CLASSES OF STEEL.
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The Selection of Specific Qualities for Particular Needs.
AT THE PRESENT TIME when material and spare parts are most difficult to obtain it is not out of place to put before our readers a few helpful notes on the selection of materials.
The rapid development of the automobile industry has awakened in manufacturers a quick and keen appreciation of the great importance in securing the best qualities of steel for their needs.
Classes of Steel.
The term steel is applied to a number of mixtures which differ greatly from each other in their chemical as well as their physical qualities. The ingredient that exerts the most influence on steel is carbon. High-grade razor steel contains about 1.25 per cent. carbon ; spring steel 1 per cent. ; steel rails from 0.50 to 0.75 per cent. ; and soft boiler plates may have as little as 0.062 per cent, of carbon. Steel which is very low in carbon can easily be welded, but. it cannot be hardened. When the carbon is above 0.33-per cent. welding is more difficult, and Can only be done by the use of borax or some other flux, or by the electric or therrnit processes. Steel with carbon above 0.75 per cent. can readily be hardened. In tool steel other ingredients than carbon are sometimes used to influence its hardness such as nickel, manganese, chromium, tungsten, etc., the last named playing an important part in so-called ." high-speed steels," that is, tool steels that will cut metals at a high speed without losing their temper or hardness.
Pig iron and cast-iron contain about 4 per cent, of carbon, and wrought iron only a trace of it, while steel is between these two extremes ; hence, in the manufacture of steel it is important to get the right proportion of carbon.
Until recently the commonly accepted belief was that strength and stiffness are co-ordinate, or "the stronger a piece of steel, the stiffer it is." To illustrate this theory it was thought that if one piece was twice as strong as another, it would bend only one half as much under a given weight; but actual test has shown that a chrome-nickel steel having an elastic limit of 150,000 lb. or more per sq in. cet section bends under a given load the same amount as a carbon steel specimen, and this condition holds true so long as the load is within the elastic limit of the weaker material. The elastic limit of a -well heattreated steel spring is about 150,000 lb. per sq. in., but a spring can be made of soft steel. If it is not loaded beyond its elastic limit the spring will return to its original shape after every deflection, but the deflection would not be sufficient to make a good
spring. .
The cost of the materials used in automobile construction amounts to about 60 per cent. of the total cost of production. In view of this fact, the kind of material best.suited for the more vital parts is highly important.
Carbon Steels.
The 0.10 per cent, carbon steel is usually known to the trade as soft, basic open-hearth steel, and is coinmanly used for seamless tubing, pressed steel frames and brake drums, sheet steel brake bands, etc. It is soft and ductile, and will stand a great deal of deformation without cracking. In its natural or annealed condition it should not be used where a great deal of strength is required. The 0.20 per cent, carbon steel is known to the trade as machine steel. This steel will case-harden. It can be drawn into tubes, and is a_good frame material and can be used for gears, cam rollers, steering wheel pivot pins, etc.
The 0.30 per cent, carbon steel is used for such forgings as axles, driving shafts, steering pivots, etc. This steel is not intended for case-hardening. The 0.40 per cent, carbon possesses great strength and a considerable degree of toughness. It is commonly used for crankshafts, driving shafts, and propeller shafts. It is sometimes used for transmission gears, but it is not quite hard enough without casehardening, and when case-hardened.not tough enough to make safe transmission gears.
Nickel Steels.
Nickel steel is the most generally used of the alloy steels. The best quality contains 0.20 to 0.25 per cent. carbon, 3.50 per cent. nickel, 0.50 to 0.80 per cent. manganese, and not over 0.04 per cent. sulphur or phosphorus. A slightly higher percentage of carbon is much used for crankshafts.
The 0.15 per cent, carbon nickel steel is suitable for carbonizing purposes, and is of exceedingly tough and strong core, and has a hard exterior.
The 0.20 per cent, carbon nickel steel may also be ease-hardened, and is very satisfactory for gears_ The 0.30 and 0.35 carbon nickel steel is primarily used for structural parts where great strength and toughness are inquired; for example, axles, crankshafts, driving shafts and transmission shafts.
Nickel Chromium Steels.
There are three types of nickel chromium steels in common use, known as low, medium and high, according to the .percentage of nickel and chromium which each contain. Nickel chromium steels are also made either with a high carbon content, and used for oilhardened gears and springs, or with a low carbon con
tent, in which case the steel is used for axles, shafts, forged parts and gears.
A special nickel chromium tungsten steel is sometimes used for springs. Chrome-vanadium steel is excellent for springs. For excellent quality the latter product constitutes the highest attainment of the steel maker's art, and it seems that for springs no material is better suited than this steel.
Necessity of Heat Treatment of Steel Alloy.
While the best alloy steels are now quite good for most of the parts in automobile construction, their qualities will not become pronounced unless they receive proper heat treatment. It is a waste of money to buy good alloy steels without knowing how toproperly treat them to bring forth their exceptional qualities. For gauging the heat a pyrometer is necessary, but it is too often supposed to take care of itself.
The heat treatment operations depend upon established scientific facts, and a lack of appreciation of this causes many people to buy high-priced alloy steel from which they get no better results than from carbon steel properly handled_ As an example of the effect of heat treatment may be mentioned a chrome steel which in its rolled condition had an elastic limit of 158,000 lb., 5 per cent, elongation, and 9.4 per cent.. reduction in area. The same steel, oil tempered and annealed, had an elastic limit of 153,000 lb., 14 per cent, elongation and 62 per cent, reduction in area. In other words, the material was transformed from brittle to tough without appreciably affecting its elastic limit.