[taz]

A blanket statement for maximum interpass temperature is not a big mistake.
It depends mostly on material and for carbon steels is 250C (482F). See EN 1011-2 - Welding —Recommendations for welding of metallic materials — Part 2: Arc welding of ferritic steels.
Obviously if you have a wps (supported by a pqr) to follow you should do so. However many people here do not and it is nice to have a base value to stick to.
So for you guys out there just messing around who just need a little guidance doing non code work stick to this:

Carbon steel =250C
Austenitic stainless steel = 150C
9Cr1Mo etc = you should not be asking on a forum

Minimum interpass temperature on the other hand is a lot more complicated and there are even occasions where minimum interpass temperature can be lower than the preheat temperature (eg on filler passes where heat input is considerably higher to a root pass).

That said part size, thickness etc also play a role in selecting a maximum interpass temperature since obviously if you are welding a small piece and start welding after it has cooled to 250C by the time you have progressed a little the rest of the part could be red hot.

[Otto Nobedder]

Taz,

Spot on, in my opinion.

I can't speak to carbon steel, but I can confirm that two WPSs I have for ASME code work call for a maximum interpass temperature of 350*F for 304SS in both GTAW and Spray-transfer GMAW. This is close to the 150*C figure you gave.

And, I also agree, NO ONE should be doing 9-chrome without a WPS. That stuff is temerature-critical.

For my particular code work on stainless, minimum preheat is 50*F for any pass, so it's pretty simple there. There's only three months out of the year where the ambient temperature may be below 50*F.

Steve S