Help selecting high stiffness material w/ good fatigue life

[bob1111]

We are working on a shift fork for a transmission. The OE part is an unknown steel alloy. Have not got everything tested yet but seems to be around RC35-38 and the shift paws that have all the sliding friction seem to have some type of hard silver coat on them that is around RC65. I grinded on the part and does not seem to have much if any case harden and probably to keep the fatigue life up.

I am trying to select a material for this. I have looked at 4340 and A-2. I would prefer not do to any super special processing with these other than a temper. I have looked at nitrocarburizing to add a thin, super hard case on the part. I am not really familiar with the process though.

Very open to ideas to get high stiffness, high fatigue life, good wear. I will be adding a lubricity ceramic on the completed part as well. my target hardness for these is RC62-68 on the case.

 

[TVP]

Stiffness is a function of geometry, not the alloy. All martensitic steels have essentially the same elastic modulus. The OE part was probably plated with hard/industrial chromium.

Nitrocarburizing is certainly capable of imparting good friction and wear properties. You can review the Kolene/Nitromet website for more information on the process, finished properties, etc. Personally I would not use a tool steel like A2 for this type of application. I would probably try a martensitic steel like 4340, Nitralloy, etc. that responds well to nitriding. Another option would be to induction harden only the surface, but 4340 is only capable of ~ 58-60 HRC after water quenching. You would need a steel with ~ 0.50-0.55% C in order to consistently achieve 62+ HRC after induction hardening.

 

[arunmrao]

My feeling is that since you have the original sample.you could get a test report and submit here. You would then get spot on advice,now it would be many conjectures and assumptions,which might only add to confusion. Please get a sample report and let us know the chemistry,hardness and microstructure.

Learn the rules,so you know how to break them properly.
Dalai Lama

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[bob1111]

The coating we will apply contains 10% Molybdenum and will apply at approx .001" thick. Our thought was to use 4340 and through harden to approx HRC58-60, then apply our anti-wear coating. I still cannot get much from the coating people to figure out if this coating can be substituted for a hard plating on the shift paws.

I realize that stiffness of most steels is basically the same and our revised design certainly addresses the stiffness problems but it is my understanding there are some high Moly alloys that have higher stiffness. I could be off target on that.

 

[EdStainless]

Check the paws, they may be coated with a hard facing, either welded, brazed or sprayed.

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Plymouth Tube

 

[tbuelna]

viper6383,

The most cost-effective hard coating on steel, for wear and low friction, is thin dense chrome. It is very hard (>Rc76), low friction, and has excellent adhesion to steel substrates.

I would stay away from ceramic coatings, since the quality of their adhesion after spraying would be questionable in the (grooved) wear areas of a typical shift fork. Sprayed coatings only adhere well when the direction of the spray is normal to the substrate surface. Thin dense chrome is electrodeposited, so the part shape is usually not an issue. Just make sure to specify a hydrogen embrittlement relief immediately after any plating is done on high strength steel parts.

4340 would be an excellent choice for the shift fork material. But I would not heat treat it to a hardness any higher than your load requirements dictate, and definitely not to Rc58-60. There is always a trade-off between strength and ductility, and a part that yields slightly before breaking is always a much safer design.

Hope that helps.
Terry

 

[bob1111]

Terry, that is super information. Upon careful discussion, we have strongly looked at 4340 and 8620 but to be honest, the 8620 is a metal we have never cut or messed with. The decision of how to heat treat us still in the air but we have plenty of experience with 4140HT but really trying to learn more about the HT processes for these alloys so we can setup a small system in the shop to HT in house if possible. I REALLY would like to start ordering annealed material so we can mill faster with less cutter wear.

I am very curious about your views on the coats as we certainly have the same reserve about it. We already have the stuff here but our way of thinking is if they require a 150 grit Aluminum Oxide blast just to get the stuff to stick, that must be a rather weak mechanical bond and that coating may just become a huge source of problems.

Regarding chroming. The only Chrome we have done here is decorative. I have some concern that our budget of $1-1.50
for a 1in3 part in lots of 150 would be stretching it. We really are going to have to drag costs down in this design this our thoughts on keeping what we can in house.

Another concern I have is we have a shaft fit on this part and do NOT want to get involved with a post grinding process. If we can get a plater to control the plate thickness, that would be great but I do not see them being able to hold .0002" but maybe. We planned to run the parts with a boring operation that would allow us to stay in our tolerance and we have been successful with that before so that is why we thought if we could throw a good alloy and HT at this with a dry film lube with Moly, it might work but we share your concerns.

 

[B1ALT]

I believe that 4340 is a good choice for this application, and would not recommend heat treating it beyond the RC35-38 hardness.
I agree with Terry using hard chrome coating but it might be important to have the wear surface area under the chrome to have a nice ground or polished surface prior to the hard chrome process.

Bob

 

[bob1111]

Thanks, I think we would like to through harden to about HRC28-32 at this time, then do a case harden to max hardness for wear protection. We really want to keep the HT in house to reduce costs and lead times.

Is there a process that can yield more than HRC60 in 4340 and is there a process that can be done in house without too much complication? I hear some carburizing can be rather hazardous so we will have to weight the application.

 

[tbuelna]

viper6383,

If you are willing to do a case hardening process, then I would recommend nitriding instead of carburizing. There will be much less HT distortion with nitriding than carburizing, since nitriding does not require a quench.

Most any alloy steel (4340, 4140, nitralloy, etc.) will respond to nitriding. The drawback of nitride versus carburize is that the nitrided case depth is much less than a carburized case. But for your shift fork application a nitrided case would still be more than adequate.

Carburized and nitrided surfaces both would require a finish grind or honing operation to clean up the wear surface. The nitrided surface would require less stock removal since it would have less HT distortion, but it would still require removal of the white layer as a minimum.

Finally, the drawback of case hardening versus thin-dense chrome is that the case hardened part cannot be reworked with the same process as the original part, like the thin-dense chrome treated part can. And since case hardening and thin-dense chrome still both require a finish grind operation, I would guess that the thin-dense chrome plated part would be less expensive to manufacture.

Good luck.
Terry