What is the typical preheat temperature range in Fahrenheit for steel patches in collision repair MIG welding?

• A. 0-32°F.
• B. None; preheating is not used.
• C. 300-500°F (150-250°C).
• D. 1000-1200°F.

Answer: (C)

Preheating before MIG welding a steel patch helps control heat flow and reduces problems like hydrogen cracking, excessive shrinkage, and distortion in the welded joint. For typical automotive patch repairs on mild steel sheet, warming the area to about 300–500°F (150–250°C) is common practice. This temperature is hot enough to slow the cooling of the weld, which lowers the risk of brittle cracking and helps the patch fuse more smoothly without pulling the surrounding metal out of alignment.

The lower option is not appropriate because freezing temperatures do not represent a preheat; you’d still be dealing with rapid cooling and potential cracking. The idea that preheating isn’t used isn’t accurate for many collision repair scenarios where moisture, coating, or steel thickness can make preheating beneficial. The high range around 1000–1200°F would overheat thin sheet metal, likely causing burn-through, excessive distortion, or damage to coatings and adjacent metal.

So, the typical preheat range for steel patches in collision repair MIG welding is about 300–500°F.