Aluminum’s Role In Steel

Aluminum is a critical ingredient of  steel in our shops, not just as a stand alone material for machining.

And it makes a darn nice container for pressurized carbonated beverages like Pepsi...

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.

This is what nitrided steel looks like under the microscope.

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

 

 

 

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