Preheating

I am currently a student and I was asked this question but don't quite understand and I was wondering if anyone can explain it to me some. Any
help will be very helpful. The questions is:
Why do you pre-heat larger pieces of steel at a higher temperature than that of smaller?
The only conclusion that I have come up with, which I am not sure if it correct, which is to help to prevent martensite getting into the steel.
Any direction to help me figure this out will be great.
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Are you talking about preheating during welding or preheating prior to heat treatment?

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I am looking at preheating before welding.
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Depending upon the alloy, one reason is this:
If the part is thin, it will most likely cool rapidly after welding and preheating (or increasing the preheat temperature) will not be effective. A post-weld heat treatment may be necessary. A thick part will retain the heat longer and preheating will help prevent martensite formation in the heat affected zone.

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Here is a start
http://www.twi.co.uk/j32k/protected/band_3/jk46.html
There are several reasons.
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Chopster wrote:

Smaller cools off much quicker. Bigger temperature gradients.

Yep. You don't want the steel to crack.

Med venlig hilsen / Best regards Martin Jrgensen
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