Heat Treatment and Metallurgy
Preheat, interpass temperature, PWHT, HAZ, carbon equivalent, and weldability of steels.
- Explain preheat and interpass temperature requirements and their purpose
- Describe PWHT and its effect on the heat-affected zone
- Calculate and interpret carbon equivalent for weldability assessment
- Explain filler metal matching and dilution concepts
Lesson 1
Preheat and Interpass Temperature
Why Preheat?
Preheating is heating the base metal before welding to slow the cooling rate. Rapid cooling after welding produces hard, brittle microstructures in the heat-affected zone that are susceptible to cracking. By slowing the cooling rate, preheat allows the microstructure to form more slowly, producing softer and more ductile structures.
Preheat is used to reduce the risk of cracking by slowing cooling and reducing thermal stress. It also drives off surface moisture and reduces the temperature gradient across the weld zone.
Interpass Temperature
Interpass temperature is the temperature of the weld area between passes. It has both a minimum (must not fall below preheat temperature) and a maximum (to prevent overheating and degraded properties). The interpass temperature is measured on the base metal adjacent to the weld, typically using a contact thermometer or temperature-indicating crayon.
Preheat Is Not Optional
When preheat is specified on the WPS, it is mandatory. Welding without required preheat on thick or high-carbon steel will almost certainly result in hydrogen cracking - potentially hours or days after the weld is completed.
Preheat slows the cooling rate to reduce risk of cracking and thermal stress. Interpass temperature is the temperature between weld passes - it must stay above the minimum preheat and below the maximum interpass limit specified on the WPS.