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 temperature. 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 steel that will chip or split when a punch, die, or 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 increases hardness and reduces impact energy. Tempering above it restores toughness at the expense of wear resistance. You can move the same grade up or down these tables with tempering alone, which is why every row includes a tempering temperature.

The spread between grades drives the selection decision. D3 and D7 absorb 6 ft lb (8 J) on this test, while S7 absorbs 240 ft lb (325 J), a forty-fold difference between two steels 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 less room for the steel to yield before it cracks.

Impact energy is one property among several, not a ranking of grades. A tool that fails by chipping or splitting is the case when moving up these tables toward the shock-resisting grades. A tool that fails by abrasive wear is the case when you 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 cover the wear side of that same decision.

Published impact figures for the same grade are not interchangeable, because the specimen controls the number. A C-notch bar absorbs more energy than a V-notch bar, and an unnotched bar even more. For A2 at 400 °F, steelmaker data sheets give 31 to 40 ft lb on a C-notch and about 80 ft lb on an unnotched bar, against the 17 ft lb in the table below. Every figure on this page comes from the single V-notch test series named under the tables, so compare it only with other V-notch figures read at the same hardness and temper.

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 grade has 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 compare case-hardened, water-hardening tool steels with a medium alloy steel.

Where two hardness values appear, in D2, L6, and the 4140 and 4150 rows, 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 their properties settle there.

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 on this list, and they absorb the least energy in 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 under those conditions and do not predict service life. Notch geometry, specimen size, specimen orientation relative to the bar, and test temperature all affect a Charpy result, so this number is not carried on a mill certificate and should not be written into a purchase specification without first agreeing the test conditions.

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. 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. The grain size-toughness relationship for a grade is set out in the ASTM grain size chart for tool steel.

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 from the source’s V-notch Charpy test series, read at the optimum tempering temperature, are marked for each grade.