yoy must get the certifing authority to aprove your procedure. generaly speaking there should be a preheat of 100F and do not exceed 300F in welding should not require a stress relieveing cycle. Ie dime sized spot welds with cooling through completion.
not knowing the particulars of this application I can only sugest that you consider nylon screws. I worked on a probe housing that was continously presurized and normalized.The steel screws would eventualy eat into the plastic cover, However, the nylon would streach and contract with the...
since you need only half a ball consider welding/ brazing or soldering a steel ball in place. hardened steel balls for use in bearings can be obtained in bulk for as low as .20 $ ea.