Company Announcement - Thermaspray, South Africa’s market leader in surface engineering and thermal spray coating technology, has optimised the parameters of its Plasma Transferred Arc process to ensure high quality, crack-free stellite hardfacing deposits on a wide range of substrates. Weld deposits of hardfacing alloys are commonly employed to increase the service life of components that are subject to abrasive wear and corrosion. Properties in the deposits vary and generally greater life is achieved with deposits of higher hardness which is obtained by the presence of hard carbides in the matrix. Because this cracking does not significantly reduce the service life of the component it is sometimes seen as advantageous in reducing residual stresses in the material.

However, Shaik Hoosain, Metallurgical Engineer at Thermaspray, points out that in many instances, cracking, whether to obtain a sealing surface or to prevent fatigue failure, is undesirable. He explains: “Cracking in stellite hardfacing alloys are essentially related to the very high strength and low tensile ductility of the weld deposit and its sensitivity to dilution. To avoid cracking in these hardfacings, it is essential to control or adjust parameters. Subsequently we have developed welding parameters in our PTA process which are strictly controlled to ensure high hardness stellite deposits that are free from cracks and flaws.”

Outlining the PTA process, Hoosain explains that this hardfacing procedure heats metals and merges them by means of arc outlines constriction. “It is a versatile method of depositing high quality, metallurgically fused deposits on relatively low cost substrates.”

The PTA process was first introduced to the welding industry in 1964 as a method to bring better control to the arc welding processes in lower current ranges and it is complimentary to both thermal spray and conventional welding. PTA 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. Hoosain adds that PTA can be applied in practically every case where hardfacing is needed.

Cracking in the subsequent deposits result from unequal cooling rates within the deposit and the expansion mismatch between the substrate and the weld. Thermaspray has addressed this through the dilution of the stellite by a steel substrate which involves the reduction of compositional mismatch, making a more ductile weld deposit by decreasing the carbide content. Furthermore, increased sensitivity to cracking as more deposits are made as a result of lower dilution and higher deposit hardness,  can be reduced by the application of a correct preheat and current level.

The cracking risk is also influenced by preheat levels and ensuing cooling rates. “Here it is  most critical to carefully control the heat input which makes it possible to control weld dilution to less than 5%, which is crucial for many high-performance alloys,” states Hoosain.

A number of important advantages of the PTA process compared to conventional arc welding processes:
• Precise control of important weld parameters - high degree of consistency.
• Controlled heat input, less than other conventional arc welding processes - ensures weld dilution can be controlled between 5-7%.
• Weld deposits are characterised by low levels of inclusions, oxides and discontinuities
• The weld hard-facing closely mimics the corrosion resistance of the equivalent alloy.
    
Thermaspray, ISO 9001 accredited and an Eskom level 1 approved supplier of coatings and PTA welding, has conducted several welding qualification procedures on various material substrates. 

Thermaspray, in a joint venture with Surcotec, offers an extensive portfolio of engineering and thermal spray coating solutions that extend component life cycles to assist OEM and end-user clients across southern Africa in reducing costs and increasing production. Based in Gauteng and the Western Cape respectively, Thermaspray and Surcotec’s world-class quality wear- and corrosion-resistant thermal spray coatings, Plasma Transferred Arc (PTA) cladding and Polymer coatings (in partnership with Plasma Coatings USA and Diamant Metalplastic Germany) are augmented by a host of specialised allied services. These include coating finishing technologies such as machining, grinding, diamond grinding, probe track burnishing, electrical run out measurements/reporting, linishing, and super finishing.