Need help picking a steel to use.

I am looking for a recommendation for a material for an application below.
The application involving two shafts. One shaft is about 35mm in diameter x 330mm long. It has a 14mm bore in one end with a 6 mm x 27mm long slot through the shaft coincident with the 14mm bore. This shaft is held is place by two ball bearings and is driven from the end without the bore by a servo motor.
The other shaft is 12mm in diameter x 150mm long. It has a 6mm hole though the shaft at one end. This shaft is inserted in the 14mm bore of the other shaft. The 6mm hole is aligned with the 6mm slot in the first shaft and a pin is pressed in the hole to transmit torque.
This assembly must be able to transmit 165Nm (110Nm with a 150% safety factor) of torque at very slow speed. The total radial travel is less that 180 degrees.
Both of these shafts are currently made from SAE 4340 and harden to Rc 48-52. I thinking maybe some other material might be better suited for this application?
The problem is we're seeing 9-12 degrees wind up in this assembly at 110Nm and need to redesign.
We don't have much room to go bigger but do plan on making the smaller shaft a little larger and the bore in the larger shaft larger accordingly.
The shafts don't see much impact but the slot in the larger one does need to be harden to transmit the load from the pin.
Any help specifying a material would be greatly appreciated.

You are aware, I hope, that a broad range of steels have practically identical values of Elastic Modulus and are essentially identical in their elastic range (non-yelding)?
So, unless the large windup is related to the slop in the interface between the two steel components, changing steels will do relatively little in reducing windup.
Have you the capabilities to do a simple strength of materials analysis of torsional deflections?
Without an ability to analyse the deflections, your designs are just drawings.
In the family of metals and alloys, there is a trend to have similar ratios of (Elastic Modulus)/(Density) and if you don't mind the extra weight you could consider Tungsten as a small shaft.
Composites with the right fiber layup function pretty well in torsion, but probably don't like that funny 6mm pin hole for fixation.
With long small diameter shafts in torsion, you should expect some windup.
I would start with some analysis of that, and a better handle on just how much windup you can actually tolerate.
"As little as possible" is not a mature designer's response.
snipped-for-privacy@gmail.com wrote:
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Thanks you for your response! No I was not aware. Please bear with me I am not an engineer and am trying to understand which steel properties dictate how much torque it will transmit. I do have a couple of CAD programs that will do simple FEA calculations. The following list are the editable properties of materials in the CAD programs.
Elastic Modulus Poissons ratio Shear Modulus Thermal Expansion Coefficient Density Thermal Conductivity Tensile Strength Yield Strength
Do understand you correctly that "Elastic Modulus" determine how much torque a material can transmit? How much does tensile strength influence how much torque a material can transmit? I did notice that as the tensile strength may very from steel to steel Elastic Modulus does seem to remain somewhat constant.
Am I wasting my time trying to find a different material? I have been told to look at Flexor or Teledyne VascoMax as possible solutions? It sound like tungsten is also a possibility. Weight is not an issue.

snipped-for-privacy@gmail.com wrote:
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Elastic modulus is STIFFNESS, it would relate to how much rotation a shaft of a certain size will rotate under load in the ELASTIC load region (the shaft will return to the original shape or position after the load is removed). Another expression for this under torsional loading would be windup.
Strength is another matter, and when the yield strength of a material is exceeded, some of the deformation put into the material is permanent. A spring loaded beyond the yield strength will permanently stretch. Similarly a spring loaded beyond the failure strength will fail or break.
You probably need to actually retain a trained engineer if you are where you seem to be in technical knowledge.
Or just keep working away trying everything everybody tells you to do.
I'm retired. I don't work under conditions like this anymore.

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Without giving you a lesson in shaft design, one method of reducing that wind up (torsional twist) is to use a larger diameter shaft. If weight is a problem, use a hollow shaft. Excessive and frequent torsional twist can and will destroy the shaft. (I say that from experience.) For a power transmission shaft length of 330 mm (13 inches) the recommended maximum twist is 0.083 degrees. I think your design needs refreshing.
You may want to examine a copy of Shaum's Outline Series "Theory and Problems of Machine Design". I believe it is now published by McGRAW-HILL. In it there is a chapter titled Power Transmission Shafting which will enable you to understand my suggestion. I strongly recommend you get specific information from an engineer knowledgeable in shaft design. In addition, you may want to look up "torsional modulus of elasticity" to be aware of the values regarding steels.
Jim Y

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Thanks Jim,
I have ordered Shaum's Outline Series "Theory and Problems of Machine Design" and will see where that takes me.
Thanks again Rick