Defects of seamless pipe heat treatment and their prevention
Defects of seamless pipe heat treatment and their prevention
The heat treatment of metal materials is divided into overall heat treatment, surface heat treatment and chemical heat treatment. Heat treatment of steel pipe generally adopts integral heat treatment. Generally, it undergoes basic processes such as heating, heat preservation, and cooling, and defects may occur in these processes.
The heat treatment defects of steel pipes mainly include unqualified steel pipe structure and performance, excessive size, surface cracks, scratches, severe oxidation, decarburization, overheating, overburning, etc.
Characteristics of heat treatment process of steel pipe:
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 steel structure and eliminate the residual stress of the steel pipe, and the second is to austenitize the steel.
The second process is heat preservation: the purpose is to even the heating temperature of the steel pipe to obtain a reasonable heating structure.
The third process is cooling: the cooling process is the key process of the heat treatment of the steel pipe, which determines the metallographic structure and mechanical properties of the steel pipe after cooling. The steel pipe cooling methods used in the actual production process are varied. The commonly used cooling methods are furnace cooling, air cooling, oil cooling, polymer cooling, and water cooling.
According to different steel pipe heating temperature, combined with different steel pipe cooling speed, it is divided into normalizing, annealing, tempering, quenching and other processes.
Normalizing: Refine the austenite grains, uniform the internal structure and change the residual stress state, and improve the overall performance of the steel pipe. Reduce the band structure and mixed crystal formed during the deformation of the steel pipe (but it cannot eliminate the band structure caused by segregation and inclusions in the steel); eliminate the network carbide in the hypereutectoid steel, which is beneficial to the ball Chemical annealing; used as the pre-treatment before quenching of medium carbon steel and alloy structural steel pipes to refine the grains to make the structure uniform and reduce the steel pipe defects caused by the quenching process; for low carbon steel and low alloy steel pipes, it is used to replace annealing, Improve the cutting performance of steel pipes; it can also be used as the final heat treatment of ordinary steel pipes that are not demanding.
Annealing: It is divided into recrystallization annealing, complete annealing, isothermal annealing, spheroidizing annealing and residual stress relief annealing. Generally, high-carbon, low-alloy and alloy steel pipes need to be annealed to reduce their hardness and strength, improve plasticity, eliminate internal stress and uneven microstructure, and refine the crystal structure to facilitate the machining of the steel pipe and lay the foundation for the final heat treatment of the steel pipe
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 the steel pipe; enable the steel pipe to obtain good comprehensive mechanical properties, reduce or eliminate the residual stress generated by the steel pipe during quenching and stabilize the steel pipe size, so that the steel pipe does not change during use. Tempering generally adopts air cooling. In order to prevent the steel pipe from regenerating internal stress, it should be cooled slowly; for high-temperature tempered brittle steel pipes, rapid cooling, such as oil cooling, should be adopted after tempering.
Quenching: The process of heating the metal material to 30-50°C above the austenite Ac3 line, and after holding for a period of time, quickly cooling the steel pipe to obtain martensite or bainite. After the steel pipe is quenched, thermal stress and structural stress will be generated, which can generally be eliminated and improved by tempering. The combination of quenching and tempering (quenching and tempering) can greatly improve the overall performance of steel.
Other processes include solid solution treatment and protective gas heat treatment.
Heat treatment defects and their prevention
Unqualified steel pipe structure and performance: After the steel pipe is austenitized, according to its different carbon content and different cooling rate, pearlite, bainite and martensitic structures can be obtained. If the heat treatment process is not properly controlled, Widmanstatten structure may be produced. Widmanstatten organization is an overheated organization. It has an adverse effect on the overall performance of the steel pipe (the organization is excellent in high temperature durability), and it will reduce the strength of the steel pipe at room temperature and increase the brittleness. The lighter Widmanstatten structure can be eliminated by normalizing at an appropriate temperature, while the heavier Widmanstatten structure 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 grain refinement.
The undercooled austenite transformation curve (TTT) and the undercooled austenite continuous cooling transformation curve (CCT) are important basis for formulating the cooling rate of heat treatment.
Unqualified steel pipe size
After the steel pipe is heat-treated, the size of the steel pipe will change significantly in some cases and appear out of tolerance, including changes in the outer diameter, ellipticity and bending of the steel pipe. Generally occurs during the quenching process. The setting diameter process is often added after the tempering process. The ovality change of the steel pipe usually occurs at the end of the steel pipe, which is mainly caused by the "burning" of the pipe end when the large-diameter thin-walled pipe is heated for a long time.
Generally speaking, the bending of the steel pipe can be corrected by a straightening machine. When the degree of curvature is large, it will cause difficulties in the transportation of the steel pipe, and the steel pipe will have a great straightening stress during straightening. Will seriously drop
The collapse resistance and corrosion resistance of low steel pipes. What's more, the steel pipe is cracked or broken during the straightening process.
Surface cracks of steel pipe
During the heat treatment of the steel pipe, excessive temperature stress will cause surface cracks in the steel pipe. The surface cracks of the steel pipe are mainly caused by the heating rate or the cooling rate too fast. When the high-alloy thick-walled pipe is heated, if the furnace temperature is too high, the steel pipe enters the furnace and encounters rapid heating. At this time, large temperature stress is prone to occur and tends to crack. In order to reduce the heat treatment cracks of the steel pipe, on the one hand, different heating systems must be formulated according to the steel type, and the appropriate quenching medium must be selected. On the other hand, the quenched steel pipe should be tempered or annealed as soon as possible to eliminate residual stress.
