Low Temperature Carbon Steel Pipe
Date:2023-04-17
Views:212
Low-temperature carbon steel pipe is a medium-carbon structural steel. It has good performance in both cold and hot heating and low-temperature steel pipes. It has good mechanical properties, low price, and wide sources, so it is widely used. Its biggest weakness is low hardenability, and it is not suitable for workpieces with large cross-sectional dimensions and relatively high requirements.
Carbon seamless steel pipe for low temperature, characterized in that the alloy composition and content (percentage by weight) of the steel pipe are:
C≤0.13, Si≤0.30, Mn:0.40-1.20, P≤0.02,
S≤0.02, Cr≤0.20, Mo≤0.10, V≤0.10,
Ni ≤ 0.20, Cu ≤ 0. 20, Al: 0.01-0.008, and the balance is iron Fe. The above components are carbon seamless steel pipes obtained after the raw materials are processed into tube blanks, thermal processing, cold processing and finished products.
The most common material for low temperature carbon steel pipe is ASTM A333.
Microstructure:
The quenching temperature of low-temperature carbon steel pipe is A3+(30-50)℃, and in actual operation, the upper limit is generally taken. A higher quenching temperature can accelerate the heating speed 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 load is large, it is necessary to extend the holding time appropriately. Otherwise, insufficient hardness may occur due to uneven heating. However, if the holding time is too long, the disadvantages of coarse grains and serious oxidation and decarburization will also appear, which will affect the quenching quality. We believe that if the amount of furnace charge is greater than the stipulations in the process documents, the heating and holding time should be extended by 1/5.
Because of the low hardenability of low-temperature carbon steel pipes, a 10% brine solution with a large cooling rate should be used. After the workpiece enters the water, it should be hardened thoroughly, but not completely cooled. If the 45# precision steel pipe is cooled thoroughly in salt water, it may cause the workpiece to crack. Tensile body caused by excessive tissue stress. Therefore, when the quenched low-temperature steel pipe is quickly cooled to this temperature range, 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 water outlet can be cooled by air (if it can be cooled by oil, it is better). In addition, it is advisable to move the workpiece into the water rather than static, and it should move regularly according to the geometric shape of the workpiece. The static cooling medium and the static workpiece lead to uneven hardness and uneven stress, resulting in large deformation and even cracking of the workpiece.
The hardness of the quenched and tempered parts of low-temperature carbon steel pipe should reach HRC56~59, and the possibility of large cross-section is lower, but not lower than HRC48. Otherwise, it means that the workpiece has not been completely quenched, and sorbite or even ferrite may appear in the structure Body tissue, this tissue remains in the matrix after tempering, failing to achieve the purpose of tempering.
For high-temperature tempering after quenching of low-temperature carbon steel pipe, the heating temperature is usually 560-600°C, 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 requirements of the drawings to ensure the hardness. For example, some shaft low-temperature steel pipes 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, 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 penetrated. Generally, the tempering holding time of the workpiece is always within more than an hour.
