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> welding a chrome moly sprocket
post Jan 24 2018, 01:53 AM
Joined: 26-Apr 13

hi guys,i bought a second hand motorcycle that i later found out the splines are worn away on the output shaft,my plan is to weld the front sprocket to the shaft until the time comes for a full engine rebuild and i can replace the shaft. yesterday i tried welding it to the shaft keeping everything cool so as not to melt the sprocket seal,i used tig with er70s-6 wire. as soon as i took the bike for a ride the welds broke, after a bit of research i found out the sprocket is made from scm420 fully hardened chromemoly,not just case hardened as i originally thought. my plan is to take the sprocket off and anneal it but i was wondering when i heat it up to anneal it should i put some weld on it while it is still red hot,almost like buttering or should i just let it cool down and weld it directly to the shaft,any help appreciated
was wanting to post a photo but seems there is no way of attaching a photo on this forum,very strange
Technic Al
post Jan 24 2018, 01:48 PM
Joined: 14-Oct 03

I should imagine its CrMo to give it some wear resistance so annealing it will soften it. It could also distort it so it doesnt fit anymore.
Have you thought about welding it with Stainless 309L....It might do it without annealing or preheat
post Jan 25 2018, 01:53 AM
Joined: 26-Apr 13

i was only going to anneal the points where it needed to be welded to the shaft not the teeth themselves. there is 6 points of contact that need welding so i could try weld 3 points of contact with the stainless 309L,would that really make a difference with the sprocket being hardened due to .5 percent carbon
Technic Al
post Jan 25 2018, 10:32 AM
Joined: 14-Oct 03

How would you anneal specific areas? Not sure where the 0.5% or is it 5% comes in.......SCM420 is 0.18-0.23%. Im confused.
post Jan 26 2018, 02:01 AM
Joined: 26-Apr 13

my mistake on the carbon content another website had it listed as 0.5 percent but all others stated the same as you. can you not anneal specific areas just as you can case harden certain areas? if i heat up the area to be welded with an oxy torch then allow it to cool slowly. anyway not alot of responses so i guess its not a common thing to do, i will try anneal and butter the area to be welded and hope for the best
post Feb 4 2018, 06:59 AM
Joined: 14-Jan 07

Tempering is something like an hour at 550C. Not going to get a soft outcome that way in this case.
You'd have to go above transformation temperature and slow-cool.
How are you going to keep sprocket teeth region cool while heating middle to red heat?
As is desperate case and sprocket it replaceable, might be worth a try - red-hot in middle and keep outer teeth area cooled. Reduce from red heat over a few minutes.
Welding - TIG - the "desperate case" material is Alloy 625. Unusually high yield strength for an austenitic material, and is very tolerant of dilution by various elements including iron, Cr, Mo, etc. You can blag some TIG rod stubs from work?
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