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.
Soldering can be considered under two classifications; soft soldering using alloys melting below 350°C, and hard soldering using stronger alloys with a higher melting point. For joining copper and copper alloys the latter groups are often called silver solders. Soft soldering using tin based solders is widely used for joining copper and brass where mechanical strength is not so important. For electrical applications, tin lead solders are at present most commonly used but for plumbing applications for potable water, lead free solders are now specified. The fear of lead poisoning which has caused the prohibition of lead in some plumbing applications is now threatening its use elsewhere.
A soldering copper (usually called a soldering iron) consists of a forged copper head and an iron rod with a handle. The handle, which may be wood or fiber, is either forced or screwed onto the rod.
Soldering heads are available in various shapes. The pointed copper is for general soldering work. The stub copper is used for soldering flat seams that need a considerable amount of heat. The bottom copper is used for soldering seams that are hard to reach, such as those found in pails, pans, trays, and other simila objects.
Also, coppers must be filed and retinned after overheating or for any other reason that caused the loss of their solder coating. The procedure for filing and tinning a copper is defined as follows:
- Heat the copper to a cherry red.
- Clamp the copper in a vice.
- File the copper with a single-cut bastard file.
There are many different types of solder being used by industry. Solders are available in various forms and include bars, wires, ingots, and powders. Wire solders are available with or without a flux core. Because of the many types of solder available, this chapter only covers the solders most commonly used by Steelworkers.
The largest group of all solders in use is solders of the tin-lead alloy group. They have good corrosion resistance and can be used for joining most metals. Their compatibility with soldering processes, cleaning, and most types of flux is excellent. In describing solders, it is the custom of industry to state the tin content first; for example, a 40/60 solder means to have 40% tin and 60% lead. Tin-lead alloy melting characteristics depend upon the ratio of tin to lead. The higher the tin content, the lower the melting temperature. Tin also increases the wetting ability and lowers the cracking potential of the solder.
The behavior of tin-lead solder is shown by the diagram in figure 2. This diagram shows that 100% lead melts at 621°F and 100% tin melts at 450°F. Solders that contain 19.5% to 97.5% tin remain a solid until they exceed 360°F. The eutectic composition for tin-lead solder is about 63% tin and 37% lead. A 63/37 solder becomes completely liquid at 361°F. Solders with lower tin content are less expensive and primarily used for sheet metal products and other high-volume solder requirements. High tin solders are extensively used in electrical work. Solders with 60% tin or more are called fine solders and are used in instrument soldering where temperatures are critical.
Figure 1: Tin-lead alloy constitutional diagram
Antimony is added to a tin-lead solder as a substitute for some of the tin. The antimony, up to 6%, increases the strength and mechanical properties of the solder. A word of caution, solders having a high antimony content should not be used on aluminum, zinc, or zinc-coated materials. They form an intermetallic compound of zinc and antimony that causes the solder to become very brittle.
Several tin-zinc solders have come into use for the joining of aluminum alloys. The 91/9 and 60/40 tin-zinc solders are for higher temperature ranges (above 300°F), and the 80/20 and 70/30 tin-zinc alloys are normally used as precoating solders.
Lead-silver solders are useful where strength at moderately high temperatures is required. The reason lead by itself cannot be used is that it does not normally wet steel, cast iron, or copper and its alloys. Adding silver to lead results in alloys that more readily wet steel and copper. Flow characteristics for straight lead-silver solders are rather poor, and these solders are susceptible to humidity and corrosion during storage. The wetting and flow characteristics can be enhanced as well as an increased resistance to corrosion by introducing a tin content of 1%.
Lead-silver solders require higher soldering temperatures and special fluxing techniques. The use of a zinc-chloride base flux or uncoated metals is recommended, because rosin fluxes decompose rapidly at high temperatures.
Tin-antimony solders are used for refrigeration work or for joining copper to cast-iron joints. The most common example is the 95/5 solder.
Tin-silver solder (96/4) is used for food or beverage containers as it must be cadmium and lead-free. It also can be used as a replacement for tin-antimony solder (95/5) for refrigeration work.