A bolt flange developed a network of short cracks during cold heading. The supplied section images and microhardness records suggest a near-surface difference in material response. They support investigation of annealing and furnace atmosphere, but do not complete a quantitative carbon or crack-origin diagnosis.
Material and investigation scope
An anonymized fastener customer asked Creation Group to analyse samples made from another supplier’s wire. The customer designated the material SCM435, at a wire diameter of 13.15 mm. Creation Group did not supply the material. The available evidence consists of appearance, section micrographs and micro-Vickers hardness records; same-batch original wire, complete heat-treatment records and quantitative carbon profiles were not available.
Near-surface microstructure
Two marked widths in the examined section are 78.56 µm and 105.64 µm, approximately 0.08–0.11 mm. These are local widths of a region with different microstructural appearance, not a measured carburization depth. Higher-magnification images show more pronounced lamellar or strip-like features near the surface than in the interior. Optical contrast does not measure carbon content.
The hardness record
The original record contains 15 HV0.1 points, labelled from the edge towards the interior. The following English table transcribes the displayed positions and Vickers values. It does not turn the software’s converted HRC column into a separate measurement or define a grade acceptance limit.
| Point | Displayed position (mm) | Hardness (HV0.1) |
|---|---|---|
| 1 | 0.0 | 260.5 |
| 2 | 0.1 | 248.5 |
| 3 | 0.2 | 240.6 |
| 4 | 0.3 | 238.5 |
| 5 | 0.4 | 241.7 |
| 6 | 0.5 | 232.1 |
| 7 | 0.6 | 237.4 |
| 8 | 0.7 | 240.6 |
| 9 | 0.8 | 231.0 |
| 10 | 0.9 | 232.1 |
| 11 | 1.0 | 234.2 |
| 12 | 1.1 | 227.9 |
| 13 | 1.2 | 226.9 |
| 14 | 1.3 | 226.9 |
| 15 | 1.4 | 235.2 |
The first point is 260.5 HV0.1, followed by 248.5 and 240.6 HV0.1. The ten displayed points from 0.5 through 1.4 mm average 232.43 HV0.1, approximately 232.4. The difference from the first point is 28.07 HV0.1, approximately 28.1. These calculations reproduce the displayed record; the measurement origin and part location still require the test record for interpretation.
Investigate residual or re-formed lamellar structure
Spheroidizing annealing must produce a suitable carbide morphology and distribution, as well as the desired hardness. The observed lamellar features could be residual pearlite that did not spheroidize sufficiently, or pearlite formed again during cooling. Review the actual combination of heating, holding and cooling. A nominal annealing operation or a longer hold alone does not establish the resulting structure.
If heating causes partial austenitization and alters carbide dissolution, later cooling can favour lamellar products rather than the intended spheroidized condition. This is a proposed thermal-history explanation. The supplied images do not reconstruct the complete furnace cycle.
Check the atmosphere and surface carbon profile
A furnace atmosphere with a carburizing potential could enrich the surface and change local carbide formation, transformation and hardness. That could interact with an unsuitable spheroidizing cycle. However, the micrographs and hardness values do not independently prove increased carbon: structure and carbide distribution also influence hardness at unchanged composition. Atmosphere-related carburization remains an inference requiring quantitative verification.
A possible route to flange cracking
As the flange expands, the near-surface region must deform with the interior. Different local flow response under tension and shear could initiate cracks at several weak locations, followed by growth and joining into a network. The proposed explanation combines the observed structure and hardness differences with that forming behaviour.
The existing sampling sketch marks the shank. It does not demonstrate that a flange crack was directly observed initiating in the examined layer. Residual or re-formed pearlite, suspected carburization and the connection to the actual crack origin remain distinct propositions to test.
Verification priorities
- Match the material and samples to the actual heating, holding and cooling records.
- Review furnace-atmosphere control and measure surface-to-core carbon, microstructure and hardness on representative same-batch original wire.
- Compare the abnormal and normal material locations and the actual flange crack origin.
- Verify any adjusted process through microstructure and representative forming trials, then sustained production records.
No post-correction retest or production record is included in the supplied material. The review priorities are proposed actions, not measures already demonstrated to work. Creation Group’s contribution in this case is sample analysis and a testable explanation, with the observed facts and unresolved causal links kept explicit.