Analysis of the Causes of Cracks at the Large End of Cold-Headed Tapered Rollers

Abstract

The causes of cracking at the large end of cold-headed tapered rollers were analyzed. The tapered rollers were manufactured from Φ20.2 mm GCr15 spheroidize-annealed bright straight bars using a single-station press for open-die blanking. The results show that the cracks at the large end of the rollers are mainly caused by severe defects, such as shear lips, collapse/rollover at the edge, and microcracks, generated on the cut surface during the blanking process. During cold heading, these defects act as stress concentrators, resulting in cracks at the large end of the rollers.

Keywords: cold heading; tapered roller; large end; crack; blanking; notch effect

1 Characteristics of Crack Morphology

Φ20.2 mm GCr15 spheroidize-annealed bright straight bars were used for cold heading of tapered rollers. During cold heading, cracks were observed at the edge of the large end and on the large-end face of the rollers. The morphological characteristics of the cracks are shown in Figures 1 and 2.

Figure 1 Crack morphology at the edge of the large end of a cold-headed tapered roller

Figure 2 Crack morphology on the large-end face of a cold-headed tapered roller

After removing the cracked area by grinding, it was observed that the cracks originated from the shear lip on the cut surface and extended gradually in the longitudinal direction. As shown in Figure 3, the angle between the crack initiation direction and the cutting surface was approximately 45°, after which the cracks gradually propagated longitudinally.

Figure 3 Morphological characteristics of crack propagation shown in Figure 2

2 Morphological Changes in the Material Cut Surface During Blanking

An open-die blanking process was used, followed by cold heading to form the tapered rollers. This process is characterized by simple tooling and convenient adjustment. A schematic diagram of the cutting process is shown in Figure 4.

During the blanking process, the stress state of the material changes continuously. The material undergoes elastic and plastic deformation, and a shear zone is formed in the contact area between the cutting blade and the material. The cutting surface of the blank develops a shear lip, while the edge of the cut section forms a shear fracture zone, with significant rounding of the cut-section edge and fracture characteristics (Figures 5 and 6). In some cases, microcracks may also be present on the cut surface (Figures 7–9).

The relevant morphological characteristics are shown in Figures 10–12.

Figure 4 Schematic diagram of the cutting process

1 — Movable blade; 2 — Cutting blade; 3 — Bar stock to be cut; 4 — Positioning block

Figure 5 Schematic diagram of the corner and B cut surface of the blank

Figure 6 Cut-surface profile
(The rounding of the A cut surface is significantly greater than that of the B cut surface.)

Figure 7 Morphological characteristics of the A cut surface
(A distinct shear-fracture zone and a significant degree of edge rounding are observed.)

Figure 8 Surface characteristics of the A cut surface after heat treatment
(The shear lip and shear-fracture zone exhibit numerous microcracks.)

Figure 9 Surface characteristics of the A cut surface of the cold-headed roller after heat treatment
(The cracked surface is located at the center of the large-end face.)

Figure 10 Morphological characteristics of the B cut surface
(The fracture surface is relatively flat and exhibits a shear-fracture morphology; the small circular area is caused by fracture at the positioning block.)

Figure 11 Surface characteristics of the B cut surface after heat treatment
(Only a small number of microcracks are present in the central area of the cut surface.)

Figure 12 Surface characteristics of the B cut surface of the cold-headed roller after heat treatment
(No obvious cracks are observed.)

Based on the above figures, it can be clearly seen that the morphological characteristics of the A and B cut surfaces after open-die blanking are significantly different. The shear lip of the A cut surface is wider and thicker, and the degree of edge rounding is greater, with a large number of microcracks present. In contrast, the B cut surface exhibits less edge rounding, with a relatively small number of microcracks.

The large-end face of the tapered roller is formed by the A cut surface. The distribution and orientation of the cracks at the large end are consistent with those of the microcracks in the shear-fracture zone of the A cut surface, indicating a strong correlation between cracking at the large end of the cold-headed tapered roller and the microcracks generated on the A cut surface during blanking.

3 Analysis of the Causes of Cracking

3.1 Material Analysis

The material grain size, morphology and distribution of non-metallic inclusions, strength and plasticity of the material, hardness of the material, cutting force, and the gap between the upper and lower cutting blades, among other factors, all affect the blanking process.The occurrence of microcracks on the cut surface has a certain influence on crack formation [1]. The causes of crack formation at the large end of the rollers are analyzed below from the perspectives of the distribution and morphology of non-metallic inclusions, grain size, hardness of the material, and the open-die blanking process.

3.1.1 Chemical Composition

An OBLF GS-1000 direct-reading spectrometer was used to analyze the chemical composition of the roller steel. The results are shown in Table 1. As can be seen from the table, the chemical composition complies with the requirements of GB/T 18254—2002.

Table 1

Chemical Composition of the Roller Steel (Mass Fraction, %)

3.1.2 Non-Metallic Inclusions and Grain Size

Specimens were prepared longitudinally from the material sections shown in Fig. 8 after hot acid etching. At a magnification of 100×, the cracks on the A cut surface were observed to be distributed at an angle of approximately 45° to the shear surface. The deeper cracks reached approximately 1 mm in depth. No obvious non-metallic inclusions were observed in either the microcrack regions or the crack-propagation regions, as shown in Figs. 13 and 14.

The grain size was examined using the oxidation method and evaluated in accordance with GB/T 6394—2002. The grain size was determined to be Grade 7, as shown in Fig. 15. The grain sizes at the surface layer and core were essentially consistent.

In addition, the microhardness values of the surface layer, subsurface layer, and core of the raw material were essentially identical, with HV0.2 values of 200–208. This indicates that the surface hardening phenomenon caused by grinding during the material preparation process was not significant.

Figure 13 Distribution and morphological characteristics of microcracks on the A cut surface (100×)

Figure 14 Local crack morphology and crack depth of the A cut surface: approximately 1 mm (100×)

Figure 15 Grain size of the spheroidized microstructure (100×)

Based on the above test results, the possibility that non-metallic inclusions, coarse grains, excessive hardness, surface hardening, surface cracking, or surface decarburization are responsible for the formation of microcracks can be excluded.

3.1.3 Inspection of Surface Cracks and Decarburization

Material sections from the same batch that had undergone single-station press open-die blanking, as well as cold-headed tapered rollers exhibiting cracks at the large end, were sampled for inspection. After hot acid etching, no obvious decarburization was observed on either side of the cracks. The samples were subsequently examined according to GB/T 18254—2002, and no surface decarburization or material surface cracking was observed.

3.2 Analysis of the Open-Die Blanking Process

3.2.1 Characteristics of the Blanking Process

During open-die blanking, the stress state of the material is complex, exhibiting the characteristics of non-uniform shear deformation.

The cutting process begins when the movable blade applies pressure to the contact area between the material and the U-shaped groove of the cutting blade, causing plastic deformation and forming a shear zone. As the cutting blade penetrates into the material, the material in the shear zone undergoes bending deformation, and a shear lip is formed at the edge of the cut section. At the same time, the pressure generated by the movable blade in the U-shaped groove produces a bending moment, causing the shear lip to expand in the direction of fracture. Consequently, the stress state at the edge of the cut section continuously changes, providing favorable conditions for the initiation and propagation of microcracks.

It can be observed that the cut surface produced during open-die blanking exhibits characteristics such as significant deformation, shear-surface inclination, and edge collapse.

3.2.2 Morphological Characteristics During Cold Heading

A press was used for cold heading of the tapered rollers. Cracks at the large end, large-end face, and R-corner region of the large-end chamfer were mainly concentrated in the edge-collapse region of the A cut surface, whereas crack propagation originating from the B cut surface was rarely observed.

The reasons are as follows:

(1) The microcracks in the edge-collapse region of the A cut surface are relatively dense, and their propagation directions are approximately perpendicular to the material surface. The shear lip and edge-collapse region undergo relatively large deformation, resulting in work hardening and a consequent reduction in the material's ability to undergo plastic deformation.

(2) During cold forming, the material must undergo a relatively large amount of plastic deformation to fill the cavity and annular space between the concave and convex dies. Multidirectional stresses generate a notch effect at the tips of the microcracks, while simultaneously promoting the preferential expansion and propagation of the microcracks at regions of stress concentration, such as the large end, the R-corner of the large-end chamfer, and the annular region.

(3) During the forming process, the small end of the roller tends to contract within the tapered die cavity. The circumferential and axial stresses exert a restraining and crack-closing effect on the microcracks, so crack propagation generally does not occur.

4 Conclusions

The large deformation, edge collapse, and inclination of the cut surface generated during open-die blanking are the primary causes of the formation of numerous microcracks on the cut surface, whereas the propagation of microcracks during the cold-heading process is the primary cause of cracking at the large end and the large-end chamfer of the tapered rollers.

The differences in morphology between the A and B cut surfaces produced during open-die blanking are significant. The A cut surface, which exhibits relatively large deformation, edge collapse, and a shear lip, contains a large number of microcracks distributed in a longitudinal direction.

During production, the A cut surface is commonly used as the large-end face of the roller for cold heading, which often causes the microcracks to propagate directly into the open end during the forming process. Therefore, during production, the orientation of the A cut surface should be changed to face inward toward the die cavity. This can effectively suppress the deformation and propagation of microcracks and prevent cracking.

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