Aluminum is a critical ingredient of steel in our shops, not just as a stand alone material for machining.
Aluminum metal is used to make many parts produced by precision machining, and is finding increasing application in automotive because of its light weight and high strength to weight ratio.
But aluminum plays a key role in some steel applications that you should know about.
- Aluminum is used as a deoxidizer. Aluminum scavenges Oxygen from the melt reducing porosity in the solidiied steel.
- Aluminum is used to produce a fine austenitic grain size. (Aluminum is the most effective element to control grain growth in steel.)
- Aluminum is also used as an alloying addition in the amounts of 0.95- 1.30 weight 5 to make Nitriding steel. Nitriding increases the hardness of the steel by the formation of a hard, stable aluminum nitiride compound.
Aluminum’s ability to scavenge Oxygen results in tiny aluminum oxide particles dispersed throughout the steel. As aluminum oxide is hard and abrasive, Aluminum is not deliberately added to free machining steels where it would destroy tool life.
Aluminum is more effective at grain growth control than elements like vanadium, titainium and zirconium. These three elements adversley affect hardenability because they form carbides that are both quite stable and difficult to dissolve in austenite prior to quenching.
In the nitriding steel, this recipe is relatively distortion free at the temperatures up to the nitriding temperature.
| Nitralloy “N” | Nitralloy 135 | ||
| C | 0.22-0.27 | 0.38-0.43 | |
| Mn | 0.50-0.70 | 0.50-0.80 | |
| P | 0.035 | 0.025 | |
| S | 0.040 | 0.025 | |
| Si | 0.15-0.35 | 0.20-0.40 | |
| Ni | 3.25-3.75 | 0.25 | |
| Cr | 1.00-1.35 | 1.40-1.80 | |
| Mo | 0.20-0.30 | 0.30-0.40 | |
| V | – | – | |
| Cu | – | 0.35 | |
| Other | Al, 0.95-1.30 | Al, 0.95-1.30 | |
| Source | ASTM A355-89 | AMS 6470J |
So yes Virginia, you may have more Aluminum in your shop than the number of aluminum bars, soda cans and foil wrappers might lead you to believe. Hiding in your steel!
Nitride structure
Nitralloy Table
