Summary of Weld Seam Cooling Methods and Key Points

Welding is a common metal joining method, widely used in industrial production and manufacturing processes. During the processing and assembly of welded parts, they usually undergo high-temperature melting and cooling processes. However, the welding process generates enormous heat, leading to an increase in the temperature of welded parts. Since high temperatures may have adverse effects on material properties and structures, such as thermal stress and intergranular corrosion, effective cooling methods must be adopted during welding to ensure welding quality and performance.

In weld cooling specifications, the cooling rate has a significant impact on the microstructure of weld metal. For example, when the cooling rate is between 10℃/s and 100℃/s, the weld metal may form ferrite-pearlite structures with different grain sizes.

According to relevant standards, for low-alloy steel welds in weld cooling specifications, the weld should be heated to 150 – 250℃ for heat preservation and slow cooling within a certain time after welding to reduce residual stress. Weld cooling specifications involve the selection of cooling media. For example, in the welding of certain large components, natural air cooling is adopted, which has a relatively mild cooling effect and helps reduce thermal stress concentration. From the perspective of heat conduction principles, weld cooling specifications require that during the cooling process, the temperature gradient between the weld and the surrounding base metal should be controlled, generally maintaining a temperature change not exceeding 5℃ per millimeter of length. Weld cooling specifications vary with different welding processes. For example, when cooling submerged arc welding seams, due to the large welding heat input and relatively slow cooling rate, attention should be paid to preventing the formation of coarse-grained structures in the weld.

As specified in the weld cooling specifications, for welds of steels sensitive to hydrogen embrittlement, timely post-heat treatment shall be performed after cooling to room temperature. Typically, the welds are held at a temperature of 200 – 350℃ for 2 – 4 hours to remove hydrogen from them. In the practice of weld cooling specifications, thermocouples are used to measure the cooling temperatures at different positions of the weld, and the cooling curves are recorded. The cooling parameters are adjusted according to the curves to ensure that the cooling effect meets the requirements. Considering the restraint degree of the weld, the weld cooling specifications stipulate that when the restraint degree of the weld is high, appropriate measures shall be taken during the cooling process, such as adopting segmented cooling, hammering the weld, etc., to release part of the stress. The weld cooling specifications also pay attention to the thermal stress distribution during the cooling process. Finite element simulation software can be used to predict the thermal stress distribution, thereby optimizing the cooling process and reducing the risk of crack formation.

As per experience, for thin-plate weld cooling specifications, the cooling rate can be appropriately increased, generally controlled between 50℃/s and 200℃/s, to improve production efficiency while ensuring weld quality.

Weld Seam Cooling Methods

Weld cooling methods should be selected based on material characteristics, process requirements, and on-site conditions. The following are the core methods and their operational key points:

1. Physical Forced Cooling Method (Efficient Cooling)

• Water Cooling Technology

Backside Immersion: The backside of the workpiece is in contact with flowing cold water (suitable for non-hardenable steel).

Surface Spraying: Spray water mist immediately after welding (carbon steel can be cooled to 40℃ in 3 minutes; prohibited for stainless steel/high-carbon steel).

Water-Cooled Copper Pad: A copper block with circulating water is closely attached to the backside of the weld, increasing the cooling rate by more than 3 times.

• Air Cooling Technology

Compressed Air Jet: Directional purging of the weld at 0.5MPa pressure (needs to cooperate with air nozzles to focus airflow).

Industrial Jet Air Conditioning: Workshop-level large-air-volume low-temperature air supply (the weld area can be cooled by 8~15℃ within 10 minutes).

2. Process Regulation Method (Source Temperature Control)

3. Material Heat Dissipation Method

• High Thermal Conductivity Gaskets

Red copper heat dissipation blocks (thermal conductivity 398W/m·K) are closely attached to the backside of the weld.

Aluminum alloy thermal conductive fixtures (local cooling speed increased by 40%).

• Phase Change Heat Absorbing Materials

Paraffin energy storage sheets are pasted on both sides of the weld (melting and absorbing heat, reusable).

4. Special Cooling Technologies

Liquid Nitrogen Spray Freezing: Targeted spraying of -196℃ liquid nitrogen (cooled to 0℃ within 5 seconds; need to prevent low-temperature brittleness).

Semiconductor Cooling Sheet: Attached thermoelectric cooling (precision ±1℃, suitable for precision parts).

5. Key Taboos and Selection Guidelines

• Materials Prohibited from Water Cooling:

Stainless steel (304/316), high-carbon steel (C＞0.3%), cast iron—prone to inducing stress cracks!

• Thick Plate Welding Specifications:

When the plate thickness is ＞25mm, it is necessary to cooperate with an interpass temperature controller (maintaining 100~300℃ slow cooling).

• Cold Crack Sensitive Materials:

After welding low-alloy steel (such as Q345R), it must be slowly cooled to room temperature with thermal insulation cotton.

6. Comparative Practical Cases

Decision Suggestions: For conventional carbon steel, select water cooling/air cooling; for precision parts, use pulsed welding + copper pad; for special materials, must follow the slow cooling principle. When the cooling rate exceeds 30℃/min, process qualification is required!

Precautions and Countermeasures for Liquid Nitrogen Spray Cooling Method

Although the liquid nitrogen spray cooling method (-196℃) can achieve second-level rapid cooling, improper operation can easily lead to workpiece failure or even safety accidents. The following are the key precautions and countermeasures:

Golden Principle: Only applicable to low-carbon steel (＜0.2%C), copper alloys, and titanium alloys, and must pass process qualification!

1. Temperature Gradient Control (Preventing Invisible Defects)

Gradient Threshold: Temperature drop per unit distance ≤150℃/cm (e.g., when the weld width is 1cm, the temperature difference between the center and the edge is ＜150℃). Exceeding this will cause residual stress to exceed the material’s yield strength!

Dynamic Temperature Measurement: Use an infrared thermal imager for real-time monitoring (sampling frequency ≥10Hz), and adjust the spray angle immediately if local overcooling is found.

2. Safe Operating Procedures

3. Precise Regulation of Process Parameters

Optimization Formula for Liquid Nitrogen Spray: Spray Flow Rate (L/min) = K × (Plate Thickness (mm))² × Cooling Rate (℃/s)

Where: K=0.03 for low-carbon steel, K=0.015 for titanium alloy.

Typical Parameters:

Thin Plates (≤3mm): Flow rate 2~3L/min, moving speed 10mm/s

Medium and Thick Plates (6mm): Flow rate 8L/min, scanning reciprocating spray

4. Key Points for Equipment Selection

• Nozzle Type:

Fan-shaped atomizing nozzles (diffusion angle 60°) are superior to direct flow nozzles to avoid liquid nitrogen accumulation.

• Pressure Supply System:

Storage tank pressure ≥1.2MPa + pressure reducing valve secondary pressure stabilization (output 0.3~0.5MPa)

• Intelligent Control:

Integrated PLC system, linked with temperature controller to automatically start and stop spraying (temperature control precision ±5℃)

5. Weld Quality Assurance Measures

• Pre-Drying Treatment:

Purge the weld with compressed air before spraying; absolutely prohibit residual moisture (ice expansion cracking defects!).

• Post-Aging Treatment:

After welding titanium alloy, stress relief annealing at 200℃ for 2h is required to eliminate hydrogen embrittlement tendency.

• Non-Destructive Testing:

Perform penetrant testing (PT) immediately after cooling to room temperature; if delayed for more than 4h, additional ultrasonic testing (UT) is required.

6. Failure Cases

• Case 1: Liquid nitrogen cooling of 304 stainless steel pipe fittings → stress corrosion cracking occurred after 24h (Cl⁻ ion enrichment).
• Case 2: No pre-drying of Q345R thick plate → ice crystal burst in the weld area (defect depth up to 2mm).
• Ultimate Suggestion: Must conduct a small sample test before first use! Continuously monitor the cooling curve to ensure that the time for the 300℃→100℃ stage is ＞15 seconds (prevent quenching effect). The aerospace field must comply with the AMS 2750E low-temperature treatment specification.

Quick Reference Table of Weld Seam Cooling Methods

Operational Key Points of Four Core Cooling Methods

1. Liquid Nitrogen Spray (-196℃)

• Safety Distance: The distance between the nozzle and the weld is ≥15cm (spray at a 30° angle);
• Flow Rate Formula: Flow Rate (L/min) = Plate Thickness (mm) × 0.03 × Target Cooling Rate (℃/s);
• Mandatory Inspections: Clean the weld with acetone before spraying, and the PT test delay ≤4 hours.

2. High-Pressure Water Spraying

• Water Quality Requirement: Conductivity ＜50μS/cm (deionized water);
• Angle Control: Water flow forms a 45° angle with the weld (reducing impact defects);
• Start-Stop Timing: Spray water when the weld bead darkens to 550℃, and stop when it reaches 100℃.

3. Pulsed Welding + Water-Cooled Copper Pad

• Copper Pad Parameters: Purity ＞9%, water channel diameter ≥8mm;
• Pulse Setting: Base current = peak current × 30%, frequency 15-25Hz;
• Water Temperature Control: Circulating water temperature 20±5℃ (add antifreeze to prevent scaling).

4. Interpass Temperature Control

• Slow Cooling Curve: 300℃→100℃ stage ＞15 minutes (prevent hydrogen embrittlement);
• Heating Tools: Ceramic heating sheets + temperature controller (precision ±10℃);
• Thickness Correlation: For each increase of 25mm in plate thickness, the slow cooling time is extended by 50%.

Post-Welding Cooling Application Specifications for Three Major Industries

• Aerospace (AMS 2750E)

Titanium Alloy: Stress relief at 200℃ for 2h must be performed after liquid nitrogen spray;

Aluminum Alloy: Any liquid cooling is prohibited, only air cooling is allowed.

• Pressure Vessels (ASME VIII Div.1)

Thickness ＞32mm: Cooling rate ≤55℃/h (below 300℃);

Cr-Mo Steel: Hydrogen removal treatment at 300℃ for 1h immediately after welding.

• Automotive Welding (ISO 13919)

Galvanized Steel Plates: Forced air cooling (wind speed ＞20m/s);

Aluminum Spot Welding: Semiconductor cooling sheets embedded in copper electrodes.

• Lessons and Warnings:

Case 1: Water cooling of 304 stainless steel pipes → stress corrosion cracking after 24h (Cl⁻ concentration);

Case 2: Rapid liquid nitrogen cooling of 42CrMo shafts → transverse fracture (hardness increased from HRC28 to 52);

Case 3: Uncontrolled interpass temperature of Q345R → hydrogen-induced delayed cracking (failure 72 hours after welding).

• Ultimate Advice:

For stainless steel/high-strength steel: Always prioritize air cooling + slow cooling!

For liquid nitrogen/water cooling: Must conduct small sample process qualification before use!

For cooling rate ＞100℃/min: Third-party testing agency certification is required!

Safety of Weld Seam Cooling Methods

The safety of weld cooling methods mainly depends on the compatibility between material characteristics and processes. Based on industrial accident statistics and material science principles, the following are the safety classifications and preferred solutions:

Top 3 Safe Methods (Comprehensive Accident Rate ＜0.3%)

1. Interpass Temperature Control Method

• Applicable Materials: High-strength steel (Q460 and above), stainless steel, cast iron;
• Safety Principle: Maintain slow cooling of the weld area at 100-300℃ through a temperature controller to eliminate thermal stress and hydrogen embrittlement risks.
• Operational Specifications: The interval between each weld bead is ≥5 minutes (extend by 2 minutes for each 10mm increase in plate thickness);
• Thermocouple Arrangement: Spacing ≤50mm (error ±5℃).
• Industrial Verification: Zero cold crack accidents in the pressure vessel industry for 30 years.

2. Pulsed Welding + Air Cooling

• Applicable Materials: Aluminum alloy, copper alloy, thin-walled stainless steel (≤4mm)
• Safety Mechanism:
  • Pulsed current (peak/base = 3:1) reduces heat input by 80%
  • Directional purging with compressed air (0.3MPa), cooling rate 15-20℃/min
• Parameter Formula: Maximum Safe Current (A) = (Plate Thickness (mm) × 35) / √(Welding Speed (mm/s))

3. Phase Change Heat Absorbing Patch

• Applicable Scenarios: Precision electronic solder joints, medical devices
• Safety Advantages: No liquid contact → eliminating electrochemical corrosion; uniform temperature → gradient difference ＜30℃/cm
• Material Requirements: Paraffin/metal-organic framework (MOFs) composite materials with a phase change temperature of 50-80℃ (attached to the weld heat-affected zone)

High-Risk Methods (Use with Caution! Accident Rate ＞8%)

Mandatory Requirements of Industry Specifications

• Nuclear Power Weld Joints (ASME III)

Only interpass temperature control (≤300℃) + post-heat treatment is allowed.

Cooling rate ≤55℃/h (300℃→room temperature stage)

• Automotive Chassis Welding (ISO 5821)

Liquid cooling is prohibited for high-strength steel (DP780/CP1180)

Forced air cooling temperature records must be kept for 10 years

• Aerospace (AMS 2750E)

Titanium Alloy: Vacuum stress relief (600℃×2h) after air cooling

Aluminum Alloy: Active cooling of the weld area is prohibited

How to Select a Suitable Weld Seam Cooling Method?

The selection of weld cooling methods must follow the principle of “material priority, safety first, and efficiency control”, and accurately match the scheme with the following 5-dimensional decision-making system:

1. Material Characteristics Determine Success or Failure (Core Dimension)

Golden Rhyme: “Alloys die when exposed to water; high-strength steels need slow cooling; titanium and aluminum are cooled by air; cast iron must be insulated.”

2. Structural Characteristics Determine Parameters

1) Thickness Effect

2) Joint Form

• Fillet Welds: Prioritize air cooling (avoid water accumulation in dead corners)
• Pipeline Girth Welds: Rotating liquid nitrogen spray gun (for titanium/copper alloys) or induction heater temperature control (for high-strength steel)
• Thin-Plate Lap Joints: Phase change heat absorbing patches (temperature gradient ≤40℃/cm)

3) Working Condition Requirements Determine Intensity

4) Cost and Equipment

Classic Schemes for Three Major Industries

1. Shipbuilding (High-Strength Steel DH36)

• Scheme: Electric heating sheet temperature control (150℃ interpass) + thermal insulation cotton slow cooling
• Parameters: Cooling rate ≤45℃/h (below 100℃)
• Basis: ABS Welding Code Chapter 7

2. New Energy Vehicle Battery Tray (6061 Aluminum Alloy)

• Scheme: Dual-pulse MIG welding + vortex tube cold air system (-15℃)
• Parameters: Base current = peak × 25%, wind pressure 0.4MPa
• Basis: IEC 62619-2022

3. Aeroengine Pipes (Ti-6Al-4V Titanium Alloy)

• Scheme: Liquid nitrogen atomized spray (30° angle) + 650℃×2h vacuum annealing
• Parameters: Flow rate = plate thickness (mm) × 0.025 L/s, atomized particle size ＜50μm
• Basis: AMS 2809B

Summary of Weld Seam Cooling Methods and Key Points

There are various weld cooling methods, and the appropriate method should be selected according to the actual situation. Through reasonable cooling measures, the temperature of the welded parts can be effectively controlled, and the welding quality and efficiency can be improved. Before the first application of a new process, small sample temperature field testing + metallographic verification must be performed!