Optimal Splined Shaft Material for Low Cycle Fatigue 3

[novateague]

We're looking to maximize the amount of torque we can apply to a short splined shaft (~10,000 reversing cycles). We have been using 4140 previously.

What would be the most important material property to achieve this? We are looking at Aermet 100, Ferrium C61, MP35N... Seems 300M is recommended for shafts frequently on here.
 
Taking some inspiration from this forum, we already changed the interface radius to an ellipse shape and increased the cross-sectional area to the maximum.

As we speak, we are machining some of these parts from Maraging C350 as a test. The parts will be heat treated for the higher end of fracture toughness and then shot peened. Hopefully we can utilize some of the insane yield strength before the lower ductility becomes an issue.

 

[mfgenggear]

Novateage
Are there issues with the current material.
4340M or 300M are readily accessible.
Compare all the stats. Hardness, tensile yield and ultimate. Charpy properties.
An other material is AISI 9310 carburize.
Run the complete numbers. With the new material.
Look at bearing loads, fretting(contact stress), tooth bending, depending on the geometry, torque and RPM.

 

[TugboatEng]

Are you finish grinding after heat treatment?

 

[mfgenggear]

Maraging steels are a pita to process, and I experience large dimensional changes after heat treating. Pickling has to be carefully processed.
Very extensive

 

[mfgenggear]

Also shot peening splines has to be done carefully.
It has to be inspect before, during and right after shot peening. Some shot peening people get to aggressive and have dimensional change accepting the no go gagae.

 

[TugboatEng]

My understanding is that maraging steels experience less dimensional change during heat treatment. I am not an expert in this matter but I have hired a company to produce splined coupling and they convinced us to change from 4140 to 4340M because it would not require additional lapping after heat treatment which significantly reduced costs.

 

[mfgenggear]

Tug
My experience has been the opposite.
Shops not experience with this material mar aging
Will scrap their first lot.
4340m and 300m depending on the thickness
Can be vacuum heat treated with gas quench
With little or no distortion.
Follow the procedure of AMS2759/3 specification.

 

[EdStainless]

What is the current failure mode?
How long are the lasting?
I have found maraging steels to not like low cycle high load conditions.
The size change is very predictable, and we could make 1.5" splines and hold 0.0005" easily after aging.
But found the performance lacking.
I would buy some 43335Vmod (or aerospace 4340mod) that is double melted for very clean structure.
And then optimize the heck out of your spline.
Full involute? Very tight tolerance? What lubricant? Supported to eliminate bending? Ground or polished after machining?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Yes let's make some analogy.
We all know 17-4 has a dimensional change about .0005 per inch after precipitation harden.
And has to be accounted for.
Same as Mar aging steels except I don't remember the exact amount but approximately change of .oo1 per Inch after initial and final aging
A 1.5 inch spline will have very little change.
Mow may change .0008 inch. However a 12 inch exter gear could change as much as 008 inch.
So which if gear is finish before final aging
It will be under size. It's been a while.
So this is an analogy.

 

[novateague]

The failure mode is always ductile torsion failure right at the radius where the input to the shaft steps up to meet the splines.
I should add that this is not a typical "shaft" - it's used in a static loading application. 0 RPM.

4140 has acceptable service life, but at lower torque than is ideal. If we can raise the torque, that would be great for us.

We're going to age the Maraging C350 and then wire EDM and finish the splines for this test.
Has to be micro shot peened (0.3 mm diameter shot) due to the tiny custom splines.

The solution annealing and aging temps were selected to maximize the fracture toughness at the expense of strength.

I'll look into 4335Vmod, I've never heard of it.

In your guys experience, is there at least a small increase in performance with ultra high strength materials, even with the more brittle properties?

 

[mfgenggear]

Novateague

Look at SN curves for suggested materials
Do you have software you can use.
Calculate torsional loads

 

[novateague]

@mfgenggear

So, focus on material fatigue strength at 1E4 cycles?
C350 has a relatively low fatigue life - 110 ksi @ 1E7 cycles I believe.

The part has been analyzed with FEA, but it fails at higher loads than calculated by hand and FEA.
Maybe it is strengthening past it's yield point?

 

[novateague]

Based on SN curves, Ferrium C61 and S54 look like they're almost as good as it gets.

I found this previously, thought it was a nice toughness comparison of high strength materials:

https://www.carpentertechnology.com/blog/toughness-index-for-alloy-comparisons

 

[mfgenggear]

This link

https://amesweb.info/Torsion/torsional-stress-formula.aspx

 

[mfgenggear]

Most common steels used for splines and gear shafts for aerospace application.
9310 carburized
4340 modified
17-4ph
Nitralloy nitrided
Ferrium C61 steel
9-4-30
Find the data sheets for each of these and sn curves.

 

[geesaman.d]

The best materials will still be notch sensitive if your surface finish and lay mark direction are not good.

 

[EdStainless]

Can you make the transition taper any longer?
Are all of the surface's ground and polished?
We used to lap parts like this aiming for surfaces that no scratches when viewed at 40x.
This is way smoother than any profilometer will measure.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Nova
A little more info if you can.

 

[MFAMET]

This attached diagram was taken from AMS Handbook, "Heat Treatment"

 

[novateague]

@EdStainless

The transition is as long as possible (mating parts dictate the maximum size).
One thing is, now that it is an elliptical transition, the machining toolpaths will be more prominent than using a standard radiused corner bullnose endmill...

It will be blended by hand to eliminate machining marks, then micro shot peened. No exotic surface treatments for this test. I understand for these high strength materials, the tolerance for any surface imperfections is very small.

I am thinking these 350 Maraging parts will be exceptionally strong on a "one-off" ultimate failure test, but it's probably not the optimal material for the highest torque over 10k cycles.

Ferrium C61 looks very nice from a fatigue standpoint:

The parts have just undergone solution annealing before machining - average hardness is 32.5 HRC. I will report results after the aging is completed.

I attached some studies on Maraging 350, some of them are quite old.

Boeing AD692165
"The Effect of Heat Treatment on the Fracture Toughness and Subcritical Crack Growth Characteristics of a 350-Grade Maraging Steel"

https://files.engineering.com/getfi...ss_and_Critical_Crack_Growth_350_Maraging.pdf

NASA Technical Memorandum 106562 AIAA-94-3 078
"Surface Fatigue Life of High Temperature Gear Materials"

https://files.engineering.com/getfi...mperature_Gear_Materials_Maraging350_gear.pdf

Army Materials and Mechanics Research Center AD-756 422
"Processing of 18% Ni Maraging Steel (350 Grade)"

https://files.engineering.com/getfi...le=Processing_of_18Ni_C350_Maraging_Steel.pdf