[time-nuts] Thunderbolt stability and ambient temperature
cfharris at erols.com
Thu Jun 11 23:59:28 EDT 2009
Tom Van Baak wrote:
>> Most people thought it was coincidence of heat propagating up the bar
>> just at that time, or steam. Could be, but I still think it is real.
>> The cold end of the bar was slowly getting warmer as I carried it, but
>> after the sudden cooling of the hot end, the cold end seemed to get
>> hot fast.
> We know each material has it's own thermal conductivity. Copper
> is 401 W/m-K and steel only about 1/10th that, etc. It would appear
> from conductivity tables in books that these are fixed constants.
> But I wonder if near-liquid (red hot) steel has a different (e.g., even
> lower) value than warm or cold steel? Is metal thermal conductivity
> actually a fixed constant or is it a function of temperature over a
> wide range of temperature?
> If so, not only is the temperature changing when you drop the
> steel in water, but also the thermal conductivity of the steel. Thus,
> depending on how fast or slowly you cool it vs. how non-linear
> the conductivity is as a function of temperature, I imagine you
> could observe the very effect you describe.
Red hot steel bars are quite far from being molten. They are
just black body emitting a more visible light range. It is not
until the bar is almost white hot that it is going to melt...
generally a very bright yellow. Your bare eyes won't like it!
I have heated and held all manner of steel bars in the process
of welding and working steel. It always seems to me that it
takes about the same amount of time for me to feel the heat.
Unlike aluminum, it takes minutes for the heat of a red hot end
of a steel bar to travel 2-3 feet and make that end too hot to
handle. It always gets there, though.
When you stick a bar in water, it lets loose a great burst of
steam. The steam is hot enough to burn you quite soundly.
Think about this:
If your threshold for heat pain is 160F, and the bar is at 130F,
how much additional heat does some 500F steam need to your hand
add to make your hand uncomfortably hot?
I never harden steel in water or oil with a bare hand. Always pliers.
Steam does a much better job of burning you than does steel.
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