I had very limited experience with an exothermic device that was used to PWHT welds on well casing attachments to casingheads. That was over 10 years ago. Destructive metallurgical evaluation of samples of the treated welds showed acceptable hardness had been obtained. Sorry, but I don't recall details of casing size and thickness, but the casinghead is typically fairly thick wall and the casing and casing head were both fairly high carbon equivalent, so effective preheat and PWHT was important. I believe that the company that made the devices at that time was "RedHead" or RedHed. Not sure if they are still around since I no longer get involved in those types of welds.
Exothermic PWHT enjoyed a certain popularity back in the '60ties and '70ties especially in the petroleum and petrochemical industry, because it was a fast method to carry out the treatment that required no electric power supply and no external temperature control, and because the ANSI (now ASME) B31.3 Code allowed its use.
One manufacturer was:
Exomet Inc.
P.O. Box 647
Conneaut, OH 44030
Phone (216) 593-1161
For a number of reasons, however, it lost its popularity and I doubt that it is still used.
The mortal enemies of exothermic PWHT were the manufacturers of PWHT electric equipment, Cooperheat among them. I've got an 8 pages publication titled "Cooperheat has a look at an exothermic kit, tries one and makes a mess of it". If you let me know your postal address, I can send you a xerox copy of it.
Giovanni S. Crisi
São Paulo - Brazil