Case 01 / Cracking in Cold Heading
The bolt flange has cracked into a network:
Tracing the cause through lamellar microstructure and surface hardening
A bolt appears normal in its shank, but fine cracks have appeared along the edge of the cold-headed flange. In such cases, simply asking “Is the material too hard?” is often not enough.
The focus of this analysis is that both microstructural abnormalities and increased hardness were observed near the surface of the material. Based on these findings, we propose that the lamellar microstructure together with suspected surface carburization jointly impaired the local deformation capability, thereby promoting cracking during the flanging process.
Below, we will clearly present our inspection findings and elucidate this failure pathway.
I. Case Background: Customer Entrusts Creation Group with Analysis of Cold-Heading Cracking
A fastener manufacturer uses wire supplied by its existing vendor to produce bolts. During the cold-heading operation, a network of cracks was discovered on the flange area, prompting the company to commission Creation Group for assistance in conducting an analysis.
The customer specifies the material as SCM435, with a wire diameter of φ13.15 mm. The material under investigation was not supplied by Creation Group; instead, Creation Group undertook the analytical work on the submitted samples. Information regarding the customer and supplier has been anonymized.
This analysis is based on the sample’s external appearance, cross-sectional metallographic images, and micro-Vickers hardness measurements. No original wire from the same batch, complete heat-treatment records, or quantitative carbon concentration data were available.
Appearance of Cracks on the Flange
The flange area of the submitted sample shows multiple short cracks, some of which intersect locally.
II. Findings: The near-surface layer not only has a different microstructure but also higher hardness
Metallographic photographs reveal distinct microstructural differences in the near-surface region of the examined cross-section, with two marked areas measuring 78.56 μm and 105.64 μm in width, roughly equivalent to 0.08–0.11 mm.
Under high magnification, this area exhibits clearly visible lamellar and striated structures, differing from the morphology of carbides found in the interior.
Near-surface microstructure and local width measurements
The measured widths of the two marked regions representing microstructural differences do not directly correspond to the determined carburized case depth.
Near-surface microstructure at high magnification
Compare with Figure 3 to observe the morphology and distribution of carbides; note that brightness contrasts should not be used as a direct substitute for carbon content analysis.
Microhardness measurements provide additional evidence. According to the test report’s stated measurement sequence “from edge toward interior”:
Comparison Item Test Record
The first measurement point near the surface: 260.5 HV0.1
The subsequent two points: 248.5 and 240.6 HV0.1
Average value within the 0.5–1.4 mm range: approximately 232.4 HV0.1
The difference between the initial near-surface point and the above average: about 28.1 HV0.1
The metallographic microstructural differences and increased hardness mutually corroborate each other: the near-surface layer is not merely “a bit darker in the photograph,” but rather exhibits a microstructure and mechanical response distinct from the interior.
Test Data
Raw data from 15 micro-Vickers hardness measurements
Hardness curve
Micro-Vickers hardness curve
The testing conditions were HV0.1, totaling 15 measurement points. Both the raw data and the curve illustrate the variation in hardness among the measurement locations and should be interpreted in conjunction with their respective positions.
III. First Line of Evidence: Why Does Lamellar Structure Appear?
The purpose of spheroidizing annealing is not merely to reduce a hardness value, but also to achieve the desired carbide morphology and distribution suitable for subsequent cold forming.
Regarding the lamellar structure observed this time, we primarily consider two possible formation pathways: either the original pearlite was not sufficiently spheroidized, or pearlite reformed during the cooling process after heating.
The main heat-treatment explanation proposed here is that improper coordination among heating temperature, holding time, and cooling conditions led to the retention or regeneration of lamellar pearlite.
If the heating temperature falls within the partial austenitization range—particularly if it is too high or the holding time at that temperature is inadequate—the dissolution state of carbides may change. Subsequent cooling, if unfavorable for the formation of spherical carbides, can result in the re-formation of lamellar structures. The spheroidizing process itself involves both carbide dissolution during heating and their reprecipitation upon cooling; therefore, one should not focus solely on a single holding parameter.
Hence, it is neither true that “the longer the holding time, the softer the material,” nor that “once spheroidizing is performed, the process will necessarily be complete.” Ultimately, the microstructure must be examined.
IV. Second Line of Evidence: Did the Furnace Atmosphere Exacerbate Surface Anomalies?
Surface carburization represents another major process-related cause identified in this case.
If the furnace atmosphere tends to increase carbon content relative to the steel, the surface layer may become enriched with carbon. This, in turn, alters the quantity, dissolution behavior, and transformation characteristics of carbides in that region, leading to distinct microstructural states between the surface and the interior.
This phenomenon could interact synergistically with abnormalities in the spheroidizing process: on one hand, lamellar structures remain inadequately eliminated or reform; on the other, surface-carbon enrichment further influences local microstructure and hardness.
However, an important distinction must be noted: while existing metallographic and hardness results support this interpretation, they do not independently confirm elevated carbon levels. Even at the same overall carbon content, variations in microstructure, grain size, and carbide distribution can significantly affect material properties.
Therefore, “atmospheric carburization” in this case constitutes a primary process-related inference rather than a quantitatively verified experimental finding.
V. Why Does It Ultimately Manifest as Flange Network-like Cracking?
During flange forming, the material expands outward, increasing its surface area, and the near-surface layers must deform along with the interior.
The presence of lamellar structures and localized hardness suggests that deformation response in these regions differs from that of the interior. Under tensile and shear stresses, areas with insufficient ductility are prone to cracking first.
As forming continues, cracks initiate sequentially at various weak points, propagate along different directions, and locally interconnect, eventually forming a network-like crack pattern.
In simpler terms: while the interior continues to expand outward, the surface layer cannot keep pace, resulting in multiple locations being pulled apart successively.
The failure mechanism proposed here is as follows: improper spheroidizing heat treatment resulted in residual or regrown lamellar structures; suspected atmospheric carburization exacerbated differences in microstructure and hardness near the surface; and during cold heading of the flange, inconsistent local deformation contributed to multi-point crack initiation.
This is the primary mechanistic assessment derived from available data. Sampling has been conducted on the shaft portion, but observations from this cross-section cannot yet be directly equated with evidence that “flange cracks initiated from the abnormal layer.”
VI. Key Points of Handling: Simultaneously Verify the Heat Treatment Process and Atmosphere Control
Since a dual-factor cause has been proposed, subsequent handling cannot focus solely on a single hardness value.
First, verify the actual heat treatment process. Compare it with the material’s actual heating temperature, holding time, and cooling curve to determine whether the lamellar structure remained insufficiently spheroidized or reformed during cooling.
Second, check the atmosphere and surface carbon distribution. Refer to the atmosphere control records and conduct corresponding tests on the same batch of raw wire rod, examining the carbon content, microstructure, and hardness from the surface inward, thereby validating the judgment of surface carburization.
Third, use microstructural and forming results to verify the effectiveness of adjustments. Do not merely assess whether the overall hardness has decreased after adjustment; also examine whether the near-surface lamellar structure has diminished, whether the internal–external condition has become more uniform, and review cold heading verification results for similar products.
The available data do not include post-correction re-inspection or mass production records; therefore, the above-mentioned handling approaches are not presented as measures that have already been successfully verified.
VII. Case Conclusion
The primary failure mechanism identified in this case is that the near-surface lamellar structure combined with localized hardening reduced deformation coordination capability, thereby promoting network cracking during flange cold heading expansion.
Regarding the origin of the anomaly, the main focus points to the combined effects of improper spheroidizing heat treatment and suspected carburization caused by the furnace atmosphere. Specifically, the detailed formation processes of regenerated pearlite and carburization require further corroboration through comparisons among the original wire rod, heat treatment, and atmosphere records.
This case demonstrates that evaluating cold heading materials should not be limited to asking “What is the average hardness?” It is equally important to consider: What is the microstructure at the surface? How does it differ from the interior? And can these regions work together to achieve the product’s required deformation?
Creation Group assists clients in conducting failure analyses by linking crack morphology, microstructure, and performance data, providing well-founded, verifiable causal assessments so that subsequent improvements have clear targets.
Observing the crack is only the starting point; explaining why the crack occurred is where the true value of failure analysis lies.
Creation Group · Cold Heading Engineer Classroom
Clarifying Material Issues