alloy steel main imageThe development of alloy steels in the past has been largely a result of trial and error. It is practically impossible to predict, with any degree of certainty, the exact properties that can be obtained by a given combination of elements. In a general way, the effect of a adding a specific alloying elements is known. This information is useful to the designer in deciding which material possesses just the right properties for the proposed design. The constituents of plain and alloy steels are discussed in detail in the following paragraphs, emphasis being placed on those properties that have a bearing on aircraft use.


carbonCarbon:- Carbon is by far the most important constituent of steel. It combines readily with iron to form iron carbide (Fe3C), which is a compound known as cementite. It is largely due to the quantity and behavior of this compound that steels can be heat treated to various degrees of strength and toughness. This fact is equally true of both plain carbon and alloy steels. Within certain limitations, the higher the carbon content of steel is, the greater will be the ultimate strength, the hardness, and the range through which it can be heat treated. At the same time, the ductility, malleability, toughness, impact resistance, and the weld ability will be reduced as the carbon increases. In selecting a steel for a given design, the carbon content must be considered:  a low-carbon steel is necessary if deep drawing or excessive mechanical working are required without excessive strength, and a high-carbon steel is necessary where great hardness is required and ductility is not important. In general, low-carbon steels are used for formed fitting and welded parts, and high-carbon steels for springs. The medium-carbon steels are used for forged fittings and tie-rods where good strength, combined with ductility, is required.


manganeseManganese:- Next to carbon, manganese is the most important ingredient in steel. Its primary purpose is to deoxidize and desulphurize the steel to produce a clean, though metal. It deoxidizes by eliminating ferrous oxide, which is a harmful impurity; and it combines with sulphur to from manganese is added to the steel to leave an excess of not more than 1% in the metal. This excess manganese exists as manganese carbide (Mn3C), which has characteristics in hardening and toughening the steel similar to those of cementite (Fe3C), although not to as great an extant. Manganese does possess the property known as “penetration hardness” which means that in heat treatment of large sections, the hardness is not merely on the surfaces but penetrates to the core as well. In addition, the presence of manganese will greatly improve the forging qualities of the steel by reducing brittleness at forging and rolling temperatures.

An excess of more than 1% of manganese will increase the brittleness of the metal. There is, however, a manganese steel containing approximately 13% manganese that is exceptionally hard and ductile; but it is too hard to cut and must be forged, rolled, or cast to practically the finished shape. Some finishing may be done by grinding. This material was used at one time for tail-skid shoes on airplanes, which were cast to size. Commercially it is used for rock-crusher jaws and railroad curves. It has the interesting property of being nonmagnetic.


siliconSilicon:- Only a very small amount, not exceeding 0.3% of silicon, is present in steel. It is an excellent deoxidizer, but it also has the property of combining with iron more readily than carbon. Therefore it must be limited. A small amount of silicon improves the ductility of the metal. Its main purpose however, is to produce a sound metal.

Silicon and manganese in large amount are used as alloying elements in the formation of silico-manganese steels. These steels have good impact resistance.



sulphurSulphur:- Sulphur is a very undesirable impurity which must be limited in amount to not more than 0.06%. The maximum permissible sulphur content is always specified in the chemical specification for any particular steel. The presence of sulphur renders steel brittle at rolling or forging temperatures. In this condition the steel is said to be “hot short”. As stated previously, manganese combines with the sulphur to form manganese sulphide, which is harmless in small amount. When too much sulphur is present, an iron sulphide is formed which, because of its lower melting point, is in liquid from at the forging temperature of the steel. This liquid ingredient breaks up the cohesion of the crystals of the metal, hence cracking and breaking result. With a minimum of 0.30% manganese present (as usually specified) and not more than 0.06% sulphur, all the sulphur will be in the form of manganese sulphide, which is harmless in such small quantities.

phosphorusPhosphorus:- Phosphorus, like sulphur, is an undesirable impurity limited in amount to not more than 0.05%. The maximum permissible content is always specified. Phosphorus is believed responsible for “cold shortness” or brittleness when the metal is cold. Below the 0.05% specified there is little, if any, brittleness in the steel. There is some evidence that very small amounts of phosphorus increase the strength slightly.





nickelNickel:- Nickel is a white metal almost as bright as silver. In the pure state it is malleable, ductile, and weldable. It does not corrode quickly, as attested by its, use in nickel plating. Nickel dissolves in all proportions in molten steel. The addition of nickel steels contain from 3% to 5% nickel. The addition of nickel to steels increases the strength, yield point, and hardness without materially affecting the ductility. In heat treatment the presence of nickel in the steel slows down the critical rate of hardening which, in turn, increases the depth of hardening and produces a finer grain structure. There is also less war page and scaling of heat-treated nickel-steel parts. Nickel increases the corrosion-resistance of the steel. It is one of the principle constituents of the so-called “stainless” or corrosion-resisting steels.


chromium2Chromium:- Chromium is a hard gray metal with a high melting point. Chromium imparts hardness, strength, wear resistance, and corrosion resistance to steel. It also improves the magnetic qualities to such an extent that chromium steel is used for magnets. Chromium possesses excellent “penetration hardness” characteristics and its alloys heat treat well. The main use of chromium in alloys is in conjunction with nickel, molybdenum, and vanadium. About 1% of chromium is present in these alloys, which are strong, hard, and have fair ductility. These alloys are also resistant to shock loads. It is possible to heat treat nickel-chromium alloys to an ultimate tensile strength as high as 250,000 p.s.i. and still retain ductility.

Corrosion-resisting steels contain large amounts of chromium. The most common of these steels is 18-8 steel – approximately 18% chromium and 8% nickel. This metal is very corrosion-resistant. At the same time, it is practically nonmagnetic although some chromium steels are used for magnets and nickel in its pure state is magnetic. This material furnishes an excellent example of the fact that the alloy does not necessarily retain the properties of the constituents.

Some chromium alloys are used where great wear resistance is required. Thus a chrome-vanadium alloy is used for ball bearings, and a tungsten-chromium alloy for high-speed cutting tools.


molybdenum 2Molybdenum:- Molybdenum is a very effective alloying element. A small percentage has as much effect as much larger amounts of other alloying elements. It improves the homogeneity of the metal and reduces the grain size. It also increases the elastic limit, the impact value, wear resistance, and fatigue strength. An exceptionally important property from the aircraft viewpoint is the improvement in the air-hardeningproperties of steel containing molybdenum. This property is particularly useful where the steel has been subjected to a welding process, as is very common with chrome-molybdenum steel in airplane construction. In general it may be said that while molybdenum is one of the most recently used alloying elements, it shows great promise and without doubt will find many new applications in the near future. The molybdenum steels are readily heat treated, forged and machined.

vanadiumVanadium:- Vanadium is the most expensive of the alloying elements. It is seldom used in amounts over 0.20%, but it is an intensive deoxidizing agent and improves the grain structure and fatigue strength. Vanadium also increases the ultimate strength, yield point, toughness, and resistance to impact, vibration and stress reversal. These latter qualities are identical with fatigue strength and are the basis for using vanadium alloys for propeller hubs and engine bolts. The vanadium alloys, as used generally, contain about 1 % chromium and are called chrome-vanadium steel. These steels have good ductility, along with high strength.



TungstenTungsten:- Tungsten steels have no direct application in aircraft construction, but they possess an-interesting property known as “red hardness”. “High-speed steel” is a tungsten-chromium steel used for tools which will retain their cutting edge even when heated to dull redness by working. This tool steel contains from 14% to 18% tungsten, and 2 % to 4% chromium.





titaniumTitanium:- Titanium is often added in small quantities to 18-8 corrosion-resisting steel to reduce the embrittlement at the operating temperatures of exhaust stacks and collectors.