Introduction to Welding Metallurgy
The materials used by pipe welders are primarily metals. It is, therefore, logical that a better understanding of the properties and the nature of metal will be useful in pipe welding and the objective of this chapter is to enlarge upon this subject.
Metals have distinguishing characteristics that are important to know. In the solid state they exist in the form of crystals and they can be deformed plastically. AH metals are good conductors of heat and electricity, and have a metallic luster that readily reflects light.
A few metals, such as copper and aluminum, are used commercially in their pure form. Most commercial metals, however, are not pure metals; they are usually a mixture of metals called alloys. For example, brass is an alloy of copper and zinc, and the so-called silver brazing alloys are mixtures of silver, copper, and zinc, with amounts of cadmium and tin sometimes added. Most of the aluminum used in industry is in the form of aluminum alloys containing manganese, magnesium, silicon, zinc, or copper. Plain carbon steel is an alloy of iron and carbon. Alloy steels contain additions of other alloying elements (chromium, tungsten, vanadium, nickel, etc.) to enhance their properties. The properties of complex metal alloys can best be learned by first studying the basic concepts of metallurgy.
Properties of Metals
In the following paragraphs the basic properties of metals are defined and discussed. However, the testing procedure used to obtain values of these properties is not treated. Information on these procedures is available elsewhere.
A characteristic of metals is their ability to resist large external loads. Although, perhaps, it is not correct to speak of internal loads in metals, it is a useful idea to consider stress an internal load. In this sense, an external load applied at one point in a metal will result in an internal load at many points inside the metal. It is really the ability to resist these internal loads, or stresses, that provides metal with the ability to resist externally applied loads.
Stress. If in Fig. 11*1 A, a load of 1,000 lbs is acting downward on the rectangular column, and if the area at the end of the column is 2 sq in., the stress on the bearing plate on which the column rests is 500 lbs per sq in. (1,000 lbs 2 sq in.).