What Is Tool and Die Steel?
Tool and die steel covers the carbon, alloy and high-speed steels used to make tools, dies and moulds that shape or cut other materials. Where a die only has to survive a few thousand cycles, a plain carbon tool steel is enough; where a stamping die, forging die or die-casting mould has to run for hundreds of thousands of cycles, an alloyed die steel with carbide-forming elements is required. The grade choice is therefore always a balance between wear resistance, toughness, hot hardness and machinability, weighted by the process the die will run.
Tool steel is grouped into cold work, hot work, plastic mould and high-speed families, and each family is defined by an alloy system rather than by a single property. Within each family, the standard grades are catalogued in ASTM A681 for tool steels, ASTM A686 for high-speed tool steels, EN ISO 4957 internationally, and JIS G4401, G4403 and G4404 in Japan.
Die Steel Families and Representative Grades
| Family | Typical grades | Defining properties | Main uses |
|---|---|---|---|
| Cold work steel | D2, D3, A2, S7 | High wear resistance and compressive strength, moderate hot hardness | Blank and form dies, punching tools, shear blades |
| Hot work steel | H13, H11, H21 | Hot hardness to about 500 degrees C, thermal fatigue resistance | Forging dies, die-casting moulds, extrusion tooling |
| Plastic mould steel | P20 and pre-hardened variants | Machinability at 28-34 HRC, polishability, uniform hardness | Injection moulds for plastics and rubber |
| High-speed steel | M2, M35, T1 | Red hardness and high cutting wear resistance | Drills, taps, milling cutters, broaches |
The practical differences inside a family are worth noting. In the cold work group, D2 has the highest wear resistance of the three common grades because of its high chromium and carbon content, A2 trades some wear resistance for better dimensional stability after hardening, and S7 is chosen where shock resistance matters more than abrasion resistance.
Chemical Composition of Common Tool and Die Steels
| Grade | C % | Cr % | Mo % | V % | Other |
|---|---|---|---|---|---|
| D2 | 1.40-1.60 | 11.0-13.0 | 0.70-1.20 | 0.50-1.10 | Mn max 0.60 |
| A2 | 0.95-1.05 | 4.75-5.50 | 0.90-1.40 | max 0.50 | Mn max 1.00 |
| S7 | 0.45-0.55 | 3.00-3.50 | 1.30-1.80 | - | Si 0.20-1.00 |
| H13 | 0.32-0.45 | 4.75-5.50 | 1.10-1.75 | 0.80-1.20 | Si 0.80-1.20 |
| P20 | 0.28-0.40 | 0.40-2.00 | 0.30-0.55 | - | Mn 0.60-1.00 |
Chromium and vanadium are the elements that create the hard carbides responsible for wear resistance; molybdenum raises hardenability and, in hot work grades, provides high-temperature strength; silicon improves tempering resistance in H13. Carbon content separates the families: cold work and high-speed grades run above 0.9% carbon, hot work grades sit near 0.35-0.45%, and plastic mould grades are lower still so that they can be supplied pre-hardened and still machined.
Key Properties That Drive Selection
Wear resistance: governed by carbide volume and hardness; needed for high-volume stamping and abrasive blanking, and best served by D2 and the high-speed grades.
Toughness: resistance to chipping and cracking under impact; S7 and H13 lead here, and toughness generally falls as hardness and carbide content rise.
Hot hardness: the ability to keep hardness at elevated temperature. H13 retains useful hardness to about 500 degrees C, which is why it dominates die casting and hot forging.
Dimensional stability: A2 and the air-hardening grades resist movement in heat treatment, which matters for dies with tight fits or complex geometry.
Machinability and polishability: pre-hardened P20 plate can be milled and then polished to a fine surface finish without further heat treatment.
These properties pull against each other. Raising hardness to fight wear usually costs toughness; raising alloy content to improve hardenability reduces weldability and machinability. The correct answer for a given die is set by the dominant failure mode, whether that is abrasive wear, chipping, heat checking or plastic deformation.
Heat Treatment and Selection Guidance
Die steels are supplied annealed for machining and are hardened and tempered after the cavity or profile has been produced. Typical practice is austenitizing between about 800 degrees C and 1050 degrees C depending on grade, quenching in oil, air or a salt bath, then tempering between about 150 degrees C and 650 degrees C to reach the working hardness while relieving stress. Double tempering is common for hot work grades to stabilize the structure against heat checking.
High-temperature work such as aluminium or zinc die casting and hot forging: select H13 for its combination of hot hardness, thermal fatigue resistance and toughness.
Cold work such as stamping, blanking and shearing: D2 where wear is dominant, A2 where dimensional stability or toughness is the limiting factor.
Heavy impact such as chipping-prone trimming or cold heading: S7, which is engineered for shock resistance rather than maximum abrasion resistance.
Plastic and rubber moulding: pre-hardened P20 plate for large moulds requiring good machinability and surface finish, with harder grades reserved for high-gloss or glass-filled polymer work.
Plates are commonly supplied pre-hardened or annealed to the ASTM A681 grade list, with cut lengths and thicknesses to drawing. Round bars follow the same families, including the carbon tool steel and alloy tool steel ranges catalogued in EN ISO 4957 and JIS G4401 to G4404, and are used for punches, inserts, cores and small die components.
Frequently Asked Questions
Q: What is the difference between tool steel and die steel?
Die steel is a subset of tool steel. The term is used for grades optimized for dies, moulds and punches, with the toughness and heat resistance needed for stamping, forging and die casting rather than for cutting tools.
Q: Which die steel is best for hot work?
H13 is the standard choice for die casting and hot forging because it keeps hardness to about 500 degrees C and resists thermal fatigue cracking.
Q: Is D2 or A2 better for cold work dies?
D2 offers higher wear resistance and suits high-volume abrasive stamping. A2 gives better toughness and dimensional stability after hardening and is preferred where the die is complex or shock loaded.
Q: What hardness do tool and die steels reach?
Cold work grades such as D2 and A2 typically reach 57-62 HRC, S7 is usually used near 56-58 HRC, H13 runs about 44-52 HRC in service, and pre-hardened plastic mould plate is supplied around 28-34 HRC.
Q: Why is die steel tempered after hardening?
Tempering relieves the internal stress created by quenching and adjusts toughness against hardness. Without it, a hardened die is brittle and prone to cracking in service.
Q: Which standards specify tool and die steels?
ASTM A681 covers tool steels, ASTM A686 covers high-speed tool steels, EN ISO 4957 is the international tool steel standard, and JIS G4401, G4403 and G4404 cover carbon, high-speed and alloy tool steels.
