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Soldering and brazing

Soldering and brazing are metal joining processes where the two parts being joined are joined by filling the gap with some metallic filler. This is done by heating the two parts to a temperature where the filler will melt, fill the gap and bond the parts together.

The difference between soldering and brazing is essentially set by the melting point of the filler. At the lowest temperature a solder is usually an alloy of tin and lead. These cover about 200º up to about 400º. at higher temperatures the solder usually contain silver. These silver solders can be used up to about 1000º. At temperatures above this the filler is often copper based, one common one would be brass alloys, hence the word brazing.

When soldering or brazing it follows that the melting point of the filler must be lower than that of either of the parts being joined.

In general the higher the melting point of the filler the stronger it is.

There are six key steps in making a soldered or brazed joint. The following section are very generalised. Exceptions are covered at the end.

 

The design of the joint

All soldered or brazed joints depend upon capillary action, that is a liquid filler will flow into a gap and fill it so long as the gap is not too narrrow nor too wide. The range is, roughly, about 0.02mm up to about 0.20 mm. This is extremely useful because it means gaps do not have to be specially made they are the sort of gap that is usually present when two surfaces are brought into contact.

In general the larger the area of contact the stronger the joint will be. For example, if two pieces of copper sheet are to be joined end to end, the end surfaces could be solder/brazed but if they overlapped the join would be stronger.

Where the thermal coefficient of expansion for the two parts is very different this can mean that, at the temperature the filler melts, the gap might be much larger or smaller than expected.

 

The surfaces must be clean

There are two types of dirt that will spoil a join. Firstly there is grease. Secondly there is scale. If both are present then it is best to degrease the surfaces first. This can be done with acetone. All metals will have some sort of oxide contamination the flux will often get rid of this but it is always worth cleaning the surface with emery paper.

 

Most soldered/brazed joints need flux

Even if a joint is cleaned with emery paper it will still need flux to be sure of making a good joint.. The right flux will have little effect till it is heated to the temperature around that at which the filler melts. At this temperature the flux will dissolves any residual oxides. It will cover the joint and prevent any further oxidation. Flux can be applied to the joint as dry power or paste. It is said that adding a drop of detergent to the paste makes the flux cover the surface more easily. As the joint is heated up the flux will dry out and then melt. It will spread over the surface of the metal near the joint and it will be sucked into the joint by capillary action. When it reaches the right temperature it will go clear and it will be possible to see through it and the metal underneath will appear bright and clean.

As the soldering proceeds the flux is used up and becomes inactive. It is necessary to make sure there is enough active flux until the joint is finished. This can be added as the job is being heated up. One way is to use a steel strip to put it on the join. It is also possible to heat a silver solder rod and dunk it in the flux powder some of which will stick to the rod.

In general the solder will only flow in area where the flux has been – no flux – no joint.

Traditionally the flux used for silver was ordinary borax. The best fluxes to use are the “easy-flo” fluxes made by Johnson Matthey.

 

The joint is assembled for brazing

Almost by definition the joint has two parts. When they are soldered/brazed they have to start in the position where they are wanted. furthermore they have to stay there till the joint has gone cold and solid. The easiest way of doing this is to design the joint so that it cannot move apart. for example, a tube could be soldered onto a flat plate. It will be found that it is possible for the tube to “float” on the solder and move. One solution would be to machine a groove in the plate. Another would be to fit three small pins in the plate to hold the tube. Another would be to have a weight, fixed at some other point, weighing down on the tube. The friction between the tube and the plate will stop it moving.

In general if the two parts can move relative to each other it can be assumed that they will.

Gravity is what makes most solderi/brazing possible.

It is often useful for the path we would like the solder to take to be horizontal.

 

Soldering/brazing

The whole area to be brazed has to be brought up to the temperature at which the filler melts. It is best to heated this area evenly. If the workpiece is very large it might require a lot of heat, not only because is heavy but because its surface are might be very large a will radiate heat away. This is a real problem when the metal is highly conductive like copper. In cases like this it can be useful to surround the workpiece with ceramic bricks or ceramic blanket.

When the joint has reached the right temperature the tip of the rod can touch the joint. It will melt and flow into the gap. It can be drawn along a gap by moving the source of the heat along the gap adding filler as required. If the source of heat hits the rod directly the rod will melt and the solder will probably fall anywhere but in the gap. The trick is always to heat the workpiece then apply the filler to the workpiece.

Where a very small amount of solder is required it can be melted onto the end of a steel rod. When the join is heated up the heated rod is applied to the joint.

 

Cleaning the joint

All fluxes by their very nature are potentially corrosive. When the joint has been finished it is essential to remove the remains of the flux used. The fluxes used at higher temperatures tend to produce a hard, glassy substance. When it is dry it appear to be harmless but when it is left it absorbs water from the air and is very corrosive. It is essential to remove it. Unfortunately it can be very hard and almost insoluble in cold water. However it is usually soluble in very hot water and if this fails it will dissolves in dilute hydrochloric acid.

 

Exceptions

 

Electronic soldering

Electronic joints are often soldered. solder is specially made for this. This solder used to contain lead. Lead is now banned and solder for electronics is largely made of tin. What the amateur uses is like wire but this “wire” contains the flux necessary to make perfect joints. Whats more when the joints are made the flux appears to vapourise so no cleaning is necessary afterwards.

The flux is usually sufficient to clean most components as supplied. However very old components can often have tarnished leads and these need a bit more effort to solder.

The procedure it hold the two parts next to each other so they can both be heated at the same time by the soldering iron. While this is being done the joint is touched with the tip of the solder wire. This will melt and solder will cover both parts and fill the gap between them.

 

Soldering carbide tips onto steel shanks

This is a useful way of making carbide tipped tools. It can be very difficult to “wet” carbide. Also not all pieces of carbide are made of the same stuff. One way of cleaning a carbide surfaces is to grind the surface off.

Carbide and steel have very different coefficient of thermal expansion. One way of getting round this is to put another metal like brass shim between them.

 

Sweating

Where two part have to be join and there is a large surface are between them then one way on making sure a good contact is made between them is by “sweating”. In this both surfaces a pre tinned. This word dates back to when this was done with tin solders. Both surfaces are covered with the filler. The surfaces are placed together and heated up. When the filler melts the surfaces are pressed together. The excess filler is squeezed out and form what looks like drops of sweat round the edges of the joint.

 

Soldering nichrome wire

Some metal like nichrome cannot be soldered with ordinary solder. But they can be soldered with silver solder. When it is silver solder it can then be soldered with tin solder.

 

Saving silver solder

Silver solder is considerably more expensive than any soldering or brazing material. It pays to collect all the little bits drops, spilt or left over. Any rod like bits can be soldered and to end and then used up as rod. Any other bit can be chopped up into small bits. These are used on small joints by placing them in the flux as the joint is heated up.

Silver solders and cadmium

Till recently, the best silver solders all contained cadmium. Cadmium is a highly toxic material and is now banned for almost all applications. The old silver solders were called Easy flo xxx. The have been replaced by silver solders with names like “Silver flo xxx”.

 

Step soldering/brazing

Where a the workpiece is made of several parts all of which are to be silver soldered together then this can be simplified by using a technique known as step soldering. This term is often used even when the process is really brazing.

In this some parts are assembled using a solder that melts at a relatively high temperature. Other parts are then soldered on using a solder with a lower melting point. This can be done any number of times so long care is taken to control the maximum temperature. Of course, the maximum working temperature of the workpiece is going to be limited by the melting point of the solder with the lowest melting point. On the other hand it is possible to assemble some parts at a much higher temperature than it would be possible to achieve with the whole assembly.

For example, on a model boiler for a steam engine, the boiler tubes can be soldered using the highest temperature solder to the front of the fire box. Then lower temperature solders can be used for other parts as they are soldered together.

In practice all model boilers are made using only silver solder. These are the only “solders” that have the strength required at the temperatures the boiler will be working at. If brazing is taken to mean a solder containing brass then these cannot be used because brass contains zinc and at high temperatures/pressures the zinc can be leached out of the soldering/brazing material.

 

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