The seamless steel pipe
is a medium carbon structural steel, its hot and cold processing functions are excellent, and the mechanical function is rather good
, moreover, its price is low, and the source is wide, so it is widely used. However, its biggest weakness is the low hardenability, large cross-section and high requirements of the workpiece should not be used.
The quenching temperature of the seamless steel tube is at A3+(30~50) °C. In actual operation, the upper limit is generally taken. The high quenching temperature can accelerate the heating of the workpiece, reduce the surface oxidation, and improve the work efficiency. In order to average the austenite of the workpiece, sufficient holding time is required. If the actual amount of furnace loading is large, it is necessary to extend the holding time appropriately. Otherwise, there may be a phenomenon in which the hardness is insufficient due to uneven heating. However, if the holding time is too long, the grain size will be coarse, and the oxidative decarburization will be severe, which will affect the quenching quality. We believe that if the furnace volume is greater than the process documentation, the heating and holding time needs to be extended by 1/5.
Since the seamless steel pipe has low hardenability, a 10% brine solution with a high cooling rate should be used. After the workpiece enters the water, it should be hardened, but not cold. If the workpiece is cold in the brine, it may crack the workpiece. This is because the austenite is rapidly transformed into martensite when the workpiece is cooled to about 180 °C. Excessive tissue stress. Therefore, when the quenched workpiece is cooled to this temperature zone, a slow cooling method should be adopted. Because the temperature of the water is difficult to grasp, it must be operated by experience. When the workpiece in the water is shaken, the water can be cooled (if it can be oil-cooled). In addition, the workpiece should not move statically, and should be regularly moved according to the geometric shape of the workpiece. The static cooling medium plus the stationary workpiece results in uneven hardness and uneven stress, which causes the workpiece to deform greatly and even crack.
The hardness of the seamless steel tube quenched and tempered parts should reach HRC56~59, and the possibility of large cross section is lower, but it can't be lower than HRC48. Otherwise, the workpiece is not completely quenched, and the structure may have sootite or even iron. The body organization, this kind of organization still remains in the matrix through tempering, and does not achieve the purpose of quenching and tempering.
The high temperature tempering of the seamless steel tube after quenching, 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 functions, the hardness range is relatively wide. However, if the drawings have hardness requirements, the tempering temperature should be adjusted according to the drawings to ensure the hardness. For example, some shaft parts require high strength and high hardness requirements. Some gears and shaft parts with keyways are milled and inserted after quenching and tempering, and the hardness requirements are lower. Regarding the tempering holding time, depending on the hardness 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 reciprocated. Generally, the workpiece is tempered and kept for more than one hour.