What is Hot Forging?

Description

Hot forging is the process of heating steel above its recrystallization temperature — typically around 1,100°C for scissor steels — and shaping it with dies or hammers. This aligns the grain flow along the blade, producing denser, stronger blanks than stamping or casting methods.

What is Hot Forging?

Hot forging is the process of heating steel above its recrystallization temperature — typically around 1,100°C for common scissor steels — and shaping it under compressive force using dies or hammers. The high temperature makes the steel plastic and workable, allowing it to flow into the desired shape while simultaneously refining its internal grain structure.

Why It Matters for Scissors

Hot forging produces scissor blanks with a grain structure that flows along the length of the blade, following its contours rather than being randomly oriented. This directional grain flow increases tensile strength along the cutting edge by 15-30% compared to stamped blanks, where the grain runs in whatever direction the sheet happened to be rolled.

The compressive forces during forging also close internal porosity and consolidate the steel, producing a denser blank. Forged blanks typically exhibit fewer inclusions at the blade edge after grinding, which translates to better edge retention and reduced micro-chipping during use.

Japanese manufacturers like Mizutani and Hikari use hot forging (鍛造, tanzou) for their premium lines. The Seki City scissor industry has maintained forging traditions alongside modern CNC methods. In contrast, most mass-market scissors worldwide use stamped blanks for cost efficiency — the forging step can add 30-50% to blank production costs due to energy, tooling, and labour requirements.

The forging temperature must be carefully controlled. Too high and the steel suffers grain growth and excessive scale formation. Too low and the steel resists deformation, risking cracking. For VG-10, the optimal forging range is approximately 1,050-1,150°C.

Technical Detail
The metallurgical advantage of hot forging lies in dynamic recrystallization. When steel is deformed above its recrystallization temperature (roughly 0.4-0.5 of the melting point in Kelvin), the distorted grains continuously reform into new, smaller, equiaxed grains. This produces a refined grain structure — ASTM grain size numbers of 7-9 are typical for well-forged scissor blanks, compared to 5-7 for as-rolled sheet stock. The forging process for scissor blanks typically follows this sequence: 1. **Billet cutting** — steel bar stock is cut to appropriate weight for the finished blank 2. **Heating** — the billet is brought to forging temperature in a gas or induction furnace 3. **Rough forging** — initial shaping establishes the basic blade and handle outline 4. **Finish forging** — more precise dies refine the shape closer to final dimensions 5. **Trimming** — flash (excess material squeezed out between dies) is removed Multi-step forging with reheating between steps produces the best results. Each forging pass further refines the grain while progressively shaping the blank. Some Japanese manufacturers use three or four forging steps for premium scissor blanks. The grain flow pattern is particularly important at the pivot hole area, where stress concentrations are highest during cutting. A forged blank has continuous grain flow around the pivot, while a stamped blank has grain abruptly cut at the hole edge. This makes forged scissors more resistant to fatigue cracking at the pivot — a failure mode that occurs in heavily-used professional scissors. Closed-die forging (where the steel is fully enclosed in the die cavity) produces tighter dimensional tolerances than open-die forging, typically within 0.5mm of final dimensions. This reduces subsequent grinding, preserving more of the refined surface grain structure at the blade edge. Open-die forging with hammers allows more flexibility but requires greater skill and produces less consistent results. The choice between forging and stamping is ultimately an economic one. For scissors retailing above approximately $200-300, the cost of forging is justified by the performance improvement. Below that price point, well-executed heat treatment of stamped blanks can produce perfectly serviceable scissors.

Sources

Frequently Asked Questions

Hot forging requires heating each blank to over 1,000°C, using heavy forging presses or skilled hammer operators, and multiple forging steps with reheating between them. The tooling (dies) is expensive, the process is slower, and the energy cost is significantly higher than simply cutting shapes from flat sheet steel.

Sometimes. Forged scissors often have a slightly different surface texture before finishing, and premium manufacturers will advertise forging as a feature. Once fully ground and polished, it becomes difficult to distinguish visually — but the internal grain structure differs, which affects long-term performance and durability.

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