Iron and steel are the most useful and the most used metals we have and, since steel is merely iron with the addition of a small percentage of carbon and a few other elements, we will consider iron first.
Iron is made by taking iron ore, placing it in a furnace with fuel (coal, coke, or charcoal) and a suitable flux (usually limestone), and then melting it with the assistance of a forced draft until the ferrite in the ore is reduced to metallic iron. This is then run out into molds to form the "pig iron" of commerce. If this pig iron is again melted in a cupola and cast into molds of various shapes, it is called "cast iron" and is the material so largely used now for machinery and other articles.
Cast iron is hard and brittle and granular in composition, because it contains so much carbon. This latter substance has come from the fuel used in melting the iron and the carbon content can be varied to suit requirements by "burning" it out. This is done by forcing a strong blast of heated air through the iron while it is still molten, causing the oxygen in the air to combine with the carbon and carry it off in the form of carbonic-acid gas, (C O2). Charcoal makes the best fuel for use in smelting because it is so free from sulphur and other impurities and it is used when making so-called Swedish iron. Good charcoal iron is easily welded and will stand more bending without breaking than any other kind of iron. Wrought iron is almost entirely free from carbon and is very malleable and ductile, hence easily welded.
Steel is wrought iron with an appreciable percentage of carbon added, the amount depending upon the use to which it is to be put. It is made by simply stopping the air blast when making iron and leaving a small amount of carbon in the metal. This is usually at the point where the mixture still possesses the ductility and malleability of wrought iron together with the hardness and brittleness due to the carbon. This is why steel is both tough and hard and the amount of carbon determines the hardness. While wrought iron is slightly fibrous, steel is crystalline, but it may be improved by working at the proper temperature, and good steel is homogeneous throughout.
Soft steel, or "mild" steel, as it is called commercially, contains very little carbon and is really on the dividing line between iron and steel. When it is made by forcing air through the molten iron to burn out the carbon, as previously described, it is called "Bessemer" steel, after the inventor of the process. The desired amount of carbon is afterwards supplied by adding an iron called "spiegeleisen" which contains both carbon and manganese, the latter enabling the iron to hold a larger amount of carbon and adding to its strength.
Open-hearth steel is made by melting cast iron in an "open hearth" or broad shallow furnace and adding the proper proportions of scrap wrought iron or steel and iron ore. This is a very tough steel and is used for boiler plates and similar articles.
In all of the processes the steel is cast into ingots and then rolled or forged into bars, sheets, or other shapes for commercial purposes. The amounts of carbon in iron and steel are approximately as follows:
Cast Iron................. 3. %to4.5%
Tool Steel.................. 5%to2 %
Mild Steel.................1% to .5%
Wrought Iron.............Less than .1%
Steel castings are usually made of mild steel and contain small amounts of manganese, silicon, sulphur, and phosphorus in addition to the carbon. Manganese and silicon improve the steel but sulphur and phosphorus are not desirable. Aluminum is also added sometimes as a solidifier or deoxidizer before pouring the castings and it improves their quality. The amounts of these elements in steel castings usually vary as follows:
Carbon................18% to .75%
Manganese.............30% to .80%
Silicon.................27% to .33%
Sulphur................032% to .056%
Phosphorus............032% to .092%
Aluminum............Traces only after melting.
The various elements used to alloy iron are technically known as "impurities", even though their addition is a distinct advantage. All of the alloys of iron and steel may be welded by any of the modern methods, although some of them cannot be welded by the blacksmith. Silicon causes brittleness and too much of it prevents welding, on account of the crystalline structure of the alloy, but the addition of manganese tends to overcome this and make it more weldable.
Manganese up to 1.5 per cent may be added to iron or steel without preventing welding, but more than that makes a brittle alloy. Manganese reduces both the sulphur and oxygen in the iron and adds greatly to its strength. Nickel steel may be welded and the addition of nickel up to 5 per cent is safe, if the amount of carbon is kept small. Nickel increases the tensile strength of steel without impairing the elasticity and also tends to prevent rusting of the iron alloy. Chrome steel may also be welded successfully.
Copper is one of the elemental metals and is found as an ore and in a pure state. The ore must be smelted and the copper refined before it is ready for use in the arts like other metals. It is cast and sold in the form of ingots and should be at least 99.5 per cent pure and entirely free from sulphur. It is used largely in the form of sheets, bars, and tubes and occasionally it is cast in molds like iron and steel.
It is the principal element in brass, bronze, gun metal, and many other alloys, and is nearly as useful to man as iron. It can be welded readily by any method, although this is rarely done, for brazing and soldering have been the processes generally used for joining or repairing pieces of it. Great care must be exercised when casting copper to insure its being properly deoxidized and the same thing applies to welding it. Silicon, aluminum, and phosphorus are used for this purpose, although aluminum alone presents the disadvantage of oxidizing rapidly when exposed to the air.
Tin and copper form a good alloy called "bronze", which is harder than either metal alone. The addition of tin increases the fluidity of copper but diminishes its ductility; the strength of copper is increased by adding up to 12 per cent of tin, and its crushing strength is increased by additions up to 18 per cent of tin. Beyond this latter point the bronze becomes hard and brittle. "Gun metal" is copper with from 8 per cent to 2 per cent of tin but the best alloy contains about 10 per cent of tin. Great care must be used when welding alloys containing tin because the latter melts at a comparatively low temperature and may easily be burned when welding.
Zinc and copper form the alloy known as "brass", and percentages of zinc as high as 40 per cent are sometimes used without serious effect on the malleability or ductility of the alloy but more zinc makes it very brittle. Tin is sometimes added to brass to increase its strength. Zinc is a good deoxidizing agent for copper but it vaporizes quite rapidly at high temperatures. This causes the zinc to pass out of the alloy, leaving the copper porous or spongy; this is why brass is so hard to weld satisfactorily. Lead is also used as an alloy for copper but not over 3 per cent can be used because it does not mix well.
Manganese is alloyed with copper in various proportions for certain purposes. This alloy is known as "manganese bronze" and may be both forged and welded by proper methods, although welding is a rather hard process to perform satisfactorily with it. The ductility and strength are both very high and the alloy does not corrode easily, even in salt water. It weakens slightly when heated and shrinks more than gun metal; hence it requires special care during welding operations.
Phosphorus is also used with copper for making bearing metals and makes a strong alloy which resists corrosion. The percentage of phosphorus used when making "phosphor-bronze" castings ranges from 2 per cent down to but a few hundredths of one per cent; frequently the phosphorus causes hard spots because it does not always combine freely and thoroughly. This alloy is also hard to weld readily, although it may be done by proper methods.
Aluminum is the lightest of the commercial metals and is very valuable for forming alloys but it is rather hard to weld or solder because of the rapidity with which it oxidizes, especially at high temperatures. Castings of aluminum are being successfully welded now, however, by several methods; and sheets of aluminum are welded by the gas methods. The various alloys are also being welded; the heat conductivity is comparatively high and it acts like solder when melted; it melts at 655 degrees centigrade; it is easily burned; and a sort of scum forms on the surface, if welded with a high-oxygen flame.
Commercial welding is confined almost entirely to the metals here described, although gold, silver, platinum, and a few others are weldable and are used in some of the finer arts. The characteristics of iron, steel, and copper, however, are most important to remember in connection with commercial welding and manufacturing.