Soldering and brazing

Soldering and brazing are the processes of joining metal pieces, making use of heat, and a filler metal whose melting point is lower than the melting points of metals to be joined. In soldering, the melting point of solder (or filler metal) is usually less than 427°C, whereas in brazing and braze welding, the melting point of filler metal is higher than 350°C. In soldering and brazing, there is no direct melting of the base metal of workpieces being joined, rather the solder or filler metal flows between the surfaces to be joined through the capillary action. The most common use of soldering and brazing is in joining two dissimilar metals. Differences between soldering and brazing are given in the following:

(a) The melting point of solder or filler metal used in soldering is lower than 427°C, whereas in brazing, the melting point of filler metal is higher than 427°C.

(b) Soldered joints do not resist corrosion to the extent that brazed joints can do.
(c) Brazing gives a stronger joint which can stand to higher temperature service.
(d) In brazing, bonding conditions are set up to allow large amount of diffusion to take place along the surfaces being joined, whereas in soldering, diffusion is of secondary importance. Diffusion bonding refers to the metallurgical joining of metal surfaces by the application of heat or/and pressure to cause co-mingling of atoms at the joint interface. The interface surfaces should be very clean and free of contamination. Diffusion bonding in short instances is accomplished entirely in solid state.


Soldering is defined as a metal joining process wherein coalescence is produced by heating the surfaces to be joined to a suitable temperature and melting the filler metal, which is a fusible alloy called solder (melting point usually less than 427°C), so that it may be distributed between properly fitted surfaces of the joint through the capillary action. Soldering operation is performed by bringing molten solder in contact with the pre-heated surfaces (being joined) and heating the joint area to a good wetting temperature (about 55 to 80°C above the melting point of soldering alloy). The solder is then left to cool and freeze as quickly as possible to avoid development of internal microcracks in the joint. The principle underlying soldering is that when the surfaces to be joined are cleaned off well from oxides, they can be joined together using molten solder that may adhere easily to the workpiece surfaces due to molecular attraction. The molecules of solder entwine with the parent metal molecules and form a strong bond.


Brazing is a technique of joining two similar or dissimilar metal pieces together by heating the surfaces and by using a non-ferrous filler metal having its melting point above 427°C but below the melting points of metals to be brazed. The molten filler metal is distributed between the joint surfaces by the capillary action, which on cooling results in a sound joint. The main advantage of brazing process is the joining of dissimilar metals and thin sections. The process i is mostly used for joining pipes and other fittings, carbide tipping on tool shanks, electrical parts, radiator, repair of cast iron parts and heat exchangers.

In brazing, bond is produced by the formation of either solid-state solution (diffusion bonding) or intermetallic compounds of the parent metal (job) and one of the metals in the filler material (brazing alloy). The strength of the brazed joint is provided by metallic bonding.

Leave a Reply

Your email address will not be published.