A technician is preparing to weld two pieces of 12 mm thick high-strength low-alloy (HSLA) steel structural members using SMAW. The manufacturer's service manual specifies preheating the material to 150°C. What is the primary reason for this preheating requirement?

Question: A technician is preparing to weld two pieces of 12 mm thick high-strength low-alloy (HSLA) steel structural members using SMAW. The manufacturer's service manual specifies preheating the material to 150°C. What is the primary reason for this preheating requirement?

Answer options:

• To increase the welding speed and deposition rate. ✅ To reduce the likelihood of hydrogen-induced cracking in the heat-affected zone (HAZ).
• To improve the aesthetic appearance of the weld bead.
• To make the material softer and easier to weld.

Correct answer: To reduce the likelihood of hydrogen-induced cracking in the heat-affected zone (HAZ).

Explanation: Preheating high-strength low-alloy steels is critical to reduce the cooling rate of the weld and the heat-affected zone (HAZ), which minimizes the formation of brittle microstructures and allows diffused hydrogen to escape, thereby preventing hydrogen-induced cracking. HSLA steels are more susceptible to this type of cracking. Preheating does not primarily increase welding speed, improve aesthetics, or soften the material.