Carburised Hardened Thread

[gearman1234]

Does the thread get stripped only beacuse it is case hardned to 0.02/0.03? All other things remaining same.
Thread size M20. Does the same logic apply to internal and external threads?

 

[Tmoose]

It sometimes (often) is best to mask or otherwise protect the threads from the carburizing bath to prevent hardening the threads. Harley used to copper plate the threaded ends of their main shafts used to create the crankshaft, which used roller bearing directly on the Rc 60 shaft journals. High strength grade 8 bolts are only RC 40 something.

 

[gearman1234]

I understand that it is advisable to keep the threads at less than 40 HRc . But is there any field evidence that 60 HRc threads have stripped off while in service?

 

[Screwman]

That hard case is going to be a set up for fatigue cracking if there is any kind of dynamic loading going on. The threaded connections usually have some localized yielding at the highest stress areas if the joint is being loaded up to it's full capacity and under those conditions there is going to be cracking in the thread flanks.
Also, a hardened section at that high hardness is going to be very susecptable to both stress corrosion cracking or hydrogen embrittelment failures in service.
I would try to locally anneal those threads to get some ductility in them.

 

[swall]

I have worked for two Tier 1 OEM automotive suppliers as a metallurgist. One made ball studs for steering gear and the carburized threads were always annealed to soften them (don't remember the exact hardness range, but it was in the mid Rc 30's, I believe).The other made CV joints and the threads on the spindle end of wheel side outer housing was annealed on those outers that were carburized. BTW, most CV joint outer housings are now induction hardened, and you can keep the hardening out of the threads.With both components, the big concern with carburized/hardened threads was poor impact resistance. In both cases, the components were safety critical items.

 

[gearman1234]

Tmoose, Screwman and Swall-- I thank you very much for your information. This answers my query.