Crack analysis of corrugating roller

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The main production equipment of the carton factory is the single-sided machine, and the heart part of the single-sided machine is the corrugated roll. However, the problem of corrugating roller cracks has been puzzling manufacturers and users, especially the manufacturers who renovate the corrugating roller. During the grinding process, a strip of teeth will be suddenly worn off, and even the operator will be injured

how is the corrugating roller crack formed? The author thinks that: first, it is caused by improper heat treatment; Second, it is caused by incorrect installation and operation; Third, the grinding process is not correct

I. the crack caused by improper heat treatment is the most serious problem, which causes unavoidable losses to the renovation work. Corrugated materials are usually 35crm and 42crm. The heat treatment method is surface induction medium frequency quenching. The highest hardness reaches HRC. The higher the hardness, the longer the service life. If the heat treatment process is improper, two kinds of quenching cracks will occur:

1 Longitudinal crack: the corrugating roller is relatively completely quenched, and the center of the tooth bottom is transformed into martensite, resulting in excessive tangential tensile stress on the surface. The relatively complete quenching is mainly caused by the high quenching temperature, the slow moving speed of the induction coil, or the insufficient power of the medium frequency motor, and the shallow critical value of martensite and sorbite, that is, the depth of the quenching layer is insufficient

2. Transverse crack: the corrugating roll is not quenched thoroughly, and there is a large tensile stress peak at the transition between the hardened zone and the unhardened zone, and the axial stress is greater than the tangential stress. The main reasons for the lack of quenching are that the special treatment process is rough, the quenching temperature is improperly selected (current intensity), the induction coil moves too fast, there is no shielding measure, or the power of the medium frequency motor is insufficient, and the critical surface of martensite and sorbite is too shallow, that is, the depth of the quenching layer is insufficient

there is also an influencing factor for the occurrence of two kinds of cracks - aging cracks: that is, the tempering process is not timely or not fully tempered, resulting in the transformation of more residual austenite and the increase of stress. As long as the heat treatment process is correct and the hardness of the corrugating roll tooth surface is at HRC, cracks can be completely avoided. In case of oil leakage, the tooth surface hardness of Shenjian corrugated roll of capital Aerospace Machinery Factory is about HRC, and our company has not found a crack during the renovation process. However, a few years ago, the Middle East region was able to import a part of the corrugated roller teeth of many other manufacturers. Either the ethane surface was not hard enough, or there were longitudinal and transverse cracks. What is the reason? The key is the incorrect heat treatment process

II. Cracks caused by incorrect installation. Whether the installation of corrugating roll and pressing roll on the single-sided machine is correct or not will directly lead to whether the corrugating roll has cracks or teeth falling or breaking. Because the pressure at both ends of the pressing roller is unbalanced or too high, it will cause local tooth falling and tooth collapse at both ends of the lower corrugating roller; The corrugating roll has medium and high parameters whether it is newly built or renovated. If the axis of the upper corrugating roll is not parallel to the axis of the lower corrugating roll, local cold work hardening will occur (stress concentration will be caused by uneven stress on the whole tooth width). Under the action of lattice distortion stress, cracks will be generated by superposition with the stress of heat treatment defects. The cracks are S-shaped on the corrugating roll. At this time, both sides of the corrugating roll paperboard are asymmetrically inverted in the opposite direction. This kind of crack caused by cold work hardening generally exists in steam heating mode, and rarely occurs in electric heating mode. Because the temperature of the steam corrugated roller is usually about 170 ℃, which is lower than the cold work hardening recrystallization temperature of the corrugated roller material, while the temperature of the electric heating corrugated roller is generally about 500 ℃. (the surface color of the corrugating roller is black brown), which is higher than the recrystallization temperature, thus eliminating the cold work hardening stress

III. grinding cracks grinding cracks are completely caused by burns, that is, secondary martensite formed by grinding burns. It is characterized by irregular fine, dense, shallow and sparse or dense complex cracks under the surface layer of the tooth surface perpendicular to the grinding trace. Grinding burn crack is a simple and complex problem. For the newly built corrugating roller, it involves a variety of stresses under the residual heat treatment and the first kind of temper brittleness caused by chromium and molybdenum; For the refurbished corrugating roll, it is not only affected by the defects of the new roll, but also threatened by cold work hardening. As long as various stresses are less than the strength limit of the material, there will be no cracks, that is, as long as the hardness of quenched steel is not greater than HRC55, there will be no grinding cracks

grinding burn is unavoidable, because the heat generated by grinding is at ℃, and the heat absorbed by grinding wheel accounts for about 12%, and the heat absorbed by grinding accounts for about 4%. All the remaining heat is absorbed by the corrugating roller. When grinding quenched corrugating rollers, if tempering occurs in the surface layer, martensite will be transformed into sorbite or troostite (both of which are pearlite with high degree of diffusion). Due to volume reduction, the surface layer will produce residual tensile stress and the inner layer will produce residual compressive stress. If secondary quenching occurs in the surface layer, secondary quenching martensite will occur in the surface layer, and its volume is larger than the tempering structure of the inner layer. Therefore, compressive stress will occur on the surface and tensile stress will occur in the inner layer. Generally, the grinding heat is high, and the tensile stress caused by phase transformation and thermoplastic deformation is the main stress, so the surface layer often has residual tensile stress. When the residual stress exceeds the strength limit of the material, cracks will appear on the surface of the corrugating roller. Some grinding cracks are not on the outer surface, but are hard to find by naked eyes under the surface layer. The direction of the crack is usually perpendicular to or shaped like the grinding direction, and the crack often exists at the same time with the burn. How to avoid the burn, the author thinks:

1 Improve the cooling effect: as the rotating grinding wheel surface produces a strong atmospheric flow layer, not much coolant can enter the grinding area, so a large amount of coolant is sprayed on the machined tooth surface that has left the grinding area. At this time, the grinding heat has entered the corrugated roller surface, causing thermal damage. Our company's improvement measures:

(a) adopt high-pressure large flow cooling. This can not only enhance the cooling effect, but also wash the surface of the grinding wheel, so that the gap is not easy to be blocked by cutting

(b) in order to reduce the effect of high-pressure attached air flow on the surface of the high-speed rotating grinding wheel, an air baffle is installed and injected into the grinding area from multiple directions, so that the coolant can be injected into the grinding area smoothly

(c) use alkaline coolant

2. The selection of grinding parameters can increase the speed of grinding wheel and corrugating roller. Increasing the grinding depth and the speed of corrugating roller during rough grinding will increase the degree of plastic deformation and increase the roughness. Finally, small cutting depth or non sparking grinding is used to improve the surface finish

3. Improve the grinding performance of the grinding wheel: the abrasive and adhesion of the grinding wheel play an important role in many application fields. The binding agent is very important in the grinding of the corrugating roller. The hardness and strength of cubic boron nitride abrasive (CBM) are slightly lower than that of diamond, but its thermal stability is good, and its chemical inertness with iron group elements is high. Therefore, the grinding corrugating roller does not produce adhesive chips, and the grinding heat is low. The ground surface is of high quality. Rubber 1. The main machine adopts the exchange servo electromechanical and driver as the power source of the grinding wheel. The adhesive can prevent the surface from being burned, because this adhesive has certain elasticity. When the abrasive grains are subjected to excessive cutting force, it will automatically yield to reduce the grinding depth. In addition, the correction of the grinding wheel is also very important, that is, the sharpness of the grinding wheel depends on the grinding depth and the speed of the corrugating roller. Not here

the above is the analysis of the crack phenomenon encountered by the author in the production practice, and the corresponding process and technical measures have been taken to prevent the formation and expansion of the corrugating roll crack. The cracks produced by the corrugating roll are firstly caused by the heat treatment process. No matter the longitudinal cracks or the transverse cracks, they are often penetrating cracks that cause the corrugating roll to lose teeth and collapse teeth; Secondly, incorrect installation and operation lead to superposition of lattice distortion stress and quenching stress of cold work hardening, and tooth dropping and tooth collapse will also occur. Incorrect grinding process may cause burn cracks, but it is shallow and sparse, so there will be no penetrating cracks, and there will be no tooth dropping and tooth collapse. (chenqian, general manager of Nanjing Xiaoxiang Electromechanical Technology Co., Ltd.)

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