Chapter 6: reducing the diameter also changes the condition of the wire.
The same final diameter can conceal a different history
Two batches can share a grade, steelmaker and final size yet behave differently in cold heading. In an illustrative example, both finish at 10 mm, but one starts at 11 mm and another at 13 mm. They reach the same diameter through different total reductions. These dimensions are examples, not a prescribed drawing schedule.
Cold drawing generally increases strength and hardness through work hardening. The incoming size and total deformation therefore matter alongside the final diameter.
The pass schedule matters
A large reduction in one pass can increase contact pressure, friction and heating and create different responses near the surface and core. More passes are not automatically better either: deformation and friction accumulate. Review each pass and particularly the final pass, rather than judge only the total reduction.
Direction changes: the Bauschinger effect
Drawing introduces a tensile deformation history; subsequent upsetting applies compression and redirects material flow. Prior plastic deformation can lower the yield stress when loading reverses. This directional response is known as the Bauschinger effect.
An increase in tensile strength after drawing does not mean that initial compressive flow stress increases in the same proportion. The effect depends on the material and loading path; results for drawing followed by upsetting should not be transferred automatically to every forward- or backward-extrusion stage.
Work hardening and reverse yielding act together
Drawing can increase resistance to further deformation while reverse loading changes the onset of compressive yielding. Their relative influence depends on the steel, starting condition and prior strain. Neither minimum drawing nor maximum drawing is a universal solution. A useful reduction range must be established for the grade, size and part.
Bainitic and non-quenched-and-tempered routes
Some bainitic high-strength wire developments use their work-hardening and reverse-loading response to balance strength and cold forgeability. This is a grade- and process-specific development route, not a guarantee that every bainitic steel behaves alike. Evaluate drawing history and actual forming response together with tensile strength and hardness.
Time after drawing can matter
The material response can also evolve with storage time and temperature. Compare drawing dates, elapsed storage periods and storage conditions when batches behave differently. Those records are investigation leads; they do not establish a universal ageing time or prove ageing as the cause.
What to compare during an investigation
- Diameter before and after drawing.
- Total reduction, pass count and pass schedule.
- The final-pass reduction and drawing equipment.
- Drawing date, storage time and storage conditions.
- Material condition and measured mechanical properties.
- Whether the cold-heading anomaly follows the drawing batch when the part, machine and dies remain comparable.
A difference in drawing parameters is not a root-cause conclusion. Check whether it corresponds reproducibly with material properties and the observed forming problem. Retain the material heat, processing batch and location references for comparison.
Technical references
The following sources support the loading-path distinction and examples of bainitic wire development; they do not provide a universal drawing schedule for every product.
- Miki and Toda: work hardening and the Bauschinger effect in cold forging
- Nippon Steel Technical Report 135: bainitic wire rod for non-heat-treatment bolts
Drawing determines the state in which the material enters the heading machine, together with the starting wire rod and any heat treatment. For an investigation, provide the grade, before-and-after dimensions, pass history, production dates and photographs of the abnormal parts. The next chapter examines phosphate-and-soap coating performance after drawing.