The basic theory and technique of soldering and brazing are the same for all diameters of copper tube. The only variables are the filler metal and the amount of time and heat required to complete a given joint. The American Welding Society defines soldering as a joining process which takes place below 840°F and brazing as a process that takes place above 840°F but below the melting point of the base metals. In actual practice for copper systems, most soldering is done at temperatures from about 350°F to 600°F, while most brazing is done at temperatures ranging from 1100°F to 1500°F.
The choice between soldering or brazing generally depends on the operating conditions of the system and the requirements of the governing construction codes. Solder joints are generally used where the service temperature does not exceed 250°F, while brazed joints can be used where greater joint strength is required or where system temperatures are as high as 350°F.
Although brazed joints offer higher joint strength in general, the annealing of the tube and fitting which results from the higher heat used in the brazing process can cause the rated pressure of the system to be less than that of a soldered joint. This fact should be considered in choosing which joining process to use.
Although soldering and brazing are the most common methods of joining copper tube and fittings, they are often the least understood. It is this lack of understanding that can develop into poor installation techniques and lead to poor or faulty joints. Investigations into the common causes of joint failures revealed several factors contributing to faulty joints, including:
- Improper joint preparation prior to soldering.
- Lack of proper support and/or hanging during soldering or brazing.
- Improper heat control and heat distribution through the entire joining process.
- Improper application of solder or brazing filler metal to the joint.
- Inadequate amount of filler metal applied to the joint.
- Sudden shock cooling and/or wiping the molten filler metal following soldering or brazing.
- Pre-tinning of joints prior to assembly and soldering.
Although soldering and brazing operations are inherently simple, the deletion or misapplication of a single part of the process may mean the difference between a good joint and a failure.
Additional information on the various steps involved in soldering and brazing copper tube are given in the Soldering and Brazing Application Data Sheet (English version in pdf format, about 700kb), Soldadura & Soldadura Reforzada para Tubo de Cobre y Enlaces (Spanish version in pdf format, about 700kb) and in the Copper Tube Handbook. In particular, the following sections of the Handbook have specific information on soldering and brazing copper tube: