Low temperature seamless carbon steel pipe is a seamless steel pipe suitable for -45?~-195? low temperature pressure vessel pipes and low temperature heat exchanger pipes. Low temperature carbon seamless steel pipe is a kind of medium carbon structural steel. It has good performance in hot and cold heating and low temperature steel pipes, good mechanical properties, low price, and wide sources, so it is widely used. Its biggest weakness is its low hardenability, and it is not suitable for workpieces with large cross-sectional dimensions and relatively high requirements.
The quenching temperature of low temperature carbon seamless steel pipe is A3+(30~50)?. In practice, the upper limit is generally taken. A higher quenching temperature can speed up the heating rate of the low-temperature steel pipe, reduce surface oxidation, and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is required. If the actual furnace capacity is large, the holding time should be extended appropriately. Otherwise, insufficient hardness may occur due to uneven heating. However, if the holding time is too long, there will also be disadvantages of coarse grains and serious oxidation and decarburization, which will affect the quenching quality.
Due to the low hardenability of low-temperature carbon seamless steel pipes, a 10% brine solution with a large cooling rate should be used. The workpiece should harden after immersion in water, but not completely cool. If cooled thoroughly in salt water, the workpiece may crack. This is because when the workpiece is cooled to about 180°C, austenite rapidly transforms into martensite, resulting in excessive structural stress. Therefore, when the quenched low-temperature carbon seamless steel pipe is rapidly cooled to this temperature range, the slow cooling method should be adopted. Since the outlet water temperature is difficult to control, it must be operated by experience. When the shaking of the workpiece in the water stops, the water outlet can be air-cooled. In addition, the workpiece should move rather than stand still when it is launched into the water, and should move regularly according to the geometry of the workpiece. The static cooling medium and the static workpiece lead to uneven hardness and uneven force, resulting in large deformation and even cracking of the workpiece.
The hardness of the quenched and tempered parts of low-temperature carbon seamless steel pipe should reach HRC56~59, and the possibility of large cross-section is low. 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. This organization remains in the matrix after tempering, failing to achieve the purpose of tempering.
Low-temperature carbon seamless steel pipe is quenched and then tempered at high temperature. 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. But if there is a hardness requirement, the tempering temperature must be adjusted according to the requirement to ensure the hardness. For example, some low-temperature steel pipes for shafts require high strength and high hardness; while some gears and shaft parts with keyways need to be milled after quenching and tempering, and the hardness requirements are low. Regarding the tempering holding time, it depends on the hardness requirements and the size of the workpiece. The hardness after tempering depends on the tempering temperature and has little to do with the tempering time, but it must be tempered. Generally, the tempering holding time of the workpiece is always more than one hour.
