Lesson 5

Read the text: Alloys

An alloy is a relatively homogeneous metallic material which is not a chemical element. The constituents of any alloy must be soluble in each other in the molten state; otherwise the resulting solid will have no uniformity of structure. Gold and lead dissolve in each other when molten, but are practically insoluble in each other when solid. On the other hand, copper and lead are insoluble in each other in the liquid state and separate into distinct layers. Copper and lead alloys are used successfully for bearings in railway service. In this case, other elements present in small amounts promote a fair distribution of the lead particles throughout the copper matrix.
Constituents of Alloys. When an alloy is examined under the microscope, it will be found to have a definite appearance, depending upon the way in which the elements constituting the alloy are combined. The constituents of an alloy must be soluble in the liquid state. This solubility may continue even after solidification has taken place, in which case the structure will appear homogeneous. If, on the other hand, the two constituents are insoluble in the solid state, microscope examination will reveal two distinct constituents. Under certain conditions three constituents may appear. The constituents of an alloy may be classified as follows:

(a) Pure metals; (b) metalloids.

Chemical compounds: (a) of pure metals; (b) of metals and metalloids; (c) of metals and non-metals.

3. Metallic solid solutions of the above: (a) soluble in each other in all proportions; (b) partially soluble in each other.

Therefore, an alloy may be made up of a single homogeneous solid solution, or it may be made up of pure metals, or chemical compounds, or solid solutions, or of various mixtures of these constituents.

Steel. Steel is an alloy of iron and carbon in which the carbon content does not exceed 1.7 per cent. It is produced from pig iron and scrap by processes which remove some carbon and a certain amount of sulphur, phosphorus, manganese, and silicon. Steels containing only iron and carbon, together with a small amount of impurities, are classed as plain carbon steels, while those containing certain elements intentionally added are classed as alloy steels. In general, plain carbon steels are classified according to the carbon content, the method of manufacture, and the uses for which their properties fit them.

Fundamental Considerations in the Manufacture of Steel. The manufacture of steel is a process by which a melted charge of pig iron or pig and scrap has the bulk of its impurities removed by oxidation.
The method and extent of removal of these impurities depend upon the process employed in refining the pig iron. The various methods of refining may be classified according to the chemical nature of the process and according to the type of furnace used in the process.

Alloy Steels. In many cases, the engineer requires steel with a tensile strength, yield point, hardness, response to heat treatment, etc., that cannot be obtained with plain carbon steels. Higher tensile strength, yield point, endurance limit, and impact strength can be obtained by adding elements such as nickel, chromium, molybdenum, and vanadium to the steel. Alloy steel may be defined as steel to which elements other than carbon are added in sufficient amount to impart desirable mechanical properties. To obtain full advantage of these properties, alloy steels should be heat-treated. The alloying elements affect the properties of the steels in several ways:

· By changing the critical temperatures at which austenite is transformed into ferrite with consequent precipitation of iron carbide;

· By slowing up the rate of transformation;

· By forming solid solutions of the alloying element and the iron to produce a stronger ferrite (M, Сu, Mn);

· By producing carbides of the alloying elements (Cr, V, Mo, W).

There are many different alloy steels and it is very difficult in many cases for the engineer to make a selection that will satisfy his requirements. In many cases, alloy steels are used where good plain carbon steel would be equally satisfactory.

Bronze is usually an alloy of copper and tin. Bronzes are commercially important because of their relatively high strength, their ability to resist corrosion, and the ease with which they may be cast and machined. Brass is usually an alloy of copper and zinc. Brasses have most of the desirable characteristics of bronzes with the added advantage that they may be drawn and rolled as well as cast. Standard brass is an alloy of 65% copper with zinc; yellow brass contains 62% copper, 2 to 4% lead with zinc and traces of tin and iron. Yellow brass is inexpensive, gives excellent castings, and is readily machined.

1. Match the left part with the right:

1. An alloy steel may be defined	a) bronzes with the added advantage that they may be drown and rolled as well as cast.
2. The solubility may continue even after	b )as a steel to which elements other than carbon are added in sufficient amount to impart desirable mechanical properties.
3 Bronzes are commercially important	c) because of their relatively high strength.
4. Brasses have most of the desirable characteristic of	d) solidification has taken place.

2. Complete the sentences with the suggested words: soluble, molten, alloy, solid

The constituents of any _______must be_______ in each other in the_______ state, otherwise the resulting _______will have no uniformity of structure.