Material Preparation for Cold-Headed Parts: Wire Rod, Annealing, and Surface Lubrication Treatment
Executive Summary
With the same cold header and the same die, wrong material still causes cracking or even die bursting. Half the quality of cold-headed parts depends on pre-line material preparation: wire rod selection, spheroidizing annealing, and phosphating/soaping film-weight control. Following the process flow, this article clarifies wire rod grades and incoming-inspection items, spheroidization criteria, the phosphating film-weight window and shelf life, and gives a cause table for cold heading cracking.
Table of Contents
When Cold Heading Cracks, the Problem Is Often Not the Die
When cracking appears on a cold heading line, the first reaction is usually to check the dies, the equipment, and the process parameters. But a significant proportion of cases have their root cause before the line: insufficient spheroidization of the wire rod, inadequate annealing, an overly thin phosphating film, or a surface decarburized layer.
Cold heading is severe plastic deformation at room temperature and places extremely high demands on the material's plasticity reserve. When the per-stroke upset ratio reaches 2.0 or more, the material's elongation, reduction of area, and spheroidized microstructure state directly determine whether it cracks. The die only determines whether it can be made; the material determines whether it can be made stably.
Step 1: Wire Rod Selection
Cold heading steel is usually supplied as wire rod, with diameters from Φ5.5 to Φ42 mm, of which Φ5.5～Φ16 mm are most commonly used.
Common Cold Heading Steel Grades and Uses
| Typical Grade | Carbon Content | Target Property Class | Typical Use |
|---|---|---|---|
| SWRCH6A～10A / Q195 | ≤0.10% | Below class 4.8 | Low-strength screws, rivets |
| SWRCH15A～18A | 0.13%～0.20% | Class 4.8～6.8 | Common screws, self-drilling screws |
Incoming-Inspection Items for Wire Rod
| Inspection Item | Required Range | Consequence of Nonconformance |
|---|---|---|
| Chemical composition | Per grade standard | Insufficient strength or poor plasticity |
| Diameter tolerance | ±0.05～±0.10 mm | Cut-off volume deviation, head dimension out of tolerance |
| Surface defects | No cracks, laps, seams, scabs | Directly induces cold heading cracking |
| Decarburized layer depth | ≤ 0.5%～1.0% of diameter | Hard, brittle surface, cold heading cracking, reduced fatigue |
Step 2: Spheroidizing Annealing
Medium-carbon and alloy steel wire rods have a lamellar pearlite microstructure after rolling. Lamellar pearlite is hard and poor in plasticity, and cold heading it directly inevitably cracks. It must undergo spheroidizing annealing to transform lamellar cementite into granular (spheroidal) cementite uniformly distributed on the ferrite matrix.
Purpose of Spheroidizing Annealing
- Hardness drops sharply and plasticity improves significantly, so cold heading does not crack;
- Improves machinability (for areas that need subsequent cutting);
- Provides a uniform microstructure basis for final heat treatment.
Process Parameters
| Material Type | Heating Temperature | Holding Time | Cooling Method | Target Hardness |
|---|---|---|---|---|
| Low carbon steel (≤0.20%C) | 680～720 ℃ | 4～8 h | Furnace cooling | ≤ 80 HRB |
| Medium carbon steel (0.30%～0.45%C) | 740～770 ℃ | 6～12 h | Slow cooling (≤20 ℃/h to 650 ℃) | 90～105 HRB |
Spheroidization Criterion
Spheroidization requirement: ≥ 90% (premium products require ≥ 95%). The direct manifestation of insufficient spheroidization is high hardness and cracking during cold heading, especially in the outer head region where tensile deformation is greatest.
Step 3: Surface Treatment (Phosphating and Soaping)
The annealed wire rod surface needs a lubricant film that can withstand high pressure, otherwise the metal will contact the die directly, causing cold welding and scoring. The mainstream process is phosphating + soaping.
Phosphating
The wire rod is immersed in a phosphate solution, where a chemical reaction on the surface forms a porous zinc phosphate crystalline film. The role of this film is:
- as a carrier for the lubricant (the porous structure adsorbs and stores the soap solution);
- to isolate the metal from the die and prevent cold welding;
- to reduce the friction coefficient.
Soaping
A layer of sodium stearate (soap) film is further deposited on the phosphating film. The soap film has a layered structure and can slide along the metal flow direction under high pressure, acting as a solid lubricant.
Film-Weight Control Parameters
| Item | Typical Control Range | Notes |
|---|---|---|
| Phosphating film weight | 3～8 g/m² | Lower limit for low-pressure forming, upper limit for high-pressure forming |
| Phosphating film thickness | 2～8 μm | Too thin → scoring; too thick → poor filling |
| Soap film weight | 1～4 g/m² | Used together with the phosphating film |
| Phosphating solution temperature | 70～85 ℃ | Temperature affects crystal fineness |
① Film too thin: under high-pressure forming the film breaks through, metal contacts the die directly, causing scoring and pickup, and accelerating die wear.
② Film too thick: the lubricant layer takes up space and the metal flow direction runs out of control, causing incomplete cavity filling and dimensional out-of-tolerance; meanwhile the thick film is locally squeezed out, forming dry-friction zones.
The film weight must fall within the process window; you cannot "apply a little more for safety."
Step 4: The Final Check Before Going Online
After the wire rod has been inspected, annealed, phosphated, and soaped, several confirmations should be completed before it enters the cold header:
- Diameter recheck: the phosphating film adds 2～8 μm to the diameter; confirm it is within tolerance.
- Film appearance: should be uniform gray-black (or dark gray) crystals, with no missed plating, no rust spots, and no buildup.
- Surface cleanliness: no oil, no dust, no residue.
- Hardness recheck: whether the annealed hardness is in the target range.
- Aging and storage: phosphated wire rod should not be stored long (the film absorbs moisture and fails); it is generally recommended to use within 7～15 days; the storage environment should be dry, avoiding condensation and rust.
| Cold Heading Cracking Symptom | Possible Cause | Priority Check Direction |
|---|---|---|
| Longitudinal crack on the outer head | Insufficient material plasticity | Whether spheroidization rate, hardness, carbon content conform |
| Surface scoring, fish-scale pattern | Lubricant film failure | Phosphating film weight, soaping quality, die wear |
| Crack near the cut-off | Worn cut-off blade, blank end-face defects | Cutting blade condition, wire rod surface defects |
Conclusion: Material Preparation Is an Underestimated Quality Lever
The stability of cold-headed part quality depends at least half on the material condition before going online. In the same workshop and on the same equipment, using qualified annealed wire rod and phosphated film can keep product yield stably above 99%; once a batch has insufficient spheroidization or an over-thick phosphating film, the scrap rate can several-fold overnight.
Therefore, for cold-headed part suppliers, wire rod incoming inspection + spheroidizing annealing control + phosphating film-weight monitoring are three process control points that must be fixed into the work standards. They do not appear on the part drawing, but they determine whether the drawing's tolerances can be stably achieved.
