D6AC steel is vacuum melted, medium carbon, low alloyed, and ultra-high-strength steel. This grade is used for high-strength applications like aircraft structures with strength levels up to 280,000 psi. With proper tempering, yield strength and hardness of this grade can reach up to 1750 MPa, and 53HRC respectively.
D6AC Steel Composition
| Elements | Weight % |
|---|---|
| C | 0.46 |
| Cr | 1.1 |
| Mn | 0.75 |
| Ni | 0.6 |
| Mo | 1 |
| Si | 0.25 |
| V | 0.1 |
| Fe | Base |
D6AC Steel Properties
Physical Properties
| Properties | Units (metric) |
|---|---|
| Melting Point | 1426 C |
| Specific Gravity | 7.8 g/cc |
| Modulus of Elastic Tension | 32 |
Mechanical Properties
We used the Rockwell Hardness test for the measurement of the Hardness of D6AC steel.
| Tempered D6AC Mechanical Properties | Tempering Temp - 316 C Units (metrics) | Tempering Temp - 510 C Units (metrics) |
|---|---|---|
| Yield Strength | 1724 MPa | 1345 MPa |
| Poisson Ratio | 0.27-0.3 | 0.27-0.3 |
| Elastic Modulus | 210 GPa | 150 GPa |
| Reduction Ratio | 23 % | 25 % |
| Elongation at break | 7 % | 7 % |
| Hardness (Rockwell C) | 53 | 46 |
Thermal Properties
| D6AC steel | Properties |
|---|---|
| Thermal Conductivity (W/m. K) | 30 /m-K |
| Specific Heat Capacity | 0.116 J/g - C |
| Mean Coef of thermal Expansion | 7 |
D6AC Steel Heat treatment
D6AC heat treatment cycle starts from the normalizing or annealing process. With annealing or normalizing, uniformity in structure and removal of previous stresses is carried out. Both Annealing and Normalizing for this grade of steel are carried out in the gamma region.
After removing stresses, hardening treatment is carried out. With the hardening process, carbides of Mo, and V nucleate at grain boundaries provide extra strength to the material. After soaking at hardening temperature, steel is immediately quenched, not allowing carbides to dissolve. This results in ultra high strength in D6AC steel.
Hardening treatment makes this grade prone to quench cracking. To prevent this defect from occurring, this grade is immediately tempered at temperatures given above in a table. The tempering process removes all stresses and also makes this steel ultra-hard as a result of secondary hardening.
The heat treatment cycle for Annealing, Normalizing, Hardening, and Tempering for this steel grade is explained in the picture below;
Forging
This steel grade is forgeable. Forging for this specific steel grade is carried out between 1090oC to 1230oC. For minimum finishing, the forging temperature should be at least 930o C. After forging, the steel part is furnace cooled up to 540oC, and later it is air quenched to room temperature.
Welding
Thin sections of D6AC are welded by TIG welding while GMAW can be applied on the thick section with filler rods of the same steel grade. Preheat steel parts that are to be welded up to 200oC to avoid weld cracks. In the case of restrained weldments, proper post-weld heat treatment should be applied.
Post welding treatment is carried out by heating this steel grade to 600F, air-cooled to 300F and then it is stress relived at 1000F. After stress relieving, normalizing and re-tempering processes are conducted to achieve desired mechanical properties.
Machining
Low alloy steel can be easily machined. The same goes for D6AC steel. Machining of this grade is carried out in normalized temper conditions. Heat-treated steel grades that have a yield strength of 170MPa should pass from the stress-relieving step after the machining process. Duration for stress relief is 3 hours at the annealing temperature.
D6AC Steel Applications
D6AC Steel has many structural and defense applications. The fields in which this structural steel is employed are vehicle, airplane, pressure vessel, power generations, and many more..
If we just summarized them, the following are the possible places where this alloy steel is employed;
- Fasteners
- Boiler rods for support
- Turbine fasteners
- Connecting ROds
- Mechanical Parts like gears and shaft
