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Common defects in fastener raw materials directly affect their machinability and reliability in use; these can be broadly categorised into two main types: surface defects and internal defects. The following is a detailed classification and description based on information from various sources:

Surface cracks

Longitudinal cracks: Caused by improper rolling processes (such as surface scratches on the billet or the exposure of internal defects), these appear as straight lines along the rolling direction and are prone to propagation during subsequent processing.

Transverse cracks: Often caused by improper cutting or extrusion processes, these are more hazardous than longitudinal cracks and are commonly found at the ends of bars.

Scratches: Caused by mechanical damage during rolling or drawing; the cross-section shows material compression rather than cracking, and may lead to head cracks during cold heading.

Surface folds

Formed when metal burrs are pressed into the surface due to wear of the roll grooves or poor alignment. Unlike cracks, folds do not follow a fixed direction in the cross-section and may extend through long sections of the material.

Surface decarburisation

Improper annealing leads to the loss of carbon from the surface layer of the material, reducing strength and increasing susceptibility to premature fracture, with particularly significant consequences for threaded sections.

Coarse-grained ring

Commonly found in extruded aluminium and magnesium alloy bars; caused by excessive cold drawing, resulting in abnormal grain growth in the surface layer. This makes the grain boundaries prone to cracking and may produce an ‘orange peel’ surface after cold heading.

II. Internal Defects

Internal Cracks

These are often associated with material porosity or inclusions, appearing tortuous with a tapered tip. If decarburisation or oxidation layers are present on either side of the crack, this indicates that the crack existed prior to heat treatment.

Residual Shrinkage Cavities

Caused by incomplete removal of the ingot cap, accompanied by inclusions and segregation; after machining, this may form through-holes or internal cracks, such as in titanium alloy screws where assembly failure occurs due to unwelded shrinkage cavities.

Inclusions

These are classified as external inclusions (residual slag/refractory materials) and matrix inclusions (chemical residues). Inclusions distributed in a chain-like pattern are prone to causing cracking during processing or use; for example, longitudinal cracks in high-temperature alloy nuts caused by inclusion particles.

III. Other Defects

Composition Inhomogeneity: Excessive carbon content or excessive sulphur and phosphorus levels (e.g. sulphur > 0.05%) can increase the risk of hydrogen embrittlement or quenching cracks.

Insufficient Lubrication: During cold forming, increased friction caused by inadequate surface cleaning or improper coating may lead to die sticking or incomplete part formation.

Microstructural Abnormalities: Uneven grain size or poor spheroidisation can affect cold forming performance; annealing processes must be strictly controlled.

IV. Inspection and Preventive Measures

Non-destructive testing: At the raw material stage, eddy current testing (for surface/near-surface defects) and ultrasonic testing (for internal defects) are recommended; during production, magnetic particle testing is used for ferromagnetic materials, and penetrant testing for non-ferromagnetic materials.

Incoming Inspection: This includes wire diameter measurement (vernier caliper), chemical analysis (direct-reading spectrometer) and visual inspection for cracks, to ensure material purity and process suitability.

The prevention and control of these defects require coordination across multiple stages, from raw material procurement and process optimisation to comprehensive testing, in order to enhance the reliability of the finished fasteners.