4000 Series Steel vs 1144 Fatigue Proof 1

[CSM11_ME]

We are currently designing a rather large stationary axle, that has a large moment on the ends of it. The axle is stationary but will see large forces, repeatedly applied, fluctuating from small nominal (almost 0) to a large value. Because of this loading the design is ok in normal loading but fatigue added to a stress concentration at one of the many step downs, fatigue is our limiting factor. We are trying to keep weight and size down for this axle. Currently we are considering using 1144 Fatigue proof or, maybe using a 4000 steel that is heat treated, we are looking for the best cost option betweeen some sort of heat treating and buying the stronger material so that we have 120ksi Yield, and 145ksi tensile, (or similar) anyone have experience making a similar decision? There is a lot of machining that has to happen to this axle which starts around 3.5" diameter down to about 1.5" at the ends, and it is almost 4 ft long, so machine-ability is important.

 

[metengr]

From the information above, I would consider a 4340 alloy steel, quenched and tempered.

 

[EdStainless]

It is difficult to buy small amounts of material with special properties, but in your case a stand-in for fatigue resistance might be transverse impact toughness.
Higher strength will improve fatigue life, but if the structure isn't clean you may not gain much. The are Ca treated steels (for inclusion shape control) that are used for high toughness applications. These would be appropriate for this application.
I would look at 4335 (maybe V mod, or Impact 7) or 4340, and look for some with high transverse toughness. I would go to higher strength also, 140-160ksi yield is easy in these alloys and delivers a good balance of strength, toughness, and fatigue resistance.
If you could find some Timken Impact 7 that would be a great option.

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P.E. Metallurgy, Plymouth Tube

 

[mrfailure]

Why would you even consider using 1144 "Fatigue Proof" for this application? This resulfurized steel is loaded with internal stress risers from all of the sulfides that make cracking easier to initiate and propagate over a properly heat treated 4000-series. Think about it: you would be using a steel that effectively has large numbers of internal longitudinal flaws in it since the sulfides do not do anything for structure. I would be very careful in utilizing such steels in applications such as this stationary axle.