When cracks appear in a cold-headed part at the head, flange edge or fillet, the first response is often to check the material or the heading die.
The location where a defect becomes visible in the finished part is often its final point of exposure, rather than the place where it originally formed.
Some defects originate in continuous casting or rolling, some arise during transport and handling, and others are introduced during pickling, drawing, wire guiding, feeding or shearing.
Surface defects can be inherited from upstream operations or introduced in intermediate processes. Cold heading changes their shape; it cannot restore material continuity that has already been damaged.
Defect pathway: continuously cast billet → rolled wire rod → transport and handling → pickling and wire processing → drawing and wire guiding → cold heading
1. Continuous casting: defects can start below the billet surface
The billet is the starting material for wire rod. Abnormal oscillation marks, subsurface blowholes, slag inclusions, scabs and pits can all become sources of subsequent surface defects.
Figure 1. Abnormal oscillation marks on a continuously cast billet. Normal shallow marks are not inherently defects. Risk increases when the marks become unusually deep or are accompanied by cracks or slag inclusions at their roots.
Figure 2. Subsurface blowholes exposed by grinding. Rolling may close the pores or elongate them in the rolling direction. Oxidation of their internal surfaces affects whether sound pressure welding can occur during subsequent rolling.
Figure 3. Slag inclusions at or near the billet surface. Slag does not deform compatibly with the steel matrix; rolling may fragment and elongate it, producing scabs, laps or linear defects.
Figure 4. Scabs on a steel billet. Flaky metal attached to the surface may be flattened and rolled into the matrix. A surface that looks smooth does not establish that the internal interface has bonded.
Figure 5. Pits on a steel billet. “Pit” describes the observed shape. Identifying the cause also requires examination of oxides at the bottom, folded edges and surrounding cracks.
Rolling may close, elongate or redirect defects at the billet surface. A smoother appearance does not establish that the original material discontinuity has disappeared.
2. Rolling: defects can elongate, and new defects can form
Billet cracks may develop into longitudinal wire-rod cracks. Scabs and slag inclusions may flatten and elongate, while subsurface blowholes may produce intermittent linear indications.
Rolling can also introduce defects. Abnormal roll-pass filling, guide alignment or roll-gap conditions may produce fins. If a fin is folded back onto the surface in a subsequent pass, it may form a lap. Scale or foreign matter rolled into the steel can also create a local discontinuity.
Figure 6. Cross-section of a lap. Surface metal has folded inward in a tongue-like shape without sound bonding at the interface. Temporary closure at the surface does not mean the internal defect has been removed.
Figure 7. Scabs on wire rod. Compared with ordinary scratches, scabs commonly show flaky metal, locally lifted edges or evidence of detachment.
Figure 8. Cross-section through a wire-rod surface crack. The crack extends from the surface into the matrix. Further examination is needed to determine whether oxidation, decarburization or an abnormal microstructure is present.
Laps and cracks can look similar, but their formation mechanisms differ. A lap forms when metal folds over without sound bonding; a crack is a separation in the continuity of the matrix. Their causes should therefore be investigated separately.
3. Transport and handling: impact damage and scratches can initiate cracking
Impacts, dragging and rubbing during lifting, coil placement, stacking or transport can cause scratches, indentations and raised metal edges. Such damage is usually local and irregular, often concentrated on the outer coil turns or lifting-contact points.
Figure 9. Scratches and impact damage from transport or handling. Their direction and position are generally irregular, unlike continuous longitudinal scoring caused by a drawing die.
Minor damage may not cause an immediate problem in parts with moderate deformation. In parts involving large deformation, complex heads or thin flanges, however, the damaged area may concentrate strain and open further during cold heading.
4. Pickling: existing defects can be exposed, and new damage can be introduced
Removing scale during pickling can expose cracks, laps, scabs and pits that were previously concealed. Finding a defect after pickling does not establish that pickling caused it.
Incomplete descaling can prevent a uniform phosphate coating from forming. Residual oxides may then break up, detach or be pressed into the wire surface during drawing.
Figure 10. Residual scale after pickling. Dark longitudinal bands must also be distinguished from oxides retained inside a lap or crack. Colour alone cannot establish the cause.
Poorly controlled pickling can also cause pitting and surface roughness. Retain specimens from before and after pickling to distinguish exposure of an existing defect from newly introduced damage.
5. Drawing and reprocessing: newly introduced longitudinal scoring is easily underestimated
Reprocessing changes more than material dimensions; it can also damage the wire surface directly.
Worn drawing dies, scale or metal particles entering a die, temporary lubricant-film failure, burrs, and abnormal contact at guide wheels or other wire-contact points can cause continuous or intermittent longitudinal scoring.
Figure 11. Local scoring introduced during drawing. The damage follows the wire length and may include grooves, raised metal or local tearing.
For short, intermittent marks, check hard particles, momentary metal pickup and local contact. Long, continuous marks at a relatively fixed circumferential position call for close examination of the drawing die, guide wheels and wire path.
Longitudinal marks are not necessarily caused by drawing. Rolling cracks, laps and residual scale can acquire a similar appearance after drawing. Determine whether the defect existed before drawing or first appeared after a particular die or wire-contact point.
This chapter addresses surface damage associated with drawing. Chapter 6 separately discusses how reduction of area, drawing passes and work hardening affect cold-heading performance.
6. Before heading: the last opportunities for new damage are often overlooked
New surface damage can occur after the wire enters the heading machine but before forming begins. Guide wheels, straighteners and feeding mechanisms can score the wire. Worn, chipped or burred cut-off knives and dies can damage the blank end or side surface.
If defects recur at a fixed orientation and position, or change markedly after replacing a guide wheel or cut-off knife, an investigation limited to the material certificate and original wire rod is insufficient.
7. Why do defects persist through to the finished part?
Drawing elongates the material, while cold heading produces axial compression and radial flow. Existing defects may elongate, narrow or close temporarily. Material flow can also carry them into the head, flange or fillet, where they reopen in a heavily deformed region.
Figure 12. Cracking at the flange edge of a cold-headed part. The photograph shows the final point of exposure. Appearance alone cannot establish that the material was the original cause.
A longitudinal defect in wire need not remain straight in the finished part. It may appear as a crack at the head edge, a lap in the flange, a fillet crack or a local depression. Its position depends on part geometry, cut-off orientation and the material-flow path.
A defect is not simply copied into the finished part. It can persist while changing shape at each processing stage.
8. Locate the stage where the defect first appears
Examining the final cracked part alone is usually insufficient to identify the originating operation. Build a specimen chain through billet, wire rod, pickled material, drawn intermediate wire, heading blank and finished part to locate the first appearance of the defect.
Is the indication confined to one length of the coil or continuous throughout it? Are its direction and circumferential position fixed? Does it change after replacing a die, guide wheel or cut-off knife? Are oxidation, decarburization or abnormal metal-flow patterns present inside the crack? These observations help guide an investigation, but they must be assessed together with process records.
Engineering perspective
Surface defects may originate in casting, rolling, transport, pickling or drawing. They may also be introduced by wire guiding, feeding or shearing in the heading machine.
The location where a finished part cracks does not establish where the defect formed.
The finished-part defect is an outcome. Locating the first operation in which the defect appears brings the investigation closer to its origin.
The purpose of defect tracing is to establish a complete chain of process evidence before assigning responsibility.
For scabs on wire rod, longitudinal scoring, laps or intermittent cold-heading cracks, provide the grade, dimensions, photographs, affected forming stage and defect distribution. We can help identify whether the initial investigation should focus on steelmaking and rolling, transport protection, pickling and drawing, or heading equipment.
Next chapter
Chapter 6: Why can drawing change the cold-heading behaviour of the same material?
The next chapter examines how reduction of area, drawing passes and work hardening change the actual condition of material entering the heading machine.