A2 and A6 Tool Steel Mechanical Properties After Tempering
Tempering is the step that decides what a cold work die can actually take, because the hardness a grade leaves the quench with is not the hardness it works at. The table below sets hardness, tensile and yield strength, elongation, reduction of area and unnotched Charpy impact beside the tempering temperature for two air-hardening cold work tool steels, A2 and A6, each on one heat at the hardening temperature named under the table. Every figure belongs to the exact hardening and tempering condition printed beside it, so read it as measured behaviour rather than as a design value. This is a reference summary of published practice and not a specification of ours.
What the figures measure
Both steels in the table are air-hardening cold work grades that are quenched and then tempered once, and the only variable across the columns is the tempering temperature. A2 was hardened from 945 °C (1730 °F) and is carried at two tempering temperatures, 540 and 595 °C. A6 was hardened from 845 °C (1550 °F) and is carried at three, 175, 315 and 480 °C. Hardness, ultimate strength and the ductility measures are the result of that one temper, so the table answers the question a die shop asks when it picks a tempering temperature for a punch, a die insert or a cutting edge.
The two grades are not tested to the same depth. The source reports hardness and ultimate strength for A2 and leaves the yield strength, elongation, reduction of area and impact columns empty, marked here with three dots exactly as the source prints them. A6 is carried right through, with a notched test left out and an unnotched Charpy value given, which is the reason the last two columns carry larger numbers than a notched impact reading would. Note also that A6 is reported at 175 °C, which is at the low end of any tempering range and belongs to a part that is being stress relieved rather than softened.
Hardness, strength and impact after tempering
A2 tool steel after hardening from 945 °C (1730 °F) and one temper
| Tempering temperature, °C | Tempering temperature, °F | Hardness, HRC | Ultimate strength, MPa | Ultimate strength, ksi | Yield strength, MPa | Yield strength, ksi | Elongation in 50 mm (2 in.), % | Reduction of area, % | Unnotched Charpy impact, J | Unnotched Charpy impact, ft·lbf |
|---|---|---|---|---|---|---|---|---|---|---|
| 540 | 1000 | 54.1 | 1858.2 | 269.5 | … | … | … | … | … | … |
| 595 | 1100 | 46.7 | 1603.8 | 232.6 | 1269 | 184 | 5.0 | 13.9 | … | … |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 11-3, Mechanical properties of A2 and A6 tool steels, printed page 199. A2 figures from Teledyne VASCO. The three dots are printed by the source where it gives no figure for that property. This is a reference summary of published practice and not an Aobo Steel specification.
A6 tool steel after hardening from 845 °C (1550 °F) and one temper
| Tempering temperature, °C | Tempering temperature, °F | Hardness, HRC | Ultimate strength, MPa | Ultimate strength, ksi | Yield strength, MPa | Yield strength, ksi | Elongation in 50 mm (2 in.), % | Reduction of area, % | Unnotched Charpy impact, J | Unnotched Charpy impact, ft·lbf |
|---|---|---|---|---|---|---|---|---|---|---|
| 175 | 350 | 59.7 | 2137 | 310 | … | … | 0.5 | 1.0 | 117 | 86 |
| 315 | 600 | 55.5 | 2016.8 | 292.5 | 1822.3 | 264.3 | 1.0 | 2.2 | 163 | 120 |
| 480 | 900 | 51.0 | 1751 | 254 | 1510 | 219 | 2.5 | 7.4 | 170 | 125 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 11-3, printed page 199. A6 figures from Universal Cyclops Steel Corp. The impact column is unnotched Charpy, so it is not comparable with notched readings for the same steel, and 175 °C is a stress relieving temper rather than a softening one. This is a reference summary of published practice and not an Aobo Steel specification.
Reading the two grades together
Take the hardness column first. A2 falls from 54.1 to 46.7 HRC across its two conditions, a drop of 7.4 points for 55 °C more temper, and its ultimate strength follows it down from 1858.2 to 1603.8 MPa. That single step is the whole of the trade a shop makes on A2 when it decides between a hard edge and a tough one, and the source gives no impact figure to price the other side of it.
A6 shows the same trade with the impact side filled in. As the tempering temperature goes from 175 to 315 to 480 °C the hardness falls 59.7, 55.5, 51.0 HRC, the unnotched Charpy energy rises 117, 163, 170 J, and elongation rises from 0.5 to 2.5 per cent while the reduction of area goes from 1.0 to 7.4 per cent. The largest single gain in toughness for the smallest loss of hardness sits between 175 and 315 °C, where 4.2 HRC buys 46 J. Past that the curve flattens, and the step from 315 to 480 °C costs another 4.5 HRC for 7 J, which is the point at which a shop usually stops and accepts the harder condition.
The yield figures deserve one caution. A6 in the 315 °C condition is printed with a yield strength of 1822.3 MPa against an ultimate strength of 2016.8 MPa, a ratio that leaves very little room between yield and fracture, and the same grade at 480 °C opens that gap to 241 MPa. A die run in the harder, lower temperature condition therefore has almost no reserve before it cracks, which is consistent with the very low elongation and reduction of area printed beside it.
Related reference data
The grade that carries most of the cold work tonnage is described with its forms, section ranges and delivery conditions on A2 tool steel, and the properties, hardness bands and uses of the same grade are on A2 tool steel properties and A2 tool steel hardness. The hardening and tempering range for the whole air-hardening group is on air-hardening tool steel, the hardness each tool steel grade keeps at a given tempering temperature is charted on tool steel tempering chart, and impact values for the tool steel grades are collected on tool steel impact toughness. How a tempering temperature is chosen for a tool that has to resist chipping is worked through on tool steels for chipping resistance.
Compiled from Tool Steels (G. A. Roberts and G. Krauss), Table 11-3, Mechanical properties of A2 and A6 tool steels, on printed page 199, the chapter on air-hardening cold work tool steels. The table is reproduced as published and is a reference summary rather than an Aobo Steel specification, so treat it as the measured behaviour of one heat under one heat treatment rather than as a design allowable for a particular die.