Heat input and cooling rate

[nabeel3]

Dear All,

I am a student of Welding. I couldn’t understand the reason for the below statement. “For high heat input the cooling rate is low and for low heat input the cooling rate is high.” As far as I know the rate of cooling depends on the temperature difference. High heat input should result in high temperature than low heat input and the temp differential should be high in high heat input. Hence the cooling rate should be high in high heat input case.

 

[EdStainless]

One way to picture this to think of TIG torch running a bead on plate, not filler and not even aiming for full penetration.
If I run low heat input I will melt very little metal, and width of the heated zone is narrow and the base of the plate is a large heat sink and it cools quickly.
Now if I run high heat input the width of metal that gets hot is much wider, and the cooling will be slower because in order to heat to leave the weld itself it must be transferred through a wider hot zone on either side.
If you could make a high heat input weld that was physically very small (laser and E-beam do this) then you would get very high cooling rates.
The physical size of the weld is important as is the heat transfer of the metal.

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P.E. Metallurgy

 

[nabeel3]

Thank you Ed stainless for the reply.