Crankshaft material question 1

[PackardV8]

Crankshafts - billet steel, forged steel, cast steel, nodular iron, malleable steel and cast iron - what else?

Recently a discussion arose concerning various materials used to make crankshafts and I realize after fifty years of hefting cranks, I don't have all the facts at hand.

We all know about billet steel and forged steel performance crankshafts. A 1970s specification book I just consulted showed OEM shafts in that era all being either forged steel, nodular cast iron or cast iron. The 1955 SAE paper detailing the new Packard V8 specifically states the engineers debated the choice of forging or casting and chose cast steel for the crank.

Here are questions; who's got hard answers with reference sources?
1. Who uses cast steel crankshafts?
2. Given the difficulty of weld repair on cast iron heads and blocks, what physical properties/alloys of the nodular iron and cast iron used in crankshafts make them so easy to weld repair?

jack vines

 

[swall]

I am not aware of any cast steel used for automotive crankshafts. It is my understanding that some cast steel cranks were produced during WW2 for military vehicles. As for cast iron cranks, the only ones I am aware of are ductile iron (aka nodular iron), austempered ductile iron and malleable cast iron. Malleable cast iron cranks were used into the early '70's on some GM vehicles. I am not aware of gray cast iron ever being used for automotive cranks. I would not agree with your statement about nodular iron being easy to repair.

 

[tbuelna]

PackardV8,

Modern production cranks are typically cast nodular iron, cast ADI or forged steel. Because production applications are so cost sensitive, cast cranks are an attractive option for lower performance engines. But even cast cranks can give an amazing level of structural performance due to their very high current state of metallurgical/process development.

I can't answer your question about weld repair of cranks, but here's an interesting tech paper comparing structural performance & manufacturing cost between cast iron and forged steel cranks.

Hope that helps.
Terry

 

[patprimmer]

Terry

The link does not work.

Regards
Pat
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[tbuelna]

Sorry Pat, I'm not really so good with this internet thingy.

The 2005 paper was authored by M. ZOROUFI AND A. FATEMI and was titled "A LITERATURE REVIEW ON DURABILITY EVALUATION OF CRANKSHAFTS INCLUDING COMPARISONS OF COMPETING MANUFACTURING PROCESSES AND COST ANALYSIS". Should be easy enough to find with google.

 

[patprimmer]

Found first hit and attempting to download now. Thank you Terry. It should be interesting reading.

Regards
Pat
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[BrianGar]

Thanks also Terry,

Brian,

 

[tbuelna]

BrianGar,

There was some interesting stuff in that paper, so I hope you had time to read it. I am always fascinated by production engineering of things like crankshafts, and how manufacturing engineers wring cost out of a part/process. It's definitely an under-appreciated skill.

I liked figure 27, which gives a cost comparison of cast vs. forged process for various annual crankshaft production rates. The as-formed cost curve for forged steel construction dips below that of ADI cast construction at about 200K pieces per year, and below cast nodular at about 600K pieces per year. It was surprising to see that the high non-recurring costs for forging/trimming tooling were offset so quickly.

I also liked table 7 which gives a cost breakdown for materials & processes of a finished forged steel crank. Some of the individual manufacturing processes cost less than one dollar to perform. Even heat treatment only cost $2.69!

Good stuff.
Terry

 

[BrianGar]

Terry,

Sure did read it, I have to study it in-depth next when I have time.
Forging cross plane cranks can also be interesting.

I found a working link here> Link
You can download by logging in with your Facebook acc, or sign up. For a fee of a days membership of 9dollars you can then start to download.
You dont need to pay fee to view.
Im sure It could be got in other places for free, but for 9 dollars its a steal.

Thanks again Terry, its good stuff, more interesting than some others,

BG

 

[JayMaechtlen]

As far as crank repair-
Some places will use a flame spray process to build up a journal.
I don't know the whole process- I suppose they grind the damaged surface down enough so the sprayed surface can be a fairly uniform thickness.
The only crank I remember having this done to is still in my garage waiting for me to finish the motor.
(old Moto Guzzi, now waiting > 30 years...)


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[tbuelna]

The weld repair of cast nodular cranks and cylinder heads usually involve different processes/consumables. The welding of a nodular iron cylinder head is usually to repair a crack or surface defect, while the welding of a crankshaft is usually to build-up and salvage a worn/damaged journal surface. If the proper procedures and materials are employed, both cast nodular iron cranks and heads can be successfully weld repaired. However, in either case the HAZ of the weld repaired nodular iron part will not have the same metallurgical properties as the original part.

Since a welded nodular iron crankshaft journal will not be heat treated to re-harden the surface, the weld wire alloy used is one that has high hardness as deposited (hard facing). Structural weld repair of a cast nodular iron cylinder head does not require hardness, so a different weld wire alloy is used. Since post weld cracking is usually the biggest concern with cylinder head repairs, the alloy used is one that resists cracking.

One reason for the apparent misconception that iron cranks are "easier" to weld repair than iron cylinder heads may be due to the fact that crank repair is mostly performed by specialists using automated processes/equipment, while cylinder head repairs are more routinely performed by less specialized/experienced shops using manual processes.

Regards,
Terry

 

[patprimmer]

Even I weld or more likely braze cracked iron heads. I would never think to try to weld a cast iron crank. I tried to weld a cracked block once and failed miserably, although I have to say doing it insitu in a paddock in the dark with oil still in the engine made it somewhat more difficult than it might have been stripped down and cleaned and on the bench.

I should add, although it may be obvious, I am not a trained welder, but I still manage simple repairs to cylinder heads. Preheating is an essential part of the job, as is preparing a clean surface. Professional shops who do cranks of course already know this.

Regards
Pat
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[patprimmer]

I know some shops also very successfully weld hard face overlay onto cast iron camshafts to provide extra material to substantially increase lift and or duration without going to a small base circle.

Regards
Pat
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[mfgenggear]

I have had good success with repairs using engineering chrome plate
regrind the journals to .002 u/s on dia.
then chrome plate with .002 min per surface buildup.
then regrind journals to size.

should work for cranks.

Mfgenggear

 

[Tmoose]

I know some premium crank makers and refinishers have used chromed journals, at least in years past.

http://www.ecgrinding.com/Performance_Racing.html

Legend has it most hard chrome has microcracks. There have been stories of cracks in chromed components serving to initiate fatigue cracks then migrating into the base metal of parts experiencing cyclid. If any of that is true I'd hesitate to chrome the hard working high stress fillets/radiuses at the edges of the rod and main bearings.

This lab of some reputation subscribes to the possibility of reduced fatigue resistance.

http://www.nhml.com/hard-chromium-plating.cfm

Failure to process (bake) the part shortly after plating would bring on the invisible risk of hydrogen embrittlement as well.

 

[mfgenggear]

Tmoose

It is a common fix for aviation/aerospace.
as long as the the chrome plated journals are NDT before & after there should be no issues.
I been using the engineering chrome for the last 35 years with out issues.
It may have to do with workmanship issues.

Mfgenggear

 

[mfgenggear]

I also left out a bunch of information.
pre bake to stress relieve then plate.
embrittlement relief is part of the process post plate.

I have never had issues as long as all the proper procedures are followed.
I know from personnel experience some shops cut corners. then thats when
trouble starts.
if all the precautions are followed to the chrome plate specification all is good.:>)

Mfgenggear.

 

[tbuelna]

mfgenggear,

Thin-dense chrome is an excellent and widely accepted way to salvage bearing journals. In fact, the thin-dense chrome plate is so durable that some rolling element bearing manufacturers use it on new alloy steel bearings races as a cheaper corrosion resistant alternative to 440C. Unfortunately, thin-dense chrome plating cannot be successfully applied much thicker than about 0.0005". So it would have some limits for journal resurfacing.

Here's a link that gives a brief overview of hard face weld repair of crankshaft journals.

Regards,
Terry