Aircraft landing gear and turbine engine shaft vacuum arc melting

How 250 ksi yield landing gear is made without hydrogen cracking — triple vacuum melting gets hydrogen below half a part per million.

For those nickel-based alloys, in many cases they're triple vacuum-melted — vacuum arc remelted, double or triple vacuum-melted, vacuum induction-melted. You're getting rid of all the nitrogen, hydrogen, oxygen, down to very low values. How do we make a landing gear at 250 ksi yield strength and not get hydrogen cracking? Because we vacuum-melt it three times — there's not a lot of hydrogen left in there. It's below half a part per million hydrogen. But if you try to do it in the air, which is where we melt most of the steel in the world — just the moisture in the air —

Commercial aircraft landing gear and engine rotors/shafts are double vacuum arc remelted to bring inclusions below 10 microns, since at 250 ksi yield, 100-micron flaws threaten fatigue life.

Once inclusions get in there, you've got to remelt to get them out. That's electroslag remelting and vacuum arc remelting. For the best — actually, even for aircraft landing gear, for the shafts and the rotors for commercial materials — it's typically two vacuum arc remeltings. You cast it once by vacuum induction melting to get the impurities down as low as possible. You then do vacuum arc remelting to vaporize off the inclusions, and then you do it again. I've heard of people trying a third time, but you get to the law of diminishing returns. Typically aircraft landing gear for a 747 or any commercial aircraft is double vacuum arc remelted. One reason is you want to get your imperfections down to less than about 10 microns in size. When you get to strength levels of 250 ksi, your fracture mechanics tells you that flaws 100 microns in size — the thickness of a sheet of paper — can be harmful for fatigue life. So the rotors for the engines, the shafts, the landing gear — you're talking about material that's going to be hundreds of dollars a pound because of all this extra processing.

Triple vacuum arc remelting as the standard for aircraft crankshafts, landing gear, transmission shafts, and turbine engine shafts. Used to illustrate why inclusion control matters for fatigue-critical components.

All the crankshafts and piston engines and aircraft have to be triple vacuum arc melted now, because vacuum arc melting burns away the inclusions formed from the regular steelmaking process. You make the steel, cast it into an ingot, and then you take that great big ingot and arc it into another mold. You've got a round ingot full of dirt and inclusions, and you arc it into a copper mold, withdrawing as you go — you just remelt it, but in a vacuum. Now in the heat of those little spatters of metal going across, they're being melted in a vacuum, and it sucks out the oxygen and nitrogen, and you end up with a very clean steel. They will triple vacuum arc remelt aircraft landing gear, transmission shafts, and the shafts for turbine engines. They're using steel for these things and triple-melting them — once, twice, and three times. You end up with things so clean you look at them at 500x and have a hard time finding an inclusion. There are standards for all these inclusion counts.