Heat-treated 4130: ability to withstand high torque with large angular deflection

[JackRu]

Hi all,
So I am trying to replicate a tool known as an increment borer. It is a hollow shaft with hollow cutting threads on one end meant to screw into live wood and extract a 16"-long core sample. I have been told by their engineers that the borer is made out of Uddeholm Rigor (XW-10) steel, also known as A2 tool steel and is hardened to 48 HRC. The best steel I can find in the dimensions of the shaft (0.375" OD by 0.259" ID) is 4130. Unfortunately I do not have access to a gun drill able to drill out solid steel rod to such lengths, so I am limited to 4130 at the moment. I created a table below with my torque and angular deflection calculations based on different steels in various conditions:

It appears that quenching 4130 at 1575°F and tempering it at 400°F would achieve similar yield and ultimate torque values to the actual material used, AISI A2. The manufactures of the increment borer claim that their tool can withstand up to 90 ft-lbs of torque (field tested). My only concern is that 4130 would become too brittle at such a low tempering temperature (near its maximum hardness) and thus will not have enough tensile toughness to withstand such a high torque given the angular deflection. Any thoughts on this? I also know that for 4130 tempered at 400°F, the elongation in 2" is ~15%, its reduction in area is ~42%, and its Izod impact is ~31 ft-lb/in.
Is there a reasonable way to calculate its maximum permissible angular deflection based on the applied torque and toughness of 4130 after heat treating at these temperatures?

Thanks,
Jack

 

[EdStainless]

You will need the same strength, and since these are steels a close approx is the same hardness.
4130 is a poor choice, you need more C.
Finding 3/8" x 1/4" ID tube in higher strength material will be difficult.
The real problem that you will have with 4130 is that it will get dull faster than a tool steel.

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[TugboatEng]

It appears you're using McMaster-Carr as a source for the material. They have 440C stainless tube at 60 HRC. It's a thinner wall. This may actually be a benefit to your application as the thinner wall will require less force to drive.

 

[3DDave]

It needs a minimum wall thickness to machine the threads that pull the cutter into the tree. Too thin and it won't work.

 

[EdStainless]

Jack, 440C can easily be tempered at higher temperature to lower the hardness/increase toughness.
With a 900F temper (2hr) you should get something like 52 HRC, Yield 212.6ksi(1466MPa), UTS 269.5ksi(1858MPa)
We always used 1000F and lived with 48HRC.
But how on earth are you making this? Are you going to anneal, machine, re-harden and temper?


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[JackRu]

Ed,
The threads are made out of a separate piece of 4340N steel, TIG welded onto the 4130N pipe, and then heat treated afterwards. I will probably QT the entire thing according to 4130, and then QT the 4340 threads separately afterwards to achieve greater edge retention.

 

[EdStainless]

Why are the threads 4340? You only need to move from 41xx to 43xx when the parts get thicker. The hardness of the 43xx will be slightly lower than 41xx with the same C.
If you made the body from 4130 and the threads from 4140, weld together, Q&T with a 500F temper, then the threads will be harder than the body (more C).

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P.E. Metallurgy, consulting work welcomed

 

[JackRu]

EdStainless,
I always thought that Q&T 4340 had higher toughness, strength, corrosion resistance, and hardness compared to Q&T 4140.

 

[EdStainless]

43xx will have higher toughness and harden deeper.
If you look at Jominy data they will either be same surface hardness or the 4140 will a couple of points harder.
The hardness of the 4140 bar at 9/16" will be the same as the 4340 at 32/16".
If you temper at the same temperature (say 1000F) the 4340 will be a couple of points harder, but this is a temper response issue.
Deep hardening and toughness are the advantages of 43xx, but of course it costs more.

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P.E. Metallurgy, consulting work welcomed