Lesson 3

Read the text: Ferrous Metals

Ferrous metals are used in industry in two general forms: cast iron and steel. These two ferrous alloys are usually produced from pig iron and they have different carbon content. Articles of cast iron are made of pig iron which is received from the blast furnace in the form of pigs, remelted in a cupola or other form of remelting furnace and poured into suitable moulds. The carbon content varies between 2% and 4°/₀. There are two principal forms: gray cast iron, in which most of the carbon is present in a free or uncombined state and which is comparatively easy to machine, and white cast iron, which is difficult to machine because most of the carbon present is in chemical combination with the iron.

Cast iron is the cheapest of the metals. Gray cast iron is the iron of general use having ultimate tensile and compressive strengths of about 18,000 psi and 90,000 psi respectively. It is comparatively brittle, however, and for that reason should not be employed for members subjected to shock.

White Cast Iron is employed principally for the production of malleable iron castings. The iron is cast into sand moulds, cooled and the casting is then packed at an elevated temperature in a material which may be of an oxidizing character; thus precipitating the chemically combined carbon into graphitic form. The average ultimate strength of the resulting malleable iron is 50,000 psi, in both tension and compression. Malleable iron parts are tough and can often be employed to replace steel forgings. Malleable iron is employed when parts of rather complicated shape are required, parts which may be more readily produced by casting than by forging, but for which gray cast iron is unsuitable - on account of its low tensile strength and brittle nature.

Alloy cast irons are coming into extensive use although they are more expensive than gray iron.

Semi-steel is made by adding from 20 to 40°/₀ of scrap steel to gray iron. The ultimate tensile and compressive strengths are 28,000 psi and 110,000 psi. Semi-steel may be rendered ductile by annealing or softening at a temperature of about 800° F, but its tensile strength is thereby reduced by about one-third.

Wrought iron is a ferrous product that is produced by oxidizing the carbon from molten iron, and then hammering or rolling the heated iron while in a plastic form, and thoroughly "working" the slag into the product. The carbon content is generally less that 0.1°/₀ and the material must contain not less than l°/₀ slag. The slag content and the comparatively pure nature of the iron provide the principal characteristic of wrought iron — its high resistance to corrosion. Wrought iron is quite ductile and can be easily rolled, drawn, forged and welded. It cannot be cast since the necessary remelting process destroys the slag-impregnated nature of the material. Its initial cost is greater than that of cast iron or steel, but wrought iron is extensively used for rivets and for steam and water pipes where its characteristic of high resistance to corrosion is of value.

Steel is a ferrous material with a carbon content intermediate between wrought iron and cast iron. The carbon in steel is a chemically combined form, and varies from 0.1% to 1.0°/₀.

Cast steel normally contains about 0.5°/₀ carbon, and is used to replace cast iron when castings of considerable strength are required. Steel castings are more difficult to produce than those of cast iron, because of greater shrinkage. They are extremely difficult to machine unless annealed.

Forged steel is steel that has been hammered, drawn, pressed or rolled in the process of manufacture of a particular part. It is classified according to its carbon content since carbon is the most important element in forged steel.

Alloy steels are those in which some alloying element in addition to the carbon is present in some appreciable quantity. There are numerous alloying materials, each of which has a definite effect on the characteristic of the steel. Nickel gives increased strength and hardness without any proportionate decrease in ductility. Silicon increases the strength if present in quantities not exceeding 2°/₀. Chromium increases the strength and hardness of the steel at the expense of a slight decrease in ductility. Vanadium increases the toughness and the resistance of the alloy steel to shock. About 14% of manganese will provide a very ductile, abrasion-resistant alloy steel. Copper is employed to provide resistance to corrosion.

1. Match the left part with the right:

1. About 14% of manganese will provide a	a) the principal characteristic of wrought iron — its high resistance to corrosion.
2. The slag content provides	b) since the necessary remelting process destroys the slag-impregnated nature of the material.
3. Wrought iron cannot be cast	c) the iron of general use.
4. Gray cast iron is	d) very ductile, abrasion-resistant alloy steel.

2. Complete the sentences with the suggested words: brittle, strengths, reason, tensile, shock

Gray cast iron is the iron of general use, having ultimate _______and compressive_______of about 18,000 psi and 90,000 psi respectively. It is comparatively_______ , however, and for that _______should not be employed for members subjected to________.