Leif,

To better understand about metal stress and Pressure vessel, you might wnat to look through a welding book that has to do with ASME Codes Section VIII (8). This my answer some of your Questions. The book I have is called "Pressure Vessels "by Robert Chuse and Stephen M. Eber. Simplified version of the Code written in 1980's. You may be able to find it at a Library or You may want to go to the American Welding Society website to ask questions
www.aws.org