What is low temperature seamless carbon steel pipe?

Low temperature seamless carbon steel pipe is a kind of medium carbon structural steel. It has good cold and hot heating properties, 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.

Cold-rolled steel that operates at low temperatures can withstand certain low-temperature shocks. A commonly used standard is ASTM A333.

The quenching temperature of low temperature carbon steel pipe is A3+(30～50)℃. In practice, the upper limit is generally taken. High quenching temperature can speed up the heating rate of low-temperature carbon 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, the holding time should be extended appropriately. Otherwise, it may cause insufficient hardness due to uneven heating. However, if the holding time is too long, the crystal grains will be coarse, and the oxidation and decarburization will be serious, which will affect the quenching quality. We believe that if the furnace load is greater than the requirements of the process documents, the heating and holding time should be extended by 1/5.

Due to the low hardenability of low-temperature carbon steel pipes, a 10% salt solution with a large cooling rate should be used. The workpiece should be hardened after being immersed in water, but not completely cooled. If the 45# precision steel pipe is completely cooled in salt water, it may crack the workpiece. This is because when the workpiece is cooled to about 180°C, the austenite rapidly transforms into mahogany. It is caused by excessive structural stress caused by strength. Therefore, when the quenched low-temperature steel pipe is rapidly cooled to this temperature zone, the slow cooling method should be adopted. Since the temperature of the water outlet is difficult to control, it must be operated by experience. After the workpiece stops shaking in the water, it can be water-cooled and air-cooled (it would be better if it can be oil-cooled). In addition, the workpiece must move when it enters the water, not static, and it must move regularly according to the geometry of the workpiece. The static cooling medium and static workpiece will cause uneven hardness and uneven force, which will lead to deformation or even cracking of the workpiece.

The hardness of the low-temperature carbon steel pipe after quenching and tempering part should reach HRC56~59, and the possibility of large cross-section is low, but it should not 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. The tempered structure remains in the matrix, failing to achieve the purpose of tempering.

The low-temperature carbon steel pipe is quenched and then tempered at high temperature. The heating temperature is usually 560-600°C, and the hardness requirement is HRC22-34. Since 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 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. We think that 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.

Tip: ASTM A333 is the standard specification for seamless and welded carbon and alloy steel pipe for cryogenic service or other applications requiring notch toughness. There are several grades of ferritic steels covered by this standard: Grade, Grade 3, Grade 4, Grade 6, Grade 7, Grade 8, Grade 9, Grade 10, and Grade 11.

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