Mastering HT alloy welding: techniques, tips, and best practices

HT alloy welding castings involve a detailed process to ensure high-quality, durable joints.

Understanding HT alloys

HT alloys are austenitic in microstructure and contain significant amounts of carbides. These alloys are known for not forming the sigma phase under any conditions, which can be detrimental to other alloys. The weldability of HT alloys is highly influenced by the ratio of silicon to carbon, with a 2:1 ratio often providing the best balance between weld soundness and ductility. Here’s what you need to know about the key components of the welding procedure:

Base metal and preparation

1. Base metal: HT alloys typically comprise 15-19% Chromium, 33-37% Nickel, and 0.35-0.75% Carbon. Both static and centrifugal castings are used.
2. Filler metal: AWS E330-15 lime-coated electrodes are preferred, modified to deposit weld metal with carbon content matching the base metal.
3. Position: Welding is generally performed in the “flat” position. An angle of ±15° from the horizontal is acceptable.
4. Base metal preparation for repair: Defects must be removed through arc-air, chipping, gouging, grinding, or machining. Inspection via visual, dye penetrant, or radiographic methods ensures defect-free preparation.
5. Base metal preparation for fabrication: Parts should be shaped to create a groove for welding, ensuring a good fit. Machining should be done dry, and components must be thoroughly cleaned before assembly.

Welding procedure

6. Preheat temperature: Preheating is generally unnecessary unless dealing with complex shapes or sections thicker than ¾ inch, where preheating to 200-400°F (93-204°C) can be beneficial.
7. Section size: For sections under ½ inch, use an electrode no larger than 1/8 inch in diameter. Thicker sections may require preheating.
8. Cavity dimensions: Maintain a minimum angle of 30° in the cavity with a root radius of 3/16 to ¼ inch. For large defects, a vertical uphill welding position might be necessary.
9. Welding technique: Ensure surfaces are dry and clean. Either stringer or weave bead techniques are suitable, with weaving limited to three times the electrode diameter. Buttering the sides of cavities can minimize base metal dilution. Remove all slag between passes to prevent corrosion.
10. Electrical characteristics: Welding is typically performed using DC reverse polarity, though AC can be used. Electrode sizes range from 3/32 to ¼ inch, with parameters set by the electrode manufacturer.

Additional considerations

11. Machined castings: Use small rods and low heat to avoid distortion. Peening can counteract contraction.
12. Post-weld heat treatment: Generally not required, but large or critical welds in complex castings may benefit from heat treatment at 1900-2100°F (1038-1149°C) for 3-4 hours followed by furnace cooling.
13. Non-destructive tests: Quality inspection methods include visual inspection, dye penetrant, radiography, or pressure testing.

Summary of best practices

To ensure high-quality welds for HT castings, follow these key precautions:

1. Defect removal: Ensure all defects are fully removed and surfaces are thoroughly cleaned.
2. Correct filler metal: Use filler metals that match the base metal composition.
3. Proper welding technique: Employ techniques that prevent porosity, undercutting, or lack of penetration.

By adhering to these guidelines, welders can achieve strong, defect-free welds that meet the rigorous demands of HT alloy applications.

This comprehensive approach ensures that HT alloy welding processes result in durable, reliable joints, enhancing the overall performance and longevity of the castings in their intended applications.