Causes and Treatment of Surface Cracks in Corrugating Rolls

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? There are three answers: one is caused by improper heat treatment; the other is due to improper installation and operation; and third, incorrect grinding process.

First, the crack problem caused by improper heat treatment is the most serious

It caused unavoidable damage to the renovation work. Corrugated material is 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: Corrugated roll is relatively completely quenched, resulting in the transformation of the core of the bottom of the tooth to martensite, resulting in excessive tangential tensile stress on the surface. 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 it more retained austenite, the organization of the transformation of increased 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

The installation of the corrugated roller and the pressure roller on the single-sided machine, whether or not the operation is correct, will directly cause the corrugating roller to crack or lose teeth and collapse. Because the pressure at both ends of the pressure roller is unbalanced or the pressure is too high, it will cause the local bottom of the lower corrugating roller to fall off and collapse.

The corrugating roll has medium and high parameters regardless of whether the corrugating roll is newly made or rebuilt. 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 crack

Grinding cracks are completely caused by burns, which are 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 two elements of chromium and molybdenum.

For refurbishing corrugating roller, it not only has the effect of new roller defects but also is threatened by cold work hardening. As long as all kinds of stress are less than the strength limit of the material, cracks will not occur, that is, the hardness of hardened steel is not greater than HRC55. No grinding cracks. 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?