Engineering materials - Ferrous metals

Engineering materials - Ferrous metals

Engineering materials - selection

In the design of machinery in general, a vast variety of materials of both organic and inorganic origin is utilized. We generally think of metals as the usual materials of design, but, although used to a lesser degree, such materials as wood, leather, rubber, and other plastics have widespread use, and others, such as fabrics, cork, special minerals, etc., have limited use.

In making a selection of a material we must first decide what constitutes a "proper materials". A proper material may be defined as one which best performs the functions required with the least total cost. This does not mean that the material having the lowest unit cost is best, because a more expensive material may permit reduction of weight, eaier heat tretment or fabrication, or it may process other advantages that make the final result less costly. And at times, of course, luxury, aooearance, or extreme safety is desired even at great expense.

Designers are interested principally in the physical properities and the cost of the finished part, and only incedentally in the chemical constituents and methods of preparation from the raw material. The physical properities of most importance are strenght, regidity, resistance to corrosion and to fatigue failure, and in some cases, weight. Other properities that may be of importance are hardness, impact resistance, heat and electrical conductivity, wear resistance, low friction, machinability, and weldability, When several of these characteristics are desired simultaneosly, selection of the most suitable and the most economical material is sometimes difficult.

Ferrous metals - Iron.

In general, we may say that certain groups of materials are used mainly because they are abundantly available and cheap. This condition is particularly true of the ferrous group of metals.

Ferrous metals are the most commonly used, and, with proper alloying and treatment, they may be adapted to almost all simple needs. The advantages of iron as a base metal, in addition to its abundance and low cost, are its strength and its adaptability to fabrication. It may be readily cast, forged, machined, and welded. Principal limitations are its weight and its susceptibility to corrosion.

Plain carbon steels

Steel differs from cast iron in that it has no carbon in free state. The percentage of carbon varies from 0.08 to 1.5 with consequent differences in properities. Steel is classified according to carbon content approximately as follows: "Very mild", "mild", or "low carbon"; "medium carbon", and "high carbon" or "hard". Both low and medium-carbon steels are generally used for machine parts, whereas high-carbon steels are used for springs or tools. Low-carbon steels are readily welded and foged since they are plastic over an extensive temperature range. They are very ductile and hence are resistant to shock and impect, but are not responsive to heat treatment by quenching. Medium-carbon steels are more difficult to forge and weld, but tensile strenght and elastic limit can be increased considerably by quenching at the expense of lessened ductility. High-carbon steels are difficult to forge and weld but may be hardened to a good cutting edge by quenching.

Alloy steels.

When metals are dissolved in each other and then solidified, an alloy results. Alloy steel is obtained when the other elements added to the iron and carbon are in sufficient quantities to influence the physical properities. Practically all alloy steels must undergo special heat treatment to obain the properities desired. The alloying elements used in steel are as follows: Nickel, silicon, chromium, vanadium, tungsten, molybdenum, manganese, and copper.

Several of these may be used simultaneously to obtain special physical properities when given a double heat treatment, High elastic limit with ample ductility, hard wearresisting surfaces combined with high core strength and toughness, high impact and fatigue resistance, are some of the properities that are readily attainable.