Company Announcement - Plasma Transferred Arc welding and hardfacing process from thermal spray and surface engineering coating specialist, Thermaspray (Pty) Ltd , provides a versatile method of depositing high quality, metallurgically fused deposits on relatively low cost substrates.

First introduced to the welding industry in 1964 as a method to bring better control to the arc welding processes in lower current ranges, the PTA process is complimentary to both thermal spray and conventional welding and is mainly used on components that are subjected to severe corrosion or abrasion, thermal shock, slurry erosion or extreme impact forces to give the necessary protection to the substrate by providing a coating that can withstand these conditions. PTA can be applied in practically every case where hardfacing is needed.

According to Shaik Hoosain, Metallurgical Engineer at Thermaspray, a wide variety of materials, ranging from soft / medium hardness (e.g. stainless steel) to very hard, wear resistant materials (e.g. carbide composites), can be deposited using the PTA process to achieve diverse surface properties such as mechanical strength, wear, corrosion and creep resistance. Bead thicknesses range from 1.2 to 2.5mm and multi-pass runs depend on the application.

Hoosain explains that the plasma arc welding process uses a plasma to transfer an electric arc to the workpiece, a process which he says is similar to Gas Tungsten Arc Welding.

Plasma Transferred Arc welding process

“To create a plasma, a gas, typically argon, is heated to a sufficiently high temperature and ionized so that it becomes electrically conductive. “ “The plasma welding torch contains a non-consumable tungsten electrode (-) located within a copper nozzle (+) with a small opening at the tip,” continues Hoosain.” A pilot arc is struck between the torch electrode and the nozzle tip and the arc is transferred to the metal to be welded. By forcing the plasma gas and the arc through the constricted orifice, the torch delivers a high concentration of heat to a small area which leads to high quality welds. The deposit alloy powder is conveyed from a hopper to the torch by a carrier gas.”

The plasma transferred arc process offers a number of important advantages over conventional arc welding processes including:
• Easily automated - high degree of reproducibility.
• Precise metering of metallic powder feedstock - less material is used compared other   traditional welding processes.
• Precise control of important weld parameters - high degree of consistency from batch to batch.
• Controlled heat input, less than other conventional arc welding processes - ensures weld dilution can be controlled between 5-7%.
• Produces a deposit of a specific alloy that is tougher and more corrosion resistant than counterparts produced through GTAW and OFW.
• PTA weld deposits have characteristically less oxides, inclusions and discontinuities.
• Post weld machining is minimised due to the smooth deposit created through the PTA process.

Typical applications of the PTA process offered at Thermaspray include Pot roll (Journals with cobalt-base process roll.

High temperature wear and corrosion resistance); Pot roll scraper blades (wear area of scraper blade PTA welded with cobalt-base alloy for high temperature wear resistance); Scraper rings (gearbox components with cobalt-base alloys. Wear and abrasion resistance and high temperature steam valve components (valve seats with cobalt base alloy). “Components used in glass manufacturing are PTA welded with a Ni-based alloy to provide impact and wear resistance at high temperatures,” adds Hoosain.

In closing, Hoosain says that because the PTA welding process has not received as much attention in South Africa as conventional arc welding processes like Gas Tungsten Arc Welding, it is unfortunately an underutilised welding process in the local market. “PTA is a versatile, high temperature, wear and impact resistant welding process that is ideally suited to a wide range of applications, guaranteeing a high quality weld deposit with excellent reproducibility from batch to batch for severe impact, wear and corrosive environments.”

In addition to PTA welding, Thermaspray also provides a comprehensive range of support coating finishing technologies including machining, grinding and burnishing from its state-of-the art facilities in Olifantsfontein, Johannesburg