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  • Suggestion on what material to use

    Discussion in 'The main mechanical design forum' started by KevinC, Jul 7, 2011.

    1. KevinC

      KevinC Well-Known Member

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      I'm designing a small part, probably weights about a pound or two that needs to take some load and needs to be corrosion resistance. I was originally thinking of using stainless steel. There is a very tight budget requirement (well, which project doesn't? :lol: ). I want some feedback in terms of what to choose based on what's best for the cost. Size is not a huge problem, neither is weight. Let me know if I'm saying something that doesn't make sense, but please be nice.

      Here are four things I'm considering right now:

      1) Use high strength stainless steel 17-4 PH (110 ksi yield strength) or something like that
      2) Use regular stainless steel 304 or something like that and probably do a heat treating to make it stronger (70 ksi yield strength)
      3) Design with just stainless steel 304 or something like that and scale up the part to the specific strength.
      4) Using Carbon Steel (because they are cheap) and probably do some heavy plating (chrome?) for corrosion resistance

      The parts don't need to be welded, the main two parts are rather complicated to be made by CNC machine (it will probably waste a lot of materials 60+% also). We'll make probably about 100-200 parts (I'll probably convey the person to do 1000) and is casting my best bet (compare to machining or any other process)? Die Casting? I can probably make a two part mold to make the parts.

      What are my options. Any help, or any light shed will be greatly appreciated!
       
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    3. Dana

      Dana Well-Known Member

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      Difficult or impossible to say without knowing more about what the part needs to do. Just how strong does it need to be? And what level of corrosion resistance, in what environment? Depending on loads, plastic or composite might be an option. Or aluminum alloy? For a run of 200 metal parts, casting is a likely option, but again, depending on the shape and required precision of the part, it can run the gamut from sand casting (cheap, loose tolerances, secondary machining required) up to investment casting expensive, close tolerances, possibly no secondaries necessary).
       
    4. KevinC

      KevinC Well-Known Member

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      Thanks for a reply, Dana! The material needs to have at least 50ksi of yield strength (30 is absolute minimum, above 100 is desired). So I don't know there are much options besides steel and titanium. Plastic and aluminum are out of option. The material needs to be able to be in salt water once every while (maybe once every week). The material need to withstand impact of dropping from above a meter or so, so it needs to be ductile enough.
       
    5. sdod

      sdod Member

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      Hi Kevin,

      For 200 parts I would probably take a look at machining 8620 or 4140 and heat treat for strength. You can reach well over 100ksi with the right process. For 800+ parts I would look at MIM (Metal Injection Molding). They inject powdered 4140 with a waxy binder into the mold then heat treat afterwards to remove binder and add strength. A lot of the small handgun parts are made with the MIM process. The wear properties should be decent but saltwater will eat it up. You will need a thin (.0005 to .001) chrome (electroless nickel) coating for wear and corrosion protection.

      my .02,

      Steve
       
    6. KevinC

      KevinC Well-Known Member

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      Thanks, sdod! That definitely sounds like an option. Does the heat treatment + chrome plating cost a lot of money? Do you know how does that compare to the stainless steel? especially the 17-4 PH. Although I'm also thinking about electropolishing it. Do you know what kind of ball part figure in terms of cost for the heat treatment, chrome, electro polishing processing?
       
    7. sdod

      sdod Member

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      I hesitate to offer estimates on cost for the HT or plating. I don't know the parts, how they will be used, conditions of use, etc. There are too many factors involved. You best best is to get quotes from your vendors. Once you have the cost of post processing, you can combine the cost of material and machining to come up with the best solution. Welcome to the life of a mechanical engineer.
       
    8. KevinC

      KevinC Well-Known Member

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      Thanks Again, sdod. You are awesome! I'll contact some vendor to see the cost differences.
       
    9. Dana

      Dana Well-Known Member

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      30-100ksi is a rather large range...

      6061-T6 aluminum is around 35ksi yield. 2024 is around 50 in some forms, and 7075 can be up around 70. If it's going in salt water I'd avoid plain alloy steel even with plating, especially if you're dropping it (damage the plating and you've lost the corrosion protection). Most of the stainless steels would probably work. Titanium probably doesn't make sense unless you need the weight savings compared to steel (roughly 2/3 the weight for the same strength).
       
    10. KevinC

      KevinC Well-Known Member

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      Thanks, Dana. I was thinking about modifying the size if the material doesn't have as much strength. Yea, Ti is definitely not what I'll choose. Because I don't know the cost differences between higher strength material, higher strength with manufacturing process, or just use 2-3x materials to compensate. So that's why the range is a bit higher, because all I want is trying to make the budget works. =)
       
    11. Dana

      Dana Well-Known Member

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      Well, stainless is relatively expensive. Steel is cheapest, of course, but aluminum, even though it may cost a bit more, might save on fabrication cost because it's so easy to machine. If you can live with somewhat lower strength, an aluminum casting in 356 alloy (more or less the same as 6061) might be a good option if your shape would otherwise require a lot of machining.
       

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