Soldering is a method of using a filler metal (commonly known as solder) for joining two metals without heating them to their melting points. Soldering is valuable to the Steelworker because it is a simple and fast means for joining sheet metal, making electrical connections, and sealing seams against leakage. Additionally, it is used to join iron, nickel, lead, tin, copper, zinc, aluminum, and many other alloys.
The two soldering methods most often used are soldering with coppers or torch soldering. The considerations and general instructions that apply to these methods of soldering are as follows:
- Clean all surfaces of oxides, dirt, grease, and other foreign matter.
- Use the proper flux for the particular job. Some work requires the use of corrosive fluxes, whilst other work requires the use of noncorrosive fluxes. Remember, the melting point of the flux must be BELOW the melting point of the solder you are going to use.
- Heat the surfaces just enough to melt the solder. Solder does not stick to unheated surfaces; however, you should be very careful not to overheat the solder, the soldering coppers, or the surfaces to be joined. Heating solder above the work temperature increases the rate of oxidation and changes the proportions of tin and lead.
- After making a soldered joint, you should remove as much of the corrosive flux as possible.
Sweat soldering is used when there is a need to make a joint and not have the solder exposed. This process is particularly suitable for electrical and pipe connections. To make a sweated joint, you should clean, flux, and tin each adjoining surface. Hold the pieces firmly together and heat the joint with a soldering copper or a torch until the solder melts and joins the pieces together. Remove the source of heat and keep the parts firmly in position until the solder has completely hardened. Cleaning any residue from the soldered area completes the job.
Seam soldering involves running a layer of solder along the edges of a joint with the solder seam joints on the inside whenever possible. The best method to use for this process is soldering coppers, because they provide better control of heat and cause less distortion. Clean and flux the areas to be soldered. If the seam is not already tacked, grooved, riveted, or otherwise held together, tack the pieces so the work stays in position. Position the piece so the seam does not rest directly on the support. This is necessary to prevent loss of heat to the support. After you have firmly fastened the pieces together, solder the seam.
Current research is aimed at the production of an alternative cheap low melting point, fluid and non-toxic solder, with tin zinc alloys appearing the most promising. This ensures an adequate area of filler metal to carry joint loads. Such joints need three conditions for successful soldering:
1. The correct heating cycle to allow the molten filler to flow and completely fill the joint.
2. The provision of the correct joint gap. Clearances of 0.07 to 0.25 mm are permissible between overlapping parts with approximately 0.1 mm being optimum for capillarity and joint strength. During soldering and solidification there should be no relative movement between joint faces and the use of jigs, self-locating designs or temporary solder-tagging is therefore recommended. The jigs and fixtures should not, however, act as local heat sinks that prevent efficient soldering.
3. Chemically clean metal surfaces obtained by degreasing, mechanically abrading and using a suitable flux. The earliest and most active fluxes are those water-based solutions such as the original `killed spirits', zinc dissolved in hydrochloric acid, latterly with additions of ammonia and perhaps alcohol and/or a detergent.
While very effective on the common metals such as steel, tinplate, copper and brass, they are also very corrosive if not washed off thoroughly immediately after soldering is completed. Aqueous solutions of orthophosphoric acid are also used for some applications, especially for the low melting point solders and for special steels. Subsequently, organic-based fluxes have been developed, now forming four main groups, resin, synthetic, synthetic resin and water-based organic fluxes.
Recommended Solder Fluxes for Engineering Materials
• Copper, solid or plates, tin-bronzes, gun metals. With red oxide tarnish, can be soldered with mildly activated resin, Black oxide only removable with activated resin, organic acids or zinc ammonium chloride solutions (e.g. for radiator plates).
• Copper-nickel alloys. Not difficult to solder if clean and well fluxed but soldering is not commonly suitable for the service conditions for which these alloys are specified.
• Copper-aluminium alloys (aluminium bronzes). Due to the tenacious alumina film which forms so rapidly, these alloys can only be soldered after copper plating or with techniques used for aluminium alloys.
• Gilding metals (CuZn10 and CuZn20). If clean, quite easy to solder with mildly activated resin. May be copper or silver-plates, but ensure good plating adhesion.
• Commercial brasses (CuZn30 and CuZn40). Use activated resin if tarnish is thin. Impossible to solder - even with inorganic flux – if significant oxidation is visible, due to surface film of zinc oxide. Barrier plating of more than 3μm nickel or copper is recommended for preserving solderability during shelf-life (should prevent zinc diffusion to surface). Barrier of 5μm necessary if brass is a lead free-machining grade.