Lesson 8 - Hardsurfacing Electrodes

torch has a non-consumable tungsten electrode, the end of which is behind a small con- stricting orifice. The electrode is surrounded by an inert gas such as argon. When the arc is established, either between the electrode and the constricting nozzle (non-transferred arc) or between the electrode and the work (transferred arc), the gas becomes ionized in the arc, forms a plasma, which is forced through the orifice by the plasma gas and im- pinges on the work piece. In plasma arc spraying, the non-transferred arc torch is used and the metal powder is introduced into the plasma gas. It is projected at high velocity against the object being surfaced. Because the metal particles are fully molten and travel at high velocity, the mechanical bond at the surface is very good and does not require subsequent fusing in most cases. 8.4.3.1 In plasma arc welding, the transferred arc method is used, which is a higher energy process. The base metal is actually melted, resulting in a fully fused surface. Both plasma arc methods lend themselves to high production, automatic surfacing applications requiring a thin overlay. 8.5 GENERAL RULES FOR HARDFACING Some general rules and precautions which will help to assure sound hardfacing deposits are listed below: a. Base Metal Identification - The base metal must be properly identified so that the proper buildup and/or hardfacing alloy can be selected. Also, base metal type will help determine the proper preheat and interpass temperature. A magnet will help to identify austenitic manganese steel since it is non-magnetic. The magnet should be tried at several locations on the part because work hardened areas will be slightly magnetic. b. Base Metal Preparation - The base metal must be cleaned with a grinding wheel and be free of rust, oil, grease, or other foreign matter. Cracks, tears, or gouges must be repaired using the proper filler metal or buildup alloy. c. Metal Removal - Rolled over and fatigued metal must removed. Work hardened surfaces of austenitic manganese steel should be ground away before buildup or surfacing. d. Buildup - Buildup of badly worn parts to within approximately ¼” of their final size with an appropriate buildup alloy prior to hardfacing is necessary.

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