Heat Treatment: Preheat, Interpass Temperature Control, and PWHT

Why Thermal Control Matters in Structural and Pipe Welding

Steel responds to heat in predictable but unforgiving ways. Weld too cold and hydrogen can crack the heat-affected zone (HAZ). Weld too hot and you coarsen the grain structure, destroy toughness, or distort a carefully fitted joint. Three thermal controls keep those problems in check: preheat, interpass temperature control, and post-weld heat treatment (PWHT). Every major welding standard — AWS D1.1 for structural steel and ASME Section IX for pressure-boundary welds — requires these controls to be specified in the Welding Procedure Specification (WPS) and documented in the record.

Preheat: Starting Warm on Purpose

Preheat is heat applied to the base metal before the arc is struck. Its main jobs are:

• Slow the cooling rate so hydrogen has time to escape the weld metal before it becomes trapped.
• Reduce thermal shock and the hardness spike that forms in the HAZ of hardenable steels.
• Drive off moisture from the joint surface, which is a hydrogen source.

For A572 Grade 50 structural steel in thicker sections (typically 1.5 in. and above), preheat requirements depend on the carbon equivalent of the heat, the process, and filler hydrogen level. Your WPS or the applicable code table tells you the minimum temperature. A common way to apply preheat on the job is a propane rosebud torch or a resistance-heating blanket. The blanket is preferred when a long, uniform soak is needed; the torch is faster for smaller areas.

How to verify: Use a calibrated contact thermometer or a Tempilstik (temperature-indicating crayon) rated to the target temperature. Per standard practice, check the temperature at least 3 inches from the edge of the weld joint on each side. If the Tempilstik melts when touched to the steel, the surface has reached that crayon's rated temperature. The contact thermometer gives you an exact reading you can record on the heat-treatment log. Do not strike an arc until the required preheat is confirmed and the instructor has signed the log — this is a safety gate in this program.

Interpass Temperature: Keeping the Lid On

Once welding begins, each subsequent pass adds heat. The interpass temperature is the temperature of the weld and adjacent base metal measured immediately before a new pass is deposited. Code procedures set both a minimum (same as preheat — the joint must not cool below preheat between passes) and a maximum (to protect the HAZ microstructure and mechanical properties).

Exceeding the maximum interpass temperature can:

• Reduce notch toughness in the HAZ.
• Cause excessive grain growth in low-alloy and alloy steels.
• Alter distortion behavior in precision-fit joints.

Check interpass temperature with a contact thermometer placed on the weld bead or within the joint. If the reading is above the maximum specified in the procedure, stop welding and allow the plate to cool naturally — never quench with water unless the WPS specifically permits it. Record the reading, the time, and your initials on the heat-treatment log. The instructor reviews and co-signs each entry.

Post-Weld Heat Treatment (PWHT): Stress Relief and Tempering

PWHT is a controlled thermal cycle applied after the weld is complete. For pressure-boundary components — such as chrome-moly (Cr-Mo) alloy pipe used in power plants and refineries — PWHT is mandatory under ASME Section IX and the applicable construction code. Its purposes include:

• Stress relief — relaxing residual stresses locked in during cooling.
• Hydrogen bake-out — driving remaining diffusible hydrogen out of thick sections.
• Tempering — softening the hard HAZ martensite in hardenable alloys, restoring ductility and toughness.

A PWHT cycle has three controlled phases recorded on the heat-treatment log:

1. Ramp rate (heat-up rate) — how fast the temperature rises, typically expressed in °F per hour. Heating too fast creates new thermal stresses.

2. Time at temperature (hold time) — the soak period at the target temperature. For P91 Cr-Mo pipe, the hold temperature and duration are critical and specified in the applicable code and the Procedure Qualification Record (PQR).

3. Cooling rate — how fast the assembly is allowed to cool after the hold. Cooling too quickly can re-introduce residual stress or cause cracking in sensitive alloys.

All three values must be filled in on the heat-treatment log. That log becomes a permanent attachment to the Procedure Qualification Record (PQR) under ASME Section IX, providing proof that the qualified procedure was followed on the actual production weld.

Safety Points for This Demo

• Resistance blankets and rosebud torches reach temperatures that cause severe burns instantly. Wear your welding jacket, leather gloves, and face shield whenever handling heated steel or operating the torch.
• Hot plate coupons look identical to cold ones. Use a Tempilstik or thermometer before touching any surface. Mark hot parts clearly.
• Propane torch use requires proper hose, regulator, and flashback-arrestor inspection before ignition. Secure cylinders upright.
• SMAW on preheated plate increases fume generation. Ensure ventilation is operating and position your head out of the fume plume.
• A missed temperature check means an automatic redo of the station — not as a punishment, but because an undocumented thermal record is worthless in the field and can mean a failed weld in a structure.

The Heat-Treatment Log

The log you complete today mirrors what is used on certified projects. Every field matters: joint ID, base metal specification and thickness, preheat method, preheat temperature measured and location, interpass temperatures by pass number, time stamps, and welder ID. Incomplete logs are rejected on ASME-governed work sites. Practice the habit now.