Welding Metallurgy and Testing
Welding Metallurgy and Testing
Welding metallurgy covers the effects of heat on the base metal and weld metal, including the heat-affected zone (HAZ), phase transformations, and defect formation. Key concepts include cooling rates, hardenability, hydrogen embrittlement, and lamellar tearing. The HAZ can experience grain growth and hardening, leading to cracking if not controlled by preheat and heat input. Testing methods include destructive tests (tensile, bend, impact, hardness) and non-destructive tests (visual, radiographic, ultrasonic, magnetic particle, dye penetrant). In Canada, CSA W59 specifies acceptance criteria for steel welds. Impact testing is required for certain applications to ensure toughness at low temperatures.
A common trap is misinterpreting the purpose of preheat: it is not just for removing moisture but primarily to slow cooling and prevent hard, crack-susceptible microstructures. Another trap is confusing the location of the HAZ; it is adjacent to the weld, not in the weld metal itself. Also, understanding that hydrogen-induced cracking can be delayed (delayed cracking) and may require 48 hours before inspection.
Worked example: Welding a thick section of A514 (high-strength low-alloy steel) requires preheat and controlled heat input to avoid cold cracking. A PQR must include impact testing if the service temperature is low. If the heat input is too high, the HAZ toughness may degrade. The welder must follow the WPS parameters exactly.
Rule of thumb: For high-strength steels, use preheat and limit heat input to balance hardness and toughness. Always wait 48 hours before inspecting for delayed hydrogen cracking.