Forged maraging steels - ASTM A579

[PerpetualDynamism]

I’m currently designing an arm to hold a system onto a military vehicle. The arm must support a large load and at the same time must remain light, hollow for cable pass through, and mass manufacturable. Although we are using aluminum throughout the system, in the arms we have decided against it for fatigue reasons.

I’m leaning toward a steel arm but based on my simulations I’m looking for a material with an infinite life fatigue strength of over 100ksi. The arm shape cannot change greatly due to space limitations in almost every direction. Not being able to find fatigue strengths for a lot of materials I’m assuming 40% of Ultimate Tensile Strength (UTS) as fatigue strength, which means I need a material with a UTS of >250ksi.

As I see it that rules out the normal steels like 4130 or 4140. Instead I’m looking at using ASTM A579 grade 72 or 73 with UTS of 255 or 280ksi, and elongation before break of 10% and 9% (not super brittle). However I’ve never designed something using a forged maraging steel and so here is my dilemma.

Is this metal in fact available in small sizes (read ~3” square stock) and likely to be in stock? Has anyone had parts machined from this material? Do I need to have it forged into the rough shape first or is taking a bar into a U-channel, for instance, an ok thing to plan on doing? Also, I anticipate the assembly would need to be welded, and therefore re-heat treated. Can anyone direct me to a good source for properties associated with different quenching/annealing properties or other post machining treatments?

Any help on this would be great. Also, suggestions for similar materials that also might be worth looking into would be appreciated. Up until now most of what I’ve designed/worked on has been aluminum or carbon steel, and this is just outside my experience so far!

[PinkertonD]

Welcome to the forum. If fatigue life is critical then you are correct in looking at steel. However havng said that there are way too many unknowns to be able to advise you. It is impossible, well, for me at least, to offer anything when you are talking "tube," "forging" and "machining" all in the same paragraph.

It gets worse.
Given that it is for the Military it is going to require documentation coming out of the walls, floors and ceilings. Asking here is probably not a good start. I would be looking for an Engineer familiar with large commercial aircraft landing gear experience to answer those questions. Even better, you should probably hire him to design it with those stress and fatigue figure requirements.

[RWOLFEJR]

My experience with maraging steels is limited but I've spent a lot of $$ on a maraging steel while learning what doesn't work.

First thing that popped to my mind was a concern for your fatigue requirements and the use of the maraging steel at what sounds like, its ultimate attainable state. We've used a material made by Teledyne Technologies (used to be called Teledyne Vasco and was Vasco something or other before Teledyne bought them...) over the years. When you go for the high end of the tensile strength, (via forging and or solution heat treat) you might possibly reduce your fatigue cycle life. (To be read... When we bumped up hardness to achieve higher tensile strength to approx. 80% of maximum the material could reach. ... our C350 rods didn't last long (month or two) due to fatigue cracks. When we considered our loads and allowed for 100% of theoretical tensile requirement... which took us to approx. 50-60% of the materials attainable tensile ... I don't recall one breaking in the last fifteen or twenty years.)

All that to say... Going full boat on hardness to get 250ksi tensile might not get you where you need to be. You might need to go 125ksi tensile and operate at 80% of UTS in order to get the fatigue life you're after.

Those numbers from our particular situation aren't etched in stone... just my recollection from maybe fifteen years ago and more of an F.Y.I. we'll say?

But that wasn't really what you asked and I'm not an expert so along with what Dave mentioned I'd also suggest talking to somebody at Teledyne Vasco or whatever they go by now... and I think ATI... Allegheny Technologies might make maraging steels also? And a good heat treater might also have information on those types of materials and their properties at different processing levels. (To be read... I always contact a guy that we've used for heat treating for eons that really knows his stuff and has kept good records of past runs and results. The guy amazes me in what he knows about all metals and the information he can dig up. He's a wealth of information ...)

Do me a favor if you would...
Let us know what you find out on this. Just curious...

Good Luck,
Bob