Auxiliary Heat Dissipation Technology for Weld Overlay Process Temperature Control

Literature Overview

This technical entry addresses auxiliary cooling methods employed during multi-layer weld overlay operations to manage layer temperature and minimize distortion. The techniques include copper backing plates, water-cooled backing systems, and compressed air cooling. These methods are particularly relevant for overlay welding on stainless steel, duplex steel, and titanium substrates where thermal expansion mismatch and distortion control are critical.

Technical Principles and Application Scope

The fundamental principle is to extract heat from the weld zone and backing side more rapidly than the natural cooling rate, thereby reducing the peak temperature of the base metal and limiting the thermal affected zone (TAZ). This is achieved through high thermal conductivity materials (copper) or active cooling media (water, compressed air) placed in contact with the backing surface.

Application Boundaries and Restrictions

A critical constraint is that these active cooling methods are strictly prohibited for quench-sensitive materials such as medium-carbon steels, Cr-Mo steels, and certain high-strength low-alloy steels. Rapid cooling of these materials through the martensite transformation range can induce hydrogen-assisted cracking, lamellar tearing, or residual stress-induced distortion. The cooling rate must be carefully controlled to remain below the critical cooling rate for martensite formation in the base metal.

The applicable material groups include:

• Austenitic stainless steels (304, 316, 321, 347): tolerant of moderate cooling rates due to austenite stability
• Duplex stainless steels (2205, 2507): require layer temperature control below 250°C to prevent sigma phase formation
• Titanium and titanium alloys: benefit from reduced TAZ width and limited intergranular oxidation
• Nickel-based alloys (Inconel, Hastelloy): generally tolerant but require control of interpass temperature

Process Implementation Details

For copper backing plate applications, the copper plate is machined to match the contour of the workpiece backing surface and clamped with thermal paste or refractory gap filler to ensure intimate contact. The copper plate thickness should be at least 30 mm to serve as an effective heat sink without overheating. The plate temperature should be monitored with a thermocouple embedded near the weld zone; it should not exceed 150°C during multi-pass overlay.

Water-cooled backing systems involve a copper or stainless steel backing plate with internal water channels. The water flow rate is typically 2–5 L/min per 100 mm of weld length, with inlet temperature below 20°C. The system must include flow switches and temperature alarms to prevent dry-out or overheating. For pressure vessels, the backing system must be designed to accommodate the internal pressure test requirements, with appropriate pressure relief provisions.

Compressed air cooling is the simplest method, involving directed air jets applied to the backing surface or the weld zone during and after each pass. The air velocity should be 5–15 m/s, and the air temperature should be ambient. This method provides modest cooling but is effective for thin overlay layers (2–5 mm) on austenitic substrates.

Integration with WPS and Quality Control

The use of auxiliary cooling must be explicitly documented in the Welding Procedure Specification (WPS). Key parameters to specify include:

1. Cooling method (copper backing, water-cooled, compressed air)
2. Backing material and thickness
3. Water flow rate and temperature (if water-cooled)
4. Air velocity (if compressed air)
5. Maximum allowable backing surface temperature
6. Interpass temperature limits

During production, the cooling system must be verified before each shift. Water-cooled systems require daily inspection of flow rate, temperature, and absence of leaks. Copper backing plates must be inspected for warping, contamination, or thermal damage.

Engineering Practice Case

In the fabrication of a hydrogenation reactor with 316L overlay on a 16Mn base plate, a water-cooled backing system was employed for the first three overlay passes. The backing plate was a 40 mm thick copper plate with serpentine water channels, providing a water flow of 3 L/min at 18°C inlet temperature. The layer temperature was maintained below 180°C, effectively preventing the formation of intermetallic phases at the interface and limiting distortion to less than 0.5 mm per meter of vessel length. The overlay was subsequently qualified through ultrasonic testing (UT) for bond strength verification, achieving 100% sound bonding across the entire overlay area.

Study Insights and Reflections

The auxiliary cooling technology is deceptively simple but requires careful engineering judgment. The primary challenge is determining the boundary between beneficial cooling and detrimental quenching. For austenitic and duplex materials, the cooling benefit clearly outweighs the quench risk, but for ferritic and martensitic materials, the situation is reversed. The engineer must understand the metallurgical response of the specific base material before selecting a cooling strategy.

Another important consideration is the impact of cooling on residual stress distribution. Rapid cooling can reduce peak temperatures and TAZ width but may increase thermal gradient-induced residual stresses. For thick-walled pressure vessels, the residual stress field must be evaluated in conjunction with the cooling strategy to ensure that the combined stress state does not exceed allowable limits during service. The cooling method should be integrated into the overall thermal management plan, including preheat, interpass temperature, and post-weld heat treatment specifications.