Introduction

A fastener producer’s journey starts not with a screw or bolt, but with wire rod—a coil of semi-finished steel typically 5.5 to 16 mm in diameter, produced by a steel mill. Before that wire can enter a cold-heading machine, it must be transformed to meet strict requirements: precise diameter, controlled hardness, and a microstructure that will deform cleanly without cracking. This transformation—drawing, annealing, and for carbon and alloy steels, spheroidizing—is invisible to the end user but essential to the part’s reliability and the producer’s profitability.

This article explains why each step exists, how it works, and what properties the wire must achieve.

Part 1: Understanding wire rod as a starting material

What the mill delivers

Wire rod arrives from the steel mill as coils of typically 500 to 2000 kg. The rod has been hot-rolled and air-cooled, giving it:

• Rough surface with mill scale (oxidized iron)
• Pearlite microstructure (in carbon steels) or mixed pearlite and ferrite (in lower-carbon grades), plus carbide particles if the steel is alloy
• Tensile strength typically 400–550 MPa for mild to medium-carbon grades
• High hardness and brittleness unsuitable for cold forming
• Surface defects that can initiate fatigue cracks

For a cold-heading machine that will squeeze and form the metal in a fraction of a second, this starting material is too hard, too brittle and too dirty. The wire preparation process softens it, cleans it, and transforms its microstructure to allow large plastic deformations without cracking.

Why cold forming requires special wire

Cold heading works by plastic flow. When a header tool presses on wire with loads of 10 to 50 tons per square millimeter, the metal does not shear or fracture—it flows outward, bulges, and fills the die cavity. This happens so fast that the wire cannot work-harden significantly; if it does, it cracks.

To survive this abuse, the wire must have:

1. Low yield strength so plastic flow begins easily
2. High ductility to deform 20, 30, even 50% without necking or cracking
3. Uniform microstructure with no hard spots or inclusions that would cause localized failure
4. Clean surface free of scale and deep defects that would propagate cracks

Wire rod from the mill has none of these. The preparation process delivers all of them.

Part 2: Wire drawing

Purpose and process

Wire drawing reduces diameter and consolidates the metal, working out porosity and controlling grain size. A rod of, say, 10 mm diameter is fed through a series of draw dies, each reducing the diameter by 5–15% until the target size is reached.

A typical sequence for a 10 mm rod aiming for a 5.5 mm header-ready wire might look like:

• 10.0 mm → 8.5 mm (first die)
• 8.5 mm → 7.2 mm (second die)
• 7.2 mm → 6.0 mm (third die)
• 6.0 mm → 5.5 mm (final die)

Each die is a hard steel or tungsten carbide block with a conical hole. As the wire is pulled through, the metal is compressed radially and elongated axially. Lubrication—typically a soap- or polymer-based compound—reduces friction and heat.

Mechanical effects of drawing

Cold drawing creates several changes:

Work hardening. The repeated plastic deformation increases dislocation density, raising tensile strength and yield strength. For annealed wire, strength typically rises from ~250 MPa to 500–700 MPa after drawing.

Grain refinement. The wire’s grains become elongated along the drawing direction, creating a fibrous texture. This actually improves toughness and ductility along the wire axis, though it reduces ductility in the radial direction (a detail cold-header operators know well).

Surface improvement. Drawing removes much of the mill scale, though not all. The wire is cleaner but still requires further treatment.

Strain hardening limit. If the wire is drawn too much in one pass (a reduction of more than ~15%) or if total reductions are extreme, the ductility falls and the wire becomes prone to cracking, either during drawing or later during cold heading.

Lubrication and temperature control

The wire heats during drawing due to plastic deformation and friction. A typical drawing operation on a carbon steel wire can raise the temperature by 50–100°C. This is usually acceptable, but if the wire is drawn too fast or with poor lubrication, localized heating can occur, potentially softening the wire unevenly or causing the lubricant to break down.

Good drawing practice:

• Applies adequate, clean lubrication to reduce friction
• Controls drawing speed (typically 50–200 m/min for large diameter wire)
• Monitors wire temperature, stopping if it exceeds ~80–100°C above ambient
• Uses water cooling between stages for high-speed operations

Part 3: Annealing (process annealing)

Why annealing is necessary

After drawing, the wire is work-hardened and strong but often too brittle for the deformations that cold heading will demand. An M8 bolt head requires pulling metal from a 5.5 mm wire diameter to fill a die pocket roughly 13 mm across—a radial strain of 100% or more. If the wire is too hard, it cracks during this flow.

Annealing heals the work-hardened structure without softening the wire too much. The goal is to restore ductility while maintaining enough strength to hold the shape of the finished fastener.

Process annealing temperature and time

For plain carbon steel and low-alloy steels, process annealing is typically done at 400–600°C, usually in the range of 500–550°C, for 15 to 60 minutes (depending on wire diameter and furnace type). The wire is heated in a controlled atmosphere (usually nitrogen or a nitrogen-endogas mix) to prevent oxidation and decarburization.

The goal is recrystallization. Above the recrystallization temperature, the work-hardened dislocation structure annihilates itself, and new stress-free grains grow. This restores ductility dramatically. But the temperature must not be so high that the existing carbide particles (which are important for the final properties) begin to coarsen, or that unwanted phase transformations occur.

Mechanical properties after process annealing

A typical annealed wire for cold heading shows:

• Tensile strength: 300–400 MPa (roughly half the drawn strength)
• Yield strength: 200–300 MPa
• Elongation: 25–35% in the wire direction
• Hardness: 95–120 HV (Vickers), depending on grade

This is soft enough to flow plastically without cracking, yet strong enough to hold its shape in the final fastener.

Furnace types

• Batch furnaces: wire coils are loaded into a chamber, heated and cooled. Common for smaller producers or varied grades. Cycle time is typically 4–12 hours.
• Continuous furnaces: wire passes through a heated chamber on a conveyor. Faster, more repeatable, better energy efficiency. Becoming standard for high-volume producers.
• Induction furnaces: for special alloys or very high temperatures. Used less frequently because of cost and complexity.

The atmosphere is critical. An oxidizing atmosphere (air) creates a blue or black oxide scale on the wire, which must be removed later. Most producers use a controlled or inert atmosphere to avoid this.

Part 4: Spheroidizing (for carbon and alloy steels)

What spheroidizing does and why

Annealing alone is often not enough. The microstructure of annealed plain carbon and alloy steel still contains pearlite—thin lamellae (layers) of ferrite and cementite (iron carbide). When the wire is cold-headed, these lamellae must deform together, but because they have different strengths, they deform unevenly. This creates internal stress and can cause micro-cracks.

Spheroidizing transforms the pearlite into isolated, rounded carbide particles embedded in a soft ferrite matrix. This structure deforms much more uniformly, improving ductility and reducing the risk of cracking during cold heading.

Spheroidizing is essential for:

• Medium-carbon steels (0.30–0.50% C)—especially for Grade 8.8 and 10.9 bolts
• Alloy steels (Cr-Mo, boron steels)—for high-strength fasteners
• High-carbon steels—for very demanding applications

It is less critical for low-carbon steels (< 0.25% C), which naturally contain less pearlite and can often be used with process annealing alone.

The spheroidizing process

Two main methods are used industrially:

Subcritical annealing (most common for fastener wire)

The wire is heated to just below the lower critical temperature (about 650–700°C for carbon steels). At this temperature, the pearlite does not dissolve, but the cementide particles begin to round up and separate. The wire is held at this temperature for a long time (typically 4–20 hours) to allow the transformation to go to completion.

Advantages:

• Lower temperature, less risk of decarburization
• Slower cooling is acceptable; no need for rapid quenching
• Less distortion and oxidation

Disadvantages:

• Longer cycle time
• Requires tight temperature control (within ±10°C) to avoid incomplete transformation

Full annealing followed by temper

The wire is heated above the upper critical temperature (typically 800–850°C for carbon steels), held briefly to dissolve carbides, then cooled slowly. This creates pearlite again. A second heat to 500–600°C (temper) then begins to spheroidize the pearlite.

Advantages:

• Faster initial heating
• Better grain refinement
• Can be combined with a temper for final hardness control

Disadvantages:

• Higher temperature, greater risk of scaling and decarburization
• More complex two-stage process
• Higher energy cost

Microstructure after spheroidizing

The target microstructure is:

• Ferrite matrix (soft, ductile)
• Spheroidal cementite particles (small, isolated, rounded)
• Grain size: fine and uniform

Under a microscope, this looks very different from pearlite’s distinct light and dark bands. The carbide particles appear as small dark dots scattered throughout the light ferrite.

Mechanical properties after spheroidizing

A fully spheroidized wire for a 10.9-grade bolt (medium-carbon alloy steel) might show:

• Tensile strength: 350–450 MPa
• Yield strength: 250–350 MPa
• Elongation: 30–40%
• Hardness: 120–150 HV

The combination of low yield strength, high ductility, and uniform microstructure makes this ideal for cold heading.

Part 5: Quality control and verification

Hardness testing

The most common in-process check is hardness. A Vickers hardness gun with a light load (typically 10 kg) is applied to the wire surface. For header-ready wire:

• Carbon steel, annealed: 95–120 HV
• Carbon steel, spheroidized: 120–150 HV
• Alloy steel, spheroidized: 150–180 HV

Wide hardness variation (> 20 HV) signals a problem: uneven annealing, cooling rate changes, or contamination.

Tensile testing

Pull testing on a sample of wire from each coil confirms strength and elongation. A typical spec for a low-carbon, annealed wire:

• Tensile strength: 350–420 MPa
• Yield strength: 250–320 MPa
• Elongation: > 25%

Microstructure inspection

Samples are sectioned, polished and etched (typically with 2% nital acid for carbon steels) and examined under an optical microscope. The metallurgist is looking for:

• Degree of spheroidization: are most carbide particles rounded, or are some still lamellar?
• Grain size: is it fine and uniform?
• Decarburization: is there a soft, carbon-depleted layer at the surface?
• Inclusions: are there non-metallic particles (oxides, silicates)?

Complete spheroidization can take time. A wire that has been subcritical annealed for only 2–4 hours might be 70–80% spheroidized; 12–16 hours is typically needed for 95%+ completion.

Cold-heading trials

The ultimate test: does the wire actually run without breaking heads off in the heading machine? Small batches are often run on a header to catch problems before a large order is committed.

Part 6: Special considerations and challenges

Decarburization

High temperature, long soak times and poor atmosphere control can cause decarburization—loss of carbon from the surface. A decarburized layer is soft and weak, and fasteners made from decarburized wire are prone to fatigue cracking.

Furnace atmosphere is the primary control:

• Endogas (a mix of CO, H₂, N₂ from partially burned methane-air) is neutral or slightly reducing, preventing carbon loss
• Nitrogen alone is inert but does not prevent carbon loss if the furnace leaks
• Air or steam cause rapid decarburization and are avoided

Inclusion cleanliness

Steel mills rate cleanliness using oxide and non-metallic inclusion content. High-cleanliness grades (typically designated “extra low carbon” or ELC for stainless, or by inclusion size limits for carbon steel) improve ductility and fatigue resistance.

For cold heading, especially of high-strength fasteners, cleanliness matters: hard inclusions can initiate cracks or cause header die wear.

Boron steel and its quirks

Boron steel (typically 0.19–0.25% C with 0.001–0.003% B) is popular for Grade 10.9 bolts because boron increases hardenability without requiring much nickel or molybdenum. But boron segregates along grain boundaries during cooling, and if the wire is annealed at too high a temperature, boron can precipitate as hard phases that embrittle the wire.

Boron steels require careful control of annealing temperature, typically in the 500–600°C range for process annealing, or special protocols if subcritical spheroidizing is used.

Hydrogen embrittlement risk

If the wire is treated in a wet environment (scale removal or pickling in dilute acids) before annealing, hydrogen can diffuse into the steel. Even small amounts of hydrogen trapped in the microstructure can cause delayed cracking, especially in high-strength grades.

Best practice: carry out acid pickling after annealing and spheroidizing, not before. This limits the time hydrogen has to diffuse back in and allows any absorbed hydrogen to be released at the high temperature.

Part 7: Linking to cold heading performance

A well-prepared header-ready wire exhibits:

1. Consistent diameter (±0.02 mm or better) for repeatable tool fill and clamp loads
2. Uniform hardness (within ±20 HV) to avoid die wear and part variation
3. High ductility (elongation > 25%) to flow without cracking
4. Spheroidized microstructure (for medium-carbon and alloy grades) for uniform deformation
5. Clean surface free of heavy scale and deep defects

When these are met, the header machine can run at high speed (up to 100+ parts per minute for smaller fasteners) with low scrap rates and consistent quality.

When they are not—when the wire is too hard, unevenly annealed, or contaminated—the consequences are immediate:

• Head breaks: metal fractures during forming, requiring production stops to clear the die
• Incomplete fill: the die pocket is not fully filled, producing undersized or misshapen heads
• Tool wear: hard spots accelerate die wear, raising changeover frequency and cost
• Fatigue failure: residual stress or defects initiated during forming cause premature failure in service

Understanding wire preparation is thus central to understanding fastener quality and cost.

Summary

The transformation from wire rod to header-ready wire is a three-stage process: drawing reduces diameter and consolidates the metal, annealing restores ductility, and spheroidizing transforms the microstructure into a form that deforms uniformly under the extreme stresses of cold heading.

Each stage is controlled by temperature, time, atmosphere and mechanical parameters. The result is a material that looks simple—a smooth coil of shiny steel wire—but whose properties are tightly engineered to enable one of the most efficient metalworking processes ever invented.

For a fastener producer, control of wire preparation is control of scrap rate, tool life, production speed and final quality. For a buyer specifying fasteners, understanding these steps provides insight into why quality differences exist and why the cheapest fastener is not always the best buy.

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References and further reading

• ASM Handbook Vol. 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials (for ferrous materials properties and processing)
• VDI 2230: Systematic calculation of high duty bolted joints (for clamp force and joint design consequences of wire metallurgy)
• ISO 8654: Fasteners—Mechanical properties of stainless steel bolts, screws and studs (material standards)
• DIN EN 10227: Steel wire rods for fasteners—Technical delivery conditions (German standard covering wire rod specifications)
• ASTM A370: Standard Test Methods and Definitions for Mechanical Testing of Steel Products (mechanical testing procedures)
• Verhoeven, J. D. Steel Metallurgy for the Non-Metallurgist. ASM International, 2007.
• Davis, J. R. (Ed.). Alloying: Understanding the Basics. ASM International, 1993.