Keep the work area neat and clean. Among other things, make it a practice to dispose of hot electrode stubs in a metal container. Proper eye protection is of the utmost importance, not only to the welding operator, but for other personnel in the vicinity of the welding operation. Eye protection is necessary because of the hazards posed by stray flashes, reflected glare, flying sparks, and globules of molten metal. HEAT TREATMENT OF METALS Learning Objective: Recognize the principles of heat treatment and identify the most common forms. This following text covers the forms and principles of heat treatment in general. Both ferrous and nonferrous heat treatment of metals is covered. Informa- tion given is for training purposes only. When actually performing heat treatment tasks, you must refer to the applicable technical publications. Heat treatment is a series of operations involving the heating and cooling of a metal or alloy in the solid state for the purpose of obtaining certain desirable characteristics. The rate of heating and cooling determines the crystalline structure of the material. In general, both ferrous metals (metals with iron bases) and nonferrous metals, as well as their alloys, respond to some form of heat treatment. Almost all metals have a critical temperature at which the grain structure changes. Successful heat treatment, therefore, depends largely on a knowledge of these temperatures as well as the time required to produce the desired change. PRINCIPLES OF HEAT TREATMENT The results that may be obtained by heat treatment depend, to a great extent, on the structure of the metal and the manner in which the structure changes when the metal is heated and coded. A pure metal cannot be hardened by heat treatment because there is little change in its structure when heated. On the other hand, most alloys respond to heat treatment because their structures change with heating and cooling. An alloy may be in the form of a solid solution, mechanical mixture, or a combination of a solid solution and a mechanical mixture. When an alloy is in the form of a solid solution, the elements and compounds that form the alloy are absorbed, one into the other, in much the same way that salt is dissolved in a glass of water. The constituents cannot be identified even under a microscope. When two or more elements or compounds are mixed, but can be identified by microscopic examination, a mechanical mixture is formed. A mechanical mixture might be compared to the mixture of sand and gravel in concrete. The sand and gravel are both visible. Just as the sand and gravel are held together and kept in place by the mixture of cement, the other constituents of an alloy are embedded in the mixture formed by the base metal. An alloy that is in the form of a mechanical mixture at ordinary temperatures may change to a solid solution when heated. When cooled back to normal temperature, the alloy may return to its original structure. On the other hand, it may remain a solid solution or form a combination of a solid solution and mechanical mixture. An alloy that consists of a combination of a solid solution and mechanical mixture at normal temperatures may change to a solid solution when heated. When cooled, the alloy may remain a solid solution, return to its original structure, or form a complex solution. Heat treatment involves a cycle of events. These events are heating, generally done slowly to ensure uniformity; soaking, or holding the metal at a given temperature for a specified length of time; and cooling, or returning the metal to room temperature, sometimes rapidly, sometimes slowly. These events are discussed in the following paragraphs. Heating Uniform temperature is of primary importance in the heating cycle. If one section of a part is heated more rapidly than another, the resulting uneven expansion often causes distortion or cracking of the part. Uniform heating is most nearly obtained by slow heating. The rate at which a part maybe heated depends on several factors. One important factor is the heat conductivity of the metal. A metal that conducts heat readily may be heated at a faster rate than one in which heat is not absorbed throughout the part as rapidly. The condition of the metal also affects the rate at which it may be heated. For example, the heating rate for hardened tools and parts should be slower than for metals that are not in a stressed condition. Finally, size and cross section have an important influence on the rate of heating. Parts large in cross section require a slower heating rate than thin sections. This slower heating rate is necessary so that the interior will be heated to the same temperature as the surface. It is difficult to uniformly 15-37