Cracks are a prominent problem that currently affects the development of forging production and the quality of forgings, and are also the focus of research and discussion in the forging industry. With the research of new materials and forging processes, cracks in forging also present many different forms, such as dense cracks, longitudinal cracks and transverse cracks.
Crack Phenomenon Explanation
In 2019, the longitudinal cracks of the support roller products were more serious during forging and upsetting, and it was difficult to clean them manually. After optimizing the production process, improving the control of inclusions and the surface quality of the steel ingots, the cracks were significantly improved, but the transverse crack problem still existed in the forging process.
The company’s steel ingots are heated by natural gas at a temperature of 1240~1260℃. The tonnage of the steel ingots is large, generally above 50t, and three-fire forging is adopted. The first fire is forging the riser with a clamp; the second fire is forging and stretching; the third fire is forging the finished product. Generally, after the steel ingot is stretched in the second fire, dense transverse cracks will appear on the surface, and they are more obvious on one side, while such cracks are not obvious during the roughening process. After the second fire stretching, the dense crack state on the surface of the steel ingot is shown in Figure 1.
After the steel ingot was forged in the second round and annealed in the furnace, the surface was sampled and tested.
Process review
It is generally believed that the appearance of transverse cracks or area cracks is directly related to two aspects.
- Excessive residual elements, including excessive Cu and As, will cause area cracks during upsetting or drawing. Therefore, samples are taken from areas with severe cracks and spectral analysis is performed. After chemical composition testing, no obvious harmful elements exceeding the standard were found.
- Overheating caused by over-high forging heating temperature. In the early days, the forging plant had a serious longitudinal cracking problem caused by overheating on one side during the heating process of the steel ingot. The steel ingot suffered serious losses after reforging. However, no cracks were found in the upsetting process of this steel ingot, and the heating temperature was not found to exceed the standard after checking.
Forging Technology of High Speed
Metallographic analysis
- The metallographic structure was detected at the crack-free position on the surface of the ingot in the normal position. The polished bright spot depth was 1~2mm. The structure was pearlite + a small amount of carbide, and no inclusions were found, as shown in Figure 2.
- The defect location was in the area with severe cracks on the surface of the ingot. The metallographic structure was detected (3 points in total). The polishing bright spot depth was 4~5mm. The structure was ferrite and no inclusions were found, as shown in Figure 3.
During on-site metallographic testing, it was found that the entire field of view was filled with ferrite structure, no pearlite structure was found, and the depth of the decarburization layer was significantly deeper than that of the normal area. - Sampling of defect positions The areas with severe cracks on the surface of the ingot were sampled, and the positions are shown in FIG4 .
The sample was subjected to high-magnification metallographic examination, as shown in Figure 5. The marked area is ferrite structure. Using a ruler to measure, as shown in Figure 6, the ferrite structure near the crack (the bright spot area in Figure 5) is 9-10 mm deep and about 10 mm wide.
Comparing the two photos in Figure 7 and Figure 8, it can be seen that the sizes of the ferrite tissue areas on both sides of the crack are obviously different, and the area marked in Figure 5 (red line) is ferrite tissue, and the rest are pearlite tissue. The metallographic structure of the sample shows that the grain size of the ferrite area is 6~10. The bright spot depth of the ferrite tissue was re-detected on the surface of the ingot, and the pearlite tissue was detected after grinding to 10~12mm.
Electron microscopy analysis
After the right side of the crack (marked by the black line at the bottom) was broken, the microscopic morphology of the fracture was detected, as shown in Figure 9.
It can be seen from Figure 9 that the metallic luster of the pearlite structure area is obviously different from that of the ferrite structure area. The fracture position is detected by electron microscope, as shown in Figure 10.
The fracture photos show that a suspected stone-like fracture morphology appears in Figure 10a, and no cleavage structure is found in the fracture of the pearlite area in Figure 10b, but the fracture shows signs of blunting.
Analyze
- On the side of the ingot surface with severe transverse cracks, a total of 5 points were detected by metallographic examination (2 points in normal position and 3 points in crack position). It was found that the 2 points in normal position were pearlite structures, while the 3 points in crack position were ferrite structures, indicating that the surface layer of the area where the transverse cracks existed on the surface of the ingot was ferrite structure.
- The fracture of the sample showed a cleavage structure that was not a brittle fracture, and both the ferrite and pearlite areas showed signs of overheating.
- The oxide scale found in the crack during metallographic examination may be due to poor fluidity caused by the surface structure problem of the ingot during the roughening process. During the subsequent drawing, part of the oxide scale may be pressed into the crack.
There is a 10mm deep ferrite structure in the transverse crack area on the surface of the steel ingot. The high-temperature forging performance is poor, resulting in dense transverse cracks on the surface. The main reason is that there is a certain relationship between the corresponding position 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 check the uniformity of the furnace atmosphere.
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