Process Description
Hardfacing can be applied by several different welding processes using Sulzer Metco Woka PTA and welding materials. The simplest is using an oxy-acetylene welding process, for which the weld torch is used to make a small molten pool on the substrate. At the same time, the hardface material, usually in the form of rod or wire, is also melted.

Oxy-acetylene weld torch

MIG (Metal Inert Gas) welding is also a common process for the application of hardface materials. A typical MIG welding gun has a contact tip, through which the electrode wire is continually fed. The tip both transmits an electrical charge to the electrode and directs it to the substrate to be coated with the hardface material. A shroud of shield gas is maintained during the process.

The PTA (Plasma Transferred Arc) process welds a metallic coating material in powder form to a substrate to produce a hard, wear-resistant coating that is metallurgically bonded to the substrate. Depending on the material used, the coatings may also be highly corrosion resistant, as they are fully dense.

A pilot arc is generated between an electrode and an anode (nozzle). The process gas, generally argon, then becomes ionized to the plasma state, which is electrically conductive. This then generates a transferred arc between the electrode and the workpiece, creating a melt pool on the workpiece surface. The coating material is injected into the stream of plasma gas, depositing it onto the workpiece.

The PTA welding process

One of the most important features of the PTA process is the control of dilution with the substrate. PTA produces dilutions as low as 5%, compared to 20 to 25% typically obtained when hardfacing by MGAW (MIG) and GTAW (TIG) processes. So it is possible to maintain the noble properties of deposit even in one single pass.

Features of PTA and Weld Hardfacing
• Produces hard, highly wear resistant coatings
• Metallurgical bond with the substrate material
• PTA process offers low degrees of dilution
• Coatings are fully dense and can be also very corrosion resistant
• Can be automated using MIG or PTA welding processes
• Very thick build up is possible
• Imperfections in the surface can be filled
• Coated surfaces can be ground to dimension
• Surfaces can be finished and superfinished
• Coatings are quickly applied