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Heat treatment defect of carbon steel tube and its prevention
Jan 24, 2019

Heat treatment defect of carbon steel tube and its prevention


The heat treatment of metal materials is divided into integral heat treatment, surface heat treatment and chemical heat treatment.The heat treatment of carbon steel tube generally adopts integral heat treatment.Generally, they go through the basic processes of heating, heat preservation and cooling, all of which may produce defects.


The defects of heat treatment of carbon steel tube mainly include unqualified structure and performance, excessive size, surface crack, scratch, severe oxidation, decarbonization, overheating and overburning.


Heat treatment process characteristics of carbon steel tube:


The first process is heating: heating under the critical point Ac1 or Ac3;The other is the heating above the critical point Ac1 or Ac3.The first is mainly to stabilize the structure of steel and eliminate the residual stress of carbon steel pipe, the second is mainly to austenitize steel.


The second process is heat preservation: the purpose is to evenly heat the temperature of carbon steel pipe, in order to get a reasonable heating structure.


The third process is cooling: the cooling process is the key process for the heat treatment of carbon steel tubes, which determines the metallographic structure and mechanical properties of carbon steel tubes after cooling.The cooling methods of carbon steel tube used in the actual production are various.Frequently used cooling methods are furnace cooling, air cooling, oil cooling, polymer cooling, water cooling.


According to different heating temperature of carbon steel pipe and different cooling speed of carbon steel pipe, it can be divided into normalizing, annealing, tempering, quenching and other processes.


Normalizing: refine austenite grain, homogenize internal structure and change residual stress state, improve the comprehensive performance of carbon steel tube.Reduce the banded structure and mixed crystal formed in the process of deformation of carbon steel pipe (but cannot eliminate the banded structure caused by segregation and inclusion in steel);The removal of reticular carbides in hypereutectoid steel is beneficial to spheroidizing annealing.It is used as the pretreatment of medium carbon steel and alloy structure carbon steel pipe before quenching to refine the grain and make the structure uniform and reduce the defects of carbon steel pipe produced in quenching process.For low carbon steel and low alloy steel carbon steel pipe to replace annealing, improve the cutting performance of carbon steel pipe;It can also be used as the final heat treatment of ordinary carbon steel pipe with low requirements.


Annealing includes recrystallization annealing, complete annealing, isothermal annealing, spheroidization annealing and elimination of residual stress annealing.Generally, high carbon, low alloy and alloy steel carbon steel pipe needs to be annealed to reduce its hardness and strength, improve plasticity, eliminate internal stress and uneven structure, refine crystalline structure, so as to facilitate the machining of carbon steel pipe and lay the foundation for the final heat treatment of carbon 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 ductility and toughness of carbon steel pipe;Make the carbon steel pipe to obtain good comprehensive mechanical properties, reduce or eliminate the residual stress caused by the quenching of carbon steel pipe and stabilize the size of carbon steel pipe, so that the carbon steel pipe does not change in the use process.In general, air cooling is adopted for tempering. In order to prevent the re-generation of internal stress on carbon steel pipe, it should be cooled slowly.For brittle carbon steel pipe with high temperature tempering, rapid cooling, such as oil cooling, should be adopted after tempering.


Quenching: the metal material heated to austenitic Ac3 line above 30 to 50 ° C, insulation rapid cooling after a period of time to make carbon steel tube and the technological process of the martensite and bainite.After quenching, thermal stress and microstructure stress will be produced, which can be generally eliminated and improved by tempering.The combination of quenching and tempering can greatly improve the comprehensive properties of steel.


Other processes include solid solution treatment and shielding gas heat treatment.


Heat treatment defect and its prevention


The microstructure and properties of carbon steel tube are unqualified:


After austenitizing, pearlite structure, bainite structure and martensite structure can be obtained according to different carbon content and different cooling rate.Wechsler microstructure may occur if the heat treatment process is not controlled properly.Wechsler's tissue is an overheated tissue.It has a bad effect on the comprehensive properties of carbon steel tubes (the microstructure is superior in the aspect of high temperature durability), and will reduce the strength and increase the brittleness of carbon steel tubes at room temperature.The lighter wechsler tissues can be eliminated by normalizing at appropriate temperature, while the heavier wechsler tissues can be eliminated by secondary normalizing.The first normalizing temperature is higher, and the second normalizing temperature is lower.At the same time also plays the role of refining the grain.


The TTT and CCT of supercooled austenite are important bases for determining the cooling rate of heat treatment.


After heat treatment, the size of carbon steel tube will change obviously under some circumstances, including the change of the outside diameter, ellipticity and bending degree of carbon steel tube.Generally occurs in the quenching process.Usually add sizing process after tempering process.The ellipticity change of carbon steel tube usually occurs at the end of the carbon steel tube, which is mainly caused by the "burning collapse" of the tube end when the large-diameter thin-wall tube is heated for a long time.


Generally speaking, the bending of carbon steel tube can be corrected by straightening machine.When the bending degree is very large, it will cause difficulties in the transportation of carbon steel pipe, and during the straightening, carbon steel pipe will produce great straightening stress.It will seriously reduce the crushing resistance and corrosion resistance of carbon steel pipe.What is more, carbon steel pipe in the straightening process occurs cracks or broken.


Surface crack of carbon steel tube:


In the heat treatment process of carbon steel tube, the excessive temperature stress will cause the surface crack of carbon steel tube.The surface crack of carbon steel pipe is mainly caused by heating or cooling speed.When the high alloy thick wall tube is heated, if the heating furnace temperature is too high, the carbon steel tube enters the furnace and encounters rapid heating, then it is easy to generate greater temperature stress and tend to crack.In order to reduce the heat treatment crack of carbon steel pipe, on the one hand, different heating systems should be made according to the steel type and suitable quenching medium should be selected.On the other hand, we should temper or anneal the hardened carbon steel pipe as soon as possible to eliminate the residual stress.