Tool Steel Hardenability Chart
The section each tool steel will harden through, the hardness it reaches at the core, and the hardening temperature that goes with it. Water-hardening grades harden a thin skin, oil-hardening grades go deeper, and the air-hardening grades such as D2, A2 and S7 harden through the largest blocks, which decides whether a thick die section can be made from a given grade at all.
| Grade | Hardens through | Quench | As-quenched HRC (a) |
|---|---|---|---|
| W2 | Chill depth 1/8–3/16 in. (3.2–4.8 mm) | Water | 68 HRC |
| O1 | Through 1-3/4 in. (44.5 mm) diameter | Oil | 64 to 65 |
| S5 | Through 3 in. (76.2 mm) diameter | Oil | 62–63 |
| A2 | Through 4-1/2 in. (114.3 mm) diameter | Air | 62–63 |
| S7 | Through 2-1/2 in. (63.5 mm) diameter | Air | 59–60 |
| D2 | Through 5 in. (127 mm) diameter | Air | 61–63 |
| M2 | Through 1-1/2 in. (38.1 mm) diameter | Air | 65–66 |
| H13 | Through 2-1/2 in. (63.5 mm) diameter | Air | 52–54 |
Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 5-6, from Bethlehem Steel grade data sheets.
| Grade | Hardens through | Working HRC | Hardening temperature | Quench |
|---|---|---|---|---|
| W2 | Chill depth approximately 1/8 in. (3.2 mm) per side | 60–62 | 1,400–1,450 °F (760–788 °C) | Water quench |
| S7 | Through 2-1/2 in. (63.5 mm) round | 56–58 | 1,725 °F (941 °C) | Air quench |
| A2 | Through 4-1/2 in. (114.3 mm) round | 58–60 | 1,775 °F (968 °C) | Air quench |
| D2 | Through 5 in. (127 mm) round | 58–60 | 1,850 °F (1,010 °C) | Air quench |
Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 5-8, quoting Bethlehem Steel data sheets for W2, S7, A2 and D2.
| Grade | Hardens through | Working HRC | Hardening temperature | Quench |
|---|---|---|---|---|
| A2 | Through 4-1/2 in. (114.3 mm) square | 58–60 | 1,775 °F (968 °C) | Air quench |
| S7 | Through 2-1/2 in. (63.5 mm) square | 56–58 | 1,725 °F (941 °C) | Air quench |
| 420 SS | Through 1-1/2 in. (38.1 mm) square | 48–50 | 1,750–1,850 °F (954–1,010 °C) | Air quench |
| H13 | Through 1-1/2 in. (38.1 mm) square | 46–48 | 1,850 °F (1,010 °C) | Air quench |
| P-20 | Prehardened, no hardening step | 28–32 | Prehardened on delivery | None |
Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 5-10, from Bethlehem Steel data sheets.
| Material or coating | Knoop (HK) | Rockwell C (HRC) (b) |
|---|---|---|
| Diamond | 9,000 | 99 |
| Cubic boron nitride | 5,000 | 94 |
| Titanium nitride | 2,800 | 87 |
| Vanadium carbide | 2,500 | 85 |
| Silicon carbide | 2,400 | 84 |
| Aluminum oxide | 2,100 | 83 |
| Tungsten carbide | 1,900 | 82 |
| Zirconium oxide | 1,500 | 77 |
| Gas nitrided case | 850–1,000 | 65–70 |
| Chrome plating | 900–1,000 | 68–70 |
| Hardened high speed steels | 800–850 | 63–65 |
| Hardened tool steels | 460–690 | 45–60 |
| Annealed steels | 185 | 20 |
Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 5-11, from Bry Coat hardness conversion estimates.
- Knoop and Rockwell C are not on the same scale. The values above 68 HRC are approximations rather than conversions from hardness data, because the relationship between the two scales loses accuracy as Knoop values climb.
- The diameters above describe a round section cooled from every side. Actual depth varies with part design, part mass, hardening temperature and quench speed, and the core of a heavy rectangular block cools more slowly than the same weight of round bar.
Reading the table
Steels with good hardenability harden more deeply, and they harden more safely. A grade that transforms at a slow cooling rate can be quenched in air, and an air quench puts far less stress into the part than water or oil, so the section is less likely to crack or to move out of shape. That is why the air-hardening grades carry the heaviest die sections, and why quench cracking is usually a story about a low-hardenability grade in a heavy section.
The quench medium matters as much as the grade. O1 hardens through 1-3/4 in. only when the oil is agitated and kept cool, and a part that is slow quenched will read low in the core while it reads full hardness at the corner. Where a part must be quenched in water to harden, the section is limited by the risk of cracking rather than by the hardness target.
Air hardening also means less movement. D2 grows within plus or minus 0.0005 in. per in. of dimension through the whole cycle, against 0.0015 in. per in. for an oil-hardening grade, which is the difference between grinding to size after hardening and hoping the block comes out close. The figures for every family are in the size change chart, and the stock to remove from as-supplied hot-rolled bar is in the machining allowance chart.
High hardenability is not the same as high working hardness. H13 hardens through 2-1/2 in. yet works at 46 to 48 HRC, because a hot work die trades hardness for toughness at temperature. The hardness these grades hold once they are hot is in the hot hardness chart.
A shallow-hardening grade is the right answer for some tools. W2 gives a hard skin over a tough core in one quench, which suits a punch that needs a wear face and a body that will not shatter. P20 arrives prehardened at 28 to 32 HRC and is never hardened by the toolmaker at all, which is why the large mold blocks are made from it. Both routes are covered in the selection by application pages.
Unless the core is checked, a low reading at the surface says nothing about hardenability. Decarburization and a slow quench both soften the skin of a part that hardened correctly inside. Cut or grind a few thousandths off before testing, and use the hardness testing methods when a file is not enough. Values read on other scales convert in the hardness conversion calculator.
Hardenability comes from carbon and from the alloying elements that hold it in solution, chromium, molybdenum, manganese and tungsten. What each grade carries is in the composition chart.
Match the quench to the section
Water quench for a thin section in a shallow-hardening grade, oil for the middle range, air for anything heavy or awkward. A quench faster than the section needs buys nothing and costs stress. The round bar and plate sizes Aobo Steel holds are listed in the tool steel catalog.
Prehardened is a different purchase
P20 and the prehardened mold grades arrive at working hardness and are machined straight into the mold. Nothing about the hardenability tables applies to them unless the block is later hardened, which most mold shops avoid. Mold grades are grouped under plastic mold steels.
Keep the cycle with the steel
Hardenability sets what is possible, the furnace cycle decides what you get. The heat treatment guide lists the stages a tool steel cycle has to include, and the mill certificate proves the analysis that the hardenability was calculated from.
Section size beats grade name
Two dies in the same grade behave differently when one is four times the mass of the other. Where the section is the problem, the answer is usually a different grade rather than a longer soak time, and the grade selection guide works through that trade in the order a die shop faces it.
Related reference pages
Tool steel heat treatment troubleshooting chart · Steel heat color chart · Tool steel tempering chart · Tool steel annealing and normalizing chart · D2 austenitizing temperature · Tool steel forging temperature guide
Sources: Tool and Die Making Troubleshooter (R. M. Leed, Hanser Gardner), Tables 5-6, 5-8, 5-10 and 5-11, themselves from Bethlehem Steel Corporation grade data sheets (1976 to 1981) and Bry Coat hardness conversion estimates. Reference data for comparison only. Confirm the quench and the depth on your own furnace with a test piece before production. Aobo Steel supplies tool steel in the annealed condition, and hardening remains the customer’s heat treatment.