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Machinability of SS

  1. Nov 20, 2005 #1
    In looking at various stainless steels, I notice that S and P are added to "improve machinability." What is the mechanism here? And what exactly makes a material "machinable"?
     
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  3. Nov 20, 2005 #2

    Gokul43201

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    At the basic level (that's as far as my knowledge goes), the addition of S and P in austenitic (or other) SS, leads to the formation of brittle non-metallic particles (primarily manganese sulfide/phosphide...actually, them's the only ones I remember) which help chips to break off. This makes it easier to take deeper cuts out of the metal.

    This is based on a foggy recollection of something I read a long time ago...so do not treat it as authoritative.
     
  4. Nov 20, 2005 #3

    PerennialII

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    Have a similar vague recollection .... would add oxides to the batch of inclusions used in improving machinability. If am not completely mistaken, some of the inclusions, in addition to improving chipping, have tool lubricating effects (as solid lubricants). The key (if memory serves) is to get inclusions of appropriate "softness", which as such help rather than deteriorate machinability (and other properties naturally, or try to limit such deterioration to minimum), and the development of such inclusions requires some specific steel making process "fine tuning".
     
  5. Nov 20, 2005 #4

    Astronuc

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    Gokul and Perennial are correct. I believe MoS2 is also one factor, particularly in 316S, where the S denotes addition of sulphide.

    While adding sulfur improves the machinability of austenitic SS, it also reduces fracture toughness, raised the NDTT (Nil Ductility Transition Temperature), and can also be a site for anaerobic MIC (Microbial Influence Corrosion).

    Nuclear grade 304 and 316 have extremely low levels of S and P.
    Most stainless steels, particularly 300 series, usually have 0.045% P and 0.03% S.

    It is better to use a good lubricant and sharp cutting tools to machine SS, rather than use a grade that is more 'machinable'. Many years ago, an SS coupling on a transfer line to a tankcar loaded with HF sprung a leak. The coupling was made of 316SS and it fractured during a cold period, when the temperature was below the nil-ductility transition temperature. The fracture occurred as a result of 'normal' operations in which the coupling was struck with a hammer or mallet in order to loosen it. This fact was never considered in the coupling design nor material selection. Engineers must expect the unexpected, and anticipate that which one would not normally anticipate.
     
    Last edited: Nov 21, 2005
  6. Nov 20, 2005 #5

    Gokul43201

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    Hmmm...didn't the same thing happen with the Titanic ?
     
  7. Nov 21, 2005 #6

    PerennialII

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    Talk about a bummer, wouldn't have thought would influence the behavior of an austenitic SS with 'machinability' tweaking that much with respect to fracture toughness (or well, have always thought the resulting decreases in toughness are around 5-30% or so), or introduce NDTT resulting transition behavior in the first place.
     
  8. Nov 21, 2005 #7

    Astronuc

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    Gokul you are correct about Titanic. While it was not stainless steel, but a marine structural steel, sulfur and phosphorus contributed to the failure, and the fact that the steel was low in Mn, which would have combined with some of the sulfur and improved the ductility.

    http://en.wikipedia.org/wiki/RMS_Titanic#The_rediscovery_of_Titanic
     
  9. Nov 21, 2005 #8

    PerennialII

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    ... "for laughs" a few years in our lab we did a reanalysis of the Titanic ductile to brittle transition impact data, and got similar values for T27J transition temperatures. One of those times after which everyone was trying to figure out how an earth it could float even that far in the first place, "a ship made of glass" by 'usual standards' :biggrin: .
     
  10. Nov 21, 2005 #9

    Astronuc

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    Yeah, after learning about the quality of the steel, I wonder if they used cheap steel on purpose. Certainly back then, they did not know as much as we know now about materials.

    In fact, anyone who has taken a materials course or more likely a course in fracture mechanics probably has heard about the infamous 'Liberty' cargo ships which split apart while sailing across the North Atlantic.

    Probably wasn't until after WWII that materials science really took off.
     
  11. Nov 21, 2005 #10
    Hey everyone, thanks!
     
  12. Nov 21, 2005 #11

    PerennialII

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    You're welcome RPI! Hope we don't distort the thread too bad :tongue2: .
    Yeah, impurity control at that time must be off the scale and rimmed steel and all, such steels still come up occationally .... when doing failure analyses :biggrin: .
    Actually while we're at it there was a piece in JOM a while back which is online:
    http://www.tms.org/pubs/journals/JOM/9801/Felkins-9801.html
    http://www.writing.eng.vt.edu/uer/bassett.html
    So the state of the art of the time is somewhat lacking in nowadays terms.

    ..... I think I'll use the opportunity to update some of my "introduction to cleavage" slides ... Titanic "sells" better than the Liberty ships :biggrin: .
     
  13. Nov 21, 2005 #12

    FredGarvin

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    The liberty ships were still an impressive manufacturing achievement. I have to marvel at the incredibly simple fix they came up with to help the problem. I think it's one of the few cases where a band-aid actually did the trick.
     
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