radial engine cam material choice

[petertha]

I'm mulling over what type of material to use for the cam rings. They need to be hardened. My original plan was O1 (oil hardening) tool steel. Now I'm wondering if I should consider air hardening. I've never used it before, in fact have done very little hardening. Only O1 on simple parts, flame & dunk method.

My understanding of the knife blade crowd is they use air hardening & clamp between 2 aluminum plates to 'quench' after heating in order to minimize distortion of thin blades. My cams are nominal 44mm (1.7") diameter but are kind of swiss-cheesed with holes. I don't have grinding facilities to true them flat if they do any appreciable degree of potato chip curl. Maybe I could tolerate a teeny bit of warpage by stoning but not much. Any feedback or experience?

This link shows similar operation, except he used hardening compound. Maybe so as not to risk quenching distortion?
http://philsradial.blogspot.ca/#!/20...-assembly.html

[petertha]

the cam & some work in progress

[Vixen]

Case hardening should be adequate for your two cam rings. This will leave the core soft and unhardened and so allow you to flatten out any distortion that may occur.

[Ramon Wilson]

Hi Petertha,

I would concur with Mike for case hardening but for another reason that's worth taking into consideration.

Looking at your drawing you have some very thin sections at the periphery which, if you 'through harden' will tend to get much hotter than the thicker areas if flame hardened. Even if done in a controlled environment (ie temp controlled oven) those areas will begin to cool immediately compared to the surrounding areas between oven and quenching. (One way to overcome this is to have a plate either side that is heated at the same time and the whole 'package' is then quenched, the plates helping to keep the temp even.

Even more issues could be experienced with tempering unless done in an oven as those thin areas will cool much quicker giving varying degrees of hardness.

The only thing I would add to either method (through or case) is that when you quench is to keep it as vertical as possible so that it cools as evenly as possible on each side which will help minimise any possible distortion.

Hope that helps a bit more - Tug

[Vixen]

Tug has a very good point about the thin section of the cam ring. You could consider combining both the inlet cam ring with the exhaust cam ring into a single part to create a thicker overall section.
Where do the cam drive gears fit? Could they also be combined as well?

[Vixen]

Here are some photos of the cam rings for my 1/4 Bristol Mercury (9 cylinders). I machined the Exhaust cam ring on the outside of the internal gear blank. This was bolted to the combined Inlet cam ring and hub.

Could you combine your gear wheel and cam rings in a similar way?

[Admiral_dk]

I know it's quite a bit of work posting here, but any change of more pictures from both of you - please .... 

Best wishes

Per

[petertha]

Excellent feedback thanks. I can obtain this Cherry Red case hardening substance in Canada.
http://www.rosemill.com/v/html/msds/cherry_howto.pdf

Maybe I misinterpreted the procedure originally. It says to quench in oil & I’ve seen others do that. But as suggested, that rapid temperature drop via liquid quench is the problem to be avoided. Which I saw as essentially the same thing as O1. But now I see in the vendors it says “OR LET COOL” meaning no quench, just ambient cooling. I wonder, would ambient cooling yield something less hard, or similar hardness but less penetration, or...?

I guess the easy thing to do is buy some compound, turn up a similar dimensioned washer without the cam profile & just try. Redesigning the now separate inlet/outlet cam plates into a single part is an excellent idea. If I went this route with conventional round stock, would a certain alloy give me a better chance with case hardening? For example 4140 over 1018?

[Vixen]

Hi petertha,

I am not familiar with those steel grades, we use different steel grades over this side of the pond and almost all of it is imported.

As I understand it, the case hardening material adds a thin layer of extra carbon to the surface layer and so allows the surface (the case) to reach higher levels of hardness. My guess is to avoid 'leaded' free cutting steel and go for more normal 'mild' steel. There must be many members out there who can advise you on which grades case harden better.

Mike

[Mcgyver]

Excellent feedback thanks. I can obtain this Cherry Red case hardening substance in Canada.
http://www.rosemill.com/v/html/msds/cherry_howto.pdf

Maybe I misinterpreted the procedure originally. It says to quench in oil & I’ve seen others do that. But as suggested, that rapid temperature drop via liquid quench is the problem to be avoided. Which I saw as essentially the same thing as O1. But now I see in the vendors it says “OR LET COOL” meaning no quench, just ambient cooling. I wonder, would ambient cooling yield something less hard, or similar hardness but less penetration, or...?

They are all quenches in fluid - air, oil, water or the fastest, ice cold brine.  The difference is the speed of the quench.  You could quench W1 in air but it would not harden very much, or you could quench A2 in water and it would harden but with a higher risk of cracks and distortion.  A2 has some big advantages in that an air quench is the least violent so distortion tends to be less that with faster quench.  I would think this has some advantages for tricky looking parts like you've shown.   The disadvantage of A2 is the cost vs say O1, but that's probably of little concern for a single small part

The Cherry Red is afaik much like the old Casenint.  With a few applications, followed by one quench you can get a couple of thou case depth.  Might be enough for a model?  An advantage with casehardening, aside from the ductile core, is that you don't temper, so the outer skin is very hard.  This is often a design advantage, however it will take dent or bruise a lot easier than something through hardened.  If you don't do a rapid quench like oil and just do an air quench, it won't be as hard as it could be - its that rapidly cooling that does the job of locking in the austenite structure that makes it hard.

I could see case hardening or A2 but am sort of like the A2, tempered, in that the case hardening is very thin and won't harden as much with an air quench. 

[johnl]

If I went this route with conventional round stock, would a certain alloy give me a better chance with case hardening? For example 4140 over 1018?

FYI. The Hodgson radial engine plans call for 4140 for the cam rings.

[petertha]

A2 costs about 43 USD for .125thick x 2" x 18"
A can of the hardening stuff is about half that.

Here are some A2 specs I found.
https://www.hudsontoolsteel.com/technical-data/steelA2
I suspect the size change chart doesn't properly address potential problems with some of my cam plate features like the holes, proximity to edges, non-symmetry etc.

I guess in my thick skull I'm still wondering - if I'm going to heat steel up to cherry red for any of these material options (O1, A1, Steel X, Steel Y with hardening powder...), wouldn't it make sense to use the alloy that yields some increased hardness but with the least amount of quenching temperature reduction shock... which seems to be what air hardening steel is all about? Maybe I'm still missing something here?

I'm still digging around the knife blade forums because I'd think if anyone would be super concerned about warping (= scrapping) a skinny & weirdly shaped part it would be them. I thought maybe I misunderstood the purpose of the ambient temp aluminum clamp plates, sometimes called quench plates. I just assumed it was to minimize distortion during cool down. Other links refer to a slight cooling acceleration affect to tweak hardness. But this link & others like it seems to suggest its more about mitigating distortion.
https://www.bladeforums.com/threads/air-quenching.570302/

[Mcgyver]

I guess in my thick skull I'm still wondering - if I'm going to heat steel up to cherry red for any of these material options (O1, A1, Steel X, Steel Y with hardening powder...), wouldn't it make sense to use the alloy that yields some increased hardness but with the least amount of quenching temperature reduction shock... which seems to be what air hardening steel is all about? Maybe I'm still missing something here?

I would say yes, and it would be a no brainer.....if not for the excellent points Tug made.   Perhaps A2 sandwiched between a moderate thermal mass (1/8 - 1/4 plate maybe?) is the answer.  Sorry, can't be more certain than that, I don't the experience with such delicate profiles, I'd have to give a try and see how it comes out.

[Ramon Wilson]

Hi Petertha

It seems (please correct me if I'm wrong) that you are confusing case hardening steels with through hardening or tool steels. In my experience (work based) you either use one or the other. Though I don't have as much experience with case hardening as I do with through hardening I have never seen such material (tool steels) used for case hardening - If you think about it it defeats the object - the steel is heated in the CH compound - taken to temperature and quenched - you've just through hardened it.

If you are going to case harden then the material should be suitable - eg (in old terms) EN3B or EN32B (you should be able to find comparable steels over there) These are 'mild steels' whose composition is designed to give the right core with a hard surface. Though not usually done they can be tempered but this is not the same as with tool steels if doing this at home by the 'colour' method. Were I to be making this part I would use this kind of material and case harden and leave as hardened.

There are differing ways to achieve the 'skin' - heat the item and roll in the powder then heat again to a good red colour, this can be repeated - the more it's done the thicker the skin though doing it this way that skin will be fairly thin. It can be quenched - preferably in clean cold water either straight from this heating or, allowed to cool, the powder residue cleaned off then the item reheated and quenched. Quenching with case hardening, unlike tool steels is best done rapidly and this is the point at which distortion is likely to occur on thin material. Assuming the item is perfectly heated if it is not truly vertical as it goes in the quench the microscopic difference side to side means one side cooling before the other - 'microscopic' as this may seem this is what will cause the item to bend.

The better way to obtain a decent thickness of skin is to pack the item in the CH powder in a small metal container and heat the entire lot together - the longer at temp the deeper the skin. The powder liquefies so the container needs to be well packed. Quenching should be as above.

Though I often advocate quenching silver steel (drill rod) in oil to avoid cracking I have never heard of quenching CH parts in oil. I see no reason why it shouldn't work but the old CaseNite powder did 'perform' best in cold water.

If on the other hand you decide to make it from through hardening tool steel then the points from my previous post re section thickness should be bore in mind. As said before and as Mcgyver suggests a 1/4" plate either side will help keep the part uniformly heated until quenched. The quenching should be done, not so much rapidly as with CH, but more positively with the accent on that vertical movement with a uniform speed of movement through the oil interface. Most definitely don't - just 'drop it in'

As with all 'new' heat treatments a test piece is a good idea and certainly something I would recommend you do here.

Tug

[petertha]

Yes I think I confused matters with my clumsy wording. Thanks for your patience. I guess was trying to say:

1) Oil hardening tool steel, yields through hardening, heat to red hot, liquid quenching = sudden temperature loss = increased propensity for distortion & cracks
2) Air hardening tool steel, yields through hardening, heat to similar red hot, air quenching = slower temperature loss = better for distortion. Possibly over & above improvement by sandwich quench plate method attempting to confine distortion in same plane. This is what I was inferring similarity to the knife blade application.
3) Lower carbon 'regular' steel, case hardening method, heat to similar temp, saturate with powder, allow to air cool. Although it can be liquid quenched according to specs, we are attempting to avoid same problems of 1) & the spec sheet confirms it is OK to air cool to ambient similar to 2). So at least to my amateur eye, the net result looks a lot like 2) other than we only have surface skin hardening.

You have raised one more point which I had not even considered - unequal heating / hot spots based on varying thicknesses of delicate features. But assuming red is +/- red in all cases (I have to confirm the various temps) that will be a common challenge across all 3 metals, no?

I’m only guessing but maybe the designer prescribed those large diameter holes more about achieving a bit of weight savings & maybe a bit of oil splashing. But if they were eliminated, or greatly reduced, would that change your opinion about some of the delicate thin areas along the edge it now presents? Or is it more about the overall aspect ratio disc-like shape?

[Ramon Wilson]

Hi Petertha,

Tool steels - through hardening, oil quenched variety - do not usually react in the way you see it with a propensity to crack and distort - because of the temperature shock of quenching providing temperatures are in the right area for the specific metal. Flame hardening can be a bit hit and miss in this direction - it is easy to over judge the degree of heat required for basic GFS or Silver steel (Drill Rod) As mentioned previously there are ways to ensure the distortion of thin parts are kept to a minimum ie virtually nil if not completely nil. Cracking will usually occur if the temperature is taken to far and the material becomes crystallised and brittle on quenching which no amount of tempering will change.

Thin sections created by 'cut outs' as in your part are difficult to deal with from an uneven temperature aspect and using side plates (heated to the same temperature) will help a great deal to overcome this. Ideally if those holes can be reduced in size or moved away from the edge (or both) or as you suggest eliminating them entirely that will improve the situation considerably.

I've carried out a lot of heat treatment of small components in tool steels and have a lot of patience so ask away if you need to

Regards - Tug

[petertha]

Thank you Tug. I think that's worth an experimental shot then. Buy a stick of A2, make some test 'washers', one with peripheral holes & the other without. As mentioned I cant really see an obvious requirement for the holes & why they couldn't be eliminated. They add up to negligible weight savings which isn't an issue anyway. If its a complete flop I guess I can use the rest to make a kitchen knife

Just a general question to tap your obvious experience. If air & oil hardening steels are about the same price, machine similar to one another in annealed form (that's a guess on my part), similar temperatures involved, both achieve similar hardness levels post heat treating... why as model engineering hobbyists do we use O1 often & muck about with oil as opposed to make things from A2 & just let it air cool?

[Ramon Wilson]

Hi Petertha,

I've no experience at all with air hardening steels so afraid I can't answer your question.

I can say that over the time I've been modelling I've heat treated (at home) using GFS (01) and Silver steel without too much problem overall though there has been the odd failure. I have a part not dissimilar to your own that I am concerned about distortion - the gear ring in the Bentley - but it has yet to be H/T. Made from GFS I intend to have that between two plates as described. If you go about it as suggested the distortion will be very small if any but the potential is there.

I made the thrust race rings from EN40b and had them Nitrate Hardened as this is virtually distortion free but then I was at work  EN40b is not a very nice material to machine to a good finish so wouldn't suggest that option.

A while back I was cleaning down a couple of diesel engines after their run at Forncett. One of them had run backwards and that action had unscrewed the rear rotor pin. Checking other the other engines pins I removed one and it broke apart in my fingers. Made from Silver Steel these pins had been heat treated with real care because of their function but I would guess the undercut for the screw thread reached a higher temp due to the smaller diameter and became crystallised. All engines are due for a replacement pins made from EN24t which is a very tough but machinable high tensile steel in it's supplied state. Whether that would be an option for a cam ring I have no idea - perhaps Mike could comment on that.

Regards - Tug

[john mills]

i have  mostly worked as a machinist mostly hot work die steals for things like your cam ring I'm no expert but i would have housed m200  and nitride  m200 like 4140 can be supplied heat treated not to bad to machine
then nitrided a hard surface with little distortion can grow in size i believe 4140 can be done the same.I have not done much with steels that can be done properly at home.