Analysis on the cause of peeling of the cold rolling work roll surface

The cold rolling roller is an important large-scale component of the cold rolling unit. Its manufacturing process is relatively complex and the working environment is harsh. It is subjected to stresses such as friction, thermal stress, and impact. It may break, peel, and wear during use, and then fail, increase the cost of consumables and affect production, causing economic losses. During the normal rolling production process of a company’s stainless steel production line, the surface of the cold rolling work roll suddenly peeled off, causing a fault and shutdown, which seriously affected normal production. This paper takes the peeling sample of the cold rolling working roller as the analysis object, and analyzes and discusses the cause of roller surface peeling through physical and chemical testing methods such as macroscopic fracture, spectral component determination, hardness, metallographic structure, scanning electron microscopy, etc., combined with daily use conditions.

1. Physical and chemical test results

Observation of the fracture of the peeled block

The peeled block of the roller was subjected to macroscopic morphology analysis (see Figure 1). There are typical fatigue fracture characteristics on the peeled block. Obvious shell lines can be observed in the central circle area, which is an important feature of the fatigue extension zone; the center of the shell line is the fatigue source, which is the initiation zone of fatigue cracks. It can be seen that the fatigue source is located inside the roller rather than on the surface; the outer ring area occupies the largest area, and the macroscopic morphology is radial, which is the instantaneous fracture zone. In the failure cases of structural materials and mechanical parts, fatigue failure is different from static load failure. Most of them occur without warning and in unexpected situations. There are no obvious signs in appearance before failure, and the damage is serious.

Using a scanning electron microscope to observe the fatigue source area, it was found that there were granular inclusions with a diameter of about 50μm. EDS results showed that it contained elements such as Ca, K, O, Mg, Al, and Si (see Figure 2), and was a large-particle oxide mixed inclusion.

Chemical composition: Samples were cut from the peeling block for spectral component analysis. The roller material is 8Cr3NiMoV. The test results are shown in Table 1. Except for the Cr element which is slightly lower than the lower limit, the other components are within the standard GB/T1299-2014 range. The hardness of the peeling block was tested using a Rockwell hardness tester. The hardness (HRC) value reached 64.5, and each position on the sample was relatively uniform, meeting the standard requirements.

Metallographische Untersuchung

Metallographic samples were cut at the fatigue source using a wire cutting machine. The fatigue source was measured to be about 8 mm away from the roller surface. After sample preparation and polishing, the samples were observed and analyzed using instruments such as ZEISS Imager.A1m metallographic microscope and ZEISSE VO18 scanning electron microscope. It was found that there were multiple root-like cracks in the spalled block (see Figure 3). The cracks originated from the internal fatigue source and extended to the roller surface.

A comprehensive observation of the sample revealed that there were many block inclusions in the peeling block (see Figure 4a), with a size of up to 90μm, and the inclusions were seriously exceeded. Using scanning electron microscope energy spectrum analysis (see Figure 4b), the blocky large particle inclusions were mainly O and Al, with a small amount of Mg, S, Ca, Mn, and Fe. It can be inferred that they are mixed inclusions with Al2O3 as the main component. This may be due to the lax control of the smelting process during the production of the roll.

2. Analyse und Diskussion

Crack generation process

From the macroscopic morphology analysis, it can be inferred that the roll spalling failure originates from the internal fatigue source. Subsequently, under the continuous cyclic stress during operation, the crack gradually expands to form a shell line; the fatigue crack expands to a certain extent, resulting in insufficient strength, unable to withstand the external force during the rolling process, and finally forming a terminal break zone, and the roll surface peels off. During the operation of the roll, due to the load of the rolling mill and the local extrusion of the roll at the contact point, the maximum combined shear stress is located in a small area below the roll surface. The preparation process before the roll is manufactured and used will produce residual stress. At the same time, although the cold rolling temperature is low, the temperature of the roll and the strip will also rise under the action of friction to produce thermal stress. If the strip breaks, tail swinging, overlap, slipping, etc. occur during the rolling process, the roll surface will be subjected to local overload heat and impact stress. Since non-metallic inclusions exist in steel in the form of mechanical mixtures, and their properties are very different from those of steel, they destroy the uniformity and continuity of the steel matrix, and cause stress concentration at this location, becoming a fatigue source. In addition, during the heating process, the linear expansion coefficients of non-metallic inclusions and the matrix are different, and an additional stress field is generated in the matrix near the inclusions. Under such complex stress conditions, if there are non-metallic inclusions in the surface layer, especially brittle inclusions, they will first peel off from the matrix at the two extremes of the maximum stress of the spherical inclusions, forming primary microcracks; before the cracks rapidly expand, the junction between the steel matrix and the inclusions gradually separates from the matrix and forms a crack channel. With the increase in the number of stress cycles, the microcracks gradually expand outward along the outer shell of the ball to tear the matrix. When the overall crack size exceeds the critical size that the roller can withstand, the fatigue crack enters the unstable expansion stage, and the roller surface eventually peels off due to instantaneous fracture. The peeling of the roller surface has gone through several processes: crack initiation caused by inclusions → crack expansion → peeling.

Influence of inclusions on fatigue properties

Abschluss