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Metrology Question… How does heat affect metal?

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  • #52350

    Dan Lensch
    Participant

    Here at the plant we recently have had a problem with an Air Gage where the master was placed in a computer cabinet with a temp of 84F.  We were off spec by less than .2 microns.  I realize that metals will expand under heat.  But, what my question is this:  If metal is exposed to heat for an extended period of time (in our case 84 degrees for about a year) will it “grow in size”?  Basically say the master was 122.001 at 69F when it left Air Gage.  It was placed in 84F here for a year and when it was chilled back down to 69F it is now 122.003 or larger… is this possible?  Thank you for your responses in advance
    -Dan Lensch

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    #184102

    Taylor
    Participant

    Dan
    Short answer to your question is yes. Conductivity of a pure, defect free metal is determined by the structure of atoms. With heat applied to the metal, Thermal energy causes the atoms to vibrate. In this excited state atoms interact with and scatter electrons. The amount of thermal energy needed to create this process is dependent upon the properties of the metal. Long term exposure will actually change the properties of the metal. In short the metal you started with a year ago is different than the one you have today. Again, how much different is dependent on the properties of the metal you have. That said almost any metal can change by .2 micron over a long term exposure to thermal energy especially in such a controlled state.
     

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    #184104

    MBBinWI
    Participant

    Chad:  While you are correct, would you really expect a delta temp of 12 to 16 degrees F (depending on whether the standard ambient is between 72 and 68 deg F) would really make that much difference?
    I’d take it to a calibrator and have it checked/adjusted and move on.

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    #184106

    Mikel
    Member

    I would suspect gage error.

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    #184108

    Taylor
    Participant

    The mere handling of the part with bare hands can change the reading .2 micron. But I suspect since this company went to the trouble of placing a part in a controlled temp for a year, they already know this.
    I do not know what the exact make up of the material so all I can do is confirm the posters question, that yes what he is seeing is a possibility.
    Gage error is a possibility, but I guess I assumed this has already been ruled out. Since it is an air gage I would also assume the temperature of the air has been monitored and is the same pressure, temp and density as when tested a year ago.

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    #184113

    Craig
    Participant

    122.001 versus 122.003
    Are we talking about a difference of .002 microns?

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    #184127

    Dan Lensch
    Participant

    I’ll try to clarify.  The error was .2 microns.  What I am asking about is permanent deformation.  Is it possible to permanently enlarge the metal master ring over after being exposed to heat over an extended period of time
    -Dan Lensch

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    #184136

    Taylor
    Participant

    yes

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    #184139

    Kluttz
    Member

    Short answer – it depends on the metal.
    Different metals have different thermal expansion properties.  Zinc, lead and aluminum (to name a few) would have greater thermal expansion for a given time period and temperature differential than say… tungsten, molybdenum or platinum (these metals are sometimes specifically selected for applications based on this property).  There are even some alloys that have negative thermal expansion (they actually shrink as temp increases) over certain temp ranges.  For shorter durations, the geometry of the metal (effective surface area exposed to the temp source) would play a factor, but I would guess you would reach equilibrium over an entire year. 
    But then again, I work for a bank so I’m not necessarily the best person to talk to about materials engineering.

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    #184140

    Atoor
    Participant

    Have you considered gravity. Depending on the geometery of the part and, geopmetry of the feature you are measuring and how it was stored, certain features of the gage can move if it is heavy enough by the prolonged influence of gravity.
    Good luck.

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    #184154

    walden
    Participant

    Dan,
    Many ring gages are made from type O6 tool steel. I used to work for a gage manufacturer back in the early 80s and the Chief Engineer I worked with had been in the gage making industry since the 40s. He always told me that in his experience O6 would grow in size over time even if not subjected to elevated temperatures. The increase was not highly significant, but if you’re talking about a 122mm (4.800″) diameter ring gage, an increase in 2 microns (.000078″) doesn’t surprise me, especially considering the fact that it was exposed to elevated temperatures for a year. However, I’m no metallurgist.
    Have you discussed the issue with the gage manufacturer?
    Chris

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    #184155

    MetEng
    Participant

    No, it will not grow over time, especially at the type of temperatures that you are talking about. If the master is solid steel, it is possible that there may be some time related stress relief that caused it to change dimensions (again, not related at all to the slight temperature difference). Another possibility would be that the master was (poorly) hardened and that residual microstructure changes occurred — but it would be expected that the size would shrink rather than expand.

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