Forged maraging steels - ASTM A579
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!