What is the HAZ and why is preheating important?

The HAZ (Heat Affected Zone) is the region of the base metal that has undergone metallurgical changes due to welding heat, without being melted.

Characteristics of the HAZ:

• A width of a few millimetres on each side of the weld — it depends on welding energy and is measured on the part
• Modified grain structure: coarse grains near the fusion line, fine grains further away
• The most brittle zone of the weld — risk of cracking if cooling is too fast

Why preheating:

• Slows down the cooling of the HAZ, allowing martensitic (hard) transformation into more ductile structures
• Reduces HAZ hardness: the maximum admissible hardness is set by the applicable code and the WPS, not by a single value
• Reduces the risk of underbead cracking

Preheat temperatures:

They are not read from a single table: they are prescribed by the applicable code and re-stated in the WPS, according to the steel grade (carbon and alloying element content), the thickness and the hydrogen introduced. Preheat rises with carbon content, thickness and joint restraint. The temperature is measured with a contact thermometer or a temperature-indicating crayon, close to the joint, before the first pass.