An off-road grader frame, made of quenched and tempered (Q&T) high-strength steel, requires a repair weld. Overheating the base metal during welding or preheating can degrade its mechanical properties. What is a critical preheating and interpass temperature control strategy for this material?

Question: An off-road grader frame, made of quenched and tempered (Q&T) high-strength steel, requires a repair weld. Overheating the base metal during welding or preheating can degrade its mechanical properties. What is a critical preheating and interpass temperature control strategy for this material?

Answer options:

• No preheating is required for Q&T steels, weld quickly to minimize heat input.
• Preheat to a maximum of 100°C and maintain interpass temperatures not exceeding 150°C to preserve heat treatment.
• Preheat to 250°C to ensure good fusion, then rapidly cool to prevent temper embrittlement. ✅ Use a non-contact thermometer to ensure preheat and interpass temperatures are strictly controlled, often below 200°C, to avoid altering the material’s heat-treated condition.

Correct answer: Use a non-contact thermometer to ensure preheat and interpass temperatures are strictly controlled, often below 200°C, to avoid altering the material’s heat-treated condition.

Explanation: Quenched and tempered steels derive their strength from their heat-treated condition. Excessive preheating or interpass temperatures can 'overtemper' the steel, reducing its strength and toughness. Precise temperature control, often below 200°C (400°F), is crucial, requiring temperature-indicating crayons or non-contact thermometers as per CSA W59 and manufacturer recommendations.