What are the difference between Stainless Steel Schedule 80 and Schedule 40 pipes.

The difference between sch. 40 and sch. 80 pipe refers to the wall thickness of the pipe. The outside diameter will be the same. Example- 3/4" sch. 40 will have a wall thickness of 0.113". Sch. 80 will have a wall thickness of 0.154". Again, the outside diameter will be the same for both allowing the pipe to be threaded so any difference in the schedule will still enable the pipes to be joined together. Thicker wall allows for higher internal pressure and a higher mechanical strength.