Interesting gearbox failure .. metallurgists take a look

I'm not a metallurgist but I play one on tv (just kidding).

Is this part aluminun, cast iron, or pot metal (ie zinc)?

If the engine it came out of had 100,000 miles on it, then it's probably fatigue.

Castings don't usually fare well in fatigue situations.

If you can tell me what the alloy and forming method is (cast or machined) and where the part is located in the engine, I can give you an opinion as to:

a) why it failed
b) if it failed prematurely, and
c) if and what you should do differently to be sure it does not fail again.

 

By the way, the grain structure is a function of how the part is cooled after it is formed or cast.

If it's a casting and is cooled slowly in air, it will have large grains. And metals with large grains are weak materials.

If it's quenched in water while it's very hot, it will have very fine, dense grains, and will be much stronger.

Forged parts are the strongest because they are quenched hot.

Cast parts are the weakest because they cool slowly at room temperature and allow large crystals to grow.

Casting are not inherently evil, they just have lower strength than machined or forged parts.

Machining raw stock, such as 6061-T6 rolled plate, has a fine grain and is a much stronger material, but machining is a much more expensive process, and therefore casting is usually used for mass production, if possible. Ferraris barely qualify as mass production, but none the less, with over 12,000 308's out there, I must call it mass production (in a very handcrafted way).

Looks to me like some part of the shift mechanism. If it's cast, I would duplicate it in 6061-T6 plate ( have it machined from scratch) and replace all the exisiting shift parts that are cast.

 

You might contact an organization called Exponent in Menlo Park CA. They analyze all sorts of failures. Not cheap, but doesn't look to be a super time consuming investigation. I'm a ME, who got some great tutoring on a bunch of failed aircraft engine parts from a very talented engineer(who btw owns a beautiful 308).

Here's my novice observations of what I think I could make out from the pix. From the left picture: It looks like the left edge of the part has "shear lips". This is indicative of fatigue failure. A crack starts, then it propogates each time the load and corresponding stress reaches a point to make the crack propogate until the toughness of the steel arrests it or the load goes away and so on. This will give a surface with changes in the appearance and texture and show some high and lo points or steps. If you can see chevron shaped features, they point to the crack initiation site. The right edge appears more uniform, and I'd call this fast-fracture. If the material were ductile, you'd see cup-and-cone features which show failure along the shear planes with the expected failure mode called slow ductile tearing.

ASSUMING what I've typed above has merit, I'd say a crack started from an edge that may have been sharp or had some surface flaw or feature to cause a stress concentration, and fatigue took over. The heat necessary to heat treat steel is serious. I've heat treated 420, 4130, 4340, 17-4 and others and during the process, they glow! Typical gearbox parts are SAE4340 or Euro equiv so I'd be surprised if this happened. Also, if the part overheated to the point of losing hardness you'd see ductile failure features.

If you get a real metalurgist to give you an opinion, please post it. I usually study these incidents as well although have never followed through with taking it to a real lab.

 

I've seen those pieces fractured before.

They look like cast steel to me that is hardened.

 

That looks like the fairly typical internal structure of cast steel to me.

Probably should be a forged part rather than a cast part, or at least a better casting process, and perhaps a different steel along with the addition of a hardening process. In addition to what Andy said about forged parts, forged parts are considerably stronger because the part is shaped via both heat and pressure - this results in a superior structure at both a molecular level and a full size level (ie. no significant voids and less embedded slag).

My company makes highly sophisticated sensors (ultrasonic, magnetic, etc.) for detecting flaws in metal and CF parts. Our sensors are used extensively by Boeing and the US Air Force. So we see a lot of broken castings in our R&D area, and what you have there looks pretty typical.

If you acquired a replacement part you could have one of the cryo shops do a freeze on it. Or have a machine shop machine one up for you out of appropriate grade steel. With an hour or so to do it I could make one on my personal lathe and mill, however, four kids under the age of five at home seems to be limiting my time somewhat....

 

I've replaced these pieces, as well as the tiny springs which hold them in place at many gearbox rebuilds...More times than not, the springs are broken, but the above pieces look fine.

Best,
David