The preheat must extend through the thickness. The preheat is measured a distance equal to the thickness of the part, but no less than 3 inches from the weld. If the part is 6 inches thick, the preheat must be measured no closer than 6 inches from the weld and must also be measured on the opposite side to very it is through thickness.

While your at it, the minimum interpass temperature is considered to be no less than the minimum preheat temperature.

Best regards - Al

I second Al's post.

Remember as well that these are 'MINIMUM' pre-heat temps.  While there are maximums involved with some codes that is usually not an issue. 

There were some good lectures and discussions at FABTECH about cracking, pre-heat, hydrogen, etc.  One of the basic points I took away from three of these that approached the subject from slightly different directions was that it is better to go even higher in the pre-heat, keep low hydrogen electrodes TRULY heated and dry, and especially note restrictions for wires.  People are ignoring how long they have flux cored wires exposed to atmosphere and are buying these 1000 lb spools that run for weeks.  Check out the manufacture's suggested storage as well as exposure and look at some of the seismic codes to see that this should quite possibly be more of a consideration than most shops, inspectors, and welders give to the issue.  I'm not telling anyone to force a fabricator to go beyond what the code states.  But apply it where and when required and if you are the fabricator, see how you can make this work without greatly increasing your costs.  I think we will find this becoming more researched and applied in the near future. 

Have a Great Day,  Brent