SS tool for usage in salt water environment 1

[Wolfy4u]

Hi, I have designed and produced (outsourcing)a tool for diving.
Till now we have made it from lost wax molded SS17400PH hardened to HRC42.
A new manufacturer suggested me to use MIM and S420 2.0 with a hardness of HRC46-54.
There is a flat driver on this tool that is bending when using lots of force and is the weakest point of the tool.
I have thickened it and widened it for the next tool design.
What are your thoughts? Suggestions, anything I should beware of or think of? etc...

Thanks in advance for your answers.

 

[TugboatEng]

Can the tool be rinsed after use or does it need to spend an extended time under water? 420 stainless, along with the rest of the 400 series, will pit in salt water. If you can rinse it after use it should be fine.

 

[EdStainless]

Why leave the 420 that hard? I would be worried about lack of ductility.
You want the tool to bend before it breaks.
Maybe if they could hold 46-50HRC it would help. Ask what tempering temperature they will use (it should be 800F or higher).
How much more expensive is the MIM part? It is an expensive process, though it might let you include details in the part that are now machined.
Have you looked at other alloys for the investment cast version?

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[arunmrao]

MIM is an expensive process compared to investment casting. The only factor favouring MIM is if the quantities are small. Also, frequent design changes can be done using MIM .

As suggested before 400 series SS can corrode or are generally more brittle.

 

[Wolfy4u]

The tools is not always rinsed as it sits in a case or pouch. That is not my concern as it will is treated with ArmorGalv process.
As for bending vs. breaking is more of my concern. Today it bends when using excessive torque and I have thickened it from 1.8mm to 2.2 and widened it from 4.6 to 6mm.
Is 420 at HRC50-54 more breakable or will also first bend, compared to s17400? it is not nice for the client to bend it but better than break it. But best is if it will hold more than the force of the hand power when used.
Till now I made 1000 pcs with lost wax and it was the best cost /unit ratio. But thinking of more nice finish and precision of small details and 3-5000pcs at least, I thought MIM would be better?

 

[Wolfy4u]

Flat driver on top left.

 

[EdStainless]

I would have put the two screwdrivers in the middle positions and the hex drives on the end.
A flat blade screwdriver on the end makes alignment difficult and lets you apply more torque, neither of which are good.
And the hex drives will handle more torque.

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[Wolfy4u]

Thanks Ed for the advice. The way it is designed is with thoughts of how it is used. Having the flat in the middle wouldn't work as it touches something.
Every mm is with usage thinking.

 

[EdStainless]

My hesitancy about this is seeing that screwdriver blade only partially engaged in the slot and then twisted hard and breaking off a part of the blade tip. This thin portion of the blade may not need to be as long as it is, keeping it shorter would help strengthen it.
One advantage to 420 is that you have pieces made and then test them. If it is too brittle you can re-temper them at a slightly higher temperature to lower the strength (and hardness) and make them tougher.

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[IRstuff]

Sort of a side question; does your MIM match the original density?

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Having the flat in the middle wouldn't work as it touches something.

With a slotted screw head the things touching most frequently will be your knuckles on metal.

But if you insist on having it, put it in the middle position but have it stick out farther than its neighbours.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Wolfy4u]

This is how it is used. But as I said it will be thickened and widened on the next production.

 

[EdStainless]

IR asked the question that I have been trying not to, is your MIM vendor assuring 100% density?
I would be more concerned about internal defects in the tool extensions with MIM than I am with investment casting.

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[Wolfy4u]

Is lost wax better in that issue?
As for now, I had no other problems with the tools and I hope to solve it with the new better design and with S420 2,0.

 

[EdStainless]

It really depends on the care of the processor. At their best both processes produce uniform fully dense parts that have good properties. At their worst they can both make junk.
I have worked with MIM in PH SS parts and for the most part I was impressed.
Your vendor should have a detailed quality system. There should be processes controlling the powder (composition and size distribution), the blending with binder (materials, ratios, methods), the actual injection process (pressure, temp, time), the bakeout and sintering operations (temp, time, atmosphere control), as well as the final Quench and Temper treatment.
How will they inspect the first parts to assure uniform properties and soundness? There should be a lot of hardness testing (take a finished part and grind 1/4 of the way into it and then check hardness in all areas),some metallography (show the uniformity of microstructure), some destructive testing (bending individual tool projections), and maybe even x-ray of a sample from the first run.

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[mfgenggear]

Wolfy4u

one of caveat of S420 it does distort during heat treat, but for that configuration it may be stable.
far as functionality my opinion it would work better for the application. nothing worse when torqueing down a fastener and the tool bends.
if the supplier can work out the density and hardness I believe it would work better. just from my actual experience from mechanical assembly.
17-4 is tough but 420 has better hardness and toughness for a tool. I do have tool design and fabricating experience and I would
make all my tools with A-2 tool steel. distorts very little and I would have it heat treated to 60 HRc , and never had an issue.
I would never use 420 because it distorts a lot. and is tough to control, but have made hardware with it and it is doable.
edit: but wanted to add all my tools were made from wrought material hogged out, and never mim or casted.

 

[EdStainless]

A-2 isn't a bad option, working for company that makes tool steels we tend to make in-house tools out of H-13. Not quite as strong but tougher than nearly anything else.
A tool (and knife) must bend before it breaks or it is a hazard to use.

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[mfgenggear]

ED
Totally agree but it depends how that tool is used, a knife for cutting only such as a fillet knife is not intended for bending, now a knife made from 420 is
tough and hard and can with stand side bending or more abuse.
for the OP's tool I would stay with the manufactures recommended hardness.
A-2 for my applications worked very well,

 

[Wolfy4u]

Till now I used s17-4PH A900 but as said, the flat driver bends with excessive torque. Thickening it might be enough. I'm not sure the current manufacturer is using S420 2.0 material and therefore I asked. If he don't I'll have to decide whether to continue with S17-4HP or look for another manufacturer that uses S420 2.0.
Also at current molding costs, MIM vs. Lost wax @ 1000 pcs as the first production batch, looks like the lost wax method is winning.
The only q. is S420 2.0 or S17400. I'll ask the manufacturer. If they do both I could have both tested and decide. If not. We'll see.

 

[mfgenggear]

I suggest do a review on other tool steels as well, example D2, H13 and on. D2 is used for dies, then look at the cost of material vs functionality