Considerations for Overlay Welding with PTAW Using Stellite 6 Powder
Overlay welding using Plasma Transferred Arc Welding (PTAW) with Stellite 6 powder is a critical process in industries requiring high wear resistance and corrosion protection. Stellite 6, a cobalt-based alloy, is known for its excellent hardness, wear resistance, and ability to withstand high temperatures. Here are some important considerations to ensure a successful PTAW overlay with Stellite 6 powder:
1. Preparation of the Base Material
Cleanliness: The base material should be thoroughly cleaned to remove any contaminants such as oil, grease, dirt, and oxides. This ensures better adhesion of the Stellite 6 overlay.
Surface Roughening: It is beneficial to roughen the surface slightly to enhance the mechanical interlock between the base material and the overlay.
2. Selection of PTAW Parameters
Current and Voltage: Proper adjustment of current and voltage settings is crucial. Higher currents can lead to increased dilution of the base material, whereas lower currents may result in inadequate fusion.
Powder Feed Rate: The feed rate of Stellite 6 powder should be optimized to ensure a consistent and uniform overlay. Too high a feed rate can cause excess material buildup, while too low a rate may lead to incomplete coverage.
Travel Speed: The speed at which the torch travels affects the overlay thickness and the heat input. A balanced speed ensures a uniform overlay with minimal defects.
3. Control of Heat Input
Preheating: For certain base materials, preheating is necessary to prevent cracking and ensure proper fusion. The preheating temperature depends on the base material composition.
Interpass Temperature: Maintaining an appropriate interpass temperature is important, especially when applying multiple layers. Excessive heat can cause cracking and affect the microstructure of the overlay.
4. Shielding Gas Selection
Type of Gas: Common shielding gases include Argon and Argon-Hydrogen mixtures. Argon provides a stable arc and minimizes oxidation. Hydrogen can be added to improve the fluidity of the weld pool.
Flow Rate: The shielding gas flow rate should be sufficient to protect the weld pool from atmospheric contamination, which can lead to defects like porosity.
5. Powder Characteristics
Powder Quality: Use high-quality Stellite 6 powder with a consistent particle size distribution. Poor quality or inconsistent powder can lead to defects in the overlay.
Storage: Store the powder in a dry environment to prevent moisture absorption, which can cause clumping and affect the powder flow during welding.
6. Post-Weld Treatment
Inspection: Conduct thorough inspections of the overlay for defects such as cracks, porosity, and insufficient coverage. Non-destructive testing methods like ultrasonic testing can be employed.
Heat Treatment: Depending on the application, post-weld heat treatment may be required to relieve residual stresses and enhance the mechanical properties of the overlay.
7. Health and Safety
Personal Protective Equipment (PPE): Operators should wear appropriate PPE, including welding helmets, gloves, and protective clothing, to safeguard against heat, sparks, and harmful fumes.
Ventilation: Ensure adequate ventilation or use fume extraction systems to protect operators from inhaling harmful fumes generated during the welding process.
By paying careful attention to these considerations, the quality and performance of the PTAW overlay with Stellite 6 powder can be significantly enhanced, leading to improved wear resistance and longevity of the components being treated.
