Welding Over Brazed Or Soldered Joints Is

Welding Over Brazed or Soldered Joints: A Comprehensive Guide

Welding over brazed or soldered joints is a practice that, while sometimes necessary, requires careful consideration and execution. It's not a straightforward process, and the success hinges on understanding the limitations and potential pitfalls. This comprehensive guide delves into the intricacies of this technique, outlining best practices, potential problems, and crucial considerations for different scenarios.

Understanding the Differences: Brazing, Soldering, and Welding

Before diving into welding over existing joints, it's crucial to understand the fundamental differences between brazing, soldering, and welding. These processes differ primarily in the melting temperatures of the filler materials and the base materials.

Brazing

Brazing uses a filler metal with a melting point above 450°C (842°F) but below the melting point of the base metals. The capillary action draws the molten filler metal into the joint, creating a strong bond. Brazing offers excellent strength and durability, often exceeding the strength of the solder joints but less than welding.

Soldering

Soldering utilizes a filler metal with a melting point below 450°C (842°F). Like brazing, it relies on capillary action. Solder joints, while convenient and relatively easy to create, generally exhibit lower strength compared to brazed or welded joints.

Welding

Welding, in contrast, involves melting the base metals themselves to create a fusion bond. The filler metal, if used, acts as an additive to fill gaps and strengthen the joint. Welded joints typically offer the highest strength and durability among the three processes.

Why Weld Over Brazed or Soldered Joints?

Several scenarios necessitate welding over pre-existing brazed or soldered joints:

• Increased Strength Requirements: When a brazed or soldered joint needs significantly increased strength to withstand higher loads or stresses, welding might be necessary. This is common in structural applications or situations demanding enhanced durability.
• Repairing Damaged Joints: If a brazed or soldered joint suffers damage or cracking, welding can provide a more robust and lasting repair.
• Modifying Existing Assemblies: Welding might be needed to add components or modify existing structures where the original brazed or soldered joints are insufficient.
• Eliminating Joint Weak Points: In designs requiring higher reliability, welding can replace weaker brazed or soldered connections to minimize failure points.

Challenges and Considerations When Welding Over Existing Joints

Welding over previously brazed or soldered joints presents unique challenges:

• Filler Metal Compatibility: The chosen welding filler metal must be compatible with both the base metal and the existing brazing or soldering material. Incompatible fillers can lead to poor fusion, weakening the joint and potentially causing cracking.
• Different Melting Points: The significantly different melting points of the base metal, brazing/soldering filler, and welding filler require precise temperature control to avoid overheating and damaging the joint. Overheating can cause the brazing/soldering material to flow excessively, compromising the integrity of the structure and potentially causing embrittlement of the base metal.
• Residual Stresses: The existing brazed or soldered joint might introduce residual stresses into the base metal. Welding over this can exacerbate these stresses, potentially causing cracking or distortion.
• Porosity and Inclusions: Brazing and soldering processes can sometimes leave behind voids or inclusions that can affect the weld quality. These defects can become trapped during the welding process and weaken the resulting joint.
• Joint Geometry: The geometry of the original brazed or soldered joint can impact the welding process. Irregular joint configurations may make it challenging to achieve a consistent weld bead.

Preparation: Key Steps for Success

Proper preparation is paramount for a successful weld over a pre-existing joint. This includes:

• Cleaning: Thoroughly clean the surfaces to be welded. This involves removing any oxides, fluxes, contaminants, or residual brazing/soldering material. Mechanical cleaning methods like wire brushing, grinding, or sandblasting are often necessary. Chemical cleaning might also be required, depending on the materials involved. Ensure that the surfaces are free from any grease or oil that might interfere with proper weld penetration.
• Preheating (if necessary): For certain materials, preheating the workpiece can reduce residual stresses and improve weld quality. The necessary preheating temperature depends heavily on the base metals and their thickness. This step is especially critical for larger sections or thicker materials to mitigate the risk of cracking during the rapid cooling phase after welding. Always consult appropriate welding codes and standards to determine the required preheating temperature.
• Joint Design: Evaluate the original joint's design and consider any modifications that might improve weldability. This might involve adding additional weld reinforcement for added strength or altering the geometry to improve heat distribution during welding.
• Fixture and Restraint: Proper fixturing and restraint of the workpiece during welding are crucial to prevent distortion and ensure that the weld bead is placed accurately. Clamps, jigs, or other methods should be used to hold the workpiece securely in place.

Welding Techniques and Considerations

The choice of welding technique will depend on factors such as the base material, joint design, and available equipment.

• Gas Metal Arc Welding (GMAW): A versatile process offering good control and penetration. However, it requires careful control of the arc length and wire feed speed to avoid overheating the existing joint.
• Gas Tungsten Arc Welding (GTAW): Ideal for precision work and applications requiring high-quality welds. Excellent control of heat input makes it suitable for welding over delicate brazed or soldered joints. However, it is slower than other methods.
• Shielded Metal Arc Welding (SMAW): A portable and robust technique, suitable for field work. However, it requires more skill to control the heat input and minimize the risk of overheating the joint.

Post-Welding Procedures

Post-weld procedures are critical for ensuring the integrity and longevity of the repaired joint:

• Stress Relief: For certain materials and applications, stress relief annealing may be necessary to reduce residual stresses and prevent cracking. This involves heating the workpiece to a specific temperature, holding it for a certain duration, and then slowly cooling it.
• Inspection: Thorough inspection, including visual inspection, and potentially non-destructive testing (NDT) techniques such as radiographic testing or ultrasonic testing, should be performed to assess the quality of the weld and ensure its integrity.
• Cleaning: Remove any slag or spatter from the weld area.

Conclusion: A Delicate Balance

Welding over brazed or soldered joints is a specialized procedure demanding precision and expertise. The process requires careful planning, meticulous preparation, precise execution, and thorough post-weld inspection. Failing to address the challenges associated with different melting points, residual stresses, and material compatibility can compromise the joint's integrity, leading to premature failure. Always prioritize safety and consult relevant welding codes and standards before undertaking such a project. If unsure, seek the assistance of a qualified welding professional to ensure a safe and successful outcome. The success of this intricate process relies on a thorough understanding of the limitations and a dedication to the highest standards of craftsmanship.