Precision seamless pipe is a kind of high-precision seamless steel pipe processed by cold drawing or hot rolling. Because there is no oxide layer on the inner and outer walls of the precision steel pipe, it can withstand high pressure without leakage, high precision, high finish, cold bending without deformation, flaring, flattening without cracks, etc. Therefore, it is mainly used to produce pneumatic or hydraulic components, such as cylinders or oil cylinders. There will be many defects in the precision seamless steel pipe during the heat treatment process, and these defects will have an important impact on the service life and accuracy of the precision seamless steel pipe.
1. Overheating
The superheating of the microstructure after quenching can be observed from the rough surface of the precision seamless steel pipe. But to determine exactly how overheated it is, the microstructure must be observed. If coarse acicular martensite appears in the quenched structure of GCrl5 steel, it is a quenched superheated structure. The cause of formation is the overall overheating caused by the quenching heating temperature being too high or the heating and holding time being too long. It may also be due to the serious banded carbides in the original organization, and the formation of local martensite needle-like coarseness in the low-carbon zone between the two bands, resulting in local overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases.
2. Underheating
If the quenching temperature is low or the cooling is poor, a troostite structure exceeding the standard will be produced in the microstructure, which is called an underheated structure. It reduces the hardness of parts and sharply reduces wear resistance, which directly affects the service life of precision seamless steel pipes.
3. Quenching cracks
During the quenching and cooling process, the cracks formed by the internal stress of the precision seamless steel pipe exceeding the tensile strength of the material at this position are called quenching cracks. The reasons for this crack are:
Because the quenching heating temperature is too high or the cooling is too rapid, the thermal stress and the structural stress of the metal mass volume change are greater than the fracture strength of the steel. The original defects on the working surface or the internal defects of the steel form stress concentration during quenching. Severe surface decarburization and carbide segregation. Insufficient tempering or not timely tempering of parts after quenching. Excessive cold punching stress caused by the previous process, forging folds, deep turning tool marks, sharp edges and corners of oil grooves, etc. The microstructure characteristic of quenching cracks is that there is no decarburization on both sides of the cracks, which is obviously different from forging cracks and material cracks.
4. Heat treatment deformation
During the heat treatment of precision seamless steel pipes, there are thermal stress and structural stress, and these internal stresses can superimpose or partially offset each other, which is complex and changeable. Because it can change with the heating temperature, heating speed, cooling method, cooling speed, part shape and size, and furnace loading method, so its heat treatment deformation is inevitable.
Understanding and mastering its changing law can make the deformation of bearing parts in a controllable range, which is conducive to the normal production. Of course, the mechanical collision during the heat treatment will also cause deformation of the parts, but this deformation can be reduced and avoided by improving the operation and so on.
5. Surface decarburization
During the heat treatment process, if the precision seamless steel pipe is heated in an oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the surface of the precision seamless steel pipe, resulting in surface decarburization.
If the depth of the surface decarburization layer exceeds the final processing allowance, the precision seamless steel pipe will be scrapped. The determination of the depth of the surface decarburization layer can be carried out by metallographic method and microhardness method in metallographic examination. The surface layer microhardness distribution curve measurement method shall prevail and may be used as an arbitration criterion.
Quenching microstructure overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots, etc., are mostly caused by insufficient heating, poor cooling or improper quenching operation. Precision seamless steel pipes, like surface decarburization, can cause a serious decrease in surface wear resistance and fatigue strength, and will cause local severe wear or failure during use.
1. Overheating
The superheating of the microstructure after quenching can be observed from the rough surface of the precision seamless steel pipe. But to determine exactly how overheated it is, the microstructure must be observed. If coarse acicular martensite appears in the quenched structure of GCrl5 steel, it is a quenched superheated structure. The cause of formation is the overall overheating caused by the quenching heating temperature being too high or the heating and holding time being too long. It may also be due to the serious banded carbides in the original organization, and the formation of local martensite needle-like coarseness in the low-carbon zone between the two bands, resulting in local overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases.
2. Underheating
If the quenching temperature is low or the cooling is poor, a troostite structure exceeding the standard will be produced in the microstructure, which is called an underheated structure. It reduces the hardness of parts and sharply reduces wear resistance, which directly affects the service life of precision seamless steel pipes.
3. Quenching cracks
During the quenching and cooling process, the cracks formed by the internal stress of the precision seamless steel pipe exceeding the tensile strength of the material at this position are called quenching cracks. The reasons for this crack are:
Because the quenching heating temperature is too high or the cooling is too rapid, the thermal stress and the structural stress of the metal mass volume change are greater than the fracture strength of the steel. The original defects on the working surface or the internal defects of the steel form stress concentration during quenching. Severe surface decarburization and carbide segregation. Insufficient tempering or not timely tempering of parts after quenching. Excessive cold punching stress caused by the previous process, forging folds, deep turning tool marks, sharp edges and corners of oil grooves, etc. The microstructure characteristic of quenching cracks is that there is no decarburization on both sides of the cracks, which is obviously different from forging cracks and material cracks.
4. Heat treatment deformation
During the heat treatment of precision seamless steel pipes, there are thermal stress and structural stress, and these internal stresses can superimpose or partially offset each other, which is complex and changeable. Because it can change with the heating temperature, heating speed, cooling method, cooling speed, part shape and size, and furnace loading method, so its heat treatment deformation is inevitable.
Understanding and mastering its changing law can make the deformation of bearing parts in a controllable range, which is conducive to the normal production. Of course, the mechanical collision during the heat treatment will also cause deformation of the parts, but this deformation can be reduced and avoided by improving the operation and so on.
5. Surface decarburization
During the heat treatment process, if the precision seamless steel pipe is heated in an oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the surface of the precision seamless steel pipe, resulting in surface decarburization.
If the depth of the surface decarburization layer exceeds the final processing allowance, the precision seamless steel pipe will be scrapped. The determination of the depth of the surface decarburization layer can be carried out by metallographic method and microhardness method in metallographic examination. The surface layer microhardness distribution curve measurement method shall prevail and may be used as an arbitration criterion.
Quenching microstructure overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots, etc., are mostly caused by insufficient heating, poor cooling or improper quenching operation. Precision seamless steel pipes, like surface decarburization, can cause a serious decrease in surface wear resistance and fatigue strength, and will cause local severe wear or failure during use.
