⚡ Key takeaways
- Hot forging means heating the steel to 1,000–1,200 °C before forming. It is the route for large sizes and materials that cannot be cold formed.
- When hot forging wins: above about M24, stainless and alloy steels, or any head with an upset ratio too severe for cold heading.
- The costs of hot forging: oxide scale, low dimensional accuracy and the need for secondary machining.
- The benefit: flow stress drops by an order of magnitude, so large and complex shapes become possible at all.
- Hot forged parts always need heat treatment — normalising or quench and temper — to refine the grain and restore properties.
- Cold heading versus hot forging is mostly a size question: small parts are cheaper and faster cold; large parts can only be hot forged.
- The black surface is oxide scale and must be removed by machining, blasting or pickling before plating.
1. When hot forging is the only option
2. The process route
3. Hot forging versus cold heading
4. Hot forging tooling
5. Post-processing
6. Decision tree and FAQ
1. When hot forging is the only option
Cold heading suits small sizes and ductile materials. The following cases have to be hot forged:
- Size above about M24, where the former simply does not have the tonnage.
- Stainless 304 or 316, which work-hardens severely and cracks before the head fills.
- Alloy steel such as 42CrMo, whose flow stress is too high at room temperature.
- A head upset ratio above roughly 3:1, where cold heading produces laps and splits.
2. The process route
- Cut off: bar stock sawn to length, rather than coiled wire.
- Heat: medium-frequency induction heating to 1,050–1,150 °C, with the temperature held in a narrow band to avoid burning the grain boundaries.
- Forge: upset the head in a die, usually on a friction or electro-hydraulic press.
- Trim: remove the flash.
- Heat treat: normalise, or quench and temper to the required property class.
- Machine: face the ends, drill centres if needed, then roll the thread.
3. Hot forging versus cold heading
| Dimension | Cold heading | Hot forging |
|---|---|---|
| Size range | M1–M24 | M12–M100 and beyond |
| Accuracy | High — 6g thread straight off the roller | Low — needs machining |
| Surface | Clean and bright | Oxide scale |
| Rate | 100–300 pieces/min | 5–20 pieces/min |
| Relative cost | 1× | 2–3× |
| Materials | Low carbon, ductile | Stainless, alloy steel, high carbon |
4. Hot forging tooling
Hot forging dies work at around 1,000 °C, so they are made from hot-work tool steel such as H13 (SKD61) at 46–48 HRC. Life is short compared with cold heading tooling — a few thousand to a few tens of thousands of pieces — because thermal fatigue cracks the die face long before wear does. Preheating the die and controlling the cooling spray are the main levers for extending it.
5. Post-processing
- Scale removal: shot blasting or pickling. Scale left in place prevents plating adhesion and holds moisture.
- Heat treatment: normalising refines the grain after forging; quench and temper delivers 8.8 or 10.9.
- Machining: face the bearing surface and the end so that perpendicularity and parallelism meet the drawing. As-forged faces are never good enough for a preloaded joint.
6. Decision tree and FAQ
Step 1: what size?
Below M24 → cold heading. Above M24 → hot forging.
Step 2: what material?
Low carbon steel → cold heading. Stainless or alloy steel → hot forging.
FAQ
Q: Can a hot forged bolt reach class 10.9? Yes, through quench and temper after forging. The forging process itself does not set the strength — the heat treatment does, and it must be verified on the finished part, not on the billet.
Q: Why is the surface black? Oxide scale formed during heating. It has to be removed mechanically or chemically before any coating, otherwise the coating will not adhere.
Q: Is hot forging cheaper than cold heading? No. For small parts cold heading is cheaper and faster. Hot forging is used only where cold heading is impossible, and the question is whether the part can be made at all, not whether it is cheap.
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