Best Brazing Practise

		copper boiler

Since we found the first evidence of brazing in the artefacts from the tomb of King Tutenkhamun and pictures of the process used on the walls, the brazing process has been used to make joints between a wide range of materials. It is a widely used process with its' own fundamental principles. Understanding these leads to the production of strong, leak free joints. Deviating from them leads to problems.

One definition in British Standards defines the process as "a process of joining generally applied to metals in which, during or after heating, molten filler is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined, by capillary attraction" 

The key words are by capillary attraction. Everything that you do, joint design, fluxing, heating is aimed at promoting capillary flow. If this is not done you are not brazing. You are not getting all the inherent benefits and advantages of the process. You are simply using a very expensive filler rod to block a hole!

Everything that you do should be aimed at maximizing capillary flow.
Controlling Joint Gaps.
Cleaning of the joint at 600 deg C - the function of the flux.
Developing heat patterns to encourage the flow of alloy.


See also the section on Joint Strength and Silver Soldering Tips.

As in all cases, maximum joint strength is achieved in a brazed joint when the silver solder fully penetrates the joint gap. To obtain that penetration, the joint gap must be controlled within certain tolerances dependent on the parent materials and silver solder. The alloy will not penetrate a gap that is too small or too large. The joint gap when using 842 or 456 to silver solder copper should be 0.002 - 0.006'' (0.05 - 0.15mm). If a wider melting point solder is being used eg 440 the gap should be 0.002 - 0.008'' (0.05 - 0.2mm)

The important thing to remember is that this is the gap at brazing temperature. Allowance must therefore be made for the effects of thermal expansion which may cause the gap to increase or decrease. This is more prevalent when brazing dis-similar materials. Brass expands more than steel so the joint gap can increase or decrease depending on the joint design.

When brazing tubes or stays into boilers it should be remembered that the recommended joint gaps are radial gaps. If one side of a tube or stay fits tight against a plate there may not be a gap to allow silver solder to flow into the joint. The alloy will flow by capillary action around the edge of the joint but will not penetrate the joint. One of the greatest contributing factors to joint strength is joint soundness ie the absence of voids or the complete penetration of the silver solder into the joint.

We do not want tight fits. We want "tight rattles"!

No gap = no flow = no joint!

Measures should be taken to ensure there is a gap even when parts are clamped together. Maintain a gap by inserting a joint gap gauge 0.13mm thick between the components or by centre punching dimples on the surfaces to be clamped.

The molten brazing alloy will flow through a gap towards the heat. Care should be taken to ensure all the joint is up to brazing temperature and the heating technique develops the ''heat pattern'' that will promote alloy flow into the joint.

What may appear visually to be a sound joint is in fact not, is purely superficial and may subsequently fail. Failure could occur at any time due to thermal stresses during cooling, reheating or in service.

When brazing take care not to overheat the alloy. This is best achieved by heating the joint - not the rod - and watching the flux. The flux melts, becomes a liquid, starts to do its job at about 570 deg C. The most popular alloy 842 melts at 610 deg C. Heat can be concentrated in the work by using refractory materials to reflect heat. Ideally these should be light eg vermiculite or kaolin wool.
Using common fire bricks can cause more problems than they solve. They invariably absorb heat, so slow the heating rate down and cause cold spots thay may hinder capillary flow.

The strongest joints made with silver solder are those where the joint overlap is approxiamately 3-4x the thickness of the boiler plates or wall thickness of the tubes.

After brazing, the assembly should be allowed to cool to ambient temperature. A common practice it to immerse it, still hot, into an acid bath to assist cleaning which it certainly does. However if it is too hot and contains tubes, steam pockets can be created that can blow the acid into the surrounding area. Far safer to let it cool and accept that it will take a little longer for the acid to clean the assembly.

Natural cooling will also prevent distortion and stresses caused by differential cooling rates that will be found if there is less mass in one part of an assembly than another. These stresses are sufficient to initiate joint failure.

Generally speaking copper is readily brazed using conventional brazing techniques but there are other factors which, if not taken into consideration, may adversely affect the formation of a satisfactory brazed joint. Take care when selecting your copper.

One feature to avoid is the presence or formation of refractory surface oxides which interfere with the wetting and bonding action of the silver solder. Don't clean components with emery cloth or ''sandpapers'' use a brass wire brush.

A second is metallurgical changes that can occur in the parent materials during the brazing cycle and which will affect the joint strength. The main problem arises in the brazing of oxygen bearing ''tough pitch'' or electrolytic grades of copper. These are designated C101 and C102 in the BS Schedules for Wrought Copper and Copper Alloys. The cuprous oxide particles in the copper are reduced by hydrogen at brazing temperature. At these temperatures, one of the products of this reaction is steam, which can seriously disrupt the structure of the copper and cause embrittlement. This is particularly critical in gas torch brazing. This phenomenon is referred to ''hydrogen embrittlement.'' It occurs most readily when oxy-acetylene torches are used. The blue part of the flame is very reducing and coupling that with the intense local heating easily induces the phenomenon.

The solution is to use only phosphor de-oxidized copper or oxygen free grades of copper. These are designated C103 and C106.

Avoid the use of plumbing grade copper that can contain lead. Lead is added to improve machinability and is present mainly as discrete insoluble particles throughout the metal structure. The particles are fully molten well before brazing temperatures are reached and this means that that under the inevitable thermal stressing conditions during the brazing cycle, parent material cracking can occur because of general structural weakness. Any lead being dissolved into the silver solder may also result in a brittle joint.