This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
E. A. Suverkrop
In spite of the fact that there are hundreds of brazed flanges on the average steam ship it is surprising how few engineers know anything at all about the job. This is no doubt due to the feeling among engineers in general of "every man to his own job." The ability to do a fair job of smith work is no doubt of great assistance to the engineer at times and why should he not be able to do a fair job of brazing? Brazing is considerably easier than smith work. I Would undertake to make a good brazer of any intelligent man in from twenty minutes to an hour, but who could make a smith of a man in that time?
I will first take the general principles of brazing under consideration. Brazing is the joining together of two pieces of metal by means of another metal having a lower melting point. It is practically the same as soft soldering excepting that it is done at a higher temperature and the solder of spelter used is an alloy of copper, tin and zinc or copper and zinc instead of tin and lead. The joints to be brazed should be as nearly clean bright metal as the job permits. Grease and dirt are antagonists to a good job. The parts to be brazed should be securely held in relation to each other, either by pins put through them or by wiring to each other or by other means. The heat applied should also be clean. Hard coal fires are fairly pood, a fire of soft coal charred is also good, but perhaps the best is a gas and air blast. Gasoline or oil blast also give good results.
In brazing a flux must be used. Formerly borax in one form or another was the only thing that would do. Some brazers used it powdered, some in crystals, some mixed it with water, some didn't. Some melted it in a crucible; it was then called "burnt" borax. It was then broken to various degrees of fineness according to the individual whim of the brazer, each one of whom would declare that his way of breaking the borax was the prime reason why he produced good work. The drawback about borax is that while it is a good flux it becomes as hard as glass after the job is cold. In this state it is difficult to remove and ruins the hardest file. A better flux than borax in any form is boris (also called boracic) acid. It comes in crystals or powder. 1 prefer the crystals for some work as the powdered form is apt to curl up and blow away while the crystals do not. Boric acid does not form a hard scale and if you know how to handle it does not leave any scale that cannot be almost rubbed off with the hand.
In jointing up ready for brazing the joints, one need not leave room for the brass to run in. They can be drive-fits, for if the heat and flux and spelter be applied in the right manner the brass will run into the tightest joint. The beginner, however, had better not fit his pieces too tight; just make them an easy drive fit, put one or more pins in to hold the pieces together.
We will assume that we have a steel flange to braze on a steel pipe. The flange has been bored or filed to fit on the end of the pipe which has also been filed or burned bright, the joint is clean, bright metal to metal. The job should be put in the fire so that the pieces are heated as evenly as possible. In this case we will assume (what is generally the case) that the flange is heavier than the pipe. It stands to reason that it will not heat as quickly as the pipe which is lighter, we therefore heat the flange first, placing it in the hottest part of the fire. While the. heat is coming up the brazer applies the flux to the joint with a brazing • spoon." I generally make my brazing spoons out of a piece of 1/4 or 5/16 inch iron rod of suitable length, say two feet, with the end heated and beaten out flat about 5/8 inch wide by 1 1/2 inches long. As the heat increases and the flange and pipe become a dull red the flux melts and runs all over the job, some of it runs into the joint where we want it but the bulk of it is lost. As it is cheap this does not signify. When the job reaches a so-called cherry red, that is to say, a heat at which one would temper a chisel, it is time to put a little spelter on as this heat is very near the melting point of the spelter. The spelter is applied with the spoon together with more flux and is guided into the top of the joint by the spoon. In a few moments the brazer will notice that the spelter has begun to flow and run about all over the joint almost the same as mercury. The heat is kept on a little longer, say a minute or so according to the size of the job, while more spelter and flux is applied. The job is then lifted from the fire, being handled carefully so as not to jar it, and while still red hot the flux is brushed off with a steel brush. This treatment applies especially when borax is used as it eliminates a good deal of hard work later. The job is then allowed to cool till it is at such a temperature (a dark red) that immersion in water will not harm it. It is then put into a saturated solution of sal soda water or strong soap water. When cool the scale is easily brushed off, leaving the steel and brass clean and bright.
The spelter to be used varies according to the material on which it is to be used. On steel any kind of brass can be used, as the melting point of any brass is below that of any steel. I have also brazed steel with copper when no brass was at hand.
For brass and copper a soft brass with a low melting point must be used and great care must be exercised in order to avoid melting the job itself, especially so if it is brass, as the chances are that there may not be much difference between the melting points of the job and the spelter.
It is often desirable to repair a piece of cast iron that has been broken. A great number of very good brazers assert that cast iron cannot be brazed, and an equally large number of people having some patent process or another assert that it can but only by their particular method. Both are wrong, cast iron can be brazed with common brass and common borax or boric acid. There is no mystery about it. The joint should be thoroughly clean, the pieces should be firmly pinned together so that they cannot alter their position with relation to each other, the heat should be applied slowly and steadily while plenty of flux and spelter are applied and the job should not be hurried in any way. Let the job stay in the fire for a long time, to use a blacksmith's phrase, "Let it soak in the fire." A braze made in this way will be just as good if not better than one made by any of the patented or secret (?) processes.-"The Marine Review."