Tool Steel Fracture Toughness Chart (K1c)
Fracture toughness in MPa root metre for eight common tool steel grades, measured on bend bars taken from three forging reductions and tempered at three temperatures, with the hardness each specimen was tested at. A reference summary of published practice and not an Aobo Steel specification.
How the values were obtained
Every figure on this page comes from one laboratory programme that ran three toughness tests side by side on the same eight grades, so the numbers can be read against each other. The steels were supplied as 432 mm round electroslag remelted ingots. Samples of each grade were cut from 146 to 173 mm round bar and part of each was forged down to 89 mm square and 38 mm square, which gives the three total forging reductions listed as 88, 95 and 99 per cent.
Fracture toughness was measured on standard bend bars 114 mm by 25 mm by 13 mm, machined slightly oversize on the 25 mm dimension, notched, heat treated and then finish machined before testing at room temperature. Duplicate and some triplicate specimens were tempered at each of three tempering temperatures per grade, which is why most rows carry two test values and the H13 rows carry three. Hardness was measured on every specimen after testing, so the HRC column is the hardness at which that specimen was actually tested rather than a nominal figure.
The toughness values are given in MPa root metre, written K1c, the plane strain fracture toughness. The austenitizing practice and quench differ by grade and are stated in the table notes under each table.
Fracture toughness of the oil-hardening grades (L6 and O2)
K1c by forging reduction, section size, tempering temperature and hardness
| Grade | Forging reduction | Section size | Temper temperature | K1c (MPa√m) | Average K1c | HRC |
|---|---|---|---|---|---|---|
| L6 | 99% | 38 mm Sq. | 149 °C, 1h | 20.86, 18.50 | 19.7 | 60.0 |
| L6 | 95% | 89 mm Sq. | 149 °C, 1h | 23.19, 22.14 | 22.7 | 59.5 |
| L6 | 88% | 171 mm Rd. | 149 °C, 1h | 26.11, 25.39 | 25.7 | 58.5 |
| L6 | 99% | 38 mm Sq. | 204 °C, 1h | 24.76, 24.79 | 24.8 | 57.0 |
| L6 | 95% | 89 mm Sq. | 204 °C, 1h | 26.04, 24.86 | 25.5 | 57.0 |
| L6 | 88% | 171 mm Rd. | 204 °C, 1h | 28.53, 28.11 | 28.3 | 56.5 |
| L6 | 99% | 38 mm Sq. | 288 °C, 1h | 27.15, 26.03 | 26.6 | 54.5 |
| L6 | 95% | 89 mm Sq. | 288 °C, 1h | 25.71, 27.42 | 26.6 | 54.5 |
| L6 | 88% | 171 mm Rd. | 288 °C, 1h | 29.12, 30.32 | 29.7 | 53.5 |
| O2 | 99% | 38 mm Sq. | 149 °C, 1h | 18.37, 17.50 | 17.9 | 62.0 |
| O2 | 95% | 89 mm Sq. | 149 °C, 1h | 19.31, 19.45 | 19.4 | 62.0 |
| O2 | 88% | 159 mm Rd. | 149 °C, 1h | 19.80, 19.84 | 19.8 | 62.0 |
| O2 | 99% | 38 mm Sq. | 204 °C, 1h | 19.82, 18.94 | 19.4 | 59.5 |
| O2 | 95% | 89 mm Sq. | 204 °C, 1h | 21.23, 20.72 | 21.0 | 59.0 |
| O2 | 88% | 159 mm Rd. | 204 °C, 1h | 21.62, 21.49 | 21.6 | 59.0 |
| O2 | 99% | 38 mm Sq. | 288 °C, 1h | 18.66, 18.37 | 18.5 | 56.5 |
| O2 | 95% | 89 mm Sq. | 288 °C, 1h | 18.77, 19.11 | 18.9 | 56.5 |
| O2 | 88% | 159 mm Rd. | 288 °C, 1h | 20.31, 20.32 | 20.3 | 56.0 |
Source, Table II, Results of Fracture Toughness Testing of AISI L6 and O2 of Tool Materials for Molds and Dies (G. Krauss and H. Nordberg, eds.).
Forging reduction is the total reduction from the 432 mm ingot. L6 was austenitized in salt at 830 °C for 5 minutes and oil quenched, O2 in salt at 802 °C for 5 minutes and oil quenched. Both values in each row are individual test results on separate specimens and the average column is the arithmetic mean of them.
Fracture toughness of D3 and A6
K1c where a specimen could be precracked and tested
| Grade | Forging reduction | Section size | Temper temperature | K1c (MPa√m) | Average K1c | HRC |
|---|---|---|---|---|---|---|
| D3 | 99% | 38 mm Sq. | 204 °C, 1h | 33.54, 33.67 | 33.7 | 58.0 |
| D3 | 99% | 38 mm Sq. | 371 °C, 1h | 32.76 | 32.8 | 55.5 |
| A6 | 99% | 38 mm Sq. | 149 °C, 1h | 21.81, 21.53 | 21.7 | 59.5 |
| A6 | 95% | 89 mm Sq. | 149 °C, 1h | 24.63, 21.80 | 23.2 | 61.0 |
| A6 | 88% | 173 mm Rd. | 149 °C, 1h | 24.34, 25.80 | 25.1 | 59.5 |
| A6 | 99% | 38 mm Sq. | 371 °C, 1h | 21.95, 20.25 | 21.1 | 53.0 |
| A6 | 95% | 89 mm Sq. | 371 °C, 1h | 21.63, 21.77 | 21.7 | 54.0 |
| A6 | 88% | 173 mm Rd. | 371 °C, 1h | 22.95, 23.17 | 23.1 | 52.5 |
| A6 | 99% | 38 mm Sq. | 496 °C, 1h | 28.41, 27.09 | 27.7 | 49.0 |
| A6 | 95% | 89 mm Sq. | 496 °C, 1h | 29.60, 29.46 | 29.5 | 49.5 |
| A6 | 88% | 173 mm Rd. | 496 °C, 1h | 30.06, 29.05 | 29.6 | 48.0 |
Source, Table III, Results of Fracture Toughness Testing of AISI D3 and A6 of Tool Materials for Molds and Dies.
D3 cracked during precracking at seven of the nine conditions tested, so only the two conditions in the table produced a valid value and the source gives no figure for the others. D3 was austenitized in salt at 968 °C for 25 minutes and oil quenched, A6 in salt at 843 °C for 25 minutes and air cooled.
Fracture toughness of the air-hardening cold work grades (A2 and D2)
K1c by forging reduction, section size, tempering temperature and hardness
| Grade | Forging reduction | Section size | Temper temperature | K1c (MPa√m) | Average K1c | HRC |
|---|---|---|---|---|---|---|
| A2 | 99% | 38 mm Sq. | 149 °C, 1h | 21.05, 20.69 | 20.9 | 63.0 |
| A2 | 95% | 89 mm Sq. | 149 °C, 1h | 21.58, 20.78 | 21.2 | 63.5 |
| A2 | 88% | 159 mm Rd. | 149 °C, 1h | 20.80, 20.90 | 20.9 | 63.0 |
| A2 | 99% | 38 mm Sq. | 204 °C, 1h | 22.08, 22.50 | 22.3 | 61.0 |
| A2 | 95% | 89 mm Sq. | 204 °C, 1h | 22.58, 21.46 | 22.0 | 61.0 |
| A2 | 88% | 159 mm Rd. | 204 °C, 1h | 21.40, 21.52 | 21.5 | 61.0 |
| A2 | 99% | 38 mm Sq. | 371 °C, 1h | 22.31, 22.30 | 22.3 | 58.0 |
| A2 | 95% | 89 mm Sq. | 371 °C, 1h | 21.41, 21.96 | 21.7 | 57.5 |
| A2 | 88% | 159 mm Rd. | 371 °C, 1h | 21.74, 22.09 | 21.9 | 57.5 |
| D2 | 99% | 38 mm Sq. | 204 °C, 1h | 28.84, 28.35 | 28.6 | 60.0 |
| D2 | 95% | 89 mm Sq. | 204 °C, 1h | 36.77, 36.58 | 36.6 | 59.5 |
| D2 | 88% | 167 mm Rd. | 204 °C, 1h | 45.35, 40.08, 41.00 | 42.1 | 59.0 |
| D2 | 88% | 167 mm Rd., transverse | 204 °C, 1h | 25.00 | 25.0 | 60.0 |
| D2 | 99% | 38 mm Sq. | 371 °C, 1h | 30.07, 30.62 | 30.4 | 56.5 |
| D2 | 95% | 89 mm Sq. | 371 °C, 1h | 38.63, 38.83 | 38.7 | 57.0 |
| D2 | 88% | 167 mm Rd. | 371 °C, 1h | 44.18, 46.32 | 45.2 | 57.0 |
| D2 | 99% | 38 mm Sq. | 538 °C, 1h | 24.85, 25.48 | 25.2 | 54.5 |
| D2 | 95% | 89 mm Sq. | 538 °C, 1h | 27.71, 29.87 | 28.8 | 57.5 |
| D2 | 88% | 167 mm Rd. | 538 °C, 1h | 33.12, 32.62 | 32.9 | 58.5 |
Source, Table IV, Results of Fracture Toughness Testing of AISI A2 and D2 of Tool Materials for Molds and Dies.
A2 was austenitized in salt at 954 °C for 25 minutes and air cooled, D2 in salt at 1010 °C for 25 minutes and air cooled. The 88 per cent row tempered at 204 °C marked transverse is a single specimen cut across the 167 mm round bar rather than along it, and it is listed separately because it is the only transverse reading in the table.
Fracture toughness of the hot work and shock-resisting grades (H13 and S7)
K1c by forging reduction, section size, tempering temperature and hardness
| Grade | Forging reduction | Section size | Temper temperature | K1c (MPa√m) | Average K1c | HRC |
|---|---|---|---|---|---|---|
| H13 | 99% | 38 mm Sq. | 538 °C, 2h + 2h | 29.76, 29.89, 29.21 | 29.6 | 53.5 |
| H13 | 95% | 89 mm Sq. | 538 °C, 2h + 2h | 28.19, 27.92, 28.18 | 28.1 | 53.5 |
| H13 | 88% | 154 mm Rd. | 538 °C, 2h + 2h | 27.63, 26.98, 27.54 | 27.4 | 53.5 |
| H13 | 99% | 38 mm Sq. | 593 °C, 2h + 2h | 41.25, 37.50, 38.71 | 39.2 | 48.0 |
| H13 | 95% | 89 mm Sq. | 593 °C, 2h + 2h | 34.42, 32.74, 31.98 | 33.0 | 47.5 |
| H13 | 88% | 154 mm Rd. | 593 °C, 2h + 2h | 35.90, 38.24 | 37.1 | 47.5 |
| H13 | 99% | 38 mm Sq. | 621 °C, 2h + 2h | 87.84, 88.85, 87.74 | 88.0 | 40.0 |
| H13 | 95% | 89 mm Sq. | 621 °C, 2h + 2h | 55.02, 80.62, 77.20 | 71.0 | 40.0 |
| H13 | 88% | 154 mm Rd. | 621 °C, 2h + 2h | 59.96, 60.61 | 60.3 | 40.5 |
| S7 | 99% | 38 mm Sq. | 204 °C, 1h | 34.67, 34.41 | 34.5 | 57.0 |
| S7 | 95% | 89 mm Sq. | 204 °C, 1h | 35.02, 35.97 | 35.5 | 56.5 |
| S7 | 88% | 154 mm Rd. | 204 °C, 1h | 32.99, 33.11 | 33.0 | 56.0 |
| S7 | 99% | 38 mm Sq. | 496 °C, 1h | 38.90, 38.41 | 38.6 | 53.0 |
| S7 | 95% | 89 mm Sq. | 496 °C, 1h | 39.48, 39.27 | 39.4 | 52.0 |
| S7 | 88% | 154 mm Rd. | 496 °C, 1h | 34.28, 35.13 | 34.7 | 51.5 |
| S7 | 99% | 38 mm Sq. | 538 °C, 1h | 40.48, 40.25 | 40.4 | 51.5 |
| S7 | 95% | 89 mm Sq. | 538 °C, 1h | 40.39, 42.93 | 41.7 | 51.0 |
| S7 | 88% | 154 mm Rd. | 538 °C, 1h | 32.21, 33.10 | 32.7 | 50.5 |
Source, Table V, Results of Fracture Toughness Testing of AISI H13 and S7 of Tool Materials for Molds and Dies.
H13 was austenitized in salt at 1024 °C for 25 minutes and air cooled, S7 in salt at 941 °C for 25 minutes and air cooled. The H13 rows were tempered twice at each temperature, two hours each time, which is the practice its data sheets call for because of its secondary hardening peak.
What the eight grades show side by side
At normal working hardness the grades separate into the three groups the tool trade already works with, and the table puts numbers on that separation. The oil-hardening L6 is the toughest of the eight, running from 19.7 MPa root metre at 60 HRC up to 29.7 at 53.5 HRC. O2 sits below it at 17.9 to 20.3 over the same hardness band. The wear-resisting chrome grades come next, D3 measuring 32.8 to 33.7 and D2 from 25.2 to 45.2 depending on section and temper, against A2 holding a tight band of 20.9 to 22.3 across the whole range. The hot work and shock grades top the set, S7 reaching 34.5 at 57 HRC and H13 running from 27.4 to 88.0 as it is tempered down from 53.5 to 40.0 HRC.
Two readings in the tables are worth flagging because they run against the usual rule of thumb. D2 measured higher fracture toughness than the trade usually credits it with, in the same region as H13, and its 88 per cent row at 204 °C reached 42.1 MPa root metre. The other is the gap between the longitudinal and transverse directions on that same D2 bar, 42.1 against 25.0. A transverse reading tells you more about the finished die than a longitudinal one does, because a tool usually fails across the direction the bar was worked in, so a specification written only on longitudinal toughness can flatter a grade.
Reading the hardness column against toughness also shows why tempering below the normal working hardness is not a way to buy safety. On the L6, O2, A2 and A6 rows the average toughness rises as the tempering temperature goes up and the hardness comes down, and the rise is steady rather than abrupt. Forcing a grade to a lower hardness to gain toughness also costs wear resistance, and the added toughness is smaller than the jump a change of grade would give.
Figures are published experimental values and are a reference summary only. Fracture toughness is sensitive to specimen orientation, section size and heat treatment, so treat the tables as a comparison between grades rather than as a design value for a particular part. Confirm against the material test certificate or contact Aobo Steel.
Related reference data
The impact side of the same subject, measured on unnotched Izod and Charpy V-notch specimens from the same programme, is on our tool steel impact toughness chart. For die selection that turns those toughness figures into a grade choice, see the pages for D2 tool steel, H13 tool steel, A2 tool steel and S7 tool steel, and the guides on tool steels for chipping resistance and tool steels for cracking resistance.
Source: Tool Materials for Molds and Dies, Application and Performance (G. Krauss and H. Nordberg, eds.).