr workpiece after high temperature tempering, the shape of the complex in the oil cooling, simple cooling in water, the purpose is to avoid the second type of temper brittleness. After quenching the workpiece after cooling, if necessary, apply stress relief treatment.
Affecting the quality of tempered parts, the level of operators is an important factor. At the same time, there are many reasons such as equipment, materials, and pre-conditioning. We believe:
(1) The workpiece is transferred from the furnace to the cooling tank at a slow speed. The temperature of the workpiece entering the water has dropped below the critical point of Ar3, resulting in partial decomposition. The workpiece is incompletely quenched and does not meet the hardness requirements. Therefore, the small parts of the cooling fluid should pay attention to the speed, large workpieces to be cold to grasp the time.
(2) The amount of the workpiece loading furnace should be reasonable. It is advisable to use 1 to 2 layers. The overlapping of the workpieces causes uneven heating, resulting in uneven hardness.
(3) The arrangement of the workpiece into the water shall be maintained at a certain distance, and the denseness of the workpiece will cause the vapor film in the vicinity of the workpiece to be broken, resulting in a low surface hardness of the workpiece.
(D) quenching furnace, can not be quenched in one go, should see the furnace temperature drop degree, the furnace closed to reheat, so that after the hardness of the workpiece before and after quenching consistent.
(e) Pay attention to the temperature of the coolant. If the temperature of 10% brine is higher than 60°C, it cannot be used. The coolant must not contain impurities such as oil, mud, etc., otherwise, there will be insufficient or uneven hardness.
(6) Hardness will not be uniform without roughening and tempering of raw materials. If good quality of quenching and tempering is to be achieved, rough blanks should be roughed and bar stock must be forged.
(7) Strictly control the quality of the product. After quenching, the hardness is low by 1 to 3 units. The tempering temperature can be adjusted to meet the hardness requirement. However, after quenching the workpiece hardness is too low, and some even HRC25~35 only, must be re-quenched, must not only apply medium or low temperature tempering in order to achieve the drawing requirements, otherwise, lost the meaning of tempering, and may have serious s consequence.
