Metal handling is very significant for engineering processes. Metals possess unique properties that make them useful in engineering processes. As a result of these properties, metals are used in various construction processes such as building of bridges, houses, automobiles, machines and various kinds of structures. There are basics need to performed before processes as metals have to be joined, cut or even bent. This brings in a number of engineering practices as soldering, brazing, welding and even cutting. Plasma cutters leveraging high end technologies are used in cutting metals and joints can be joined using soldering, brazing or welding. Soldering, brazing and welding are very close kinfolks. The three processes can be used in joining metals, but each of them leverages different metal joining principles.
In the soldering, two or more metals are joined by melting a filler metal, also known as solder, and it flows into the joint. In this metal joining process, the filler metal has lower melting point than the various metals involved in the joining. This is a major difference between soldering and brazing. In brazing, the filler metal should be able to melt at a temperature higher than that of the adjoining metals but the reverse is the case in soldering. In welding, the adjoining metals have to be melted and then joined but this is not the case in soldering. Here, only the filler metal is melted which is then used in joining the work piece.
History has it that soldering and brazing are age long practices. Historical evidences even state that these metal joining practices were practiced in about 5000 years ago in 4000BC. In the ancient time, the metal joining process was employed in producing cooking utensils, jewellery, and so forth ("Soldering”).
In the subject of soldering, it is very important to understand its subdivision. Soldering is a very broad subject, moreover; it has two major applications which also have sub-applications called plumbing soldering and electrical soldering
The major difference here is the chemical contained in the solder. Solders contain a chemical that is used in cleaning the surface of the metal after the joining processes. In the case of the plumbing soldering, the chemical used is an acid known as acid core solder. The acid will not work well for electrical systems. If the solder is used in an electrical system, the acid will corrode the electrical wiring. In the case of electrical systems, the solder used is known as rosin core solder. This is simply because pine rosin was formally used as a chemical for cleaning the joined metals, although it is not mostly used any longer ("Soldering" )
Figure 1 A Soldered Pipe
Oxidation is a very common process that occurs in heated metals. Oxidation is simply the reaction of a substance (usually a metal) with oxygen. When it occurs, an oxide is said to be formed. This is a chemical process which changes the composition and properties of the substance. Consider this chemical equation below for the concept of oxidation.
2Cu + O2 → 2CuO
Here, copper reacted with oxygen and copper (II) oxide is formed. This usually occurs when the metal is heated and exposed to air. Of course, exposure to air can easily happen except in an absolute vacuum. Oxidation will mar the soldering process. It will make the bonds weak and, therefore, the soldered metal can easily disintegrate.
In order to preventing this from taking place, the metal is first treated with soldering flux before soldering begins. Soldering flux is formulated to get rid of oxides from the metal before the application of solder. This will help in ensuring that the metal will not oxidize during process of heating. The flux helps to prevent oxidation and ensured that the metal and the solder can dissolve into each other. Consequently a tight seal will be created.
There are lots of downsides of not using flux before soldering. Corrosion (partial or complete degeneration, softening or dissolving of a substance through electrochemical or chemical reaction with the environment) can likely set in. The seal between the two metals prevents any breaking. In pipes, such breakaways can result to leaks, causing damage in homes. This is often seen on a daily basis, and it is likely going to happen if flux is not used in soldering.
The flux used in soldering should be dependent on the purpose of the metal or structure. For instance, metals used in water-based systems require a water-based flux. Water-based fluxes are fluxes that are most suitable in pipes that are connected to drinking water outlets of a home. It is significant to ensure that the flux and other materials used in the soldering processes are safe for use in order to make sure the health and safety of the users do not suffer. This is even why the EU standard recommends the use of lead-free solders in soldering ("Soldering”; "Why do I need to use flux when soldering metal plumbing”).
The flux used in cleaning the metal prior to welding is a major difference when it comes to the differentiating plumbing soldering and electrical soldering. In the plumbing soldering, the flux cores are an acid, however, for electrical soldering; the flux core used is a rosin acid. The rosin acid here is a mild acid that will not damage the electrical system.
Solder is an alloy of tin (Sn). In most cases, (especially in electronics) an alloy of tin (Sn) and lead (Pb) is used for the process. The lead melts at a lower temperature comparative to tin and thus by using more lead, a lower temperature can be easily achieved. In normal practice, proportion of lead and tin in the solder varies. In some cases it is 60 Sn/40 Pb, 63 Sn/37 Pb and so forth.
Figure 2 Lead-Free Solder
According to the EU standard, lead-free solders should be used for plumbing as well as electrical soldering. Most especially, soldering of water-based systems, pipes and accessories should be done with lead-free solders. This standard is based on environmental consideration. The standard ensures environmental friendliness.
What is the essence of lead in soldering? Lead is used to lower the melting point of the solder as it has lower melting point than tin. However, lead is poisonous. Taking lead into the body can result to a condition known as lead poisoning which has symptoms like anaemia, constipation, palsy, weakness, colic and paralysis of the wrists and ankle. Nonetheless, using lead in solders makes the soldering process easier than when it is not used. Today, many lead-free solders are manufactured and they are advisable for soldering ("What are the different types of solder used for?”).
Another issue that calls for consideration is the filler metal used in soldering. It is a well known fact that soldering is done by filling two metals with a filler metal. The question is: which filler metal is most suitable for the process? Obviously, the strength of the joint will be largely determined by the filler metal used. Previously, the filler metal used for soldering was an alloy of tin and lead. However, in line with the EU standard, lead based filler metals are proscribed.
The concept of soldering remains the same. The filler metal should be one below the melting points of the adjoining metals. This is of course the basic difference between soldering and brazing. According to American Welding Society, the filler metal used in soldering should be able to melt below 840oF/450oC. When the filler metal melts below this temperature, soldering can be said to take place but when it melts above the temperature, brazing can be said to occur.
Alloys are the best for filler metals because of their strength and workability. Pure metals are very difficult to work with and hence in such a system, they cannot be used to produce the required strength for soldering. Soldering is still based on tin, however, because of the Safe Drinking Water Act, lead bearing solders are no longer allowed. Prior to 1986, lead and tin combination was in use, and the alloys often consist of 50 Sn/50 Pb. However, today, other metals can be used to alloy with tin rather than lead. The filler metals today could be an alloy of tin and nickel, silver, bismuth, antimony or probably copper. In the plumbing, an alloy of copper and tin is often used as the filler metal.
The filler metals used in brazing and soldering are not the same. In brazing, the filler metal used could be BCuP alloy (consisting copper and phosphorus) or BAg alloy (consisting silver and some other metals in minor quantity) ("What are the different types of solder used for?" ).
Strong joint is the primary aim of metal joining process. Therefore, whether you are using soldering or any other metal joining process, you must ensure that the joint is strong. There are some specific processes to ensure strong joint and brazing in soldering and they should be followed. These processes have to do with the end preparation of the metal, cleaning of the metal and the appropriate application of the filler metal and heat during the process. There is a need to insert a tube in the fitting at the back fitting cup during soldering.
The manufacturing standards ASME/ANSI B16.18 and B16.22 specified the required standard for solder pressure fittings and also the standard for the depth of overlap/or depth of the socket in a capillary or lap type joint fitting. This dimension is very important because the filler metal ought to be melted into the capillary space in order for it to flow to the back of the fitting cup and join the tube and the fitting. The ideal in this case is 100% fill of the capillary space fitting and penetration. However, if 70% filling is obtained or the void in the tube is less than 30 percent, the joint formed will be able to withstand any form of pressure and, therefore, is the optimum for soldered copper tube and fitting systems ("Why do I need to use flux when soldering metal plumbing?" ).
Figure 3 Tube
Soldered and brazed joints differ when it comes to the amount of joint overlap or fill required for developing full strength of the joint. Complete fill of the jointing space is not necessary throughout the length of the tube for providing strength in brazing, but insertion of the tube is still compulsory at the back of the fitting cup. In the soldering, however, it is necessary to fill the joint space throughout the length of the tube for maximum strength to be achieved and for the joint to be very strong ("What are the different types of solder used for?”).
Solders can be sub grouped as soft and hard solder. This classification is based on their strengths and melting points. Soft solders are simply alloys of tin and lead, nowadays other metals like bismuth are used in the place of lead. Hard solders are alloys of copper ,silver and zinc or simply copper and zinc. Soft soldering is commonly used in the plumbing soldering ("Clean, Flux, Heat, and Solder”).
Soldering can be summarized into six steps which include:
- Mechanically clean the surfaces to be soldered,
- Ensure that they fit each other; every gap has to be closed,
- Apply flux on the join and ensure that it covers the join on both sides. You can even rotate your join and do this,
- Heat the joint vigorously and when it is hot enough, dip the end of your solder wire in the flux paste. The solder will flow into and around of the join; apply more and ensure that the solder is visible around the join,
- Remove the heat and cool,
- Wash the composite in a detergent to remove flux traces. You can remove solder by scrapping and sanding.
Soldering is one of the basic and principal metal joining processes which can be used in joining similar or dissimilar metals. Extensive understanding of the principles involved in this metal joining process is the highway to becoming excellent in the process. Plumbing soldering is not the same with electronic soldering, and you must not use the same flux and filler metals. It is important to follow the basic precautionary measures to achieve the required result when soldering.