Case Hardening Push Rod Ends

Hello All,

I am a newbie to hardening and am making some push rod ends for a vintage motorcyle club. The parts were turned from EN8 medium carbon steel (0.35-0.45% C) as I thought this would heat treat nicely. However, although a good hardness is achieved after quenching the subsequent tempering to reduce the brittlenes (for a part subjected to constant impact) reduced the hardness considerably....so I thought I'd case harden them.

Case hardening and quenching the wearing end of the part only worked perfectly from a hardening point of view but the surface finish became pitted/cratered which is not acceptable for the wearing surface. This appeared to happen when dipping in the comound rather than the quenching.

I have created a link (I hope) to a pdf file with photos and more detail.

Link to more detail

Does anyone have any tips on how to case harden this EN8 part without degradation to the machined surface finish ??? I will obviously expect to clean off scale, etc but cannot accept pitting/craters.

Thanks in advance.

the trick is to remove oxygen from the equation, encase on a metal box with a piece of brown paper and the case hardening medium soak at red heat. only quench the parts to be hard .

Thanks Bernard for the reply.

I guess the metal box needs to have a lid and the purpose of the brown paper is to burn the remaining oxygen in the box?

Use silver steel, then you can harden and temper to whatever value of Rc you want, up to a maximum of high 60s.

Andrew

Shouldn't pushrods be tubes with hardened ends?

As I see it, case hardening followed by quenching is going to leave the steel through-hardened. Oil quench might be acceptable, but I’m not too expert on this. Case hardening is usually carried out on non-hardenable steel.

Unless there is a large surface contact area, between the cam follower and push rod, point contact may well lead to rapid wear if the case hardening fails at a highly loaded point contact.

Tempering at 550 Celsius seems rather high to me.

Andrew’s suggestion seems the best option.

Hardening is always prone to distortion. Surface finish should be after, heat treatment, not before.

How are the rods fitted? It looks like that end, at least should be tempered - more so than the follower end.

That was my initlal thought but it does tie in the with datasheet.

I think the OP is under a misapprehension, medium carbon steels are not intended for hardening and tempering in the same way as silver steel or gauge plate. Instead the heat treatment is more about optimising impact resistance and yield point. For medium carbon and alloy steels tempering at temperatures normally used for silver steel can reduce the impact resistance.

Before deciding upon a material what are the important qualities? For instance surface hardness, ductility or impact resistance?

Andrew

EN8 is in the bottom range of what is considered hardenable (I think that's a word even though the spellcheck flags it), don't think you can expect more than 1/4" depth even in water (Jominy test). But it should work on small parts.

First thing I would try is to reduce the tempering down to 300-350C. If that's not good enough I would start looking for a better steel. Carburizing isn't that hard, but you really need a furnace with some sort of temperature control.

Thank you all for your support and replies.

I would really like to achieve a good case hardening result if possible. Not least of all because I have already machined a far few in the EN8 material but also because I think the part would perform well with a glass hard dome surface but ‘tough’ (need a better descriptive word for non brittle non soft!) body.

I do have a temperature controlled electric furnace capable of 1600 deg C so will be able to accurately control the process.

When I initially through hardened the EN8 I oil quenched and then tempered at 550 as the datasheet. I think I will try water quenching and tempering at 325 to see if that maintains a hard surface but non brittle body.

I will also see if I can soak heat in the casing compound without oxygen but that seems tricky to open a red hot metal box, take hold of the part in the correct orientation and then to the quench tank before it cools any but I’ll see how it goes.

These ends push fit onto aluminium tubes for the rods.

Get hold of a copy of Tubal Cain (TDWalshaw) book on hardening and heat treatment, it explains it all in language even I understood.

Presumably, you'll have to grind the domed surface after hardening. In which case, an alternative might be to (weld) hard-face the ends, then form the domes.

.

I doubt that you will ever find, or invent, a better word, Pete

Ref. **LINK**

http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/strength-toughness/basic.html

MichaelG.

.

Note: __ here’s a link to the parent page :

http://www-materials.eng.cam.ac.uk/mpsite/

… it seemed like a great initiative, but hasn’t had the updating that it merits.

Edited By Michael Gilligan on 21/07/2023 06:19:25

I used to make replacment push rods for Triumph and BSA bikes from silver steel ends with aluminium rods. The cup end was drilled and finnished with a ball end milling cutter then the other end was drilled and bored flat bottomed to take the alloy rod which was to be locktited on after finnishing. The ends were hardened by heated in a furnace to temp 860 -900 deg C. Wrapped in a stainless steel foil wrapper to prevent scale forming then quenching in oil to harden. They were then re-heated to about 280 deg C and quenched in water to temper them back to hard but not brittle state. A quick polish and clean before assembly. We made small batches of 20 to 30 at a time.

David

if you have oxy-acetylene gear you can melt the end of a Stellite lathe tool blank and "drip" molten Stellite onto the end of the rod, which can be of any steel, and get a very hard but just about polishable surface. I have never done this myself but I have seen it done on a commercial diesel repair, and which engine went back into service without apparent problem.

During WWII, engine parts - particularly valves - and other wearing items were welded up with stellite, before being ground back to service tolerance, as new parts were in short supply or unobtainable.

I have quite a few service data sheets from WWII on the subject of maintenance during wartimes (and likely the few years after).

If you have a furnace I would give carburizing a try. All you need is a metal box with a removable lid, bbq charcoal and soda.

Fill the box with ground up charcoal mixed with 10-20% soda. Bury the parts completely, heat to 800-850C or so for a couple of hours, then quench and temper.

Based on this (engineersedge.com) you can expect the surface to be 10HRC harder than the core.

I'm with the 2 Davids (Halford and George1) - the pushrods on my old Beeza A10 were hollow and very light, though I don't know if the tubes were alli.

I wouldn't be surprised if a solid pushrod, with its much increased inertia, would upset the timing by throwing the valve open further and/or delaying its closure, as the valve spring would have more work to do changing its direction and accelerating it back down the pushrod tunnel.

A long time ago a friend working in the textile industry gave me some worn pushrods retired from knitting machines. I think he said they were nitrided - they had a very thick and intensely hard outer layer with only a thin core of 'normal' steel in the centre. Eventually I used them as seatbelt retainers in my old Moggie Traveller when I had to fit belts to it in the mid '70s - the only way I could cut them up was with a grinder.

The OP did say pushrod ends, but I don't think case hardening EN8 is ideal 

Edited By duncan webster on 21/07/2023 19:03:08