Tool Steel Reference Data

Tool Steel Carbide Types and Hardness

The particles that carry the wear resistance of a tool steel are alloy carbides, and every type has its own chemistry, crystal structure and hardness. This page lists the carbides found in cold work and high speed tool steels, the hardness each one reaches, the alloy content that puts it there, and the way carbide type, size and distribution decide what the finished tool will survive.

MC carbide rich in vanadium or niobium M2C, M6C carbides rich in molybdenum or tungsten M7C3, M23C6 carbides rich in chromium M3C cementite, the iron carbide HV Vickers hardness, measured on the carbide vol.% carbide volume fraction of the microstructure

Carbide types, crystal structure and hardness The carbides printed in the MxCy notation used for complex carbides, where the metal side is a mixture that is rich in one element rather than a pure compound. The martensite line is the hardened matrix the carbides sit in, shown for scale.

Carbide	Main element	Pure carbide	Crystal system	Lattice parameters	Hardness
MC	Niobium	NbC	Face centered cubic	a = 4.47 Å	2300 HV
MC	Vanadium	V4C3	Cubic	a = 4.16 Å	2000 HV
M2C	Molybdenum	Mo2C	Hexagonal	a = 3.01 Å, c = 4.74 Å	1800 HV
M7C3	Chromium	Cr7C3	Hexagonal	a = 13.90 Å, c = 4.52 Å	1600 HV
M6C	Tungsten or molybdenum	Fe3Mo3C	Cubic	a = 11.12 Å	1500 HV
M3C	Iron	Fe3C	Orthorhombic	a = 5.06 Å, b = 6.74 Å, c = 4.50 Å	1100 HV
Martensite	Matrix reference	0.8% C steel	Tetragonal	a = 2.85 Å, c = 2.95 Å	900 HV

Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Table 5.2, which collects the values from the crystallography and hardness references listed there. Lattice parameters are room temperature values for the pure carbide.

Which element holds its carbon hardest The free energy of formation ranks the carbide forming elements. A lower value means the element pulls carbon out of solution more strongly, so the carbide forms earlier and dissolves less during austenitizing.

Element	Carbide	Free energy of formation (kJ per mol C)
Titanium	TiC	−170
Niobium	NbC	−135
Vanadium	VC	−42
Tungsten	WC	−35
Chromium	Cr7C3	−33
Chromium	Cr23C6	−20
Molybdenum	Mo2C	−5
Iron	Fe3C	+15
Manganese	Mn3C	+20
Cobalt	Co2C	+35
Nickel	Ni3C	+65

Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Figure 3.10. Values are for room temperature and 1% activity of the metal, expressed per mole of carbon. The lower the value, the stronger the carbide.

Reading the ranking

The order explains the shop floor behavior of these steels. Chromium carbides are the easiest to dissolve, molybdenum follows, and the MC carbides resist dissolution the most, with niobium carbide above vanadium carbide. That is why a D2 die can be austenitized in the usual range and still be full of chromium carbides, while the vanadium carbides in a high speed steel need temperatures near the solidus to go into solution.

Two consequences follow for the finished tool. Carbides that stay undissolved hold the wear resistance, and carbides that dissolve feed the tempering response, which raises hardness after tempering. The balance between the two is set by the austenitizing temperature, and the effect of that temperature on carbide dissolution is worked through on the D2 austenitizing page.

When a carbide stays in the steel An alloy addition dissolves in austenite only up to a limit. Everything above that limit stays as undissolved carbide, which is how the wear resistant carbides survive heat treatment. At 1% C the approximate limits are these.

Element	Stays in solution up to about	Excess forms
Tungsten	3% W	M6C
Molybdenum	3% Mo	M2C and M6C
Vanadium	0.5% V	MC
Niobium	0.01% Nb	MC

Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Section 5.3.2. The limits fall as the carbon content rises, because carbon competes with the alloying elements for the same solution sites.

Which carbides run in which grades The families below cover the tool steels in normal supply. The carbide content quoted is for the hardened and tempered condition at the usual working hardness.

Grade	Type	Carbide content in the finished tool	Carbide type
Cold work tool steels
O1, O2	Oil hardening	Practically none. The tempered martensite carries the wear resistance on its own.	Fe3C from tempering, no undissolved particles
A2	Air hardening, medium alloy	Low. The 5% Cr and 1% C stay mostly in solution at normal hardening temperatures.	M7C3
DC53 and 8% Cr grades	Air hardening, 8% Cr	Finer and fewer carbides than D2, which is why toughness is higher and the microstructure polishes better.	M7C3
D2	High chromium, 12% Cr	About 13 vol% in the hardened and tempered condition, and the most common carbide carrying cold work steel.	M7C3
D3, D6	High carbon, high chromium	Up to about 20 vol%. The most abrasion resistant of the cold work grades and the least tough.	M7C3
High speed steels
M2, M42	Molybdenum high speed	Up to about 15 vol%. In the molybdenum grades the M2C eutectic is metastable and converts to M6C.	MC with M6C
T1, T15	Tungsten high speed	Up to about 15 vol%. The tungsten grades form M6C directly during solidification.	MC with M6C
PM grades	Powder metallurgy	Same carbide types as the conventional grades of the same chemistry, two to ten times finer and evenly distributed.	MC with M6C
Hot work and mold steels
H13, H11, P20	Hot work and plastic mold	Designed without large undissolved carbides. Alloy additions sit close to the solubility limit and work through fine precipitates during tempering.	Fine secondary carbides and nitrides

Sources: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Sections 3.3 and 5.3, and the microstructure comparison in Figures 5.14 and 3.25.

Amount, size and distribution set the wear resistance Three factors decide how a population of carbides performs, and they are decided at two different stages. Amount and type come from the chemistry of the grade, and size and distribution come from the way the steel was made.

What the three factors do

Amount. More carbide means better wear resistance and lower toughness. The practical ceiling is around 15 vol% in normal grades, with 20 vol% the maximum seen in supplied steels. Above that the microstructure becomes so brittle that the ingot cannot be forged or rolled.

Size. Coarse carbides resist coarse abrasion best, because the abrasive particles can no longer cut the matrix without meeting a hard phase of comparable size. Fine carbides suit adhesive wear and polishing, where a coarse particle acts as a crack starter or tears out of the surface. That is the working trade behind abrasive wear grades sitting at one end of the scale and adhesive wear grades at the other.

Distribution. The same volume of carbide works much better when it is spread evenly. A steel made by powder metallurgy holds its carbides two to ten times finer than a conventionally cast grade of identical chemistry, which raises toughness and polishability without touching the chemistry.

A single comparison from the source shows how type beats volume. A 10% V experimental steel carrying about 15 vol% of MC carbides outwears D6 carrying about 20 vol% of M7C3 carbides under coarse sandpaper abrasion, because MC is roughly twice as hard as M7C3. Wear resistance rises with carbide hardness before it rises with carbide volume.

Carbides are the particle half of the microstructure

A tool steel is best read as a hardened matrix plus a population of particles. The matrix sets the strength and the tempering response, and the particles set the abrasive wear resistance. The grades built around each side of that split, with hardness and wear ratings, are collected in the tool steel properties chart.

Niobium carbide is the hardest one and the least used

Niobium forms the hardest carbide in the table and starts forming it at an addition of a fraction of a percent, so it works in grades with very little alloying room. Most tool steels were developed before niobium was widely available, and the source treats it as a promising direction more than a current practice.

Mold steels go the other way

Plastic mold steels are designed without large undissolved carbides, because every particle is a potential polishing defect and an etch line. How that plays out in surface finishing, including the grades that trade machinability against polishability, is covered in the polishing and texturing guide.

Powder metallurgy changes the size, not the type

A PM grade keeps the carbide chemistry of its conventional twin and only sharpens the distribution. If the question is wear against a fine abrasive or polishability, PM answers it. If the question is coarse abrasion, the chemistry answers it.

Confirm before quoting

Composition, carbide type and carbide volume are for general reference only. Actual values vary by standard, mill, heat number and heat treatment, and the austenitizing temperature alone can dissolve or retain a visible fraction of the carbide population. Confirm against the material test certificate (MTC) or contact Aobo Steel.

Related reference pages

Tool steel composition chart · Tool steel material finder · How to select tool steels · Tool steel machinability rating chart · Tool steels catalog · Tool steel equivalent grades

Sources: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press, 2017), Table 5.2, Figure 3.10, and Sections 3.3 and 5.3. Values cross checked against the micrographs and wear data in the same chapters. Reference data for comparison only. Confirm the grade and the heat treatment with your tool shop and the mill data sheet before production. Aobo Steel supplies tool steel in the annealed or prehardened condition.