Failure Caused by Overheating During Secondary Soldering – Damage to Brazed Structure at Nozzle Connection

Overview

In copper-brazed plate heat exchangers, the internal plate stack is joined through a controlled vacuum brazing process.

The integrity of the unit depends on maintaining the metallurgical stability of these brazed joints.

However, secondary soldering or welding operations performed at the nozzle connection can introduce excessive localized heat.

If proper temperature protection is not applied, overheating may occur, resulting in structural damage to the original brazed joints.

Observed Failure Characteristics

Based on inspection and leakage pattern:

• Leakage occurred at the nozzle interface

• No signs of corrosion-induced degradation

• No evidence of pressure overload

• Localized joint weakening around the connection

The failure mode is consistent with thermal degradation rather than material defect.

Root Cause Analysis

1️⃣ Copper Brazed Plate Structure

Copper-brazed plate heat exchangers are manufactured in a vacuum furnace at controlled temperatures (typically around 1100°C).

The brazed copper bonds:

• Join the plates

• Seal the internal channels

• Provide structural integrity

After brazing, the copper joints remain stable only within defined thermal limits.

2️⃣ Secondary Soldering Without Thermal Protection

During external nozzle soldering:

• Direct flame heating was applied

• No thermal sink or cooling protection was used

• Local temperature exceeded safe threshold

Excessive heat can:

• Re-melt or soften internal brazed copper

• Disturb capillary bonding

• Create micro-voids

• Reduce joint strength

This compromises the seal between plates and end plate near the nozzle area.

3️⃣ Mechanism of Leakage

When the internal brazed seam near the nozzle is thermally disturbed:

• Micro-cracks develop

• Capillary seal weakens

• Internal cross-leakage or external seepage appears

This type of failure typically occurs shortly after commissioning.

Why This Is Not a Manufacturing Defect

The original brazed structure is stable under specified operating conditions.

The failure mechanism observed is consistent with:

Localized overheating introduced after production, during secondary soldering.

Such overheating alters the metallurgical integrity of the brazed joint.

Preventive Recommendations

To avoid similar failures:

• Use temperature-controlled soldering techniques

• Apply heat sinks or cooling wraps during nozzle work

• Avoid prolonged direct flame exposure

• Monitor surface temperature during operation

• Consider factory-assembled connection options where possible

Engineering Conclusion

Copper-brazed heat exchangers are sensitive to uncontrolled secondary heating at connection points.

Failure in this case is consistent with:

Overheating during soldering without adequate thermal protection, leading to degradation of the original brazed joint.

Proper temperature management during field modifications is essential to maintain structural integrit