Analysis of Surface Cracks on Work Roll Forgings

The working roll is an important component of the rolling mill. It is in direct contact with the billet during the rolling process and is subjected to high temperature, high speed, high load and harsh working environment. After quenching and tempering heat treatment, small cracks were found on the surface of the working roll. In order to determine the nature of the cracks and the cause of the cracks, the working roll was inspected and analyzed.

Technical requirements for working rolls

The material of the working roll is X40CrMoV51, and the production process is: ingot casting → forging → annealing → rough machining → quenching and tempering → semi-finishing → stress relief → finishing. The technical requirements are:
⑴ Quenching and tempering, the surface hardness requirement is 37 ~ 58HRC.
⑵ UT test according to JB/T 5000.15-2007 Level II.
⑶ The steel ingot adopts electric arc furnace + refining + vacuum degassing. The chemical composition of the working roll is shown in Table 1, [H] ≤ 2.5ppm, [O] ≤ 40ppm, [N] ≤ 70ppm, and the size of the working roll is shown in Figure 1.

Working roll production process

Forging uses a 4t steel ingot, which is heated to 1200℃ and kept warm for 3h. After blanking, it is sawn and forged. The forging process adopts the process of upsetting first and then drawing and forming. After forging, it is normalized to eliminate residual stress, reduce hardness, and facilitate rough processing. The heat treatment curve after forging is shown in Figure 2, and the tempering curve of forgings is shown in Figure 3.

Physical and chemical testing

⑴ Macro analysis: There are two small cracks on the surface of the working roll, distributed along the axial direction, the longest one is about 8mm, and the surface is obviously open.

(2) Chemical composition analysis. Samples were taken from the cracks to test the chemical composition. The test results are shown in Table 2. Trace elements [H] ≤ 0.78 ppm, [O] ≤ 15.1 ppm, [N] ≤ 65 ppm. The chemical composition meets the user’s requirements.

Table 2 Chemical composition analysis (mass fraction, %)

⑶ Sulfur print test. The sulfur print test result is level 2.5. The test results show that the sulfur segregation of the working roll forging is relatively serious.
(4) High and low magnification tests. The results of the low magnification test analysis are shown in Table 3. It can be seen from the table that the forgings have typical slag inclusion defects, are unqualified products, and have obvious dendrite segregation, see Figures 4 to 7.

Table 3 Macroscopic test analysis results

The test results show that the general spot segregation of the work roll macrostructure is unqualified at level 2.5. The general spot segregation is mainly distributed in the core area of ​​the test piece. The general spot segregation is usually caused by poor crystallization conditions of the ingot. The molten steel cools slowly during the crystallization process, resulting in component segregation. There is coarse dendrite segregation at R/2, the forging surface is not fully forged, and the coarse dendrites in the cast state of the ingot are not broken. The results of the macroscopic inspection are shown in Table 4 and Figures 8 to 13.

The high-magnification test results show that the purity of the forgings is within the qualified level range, but there are a large number of TiN inclusions, which will have a certain impact on the mechanical properties; the structure is tempered troostite or tempered troostite + bainite, which is a normal quenched and tempered structure, but the structure distribution is uneven and there is serious banded segregation, indicating that normalizing was not carried out or normalizing was insufficient before quenching and tempering, and the serious banded structure has a certain impact on the tangential properties of the forgings.

⑸ Fractographic test. A new fracture was made on the working roll test piece, and it can be seen that the fracture is fibrous + a small amount of lamellar. The lamellar fracture is caused by multiple parallel non-metallic inclusions. After the fracture, a bluing test was performed, and the level was 1A/2A, which is within the qualified level range, as shown in Figures 14 and 15.

Failure Analysis

From the transverse specimen observation of the small crack on the surface of the working roll, it can be seen that the crack is open and shallow in depth. The bottom of the crack is forked and not round and blunt. There are no inclusions filling the cracks and inside the cracks. The boundaries on both sides of the cracks are not smooth. From the longitudinal specimen observation, it can be seen that the cracks are of different widths and intermittently forked. There are no inclusions filling or accompanying the cracks. Many fine cracks can be seen on one side of the coarser crack that are connected to the main crack. After the sample is etched with 4% nitric acid alcohol solution, there is no oxidation and decarburization on both sides of the crack.

⑴ The chemical composition test of the working roll is qualified, indicating that the material meets the requirements.

(2) The results of low-power acid leaching test, blue fracture and micro-inclusion test all meet the requirements, indicating that the purity of the working roll is qualified, but the general point segregation level 2.5 in the low-power acid leaching test is unqualified. The cause of the point segregation is generally believed to be the poor crystallization conditions of the ingot. The molten steel cools slowly during the crystallization process, resulting in component segregation. However, when there are a large amount of gas and inclusions, the point segregation will be more serious. The existence of point segregation will significantly reduce the plasticity and fatigue strength of the steel.

There is obvious coarse dendrite segregation in the R/2 area outside the specimen. Severe dendrite segregation can cause segregation and unevenness of micro-region components in the steel. Usually, the dendrite segregation is not graded. If there are special requirements, severe dendritic segregation is not allowed. These indicate that the forging is insufficient, the coarse dendrites in the cast ingot are not broken, and the density and uniformity of the forging are not effectively improved.

⑶ The microstructure of the working roll is tempered troostite or tempered troostite + bainite, which is a normal quenched and tempered structure, but the distribution is uneven, especially there is serious banded segregation in the core, indicating that normalizing treatment was not carried out before quenching and tempering or the normalizing treatment was insufficient. The serious banded structure will have a certain impact on the tangential properties of the forging.

⑷ There are no inclusions filling or accompanying the cracks beside or inside the surface cracks, and the cracks are of different widths and intermittent bifurcations. There is no oxidation and decarburization on both sides of the cracks. The cracks on the longitudinal specimens are basically located in the darker strips of banded segregation. These characteristics indicate that the small cracks are neither inclusion cracks nor typical forging folds or forging stress cracks. The cracks occur during the quenching and cooling process after quenching heating and equalization.

Therefore, the small cracks on the surface of the working roll are most likely forging cracks formed during the forging process. The original cracks are affected by the combined factors of thermal stress and structural stress during the quenching and cooling process, and the original crack area is processed away in the subsequent cold working process, leaving only its extended part, that is, the cracks are not completely processed away.

Improvement measures

⑴ Improve the purity of steel ingots and adjust the content of titanium during smelting.

⑵ Modify the forging process to two upsetting and two drawing, increase the forging ratio, the total forging ratio is not less than 6, the drawing ratio is not less than 4, and the repeated upsetting and drawing process is adopted to improve the anisotropy of the forging, avoid the serious banded structure, break up the dendritic segregation of the ingot, eliminate the cast structure, uniform the structure, and ensure the density of the forging.

⑶ Extend the holding time of normalizing treatment to ensure sufficient organizational transformation.

(4) After rough machining, perform flaw detection and magnetic particle inspection to grind off small cracks in time to avoid stress concentration at the cracks during subsequent quenching and tempering treatment, which may cause cracking of forgings.

निष्कर्ष

After the smelting and forging process of the work roll forgings were improved, the metallographic inspection and performance inspection met the user’s requirements, the flaw detection met the requirements of JB/T 5000.15-2007 Level II, the qualified rate reached 100%, and no cracks or other defects appeared, meeting the delivery conditions. In the process of solving the crack problem of the work roll forgings, we have accumulated relevant experience, formed a set of standardized process flow for the production of work rolls, and improved the quality of forgings.

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