Upsetting Pressure

The upsetting pressure presses the heated and plastic ends of the workpieces together. Impurities and oxides are squeezed out of the joint zone, creating a clean, tight joint. Too little pressure leaves pores and slag in the weld – the joint won't be tight. Too much pressure squeezes out too much material and makes the structure at the weld hard and brittle. You have to choose the pressure so that the ends upset cleanly, but the material isn't overstressed. Run the machine up slowly and watch the upsetting flash – it tells you if the pressure is right.