Abstract: An analysis was conducted on the phenomenon of cracks occurring at the large end when using a single station press for open cutting and cold heading to produce tapered rollers for the spheroidized, annealed, and polished straight strip material of Φ 20.2 mm GCr15. The results indicate that this type of crack is caused by severe defects such as shear lips, collapse angles, and microcracks at the fracture surface of the material section during open cutting. During roller cold heading forming, cracks are generated at the large end of the roller due to the notch effect of these defects.
Keywords: tapered roller; Cold heading; Open type feeding; crackle; Gap effect
1. Characteristics of crack morphology
When using a single station press for open cutting and cold heading to produce tapered rollers, cracks appeared on the ring belt and end face of the roller at the large end of the GCr15 spheroidized, annealed, and polished straight strip material with a diameter of 20.2 mm. The morphological characteristics are shown in Figure 1 and Figure 2.
Figure 1 Crack morphology at the large end ring of cold heading tapered roller
Figure 2 Crack morphology on the large end face of cold heading tapered roller
After removing its ring and undergoing hot acid washing corrosion, it can be observed from the direction of fiber flow on the end face that cracks are mostly concentrated on the side of the shear lip formed by cutting and cutting. The crack was observed by grinding the sample, and the starting angle of the crack was about 45 ° from the cutting surface, gradually expanding along the longitudinal direction, as shown in
Figure 3 The morphological characteristics of crack propagation shown in Figure 2
2. Changes in the shape of the material cross-section during the cutting process
The production method adopts open cutting and cold heading to form tapered rollers, which has the characteristics of simple tooling and easy adjustment. The cutting schematic diagram is shown in Figure 4. However, the stress state during the cutting process is complex, and the disadvantages such as material strain and plastic deformation are prominent. At section A, the contact area between the movable blade and the material forms a collapsed angle, the cutting edge of the cutting cylinder forms a shear lip, the fracture surface is inclined, and the section is significantly elliptical (Figure 5, Figure 6). There are even a large number of microcracks on this section (Figure 7-9). No cracks were found at section B (Figures 10-12).
1- Activity blade; 2- Cut the cylinder; 3- Cold straightening strip material; 4- Positioning anvil
Figure 4 Schematic diagram of cutting materials
Figure 5 Schematic diagram of incision collapse angle and inclination
Figure 6 Elliptical section (the trend of ellipticization in section A is significantly greater than that in section B)
Figure 7 The morphological characteristics of section A (with obvious shear lip and brittle fracture zones, and a clear trend of ellipticization in the section)
Figure 8 Surface features of section A after hot acid washing (with numerous microcracks in the shear lip and brittle fracture zone)
Figure 9 Surface characteristics of section A after cold heading and hot acid washing (cracks on the roller end face are concentrated in the shear lip area)
Figure 10 The morphological characteristics of section B (the fracture surface is flat and brittle, and the small lunar tooth surface is caused by the scraping of the positioning anvil)
Figure 11 Surface features of section B after hot acid washing (only a small number of microcracks exist in the center of the section)
Figure 12 Surface characteristics of section B after cold heading and hot acid washing (no obvious cracks observed)
From the above picture, it can be clearly seen that the morphological characteristics of the two sections after open cutting are significantly different. The cutting lip of section A is wide and thick, with a large amount of deformation and numerous microcracks; The deformation of section B is small, and the number of microcracks is significantly less than that of section A; The location and opening direction of the cracks in the tapered roller with large end cracking after cold heading forming are consistent with the microcracks in the shear lip area of section A, indicating a high correlation between the cracks on the large end face of the tapered roller and the microcracks generated in section A.
3. Analysis of Cracking Causes
3.1 Material analysis
The size of material grain size, distribution pattern of non-metallic inclusions, strength, and plastic deformation ability; The sharpness of the cutting blade, the spacing between the cutting blade and the sleeve, and other factors can all have a certain impact on the occurrence of microcracks in the open cutting surface. The reasons for the formation of roller large end cracks are analyzed from the inspection of non-metallic inclusion distribution morphology, grain size, hardness, and open cutting process of the material.
3.1.1 Chemical Composition
The chemical composition of the rollers was determined using the OBLFGS-1000 direct reading spectrometer, and the results are shown in Table 1. According to Table 1, the chemical composition meets the requirements of GB/T 18254-2002.
Table 1 Chemical composition of rollers (mass fraction)
3.1.2 Non metallic inclusions and grain size
Samples were prepared longitudinally from the section of material in Figure 8 after hot acid washing. When observed at 100 times magnification, cracks in section A were distributed at a 45 ° angle to the shear plane, with deeper cracks reaching about 1 mm. No obvious non-metallic inclusions were found in the microcracks and crack propagation areas, as shown in Figures 13 and 14. The grain size was tested by oxidation method and judged according to GB/T 6394-2002. The grain size was classified as Grade 7, as shown in Figure 15, and the grain size of the surface and subsurface layers was basically consistent with that of the core; In addition, the micro Vickers hardness values of the surface and subsurface layers of the raw material are consistent with the micro Vickers hardness values of the core, with HV 0.2 ranging from 200 to 208, indicating that the grinding hardening phenomenon on the surface of the strip during grinding is not significant.
Figure 13 Distribution and morphological characteristics of microcracks in section A (100 ×)
Figure 14 The depth of microcrack propagation in the local area of section A is about 1 mm (100 ×)
3.1.3 Material surface crack and decarburization inspection
Metallographic specimens were prepared from the open cut section of the same batch of single station press and the cold heading tapered roller blank with large end face cracking in the transverse direction perpendicular to the crack. After corrosion with 2% nitric acid alcohol, no decarburization was found on both sides of the crack; The samples that have undergone metallographic examination were subjected to hot acid washing inspection according to GB/T 18254-2002, and no longitudinal through type material cracks were found on the surface.
Figure 15 Grain size of spheroidized annealed microstructure (100 ×)
Based on the above inspection results, it can be ruled out that the material may have excessive non-metallic inclusions, coarse grains, high hardness, and the presence of grinding hardening layers, surface cracks, and surface decarburization on the surface of the material, which may induce microcracks in the cutting surface.
3.2 Analysis of Open Cutting Process
3.2. Characteristics of stress during material cutting
The stress state during the open cutting process is complex and exhibits the characteristic of non single shear stress action. During the shearing process, the U-shaped groove of the movable blade applies positive pressure to the material contact area, causing plastic deformation of the material and forming a collapse angle; The cutting edge of the cutting cylinder is embedded in a crescent shaped cutting lip, and its size is related to the sharpness of the cutting edge and the size of the gap between the cutting edges. Secondly, the positive pressure on the U-shaped slot of the movable blade generates a torque effect, which causes the expansion direction of the shear lip to deviate, resulting in the stress components of shear stress and compressive stress constantly changing with the cutting of the material segment, providing conditions for a large number of microcracks to initiate and propagate in the lip area. It can be seen that the open cutting surface has disadvantages such as large deformation, inclined shear surface, and dense microcracks.
3.2.2 Deformation characteristics of cold heading process
The use of a pressure machine for cold heading tapered rollers results in cracks concentrated in the lip area of section A at the chamfer R of the large end and large end surface, while crack propagation is rare in section B. The reason is that: (1) microcracks are dense in the lip area of section A, and the crack propagation direction is perpendicular to the material surface; The large deformation in the shearing lip and collapsed corner area, as well as the reduced plastic deformation ability of the material due to cold work hardening. (2) In the stamping process, the material must be filled with large plastic deformation to fill the cavity and annular space between the concave and convex molds. Multi directional stress produces a notch effect at the sharp corners of microcracks, which promotes the expansion and cracking of microcracks at stress concentrated areas such as the large end, the chamfer R of the large end surface, and the annular zone. (3) During the forming process, the small end of the roller tends to shrink inside the conical die, and the circumferential and positive compressive stresses have a restraining and suturing effect on microcracks, usually without crack propagation.
4. Conclusion
The open cutting surface has disadvantages such as large elliptical deformation, collapse angle, and inclined cutting surface, which are the main factors that promote the generation of a large number of microcracks on the fracture surface; The expansion of microcracks during cold heading is the main cause of cracks in the large end and chamfer R of tapered rollers. There is a significant difference between the two ends of the material section during open cutting, with a large amount of deformation and a large number of radial distributed microcracks on the A-section containing collapse angles and shear lips. In production, it is customary to use the A-section as the large end face of the roller for cold heading forming, which often leads to the expansion of microcracks until open cracks appear. When cold heading, if the A section is turned towards the concave mold cavity, the stress state during the forming of this section can be changed, which can suppress and suture microcracks and avoid crack occurrence.
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