Plasma Transferred Arc (PTA) welding is a versatile method of depositing high-quality metallurgically fused deposits on relatively low cost surfaces. Soft alloys, medium and high hardness materials, and carbide composites can be deposited on a variety of substrates to achieve diverse properties such as mechanical strength, wear and corrosion resistance, and creep.
PTA weldinging has significant advantages over traditional welding processes such as oxyfuel (OFW) and gas tungsten arc welding (GTAW).
Benefits of PTA:
• PTA is easily automated, providing a high degree of reproducibility.
• PTA allows precise metering of metallic powder feedstocks, resulting in lesser material quantity used compared to other welding processes.
• PTA permits precise control of important weld parameters (i.e. powder feed rates, gas flow rates, amperage, voltage, and heat input), ensuring consistency from lot to lot. Controlled heat input ensures weld dilutions that can be controlled from 5-7%.
• PTA produces alloy deposits that are tougher and more corrosion resistant then counterparts laid down by GTAW or OFW processes. Weld deposits are characterized by very low levels of inclusions, oxides, and discontinuities.
• PTA produces smooth deposits that significantly reduce required post weld machining.
• PTA parameters can be adjusted to provide a variety of deposits in thicknesses from 1.2 to 2.5 mm (0.05 to 0.10 in.) or higher. These can be deposited by a single pass at a rate of 1 kg/h up to 13 kg/h depending upon torch, powder and application.
Oil & Gas
The perpetual demand for oil and gas pushes companies to optimize the extraction process to be as efficient as possible. Various components of drilling hammers used in oil extraction experience abrasion from mud, metal-to-metal wear and erosion. To combat excessive wear, drilling hammers are hardfaced with composite alloys of tungsten carbide, dispersed in a nickel or cobalt-based matrix. Plasma transferred arc (PTA)deposits of Ni-Cr-B-Si alloys are used to combat wear on plungers, sleeves, sucker rods and seals of mud pumps and submersible pumps.
In the refining sector, cobalt-based alloys applied by PTA are used extensively to combat wear, erosion, abrasion and corrosion.
The hardfacing of engine valve seats, which is a high volume process, was originally done using Oxyfuel welding (OFW) and gas tungsten arc welding (GTAW) processes. Since the 1980s, hardfacing of engine valves has moved steadily to PTA due to consistently repeatable quality, productivity and enhanced deposit characteristics.
Engine valve seats experience a variety of wear modes such as erosion, adhesion, galling, corrosion and fatigue. Demands like fuel efficiency, power-to-volume rating increase, and fuel quality impose further strain on the valves. Cobalt-based alloys, such as Stellite® F and Stellite® 6, have proven effective under these circumstances. Now, a host of cobalt-based alloys are used in the automotive industry for wear resistance.
Internal combustion engine valve manufacturers are consumers of cobalt alloys for hardfacing applications. Precise control of the hardfacing alloys that go into each valve is of paramount importance from a cost standpoint. The metering of the alloy must be controlled to a fraction of a gram, and PTA offers the advantage of precise feed stock delivery, consistent hard face quality, and low rejection rates.
In addition to cobalt-based alloys, several nickel-based alloys that depend on borides and carbides for hardness are also used for hardfacing engine valves.
The steam cycle of power generators contains generators, turbines, and pumps that handle steam and water. This requires a variety of control, safety, and shut off systems. The most significant wear occurs in steam and water valves, which experience high-pressure, high temperatures, and metal-to-metal wear at seating areas. For efficiency, valve seats and spindles need to inhibit oxidation on the surface, which otherwise causes sticking.
PTA deposits of cobalt-based alloys are used extensively on valve seats and faces due to high resistance to metal-to-metal adhesive and erosive wear. Cobalt alloys also retain hardness at temperatures ranging from 550°C or higher.
Cobalt alloys are deposited on the leading edges of turbine blades to combat the effects of liquid droplet erosion due to condensing steam. Cobalt-based alloys perform well on high-pressure steam turbine nozzles and induced draft fan blades.