The seamless expansion of the seamless steel tube is divided into an overall thermal expansion treatment, a surface thermal expansion treatment, and a chemical heat treatment. The seamless expansion of the seamless steel pipe is generally treated by integral thermal expansion. Generally, they are subjected to basic processes such as heating, heat preservation, and cooling, and defects may occur in these processes.
Heat treatment defects of seamless steel tubes mainly include unqualified structural properties of steel pipes, excessive size and surface cracks, scratches, severe oxidation, decarburization, overheating, and over-burning.
Features of thermal expansion processing of seamless steel tubes:
The first process is heating: one is heating below the critical point Ac1 or Ac3; the other is heating above the critical point Ac1 or Ac3. The first is mainly to stabilize the structure of the seamless steel pipe and to eliminate the residual stress of the seamless steel pipe, and the second is mainly to austenitize the steel.
The second process is thermal insulation: the purpose is to uniformly heat the heating temperature of the steel pipe to obtain a reasonable heating structure.
The third process is cooling: the cooling process is a key process in the heat treatment of seamless steel tubes. He determines the metallographic structure and mechanical properties of the seamless steel tubes after cooling. The seamless steel tube cooling methods used in the actual production process are various. The cooling methods often used are furnace cooling, air cooling, oil cooling, polymer cooling, and water cooling.
According to different heating temperature of seamless steel pipe, combined with different cooling speed of seamless steel pipe, it is divided into normalizing, annealing, tempering, quenching and other processes.
Normalizing: refining austenite grains, uniform internal structure and changing residual stress state, improving the overall performance of seamless steel tubes. Reduce the banded structure and mixed crystal formed during the deformation process of the seamless steel pipe (but can not eliminate the banded structure caused by segregation and inclusions in the steel); eliminate the network carbide in the hypereutectoid steel, Conducive to spheroidizing annealing; used as pretreatment before quenching of medium carbon steel and alloy structural seamless steel tubes to refine grains to make the structure uniform and reduce the defects of seamless steel tubes produced by quenching process; for low carbon steel and low alloy steel Seamless steel tubes are used to replace annealing to improve the cutting performance of seamless steel tubes; they can also be used as the final heat treatment for less demanding ordinary seamless steel tubes.
Annealing: divided into recrystallization annealing, complete annealing, isothermal annealing, spheroidizing annealing, and elimination of residual stress annealing. Generally high-carbon, low-alloy and alloy steel seamless steel tubes need to be annealed to reduce their hardness and strength, improve plasticity, eliminate internal stress and uneven structure, refine the crystal structure to facilitate the seamless machining of seamless steel tubes and to be seamless. The final heat treatment of the steel pipe lays the foundation.
Tempering; generally divided into low temperature tempering (150-250 ° C), medium temperature tempering (350-500 ° C), and high temperature tempering (500-650 ° C). Improve the plasticity and toughness of seamless steel tubes; obtain good comprehensive mechanical properties of seamless steel tubes, reduce or eliminate the residual stress generated by seamless steel tubes during quenching and stabilize the size of seamless steel tubes, so that the steel tubes do not change during use. . The tempering is generally air-cooled. In order to prevent the internal steel from re-generating internal stress, it should be slowly cooled. For high-temperature temper brittle seamless steel pipes, rapid cooling should be adopted after tempering, such as oil cooling.
Quenching: heating the metal material to 30-50 ° C above the austenite Ac3 line, and after a period of heat preservation, the seamless steel tube is rapidly cooled to obtain a Martensitic or Bainite process. After the quenching of the seamless steel tube, thermal stress and tissue stress are generated, which can generally be eliminated and improved by tempering. The combination of quenching and tempering (tempering) can greatly improve the overall performance of steel. Other processes include solution treatment and protective gas heat treatment.
Heat treatment defects and their prevention:
Unsatisfactory structural properties of seamless steel tubes: After austenitizing seamless steel tubes, pearlite, bainite and martensite structures can be obtained according to their different carbon contents and different cooling rates. Weiss tissue may be produced if the thermal expansion process is not properly controlled. The Wei’s organization is an overheated organization. It has an adverse effect on the overall performance of the seamless steel pipe (the structure is excellent in high-temperature durability), which causes the seamless steel pipe to have lower room temperature strength and increased brittleness. The lighter Wei’s tissue can be eliminated by normalizing at a suitable temperature, while the heavier Wei’s tissue can be eliminated by secondary normalizing. The first normalizing temperature is higher and the second normalizing temperature is lower. At the same time, it also plays a role in refining the grains.
The supercooled austenite transformation curve (TTT) and the supercooled austenite continuous cooling transition curve (CCT) are important basis for the heat treatment cooling rate.
The seamless steel pipe is unqualified. After the heat treatment of the seamless steel pipe, the size of the seamless steel pipe will change significantly in some cases, including the outer diameter, the ellipticity and the curvature of the seamless steel pipe. Generally occurs during the quenching process. The diameter setting process is often added after the tempering process. The ellipticity change of the seamless steel pipe usually occurs at the end of the seamless steel pipe, mainly because the large-diameter thin-walled seamless pipe is caused by the “burning collapse” of the seamless pipe end during long-time heating.
In general, the bending of the seamless steel pipe can be corrected by a straightener. When the degree of curvature is large, the seamless steel pipe is difficult to transport, and the straight steel pipe will have a large straightening stress during straightening. It will seriously reduce the anti-crushing performance and corrosion resistance of seamless steel tubes. What is more, the seamless steel pipe is cracked or broken during the straightening process.
Surface cracks in seamless steel tubes: Excessive temperature stress causes surface cracks in seamless steel tubes during heat treatment of seamless steel tubes. Surface cracks in seamless steel tubes are mainly caused by excessive heating rate or cooling rate. When the high-alloy thick-walled seamless pipe is heated, if the temperature of the heating furnace is too high, the seamless steel pipe enters the furnace and encounters rapid rapid heating. At this time, large temperature stress is easily generated and tends to crack. In order to reduce the heat treatment crack of the seamless steel pipe, on the one hand, different heating systems should be established according to the steel grade, and a suitable quenching medium should be selected. On the other hand, the quenched seamless steel pipe should be tempered or annealed as soon as possible to eliminate residual stress.