Strongest material

350 grade maraging? No way. I wouldn't trust it to take any shock loading, and I wouldn't trust it to stand up to hydrogen embrittlement potential problems during manufacture. (i.e., did you pickle that puppy? Or perhaps electroplate it to increase corrosion resistance? If so, make sure you've got a good test program, with enough samples to give good statistics. And stand back from your loading apparatus.)

As a practicing metallurgist (25 years) I'd first consider AerMet

100, a proprietary alloy from CarTech. Yeah, the multiphase alloys of Latrobe (MP35N and MP159) will possibly have higher strength, but they depend on work hardening- and you'll only get that in smaller sizes. The AerMet, being a vacuum melted PH alloy, will strengthen in even large (6 inch) sections. It's available, machinable, less expensive than the MP alloys, and readily heat treated. So, I suggest you use that as your starting point, and try to find something that'll beat it.

Depending on your loading conditions, carburized 9310 might be the best. Remember that carburized steels have that sweet compressive residual stress layer. Take a look at the November 1978 issue of Metallurgical Transactions (told you I was an olde fart) for a lovely article on the topic.

Remember that mechanical properties are statistical in nature, that many failure situations are flaw-size controlled, so if you go up in size the wonderful properties available in the laboratory don't translate into reality. If you're making a real part, make it of a real material.

Good luck!

gearloose

Thanks for the reply. That's the kind of help I was hoping to get. I know experience is often discounted these days but I still have a lot of respect for it.

I was aware of AerMet 100 and MP35N and looked them up before. Their ultimate strength is both the same at 300 kpsi. The only reason I suggested 350 maraging steel is that at 355 kpsi it seemed to be the strongest thing around. But as you say, perhaps there are other factors I should consider. I'm not too concerned with brittleness in my application. In fact, it is preferable that it break first without bending much beforehand. Although, it doesn't really matter either way because I would also use a very elastic material if it had an even higher ultimate strength. Corrosion resistance is probably not critical either because the device is to operate in a benign room temperature environment.

To be more specific about the actual part, it is a gear-like disk about

2" in diameter and 5mm thick. The teeth are used to lock a position and not transmit motion so there are no restrictions on the shape of the teeth and they will be designed for maximum strength only, which I am also working on at the moment. The design of the part and the selection of the material are two separate issues and I simplified my initial posting so it wouldn't confuse the two. The normal load profile will be a uniform increase from 0 to max in about 1 second. I don't know if you would consider that shock loading, but applying maximum force instantaneously will not occur.

So I guess my question is, given this further information and these specific dimensions, assuming I could find a manufacturer with enough expertise in maraging steel to be able to avoid any of the potential problems you mentioned - given all this would you still recommend AerMet

100?

Regards, snipped-for-privacy@ion.com.au

Gearloose is correct. Other carburizing grades like 4320 and 8620 are easier to work with and have tougher cores tah 9310. In your application shear force is constant, bending stress decreases with depth, and contact force dependents on thw contact geometry. You must design for each stress and compromise - that's the sign of a true engineer. Each of these forces can be calculated analytically or with FEA - preferably both - based on geometry.

E Vojcak PE

Yeah, Ed and Gearloose have the right idea. When I was a student metallurgist 7 years ago I often wondered why we didn't make everything out of maraging steels - going by Ashby's diagrams, they were the solution to all metallurgical problems. Now that I am a little older (and a little wiser) I'll say give me a

8620-ish composition steel(anything with about 0.2C, 3 Ni, 1.3Cr with a dash of Mn, Si and Mo) with a 410HV core and 1.5mm carburised case and it'll solve 98% of steel-related problems that I see in my line of work of transmission drives. I've had some competitors get a 100J charpy toughness doing that - still stumped on that one (any ideas anyone?).

Unless you're looking at a very exclusive scenario e.g. extremely high temps or corrosive conditions (I'm afraid the component that started this thread doesn't fit category in my books) you are best off selecting what Ed and Gearloose suggest. And best of all, it won't cost much either and is easy to obtain.

Gearloose: I've always held the candle for 9310 / 9315 / 9317 but my Timken maunal says they're "Former standard steels" - can one still get the stuff nowadays? Ed: Surely 9310 should have a higher toughness than 4320 & 8620?

Adios

Doug- for your part, I'd still not recommend the Maraging 350, due to concerns about toughness (i.e. resistance to catastrophic crack propagation). I'd go with the lower strength AerMet 100 in preference to the maraging 350, or even over AerMet 310. Yet if you are not going to worry about fracture toughness, then something else to consider would be tool steels. A2 is a fine grade, as it doesn't have a lot of carbides to act as crack initiation sites and air hardens to above Rc60: it is 'stronger' than either AerMet or maraging 350. The tensile strength of hardened tool steels is rarely published since getting good test values is difficult: "?nonaxiality of specimen alignment causes erratic results." (Roberts & Cary, TOOL STEELS, 4th Edition) If resistance to deformation is one thing you seek, then tool steels can provide.

I prefer air hardening steels when I can use them, as they have less residual stress that can hurt your application. I also feel that the lack of carbides in the A2 grade will help in your application, akin to the theory of the olde 'matrix steels' of Teledyne Vasco, who adjusted M2 tool steel composition so as to remove the carbides. Lower wear resistance, higher toughness was the grail they got.

That said, I've got to agree with Ed and Ian who propose carburized steel, if the part is appropriate for the process. Remember you have to put it in a carburizing furnace and at some point quench in oil. Given your part is 5 mm thick, a standard rule of thumb would be 0.8 mm case thickness, giving you about twice as much core as case. I've long preferred 9310, but recognize that perhaps I've bought into the hype. 9310 is commonly available as a vacuum melted grade, which looks good for us aerospace guys, and has lower carbon content in the core, AND in the case- the nickel reduces the eutectoid carbon content. I interpret lower carbon content as having higher toughness, yet I have no direct evidence of that. Ed, do you believe the 9310 mystique more smoke than reality?

It is readily available, by the way. We get it from Latrobe, Atlas, and Republic, per AMS 6260 or AMS 6265, the latter being the vacuum melted grade.

gearloose

9310 is a difficult steel to machine and heat treat (must anneal for microstructure and stress relieve for hardness) and in my opinion not the best choice for the part proposed in the orginal post - 9310 hardens too deep for such as small sized application. 8620 or 4320 would be more appropriate. The strongest material does not necessarily make the stongest part.

In general 9310 has a core toughness of about 140 Mpa sqrt(m)or 125 ksi sqrt(inch)see Thomas/Patel CARBURIZING Processing and Performace ASM. I can't find data on 8620 or 4320.

Ed Vojcak PE