material behavior

This is covered by the subject of metallurgy, on which are based many textbooks.

Are we missing the basics?

a) Why do we pre heat?

The pre-heating is carried out to reduce the stresses during welding due to solidification of the molten metal poodle. We do not want the rest of the metal work like a heat sink and quickly draw the heat off, creating not only thermal stresses but also microstructural problems.

b) What is the temperature of pre heat? - If you check any of the welding procedures, you will find the pre-heat temperature is much much below the critical point (ie at which the phase change will start). Hence the

grain size at the very first time of pre heating changed from fine to coarse

is unfortunately misinformed. In fact if this happens, I don't see why we usually buy all normalised plates for our equipments (since after welding anyway it will be annealed ?)

c) Everytime you do the welding (including interpass in case the temperatures are not maintaines eg you do the root layer today, and the others later, which is usually done in our vessels invariably - to account for the back gougilng and LPI), again pre-heating is necessary. And that goes for the repair welds too.

d) The requirements of pre and post heat depends on the thick-ness, material and the codes.

e) The pre and post heats do have a slight bit of loss in mechanical properties. But they are not significant enough to be that of concern. In fact I have gone through a few papers, as per that the major loo (a few % only) is in the first post heat (no literatures on pre-heat since these are only a order of a few hundred of ^oC unlike post heat of about 600^oC) and in the subsequent post heats it rapidly tapers off to insignificance.

Of course the ASME code limits total duration of post heat duration.

I think that clarifies most of the points?