A method for welding high-pressure thick-walled pipelines of domestic A335 P91 steel: Technical Field – The present invention pertains to the field of welding technology, particularly relating to a method for welding high-pressure thick-walled pipelines of domestic A335 P91 steel. We have Largest inventory of ASTM A335 P91 Alloy Steel Seamless Pipe Alloy Steel Pipe A 335 P91: Size : 1/2″ TO 24 “IN OD & NB Schedule: SCH20, SCH30, SCH40.