Bell Helicopter mast hydrogen cracking prevention

Tom's gold-standard example of disciplined hydrogen control: a 200 ksi steel mast electroplated and oven-baked within five minutes to drive off moisture. Used to set the bar before introducing the peashooter counter-example.

The most rigorous hydrogen control I ever saw was at Bell Helicopter, where they make the mast for the helicopters. You lose your mast and it's not a good day. The rotor goes one way and the fuselage goes the other way, and the fuselage tends to go down. The rotor goes up before it goes down, just like those little toys.

Critical non-welded high-strength steel part. Electroplated for corrosion resistance; within five minutes of leaving the plating bath it goes into the hydrogen bakeout oven, because the steel is high-strength and there's no redundancy — mast failure means loss of blades and no possibility of auto-rotation.

I think I may have told you about the Bell Helicopter mast. They actually electroplate it for corrosion resistance. It doesn't have any welds in it, but they electroplate it for corrosion resistance, and when they take it out of the electroplating bath, within five minutes it's in the hydrogen bakeout oven. They don't wait an hour, because it's a really high-strength steel and it's a very critical part. You've only got one — there's no redundancy — and if it fails you go down. Most of the time when you go down in a helicopter, you die. And you can't even auto-rotate. Anybody know what auto-rotation is in a helicopter?

Student: [response about tilting]

Brief but specific. 4340 steel mast at 240 KSI, electroplating-induced hydrogen baked out at 375°F for 23 hours, transferred from plating bath to furnace in under 5 minutes. Used to illustrate (a) that hydrogen sources are not only welding and (b) that procedural rigor matters in proportion to consequence of failure ("you've only got one mast on a helicopter").

If you have a very high hardness steel like a 4340, take Bell Helicopter in Fort Worth, Texas — they build the military helicopters there. Can't build military helicopters in another country, at least not for the U.S. Department of Defense, so they still build the military helicopters in Fort Worth. The civilian helicopters are in Mirabel, Quebec. It's not always welding that introduces the hydrogen — I mentioned that the electroplating process can introduce hydrogen. They actually go from the electroplating bath directly into the furnace that's going to bake the hydrogen out at 375 degrees for 23 hours. It's less than five minutes between bath and furnace — they don't wait a full hour — because now we're talking about a mast made out of 4340 steel at a strength level of 240 KSI. And they are worried enough, because if you lose your mast — you've only got one mast on a helicopter, and it's not a good day if you lose the rotor. You drop like a rock. Can't auto-rotate to ground.

Used to illustrate the <1 mL/100g hydrogen tolerance at 250 ksi for the highest-strength aerospace steels.

It turns out the ASTM specs, if you're heat-treating, say, a helicopter rotor blade or mast — which is very high-strength steel — can tolerate not four milliliters of hydrogen but less than one milliliter of hydrogen when welded. One of you asked where the hydrogen comes from. For a typical steel coming out of the steel mill, just because of the humidity in the air when they were melting it, you'll have one or two parts per million hydrogen out of the mill. Not a problem for lower-strength steels. When you weld, you can get that up to 30 parts per million. Grams per hundred is about a 1.1 conversion factor, so just take it as the same. That four-milliliter stick electrode is the best you can buy in normal commercial practice. Gas metal arc welding, which we're going to discuss, has even lower hydrogen — that's one of the reasons to go to it. But at very high-strength steels like aircraft landing gear or helicopter rotor mast, when you're at 250 ksi, you tolerate less than one part per million. That can be from the steel-making process alone.