A method of heat treatment that requires the component to be thermally insulated completely on its external surface. Gas fired portable high velocity burners are inserted into manways at strategic locations and fired directly into the components atmosphere. This virtually converts the component into its own furnace. By maintaining a constant positive pressure from within the vessel and effecting an intensive scrubbing action of the hot gases caused by the high velocity of the gas burners, temperature distribution and uniformity is maintained at all times.