4140 Hardened Cylinder

[micro72w]

I am trying to make a high precision wear resistant cylinder. Bore size is .5000 +.0000/-.0002 x 6" long, it also needs to be circular and cylindrical within .0002. I am also looking at a surface finish of 4-8 microinch. The part will be pressurized to approx 38 ksi. I have selected 4140 at Rc 30-32 for its mechanical expansion properties. My problem is the cylinder is getting damaged during use. I need a more wear resistant surface, I am currently trying to make the part in the range of Rc 45-47 this is presenting numerous manufacturing problems in achieving the final bore size.

I would consider coatings, but I am not sure I can maintain or achieve the bore dimensions, also afraid, maybe warranted that they will flake of over time due to different expansion rates. Not very familiar with with case hardening or other processes. The cylinder needs to last about 2 years with approximately 100k cycles, so brittleness is somewhat of a concern.

Any thoughts on material, heat treating, coatings or manufacturing processes would be greatly appreciated.

Thanks

 

[NickE]

Use tool steel, D6 for wear, or D2 or A2 since they are more common, if you really need impact resistance too try S7.

Lathe it oversize, have it professionally heattreated, use cryo adn several tempers to assure complete transformation and full hardening/tempering.

D2 can hit HRc 60-64, A2, normally 58-62, I've not seen too much about D6. S7 will be at 56-60.

After hardening then have it OD ground to size. That is a tight tolerance, however we can hold it on big punches, I'm sure a good tooling shop can make this part.

You talk about bore size, is that the dimension and tolerance on the mating bore for this cylinder? Then I woudl change your cylinder to .4998 +0.0000/-0.0002

Normally I try to tolerance holes using plus and pins to minus, that way they fit together.

Any other questions?

Nick
I love materials science!

 

[swall]

Is the damage you are seeing normal wear, or is it something else?

 

[TVP]

You haven't really described the entire problem sufficiently, so it would be helpful if you could elaborate on exactly what the load condition is (internal pressure from what? what are the stresses? what is the part that introduces the wear? A piston reciprocating within the ID?). Also, what is the expected temperature range? Corrosion environment?

Wear resistance can be obtained from a number of different processes that may or may not affect the base strength. I would investigate two different types of surface treatments:

1. Hard ceramic coatings deposited by CVD such as TiN, TiCN, etc. These types of coatings can be deposited directly onto the steel substrate, although the heat treating cycle needs to optimized with the coating process.

2. Thermoreactive deposition/diffusion process (also called TRD or Toyota Process). This process creates a VC surface layer (not really a coating, more of a diffusion layer like nitriding) that is extremely wear resistant. Frequently used for coating metal forming tools. Visit the TD Center website for more information.

Nitriding may also be an option. Feel free to post additional information in order to clarify the application requirements.

 

[micro72w]

Thank you for the comments;

To better define the problem. Basically, I am compressing a material abrasive like alumina in a 2.00 OD cylinder with a close fit .500 bore and piston arrangement. The max pressure within the cylinder will be 38 ksi. The process requires the ID to be finished to the tight tolerances to resist blow by in the system. The OD of the cylinder has been sized for the same reason, mainly to restrict deflections of the ID.

The wear resistance hardness is to prevent axial scrapping damage as the material is being compressed in addition to damage from impregnation of the material from the radial forces. Currently I am making a shot in the dark guess for the hardness to be Rc 45-47 as I do not have a comparable material hardness for the abrasive. I have made some at Rc 32 but am experiencing radial damage. Haven't been able to achieve the tolerances via mechanical removal, we are having drifting problems, due to the shank size of the tooling. Tried wire EDM and polishing with little success. The 6 inch long bore is playing havoc on manufacturing. Increasing the harness is just making things worse.

TVP - you mentioned ceramic coatings. First, is it possible to achieve the ID tolerances for the long bore in a one pass process or do we have to include a secondary step to get the required consistancy. Curious, once bonded if this is the correct term do the material and coating deflect at the same rate, or will they de-laminate over time.

I expect room temperature during operation and the material is not corrosive.

Again thank you for the assistance!

 

[arunmrao]

Alumina is a very hard material (can't recall must be between 7-9on Moh's scale.

In that case you certainly need to use some of the tool steels suggested earlier along with suitable surface treatment. 4140 material will be unsuitable for this application.

" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke

 

[metengr]

micro72w;
After reading your OP, I believe your material selection and dimension requirements for this high pressure cylinder need a much closer look, like from a pressure vessel design standpoint.

First, I would consider this as a small pressure vessel. I believe using ASME Boiler and Pressure Vessel Code, Section VIII, Div 1 for design guidelines would be very useful for optimizing cylinder thickness AND for appropiate material selection. You can use an alternative approach to size the cylinder. However, I found that the formula's used in Section VIII are design by rule and have proven themselves for applications like yours.

In this case, I would consider one of the forging specifications like ASME SA 336, using carbon or low alloy steel. You may need to increase the wall thickness and OD. However, you now have a basis for design using the minimum wall thickness calculation (thick cylinder formula) and associated allowable stress values in Section VIII, Div 1.

Surface hardness can be increased locally using one or more surface hardening heat treatments (like nitriding).

 

[NickE]

I would like to add to my above post that you also consider carbide.

This seems very similar to a punch and die used to compress powder metals (or anything else) to create green bodies prior to sintering.

(Alumina is one step under diamond in hardness, even the tool steels I mentioned above are easily cut using alumina paste, paper, and/or grinding wheels. Only carbide is harder.)

HRc 45-47 is not hard at all and will quickly be worn away by your abrasive, even the chunky high CrC tool steels like CPM10V, D6, etc.. will not hold the tolerances for long. You could try TiC, TiN, or even diamond like coatings, most suppliers can get very tight tolerances on these coatings, I've seen them wear away, but even in double bend fatigue testing they haven't flaked off.

If this is R&D for a production process you could use tool steel until you get teh design completed, then have components made from cemented carbides.

If you see failure due to impact you will have to talk to a good carbide vendor to suggest alternate grades that keep good wear resistance along with impact strength.

Some benefits of carbide in this application are the lack of heat treat and the much higher modulus, that will help you get and hold your tolerances.

Nick
I love materials science!

 

[TVP]

micro72w,

First, I don't think you will be able to achieve the desired tolerances using either a conventional CVD or TRD process, due to the fact that the parts must be quenched from the CVD/TRD temperature, which is designed to double as the austenitizing temperature. This means that the quenching distortion will need to be corrected, which will probably result in a coating defect somewhere along the ID/bore. The PVD process is a low temperature process, but it is limited to "line-of-sight", so long, deep bores cannot be done.

What I would recommend evaluating is a duplex process that consists of nitriding and plasma-enhanced CVD (PECVD), also called plasma-assisted CVD (PACVD). The additional energy from the plasma means that the processing temperature can be reduced to ~ 500 C, which is just below the nitriding temperature. This means that the part can be quenched & tempered, finish machined, then nitrided, and then have the PECVD coating applied. This process is now being used for precision forging/extrusion tools that require exceptional hardness/wear resistance while also maintaining good strength and toughness. Use the following links for more information:

http://www.rubig.com/index.cfm/sprache/en/seite1/haertetechnik/seite2/dienstleistungen_ht.htm

http://www.eltro.co.uk/coating.htm

http://www

tecvac.co.uk/

 

[micro72w]

I want to thank everyone for their posts, gives me a great starting point on solving my problem.

 

[unclesyd]

One material not mentioned is the machinable carbides such as Ferro-Tic.

http://www.pacificsintered.com/alloybonded/punches.htm

 

[EdStainless]

Ferro-Ti is softer than most WC grades, but it would make fine punches.
When I was making ceramic compacts we were using ceramic tooling. We had Boron Carbide dies, and punches of a softer material such as Silicon Nitride or Zirconia. This put almost all wear on the punch which was easier to replace.

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[unclesyd]

Just had one of those brain thingee's after reading the last post.

You can make the punch out of a Tool Steel and just use a carbide tip. We had a 1/4" dia. punch with a 1/4"x 1/4" carbide tip. Just make sure the punch is loaded axially if you should try this approach. We used this approach for many years make catalyst pellets from cobalt oxide in Stokes Pill Press.

 

[tripleZ]

I'll build on unclesyd's comment...if you go the way of carbide tips, use a sandwich braze such as Trimet. Steel and carbide have a different coefficient of expansion. The sandwich-style braze will offer a little give that standard silver solder will not. Make sure to put enough relief on the steel base to allow the braze to expand (0.008-0.010" per side usually works), and make sure to "ping" test the assembly after building to make sure the joint is sound.

Now for the long-winded response on alumina. I have some dies and punches that I use to coin/size powder metal parts after sintering for both part density and size qualification. These parts are high temp sintered, and often carry some alumina oxide with them into the coining tooling. If my operators over-salt the parts, I can wear out a carbide die in less than 80,000 cycles. 4140 will definitely NOT do the trick.

Let's now assume you're going with a carbide-inserted die (and potentially carbide tipped punches). If you go with the wire EDM method, you will need to make a series of round laps to lap the cavity to size after EDM. I'd figure you'd leave roughly .0015"-.002" stock total with the barrel or bell-mouthing you'll likely see. You have several other options though. Find a shop with honing capabilities (see Sunnen Products) and they should be able to hone the ID close, then use a round lap to final size. Or, find a shop with a larger Heald style ID grinder. Again, you'll be able to get very close to the final size & tolerance and will likely need only (1) -.001" lap to finish off the high spots and true up the I.D.

Next, it sounds like you're trying to eject an alumina compact from a 6" long bore? If I'm reading that right, you're likely going to need some draft in that I.D. somewhere to permit the part to expand as it's ejected. This isn't the greatest solution if your piston wants to "walk", but is often required in powdered metal compacting to avoid cracking the compact during ejection.

Finally, you probably ought to start without the coatings and stick with the carbide. As the standard PVD coating is less than .0002" thick, the alumina will likely rub the coating off in less than 1,000 cycles. I'll also add that you'll want some interim inspections of that bore, especially where the alumina is being compacted. It will have a tendency to wear more heavily in that area first.

Sounds like an interesting project. Have pics for those of us who are visually oriented?