Brazing / Silver Soldering.

The terms Brazing and Silver Soldering are often used synonymously by the model engineer.
Correctly, brazing is the use of brass as the filler material, whilst silver soldering uses a rod which is composed of a mixture of silver /copper /zinc and possibly cadmium.
This is almost always what is used today.

There are no "secrets" to silver soldering and it is not as difficult as is sometimes supposed.
However, to make a satisfactory joint does require a certain amount of attention to the following details, and practice always helps.

1. GAP. Silver soldering works by capillary action. Therefore there must be a gap to allow the solder to flow. Whilst the ideal gap varies for different solders, typically it is between 1.5 and 3 thou. If the assembly which is to be soldered is bolted or riveted together before soldering, ensure that there is a gap for the solder to fill. A piece (or pieces) of shim steel placed in the gap will enable part of the joint to be soldered an these can then be removed so that the joint can be finished.

2. CLEAN. Cleanliness is essential. All surfaces to be joined should be cleaned thoroughly, during manufacture and
immediately prior to assembly. A good scrub with steel wool so that all surfaces are bright, just before soldering is the
traditional method.

3. FLUX. The correct flux must be used, and in the right quantity. Sufficient should be applied to ensure that the surfaces to be joined are well coated with flux, but if the parts to be joined are adjacent to items where solder is not required, it is best if the flux can be kept off them to prevent the solder flowing where it is not wanted. It is better to use too much to ensure the joint is sound rather than too little.

4. RETAIN. The parts to be joined will need to be retained in place whilst they are soldered. Temporary bolts or rivets, wire twited round them, weights placed on parts, and similar methods can be used according to the circumstances.

5, HEAT. Silver solder requires temperatures of 600 ^o C up to 800 ^o C, or in some cases, even higher. The two usual heat sources are propane and oxy-acetalene, with propane being generally considered to be easiest for the less-experienced
constructor. The heat should be applied to the metal, not to the solder. The metal to be joined should be heated to about a red heat. The flux will bubble and eventually become clear. That is about the right temperature for soldering, and the solder should be applied to the work. It often helps to make a neat joint is the heat is applied underneath the metal which the joint is on top.
With small items, it is sometimes possible to place them on a sheet of stainless steel and heat that from below.
The solder will flow towards the hottest area, so by applying heat directly below the parts to be joined it will help to ensure that the solder flows into the joint.

With large areas of copper, such as a boiler, it will be found that the copper conducts the heat away so rapidly that it is often hard to achieve a high enough temperature. A second heat source can be used, or the whole assembly placed in a hearth filled with coke, or ceramic pellets, or else a screen of firebricks placed around the assembly to contain the heat.

Most people have their own method of working. I prefer to use one or two sources of heat, according to the size of object, and use these to raise the temperature of the whole assembly to, perhaps 400 degrees, and then use a smaller burner than would normally be used to add "spot heat" to the item to be joined.


6. CLEAN. After all soldering is complete, allow the assembly to cool and then place in a bath of "pickle" to remove the remnants of flux. Dilute sulphuric acid is the most commonly used.

The strength of a properly made silver soldered joint will be close or equal to the strength of the parent metal.

Two safety notes, always add the acid to the water, and, do not put the assembly in the acid whilst it is hot.
Boiling acid sprayed around is not good for one's  health!

Silver Solder.

Silver solder is an expensive item. The actual price varies according to the proportion if silver, and
unfortunately, for much of model engineering, the requirement is for those silver solders which have a high silver content.

This is particularly so for copper boilers. The designers usually specify the silver solder which they use, and this will almost always be Easyflo or Easyflo2. (Johnson Mathey designations).

Even though it is expensive, it is false economy to use anything else. The cost of the boiler, and the
completed is such that the saving in using a cheaper alternative, which may not last, is minimal.

In particular, do not be tempted to use a silver solder containing phosphorous. It will cause corrosion and fail, often quite quickly.

The have been many warnings about the hazards of cadmium, and the makers of silver solder now produce cadmium-free variants. For the small amounts that most model engineers use, and provided it is in well ventilated conditions, the actual risk from using the cadmium containing solders is minimal.

ALTERNATIVES FOR PICKLING.
The usual choice for picking after silver soldering is dilute sulphuric acid.

If you cannot get,, or do not like, using sulphuric, acetic acid works, albeit more slowly.
Acetic acid is available from chemists or winemaking stores.

At a pinch, vinegar can be used.

HEAT TREATMENT.
There are many occasions when one needs to heat-treat steel, especially to form a cutting surface.

The basic process is as follows.
The first stage is to heat the steel to a medium red then plunge into water. Then clean it and heat again to a lower (tempering) temperature, and allow it to cool.

After the initial heating and cooling it will be hard, probably too hard and brittle to be useful. Tempering will leave the steel with the correct hardness to withstand the intended use.

1. Heat the steel slowly, and keep it at red heat for some time to ensure that it has reached the same temperature throughout.

2. Immerse in a bath of water. The speed of cooling is important to the final characteristics of the steel. In many cases, brine is a better coolant than plain water, whilst with some thin sections and steel with some additives, even water is too effective a coolant and can cause the material to develop cracks. In these cases, a thin layer of oil on top of the water will give a protective film to the steel that will prevent this happening.

3. When the steel has cooled, clean and polish it thoroughly so that you can see the surface of the metal.

4. Then slowly heat it up to the TEMPERING temperature,

The biggest problem is gauging the correct tempering temperature, which is far lower than many people imagine. Depending upon the intended use, it may be as little as 200^o C, compared to the 850^o C or 900^o C of red heat.
(Ordinary 60:40 soft solder melts at less than 200^o C).
Many deep-fat fryers reach a high enough temperature and can be used to temper tools, and do many of the heat guns which are used to remove paint can reach 250^o C or so.
The easiest way to gauge the temperature is by examining the colour of the steel. A change will not be seen whilst it is in a flame, but by looking at the steel in subdued lighting, a band of colours will be seen to develop as the temperature rises.
It is usually best to apply the heat away from the cutting edge so that the colour can be seen more readily.
With experience one can gauge the colour easily, and as a guide, there is a colour-comparison chart,
in the DOWNLOAD pages.

5. Allow it to cool slowly. If you wish to delay the rate of cooling, the tool can be buried in sawdust or warm dry sand.