FERROUS AND NON FERROUS METALS
PHYSICAL TERMS
Hardness: Hardness is the property of resisting penetration or permanent distortion. The hardness of a piece of metal can usually be increased by hammering, rolling or otherwise working on it. In the case of steel, some aluminum alloys, and a few other metals, hardness can also be increased by a heat treatment. Strength of the material increases with the increased hardness.
Brittleness: Brittleness is the property of resisting a change in the relative position of molecules, or the tendency to fracture without change of shape. Brittleness and hardness are very closely associated. Hard material is invariably more brittle than soft material. In aircraft construction the use of too brittle material must be avoided or failure will be caused by the shock loads to which it will be subjected.
Plasticity: A very important mechanical property of metals is that of plasticity. Plasticity is the ability of a metal to be deformed extensively without rupture.
Ductility: Is the plasticity exhibited by a material under tension loading. It is measured by the amount the material can be permanently elongated. This ability to elongate permits a metal to be drawn from a larger size to smaller size of wire. Copper and aluminum have ductility.
Malleability: Which is another form of plasticity, is the ability of a metal to deform permanently under compression without rupture. It is this property which allows the hammering and rolling of metals into thin sheets. Gold, Silver, Tin and lead are examples of metals exhibiting high malleability. Gold has exceptional malleability and can be rolled into sheets thin enough to transmit light.
Elasticity: Elasticity is the property of returning to the original shape when the force causing the change of shape is removed. All aircraft structural design is based on this property, since it would not be desirable to have any member remain permanently distorted after it had been subjected to a load. Each material has a point known as the elastic limit beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and parts are so designed that the maximum applied loads to which the airplane may be subjected will never stress them above their elastic limit.
Density: Density is the weight of a unit volume of the material. In aircraft work the actual weight of a material per cubic inch is preferred since this figure can be used in calculating the weight of a part before actual manufacture. The density of a material is an important consideration in deciding which material to use in the design of a part.
Fusibility :Fusibility is the property of being liquefied by heat. Metals are fused in welding. Steel fused around 2500 degree’s F, aluminum alloys around 1100 degree’s F.
Conductivity: Conductivity is the property of transmitting heat or electricity. The conductivity of metals is of interest to the welder as it affects the amount of heat he must use and, to a certain extent, the design of his welding jig. Electrical conductivity is also important in connection with the bonding of airplanes to eliminate radio interference.
Contraction and Expansion: Contraction and expansion are caused by the cooling or heating of metals. These properties affect the design of welding jigs, castings, and the tolerances necessary for hot rolled material, assembly of bearing components.
Toughness: Although there is no direct and accurate method of measuring the toughness of metals, toughness involves both ductility and strength and may be defined as the ability of a metal to absorb energy without failure. Toughness may be expressed as the total area under a stress-strain curve. Often the impact resistance or shock resistance of a material is taken as an indication of its inherent toughness.
HEAT TREATMENT TERMS
Soaking During the soaking period the temperature of the furnace must be held constant. It is in this period that rearrangement of the internal structure is completed. The time of soaking depends upon the nature of the steel and the size of the part. Heavier parts require longer soaking to ensure equal heating throughout. In specifying hardening temperatures, it is customary to give a range of from 50 deg. to 75 deg. F, within which the material must be soaked. Light parts are soaked in the lower part of this range and heavy parts are in the upper part of the range. For the steels and sizes normally used in aircraft construction a soaking period of from 30 to 45 minutes is sufficient. During the tampering operation the steel is soaked from 30 minutes to one hour, depending on the thickness of the material.
Tampering is the second operation required to develop high strength, heat-treated steel. It consists of heating hardened steel to a temperature well below AC3, soaking at that temperature, and then quenching in oil or air. This treatment relieves the strains in hardened steel, decreases the brittleness and restores ductility. In addition, the strength and hardness are some what reduced. The strength, hardness and ductility obtained depend upon the temperature to which the steel was reheated. The higher the temperature, the lower the strength and hardness but greater the ductility. By decreasing the brittleness of hardened steel, tempered steel is made tough and still retains adequate strength. Tempered steels, as used in aircraft work have from 125000 to 200000 p.s.i. ultimate tensile strength. When hardened steel is reheated as in tempering, the transition from austenite to pearlite is continued further, and the martensite is converted to troostite and then sorbite. Tempered steel is composed largely of sorbite, which gives it toughness. Hardened steel, reheated to a lower temperature and quenched, is composed of troostite and sorbite, and is still very hard and strong but more ductile than hardened steel; hardened steel reheated to a higher temperature and quenched is composed of sorbite and some pearlite and is tougher and more ductile but still retains considerable strength and hardness.
Hardening is the first two operations required for the development of high strength steels by heat treatment. Hardening consists of heating above AC3, soaking at that temperature until the mass is uniformly heated, and than quenching in brine, water or oil. This treatment produces a fine grain, maximum hardness and tensile strength, minimum ductility and internal strains. In this condition the material is too hard and brittle for practical use. Heating is conducted as little above AC3 as is practical, in order to reduce wrapping and the possibility of cracking when the material is quenched. On the other hand, large objects are heated to upper limit of the hardening range in order to as sure through heating. For the materials and sections used in aircraft work, quenching in oil invariably the method employed. The heat absorption is slower than that of water or brine, and consequently the cooling operation is more gentle. Less wrapping and cracking occurs and sufficient hardness is obtained.