Retained Austenite in M2 Tool Steel and How to Reduce It
What M2 high speed steel keeps of its austenite after hardening, and what single, double, triple and quadruple 2-hour tempers at 965 °F (518 °C) each leave behind, from a tool and die shop handbook. A reference summary of published practice and not an Aobo Steel specification.
Why retained austenite causes trouble in a tool
Hardening does not finish the job. The quench turns most of the austenite into martensite and the rest stays as austenite, because the cooling stopped the transformation before it was complete. How much is left depends on the austenitising temperature, the alloy content and the cooling rate, and M2 is heavily alloyed, so it starts high. The figure in the table below is 24 percent after the quench.
The leftover austenite is softer than the martensite around it. A surface that still carries it deforms plastically under polishing pressure, which is what appears first as orange peel and then as pits, and it breaks away more easily than a fully martensitic surface. The second problem is timing. Austenite left in the part can transform later, under grinding heat or in service, and because that transformation grows the steel, a die finished to size can move after it leaves the grinder.
How each temper changes the figure
Retained austenite left after each tempering step
| Condition after hardening | Retained austenite, % |
|---|---|
| As quenched | 24.0 |
| Single 2-hour temper | 16.1 |
| Double temper, 2 + 2 hours | 4.7 |
| Triple temper, 2 + 2 + 2 hours | 2.7 |
| Quadruple temper, 2 + 2 + 2 + 2 hours | 1.7 |
Source, Table 4-7, effect of tempering M2 high speed steel on austenite retention, printed page 64 of Tool and Die Making Troubleshooter (Richard M. Leed), PDF page 81 of the copy consulted. The book credits Teledyne Vasco, 1968.
Every temper in the table was run at 965 °F (518 °C) for two hours, and the austenite was measured after each step. The figures come from one mill’s test on M2, so they show what repeated tempering does rather than what a given heat will do; the starting figure after the quench depends on the austenitising temperature and the soak the shop used.
What the numbers mean for a heat treat
The first temper at 965 °F (518 °C) removes a third of what the quench left. A second two-hour temper takes the figure below five percent, which is why double tempering is standard practice on air-hardening and high speed grades rather than a matter of preference. The third and fourth tempers keep working, but each returns less than the one before, and by the fourth the figure is under two percent.
Two things follow from that. A single temper is short on M2 even when the measured hardness already looks right, because the hardness comes from the martensite and says nothing about the austenite still sitting in the part. The tempering temperature also matters as much as the count. This table holds the temperature at 965 °F (518 °C), which sits in the secondary hardening range for M2, and running the same number of tempers lower leaves more austenite behind.
The other route, subzero treatment
Cooling below room temperature continues the transformation the quench interrupted, so a subzero step taken after hardening and before tempering lowers the figure the tempers then work on. It is used where the section is heavy, where dimensions have to hold through grinding, or where the grade is so heavily alloyed that a double temper alone does not bring the austenite down. It is common on the high carbon high chrome and high speed grades, and it is followed by tempering in every case. The treatment itself and the temperatures used are on our page on cryogenic treatment of tool steel.
Which grades this affects
The table is for M2 high speed steel, the general purpose molybdenum high speed grade. The same behaviour drives tempering practice on the high alloy cold work grades, D2 tool steel and the other air-hardening grades, and on hot work grades such as H13 tool steel, where a double temper is standard. Where the austenitising temperature, the soak and the tempering range sit for a given grade, the tool steel heat treatment guide carries the cycles, and the M2 chart is on our M2 heat treatment page.
Compiled from Tool and Die Making Troubleshooter (Richard M. Leed), Table 4-7 on printed page 64 and the discussion of transformation, stress and retained austenite on printed pages 63 to 66. The measurements are reproduced from the published table and are a reference summary rather than an Aobo Steel specification, so treat them as an indication of what multiple tempers achieve on M2 rather than as a heat treatment instruction for a particular part.