Microstructure Evolution of Plasma Arc Welding Overlay Layers Under Different Welding Processes

Literature Overview and Research Background

Plasma arc welding (PAW) is a versatile cladding process that offers high energy density, deep penetration, and excellent control over the weld geometry. The literature reviewed here investigates the microstructure evolution of overlay layers produced by different variants of plasma arc welding, including transferred arc plasma welding, non-transferred arc plasma welding, and pulsed plasma arc welding. The research is conducted on a stainless steel overlay material deposited on a carbon steel substrate, with the objective of understanding how the welding process variant influences the microstructure, mechanical properties, and corrosion resistance of the overlay layer. This study is of significant practical importance, as the selection of the appropriate plasma arc welding variant can dramatically affect the performance of the cladding in service.

Core Technical Findings

The study compares three plasma arc welding variants for the deposition of a 316L stainless steel overlay layer on a Q345 carbon steel substrate. The key findings are as follows:

• Transferred arc plasma welding (TAPW) produced the deepest penetration into the substrate, with a dilution ratio of 22 to 28 percent, resulting in a mixed microstructure of ferrite and austenite in the overlay layer.
• Non-transferred arc plasma welding (NTPW) produced a shallower penetration with a dilution ratio of 12 to 18 percent, resulting in a predominantly austenitic microstructure with minor ferrite.
• Pulsed plasma arc welding (PPAW) produced the finest microstructure with a dilution ratio of 15 to 20 percent, resulting in a fine-grained austenitic microstructure with reduced intergranular corrosion susceptibility.
• The hardness of the overlay layer varied from 210 HV for TAPW to 280 HV for PPAW, with the hardness increasing as the grain size decreased.
• The intergranular corrosion resistance, evaluated by the 6% oxalic acid test in accordance with ASTM A263, was excellent for PPAW, fair for NTPW, and poor for TAPW.

Interpretation of Technical Points

The microstructure evolution in plasma arc welding overlay layers is primarily governed by the solidification conditions, which are determined by the heat input, cooling rate, and thermal gradient at the solid-liquid interface. In transferred arc plasma welding, the high current density and deep penetration result in a high thermal gradient and a relatively low cooling rate at the overlay-substrate interface, promoting the formation of coarse columnar grains and a higher dilution ratio. The high dilution ratio introduces carbon and manganese from the carbon steel substrate into the overlay layer, promoting the formation of ferrite phases and increasing the susceptibility to intergranular corrosion.

In non-transferred arc plasma welding, the arc is established between the electrode and the workpiece, resulting in a lower heat input and shallower penetration. The lower dilution ratio preserves the austenitic composition of the overlay material, but the microstructure is still relatively coarse due to the moderate cooling rate. Pulsed plasma arc welding, with its periodic variation of the current, produces a unique solidification pattern characterized by a high peak current for penetration and a low background current for cooling. This pulsed thermal cycle promotes the nucleation of new grains at each pulse cycle, resulting in a fine-grained microstructure with excellent mechanical properties and corrosion resistance.

Process and Standards Analysis

The qualification of plasma arc welding procedures for overlay applications must comply with the requirements of NB/T 47014 and ASME IX Section 8. The essential variables for plasma arc welding include the plasma gas flow rate, electrode material, nozzle diameter, and the type of plasma arc (transferred, non-transferred, or pulsed). The process parameter ranges for the three variants are as follows:

The weld procedure qualification must include macrograph examination to assess the penetration depth and dilution ratio, metallographic examination to characterize the microstructure, hardness testing to verify the mechanical properties, and intergranular corrosion testing to assess the corrosion resistance. The acceptance criteria for the overlay layer should include a maximum dilution ratio of 20 percent, a minimum bond strength of 150 MPa, and a pass in the intergranular corrosion test.

Integration with Engineering Practice

The selection of the appropriate plasma arc welding variant for overlay applications depends on the specific performance requirements of the component. For applications where corrosion resistance is the primary concern, such as the cladding of chemical processing equipment, pulsed plasma arc welding is the preferred method due to its fine-grained microstructure and excellent intergranular corrosion resistance. For applications where penetration depth is critical, such as the repair of heavily corroded components, transferred arc plasma welding may be more appropriate despite its inferior corrosion performance.

A practical case involves the cladding of a 304 stainless steel heat exchanger tube sheet with a 316L overlay layer using pulsed plasma arc welding. The overlay layer was deposited in three passes with a total thickness of 4.5 mm. The dilution ratio was measured at 16 percent, and the overlay hardness was 275 HV. The intergranular corrosion test in accordance with ASTM A263 showed no intergranular attack, and the bond strength was measured at 220 MPa. The heat exchanger was subsequently put into service in a chloride-containing environment at 80 degrees Celsius, where it has operated for over 3 years without any signs of corrosion degradation.

Key Questions and Reflections

The study raises several important questions that warrant further investigation. First, the effect of the pulse frequency and duty cycle on the microstructure and properties of the overlay layer is not systematically optimized; different pulse parameters may produce different grain sizes and phase distributions, and the optimal parameters may vary with the overlay material composition and substrate type. Second, the long-term performance of the overlay layer under combined mechanical and corrosive loading is not well characterized; the fatigue-corrosion interaction at the overlay-substrate interface may lead to premature failure under cyclic loading. Third, the scalability of pulsed plasma arc welding for large-area overlay applications is limited by the relatively low deposition rate, and the economic feasibility of this process for large components must be evaluated.

Study Insights and Implications

The most significant insight from this study is that the microstructure and properties of plasma arc welding overlay layers are profoundly influenced by the welding process variant, and that the selection of the appropriate variant is critical for achieving the desired performance in service. Pulsed plasma arc welding offers the best combination of microstructural quality, mechanical properties, and corrosion resistance, making it the preferred process for critical overlay applications. However, the relatively low deposition rate of PPAW must be balanced against the performance requirements of the application; for non-critical applications where corrosion resistance is not a primary concern, the higher deposition rate of transferred arc plasma welding may be more economical. For engineers involved in the design and fabrication of clad components, the key takeaway is that the welding process variant must be selected based on a thorough understanding of the service conditions and performance requirements, and that the weld procedure qualification must include comprehensive testing of the overlay layer properties. The development of hybrid plasma arc welding processes that combine the advantages of different variants is an area of active research that holds promise for further improving the performance of plasma arc welding overlay layers.