Large nuclear reactor vessels generally have a number of large nozzles attached to them, through which the fluid can enter and leave. These nozzles are welded into the reactor vessel. A typical 10 to 20 foot diameter vessel may have four equally spaced, 3 foot diameter nozzles welded therein. The welding of these nozzles into the vessel is a time consuming process, generally taking from four to 6 weeks. In order to prevent undue thermal stresses in the weld metal which could cause later cracks or failure of the welds, it is essential that all of the welds be maintained at a predetermined temperature during the 4 to 6 week period until all of the welding has been completed, and the finished vessel can be properly heat treated in a large heat treating furnace. During this 4 to 6 week welding period it is desirable to maintain the nozzle welds within the range of 350.degree. - 500.degree.F with a 50.degree. maximum differential between hottest and coldest point. This has presented problems in the past because of the largeness of the welds, and because the vessel is rotated to different positions during the welding of the nozzles.