After rough machining, a large ring manufactured by a certain company was dissected and analyzed for holes and defects found on its surface. The results indicate that many internal porosity defects and leakage cause the surface porosity defects of large ring components. In contrast, the internal defects of the ring components are caused by low-temperature cold forging.
1. Experimental analysis
1.1 On site macro analysis
The rings are placed in the open air, and the surface has been corroded. Many holes are densely distributed on a section of the circular surface about 50cm long, ranging in size from 2mm to 15mm, with irregular shapes, mainly concentrated in the middle part of the circular surface, as shown in Figure 1.
Figure.1 Physical object of large ring components
1.2 Analysis of low magnification experiments
Anatomically dissect the ring components, and take cross-sectional and circumferential specimens for experimental analysis. After dissecting the defect area, many densely distributed pores were found. After hot acid immersion, it was found that the pores were mainly concentrated in the middle part within a range of about 70mm, and the pores were connected in one piece, as shown in Figure 2.
Manually open the fracture surface, the matrix is a crystalline fracture surface, and the defects in the pores are adhered to by black and red rust products. The texture of the hole is geometric in shape, and there are friction marks on the cross-section, as shown in Figure 3.
Crystalline morphology belongs to a normal fracture surface without overheating or burning characteristics. The friction marks at the holes are one of the characteristics of forging internal cracks.
Figure.2 Macroscopic morphology
Table.1 Chemical composition analysis results (mass fraction, %)
1.4 Chemical composition analysis
The results of the chemical composition analysis are shown in Table 1.
Figure.3 Fracture morphology
Figure.4 Crack morphology
Table.2 Results of non-metallic inclusion detection
|Class A||Class B||Class C||D级||Class DS|
The chemical composition analysis results indicate that the material meets the requirements of C-grade steel.
1.5 Microstructure Analysis
Observing the polished state, the steel has non-metallic inclusions, rated according to GB/T10561-2005 “Determination of non-metallic inclusion content in the steel.” The results are shown in Table 2.
The detection results of non-metallic inclusions in steel meet the requirements.
After being etched in a 4% nitric acid alcohol solution, microscopic observation shows that the microstructure is pearlite + fine ferrite along the grain, with a grain size of 0.
1.6 Microscopic observation of cracks
Observing the defects in the holes, it was found that the geometric characteristics of the holes are obvious, and cracks continue to propagate at the sharp geometric corners. No inclusion features were found on the inner wall or internal oxidation particle features. After being etched with a 4% nitric acid alcohol solution, the inner wall of the pores does not decarburize, and the transgranular expansion morphology of the pores can be observed. The tearing characteristics of the same grain can be observed in areas with crack morphology. In contrast, in finer cracks, the morphology of the crack penetrating the pearlite layer can be observed, as shown in Figure 4.
Figure.5 SEM Morphology of Holes
The experimental results indicate that the structure belongs to the forged structure with coarser grains. The transgranular characteristics of the inner wall of the hole indicate that the cracking period is below the phase transition temperature point, which is formed when the rolling ring temperature is lower than Ar1.
1.7 Scanning electron microscopy fracture analysis
Under scanning electron microscopy, it was found that the matrix exhibited typical river like cleavage fractures. The edge of the hole defect shows a few shaped transgranular features with friction marks on them. At the bottom of the hole, obvious angular cracking patterns with certain displacement and deformation are observed. In addition, rounded crystal features were found in very few pore areas, as shown in Figure 5.
The experimental results indicate that most of the cracking in the pore area belongs to stress brittle cracking. Very few rounded holes are characterized by loose defects in raw materials or internal crack surfaces that crack during early forging.
- (1) The chemical composition of the raw materials meets the requirements of C-grade steel, and no macroscopic defects, such as inclusions, were found.
- (2) Ring forging adopts Φ 495mm round steel forged, the material selection is thin, the height diameter ratio is large, and the drum shape easily appears in the upsetting process, which requires repeated straightening. When the reduction rate is not well controlled in the forging process, it is easy to cause internal cracks in the center due to Shear stress.
- (3) After the ring is rolled, the grain size of the ring is relatively large, which is usually caused by the phenomenon of high heating temperature or long insulation time during the heating of the ring. However, the normal area of the macroscopic fracture is a crystalline fracture, and no melt pits are found in the microstructure. Cracking is transgranular, indicating no obvious overheating or burning phenomenon in the ring.
- (4) The geometric cracking characteristics and pearlite layer tearing characteristics at the defects of the ring holes indicate that the formation time of a large number of macroscopic hole defects is below Ar1 during the rolling ring process, which is formed by continuing low-temperature cold forging after the phase transformation process is completed. The possible reasons for the formation of low-temperature cold forging include local cooling caused by abnormal water spraying during ring rolling or incomplete heat penetration at the core of secondary rolling.
- (5) The deformation speed significantly impacts low-plastic materials. The metal flow pattern during ring rolling is similar to the elongation process, with good edge metal flow and poor core. When the rolling ring undergoes low-temperature cold forging, the plastic deformation ability of the core metal is poor, especially when the deformation speed of the rolling ring part is too fast, which will exacerbate the generation of core defects. Therefore, appropriate forging processes should be selected based on the characteristics of the raw materials to prevent forging cracks from occurring.
Many internal porosity defects and leakage cause the surface porosity defects of large ring components, while the internal defects of the ring components are caused by low-temperature cold forging.
Author: Wang Xudong