Microconstituents in Tool Steels After Hardening
A hardened tool steel is not one structure. It is martensite, retained austenite and undissolved carbides together, and the share each one takes decides the hardness the part reaches, the toughness it keeps and how much it moves during the treatment. This page gives the measured volume percentages for four tool steels quenched from their recommended austenitizing temperatures.
The three constituents
Martensite is the hard constituent and it supplies the as-quenched hardness. It forms during quenching, when the austenite that has been held at the austenitizing temperature cools fast enough to transform without diffusing carbon.
Retained austenite is the part of that austenite which never transformed, and it is still present at room temperature. It is softer than martensite, and because it can transform later it is the constituent that moves when the steel is tempered or used at temperature.
Undissolved carbides are the alloy carbides that never went into solution during austenitizing. They carry the wear resistance of the grade, and the volume left undissolved rises with the carbide-forming alloy content, mainly chromium, molybdenum, tungsten and vanadium.
The balance between the three is set by the alloy content of the grade, the austenitizing temperature and the quench. Alloy content is the largest of the three levers, because dissolved alloy lowers the temperature at which martensite starts to form and therefore leaves more austenite untransformed at room temperature.
Volume percentages after hardening
| Steel | Hardening treatment | As-quenched hardness, HRC | Martensite, vol% | Retained austenite, vol% | Undissolved carbides, vol% |
|---|---|---|---|---|---|
| W1 | 790 °C (1450 °F), 30 min; WQ | 67.0 | 88.5 | 9 | 2.5 |
| L3 | 845 °C (1550 °F), 30 min; OQ | 66.5 | 90 | 7 | 3.0 |
| M2 | 1225 °C (2235 °F), 6 min; OQ | 64 | 71.5 | 20 | 8.5 |
| D2 | 1040 °C (1900 °F), 30 min; AC | 62 | 45 | 40 | 15 |
Microconstituents in W1, L3, M2 and D2 tool steel after hardening from the recommended austenitizing temperature. WQ, water quench; OQ, oil quench; AC, air cool. Source, Table 1 of Size Distortion in Tool Steels, in ASM Handbook, Volume 4, Heat Treating.
Hardened structure of four tool steels, printable PDF
The three volume percentages for W1, L3, M2 and D2, with the hardening treatment each figure applies to, in one PDF with our contact details.
What the numbers say
W1 is a plain carbon water-hardening grade, and it comes out of the quench as almost nothing but martensite. It measured 88.5 vol% martensite with 9 vol% retained austenite and 2.5 vol% undissolved carbides at 67.0 HRC. L3 behaves in much the same way at 66.5 HRC, with 90 vol% martensite and 7 vol% austenite left.
M2 and D2 move in the other direction. Their alloy content holds the martensite start temperature down, so a much larger share of the austenite survives the quench. The measured retained austenite is 20 vol% in M2 and 40 vol% in D2. The same alloying leaves 8.5 vol% of carbides undissolved in M2 and 15 vol% in D2. Neither grade reaches the as-quenched hardness of W1, and the retained austenite is the reason.
Both readings are needed together. The carbide volume is what makes D2 tool steel hold its edge in abrasive work, while the austenite volume is a large part of why the same grade is tempered, and on many jobs tempered twice, before it is put into service.
How the retained austenite is dealt with
Tempering is the usual answer. As the tempering temperature rises through the range 120 to 260 °C (250 to 500 °F), part of the retained austenite transforms to bainite, and that transformation is accompanied by an expansion in volume. In the highly alloyed grades not all of it transforms, so some austenite is left in the structure after a single temper. Above roughly 540 to 595 °C (1000 to 1100 °F) the cementite formed during tempering redissolves into alloy carbides, which adds a further expansion in volume.
Those two effects are why the high alloy grades are the ones where the tempering sequence is planned rather than guessed. The structure the four grades start from is described grade by grade on the tool steel microstructure page, the way retained austenite is reduced is set out on the retained austenite page, and the movement the transformation produces is collected on the volume change page. Where the austenite has to be removed before tempering rather than during it, the treatment is described on the cryogenic treatment page. The high speed grade in the table, M2 tool steel, is the one where retained austenite is most often quoted.
Before you use these values
This page is a reference summary of published practice and it is not an Aobo Steel specification. The volume percentages are the published figures for the four grades named, at the hardening treatment given, and they will not be reproduced exactly by a different furnace, quench medium or austenitizing temperature. The treatment a tool receives remains the responsibility of the heat treater.
Source: ASM Handbook, Volume 4, Heat Treating, ASM International, 1991.