Tool steel grades — when to use A2, D2, O1, S7, H13.

Tool steels are classified by their primary application:

• W-grades (water-hardening): carbon tool steels, low cost, limited hardenability
• O-grades (oil-hardening): moderate hardenability, general-purpose
• A-grades (air-hardening): cold work, low distortion, wear-resistant
• D-grades (die steel): high chromium, highest wear resistance, cold work
• S-grades (shock-resisting): impact applications, lower hardness, high toughness
• H-grades (hot work): tolerates elevated temperatures during service
• M-grades (high-speed): tool bits, cutting tools (HSS)

This guide covers the 5 most commonly used grades in CNC machining work.

Best for: blanking dies, punches, forming tools, gauges, chamfers, arbors. The most popular general-purpose tool steel because it hardens uniformly with minimal distortion during heat treat.

Air hardening means the part is heated to ~960 °C, then simply cooled in still air. No oil or water quench needed, which reduces distortion dramatically. Critical feature for precision parts.

Best for: high-volume blanking and forming dies (10,000+ cycles). Long-run punches for stamping thin sheet. Knife blades (specialty outdoor/industrial knives).

The 12% chromium content creates hard chromium carbides in the steel matrix. These provide extraordinary wear resistance but make the steel more brittle — so D2 is NOT for impact applications. For 1 million-cycle production dies in stamping, D2 is the dominant choice.

Best for: simple punches, fixtures, arbors, bushings, small tools. General-purpose tool steel when A2's air-hardening advantage isn't needed.

O1 requires oil quench for hardening, which causes more distortion than A2. For precision parts, A2 is preferred. For simpler shapes where distortion is less critical (or can be compensated for by grinding after heat treat), O1 is more economical.

O1 is widely stocked in common sizes. Fast sourcing if you need it in a hurry.

Best for: chisels, hammer heads, impact punches, shear blades, breaker bars. Any application where the tool receives sharp impact loads that would crack D2 or other high-carbide steels.

S7 runs at lower hardness (54-58 HRC vs 58-62 for A2/D2) specifically to trade hardness for toughness. The lower hardness gives the steel enough ductility to absorb impact without chipping or cracking.

For punches on soft materials (mild steel sheet), D2 is better. For punches on hard materials (thick stainless, high-strength steels), S7's toughness beats D2's wear resistance on net tool life.

Best for: die-casting dies, hot forging dies, extrusion dies, injection molding tooling. Applications where the tool is heated during service and must retain hardness at elevated temperature.

H13 also has exceptional thermal fatigue resistance — it can endure thousands of hot/cold cycles without cracking. Used for aluminum die-casting dies that see 650 °C + water quench on every cycle.

Running at lower hardness (48-54 HRC vs 58-62 for cold work tools) reflects the trade-off: hot work dies need toughness more than hardness because the metal they're forming is already softer.