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From cold-work steel to powder metallurgy — how you choose the right steel for the task
A tool is only as good as the steel it is made of. Whether you are building an injection mould or a punching tool, the task starts with a choice: which tool steel suits the load, the temperature and the number of parts the tool must handle? According to the training regulations, on the tool designer specialisation you must be able to choose tool materials, heat treatment and any surface coating for a manufacturing task, taking given production and application parameters into account — and that is a decision that determines whether the tool will last for 10,000 or 10 million parts.
Stamps and dies that cut and blanks sheet metal hour after hour must withstand enormous wear without chipping. A typical cold-work steel with high chromium and carbon content is hardened in oil or water and tempered afterwards so it ends around 58–62 HRC on the Rockwell scale – hard enough to hold a sharp edge but with the tempering the steel retains enough toughness not to become brittle and crack under the repeated punching in the press.
When plastic pellets are melted and pressed into a mold at high temperature again and again, the mold's surface is exposed to both heat and pressure. Here tool steel and special mold steel qualities are used, developed to resist temperature swings without deforming or cracking—and which can also be polished to the surface finish the finished plastic part should have.
| Steel family | Typical use | Tempered hardness | Characteristic |
|---|---|---|---|
| Cold work steel | Stamps and dies in punch tool | ca. 58-62 HRC | High wear resistance sharp edge lasts a long time |
| Hot work steel/mold steel | Spray-cast moulds, pressure-cast tools | typically lower, depending on type | Withstands temperature fluctuations, can be polished to high gloss |
| High-speed steel (HSS) | Cutting tools: drills, mills | typically high, preserved by heat | Maintains hardness at high cutting temperature |
| Powder metallurgical steel | Highly loaded complicated die tool | high with extra toughness | Fine-grained structure provides extra toughness and wear resistance |
Steel does not become hard by itself — it is the heat treatment that makes the difference. During hardening, the steel is heated to a certain temperature and cooled quickly in oil or water, which makes it hard but also brittle. Therefore, it is tempered afterwards: a renewed heating to a lower temperature, which removes stresses and gives the steel the toughness it needs not to break under load. For cold-work steel, a typical low tempering temperature is around 160-300 °C, while hot-work steel is often tempered at higher temperatures to withstand the operating heat in the mould.
Even the best tool steel can have its life extended with a surface coating. PVD coatings such as TiN (titanium nitride) and TiAlN (titanium-aluminium-nitride) apply a thin extremely hard layer on the steel which reduces wear and friction — particularly valuable on cutting tools and dies that work under constant mechanical stress.