African Fusion March 2015

Today’s technology

Refurbishment using the PTA process

T he plasma transferred arc (PTA) welding and hardfacing process from thermal spray and surface en- gineering coating specialist, Ther- maspray, provides a versatilemethod of depositing high quality, metallurgically fused deposits on relatively low-cost substrates. First introduced in 1964 to better control lower current arc welding, the PTA process is complimentary to both thermal spray and conventional weld- ing and is mainly used on components that are subjected to severe corrosion or abrasion, thermal shock, slurry erosion or extreme impact forces. The process offers the necessary protection to the substratebyproviding a coating that can withstand these conditions. PTA can be applied in practically every case where hardfacing is needed. According to Shaik Hoosain, metal- lurgical engineer at Thermaspray, a wide variety of materials, ranging from soft to medium hardness (e.g., stain- less steel) to very hard, wear resistant materials (e.g., carbide composites), can be deposited using the PTA process to achieve diverse surface properties such asmechanical strength, wear, corrosion and creep resistance. Bead thicknesses range from1.2 to 2.5 mmandmulti-pass

runs depend on the application.

resistant than those producedusing GTAW or oxy-fuel processes. PTA weld deposits have character- istically less oxides, inclusions and discontinuities. Post weld machining is minimised due to the smooth deposit created through the PTA process. Typical PTAapplications successfully un- dertakenby Thermaspray include the re- furbishment of the process rolls used in the steel industry. Journals and pot roll scraper blades for the zinc galvanising process, for example, are coated with a cobalt-based layer for high temperature wear and corrosion resistance. “Scraper rings formanufacturing gearbox compo- nents are also PTA-coated with cobalt- base alloys, as are valve seats used for high temperature steam valve compo- nents. And components used in glass manufacturing are PTA-coated with a nickel-basedalloy toprovide impact and wear resistance at high temperatures,” adds Hoosain. In closing, Hoosain says that be- cause the PTA welding process has not received as much attention in South Africa as conventional arc weld- ing processes such as GTAW, it is an under-utilised 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, ensuring highqualitywelddepositswith excellent reproducibility frombatch to batch. It is ideal for severe impact, wear and cor- rosive environments.” In addition to PTA welding, Ther- maspray also provides a comprehensive range of support coating finishing tech- nologies including machining, grinding and burnishing from its state-of-the art facilities in Olifantsfontein, Johan- nesburg. www.thermaspray.co.za • •

Hoosain explains that the plasma arc welding process uses a plasma to transfer an electric arc to theworkpiece, a process which he says is similar to gas tungsten arcwelding (GTAW). “To create plasma, a gas, typically argon, is heated to a sufficiently high temperature and ionised so that it becomes electrically conductive. The plasma welding torch contains a negatively charged non- consumable tungsten electrode within a positively charged copper nozzle with a small orifice 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 constricting the plasma gas and the arc through the orifice, the torch delivers a high concentration of heat to a small area, which leads to high quality welds.” For hardfacing, an alloy powder froma hopper is introduced, via a carrier gas, into the plasma stream. This forms a fused layer on the surface being coated, with the required alloy properties. The plasma transferred arc process offers a number of important advan- tages over conventional arc welding, including: • It is easily automated and offers a high degree of reproducibility. • Precisemetering ofmetallic powder feedstock allows lessmaterial to be used compared to traditional weld- ing processes. Precise control of the welding pa- rameters also offers a high degree of consistency. Controlled heat input, less than conventional arcweldingprocesses, ensures weld dilution can be con- trolled to between 5.0 and 7.0 %. The deposit of a specific alloy is tougher and more corrosion • • •

Journals and pot roll scraper blades for the zinc galvanising process, for example, are coated with a cobalt-based layer for high temperature wear and corrosion resistance.

Afrox ......................................................................8 Air Liquide...........................................................20 Air Products ........................................................23 BED Holdings: Fronius........................................10 Crown Publications.......................................... IBC ESAB....................................................................22 Lincoln Electric.......................................OFC, OBC

Probraze Metals..................................................15 Renttech .............................................................14 SAIW ......................................................................2 SKS Welding Systems.........................................13 Smarter Welding Africa .....................................IFC voestalpine Böhler Welding SA..........................34 Yaskawa Motoman SA ........................................30

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March 2015

AFRICAN FUSION