D2 Austenitizing in Salt Bath and Atmosphere Furnaces
D2 is an air hardening cold work tool steel, and a heat treater can austenitize it in a molten salt bath or in an endothermic atmosphere furnace. The austenitizing temperature is the same in both routes. The cycle around that temperature is not, and the published procedures for two production die inserts show how far apart the two routes sit.
Two furnace routes for the same temperature
D2 is a high carbon, high chromium cold work tool steel, and austenitizing is the step that takes its carbides into solution. The salt bath and the endothermic atmosphere furnace are both used for the A and D groups, and both keep the surface free of scaling and decarburization. That is the reason a plain open furnace is not used for these grades, and it is why the choice between the two routes is a shop decision rather than a grade decision.
The procedures below were recorded for a bending die insert and a trim die insert, both made from D2 steel and both hardened from 1010 °C (1850 °F).
Austenitizing procedures for D2 inserts
The table gives the complete cycle for each part in each furnace route, from the preheat to the cooling step.
| D2 insert | Salt bath route | Endothermic atmosphere route |
|---|---|---|
| Bending die inserts | Preheat in air furnace at 650 °C (1200 °F) for 1.5 h; austenitize at 1010 °C (1850 °F) for 35 min; air cool; remove salt | Charge directly into furnace at 705 °C (1300 °F) and preheat for 1.5 h; austenitize at 1010 °C (1850 °F) for 2 h; air cool |
| Trim die inserts | Preheat in air furnace at 650 °C (1200 °F) for 4 h, then in salt bath at 845 °C (1550 °F) for 1 h; austenitize at 1010 °C (1850 °F) for 1 h; air cool; remove salt | Charge directly into furnace at 705 °C (1300 °F) and preheat for 4 h; raise furnace temperature and austenitize at 1010 °C (1850 °F) for 4 h; air cool |
Austenitizing procedures for two D2 die inserts, by furnace route. After austenitizing, the bending die inserts were double tempered and then nitrided, and the trim die inserts were double tempered. The inserts measured 200 by 305 by 38 mm (8 by 12 by 1.5 in.). Source, Table 7 of Heat Treating of Tool Steels, in ASM Handbook, Volume 4, Heat Treating.
What the two routes change
The austenitizing temperature is identical in all four cycles. The time at temperature is not. On the bending die insert, the salt bath holds the part at 1010 °C (1850 °F) for 35 min while the endothermic atmosphere furnace takes 2 h. On the trim die insert the gap is wider, 1 h against 4 h.
The salt bath transfers heat faster, so the part comes up to temperature sooner. That same faster heating is why the trim die insert takes a second preheat in the salt bath route, at 845 °C (1550 °F) after the air furnace preheat at 650 °C (1200 °F). The endothermic furnace route reaches the same result with a single preheat, charging the part directly into the furnace at 705 °C (1300 °F).
The salt bath also leaves salt on the part, and the published cycle lists a salt removal step after the quench. The atmosphere route has no equivalent step. Against that, the published note is that austenitizing will cost more in an atmosphere furnace than in a salt bath in some shops and less in others, so the choice is usually made on part size, distortion limits and the equipment already on the floor.
Equipment the procedures were recorded on
| Description | Salt bath furnace | Endothermic atmosphere furnace |
|---|---|---|
| Furnace type | Immersed electrode salt bath | Radiant tube furnace |
| Working chamber | 380 by 760 by 915 mm (15 by 30 by 36 in.) deep | 610 by 915 by 455 mm (24 by 36 by 18 in.) high |
| Manual loading | 1.5 min per piece | 1.5 min per piece |
Furnace and handling details recorded with the cycles above. Source, Table 7 of Heat Treating of Tool Steels, in ASM Handbook, Volume 4, Heat Treating.
After austenitizing
The published procedures continue past the quench. The bending die inserts were double tempered, 2 h at 510 °C (950 °F) and an air cool, then the same cycle again, and then nitrided for 48 h at 510 °C (950 °F). The trim die inserts were double tempered at a much lower temperature, 4 h at 190 °C (375 °F) and an air cool, then the same cycle again. The two parts were run to different working hardness levels, and the tempering temperatures follow that difference rather than the furnace route.
Holding time in the furnace matters on these grades. The published note is that the A and D steels must be held at their austenitizing temperature long enough to dissolve the carbides needed for maximum hardness, and that hardening from an excessively high austenitizing temperature increases the retained austenite. Retained austenite can be reduced afterwards by repeated tempering or by subzero cooling, but the recommendation is to avoid producing it.
What this means for an order
The furnace route is chosen by the heat treater and it does not appear on a purchase order for the steel. D2 is supplied in the annealed condition for the heat treater to harden, and the two parts in the table are separated by size and by working duty rather than by grade.
For the steel itself, the D2 tool steel page carries the supply condition and the equivalents, and the DC53 tool steel page is the grade we hold for shops that want more toughness at a similar wear level.
Two pages go with this one. The D2 heat treatment guide page carries the full cycle and the hardness range, and the salt bath and furnace page covers the equipment side.
Before you use these procedures
This page is a reference summary of published practice and it is not an Aobo Steel specification. The cycles are the values published for the two D2 inserts described in the source, and a different section, a different insert geometry or a different furnace will move them. Final cycle parameters are set by the heat treater.
Source: ASM Handbook, Volume 4, Heat Treating, ASM International, 1991.