Hard steel materials: families, grades and hardness

High-carbon, high-chromium steels for wear resistance at room temperature. D2 and D3 carry a large volume of chromium carbides. A2 gives up some wear resistance for toughness and very low distortion. O1 is the low-cost oil-hardening option.

Used for: Blanking and piercing punches and dies, forming tools, shear blades, thread rolls, gauges.

Watch for: D2 chips before it wears on heavy-impact jobs. For wire EDM work, specify the high-temperature temper (about 500-540 °C). Low-tempered D2 can crack when a large slug is cut out.

• D2 in the grade table
• A2 vs D2
• O1 vs A2

Medium-carbon Cr-Mo-V steels that keep their hardness at 500-600 °C and resist heat checking. Carbon is lower than in cold work steel. The result is higher toughness and easier hard milling.

Used for: Die casting dies, forging dies, extrusion tooling, hot stamping and high-volume plastic moulds.

Watch for: Often nitrided after machining for wear resistance. Specify the nitriding depth and the surface hardness on the drawing.

• H13 in the grade table
• H11 vs H13

Low-carbide steels designed for impact toughness instead of wear resistance. They take impact loads that crack D2.

Used for: Heavy punches, chisels, shear blades for thick plate, rivet sets, ejector parts that take impact.

Watch for: Wears faster than cold work steel under sliding abrasion. A tool that fails by wear, not by cracking, needs a different grade.

• S7 in the grade table
• Machining S7

Tungsten-molybdenum steels with vanadium. M35 and M42 add cobalt. They keep their hardness at red heat. Originally a cutting tool material, now also used for punches in thin high-strength sheet.

Used for: Cutting tools, punches and buttons for high-strength sheet, cold forming tools, wear parts that need more than D2.

Watch for: Above about 64 HRC, hard milling is rarely economic. Plan grinding and EDM for the finished features.

• M2 in the grade table
• Machining M2 and M42

Made from atomised powder. Carbides are small and evenly spread, without the coarse bands of ingot steel. Wear resistance and toughness are both higher. Price is about 2-4x conventional grades.

Used for: Long-run punches and dies, slitter knives, tooling for abrasive or high-strength materials.

Watch for: The vanadium carbide that gives the wear resistance also wears grinding wheels and cutters. Finishing times are longer.

• PM tool steels

Steels selected for machinability, polishability and size stability in large blocks. Pre-hardened grades are machined to size with no furnace step.

Used for: Injection mould cavities, cores, inserts, mould plates; 420 for PVC, medical and optical parts that need corrosion resistance and a mirror finish.

Watch for: Pre-hardened P20 wears and dents in long runs or with glass-filled resins. The usual fix is a hardened insert in H13 or 420.

• P20 in the grade table
• Hardened mould inserts
• 420 mould steel

Medium-carbon Cr-Mo steels. They are not tool steels, but most holders, shafts and structural tooling parts use them. Through-hardenable in moderate sections and suited to induction hardening.

Used for: Die shoes, holders, fixture plates, shafts, gears, pins, induction-hardened wear surfaces.

Watch for: "4140" on a drawing without a hardness is incomplete. Annealed, pre-hardened and fully hardened 4140 are 3 different machining jobs.

• 4140 in the grade table
• Pre-hardened 4140 and P20

Low-carbon steels for carburising. Carbon is diffused into the surface. After quenching only the outer 0.5-2 mm is hard and the core stays tough.

Used for: Gears; splined shafts; pins; cams; rollers. Parts that need a wear surface and impact strength together.

Watch for: Every 0.1 mm ground off the surface comes out of the case depth. Specify the effective case depth after grinding.

• Case vs through hardening

A clean high-carbon chromium steel with a fine, uniform carbide structure. Hardens fully in moderate sections. Takes a ground or turned finish down to Ra 0.2 µm.

Used for: Bearing races and rollers, guide pins and bushings, gauge pins, precision shafts.

Watch for: The most common grade for hard turning in place of grinding. Fatigue-loaded surfaces need a white layer check.

• 52100 in the grade table
• Hard turning 52100

Stainless steels with enough carbon to harden like tool steel. Corrosion resistance is below 304 or 316. It also depends on the heat treatment.

Used for: Corrosion-resistant moulds, knives and blades, valve seats, pump parts, bearings in wet service.

Watch for: Tempering in the 425-550 °C range lowers corrosion resistance and toughness in these grades. The heat treat sheet should show the temper temperature.

• 440C in the grade table
• 420 in the grade table

Steels hardened by ageing at about 480-620 °C instead of a quench. Without a quench there is almost no distortion. Size change is a small, predictable shrinkage.

Used for: Precision tooling and fixtures, aerospace and medical parts, conformal-cooled mould inserts (maraging is common in metal 3D printing).

Watch for: Maraging steel is expensive and not widely stocked. Confirm availability before the design is fixed.

• 17-4 PH in the grade table
• Machining maraging steel

Low-alloy plate, quenched and tempered at the mill. It is not heat treated again. Every hole, bevel and profile is cut at full hardness.

Used for: Chute and hopper liners, bucket edges, crusher and conveyor wear parts, skirt boards.

Watch for: Thermal cutting and welding soften the edge zone. Hole patterns close to a flame-cut edge can land in a softened band.

• Wear plates and liners