Farm Machinery and Equipment. Harris Pearson Smith
are shown in Fig. 2–3.
FIG. 2–1. Different types of soft-center steel.
FIG. 2–2. Types of structural steel.
Hardening of Finished Steels. In many cases where long-life service is desired, extremely hard steels cannot be forged and machined to the required shape and finish. Under these conditions a softer steel is shaped and finished, then given a hardening treatment. The most common hardening processes are casehardening and hardening by heat-treatment.
Casehardening. This is a process of hardening a ferrous alloy so that the surface layer or case is made substantially harder than the interior or core (Fig. 2–4). Casehardening can be done by several processes, such as carburizing and quenching, carbonitriding, nitriding, cyaniding, induction hardening, and flame hardening.
Carburizing is a process in which steel is packed in charred peach pits or charcoal and heated at about 1600°F. for a long enough period to give the desired depth of hardness. It is then removed, quenched, and tempered to give the desired hardness.
Nitriding is a process of casehardening by placing the finished heat-treated steel in an airtight box and heating to 1000°F. as ammonia gas is injected into the chamber.
FIG. 2–3. Sheet metal can be pressed into many different shapes.
Carbonitriding is a process of hardening steel by the addition of a carbon-rich gas, as well as ammonia.
Cyaniding is a process where the steel is dipped into a molten bath of potassium cyanide for a short time. Some carbon and nitrogen are absorbed by the steel, which results in the hardening of a thin surface layer.
Induction hardening is accomplished by the use of a high-frequency alternating electric current for a short period. A current is induced on the surface of the steel, which causes localized heating. After heating, the surface is flooded with water to quench and harden it.
FIG. 2–4. Casehardened steel.
Flame hardening is a process in which an oxyacetylene torch is used to heat the surface quickly to a temperature above the critical temperature, after which the surface is quenched with water.
Hardening by Heat-treatment. Heat-treatment is a term used to describe the application of heating and cooling processes to steel, through a range of temperatures, to improve the structure and produce desirable characteristics. Such treatments include annealing, hardening, tempering, and casehardening.
Plow beams, plow disks, and disk-harrow blades are examples of parts of agricultural machines that are heat-treated in order to make more serviceable implements.
Hard Facing or Surfacing. The application of a hard surface, or face, by welding is not to be confused with the hardening of finished surfaces. Hard facing, or surfacing by welding, is the addition of a hard metal over the base metal by applying a welding-rod deposit to provide a final surface that is harder than the original surface.
Hard facings are applied to parts for wear resistance, heat resistance, corrosion resistance, or combinations of the three. Most hard facing is done to prevent wear. In hard-facing parts, it is essential that the correct hardening material be selected to suit the base metal.
There are possibly hundreds of different hard-facing alloys available, and these are manufactured in three forms: as welding rods, as insert shapes, and in powdered forms. There are many types of welding rods. The rods used with the oxyacetylene torch are not coated. They are heated and dipped into a special flux. Electric rods usually have a flux coating.
Inserts and filler bars are welded on surfaces where extra-heavy hard facing is required.
Hard-facing powders are spread over the base metal, which is heated to the melting point to embed the powders firmly.
REFERENCES
Brady, G. S.: Materials Handbook, McGraw-Hill Book Company, Inc., New York, 1944.
Clapp, H. W., and D. S. Clark: Engineering Materials and Processes, Metals and Plastics, International Textbook Company, Scranton, Pa., 1949.
Du Mond, T. C.: Engineering Materials Manual, Materials and Methods, Reinhold Publishing Corporation, New York, 1951.
Geiger, H. L., and H. W. Northrup: A New Metal for Farm Tool Components, Agr. Engin., 32(3):143–147, 1951.
Lyman, Taylor: Metals Handbook, The American Society for Metals, Cleveland, 1948.
Marks, Lionel S.: Mechanical Engineers’ Handbook, McGraw-Hill Book Company, Inc., New York, 1951.
Oberg, Erik, and F. D. Jones: Machinery’s Handbook, The Industrial Press, New York, 1949.
Ryerson Steels, Joseph T. Ryerson & Son, Inc., St. Louis, 1953–1954.
QUESTIONS AND PROBLEMS
1. Classify and give examples of construction materials.
2. Discuss the various nonmetallic materials and give uses of each.
3. Name the nonferrous metals and give uses of each.
4. Define an alloy.
5. What are the ferrous metals?
6. Explain the differences in the various types of castings.
7. Discuss the influence of carbon content on the hardness of steel.
8. Discuss the differences in structure and metals, in soft-center steel and in the clad steels.
9. Describe the various methods of hardening finished steels.
10. Discuss hard facing of metals.
11. Name the common steel alloys and their uses.
CHAPTER 3
MECHANICS
A clear conception of the fundamental principles of mechanics, as well as of their practical application to machinery, is necessary to a comprehensive study of farm machinery.
Force. Mechanics is the science that treats of forces and their effect. Force is the action of one body upon another which tends to produce or destroy motion in the body acted upon. Force may vary in magnitude and in method of application. In general, force is associated with muscular exertion. This, however, does not completely cover the scope and action of force because flow of an electric current, freezing of a liquid, and ignition of explosives may exert a certain amount of force. In order to compare different forces, they must all be in terms of the same unit. One such unit is called the pound weight.
Work. Whenever a force is exerted to the extent that motion is produced, work is performed. Work