The main production equipment of the carton factory is a single-sided machine, and the heart part of the single-sided machine is a corrugating roller. However, the problem of corrugating roll cracks has been plaguing manufacturers and users. In particular, it is difficult for manufacturers to renovate corrugating rolls. They often suddenly become stripped and may even injure operators during the grinding process.
How does a corrugating roll crack form? The author believes that: First, it is caused by improper heat treatment; Second, it is caused by improper installation and operation; Third, it is caused by incorrect grinding process.
First, the crack problem caused by improper heat treatment is the most serious and it causes unavoidable losses for the renovation work. Corrugated materials are usually 35CrM, 42CrM. The heat treatment method is surface induction medium frequency quenching. The highest hardness is HRC55-60. The higher the hardness, the longer the service life. If the heat treatment process is improper, there will be two kinds of quench cracks:
1. Longitudinal cracks: The corrugating roll is relatively completely hardened, causing the tangential tensile stress of the surface to become too large due to the transformation of the core of the tooth bottom to martensite. Relatively complete hardenability occurs, mainly because the quenching temperature is too high, the speed of the induction coil is slow, or the power of the intermediate frequency motor is not enough, and the margin between martensite and sorbite is too shallow, ie, the depth of the hardened layer is insufficient.
2. Transverse cracking: The corrugating roll is not hardened, and the excessive part of the hardened area and unhardened area has a large tensile stress peak, and the axial stress is greater than the tangential stress. The main reasons for undetermined hardenability are that the special treatment process is rough, the quenching temperature is not properly selected (current strength), the moving speed of the induction coil is too fast, there is no shielding measure, or the power of the intermediate frequency motor is not enough, and the critical interface between martensite and sorbite is too shallow. That is, the depth of the hardened layer is insufficient.
There are two kinds of cracks also have an influencing factor - aging cracks: that is, tempering process is not timely or not fully tempered to make more retained austenite in the tissue transformation and increase stress. As long as the heat treatment process is correct, the hardness of the corrugating roll tooth surface is HRC55-60, which can completely avoid cracks. The hardness of the teeth surface of the Wrigley's corrugated roller of the Capital Aerospace Machinery Plant was around HRC 55-60. No cracking phenomenon was found during the renovation of our company. And many other manufacturers of corrugated roll tooth surface or hardness of the pit is not enough, or there are vertical and horizontal cracks, what is the reason? The key is that the heat treatment process is not correct.
Second, the installation of incorrect cracks on the single-sided machine corrugated roller and pressure roller installation, the operation is correct or not, will directly lead to whether the corrugated roller cracks or teeth, broken tooth. Because the pressure at both ends of the pressure roller is unbalanced or the pressure is too high, the two ends of the lower corrugating roller will be partially lost and avalanche phenomenon will occur. The corrugating roller has a middle-high parameter regardless of whether the corrugating roller is newly built or the upper corrugating roller is overhauled. If the upper corrugation roller axis is not parallel to the axis of the lower corrugating roller, local cold hardening will occur (the stress concentration of the whole tooth width will cause the stress to be uneven), and the cracks will be superimposed with the heat treatment defect stress under the action of the lattice distortion stress. The corrugating roll is S-shaped, in which case both sides of the corrugated rollboard are inverted in opposite directions. This type of crack caused by cold hardening is commonly found in vapor heating methods and is generally less common in electrical heating methods. Because the temperature of the steam corrugating roll is usually around 170°C, it is lower than the cold hardening recrystallization temperature of the corrugating roll material, and the temperature of the electric heated corrugating roll is generally around 500°C. (The surface of the corrugating roll is dark brown in color.) This temperature is already greater than the recrystallization temperature, thus eliminating the cold hardening stress.
Third, grinding cracks Grinding cracks caused entirely by the burn, which is caused by the secondary martensite formed by grinding burns. It is characterized by being perpendicular to the grinding marks, showing irregular fine, dense, shallow and sparse or dense network-like cracks under the epidermis layer on the tooth surface. Grinding the burn crack is a simple and complex problem. For the new corrugating roller, it involves many kinds of stress under heat treatment residue and the first type of temper brittleness caused by chromium and molybdenum elements; In the case of refurbishing corrugating rollers, not only does the defect of the new roller also suffer the threat of cold work hardening, as long as the various stresses are less than the material's strength limit, cracks will not occur, that is, if the hardness of the quenched steel is not greater than HRC 55, it is not Grinding cracks will occur.
Grinding burns is unavoidable, because grinding generates heat at 800-1200°C, the grinding wheel absorbs about 12% of the heat, the grinding heat absorbs about 4%, and the remaining heat is absorbed by the corrugating roller. When the quenched corrugating roller is subjected to tempering at the surface layer, martensite is transformed into sorbite or troostite (both types of structures are highly diffusive pearlite). Due to the shrinking volume, the surface layer produces residual Tensile stress generates residual compressive stress in the inner layer. If the surface layer produces secondary quenching, the surface layer produces secondary quenched martensite, which is larger in volume than the tempered structure of the inner layer, and thus generates compressive stress on the surface and tensile stress in the inner layer. Grinding is generally due to higher grinding heat, often based on the tensile stress generated by phase transformation and thermoplastic deformation, so the surface layer often has residual tensile stress. When the residual stress exceeds the strength limit of the material, cracks will form on the surface of the corrugating roller. Some grinding cracks are not on the outer surface, but are defects that are difficult to detect under the surface layer. The direction of the crack is often perpendicular or mesh to the grinding direction. Cracks often occur at the same time as burns. How to avoid burns, the author thinks:
1. Improve the cooling effect: Because the surface of the rotating wheel generates a strong layer of air flow so that not much coolant can enter the grinding zone, a large number of injections are made on the surface of the machined tooth that has left the grinding zone while grinding heat. Thermal damage has been caused on the surface of the corrugating roller. Our company improvement measures:
(a) Use high pressure and large flow cooling. This not only enhances the cooling effect, but also allows the wheel surface to be flushed so that the gap is not easily blocked by cutting.
(b) In order to reduce the effect of high-pressure air flow on the surface of a high-speed rotating grinding wheel, an air baffle is installed and injected from a multi-directional grinding zone so that the cooling liquid can be smoothly injected into the grinding zone.
(c) Use alkaline coolant.
2. The choice of grinding amount, increasing the speed of the grinding wheel and the speed of the corrugating roller, increasing the grinding depth and the speed of the corrugating roller when rough grinding, will increase the degree of plastic deformation and thus increase the roughness, and finally adopt a small depth of cut or no Spark grinding to improve surface finish.
3. To improve the grinding performance of the grinding wheel: Abrasives and adhesives of the grinding wheel are extremely important in the grinding of corrugating rollers. The hardness and strength of cubic boron nitride abrasive (CBM) is slightly lower than that of diamond, but its thermal stability is good. Because of its high chemical inertness with the iron group elements, grinding of the corrugating roller does not generate scum, and the grinding heat is also low. The ground surface has a high quality. The rubber adhesive grinding wheel can prevent the surface from generating burns because the adhesive has a certain elasticity, and when the abrasive particles are subjected to excessive cutting force, they will automatically recoil and 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. It will not be elaborated here.
The above is the analysis of the crack phenomenon encountered by the author in the production practice, and adopts the corresponding technological measures to solve to prevent the formation and expansion of cracks in the corrugating roller. The cracks produced by the corrugating roll are firstly the heat treatment process problems. Whether it is a longitudinal crack or a transverse crack, penetrating cracks often cause the corrugating roll to lose teeth and break teeth; secondly, improper installation and operation cause distortion of the cold work hardening lattice. Stress and quenching stress are superimposed, and tooth loss and tooth collapse can also occur. The grinding process is incorrect. Although a burn crack occurs, it is shallow and sparse, and no penetrating cracks will occur. The tooth will not be lost or broken. (Chen Qian, General Manager, Nanjing Xiangxiang Electromechanical Technology Co., Ltd.)
How does a corrugating roll crack form? The author believes that: First, it is caused by improper heat treatment; Second, it is caused by improper installation and operation; Third, it is caused by incorrect grinding process.
First, the crack problem caused by improper heat treatment is the most serious and it causes unavoidable losses for the renovation work. Corrugated materials are usually 35CrM, 42CrM. The heat treatment method is surface induction medium frequency quenching. The highest hardness is HRC55-60. The higher the hardness, the longer the service life. If the heat treatment process is improper, there will be two kinds of quench cracks:
1. Longitudinal cracks: The corrugating roll is relatively completely hardened, causing the tangential tensile stress of the surface to become too large due to the transformation of the core of the tooth bottom to martensite. Relatively complete hardenability occurs, mainly because the quenching temperature is too high, the speed of the induction coil is slow, or the power of the intermediate frequency motor is not enough, and the margin between martensite and sorbite is too shallow, ie, the depth of the hardened layer is insufficient.
2. Transverse cracking: The corrugating roll is not hardened, and the excessive part of the hardened area and unhardened area has a large tensile stress peak, and the axial stress is greater than the tangential stress. The main reasons for undetermined hardenability are that the special treatment process is rough, the quenching temperature is not properly selected (current strength), the moving speed of the induction coil is too fast, there is no shielding measure, or the power of the intermediate frequency motor is not enough, and the critical interface between martensite and sorbite is too shallow. That is, the depth of the hardened layer is insufficient.
There are two kinds of cracks also have an influencing factor - aging cracks: that is, tempering process is not timely or not fully tempered to make more retained austenite in the tissue transformation and increase stress. As long as the heat treatment process is correct, the hardness of the corrugating roll tooth surface is HRC55-60, which can completely avoid cracks. The hardness of the teeth surface of the Wrigley's corrugated roller of the Capital Aerospace Machinery Plant was around HRC 55-60. No cracking phenomenon was found during the renovation of our company. And many other manufacturers of corrugated roll tooth surface or hardness of the pit is not enough, or there are vertical and horizontal cracks, what is the reason? The key is that the heat treatment process is not correct.
Second, the installation of incorrect cracks on the single-sided machine corrugated roller and pressure roller installation, the operation is correct or not, will directly lead to whether the corrugated roller cracks or teeth, broken tooth. Because the pressure at both ends of the pressure roller is unbalanced or the pressure is too high, the two ends of the lower corrugating roller will be partially lost and avalanche phenomenon will occur. The corrugating roller has a middle-high parameter regardless of whether the corrugating roller is newly built or the upper corrugating roller is overhauled. If the upper corrugation roller axis is not parallel to the axis of the lower corrugating roller, local cold hardening will occur (the stress concentration of the whole tooth width will cause the stress to be uneven), and the cracks will be superimposed with the heat treatment defect stress under the action of the lattice distortion stress. The corrugating roll is S-shaped, in which case both sides of the corrugated rollboard are inverted in opposite directions. This type of crack caused by cold hardening is commonly found in vapor heating methods and is generally less common in electrical heating methods. Because the temperature of the steam corrugating roll is usually around 170°C, it is lower than the cold hardening recrystallization temperature of the corrugating roll material, and the temperature of the electric heated corrugating roll is generally around 500°C. (The surface of the corrugating roll is dark brown in color.) This temperature is already greater than the recrystallization temperature, thus eliminating the cold hardening stress.
Third, grinding cracks Grinding cracks caused entirely by the burn, which is caused by the secondary martensite formed by grinding burns. It is characterized by being perpendicular to the grinding marks, showing irregular fine, dense, shallow and sparse or dense network-like cracks under the epidermis layer on the tooth surface. Grinding the burn crack is a simple and complex problem. For the new corrugating roller, it involves many kinds of stress under heat treatment residue and the first type of temper brittleness caused by chromium and molybdenum elements; In the case of refurbishing corrugating rollers, not only does the defect of the new roller also suffer the threat of cold work hardening, as long as the various stresses are less than the material's strength limit, cracks will not occur, that is, if the hardness of the quenched steel is not greater than HRC 55, it is not Grinding cracks will occur.
Grinding burns is unavoidable, because grinding generates heat at 800-1200°C, the grinding wheel absorbs about 12% of the heat, the grinding heat absorbs about 4%, and the remaining heat is absorbed by the corrugating roller. When the quenched corrugating roller is subjected to tempering at the surface layer, martensite is transformed into sorbite or troostite (both types of structures are highly diffusive pearlite). Due to the shrinking volume, the surface layer produces residual Tensile stress generates residual compressive stress in the inner layer. If the surface layer produces secondary quenching, the surface layer produces secondary quenched martensite, which is larger in volume than the tempered structure of the inner layer, and thus generates compressive stress on the surface and tensile stress in the inner layer. Grinding is generally due to higher grinding heat, often based on the tensile stress generated by phase transformation and thermoplastic deformation, so the surface layer often has residual tensile stress. When the residual stress exceeds the strength limit of the material, cracks will form on the surface of the corrugating roller. Some grinding cracks are not on the outer surface, but are defects that are difficult to detect under the surface layer. The direction of the crack is often perpendicular or mesh to the grinding direction. Cracks often occur at the same time as burns. How to avoid burns, the author thinks:
1. Improve the cooling effect: Because the surface of the rotating wheel generates a strong layer of air flow so that not much coolant can enter the grinding zone, a large number of injections are made on the surface of the machined tooth that has left the grinding zone while grinding heat. Thermal damage has been caused on the surface of the corrugating roller. Our company improvement measures:
(a) Use high pressure and large flow cooling. This not only enhances the cooling effect, but also allows the wheel surface to be flushed so that the gap is not easily blocked by cutting.
(b) In order to reduce the effect of high-pressure air flow on the surface of a high-speed rotating grinding wheel, an air baffle is installed and injected from a multi-directional grinding zone so that the cooling liquid can be smoothly injected into the grinding zone.
(c) Use alkaline coolant.
2. The choice of grinding amount, increasing the speed of the grinding wheel and the speed of the corrugating roller, increasing the grinding depth and the speed of the corrugating roller when rough grinding, will increase the degree of plastic deformation and thus increase the roughness, and finally adopt a small depth of cut or no Spark grinding to improve surface finish.
3. To improve the grinding performance of the grinding wheel: Abrasives and adhesives of the grinding wheel are extremely important in the grinding of corrugating rollers. The hardness and strength of cubic boron nitride abrasive (CBM) is slightly lower than that of diamond, but its thermal stability is good. Because of its high chemical inertness with the iron group elements, grinding of the corrugating roller does not generate scum, and the grinding heat is also low. The ground surface has a high quality. The rubber adhesive grinding wheel can prevent the surface from generating burns because the adhesive has a certain elasticity, and when the abrasive particles are subjected to excessive cutting force, they will automatically recoil and 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. It will not be elaborated here.
The above is the analysis of the crack phenomenon encountered by the author in the production practice, and adopts the corresponding technological measures to solve to prevent the formation and expansion of cracks in the corrugating roller. The cracks produced by the corrugating roll are firstly the heat treatment process problems. Whether it is a longitudinal crack or a transverse crack, penetrating cracks often cause the corrugating roll to lose teeth and break teeth; secondly, improper installation and operation cause distortion of the cold work hardening lattice. Stress and quenching stress are superimposed, and tooth loss and tooth collapse can also occur. The grinding process is incorrect. Although a burn crack occurs, it is shallow and sparse, and no penetrating cracks will occur. The tooth will not be lost or broken. (Chen Qian, General Manager, Nanjing Xiangxiang Electromechanical Technology Co., Ltd.)
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