psi yield) and Welmonil (71,500 psi yield) 

 were used in the cylinder and hemispherical 

 endcaps, respectively. These possess proper- 

 ties quite similar to HY steels. The cylindri- 

 cal portion of the hull was fashioned from six 

 sections of rolled steel cold-formed to cylin- 

 drical shape in a plane-rolling machine and 

 the longitudinal joint hand-welded. Sixteen 

 stiffener rings were fabricated of Aldur steel 

 and, when finished, placed on a special jig 

 where the cylinders were heated to 200°C 

 and lowered over the ring stiffeners. The 

 rings were welded to the cylinders at an 

 ambient temperature of 150°C. The endcaps 

 were formed of seven orange-peel sections 

 cut out of plate steel and forged to the shape 

 of a hemisphere at 900°C. The seven cylindri- 

 cal sections and endcaps were tack welded 

 together and then the main circumferential 

 welds made automatically outside and by 



hand inside. Brackets and clips were welded 

 at various locations outside the hull for later 

 attachment of ballast tanks, motors and the 

 like. Extra attachment points were included 

 to provide for future growth or modifications 

 of the vehicle. Welding such brackets or clips 

 to the hull after it has been completed should 

 be avoided as it introduces high local resid- 

 ual stresses which, in general, are impracti- 

 cal to stress relieve by heat treatment after 

 the hull has been finished and outfitted. 



When all welds were completed and 

 checked, the two sections were stress re- 

 lieved, or heated, to remove residual stresses 

 in both the parent and weld materials. This 

 procedure consisted of heating the hull to 

 525°C and holding it there for 3.5 hours (2 

 minutes for each mm of thickness) and then 

 slowly cooling it in still air. 



As described, BEN FRANKLIN's endcaps 



Fig 5 5 BEN FRANKLIN'S pressure hull (Grumman Aerospace) 



251 



