Toughness vs wear resistance

Carbides, matrix and the trade-off

A hardened tool steel is a matrix of tempered martensite with hard carbide particles spread through it. The carbides resist wear and the matrix holds them and absorbs shock. More carbide, larger carbide and a harder matrix all raise wear resistance, and all three lower toughness.

The chart above places the common grades on that trade-off. It is schematic and relative. Positions shift with hardness, heat treatment and bar size.

No grade is best overall. The right grade sits at the point that prevents the failure seen on the last tool. The overview of hard steel families introduces the groups.

Sources of wear resistance

Carbide hardness

Carbon not dissolved in martensite combines with chromium, vanadium, molybdenum and tungsten to form carbides. These carbides are about 1.5-4 times harder than the matrix.

Phase	Typical hardness	Found in
Tempered martensite at 58-62 HRC	About 650-800 HV	Matrix of every hardened tool steel
Iron carbide (cementite, M3C)	About 1000-1150 HV	O1, 52100, plain carbon grades
Chromium carbide (M7C3)	About 1500-1800 HV	D2, D3, less in A2
Tungsten-molybdenum carbide (M6C)	About 1500-1800 HV	M2, M42
Vanadium carbide (MC)	About 2500-3000 HV	M2, PM high-vanadium grades
Silica sand, quartz (abrasive)	About 1000-1200 HV	Abrasive in many wear jobs

Silica is harder than martensite and softer than vanadium carbide. In abrasive wear the matrix wears around the carbides, and only carbides harder than the abrasive stop it. A PM grade with about 10% vanadium therefore outlasts D2 on glass-filled plastics and abrasive powders at the same HRC.

Carbide volume and size

D2 carries about 12-15% carbide by volume, mostly primary chromium carbide formed as the ingot solidified. A2 has about 5-7%, O1 less and S7 very little. More carbide gives more wear resistance and more crack starters.

In conventional ingot steel, primary carbides form in the last liquid to freeze. Rolling and forging break them up and stretch them into bands along the bar axis. In a large D2 bar, carbides of 10-40 µm and bands several millimetres long are normal.

Powder metallurgy (PM) steel is atomised into fine powder and then compacted. The carbides have no time to grow, so they are typically 1-3 µm and evenly spread.

PM grades therefore sit above and to the right of D2 on the chart. Conventional D2 is also less tough across the bar than along it. A crack running along the carbide bands follows them.

Sources of toughness

Martensite matrix and tempering

Matrix hardness comes from carbon trapped in martensite. Above about 0.6% dissolved carbon the matrix gains little hardness, and the extra carbon forms carbides. Tempering trades hardness for toughness.

Each HRC point at the top of a grade’s range costs a clear share of impact toughness. For a chipping D2 punch, 61-62 HRC → 58-59 HRC is the usual first change.

Heat treatment faults that reduce toughness without changing the hardness reading:

• Untempered martensite. A single short temper, or service straight after a sub-zero step. The part cracks in handling.
• Coarse grain. Overheating during austenitising. The part meets the hardness spec and fractures at low energy.
• Decarburised skin. A soft surface layer left on the part. It wears and deforms first.
• Tempering in the embrittlement band. Many grades have one in the middle temperature range. The grade datasheet gives the band to avoid.

The cycle itself is covered on the heat treatment page.

Retained austenite

Some austenite does not transform when high-carbon, high-alloy steel is quenched. In D2 hardened from the top of its range it can exceed 15% of the structure. Under load or over time it turns into fresh, untempered martensite, and the part grows slightly and becomes brittle.

Process sequence for conversion: harden → sub-zero treatment (optional) → temper → temper → temper (if specified). For gauges and precision die plates the conversion matters more than the hardness reading.

Impact toughness testing

Tool steels are tested with unnotched or C-notch Charpy specimens. A sharp V-notch already acts as a crack, and at 62 HRC it gives values too low to separate grades. Background: Charpy impact test.

Unnotched values from different steel makers are not directly comparable. Specimen size, orientation and heat treatment differ. Use them to rank grades from one source only.

Tool failure modes and their cures

Failure mode	Appearance	Cause	Direction of the fix
Abrasive wear	Rounded, polished edge, fine scratches in the flow direction	Hard particles cutting the matrix	More or harder carbides, higher hardness, coating
Adhesive wear (galling)	Workpiece material welded to the tool, torn surface	Metal-to-metal contact on stainless, aluminium, coated sheet	Coating (PVD, nitriding), smoother finish, lubrication
Chipping	Small fragments missing along the edge	Local cracks at carbides or edge defects	Lower hardness, finer carbides, edge hone
Cracking, gross fracture	Tool broken through the section	Overload, shock or a stress raiser	Shock grade, larger corner radii, heat treatment check
Plastic deformation	Mushroomed face, rolled edge	Steel yielding under load	Higher hardness or compressive strength
Heat checking	Fine crack network on hot work surfaces	Thermal cycling	Hot work grade at controlled hardness, die preheat

Two failure modes on one tool are common, and they pull in opposite directions. A punch that chips and wears fast is the typical case for a PM grade, or for a coating on a tougher base steel.

Grade changes by failure mode

Observed failure	Typical change	Trade-off
D2 chips, wear acceptable	A2 at 58-60 HRC, or a PM cold work grade	Less wear life with A2
D2 wears, never chips	PM high-vanadium grade, or D2 + PVD coating	Higher bar and grinding cost
Cracks under shock (chisels, heavy shear blades, heading punches)	S7 at 54-58 HRC	Low wear resistance
S7 or A2 mushrooms	Top of the hardness range, then M2 at 60-62 HRC	Less toughness
O1 wears in production	A2 or D2	Higher machining cost
Hot work die heat checks or cracks	H13 at lower hardness, or ESR H13	Faster wear
All tool steels wear too fast	Cemented carbide	Very low toughness, design must avoid bending and impact

The A2 vs D2 comparison covers the cost side of the first change.

Some causes are outside the steel. Typical examples: wrong die clearance, a sharp corner at a loaded section, misalignment, grinding burn, EDM recast on a working edge. These are checked before the grade is changed.

Drawing and purchase requirements

• Grade and hardness band together. “D2, 58-60 HRC”, not “D2, hardened”. D2 at 58 and at 62 HRC behave as 2 different tools.
• Failure description of the old tool. “Chipped at the cutting edge after about 20,000 hits” gives the shop enough to propose an alternative.
• Retained austenite control on precision parts. Specify multiple tempers or a sub-zero step, and request the tempering record.
• PM or remelted grades named in plain terms. “PM cold work steel, fine carbide” or “ESR H13”. Verification is covered on the material certificates page.
• Cost and lead time. Both rise with carbide content, because D2 and PM grades machine and grind slower. Confirm per part with the shop.

Terms such as carbide, retained austenite and secondary hardening are defined in the glossary. A drawing with a failure description can be sent through the RFQ form.