The Weld Thermal Cycle: HAZ Formation, Grain Growth, and Cooling Rate

The Weld Thermal Cycle: HAZ Formation, Grain Growth, and Cooling Rate

Every time you strike an arc, you create a rapidly moving heat source that heats the base metal, then lets it cool. That sequence—heat up, peak, cool down—is called the weld thermal cycle. Understanding it is not academic busywork. It explains why a weld can crack the next morning, why some welds are tough and some are brittle, and why preheat and interpass temperature controls are written into AWS D1.1 and ASME Section IX.

What Is the Heat-Affected Zone?

The heat-affected zone (HAZ) is the portion of base metal that was not melted but was heated enough to change its microstructure. The fusion zone at the center melted completely. Just beyond it, the HAZ extends outward through a temperature gradient that can range from near the melting point down to temperatures that cause almost no metallurgical change at all.

Because the temperature drops steadily with distance from the fusion line, different bands of metal experience different peak temperatures. Each band ends up with a different grain structure and different mechanical properties. Metallurgists divide the HAZ into four sub-zones.

The Four HAZ Sub-Zones

Coarse-Grained HAZ (CGHAZ): This band sits immediately adjacent to the fusion line. Peak temperatures here are very high—well above the upper critical temperature (Ac3) for steel, approaching 1300 °C or higher. At those temperatures, austenite grains grow rapidly. When the weld cools, those large prior-austenite grains transform into coarse microstructures. If cooling is fast, martensite forms; if slower, bainite or a coarse ferrite-pearlite mixture results. The CGHAZ is the most susceptible zone to hydrogen-induced cracking and reduced toughness because of grain coarsening.

Fine-Grained HAZ (FGHAZ): Farther from the fusion line, peak temperatures are still above Ac3 but not extreme—roughly 900–1100 °C for low-carbon steel. The steel fully austenitizes but grains do not have time to grow large. On cooling, this produces a fine-grained, tougher microstructure. The FGHAZ often has better toughness than the original base metal.

Intercritical HAZ: Peak temperatures in this band fall between the lower critical temperature (Ac1) and Ac3—roughly 730–900 °C for plain carbon steel. Only part of the microstructure transforms to austenite; the rest remains as ferrite or pearlite. The result is a mixed, partially refined zone with properties between the other sub-zones.

Subcritical HAZ: The outermost band never exceeds Ac1. No phase transformation occurs, but tempering, stress relief, or—in previously cold-worked metal—recrystallization may happen. Hardness in this zone is usually close to the base metal.

How Cooling Rate Controls Microstructure

Peak temperature determines which austenite grain size you start with. Cooling rate determines what that austenite transforms into. This is captured in a continuous cooling transformation (CCT) diagram. For A36 and similar structural steels:

• Slow cooling → ferrite and pearlite → relatively soft, ductile.
• Moderate cooling → bainite → moderate hardness and toughness.
• Fast cooling → martensite → hard and brittle; highly susceptible to hydrogen cracking.

The cooling rate in the HAZ depends on three things: heat input to the weld, base-metal thickness (which acts as a heat sink), and preheat/interpass temperature. A thick plate pulls heat away fast; preheat slows the rate by reducing the temperature difference between weld and base metal.

Heat Input and the AWS Formula

Heat input (H) quantifies the energy delivered to the weld per unit length. The standard AWS formula is:

H = (60 × A × V) / TS

Where:

• H = heat input in joules per inch (J/in)
• A = welding current in amperes (A)
• V = arc voltage in volts (V)
• TS = travel speed in inches per minute (in/min)
• 60 = conversion factor (seconds per minute)

Higher heat input means slower cooling in the HAZ, which reduces the risk of martensite but increases grain growth. Lower heat input means faster cooling—helpful for avoiding grain coarsening but raising the risk of hard, brittle HAZ microstructures. Controlling heat input is therefore a balancing act addressed directly in welding procedure specifications (WPS) qualified under AWS D1.1 and ASME Section IX.

Preheat and Its Role

Preheat slows the cooling rate by raising the starting temperature of the base metal before welding. AWS D1.1 and ASME Section IX both include preheat requirements based on material type, thickness, and carbon equivalent. When the cooling rate simulation in today's lab predicts martensite in the CGHAZ, the corrective action is usually to increase preheat, increase heat input within WPS limits, or both.

Safety in the Lab

During the demonstration and HAZ examination exercises, follow all standard safety protocols:

• PPE: Wear your welding helmet (minimum shade 10 for SMAW), leather gloves, flame-resistant clothing, and leather boots whenever you are near an active arc or hot metal.
• Hot metal: A36 plate retains dangerous heat long after the arc stops. Use pliers or tongs; never assume metal is cool. A thermocouple or contact thermometer confirms temperature before handling.
• Fumes: SMAW on A36 generates iron oxide and manganese fumes. Work with local exhaust ventilation and keep your head out of the fume plume.
• Electrical safety: Inspect leads and electrode holders before use. Do not change electrodes with bare hands. Keep the work area dry.
• Macrograph specimens: Ground and etched specimens may have sharp edges and acid residue. Wear chemical-splash goggles and nitrile gloves when handling etched cross-sections.