Stainless steel selection

[prdave00]

I have attached a simple surgical tool for your review. It is essentially a handle with a pin installed via a transition fit. The smaller diameter tip protrudes in through a bore until it contacts the back face of a member press fit into place via a Morse taper. The job of the tool is to break the taper lock to eject the inner member. I've been asked to consult on material for the pin, but I'm at a loss right now. Given the environment that the tool will be used and due to various regulations, the material should be a stainless steel alloy or corrosion resistant exotic alloy (L-605, CoCr, etc.) commonly used in surgery.

The engineer arbitrarily selected XM-16 stainless in the H900 condition since we have other tools made of this alloy that rely on its material strength. Our machinist suggested Type 316 in the annealed condition (although I'm not sure why). The contacting tip of the 455 pin started mushrooming within the first 4 or 5 uses and eventually broke at the start of the taper.

Any suggestions? Any thoughts on what mechanical property I should rank candidate alloys by to (1) increase life before the tip deforms and (2) increasing the life of the tool. I apologize if this leaves the door too open.

 

[Metalhead97]

prdave00,

Was the 455 in the aged condition? Have you considered an Inconel alloy?

Metalhead

 

[dgallup]

Most surgical instruments are made of martensitic stainless as it can be heat treated to high strength. 420 up to 440C.

As a design engineer, I would recommend a fillet at the transition to the taper and a bigger diameter of the tip if you can. The sketch suggests it could be significantly bigger but there may be reasons why it can't.

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[prdave00]

Metalhead97: The 455 PH stainless was in the H900 cobdition as far as I know (or at least that's what it was supposedly certified too). I'll look into Inconel. I've never had the pleasure of looking at it for my applications and it seems it's been used for surgical blades. Is Inconel hard toconnect by? Any grade you'd suggest looking at?

dgallup: We do use 420 quite a bit and I've been pushing for 440C for our single use tools that need a good hard cutting edge. There has been a legacy of using PH stainless steels like 17-4, XM-16 (455), and the hard to come by 465 for our instruments. However I'm not sure how much research went into material selection. I'll go back and look at the mechanicals for the martensitic stainless steels.

Also good thoughts on simple design changes. I'm also contemplating proposing that the tip be radiused so there is no edge to deform from off axis impact loading.

 

[Metalhead97]

prdave,

455 in the H900 condition has a UTS of 245 ksi and 500 vickers. This is hard stuff. I think 440C has higher hardenability.

You are going to have a hard time finding an Inconel alloy with a similar UTS/hardness in plate/rod stock.

Metalhead

 

[EdStainless]

Are you sure that the 455 failed because of lack of strength and not a lack of ductility?
I do find it hard to believe that it was H900 and that is not hard enough.
Stronger than 240ksi with any usable ductility? Not 420.
Elgiloy, C-300, It is a short list.

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Plymouth Tube

 

[prdave00]

Ed: Thanks for the input and you have me double guessing whether I have all the correct information. The test part was supposedly machined from an obsolete tool that should have been Type 455 in the H900 condition but I don't think anyone confirmed that to be 100% true. It may very well be a ductility issue. I'll need to do a bit more homework on the problem.