Broken crank and high compression

jseng1 said:

There's plenty of "deep rolling" info online but the drawback is that it can reduce the diameter of the shaft and you don't want that. Ken Canaga once showed a Ron Wood crank that used stock nitrided cheeks. Maybe you could dig into your photo bucket Ken and show us again. If you could cut the PTO radius and get it Nitrided then it should be good with no crack worries.

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Not sure how cast steel responds to Nitriding. My hunch is that it would offer some improvement but you are still dealing with cast steel which is inferior when compared to a typical 4340 billet crank. Nitriding the cast steel crank cheeks would be, in my opinion, like putting lipstick on a pig when you have superior billet steel options.

The only reason I persevered with racing my old 500cc short stroke Triton for 12 years was it had a billet crank. Otherwise it was a bastard of a motorcycle. I held off racing the Seeley for 20 years because I knew it had the standard crank in the motor. I use it now, but always with a bit of trepidation. With a Commando-based race bikes, two things are essential - the gearbox and the billet crank. Otherwise you are just playing at racing.

Further to this thread, I read where the nitriding "grows" the steel (my words) with the infusion of Nitrogen molecules within the surface of the part, thus leaving a residual surface compressive stress. Since this is a surface treatment one will have essentially placed the surface in residual compression which is amenable to reducing fatigue cracking (i.e. increasing durability). Rolling the fillet is a mechanical method of achieving similar results in target areas and shot peening essentially achieves the same (similar) effect on all surface exposed to the treatment.

My understanding of this is that the nitrogen affected layer is probably only one molecule thick, so what would shot peening do ? It is not case hardening, which might be an option. A flash of hard chrome might be much better ? The way to find out if nitriding adds dimension, is to have a shaft nitrided and carefully measure it before and after. I don't think it does because whenever I used to have my Triumph shaft treated, it always used to be amenable to setting clearances using plasti-gauge.

A doubt a "flash of hard chrome" will do anything for flexural duarability (what we are talking about here) and may be detrimental due to hydrogen embrittlement.

Nitrogen one molecule thick?

Your understanding is way off.

acotrel said:

My understanding of this is that the nitrogen affected layer is probably only one molecule thick.

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Like Dances said, your understanding is a bit off.

As I explained in an earlier post, the typical thickness of the hardened layer in a nitrided crankshaft is .010" to .030" (0.25 mm to 0.75 mm for most of the world). Do you really think it's fair to describe that as one nitrogen molecule thick?

Ken

Further to that, we are talking about microstress and microstrain, something that would be very difficult to measure across fillet radius of a crankshaft but it is very effective in increasing components flexural durability.

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I know almost nothing about this subject, but perhaps acotrel was referring to the affected depth of the metal on the object being nitrided, rather than the thickness of the nitriding itself. Acotrel's follow-on implies the latter, but if the former, just to what depth is the subject object effected?




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Robert_Norton said:

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I know almost nothing about this subject, but perhaps acotrel was referring to the affected depth of the metal on the object being nitrided, rather than the thickness of the nitriding itself. Acotrel's follow-on implies the latter, but if the former, just to what depth is the subject object effected?.

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Actually, the affected depth and the thickness of the nitriding are the same thing. The nitride process diffuses nitrogen into the steel surface to produce a hardened layer. The depth of the diffusion is the depth of the hardened layer. There is a small transition zone between the core material and the nitride layer, as well as a small dual phase zone on top of the nitride layer. The latter is frequently referred to as the "white layer" in the trade, and is normally machined off after treatment. These are of small depth compared to the diffusion zone.

Gas nitriding of steel has been around for well over 100 years now, with salt and ion nitriding process a bit newer, and it's not really rocket science. It's a popular method of surface hardening because it is done at relatively low temperatures compared to other heat treating methods, and also doesn't require a rapid cooling process. As a result it doesn't usually create distortion of the parts.

One of the best historical and technical summaries of the process that I know of can be found here.

http://www.asminternational.org/documen ... pter_1.pdf

Ken

The link below compares and contrasts the various surface treatments (mechanical, thermal, chemical...) and their impacts on component flexural durability.

https://www.efatigue.com/training/Chapter_8.pdf

Interesting the cautionary statements about the use of plating; particularly chrome where they go on to state that besides the detriment of hydrogen embrittlement the hard chrome leaves a residual tensile stress rather than the desirable residual compressive stress.

Perhaps we are talking about two things here. How many of you guys have ever broken a billet crank ? The reliability the billet crank gave my short stroke 500cc Triumph was the only reason I continued to race it for 12 years. In every other way it was a nasty piece of shit. If I was going to race my Seeley 850 seriously, the billet crank would be an investment - the cost would be justified by the lack of blow-ups. Probably more essential than a Fullauto head ?

Metaphorically speaking, it seems like we have no clue what we are talking about.

I have broken a billet crank and have seen, first hand, the damage as a result of a Nourish billet crank breaking in a Triumph. The billet crank I broke was a result of, amongst other things, not redoing the nitride treatment after an undergrind of the rod journals so this is all relevant. We could have tried hard chrome but that would have been a disaster. Besides, who would think of doing something like that?

In the 63mm stroke billet crank I had, the big end journals overlapped the mains and it had been forged during manufacture to get the grain flow right. I would have thought it impossible to break. I hard-chromed the journals to stop the wear which was very rapid when the motor was revved regularly to 10,500 RPM. I suggest that when a billet crank breaks - it is more likely to be due to sulphide inclusions in the steel than any surface treatment, including chrome plating, - especially when you bake the crank after plating it.
What do you do ? - If you run with excessive big end clearance and low oil pressure, you can probably blow the motor up just as easily as you can by breaking the crank due to hydrogen embrittlement and stress-risers.

Your suggestion about the sulphide inclusions has been brought up by you numerous times and dismissed numerous times due to modern steel VAR manufacturing process unless you are making your cranks out of connecting rods from early 1900 steam locomotives connecting rods.

Use of "a flash" of hard chrome, as you had suggested" to increase flexural durability has been dismissed based on various citations due to hydrogen embrittlement and surface crack propagation attributes.

In Australia our commercially available alloy steel is unsuitable for making gun barrels due to the high sulphur content. It is possible that imported steel might be better but not very likely. In America or the UK, you might do better because you have aircraft industries, however our technology base has been devolved - we don't make anything trick.

Hydrogen embrittlement is usually only a problem in high strength steel which is subject to a constant load and in any case it is not much of a problem with chromium plating. Cadmium and nickel plating are much worse, especially when applied to high strength fasteners which are under tension when in service. Even in that situation, the problem can be overcome with appropriate post-plating heat treatments. We have successfully used cadmium plated hi-tensile allen-headed cap screws to hold the two halves of rocket motors together.

acotrel said:

In Australia our commercially available alloy steel is unsuitable for making gun barrels due to the high sulphur content. It is possible that imported steel might be better but not very likely. In America or the UK, you might do better because you have aircraft industries, however our technology base has been devolved - we don't make anything trick.

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It is possible??? Really?

It's like complaining about poor quality cast iron vises when you keep on purchasing from off-brand Chinese suppliers. No offense to the Chinese. I really don't think we are talking about "trick" anything, just quantum leaps from the steam age to today. By the way, where are all the notable sources for billet cranks for our Nortons sourced from??????

acotrel said:

Hydrogen embrittlement is usually only a problem in high strength steel which is subject to a constant load and in any case it is not much of a problem with chromium plating. Cadmium and nickel plating are much worse, especially when applied to high strength fasteners which are under tension when in service. Even in that situation, the problem can be overcome with appropriate post-plating heat treatments. We have successfully used cadmium plated hi-tensile allen-headed cap screws to hold the two halves of rocket motors together.

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Read up on hydrogen embrittlement with respect to your "splash of hard chrome" recommendation. Aside from that, I will keep your experience in mind when I decide to cadmium plate my crankshaft.

Relevance, please.

When you speak of hydrogen embrittlement - with hard chromium plating, it is almost irrelevant - a simple soak in a 200 deg. C oven for a few hours will fix it. It is nothing like cadmium - that is the worst case and the sort of thing you seem to be trying to attribute to chrome. If you think hard chroming crank journals of a billet crank might cause a failure due to embritttlement, you are kidding yourself. It is the sort of thing we would do to a ship's crankshaft.
About making a billet crank - if I tried to do it in Australia, I would have an immediate problem. Unless the steel has sulphur and phosphorus contents each less than 0.01 %, it is too dirty and won't give the physical properties. So what I'd be looking for would not be our common commercial quality alloy steel. That means I'd have to find an importer and sample and test their product - do a sulphur print on the end of the billet or bar stock. The whole thing would become a bloody painful exercise.
The alternative would be to buy an American or British manufactured shaft and even then, you take your chances.
As far as I am concerned, I am now in the situation where I cannot afford to race seriously, so billet cranks and long rods with light pistons are out of the question. I persevere with the standard crank etc. and always expect the big bang. That is why I will never go near Phillip Island. - I know I can crash safely at Winton.

Regarding the hydrogen embrittlement, no doubt there are well-known remedies for that but it does not get you past the surface conditions (from what I am reading) and lacks any significant surface residual compressive stress; in fact, it may go in the opposite direction! Hard chroming has been used to reclaim wear or journal surfaces.

As for billet crankshafts, you write as though you would machine your own. You buy mill certified steel and run with it; that's the beauty of modern day QA/QC and certifications. Unless you have a suitable set of machines at your disposal, this talk of verifying steel for metallurgy is a bit silly. Your cost to have someone build a billet crank would probably eclipse the cost of buying a billet from one of the several manufacturers who are fabricating billet crankshafts on a fairly regular basis. There are quite a few choices out there, and most of them nitiride (if not all of them) and I doubt any of them provide "a flash of hard chrome" for whatever reason.