Low Temperature Carbon Steel Pipe

Low temperature carbon steel pipe is a medium carbon structural steel. It has good performance in cold and hot heating, good mechanical properties, low price and wide source, so it is widely used. Its biggest weakness is low hardenability, and workpieces with large cross-sectional dimensions and relatively high requirements should not be used. 

Cold steel running at low temperature can withstand a certain low temperature impact. A commonly used standard is ASTM A333, Material: Gr.6.

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.

Low temperature steel pipe heat treatment:

The quenching temperature of low temperature carbon steel pipe is A3+(30～50)℃. In practice, the upper limit is generally taken. The high quenching temperature can accelerate the heating rate of the low temperature carbon steel pipe, 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 furnace load is large, it is necessary to appropriately extend the holding time. Otherwise, there may be insufficient hardness due to uneven heating. However, if the holding time is too long, there will also be coarse grains and serious oxidative decarburization, which will affect the quenching quality. We believe that if the furnace loading is greater than the requirements of the process document, the heating and holding time should be extended by 1/5.

Because the low temperature carbon steel pipe has low hardenability, a 10% salt solution with a large cooling rate should be used. After the workpiece enters the water, it should be hardened, but not cold through. If the 45# precision steel pipe is cold through in salt water, it may crack the workpiece. This is because when the workpiece is cooled to about 180 ℃, the austenite rapidly transforms into mahogany. It is caused by excessive structural stress caused by intenite. Therefore, when the quenched low-temperature steel pipe is rapidly cooled to this temperature region, a slow cooling method should be adopted. Since it is difficult to control the temperature of the water outlet, it must be operated with experience. When the workpiece in the water stops shaking, it can be cooled by water and air (if it can be oil-cooled, it is better). In addition, the workpiece should be moved and not static when it enters the water, and should be moved regularly according to the geometric shape of the workpiece. The static cooling medium and the static workpiece will cause uneven hardness and uneven stress, which will cause the workpiece to deform and even crack.

The hardness of the quenched and tempered part of the low temperature carbon steel pipe after quenching 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 or even ferrite may appear in the structure. The tissue, through tempering, still remains in the matrix and cannot achieve the purpose of tempering.

High temperature tempering after quenching of low temperature carbon steel pipe, the heating temperature is usually 560 ~ 600 ℃, and the hardness is required to be 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; while some gears and shaft parts with key grooves 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, which has little to do with the tempering time, but it must be back through. Generally, the tempering holding time of the workpiece is always more than an hour.

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