Blue Crankshaft

Submitted: Monday, Feb 08, 2010 at 11:31
ThreadID: 75871 Views:2694 Replies:6 FollowUps:3
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I am pulling down a diesel engine and have noticed that the cast iron crankshaft is uniformly dark-ish blue from one end to the other. The crank spins freely and there is no bluing on the bearing caps. It looks like the crank has been heated in a previous life, to maybe 300 degrees or more, to produce the bluing.
The manufacturer's service people don't seem to know what to do. Should I:
1. Get it re-machined and put it back?
2. Get it heat treated first?
3. Chuck it away and buy a new crank?
Any informed comment would be greatly appreciated.
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Reply By: Member - ross m (WA) - Monday, Feb 08, 2010 at 12:20

Monday, Feb 08, 2010 at 12:20
It might help if you told us what it is from. Someone may have some experience with that particular engine.

Ask an engine machinist,maybe it can be load tested for flex.
AnswerID: 403246

Follow Up By: Keith Berg - Monday, Feb 08, 2010 at 12:48

Monday, Feb 08, 2010 at 12:48
It's actually a Volvo diesel from my boat. But a diesel is a diesel and I can't get any joy on this anywhere. I put up the post because I know there are quite a few members with a lot of diesel experience.
It's a TAMD 41B Volvo diesel, similar to what has gone into ma lot of trucks. 3.6L 6cylinder intercooled turbo. Max RPM is 3,800.
It's going to a specialist machine shop.This engine has a sad history after coming from a well known Volvo marine reconditioner in Sydney; it dropped a valve a few years ago with just 80 hours on the clock. It turned out the reconditioner had used second hand valves. Now the replacement engine has done the same thing and it looks like lawyers at fifty paces. I have a metallurgist looking at the valves, an independent machine shop going through the motor plus an expert scrutineer coming in to double check. It'll be a $20,000 repair, plus legal fees, so you can understand me wanting to do a bit of homework.
I really would appreciate any second opinions on this blue crank.
FollowupID: 672724

Reply By: Malleerv - Monday, Feb 08, 2010 at 13:31

Monday, Feb 08, 2010 at 13:31
When ever I send a crank out for machining the first thing that happens is they crack test and check the hardness of the journals. If the crank has been heated it may be soft and unserviceable. If it comes up ok after the hardness test then it will be machined.

Cheers Matt
AnswerID: 403256

Follow Up By: Keith Berg - Monday, Feb 08, 2010 at 14:37

Monday, Feb 08, 2010 at 14:37
Thanks Matt,
I have read about a process caused 'blue embrittlement" in some steels, which does not reduce the hardness, but reduces the ductility, so that it will snap more easily. Maybe the crack test you suggested will answer everything. I'll certainly have to trust the machine shop people but the blue thing remains a mystery.
FollowupID: 672745

Reply By: Member - Chris & Debbie (QLD) - Monday, Feb 08, 2010 at 13:42

Monday, Feb 08, 2010 at 13:42
I have rebuilt a few different volvo engines in my time but cannot every remember the cranks being blue, may be from being ground and Nitrided?
I have seen a few with broken crankshafts due to the block being warped, so if wanting to do the job properly, the block would be need to be lined bored and the crank replaced as if I remember correctly they dont take kindly to being ground.
At a minimum I would at least get the crank bore alignment checked, and the crankshaft journals measured.

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AnswerID: 403257

Reply By: Keith Berg - Monday, Feb 08, 2010 at 14:30

Monday, Feb 08, 2010 at 14:30
Thanks Chris. Good advice.
I'll make sure the machine shop does all of that. The blue crank is a bit of a mystery though. Should be grey. My inclination is to scrap it, but we'll see.
I do appreciate you comments.
AnswerID: 403264

Reply By: ob - Monday, Feb 08, 2010 at 18:48

Monday, Feb 08, 2010 at 18:48
The blue colour could be caused by a number of things including the original heat treating of the main and con rod journals for induction hardening before finish grinding. If the journals have been ground undersize and the machine shop didn't keep the coolant flow up to scratch. Admitedly this would be extreme. Or possibly a lube oil failure causing galling or seizing and then an attempted recovery of the crank by grinding undersize. This would leave a nice finish on the journal surfaces but a possible loss of hardness.
I would get the crank hardness, crack and alignment tested before going any further

Cheers ob
AnswerID: 403298

Reply By: Keith Berg - Monday, Feb 08, 2010 at 20:00

Monday, Feb 08, 2010 at 20:00
Will do OB
It's strange that the blue is absolutely uniform, like it's been in a furnace.
Many thanks
AnswerID: 403305

Follow Up By: furph - Tuesday, Feb 09, 2010 at 07:56

Tuesday, Feb 09, 2010 at 07:56
I know its a long shot, but could the crank have been "blued" as in rifle/shotgun barrels and parts.
FollowupID: 672845

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