Comparison of Steel with Aluminum

Comparison of Steel with Aluminum

Steel is an alloy of iron and other elements, primarily carbon. It is most commonly produced by reduction of iron ore. Carbon, the most common alloying material in steel, acts as a hardening agent, preventing any dislocations within the iron atom crystal lattice from separating and sliding past each other thus making steel more durable. By varying the amount of alloying elements and the form of their presence in the steel, one can control qualities such as hardness, ductility, and tensile strength of steel. Though, steel has been known to be around since 4,000 years ago, it was not widely produced until the 17th century. Its mass production started due to the introduction of the Bessemer process during 1850s. This process made steel production cheaper, efficient and easier. Production of steel is a two stage process. First iron is produced by reduction of iron ore. This iron is then converted into steel by oxidizing the impurities. (Fig 1)

Production process for making steel

Fig 1 Production process of steel

Steel is widely used in construction and other applications because of its high tensile strength and low cost. Iron is the basic component of steel. Composition of steel mainly consists of iron and other elements such as carbon, manganese, silicon, phosphorus, sulphur, and alloying elements. A large number of elements in wide ranging percentages are used for the purpose of alloying of steels.

Variations in chemical composition of steels are responsible for a great variety of steel grades and steel properties. Each element that is added to the basic steel composition has some effect on the properties of the steel and how that steel reacts to the processes of working and fabrication of steels. The chemical composition of steel also determines the behaviour of steel in different environments. Steel standards define the limits for composition, quality and performance parameters for various steel grades.

Iron is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal in the first transition series. It is by mass the most common element on earth, forming much of earth’s outer and inner core. It is the fourth most common element in the earth’s crust. Iron is commonly found in the earth’s crust in the form of an ore, usually an iron oxide, such as magnetite, and hematite etc. Iron is extracted from iron ore by removing the oxygen through combination with a preferred chemical partner such as carbon that is lost to the atmosphere as carbon dioxide.

Aluminum, rare and expensive a century ago, has since been identified as the most common metal on earth, forming about 8 % of the earth’s crust. It is the third most plentiful element known. Only oxygen and silicon exist in greater quantities.

Aluminum is an element that is found in the earth’s crust. It is not soluble in water and ranges from silver to dull gray in colour. Aluminum is soft, durable, lightweight, non-magnetic and ductile in nature and because it is highly reactive in pure form, it is found in combined form in over 270 different minerals. The most common mineral for aluminum is bauxite. Aluminum was believed to be used by ancient Greeks and Romans as dyeing mordants and as astringents.  It was only in 1808 that Sir Humphrey Davy, the British electrochemist, established the existence of aluminum, and it was not until 17 years later that the Danish scientist Oersted produced the first tiny pellet of the metal. It was successfully extracted to its pure form by Friedrich Wohler in 1827.

Presently aluminum is produced from the mineral bauxite (Fig 2). Bauxite is converted to aluminum oxide (alumina) via the Bayer Process. The alumina is then converted to aluminum metal using electrolytic cells by the Hall-Heroult Process. Electrolytic process for the production of aluminum was discovered in 1890.

Production process for making aluminum

Fig 2 Production process for aluminum

The element aluminum, chemical symbol Al, has the atomic number 13. According to present concepts, this means that an aluminum atom is composed of 13 electrons, each having a unit negative electrical charge, arranged in three orbits around a highly concentrated nucleus having a positive charge of 13. The three electrons in the outer orbit give the aluminum atom a valence or chemical combining power of +3. The metal has an atomic weight of 26.98.

Due to its low density and high resistance to corrosion, aluminum is most commonly used in applications such as transportation vehicles, aerospace and structural materials. Also because of its reactive nature, it is used as a catalyst or an additive in explosives. Aluminum is also used in household supplies such as utensils and packaging (aluminum foils). Aluminum use is also beneficial in building cars as it is considered to have a better weight/strength ratio. Aluminum is also a good reflector and a good conductor of electricity. It also has about one-third the density and stiffness of steel and can be easily machined, cast, drawn and extruded. While aluminum is widely used in airframe structures and increasingly being used in automotive frames to decrease weight and improve fuel efficiency, it is not widely used as a civil structural material.

Steel and aluminum both are common substances that are used in everyday life and in almost everything being used today. While steel is the most popular alloy, aluminum is the most abundant metal on the earth. The amount of aluminum produced annually worldwide is second only to that of steel. Though these two are used in similar applications, they are completely different from each other.

Steel and aluminum are both versatile metals. These materials come with their advantages and disadvantages. Over the past several decades, advancements have been made in the production of steel and aluminum. Steel and aluminum are the two most popular materials used in various applications. They are now more widely used than ever. They feature in a wide range of products from drink cans to vehicles and buildings. Each material has a defined and distinct set of characteristics that make it the right or the wrong material for the job.

Comparison of properties of steel and aluminum

  • Density – The density of steel varies based on the alloying constituents but usually ranges between 7.75 and 8.05 tons/cum.  Lightness is the outstanding and best known characteristic of aluminum. It has a density of 2.70 tons/cum which is around one third that of steel. The low density of aluminum accounts for it being lightweight but this does not affect its strength.
  • Melting point – The melting point of aluminum is sensitive to purity, e.g. for 99.99 % pure aluminum at atmospheric pressure, it is 660 deg C but this reduces to 635 deg C for 99.5% commercial pure aluminum. The addition of alloying elements reduces this still further down to 500 deg C for some magnesium alloys under certain conditions. The melting point of steel varies with its composition mainly on its carbon content. Melting point of iron is around 1540 deg C and that of steel varies from 1370 deg C to 1510 deg C.
  • Strength – Pure aluminum does not have a high yield strength and tensile strength. However, the addition of alloying elements like manganese, silicon, copper and magnesium can increase the strength properties of aluminum and produce an alloy with properties tailored to particular applications. The yield strengths and tensile strengths of aluminum and its alloys vary from 30-500 N/sq mm and 79 -570 N/sq mm respectively. The yield strengths and tensile strengths of steel are higher than aluminum and are in the range of 250-1000 N/sq mm and 400-1250 N/sq mm respectively. The yield strength and tensile strength of steels are also dependent on its composition as well as on the heat treatment of the steel.
  • Magnetic property – Aluminum and its alloys are very slightly paramagnetic, as it has a magnetic permeability slightly greater than one. The permeability of aluminum is 0.00000126 henries per metre. The magnetic susceptibility, degree of magnetization/applied magnetizing force, of 99.99 % purity aluminum is only 0.000000623, which for practical purposes is regarded as non-magnetic. Steel is ferromagnetic, although its magnetic properties are different from iron. Iron substances are magnetized relatively quickly. However, iron types including soft iron lose its magnetism just as fast. These characteristics are particularly useful when strong but temporary magnets are needed such as electromagnets. Steel takes considerably longer to get magnetized, but retains its magnetism for much longer periods. This property, called high retention, enables steel to be used as permanent magnets. The permeability in henries per metre of carbon steel is 0.000126 and electrical steel is 0.005. The permeability values in henries per metre for the austenitic stainless steels are in the range of 0.00000126-0.0000088 while those of other stainless steels (ferritic and martensitic) are in the range of 0.00005- 0.00126.
  • Electrical conductivity – The electrical conductivity of 99.99 % pure aluminum at 20 deg C is 63.8 % of the International Annealed Copper Standard (IACS). Because of its low specific gravity, the mass electrical conductivity of pure aluminum is more than twice that of annealed copper and greater than that of any other metal. The value of electrical conductivity of carbon steel (carbon around 0.1 %) is around 12.1 % of the IACS. The value for electrical conductivity for stainless steel (18 Cr-8 Ni) is around 2.5 % of the IACS.
  • Thermal conductivity – The thermal conductivity of 99.99% pure aluminum is 244 W/m-K for the temperature range 0-100 deg C which is 61.9 % of the IACS, and again because of its low specific gravity its mass thermal conductivity is twice that of copper. In case of steels, the thermal conductivity varies in the range of 24.3 to 65.2 W/m-K for carbon steels, 26-48.6 W/m-K in case of alloy steels, 11.2 -36.7 W/m-K in case of stainless steels, and 19.9-48.3 W/m-K in case of tool steels.
  • Specific heat – Aluminum has a relatively high specific heat when compared with other metals on a weight basis, i.e. 921 J/kg at 100 deg C which is higher than that of any common metal except magnesium. On a volume basis, however, the heat capacity of aluminum is less than any of the heavier metals. Specific heat of steels is 490-500 J/kg.
  • Modulus of elasticity or Young’s modulus – Young’s Modulus or Modulus of elasticity is a measure of stiffness of an elastic material. It is used to describe the elastic properties of objects like wires, rods or columns when they are stretched or compressed. Modulus of elasticity for aluminum is 69 GPa while that of steels is in the range of 190-210 GPa. (1 GPa=1,000,000,000 N/sq mm).
  • Poisson’s ratio – Poisson’s ratio is the ratio of the relative contraction strain (or transverse strain) normal to the applied load – to the relative extension strain (or axial strain) in the direction of the applied load. Poisson’s ratio for aluminum is 0.334 and for steel it is in the range of 0.27 to 0.3.
  • Corrosion resistance – Aluminum has a higher resistance to corrosion than many other metals owing to the protection conferred by the thin but tenacious film of oxide. This oxide layer is always present on the surface of aluminum in oxygen atmospheres. Aluminum is, however, a very reactive chemical element and its successful resistance to corrosion depends on the completeness with which the protective film of aluminum oxide prevents this underlying activity coming into play. Steel corrodes in many media including most outdoor environments. When unalloyed or alloyed steel without corrosion protection is exposed to the atmosphere, the surface takes a reddish brown colour after a short time. This reddish brown colour indicates rust is forming and the steel is corroding. While corroding the steel is getting oxidized to produce rust, which occupies approximately 6 times the volume of the original material consumed in the process. The corrosion process begins when a corrosive medium acts on the steel. The corrosion can be either chemical corrosion or electrochemical corrosion. Chromium is normally added to steel for increasing oxidation resistance. Corrosion resistance of chromium steels increases sharply at a chromium level of greater than 12 %. Chromium forms a very coherent oxide layer on the steel surface that prevents further oxidation and thus provides resistance to corrosion in the steels. Mild steels with around 0.25 % copper have improved atmospheric corrosion resistance. This improvement is especially notable in industrial atmospheres where corrosion rates of copper bearing steels may be two to four times less than for comparable carbon steels. Copper also forms a very coherent oxide layer on the steel surface that prevents further oxidation and thus provides resistance to corrosion in the steels.
  • Fatigue – In a steel structure, fatigue is normally not considered for general structure (fatigue is not usually the limiting criteria). Aluminum however, is subject to fatigue failure (referred to as its endurance limit) more readily than mild steel. In case of alloyed aluminum, endurance limit considered more carefully wherever there will be vibration, and high stress points.

In case of aluminum, the coefficient of thermal expansion is non-linear over the range from minus 200 deg C to plus 600 deg C but for practical purposes is assumed to be constant in the temperature range of 20 deg C to 100 deg C. The coefficient of thermal expansion of alloys is affected by the nature of their constituents. The presence of silicon and copper reduces expansion while magnesium increases it. For the common commercially used wrought aluminum alloys, the coefficient of expansion varies from 0.0000235 /K for 4.6 % Cu aluminum alloy to 0.0000245 /K for 4.5 % Mg aluminum alloy, i.e. twice that of steel. For steel it is 0.000012/K.

The differential coefficient of expansion is required to be taken into consideration when aluminum is used in conjunction with other materials, e.g. large aluminum/steel structures. However, the stresses induced are moderated by aluminum’s low elastic modulus which is one third that of steel. Only where dimensions are really large and the structural members slender (laterally unstable) does the connection to steel pose a differential expansion problem. This would apply with curtain walls for high rise buildings and parapets for bridges where long slender aluminum extrusions are set on steel frameworks. In these cases slip joints, plastic caulking and other stress-relieving devices are usually needed. In cases where the structure is stiff and unlikely to buckle such as an aluminum superstructure on a steel hulled ship all joints are now made rigid and the differential expansion is accepted as a compressive or tensile stress.

Another form of dimensional change, which does not directly affect the user of aluminum but is important in the production of castings, is the contraction of the metal on solidification; this is dependent upon alloy and is between 1 % and 2 % (comparative figures for steel is 2%).

A Massachusetts Institute of Technology study and related cost models demonstrate aluminum to be significantly more costly than steel. For example (i) production of aluminum is two to three times more expensive than steel, (ii) manufacturing and assembly with aluminum is 20 % to 30 % more expensive than steel, and (iii) the mass reduction with steel can be achieved at nearly zero cost, while engineering studies show low-density materials like aluminum cost some money in engineering per kilogram saving in weight.

Worldwide annual demand for aluminum is met by around 75 % with newly smelted aluminum and balance 25 % by the recycled aluminum scrap. The use of recycled aluminum is economically and environmentally compelling. It takes 14,000 kWh to produce 1 ton of new aluminum. Conversely it takes only 5 % of this to remelt and recycle one ton of aluminum. There is no difference in quality between virgin and recycled aluminum and its alloys.

In case of steel, more than 33 % of the worldwide demand for steel is met by the recycled steel. Steel recycling accounts for significant raw material and energy savings. Over 1,200 kg of iron ore, 7 kg of coal, and 51 kg of limestone are saved for a ton of steel scrap used. From an environmental point of view, steel recycling has an enormous impact on the reduction of CO2 emissions. If 1 ton of hot rolled steel is produced from 100 % scrap rather than new materials, the total CO2 savings is approximately 1.8 tons. There is no difference in quality between virgin and recycled steel.

Pure aluminum is soft, ductile, and corrosion resistant. It has a high electrical conductivity. It is widely used for foil and conductor cables, but alloying with other elements is necessary to provide the higher strengths needed for other applications. Aluminum is one of the lightest engineering metals, having strength to weight ratio superior to steel.

By utilizing various combinations of its advantageous properties such as strength, lightness, corrosion resistance, recyclability and formability, aluminum is being employed in an ever-increasing number of applications. This array of products ranges from structural materials through to thin packaging foils.

Like steel, aluminum can be severely deformed without failure. This allows aluminum to be formed by rolling, extruding, drawing, machining and other mechanical processes. It can also be cast to a high tolerance. Alloying, cold working and heat-treating can all be utilized to tailor the properties of aluminum as it is normally done in steel.

Steel is made by putting iron through a process known as smelting, where iron is extracted from the iron ore and excess oxygen is removed and the iron is combined with chemical partners such as carbon. Steel, compared to pure iron is more rust resistant and has a better weldability. Other metals are added to the iron/carbon mix in order to affect the properties of steel. Metals such as nickel and manganese add to the steel’s tensile strength and make austenite form of the iron-carbon solution more chemically stable, while chromium can increase the hardness and melting temperature. Compared to aluminum, steel is very malleable. Steel is one of the most common used alloys in today’s world. It is found in various applications such as tools, utensils, appliances, transportation, weapons, aerospace, buildings, and infrastructure etc. It is also the most commonly produced alloy with almost more than 1.66 billion tons produced annually.

Aluminum is the backbone of the aerospace industry, is used to assist with cooking and packaging, assist in the manufacture of fully killed steel and is the base for a versatile paint. Aluminum is a light and attractive metal exhibiting a high degree of corrosion resistance in normal corrosive environments. It is also soft, hard, easy to weld, difficult to weld, and a host of other seemingly conflicting characteristics. If this sounds confused, it is. The properties of a particular aluminum product depend on the alloy chosen. The term aluminum refers to a family of alloys. Knowledge of these alloys is the key to the effective use of aluminum.

Steel is harder than aluminum. Most tempers and alloys of aluminum dent, ding, or scratch more easily as compared to steel. Steel is strong and less likely to warp, deform or bend under application of weight, force or heat. Nevertheless the strength of steel’s tradeoff is that steel is much heavier /much denser than aluminum.  Steel is typically 2.5 times denser than aluminum.

Aluminum is a lighter metal in terms of density.  However, because it cannot take the same stress as steel, one typically has to use more of it, sometimes to the point of negating the cost difference. Aluminum is sometimes cheaper.  If one is looking for an anti-corrosive metal, then aluminum is cheaper than stainless steel.  However, compared to basic steel, it often costs more.

Naturally aluminum’s anti-corrosive qualities are one of its best features.  Aluminum does not give or bend as much as steel, meaning it is more prone to breaking out right.  It also does not absorb vibrations as well as steel does, which can be good or bad depending on the situation.

Steel is known for being tough and heavy.  Because of how tough it is, one can often use less, partially negating the weight.  It can be a little harder to work with, but once one has it set in place and in shape, one can expect it to stay that way. Strength is the biggest advantage of steel. Aluminum is a great choice for many situations, but if one absolutely has to choose one or the other, the steel is preferable.

Like most metals, aluminum and steel can be melted, cast, formed and machined into different shapes. Steel has the advantage over aluminum in formability; aluminum has less forming range than steel. Also like most metals, aluminum and steel can conduct electricity. Aluminum is non-toxic, making it ideal for packaging food products. Both materials are 100 % recyclable.

Aluminum generates a protective oxide coating naturally, which makes it corrosion resistant. Aluminum is a good conductor of heat and electricity; it is also twice as good a conductor as copper. Aluminum is a good reflector of light and heat, making it ideal to use in the manufacturing of reflectors. This material is ductile with a low melting point and density. An advantage that steel has over aluminum is its surface hardness, which is much harder than aluminum.

Materials that are not affected by magnetic fields are considered to be ‘non-magnetic’. Aluminum is a metal that is considered to be non-magnetic; it is not affected by magnetic fields. Steel on the other hand is magnetic in that it is affected by magnetic fields. Both materials are recyclable but the magnetic property of steel makes it much easier to separate from other substances in the recycling process.

Aluminum is much lighter than steel. The light weight of aluminum makes it easier and more efficient to machine than steel. Aluminum has a lower melting point than steel, which also makes it easier to machine. Aluminum’s fatigue performance happens to be half that of steel, which is an advantage steel has over aluminum in vehicle life durability.

There are countless applications for both types of metals. Steel and aluminum are employed in the marine industry, the automotive industry, the mould-making industry, machining industry as well as many others.

The similarities of aluminum and steel are namely (i) structural applications of aluminum and steel are mostly similar, (ii) design problems/processes are similar so an identical approach is used, (iii) the design rules for aluminum and steel are purposely very similar.

However, there are important differences in physical as well as mechanical properties which have to be accounted for in the design process. The differences in properties, the consequences for structural behaviour and how to deal with that in structural design are described below.

First of all, the low density of aluminum is the main driver for using it in many structural applications. The high strength to weight ratio is the number one reason for the development of the aircraft industry. Although its low weight is a favourable property, it can in some cases be a disadvantage; for example with cyclic loading the ratio live load/dead load is disadvantageous as compared to steel and so fatigue must be considered early in the design stage.

The low density makes an aluminum structure prone to vibrations and in these cases the dynamic behaviour of the structure has to be considered. The Young modulus is important for the structural behaviour. Its value is about one third that of steel, but contrary to density, this is a disadvantage compared to steel.

The low value of the Young modulus has a big influence on the deformations of an aluminum structure. A well-known example is the bending of beams, where the stiffness is the governing factor.

The above indicates that in designing aluminum structures, it is often not the strength, but in many cases the deformation, that is the governing factor. So in building and civil engineering it is frequently the alloy which does not have the highest strength that has to be considered.

The low Young modulus is also responsible for the higher sensitivity to stability problems in aluminum structures (buckling). The critical stress for buckling is linearly related to the Young modulus. Moreover, aluminum designs often have very slender, thin walled sections which make it even more important to consider their stability in designing structures.

Finally, there is cyclic loading, where the Young modulus is responsible for the lower fatigue strength of aluminum which is about half that of steel. This, in combination with the low density, means that fatigue design should be considered more carefully than with steel structures. Similar to the Young modulus is the shear modulus which is also about one third of that for steel. This means that the resistance against shear forces, shear deformations and shear stability (for example lateral torsional buckling of beams) can be an important aspect in the design.

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