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.
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.
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.