Welding Tips and Tricks

To all the engineers in here,

I have always operated under print specs, which often come stapled with a WPS that includes preheat temperatures, etc.. But I have been curious to know, what are the "rules" (loosely worded) to determine what needs preheat and to what temperature. Of course, there are some obvious scenarios when preheat is required, such as welding in winter wonderland with frosty the snowman looking over your shoulder, welding thick plate with large surface areas, situations where lots of heat on one end and no heat on the other end causing warp, etc..

What input can you give as a "rule" to consider when fabricating to determine preheat and to the correct temperatures?

I'm no engineer but 1 scenario I've learned watching Jody's videos is if the welder your using is maxed out on a work piece. Some preheat can be the difference between a good and bad weld. Other than that you'll have to wait for someone with much more experience than me.

Preheat is often determined by the "thermal transfer" properties of the metal to be welded. The best example is heavy aluminum, where your initial weld heat is carried away very rapidly causing a "cold start" in the absence of preheat.

The most extreme example that comes to mind is P91 chrome. This is common in co-generation powerplants, as one example. It's often large piping, to the 4' range plus, and requires a preheat to 375F, and maintained interpass temperatures. To do this in July in Texas is a special kind of Hell, and I respect the masochists willing to do it.

Steve S