Part II. Cold working of steel

Part I

COLD WORKING OF STEEL

NONFERROUS FORGINGS

Nonferrous forgings are metal shapes produced by hot-working nonferrous metals, subjecting them to hammering and pressing operations. The result is the compression, bending, twisting or extrusion of the metal so that various parts of the forging are formed by pressure against dies.

Die-pressed or hammered forgings offer an efficient and economical method of producing irregularly-shaped metal parts from slugs cut from ingots and rods. The result is a strong, dense metal part, closely resembling the shape and size of the finished product, thus insuring a minimum of scrap metal in the final procession operation. Such forgings are freed of excess material and are ready for machining or further finishing operations.

Grain structure is uniform and dense, eliminating the disadvantages of porosity and rough surface finish. Nonferrous forgings also have high tensile strengths; the great strength and nonporosity often permit reduction in weight of parts previously produced by other processes. Fewer finishing operations are necessary, and the required machining may be performed with maximum speed. However, greater strength naturally reduces machinability.

Many nonferrous alloys are readily adaptable to the forging process and have been successfully used. Among them are forging brass, nickel, silver, leaded brasses, aluminum bronze, manganese bronze, silicon bronze and several aluminum alloys.

At present, nonferrous forgings are used for many purposes: for electrical and chemical equipment, in welding and in many other cases. Brass forgings have an important part in air compressors, compressed gas valves, gas and water meters, oil burner equipment, etc.

Nonferrous forgings can be provided in a great variety of finishes such as bright polished, plated with nickel, chromium, copper or other metals.

Steel is hot worked when it is in a homogeneous or heterogeneous austenitic condition. Hot working is the working of metals above the annealing temperature, so that the deformed metal becomes annealed before cooling to room temperature, and therefore has a normal grain structure with normal ductility and toughness. Cold working passes under quite different conditions; it is generally accompanied by many annealings of the metal being cold-worked.

Cold rolling, for example, like hot rolling may be carried out on a two-high or four-high rolling mill, or in a continuous rolling mill. Cold rolling is continued until the rolled section becomes too hard to continue the process, or until it reaches its final size. It may become necessary to anneal the metal after several passes through the cold rolling mill, in order to keep it in a workable condition. If annealing is carried out in an open furnace, pickling again is necessary before re-rolling to remove the scale and clean the metal. Today annealing of cold-rolled products is usually carried out in special furnaces that complete the annealing without the formation of scale.

Cold drawing is carried out by drawing the metal through a succession of conical tapering holes in a die plate. Die materials are: steel, cast iron, tungsten carbide and diamond. Shapes varying in size from the finest wire to those having a cross-sectional area of several square inches are commonly drawn. Due to the difficulty of making dies and small need for any other form, the fine sizes are drawn only to a round cross-section; larger sizes may be drawn to square, round, or irregular cross-sections, the larger sections being drawn on a drawbench. Metals can be formed to much closer dimensions by drawing than by rolling, and for this reason large quantities of steel and brass are cold drawn.

Multiple die machines are often used to produce wire. In these machines the wire passes through one die, around a capstan, through a second die and round another capstan, etc. As many as twelve dies may be used in a machine. Having gone through each die, the wire of course, is greatly elongated. The speed of drawing in multiple die machines may reach 10 000 feet per minute on fine wire. The die, or drag plate as it is often called, may be made of a number of materials, tungsten carbide having largely replaced other die materials because of its great ability to retain its shape during the drawing operation.

In drawing seamless tubing the metal is forced between the die and the mandrel, and in this way the wall thickness, the outside diameter, and the inside diameter may be controlled. As in any cold-working operation, the metal should be free from scale and other defects before it is cold-drawn.

Cold-working operations may be divided into two large classes:

1. Where the cold working is carried out for the purpose of shaping the articles only; and where the hardening effect is not desired and must be removed at various stages of plastic shaping as well as from the finished article by annealing.

2. Where the object of cold working is not only to obtain the required shape but to harden and strengthen the metal, and where the final annealing operation is not carried out.

In order to shape metals by cold working they must be annealed at proper intervals. Otherwise deformation must be carried out at temperatures where annealing is simultaneous with hardening, as in the hot working of metals. The selected method will depend on the individual metal as well as on the desired product. Metals vary greatly in the ease with which they deform. Copper, for example, may be worked readily at room temperatures, whereas some steels can only be worked at a red heat. Practically all metals and alloys become brittle very near their melting points and hence must not be worked at too high temperatures. There are metals that can be worked only in certain temperature ranges without cracking. Thus zinc must be worked at 200 to 300 degrees of Fahrenheit; nearly pure iron must not be worked in the blue heat range. Brass must not be heated too near its melting point in annealing, also, or it becomes «burnt».

TOPIC: «GEOGRAPHICAL POSITION OF GREAT BRITAIN»

Дата добавления: 2014-11-25