Abstract: An insulated well casing is described that includes an inner tubular member and an outer tubular casing member joined together at their ends by a weldment, typically formed into a thrust cone suitable for joining strings of insulated casings to insulate and encase the entire depth of a well for delivery of steam, for example, into a producing formation. A key element of the invention is pre-stressing the integrated casing by tensioning the inner tubular member at a level less than the yield strength of the tubular material prior to joining the weldments of the unit together. The resulting integrated casing includes a tensioned inner tubular member and an outer member that is in compression. The tensioning load is selected such that at well operating temperatures and pressures, the compressive stresses exerted upon the casing at the bottom of the hole approach the compressive yield strengt of the inner tubular member, but leave a desired margin of safety.