Material science: tool steel and heat treatment
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 mold or a punch die, the task starts with a choice: which tool steel fits the load, temperature, and number of parts the tool must handle? According to the education's decree, you must be able to choose tool materials, heat treatment, and possible surface coating for a manufacturing task considering given production and application parameters—and that is a decision that determines whether the tool will hold up for 10,000 or 10 million parts.
§Four steel families you encounter in the workshop
- 01Cold work steel — works at room temperature, high hardness and wear resistance, typically for punches and dies in stamping tools
- 02Hot work steel and mold steel — withstands elevated temperature from molten plastic or metal, used in injection molding dies and die casting tools
- 03High-speed steel (HSS) — retains hardness at high cutting temperatures, typically used for cutting tools such as drills and cutters
- 04Powder metallurgical steel — made from fine-grained steel powder, gives extra wear resistance and toughness to demanding, heavily loaded punch tools
§Cold work steel for stamping dies
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.
§Hot work steel and mold steel for injection molding dies
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 |
§Heat treatment: hardening and tempering
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.
§Surface coating extends tool life
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.