Welding Digest

The process offers consistent deposit thickness, controlled dilution, and other beneficial functions.

Plasma transferred arc welding (PTAW) hardfacing of components is a vital and complex science. Numerous military, industrial, and commercial products (e.g., pumps, augers, valves, etc.) depend on the hardfacing process to provide wear-resistant surface components. Hardfacing establishes an intimate relationship between respective welding processes, component design, and the hardfacing alloy/base metal metallurgy.

The Benefits of PTAW

AWS A3.0M/A3.0:2020, Standard Welding Terms and Definitions, defines hardfacing as “a surfacing variation used to improve wear resistance.” PTAW process technology offers distinct operational attributes as opposed to a wire/rod-fed welding process and is utilized in crucial hardfacing applications, such as high-performance nuclear components for commercial reactor power plants. Based on equipment capabilities, process reliability, and alloy powder availability, the PTAW process is utilized due to the following reasons:

1. PTAW hardfacing systems are adaptable to automation/mechanization via programmable logic controller (PLC)-based controls where process performance and repeatability are characterized. PTAW systems are employed where precise process parameter control, hardface deposit size, geometry, and placement are required.

2. PTAW systems can obtain the desired hardface deposit microstructure. Metallurgically, the PTAW process allows for the usage of alloy powders/feedstocks involving combinations of diverse materials that produce deposits exhibiting a wide range of hardness, mechanical, and chemical properties. Alloy materials include cobalt, tungsten, chromium, nickel, boron, iron, carbon, molybdenum, silicon, manganese, vanadium, and niobium. Powders are gas atomized to control particle size and the spherical shape; for example, sieve mesh sizes 80–325 and particle sizes 44–212 μm.

3. PTAW systems provide for high deposition rates through the controlled feeding of powder in conjunction with lower heat input via the precise control of hardfacing variables, such as the plasma gas flow rate, powder feed rate, amperage, voltage (arc length), travel speed, etc. With increased heat input control, there is increased regulation of the base metal’s heat-affected zone and the amount of dilution.

4. PTAW systems have a precision powder feeding mechanism that provides for the injection of a specific amount of powder; for instance, 40 g/minute directly into the plasma gas column via the argon powder carrier gas. With a controlled powder feed rate, the hardface deposit thickness is consistent. Managing the thickness of a hardfacing deposit is essential. Cracking propensities increase as the deposit hardness and thickness increase. Furthermore, with increased control of the deposit thickness, production costs are better regulated with respect to alloy powder usage (i.e., less wastage) and due to less posthardfacing deposit machining.

5. During hardfacing, the PTAW weld pool is clear and reflective, and its visibility aids the operator in observing the weld pool. Without fluxes, slag inclusions are eliminated. Also, the weld pool is dynamic where pool movement facilitates deposit soundness by allowing gases to flow to the surface and escape more readily prior to weld pool solidification. Based upon accurate programming of process variables, dilution is better controlled. The PTAW process produces quality hardfacing on a consistent basis to meet international commercial nuclear welding and nondestructive examination (NDE) acceptance criteria entailed in the ASME Boiler and Pressure Vessel Code; AFCEN RCC-M, Design and Construction Rules for Mechanical Components of PWR Nuclear Islands; ISO 3834, Quality requirements for fusion welding of metallic materials; ISO NDE acceptance standards; Japan Society of Mechanical Engineers codes; etc.

The PTAW process provides for the precise control of hardfacing variables. A hardfacing execution plan outlines production requirements and process practices to facilitate the quality hardfacing of components to meet stringent nuclear welding and NDE acceptance criteria.

This article was written by William C. LaPlante (welding engineer, AWS CWI, SCWI, and CWE) for the American Welding Society.