Tool Steel Impact Toughness Chart
V notch Charpy impact energy for the principal tool steels, read at the tempering temperature the source marks as optimum for each grade, with the hardness the part carries at that temper. Figures are given in foot pounds and joules. This is a reference summary of published test data and not an Aobo Steel specification.
How the Charpy value is read
The V notch Charpy test breaks a notched bar with a single swing of a weighted pendulum and records the energy the bar absorbs. A higher number means the grade takes a heavier blow before it breaks. A figure near the bottom of these tables describes a steel that will chip or split when a punch, a die or a blade edge meets a shock load.
Every value here was read at the tempering temperature the source marks as optimum for that grade, so the figure belongs with the hardness printed in the same row. Tempering below that temperature raises hardness and lowers the impact energy. Tempering above it buys toughness back and gives away wear resistance. The same grade can be moved up or down these tables by the temper alone, which is why the tempering temperature is in every row.
The spread between grades is what the selection decision turns on. D3 and D7 absorb 6 ft lb (8 J) on this test while S7 absorbs 240 ft lb (325 J), a difference of forty times between two steels that are both sold as tool steel. The shock resisting and hot work grades sit between them. The high speed steels sit at the bottom, because their carbon and carbide content leaves the steel less room to yield before it cracks.
Impact energy is one property among several and it is not a ranking of grades. A tool that fails by chipping or splitting is the case for a move up these tables, towards the shock resisting grades. A tool that fails by abrasive wear is the case for a move the other way, into the high carbon and high carbide grades, and the tempering decision follows that choice. The pages linked at the end of this page set out the wear side of the same decision.
Cold work and general purpose tool steels
The water hardening, oil hardening and air hardening grades used for punches, dies, forming tools and general purpose shop work. They are read at the low tempering temperature where each one carries its working hardness.
Impact energy at the optimum tempering temperature
| Grade | Type | Optimum temper, °F | °C | Hardness at that temper, HRC | Charpy impact, ft lb | J |
|---|---|---|---|---|---|---|
| W1 | Water hardening | 300 | 150 | 64 | 80 | 108 |
| W2 | Water hardening | 300 | 150 | 64 | 77 | 104 |
| O1 | Oil hardening | 300 | 150 | 63 | 33 | 45 |
| O6 | Oil hardening, graphitic | 300 | 150 | 63 | 33 | 45 |
| L6 | Oil hardening, nickel | 300 | 150 | 60/61 | 72 | 98 |
| A2 | Air hardening, 5% chromium | 400 | 205 | 60 | 17 | 23 |
| A6 | Air hardening, 1% chromium | 400 | 205 | 59 | 28 | 38 |
| D2 | Air hardening, 12% chromium | 960 | 515 | 58/60 | 8 | 11 |
| D3 | High carbon, high chromium | 400 | 205 | 62 | 6 | 8 |
| D7 | High carbon, 4% vanadium | 400 | 205 | 62 | 6 | 8 |
| 4140 / 4150 | Medium alloy, reference grades | 300 | 150 | 53/54 | 65 | 88 |
Source, tool steel selection chapter, printed pages 142 to 149. W1 and W2 share one chart in the source, as do 4140 and 4150, which are shown there as a reference point against the case hardened water hardening grades.
4140 and 4150 are not tool steels. The source prints them in the same chart to place the case hardened water hardening tool steels against a medium alloy steel.
Where two hardness values appear, in D2, L6 and the 4140 and 4150 row, the source prints a span rather than a single figure.
Shock resisting tool steels
S7, S1 and S5 are the grades for chisels, punches, pneumatic tools and any other work where a blow is part of every cycle. S7 is the air hardening grade here, and it holds the highest figure on this page.
Impact energy at the optimum tempering temperature
| Grade | Type | Optimum temper, °F | °C | Hardness at that temper, HRC | Charpy impact, ft lb | J |
|---|---|---|---|---|---|---|
| S7 | Shock resisting, air hardening | 450 | 230 | 58 | 240 | 325 |
| S1 | Shock resisting, oil hardening | 400 | 205 | 56 | 185 | 250 |
| S5 | Shock resisting, oil hardening | 400 | 205 | 60 | 170 | 230 |
Source, tool steel selection chapter, printed pages 150 to 153.
The 240 ft lb (325 J) figure for S7 is the maximum level the test equipment could record. The source states that calculations put the value nearer 280 ft lb (380 J).
Hot work tool steels
The chromium and tungsten hot work grades are read at a higher optimum temper, between 1000 and 1100°F (540 and 595°C), because they work hot and that is where the properties that matter to them settle.
Impact energy at the optimum tempering temperature
| Grade | Type | Optimum temper, °F | °C | Hardness at that temper, HRC | Charpy impact, ft lb | J |
|---|---|---|---|---|---|---|
| H13 | Chromium hot work | 1000 | 540 | 54 | 68 | 92 |
| H11 | Chromium hot work | 1000 | 540 | 57 | 85 | 115 |
| H12 | Chromium tungsten hot work | 1000 | 540 | 54 | 96 | 130 |
| A8 | Air hardening, tungsten | 1000 | 540 | 57 | 42 | 57 |
| H19 | Chromium tungsten cobalt hot work | 1100 | 595 | 52 | 43 | 58 |
| H21 | Tungsten hot work | 1000 | 540 | 53 | 34 | 46 |
Source, tool steel selection chapter, printed pages 154 to 157. The source lists A8 with the hot work group, and it is also supplied as a mold steel.
High speed steels
The molybdenum and tungsten high speed grades carry the most carbon and the most carbide of any tool steel in this list, and they absorb the least energy on this test. T15 gives 3 to 4 ft lb (4 to 5 J). Their place in a cutting tool rests on hot hardness and wear resistance rather than on impact energy.
Impact energy at the optimum tempering temperature
| Grade | Type | Optimum temper, °F | °C | Hardness at that temper, HRC | Charpy impact, ft lb | J |
|---|---|---|---|---|---|---|
| M1 | Molybdenum high speed | 1000 | 540 | 65 | 12 | 16 |
| M2 | Molybdenum high speed | 1000 | 540 | 66 | 10 | 13.5 |
| M3, class 1 | Molybdenum high speed | 1000 | 540 | 67 | 7 | 9 |
| M4 | Molybdenum high speed, vanadium | 1000 | 540 | 66 | 7 | 9 |
| M7 | Molybdenum high speed | 1000 | 540 | 67 | 11 | 15 |
| M42 | Cobalt high speed | 950 | 510 | 68 | 10 | 13.5 |
| T1 | Tungsten high speed | 1000 | 540 | 64 | 9 | 12 |
| T5 | Tungsten high speed | 1000 | 540 | 64 | 5 | 7 |
| T15 | Tungsten high speed, high vanadium | 1000 | 540 | 67 | 3–4 | 4–5 |
Source, tool steel selection chapter, printed pages 158 to 162.
What the figures do not cover
The figures come from one test series reported in the source, measured at room temperature. They compare grades against each other under those conditions, and they do not predict a service life. Notch geometry, specimen size, the direction of the specimen relative to the bar and the test temperature all move a Charpy result, so this number is not carried on a mill certificate and it should not be written into a purchase specification without agreeing the test conditions first.
The hardness in each row is the hardness that grade carries at the optimum temper, taken from the same chart in the source. Where a working range is wanted rather than one figure, the tempering chart linked below gives the window each grade is normally put into service at.
Related reference data
Tool steel is supplied in the annealed condition, which is described on the annealed condition of tool steel page. The wear and toughness ratings for the same grades are on the tool steel properties chart, and the grades chosen when chipping or cracking is the failure mode are on the tool steels for chipping resistance and abrasive wear resistant tool steels pages. Impact wear and the subsurface damage it leaves are measured on the impact wear and spalling page. The tempering window for each grade is on the tool steel tempering chart, the full schedule on the tool steel hardening and tempering chart, and hot hardness on the tool steel hot hardness chart. The grade most often specified for impact duty in our stock is on the S7 tool steel page, and the sheet metal end of the range on the tool steel for shear blades and slitter knives page.
Reference data compiled from the tool steel selection chapter of Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, 2nd edition, Hanser), printed pages 139 to 163. The impact energies are the source’s own V notch Charpy test series, read at the optimum tempering temperature it marks for each grade.