4340 & Stress Concentrations

[drillrig]

Hello all,

This is my first post in this forum so please forgive my ignorance.

I am designing a large shaft (17.75" OD) made of 4340 alloy steel. See attached picture. The shaft undergoes both axial and torsional loads as well as internal pressure. My analysis indicates a stress concentration of 45,000psi at the fillet where the "flange" starts, as would be expected. The spec I am designing to stipulates that for stress concentrations I need a safety factor of 2.8 on the material's ultimate strength. My shaft would therefore have to be made from a material with 126,000psi ultimate, in this case 4340 in some sort of quenched and tempered condition.

With a shaft this size my experience indicates that even if I could find a vendor that produced a ~18" OD bar quenched and tempered, most of the heat treated steel would be machined away and the material at the 9" OD would be more like the annealed condition (~60,000psi yield, ~95,000psi ultimate).

My question is this: Just below the fillet in question we have to induction harden the shaft to 58/62 Rc. Is it possible to also heat treat the fillet to attain the required ultimate strength? Is this common practice? Can you directly correlate a yield strength with a hardness value? Most of the stresses in the shaft are very low, and I'd rather not rough machine/heat treat/final machine the entire thing.

Any comments would be most helpful.

Thanks in advance.

- Chris

 

[unclesyd]

It would be WAG for me to address the hardness at the 9" diameter. I have been try to find charts that were given to me by Bethlehem steel that gives expected hardness for thick sections, especially roll alloys.
My literature gives UTS of 108,500 and a YS 68,500 for annealed material. That said you will be above these numbers. Looking at the end quench cure which is relatively flat to 4". Looking at this and plotting out the hardness curves for different sizes and heat treatments I would look for 115,00, 120,000 UTS at 4"-5" in

If you go the 2 part construction you should get with your NDT personnel about the RT of the welded part. The reason is that you are approaching the limit on thickness with Ir192 without screens. Your geometry doesn't lend it's self to getting a sharp radiograph of the root section of the weld. If possible the weld zone should be as far away from the flange as practical.

I would still get someone like Liberty to look at the prints. Make sure you let them know that this a prototype an if it proves out there could be additional work. Your part could be made the same way as the picture shows some long weldneck flanges.

Based on the post by EdDanzer

Do you have the option of getting a larger radius at the change in section. Ideally it should be a 3:1 elliptical.

Would it be possible to put another step in the shaft, say a short 12"

 

[Tmoose]

With that lovely solid model it seems likely the stresses were predicted with FEA. is the peak stress confined to a region near the surface? I'd look at the possibility of modifying the geometry before going for a boosted material properties.

A bigger radius could be used if the flange was undercut.

Attached is an image from Taylor's "Internal combustion engine" Volume II from MIT Press. It demonstrates the somewhat counter-intuitive fact Material removal can provide big improvements in fatigue strength for complicated shafts in bending and torsion.