AWS Certified Welding Inspector — FundamentalsWelding ProcessesMedium
A CWI is inspecting a Submerged Arc Welding (SAW) operation on a thick-section steel pipe. The procedure specifies a high heat input to ensure proper fusion and minimize cracking. However, the inspector observes excessive grain growth in the heat-affected zone (HAZ) and reduced toughness in impact tests. Which of the following is the most likely cause, given the welding parameters?
- AInsufficient preheat temperature.
- BExcessively low travel speed.
- CIncorrect electrode stick-out.
- DToo high arc voltage.
Show answer & explanationAnswer & explanation
Correct answer: B. Excessively low travel speed.
Excessively low travel speed, especially with high current and voltage, directly leads to very high heat input. This prolonged exposure to high temperatures can cause significant grain growth in the HAZ and weld metal, which degrades mechanical properties like toughness.
Why the other options are wrong
- A. Insufficient preheat usually leads to hydrogen cracking or lack of fusion, not primarily excessive grain growth and reduced toughness from high heat input.
- C. Incorrect electrode stick-out affects current and penetration but is not the primary driver of overall heat input to the extent of causing significant grain growth from excessive heat.
- D. Too high arc voltage typically results in a wider, flatter bead and increased flux consumption, but its primary effect on heat input (and thus grain growth) is less direct than travel speed.
SAW Heat Input Control
Heat input in SAW is critical for controlling microstructure and mechanical properties. It is directly influenced by current, voltage, and travel speed.
- High heat input: Larger HAZ, coarse grain structure, reduced toughness.
- Low heat input: Smaller HAZ, finer grain structure, risk of lack of fusion or cracking.
- Travel speed is a major factor in heat input.
Memory trick: Heat input is the oven, travel speed is the timer; too slow, and it's overcooked.