Stress relieved welds in positive expulsion fuel tanks with elastomeric diaphragm

A method of forming a metallic positive expulsion fuel tank, the method comprising:

• forming a first hemispherical shell with a first edge;
• attaching a pressurized gas inlet fixture to the first hemispherical shell;
• forming a metallic cylinder with first and second edges;
• welding the metallic cylinder to the first hemispherical shell along the first edge of the first hemispherical shell and the first edge of the metallic cylinder to form a first weld seam;
• forming a second hemispherical shell with a first edge;
• attaching a fuel outlet fixture to the second hemispherical shell;
• forming an elastomeric diaphragm;
• attaching the elastomeric diaphragm to the fuel outlet fixture;
• welding the second hemispherical shell, elastomeric diaphragm and fuel outlet fixture to the cylinder along the first edge of the second hemispherical shell and the second edge of the metallic cylinder to form a second weld seam and a container with two interior chambers separated by the elastomeric diaphragm;
• performing a localized post-weld stress relief heat treatment on the first and second weld seams in which heating of the tank is confined to a distance of 2 inches (5.08 cm) from the first weld seam and a distance of 2 inches (5.08 cm) from the second weld seam such that stresses in the first and second weld seams are relieved and the elastomeric diaphragm is unaffected by the heat treatment.