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I see. Here's another one, which one is correct? If all cooling process is done by air (room temperature), will higher preheating temperature cause the cooling rate to increase or not?

Why is preheating no recommended? I thought preheating should be done to prevent hot cracking on HAZ.

I've done a similar test before. I used Vickers Microhardness test and works out well. Use 500g load. Test was done with a Zwick/Roell Indentec ZHV. Link

I've welded an IN-792 with an IN-625. I tried 3 type of preheating before welding. 1st preheated to 200 celcius, 2nd preheated to 400 celcius and 3rd preheated to 400 celcius. After done welding it, I did a vickers microhardness test on the fusion line/HAZ. The results was a decrease of hardness...

I know it's to complicated for this forum. Yet, all these answers gives great insight. Anyways, does grain size really affect the strength?

My bad. The quotation I meant was this, " The coarse grain size of PC (polycristalline) castings, as compared to finer-grained forgings, favors strength at high temperatures". So, I guess it's not because of the single crystals.

What is the difference between Cast and Wrought on a Nickel-base Superalloy? I read on "Superalloy: The Technical Guide" by Michael J. Donachie stating that "the highest creep-rupture strength at elevated temperatures can be achieved in nickel-base superalloy castings for high-stress...