Steel tube heat treatment defects and their prevention
Steel tube heat treatment defects and their prevention
Heat treatment of metal materials is divided into overall heat treatment, surface heat treatment and chemical heat treatment. Steel tube heat treatment generally adopts overall heat treatment. Generally, the basic processes such as heating, heat preservation, and cooling are carried out, and defects may occur in these processes.
The heat treatment defects of the steel pipe mainly include the unqualified structure performance of the steel pipe, the size exceeding the standard, and surface cracks, scratches, severe oxidation, decarburization, overheating, and overburning.
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 to stabilize the structure of the steel 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 uniform 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 for heat treatment of steel pipes. He determines the metallurgical structure and mechanical properties of steel pipes after cooling. In the actual production process, the steel pipe cooling method is varied. Commonly used cooling methods include furnace cooling, air cooling, oil cooling, polymer cooling, and water cooling.
According to different heating temperature of steel pipe, combined with different cooling speed of steel pipe, it is divided into normalizing, annealing, tempering, quenching and other processes.
Normalizing: refine the austenite grains, uniform internal structure and change the residual stress state, improve the overall performance of the steel pipe. Reduce the band structure and mixed crystals formed during the deformation of the steel pipe (but cannot eliminate the band structure caused by segregation and inclusions in the steel); eliminate the network carbide in the eutectoid steel, which is beneficial to the ball Annealing; used as pretreatment before quenching of medium carbon steel and alloy structure steel pipes to refine the grains to make the structure uniform and reduce the defects of steel pipes produced during the quenching process; for low carbon steel and low alloy steel steel pipes to replace annealing, Improve the cutting performance of steel pipes; can also be used as the final heat treatment of ordinary steel pipes with low requirements.
Annealing: divided into recrystallization annealing, complete annealing, isothermal annealing, spheroidizing annealing and residual stress relief annealing. Generally, high carbon, low alloy and alloy steel steel pipes need to be annealed to reduce their hardness and strength, improve plasticity, eliminate internal stress and uneven structure, 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; make the steel pipe obtain good comprehensive mechanical properties, reduce or eliminate the residual stress generated by the steel pipe during quenching and stabilize the size of the steel pipe, so that the steel pipe will not change during use. Air cooling is generally used for tempering. 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 should be used after tempering, such as oil cooling.
Quenching: the process of heating the metal material to 30-50°C above the austenite Ac3 line, holding it for a period of time and cooling the steel pipe quickly to obtain martensite or bainite. After quenching, the steel pipe will generate thermal stress and structural stress, which can be generally eliminated and improved by tempering. The combination of quenching and tempering (tempering) can greatly improve the overall performance of the steel.
Other processes include solution treatment and shielding gas heat treatment.
Heat treatment defects and their prevention
Unsatisfactory performance of steel pipe organization:
After the steel tube is austenitized, according to its different carbon content and different cooling rate, the pearlite structure, bainite structure and martensite structure can be obtained. If the heat treatment process is not properly controlled, it may produce a Wei structure. The Wei's structure is an overheated tissue. It has an adverse effect on the overall performance of the steel pipe (the organization is excellent in high-temperature durability), which will reduce the strength of the steel pipe at room temperature and increase the brittleness. The lighter Wei's structure can be eliminated by normalizing at an appropriate temperature, while the heavier Wei's 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 the role of grain refinement.
Undercooled austenite transformation curve (TTT) and undercooled austenite continuous cooling transformation curve (CCT) are the important basis for formulating the cooling rate of heat treatment.
Unqualified steel pipe size:
After the heat treatment of the steel pipe, in some cases its size will change significantly and appear out of tolerance, including the change of the outer diameter, ellipticity and bending degree of the steel pipe. Generally occurs during the quenching process. After the tempering process, the diameter setting process is often added. The change of the ellipticity of the steel pipe usually occurs at the end of the steel pipe, mainly because the large-diameter thin-walled pipe is "burned" at the end of the pipe when it is heated for a long time.
In general, the bending of the steel pipe can be corrected by a straightener. When the degree of bending is large, it will cause difficulties in transportation of the steel pipe, and the steel pipe will have a great straightening stress during straightening. Will seriously reduce the anti-collapse performance and corrosion resistance of steel pipes. What's more, the steel pipe is cracked or broken during straightening.
Surface crack of steel pipe:
During the heat treatment of the steel pipe, excessive temperature stress will cause surface cracks in the steel pipe. The cracks on the surface of the steel pipe are mainly caused by the rapid heating or cooling rate. When the high-alloy thick-walled tube is heated, if the temperature of the heating furnace is too high, the steel tube enters the furnace and encounters rapid and rapid heating. At this time, it is easy to generate large temperature stress and tend to crack. In order to reduce the heat treatment cracks of the steel pipe, on the one hand, different heating systems should be formulated according to the steel type, and the appropriate quenching medium should be selected. On the other hand, the quenched steel pipe should be tempered or annealed as soon as possible to eliminate residual stress.
