Heat treatment defect of carbon steel tube and its prevention

- Jan 24, 2019-

Heat treatment defect of steel tube and its interference

The heat treatment of metal materials is split into integral heat treatment, surface heat treatment and chemical heat treatment.The heat treatment of steel tube typically adopts integral heat treatment.Generally, they are going through the fundamental processes of heating, heat preservation and cooling, all of which can turn out defects.

The defects of warmth treatment of steel tube principally embrace unqualified structure and performance, excessive size, surface crack, scratch, severe oxidisation, decarbonization, warming and overburning.

Heat treatment method characteristics of steel tube:

The first method is heating: heating underneath the juncture Ac1 or Ac3;The different is that the heating on top of the critical point Ac1 or Ac3.The first is principally to stabilize the structure of steel and eliminate the residual stress of steel pipe, the second is principally to austenitize steel.

The second method is heat preservation: the aim is to equally heat the temperature of steel pipe, so as to induce an inexpensive heating structure.

The third method is cooling: the cooling process is that the key process for the warmth treatment of steel tubes, that determines the metallographic structure and mechanical properties of steel tubes once cooling.The cooling ways of steel tube employed in the particular production are varied.Frequently used cooling ways are chamber cooling, air cooling, oil cooling, compound cooling, water cooling.

According to completely different heating temperature of steel pipe and different cooling speed of carbon steel pipe, it are often divided into normalizing, annealing, tempering, conclusion and different processes.

Normalizing: refine solid solution grain, homogenize internal structure and alter residual stress state, improve the excellent performance of steel tube.Reduce the banded structure and mixed crystal fashioned within the method of deformation of steel pipe (but cannot eliminate the banded structure caused by segregation and inclusion in steel);The removal of latticed carbides in hypereutectoid steel is helpful to spheroidizing hardening.It is used because the pretreatment of medium steel and alloy structure carbon steel pipe before conclusion to refine the grain and create the structure uniform and cut back the defects of carbon steel pipe created in quenching method.For low steel and low steel carbon steel pipe to interchange hardening, improve the cutting performance of steel pipe;It may also be used because the final heat treatment of normal carbon steel pipe with low necessities.

Annealing includes recrystallization hardening, complete hardening, equal hardening, spheroidization hardening and elimination of residual stress annealing.Generally, high carbon, low alloy and steel steel pipe has to be toughened to cut back its hardness and strength, improve malleability, eliminate internal stress and uneven structure, refine crystalline structure, therefore on facilitate the machining of steel pipe and lay the inspiration for the ultimate heat treatment of carbon steel pipe.

Tempering: typically divided into temperature tempering (150-250 ° C), medium temperature tempering (350-500 ° C), and hot temperature tempering (500-650 ° C).Improve the plasticity and toughness of steel pipe;Make the carbon steel pipe to get sensible comprehensive mechanical properties, cut back or eliminate the residual stress caused by the conclusion of steel pipe and stabilize the scale of carbon steel pipe, so the steel pipe doesn't modification within the use method.In general, air cooling is adopted for tempering. so as to stop the re-generation of internal stress on steel pipe, it ought to be cooled slowly.For brittle steel pipe with hot temperature tempering, fast cooling, like oil cooling, ought to be adopted once tempering.

Quenching: the metal material heated to solid solution Ac3 line on top of thirty to fifty ° C, insulation fast cooling once a amount of your time to form steel tube and also the technological method of the primary solid solution and bainite.After conclusion, thermal stress and microstructure stress are going to be created, which might be typically eliminated and improved by tempering.The combination of conclusion and tempering will greatly improve the excellent properties of steel.

Other processes embrace primary solid solution treatment and shielding heating plant treatment.

Heat treatment defect and its interference

The microstructure and properties of steel tube are unqualified:

After austenitizing, lamellar mixture structure, bainite structure and primary solid solution structure are often obtained consistent with completely different carbon content and different cooling rate.Wechsler microstructure could occur if the warmth treatment method isn't controlled properly.Wechsler's tissue is Associate in Nursing hot tissue.It has a foul impact on the excellent properties of steel tubes (the microstructure is superior within the side of hot temperature durability), and can cut back the strength and increase the crispiness of steel tubes at temperature.The lighter wechsler tissues are often eliminated by normalizing at acceptable temperature, whereas the heavier wechsler tissues are often eliminated by secondary normalizing.The first normalizing temperature is higher, and also the second normalizing temperature is lower.At constant time conjointly plays the role of processing the grain.

The TTT and CCT of supercooled solid solution are vital bases for decisive the cooling rate of warmth treatment.

After heat treatment, the scale of steel tube can modification clearly underneath some circumstances, as well as the modification of the surface diameter, roundedness and bending degree of steel tube.Generally happens within the conclusion method.Usually add size method once tempering process.The roundedness modification of steel tube typically happens at the tip of the carbon steel tube, that is principally caused by the "burning collapse" of the tube finish once the large-diameter thin-wall tube is heated for an extended time.

Generally speaking, the bending of steel tube are often corrected by straightening machine.When the bending degree is incredibly massive, it'll cause difficulties within the transportation of steel pipe, and through the straightening, steel pipe can turn out nice straightening stress.It will seriously cut back the crushing resistance and corrosion resistance of steel pipe.What is additional, steel pipe within the straightening method happens cracks or broken.

Surface crack of steel tube:

In the heat treatment method of steel tube, the excessive temperature stress can cause the surface crack of steel tube.The surface crack of steel pipe is principally caused by heating or cooling speed.When the high alloy thick wall tube is heated, if the heating chamber temperature is just too high, the steel tube enters the chamber and encounters fast heating, then it's straightforward to come up with larger temperature stress and have a tendency to crack.In order to cut back the warmth treatment crack of steel pipe, on the one hand, completely different heating systems ought to be created consistent with the steel sort and appropriate conclusion medium should be elite.On the opposite hand, we should always temper or toughen the hardened steel pipe as before long as doable to eliminate the residual stress.

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