Bevelling of pipes or plates is usually done to prepare the ends for welding.
There are situations when steel plates are so thick that it would be impossible to obtain a good weld without weld preparation, i.e. without bevelling. Normally, plates thicker than 4 mm need to be bevelled before welding.
What is bevelling, actually? A bevel is the inclined shape that needs to be formed on one or both sides of the plate. If you consider a section of a plate, then the shape BEFORE bevelling and shape AFTER bevelling look as follows:
There are various types of bevelling tools, for example Shearing tool or Milling head with inserts.
Similarly, with pipes whose walls are of a certain thickness, say more than 3 mm, bevelling machines are needed for weld preparation. Therefore, on pipes with walls thicker than 3 mm, bevel is made to make the outer edge inclined so that two pipes can be joined together by welding. A weld made on bevelled pipe ends is made with much more precision and the joint is very strong and resistant to pressure from the inside of the pipeline. As a matter of fact, x-ray tests are carried on after the pipe joint is welded to check if the weld is perfect. This is logical when you consider that these joints are used in power stations where enormous pressures exist inside pipelines, in chemical factories where pipelines contain sensitive liquids, or in refineries and pipe ducts for gas and oil which are highly flammable.