D5 and D7 Tool Steel Heat Treatment
The preheat, the hardening temperature and the quench medium for D5 and D7, with the hardness each tempering temperature leaves on them, set out as the source publishes it. A reference summary of published practice and not an Aobo Steel specification.
The two grades
D5 and D7 are high carbon, high chromium cold work tool steels. Both carry about 12% chromium, which is where their abrasive wear resistance comes from, and both are bought at high hardness for long run blanking, forming and shear work. D5 carries 3.10% cobalt and hardens from 1850 °F (1010 °C), and the card gives oil or air as the quench. D7 carries 4.00% vanadium and hardens from a higher 1975 °F (1080 °C) in air.
The vanadium line is the difference between the two. The source describes the 4.00% vanadium in D7 as the grade’s defining addition, because vanadium holds the grain size down and leaves a very fine grain structure in the hardened steel, and it puts the wear resistance that comes with it at 5 to 7% over the grades without it. D7 also carries 1.10% molybdenum against 0.85% in D5, and the source credits the molybdenum with a slight heat resistance.
Hardening the two grades
Preheat, hardening temperature and quench
| Step | D5 | D7 |
|---|---|---|
| Preheat | 1200 °F (650 °C) | 1400 °F (760 °C) |
| Harden | 1850 °F (1010 °C) | 1975 °F (1080 °C) |
| Quench | Oil or air | Air |
Source, heat treating reference information for D5 and D7, printed pages 185 and 186.
The card gives one preheat temperature, one hardening temperature and one quench medium for each grade. The selection chapter of the same source describes D7 as an oil hardening grade that can be quenched in air at a sacrifice of hardness, while the card lists air. The card value is printed in the table and the second reading is recorded here.
The two grades do not share a hardening window. D5 hardens from 1850 °F (1010 °C) and D7 from 1975 °F (1080 °C), which is 125 °F (70 °C) higher, and the preheat follows the same order at 1200 °F (650 °C) against 1400 °F (760 °C). The quench line is the one place where the source reads two ways, and both readings are printed on this page. For a shop choosing between the two, D5 is the one the card allows to be quenched in oil or in air, and D7 comes out of the air quench at the higher as quenched hardness of the two, 67 HRC against 64 HRC.
Tempering response of D5 and D7
Hardness left by each tempering temperature
| Temper, °F | °C | D5, HRC | D7, HRC |
|---|---|---|---|
| As quenched | No temper | 64 | 67 |
| 300 | 150 | 62 | 65 |
| 400 | 205 | 61 | 63 |
| 500 | 260 | 61 | 62 |
| 600 | 315 | 60 | 61 |
| 700 | 370 | 60 | 61 |
| 800 | 425 | 61 | 61 |
| 900 | 480 | 61 | 62 |
| 1000 | 540 | 57 | 62 |
Source, heat treating reference information for D5 and D7, printed pages 185 and 186.
The card prints a single Rockwell C figure for each tempering temperature. The D5 line falls to 60 HRC at 700 °F (370 °C) and then rises again, which is secondary hardening.
D7 holds its hardness across the whole range. It comes out of the quench at 67 HRC, loses four points by 400 °F (205 °C), and then sits between 61 and 62 HRC from 600 to 1000 °F (315 to 540 °C), so the top of the tempering range is available to it without a hardness cost. D5 starts lower at 64 HRC, falls to 60 HRC at 700 °F (370 °C) and then rises again to 61 HRC at 800 and 900 °F (425 and 480 °C) before it drops to 57 HRC at 1000 °F (540 °C). That second rise is secondary hardening, and it is the reason the D5 line is not a straight decline.
Chemistry of the two grades
Carbon and alloy content, percent
| Grade | C | Mn | Si | Cr | Mo | V | W | Co |
|---|---|---|---|---|---|---|---|---|
| D5 | 1.50 | 0.50 | 0.50 | 12.25 | 0.85 | Not given | Not given | 3.10 |
| D7 | 2.30 | 0.40 | 0.40 | 12.50 | 1.10 | 4.00 | Not given | Not given |
Source, heat treating reference information for D5 and D7, printed pages 185 and 186.
Both lines are printed as single figures in the reference section, not as the specification ranges. Cells the card leaves empty are shown as Not given rather than as a zero. The ranges for the same two grades are on the tool steel composition chart.
D5 carries 1.50% carbon and 12.25% chromium with 3.10% cobalt, and D7 carries 2.30% carbon, 12.50% chromium and 4.00% vanadium. The card leaves the vanadium and tungsten cells empty for D5 and the tungsten and cobalt cells empty for D7, and those cells are shown as Not given rather than as a zero. Every figure in the table is printed as a single number in the source, so it is a condition to start from and not a specification range.
Related reference data
Heat treatment data for the other cold work grades of the group is on the D2 heat treatment guide, the D3 heat treatment guide and the D6 heat treatment guide, and the tempering bands across the whole grade range are on the tool steel tempering chart. The specification ranges and UNS numbers for D5 and D7 are on the tool steel composition chart, the hardening and tempering bands together are on the tool steel hardening and tempering chart, and soak times by section are on the tool steel soak time chart.
Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, second edition, Hanser). The hardening card and the tempering response for D5 and D7 come from the heat treating reference information on printed pages 185 and 186, and the grade notes from the tool steel selection chapter on printed pages 148 and 149.