Analysis on the Causes of Surface Spalling of Work Rolls

Cold rolling rolls are important large parts of cold tandem mills. Their manufacturing process is relatively complex and the working environment is harsh. They bear the effects of friction, thermal stress, impact and other stresses. During use, they may break, peel, wear, etc., and then fail. , Increase the cost of consumables and affect production, causing economic losses. A company's stainless steel production line suddenly peeled off the surface of the cold tandem work roll during the normal rolling production process, causing a breakdown and shutdown, which seriously affected the normal production. This paper takes the peeling sample of the cold tandem rolling work roll as the analysis object, and analyzes and explores the reasons for the peeling of the roll surface through physical and chemical testing methods such as macroscopic fracture, spectral composition determination, hardness, metallographic structure, and scanning electron microscope, combined with daily use conditions.

Physical and chemical test results 

Fracture observation of spalling block

The macroscopic morphology analysis of the roll peeling block (see Figure 1). There are typical fatigue fracture characteristics on the spalling block. The obvious shell pattern line can be observed in the central circle area, which is an important feature of the fatigue growth zone; the center of the shell pattern line is the fatigue source, which is the initiation area of the fatigue crack. Fatigue can be seen The source is located inside the roll 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 unpredictability. There is no obvious sign before failure, and the damage is serious.

Observing the fatigue source area with scanning electron microscope, it is found that there are granular inclusions with a diameter of about 50μm. The EDS results show that they contain Ca, K, O, Mg, Al, Si and other elements (see Figure 2), which are large-particle oxide mixed inclusions. Things.

Chemical composition A sample is cut from the exfoliated block for spectral composition analysis. The material of the roll is 8Cr3NiMoV. The test results are shown in Table 1. Except for the Cr element which is slightly below the lower limit, the other components are all within the standard GB/T1299-2014 range. Using Rockwell hardness tester to test the hardness of the peeling block, the hardness (HRC) value reaches 64.5, and the positions on the sample are relatively uniform, which meets the requirements of the standard.

Metallographic observation 

A wire cutting machine was used to intercept the metallographic sample at the fatigue source. After measurement, the fatigue source was about 8mm away from the surface of the roll; after sample preparation and polishing, the ZEISSImager.A1m metallurgical microscope, ZEISSEVO18 scanning electron microscope and other instruments were used for observation and analysis. It is found that there are many root-shaped cracks in the spalling block (see Figure 3). The cracks originate from the location of the internal fatigue source and extend to the surface of the roll.

A comprehensive observation of the sample reveals that there are more massive inclusions in the peeled block (see Figure 4a), with a size of 90 μm, and the inclusions are seriously exceeding the standard. Using scanning electron microscope energy spectrum analysis (see Figure 4b), the bulky large-particle inclusions are mainly O, Al, and 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. It may be caused by inadequate control of the smelting process in the production of rolls.

Analysis and discussion 

Cracking process

From the macroscopic analysis, it can be inferred that the failure of roll spalling originates from internal fatigue sources. Then, under continuous cyclic stress during work, the cracks gradually propagate to form shell lines; fatigue cracks propagate to a certain extent, resulting in insufficient strength and unable to withstand rolling. The external force during the manufacturing process will eventually form a terminating zone and the surface of the roll will peel off. During the operation of the roll, due to the load of the rolling mill and the local squeezing of the roll at the contact point, the maximum combined shear stress is located in a small area below the surface of the roll. The preparation process of the roll before manufacturing and use will produce residual stress. At the same time, although the cold rolling processing temperature is low, the temperature of the roll and the strip steel will also increase under the action of friction to produce thermal stress. If strip breakage, tail flicking, overlap, slippage, etc. occur during the rolling process, the surface of the roll will be subject to local overload heat and impact stress. Because non-metallic inclusions exist in steel in the form of mechanical mixtures, and their performance is very different from steel, they destroy the uniformity and continuity of the steel matrix, and cause stress concentration at the place, which becomes a source of fatigue. In addition, during the heating process, non-metallic inclusions

The linear expansion coefficients of the object 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 peel off from the matrix at the two extremes of the maximum stress of the spherical inclusions, forming primary microcracks; Before the rapid expansion, the junction between the steel matrix and the inclusions gradually separated from the matrix and connected into a crack channel. With the increase of the number of stress cycles, the microcracks gradually expand outward along the shell of the ball torn the matrix. When the overall size of the crack exceeds the critical size that the roll can withstand, the fatigue crack enters the instability growth stage, and the surface of the roll is finally caused by the occurrence of fatigue cracks. It breaks instantly and peels off. The surface peeling of the roll has undergone the processes of crack initiation → crack propagation → peeling caused by inclusions.

The influence of inclusions on fatigue performance

The influence of inclusions on the fatigue life of the workpiece is related to the nature, size, quantity and distribution of the inclusions. Generally speaking, hard and brittle blocky or spherical inclusions that have poor connection with the matrix and do not deform, such as TiN and Al2O3, are more harmful than ductile and elongated inclusions. When the number of inclusions is large, when they are concentrated and distributed, or when they are on the surface of the part or in the high stress area, the fatigue life is most severely affected. At the same time, the influence of inclusions on fatigue performance also depends on the structure and properties of the matrix. Experiments indicate that the fatigue strength of mild steel has a smaller relationship with inclusions. As the strength of steel increases, the harmful effects of inclusions become more and more. serious. In metal materials with high hardness and high strength, the influence of inclusions on fatigue strength becomes a more prominent problem.

According to relevant research data, fatigue life is very sensitive to the size of inclusions. Reducing the size of inclusions can significantly increase fatigue life. For high-hardness and high-strength workpieces, the critical size of surface inclusions is 8-10μm, and it decreases as the hardness increases, and increases as the depth increases. Inclusions smaller than the critical size can avoid the fatigue fracture caused by the inclusions, and the fatigue performance will be better; when the inclusions are larger than the critical size, as the size of the inclusions increases, the fatigue strength and fatigue life of the steel decrease sharply. According to the literature, for high-strength steel, if the size of the inclusions is reduced by 1/3, the fatigue life will be extended by 10 times, and if the size of the inclusions is reduced by about half, the fatigue life will be extended by 100 times. At the same time, if the size of the inclusions is reduced by half, the fatigue strength can be increased by 1.12 to 1.15 times.

To sum up, the roll is under the action of complex stress during use, which produces fatigue crack sources at large-scale inclusions. With the continuous change of alternating stress, fatigue cracks propagate and generate secondary cracks, which are formed on the surface of the fracture. Fatigue bands and radial bands. When the overall size of the crack exceeds the critical size, the fatigue crack enters the instability growth stage, and the roll eventually peels off due to instantaneous fracture.

Conclusion 

(1) The surface spalling of the cold tandem rolling work roll is the fatigue fracture caused by the large-size Al2O3 brittle and hard inclusions near the surface.

(2) Strengthen the inspection of rolls. Ultrasonic testing can detect internal defects, and magnetic powder and eddy current can detect surface defects; use different methods or combinations according to the situation to ensure the quality of the rolls.

(3) Develop a scientific and reasonable roll use and maintenance system to ensure cooling and lubrication during use and prevent overheating.