We have a part that we are designing that will require a very hard surface for anti-wear properties. Roughly RC62-68. We are considering A2 but never machined the stuff. Just curious how it runs. We run a lot of 4140HT and 17-4PH and the annealed hardness is less but that does not mean easier sometimes.

We are also looking at an 817M40 which I personally know nothing about and not sure if that is considered a tool steel or not. I guess we are looking for an alloy with very high hardness ability, good machining properties when annealed, and very abundant in sizes.

A-2 is one of my fav's. I think it is easier on tooling than 4140HT, and it is easy to get a great finish on. You can push it till you hear it almost make a "tearing" sound, then back off a little and just sail on.

Like Spock says, A2 is decent to machine, but I don't think you can run it in that hardness range. By the time you get a minimal temper on it, it'll be more like 60-62. If the part requires any toughness, then A2 needs to be tempered into the secondary hardening range, up around 800-900*F, where you'd end up with a hardness around Rc57-58.

You're more into the range of D2, and it sure as hell isn't easy to work.

A bit more description of the application might bring some surface treatment suggestions if its strictly anti-wear. These would almost always result in a far cheaper part as opposed to starting with a tool steel.

This part will be a shift fork for a transmission. High load, high surface hardness is required. Again, I have not assessed all the steels applicable yet but tool steels came up in the meeting earlier. We are also considering a hard face plate on the high friction areas but that will prove pricey so I would rather find a system to full harden or case harden the parts to the target hardness. We obviously need really good fatigue life here as well. Deflection is inevitable but actual load range has not been determined just yet.

Maybe D3, even though it is a "D" series, it is not through-hardening, but a deep case hardening steel. Tougher than other "A" or "D" tool steels. If I remember correctly, it is an oil quench. (even though as a "D", you would think it would be air quench.)

Or maybe 8620?

Jeff

A shift fork will typically have raised pads at the ends of the fork for a fairly small contact area. A part like that would typically be selectively hardened in the contact area, and heat treated for toughness in the remainder of the fork since its primarily structural in nature.

I've never checked the hardness on the pads on a shift fork, but I'd imagine its going to be somewhat dependent on the hardness of the shift collar it engages. Since this is a pure sliding friction application, there will likely be a significant difference in the hardness of the two parts to prevent galling.

It would be rare to find a part like this that required mid-60's hardness because that's a tough goal when toughness is a consideration as well. Just a wild guess, but I'll bet some aluminum bronze inserts would be better than any sort of steel contact area. This would allow you to make the bulk of the fork from a material optimized for its structural qualities, like 4140 or 4340, and still have contact pads made from a material that'll outwear just about anything else that's commonly available.

hmm. a bronze type pad will surely fail in this application. Some comparables are using a type of hard coating on the tips which I think is nickel or hard chrome. It is more silver than shiny so who knows but we have a couple engineers working that that. We don't do steel parts much so having to drag the books out.

I will agree that 4140 seems a better option structurally and fatigue life would be much better but if we throw too many operations at this part, the budget will be blown and we will have to cancel the project. A full hard part was a discussed option if we can determine actual loads and that really becomes difficult to calculate due to some unknowns.

I am curious what the alloy is that I listed above?? I have never heard of it and not sure what it is used for but someone brought it up as a potential.

The 817M40 is also called EN24 by the Brits. It has slightly more chrome and slightly less nickel than 4340. Has the same amount of Molybdenum and Carbon as 4340. Should respond to heat treatment much the same as 4340. Perhaps some of the guys from across the pond will chime in with more info on it.

Aluminum bronze is a totally different animal than normal bronze. Vulcan Materials has one of the country's largest jaw crushers here locally. They were replacing the bronze guides on its 85,000 lb toggle plate every 3 months. I remade the guides for them from aluminum bronze. That was 8 years ago, and they're still running with negligible wear.

No doubt some sort of wear resistant overlay or plating would be a cheaper option for the fork. There's lots of different ones out there, but one of the first steps would still be to determine the hardness of the shift collar it runs against. Have you checked the hardness of the coating or overlay you mentioned in the previous post?

The gear it will ride in was tested at RC 60 IIRC. The coating on similar forks was about RC65. If we are to approach the coating method for wear reduction, I would really rather bring the operation in house if practical. obviously chrome or comparable would not be practical but I am not sure what is out there.

I concur on localized case hardening and heat treating in other areas for strength. Not sure on how to process that, all I know is it is easy to mask case hardened parts. Also what type of material strength are we looking at here. 90-110 ksi or 180-215 ksi. That will make a huge difference in your material selection.

My personal suggestion would be to through harden 4340 forks (tempered back to about 44-46 rc and mechanically attach case hardened 8620 wear pads at 60-62 rc onto the 4340 forks. Should hold up well. If you need a stronger fork, make them from 300M, it has anywhere from 20-30% higher strength and a much more predictable fatigue life, though can require a pretty special heat treating process to get the extra strength from it.


Husker

One possibility is liquid nitrocarburizing. You can get to the mid 60's easily on low alloy steels, with enhancement of lubricity and fatigue properties as well. Not something that can be done in house, but hardly any of the truly effective processes will be suitable for in house application unless at the big factory level. I've had that process applied to some parts that had previously been spec'd as carburized, and the difference in durability was remarkable. Price wasn't too bad either, in the range of 50% more than typical carburize, quench, and temper IIRC.

Plow around on this site http://www.nitromet.com/ and there's most any kind of info you could want on the process. Although not applicable in this case, another use for this process is corrosion resistance. It'll outperform hard chrome in ASTM salt spray tests by a wide margin, and some suppliers of hydraulic cylinder rod now offer nitrocarburized rod as an alternative to chromed rod.

That is a really good call Met. I will admit that I am not super familiar with the process but IIRC, this is a process specifically designed to bring in a super hard case or surface layer while minimally affecting the fatigue life of a part. If we can hit our target RC62-68 with a basic material like 4340, that would be great. I will have to read some more on the procedure and see if it is done locally.

You might want to see if you can find someone that does laser hardening. (If you do find someone, let me know too, our vendor went belly up 3 years ago.)
They can get it to HRC 90 +/-2 about 20-40 microns deep. This is a *very* durable surface. The base steel must be a tool steel, and core hardness should be in the at least to HRC 48-50 before it goes to them.
Make sure whatever surface finish you want is on it before you send it, as you won't be changing it when it gets back....

We made all kinds of fixturing with this treatment and it lasts 10x longer than any other treatment, coating or what have you in our application (5+ clampings / minute, high Si aluminum parts, 24/7.....)

For comparison, in one application, 0.8 mm deep HRC 60 case hardened 8620 will last about 6-8 weeks, thru hardened A2 in same application, 3 months. D2 - 3-4 months.
Laser Hardened H13 with a 50-52 core hardness were lasting 22 months. Really wish that company hadn't went bankrupt.....

I really doubt we can fit laser in our budget but that does sound like the right avenue to take. We have 1.50 budgeted for surface treatments. Very good idea though Tony. I might find someone and get a quote just so I know what the service is worth for future projects.