Analysis of transverse cracks in forging of backup rolls

Cracks are currently a prominent issue affecting the development of forging production and the quality of forgings, and are also a focus of research and discussion in the forging industry. With the research on new materials and forging processes, cracks in forging also show many different forms, such as dense cracks, longitudinal cracks and transverse cracks. In 2019, vertical cracks were more serious during forging and upsetting of VC backup rolls products, making manual cleaning difficult. By optimizing the production process, improving inclusion control and improving the surface quality of the steel ingot, the cracks have been significantly improved, but the problem of transverse cracks still exists in the forging process.

The company’s VC steel ingot are heated by natural gas, with a heating temperature of 1240~1260°C. The tonnage of VC steel ingot is relatively large, generally more than 50t. It is forged with three fires, with the first fire rising and pressing the clamp; the second fire pier is thickened and lengthened. ; The third fire finished product is forged. Generally, dense transverse cracks will appear on the surface of VC steel ingots after the second fire, and are more obvious on one side of the surface. However, such cracks are not obvious during the roughening process. After the second fire elongation, the dense cracks on the surface of the steel ingot are as shown in Figure 1.

Figure 1 The state of dense cracks on the surface of steel ingot

The steel ingot was forged in the second fire, loaded into the furnace and annealed, and then the surface was sampled and tested.

Process review

It is generally believed that the occurrence of transverse cracks or area cracks is directly related to two aspects.

1) If the residual elements exceed the standard, including Cu and As, etc., area cracks will appear during the upsetting or drawing process. Therefore, the areas with severe cracks were sampled, and the spectral analysis results are shown in Table 1.

Table 1 Spectral analysis results of severe crack areas (mass fraction) (%)

PSCuAsSbSn
0.0080.0030.050.0050.0020.002

After chemical composition testing, no obvious harmful elements were found to exceed the standard.

2) Overburning caused by excessive forging heating temperature. In the early days, the forging branch experienced serious longitudinal cracking problems caused by overburning on one side during the heating process of the steel ingot. The steel ingot suffered serious losses after forging. However, no cracks were found during the upsetting process of this steel ingot, and the heating temperature was not found to exceed the standard when checked back.

Forging technology of high-speed tool steel

Metallographic analysis

(1) Detect the metallographic phase at the normal position where there are no cracks on the surface of the steel ingot. The depth of the polished bright spot is 1~2mm. The structure is pearlite + a small amount of carbide, and no inclusions are found, as shown in Figure 2.

Figure 2 Metallographic inspection of crack-free locations on the surface

(2) Defect location: Detect the metallographic structure in the area with severe cracks on the surface of the steel ingot (3 points in total). The depth of the polished bright spot is 4~5mm. The structure is ferrite and no inclusions are found, as shown in Figure 3.

Figure 3 Metallographic structure detection in severe areas

During on-site metallographic inspection, it was found that the entire field of view was composed of ferrite structure, no pearlite structure was found, and the depth of the decarburization layer was obviously deeper than the normal part.

(3) Defect location sampling: Sampling the area with severe cracks on the surface of the steel ingot, the location is shown in Figure 4.

Figure 4 Surface cracks in steel ingot

High-magnification metallographic examination was performed on the sample taken, as shown in Figure 5. The marked area is ferrite structure. Use a ruler to measure, as shown in Figure 6, the ferrite structure near the crack (the bright spot area in Figure 5) has a depth of 9~10mm and a width of about 10mm.

Figure 5 The marked area is ferrite structure

Figure 6 Ferrite structure depth

Comparing the two photos of Figure 7 and Figure 8, it can be seen that the size of the ferrite tissue area on both sides of the crack is obviously different, and except for the area marked in Figure 5 (red line), which is the ferrite structure, the others are pearlite structure. . The metallography of the sample shows that the grain size of the ferrite region is 6 to 10 levels. Re-detect the bright spot depth of the ferrite structure on the surface of the steel ingot, polish it to 10~12mm, and detect the pearlite structure.

Figure 7 Microstructure changes at the crack location

Figure 8 Ferrite structure

Electron microscopy analysis

After the right side of the crack (marked by the black line in the lower part) is crushed, the microscopic morphology of the fracture surface is detected, as shown in Figure 9. It can be seen from Figure 9 that the metallic luster of the pearlite structure area and the ferrite structure area are obviously different. Conduct electron microscopy inspection of the fracture position, as shown in Figure 10.

Figure 9 Crack morphology

Figure 10 Fracture position detection

The fracture photos show that there is a suspected stone-like fracture morphology in Figure 10a. In Figure 10b, no cleavage structure is found on the fracture in the pearlite area, but the fracture shows signs of passivation.

Analysis

1) On the side with serious transverse cracks on the surface of the steel ingot, a total of 5 points were detected by on-site metallography (2 points at the normal position and 3 points at the crack position). It was found that the two points at the normal position are all pearlite structures, while the three points at the crack position are all ferrite structures, indicating that the surface layer of the area where transverse cracks exist on the surface of the steel ingot is ferrite structure.

2) The fracture surface of the sample shows a cleavage structure that is not a brittle fracture surface, and both the ferrite and pearlite areas show signs of overheating.

3) The oxide scale found in the cracks in the metallographic examination may be caused by poor fluidity due to problems with the surface structure of the steel ingot during the roughening process. During subsequent elongation, part of the oxide scale may be pressed into the cracks.

Conclusion  

There is a 10mm deep ferrite structure in the transverse crack area on the surface of the steel ingot, and the high-temperature forging performance is poor, resulting in the formation of dense transverse cracks on the surface. The main reason is that there is a certain relationship between the corresponding positions of the steel ingot and the burner and the local atmosphere of the furnace. It is necessary to standardize the position of the steel ingot and regularly test the uniformity of the furnace atmosphere.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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