The Properties of Steel - Essay Example

STEEL MAKING Academia-Research Order No. 1168745 Writer Id. # 21516 INTRODUCTION: Steel, which is an alloy of carbon and iron and other alloying elements, with carbon content up to around 2% is currently one of the most widely manufactured and used engineering materials of construction. Although iron making and its heat treatment to some extent were known to mankind since thousands of years ago, the industrial manufacture of steel started only after the discovery in 1746 by Benjamin Huntsman of the crucible steel process. In 1856, Sir Henry Bessemer announced the discovery of the revolutionary Bessemer Process in which oxygen contained in the air blown through molten pig iron burned off its impurities such as carbon and silicon, in exothermic reactions fueling the process, enabling the production of commercial grade steel at affordable costs(Cottrell,131). Siemens and Martin invented the Open Hearth Process around 1865 which is still surviving to this day. To cut a long story short, these early steel making processes have now been made almost obsolescent over the years, by the rapid advancements made in thermodynamics, electrical engineering, metallurgy, extractive metallurgy and computer and information technology which have transformed the art of steelmaking into quite modern day steel making processes such as the Basic Oxygen Steelmaking (BOS) Process and the Electric Arc Furnace (EAF) Process. While the steel industry has thus undergone gradual but sweeping changes over time, most of the current processes of steel making still involve producing steel from either pig iron or from a mixture of pig iron and steel scrap. All steel making processes deal with the removal of excessive impurities from the melt by means of slag formation in the furnace. The impurities are removed by formation of either an acid slag or a basic slag. The acid or siliceous slag removes the impurities silicon, manganese and carbon by oxidation and also enables the addition of alloying elements such as nickel, chromium, manganese vanadium, molybdenum, tungsten, niobium and titanium as ferroalloys for purposes of alloy steel making. The basic or limey slag removes the impurities phosphorous and sulfur in addition to silicon, manganese and carbon in the bath of metal which is oxidized to a greater extent than in acid process. SOME UNDERLYING MATERIAL SCIENCE CONCEPTS WHICH EXPLAINS THE PROPERTIES OF STEEL: The carbon present as the alloying element in iron matrix imparts to it a reduction in ductility but causes considerable increase in strength. Besides, the alloyed carbon in steel is responsible for various other properties of the steel: its property of getting hardened when subjected to heat-treatment followed by quenching, its property of getting toughened by tempering and its property to get softened by annealing. The carbon also enables a steel of one and the same composition to attain various microstructures when subjected to different heat-treatment cycles. Figure-1 The Fe-Fe3C Phase Diagram showing the different Constituent Phases of Steel (Adapted from Metals Handbook 8th Edition, Vol 8, American Society of Metals, Metals Park, Ohio(1973) The important phases in steel shown in the Fe-Fe3 C phase diagram at Figure 1 are: A liquid solution of iron and carbon, the ductile interstitial solid solution of carbon in Fe (bcc) known as Ferrite (α) phase, the interstitial solid solution of carbon in Fe (fcc) called Austenite (γ) phase, and a hard and brittle carbide with chemical formula Fe3 C called Cementite. The alloy compositions in Fe-Fe3 C diagram with Carbon contents less than 1.7 (weight %) are steels which freeze to form the Austenite (γ)phase while cooling from the liquid phase. Another important constituent of steel is the Pearlite, whose microstructure consists of alternate layers of ferrite and cementite in the proportion 87:13 by weight, and is formed from austenite at eutectoid temperature (A1) 727°C upon slow cooling. The Fe-Fe3C equilibrium diagram shows a Peritectic transformation, an Eutectic transformation and an Eutectoid transformation all of which occur under conditions of slow temperature changes. Constituents such as Martensite, the hardest phase in steel, can be formed under rapid cooling, and hence are not shown as a constituent in Figure 1. Based on carbon content, the steels can be divided into three groups: Hypoeutectoid steels containing 0 to 0.83 % carbon, Eutectoid steel of carbon content around 0.83% and Hypereutectoid steels containing 0.83 % to 2.06% C .The hypoeutectoid steels on freezing from liquid state finally transform to pearlite structures in primary ferrite matrix, the eutectoid steel transform to a completely pearlitic structure and the hypereutectoid steels, on solidification form free cementite along with primary ferrite. The versatility of steel is due to the fact that the crystalline structures and the varying amounts of the of its constituents such as ferrite, austenite, pearlite, martensite and cementite present in the different types of steel can be altered by treating the steels, either by heat-treatment or by mechanical working, in different ways, resulting in its utility as a material capable of exhibiting a wide range of mechanical properties as desired for specific applications. For example, austenite can be made to persist to a low temperature by rapidly cooling the steel and then be transformed to martensite, by quench-hardening. The hard and brittle martensite can be thus tempered by heating the steel article to relieve the stresses and holding it at that temperature so that precipitation of cementite can take place.(Rollason, 183 ) Tempering process is carried out to toughen the steel and bring down its hardness to the required levels. Mild Steel and Medium Carbon Steel contain carbon levels respectively, of 0.1-0.25% and 02-0.5%. Different grades of steels containing 05-1.4% carbon are classified as High Carbon Steels. Each of the above grades of steel are used for a specific application since increasing carbon content is accompanied by higher tensile strength and lowered ductility. While Mild Steel finds extensive use as Section steel for joists, channels, angles etc., Medium Carbon Steels are used to make forgings, shafts, rotors, gears etc. High Carbon steels are used to make Railway rails, Saws, Razors, drills and other tools where high hardness is required.(Rollason,172) Another factor which makes steel the most popular engineering material of choice is its tremendous capability to form high-strength alloys with even relatively small additions of alloying elements. For example, mild steel containing 014% to 0.25% carbon has a Tensile Strength of 216 to 309 N/mm2 after being normalized and oil-quenched. By adding 0.5% molybdenum and 0.003% boron, to a 0.11 % carbon steel, its Tensile strength is increased to 618 N/mm2 (Rollason,228).A typical ultra-high tensile structural steel developed for use in aircraft industry has Tensile strengths of 1544-2160 N/mm2 . It contains 0.2%silicon,0,5% manganese,0.2% nickel,3% chromium,0.9% molybdenum and 0.2% vanadium as alloying elements. (Rollason,232) STEEL MAKING PROCESSES The Basic Oxygen Steelmaking (BOS) Process: The basic oxygen steel making method shown at Figure-2 is among the primary steel making techniques, where molten pig iron that is rich in carbon is converted into steel. Oxygen is blown into the molten pig iron in order to cause oxidation of impurities including excess carbon so that steel is produced. The term “basic” denotes the fact that the slag used while removing the impurities is a basic or limey slag. Moreover, the linings of the furnace meant for basic steel making are made of “basic” or “alkaline” refractory materials such as magnesium oxide (or magnesite and dolomite) which will not be attacked by the “basic” slag. The BOS process of steel making constitutes of an approximate 60% of the total crude steel output in the world, hence making it the dominant technology for steel making. Figure 2: The BOS Process (Adapted from The Basic Oxygen Steelmaking (BOS) Process By John Stubbles) The BOS technology varies from the EAF process in that the former process is self-sufficient in meeting the requirements of heat energy which is generated autogenously. The major raw materials for use in the BOS are hot metal from the blast furnace and steel scrap which are charged into the vessel of the Basic Oxygen Furnace (BOF) with oxygen being blown at supersonic velocity into the BOF. The oxygen oxidizes silicon and carbon present in the hot metal, and the accompanying exothermic reactions release high amounts of heat that melt the scrap. The oxidation reactions of manganese, phosphorous and iron also occur in a less exothermic manner with less emissions of heat energy. Both carbon and phosphorous are removed simultaneously in BOS process. Combustion of carbon monoxide formed inside the oxygen converter vessel provides additional heat input to the metal bath. The steel produced by the BOS process is subjected to a chemical analysis at 2900°F-3000°F. Based on the chemical analysis report, the steel collected in ladles (Figure 3) is either subjected to a secondary refining process or directly sent to a continuous caster for producing slabs, blooms and billets. Figure 3: Molten Steel output from a BOS process furnace being transferred in a Ladle (Adapted from The Basic Oxygen Steelmaking (BOS) Process By John Stubbles) The basic oxygen converter, is pear-shaped and has an oxygen lance that is positioned concentrically. In addition, the shell made of steel is lined with refractories such as magnesia (MgO) that are basic in nature. The ferro-alloys, scrap, fluxes, and the hot metal get charged down through the throat and into the converter.The BOS process was developed from the earlier Linz-Donawitz process and has revolutionized the steel making industry by making use of tonnage oxygen. The Electric Arc Furnace (EAF) Process: Electric arcs struck between carbon electrodes and the metal bath provide the heat energy required for the EAF process shown at Figure 4. The process consists of operations of furnace charging, melting, refining, de-slagging and tapping. The charge generally consists of graded steel scrap. The Figure 4. Electric Arc Furnace Steel Making process (Courtesy of Mannesmann Demag Corp) Jeremy A. T. Jones, Nupro Corporation charge is first melted under an oxidizing slag to remove phosphorous. After de-slagging operation, sulfur is removed and the liquid metal is deoxidized by using another basic slag. Stainless steel scrap charge requires oxygen lancing to remove excess carbon in presence of chromium.(Rollason).High grade steels and alloy steels can be made by the EAF process by first refining the melt formed from steel scrap, removing the first slag by adding ferro-alloys of elements such as nickel, chromium, manganese, vanadium, molybdenum, tungsten, niobium and titanium to form a second reducing slag . Alloy steels can be made in this reducing atmosphere at very high temperatures of above 3270o F by EAF process. The high temperatures do not damage the furnace roof, as no flames are present and heating is mostly contained inside the charge.(Cottrell,139) Other Processes of Steel Making and Refining Vacuum degassing, Vacuum Induction Melting, Consumable Electrode Vacuum Arc Remelting and Electroslag refining (ESR) are special prcesses of steel making. Vacuum is used in Vacuum Degassing Process to remove hydrogen, volatile impurities such as tin, copper, lead antimony etc, to reduce metal oxides and to control alloy composition. Vacuum Induction Melting is used to produce Nickel base and Cobalt base superalloys for aircraft engines. Consumable electrode vacuum arc re-melting is used especially to remove hydrogen and for refining titanium alloys for aircraft applications. Electroslag refining process remelts a preformed electrode of the metal into a water cooled crucible by means of heat from electrical resistance developed in molten slag and further refines the molten steel which settles to the bottom of the crucible in drops.(Rollason,159). Steels manufactured by the above processes contain very low levels of deleterious elements, have good elevated temperature properties and are hence used for aerospace and other strategic applications. CONCLUSION: Carbon and iron combine in unique and spectacular ways as described by their phase diagram which has resulted in myriad possibilities of properties and uses for their resultant alloy called steel, the major processes for manufacture of which, at present, are the BOS process and the EAF process in the world. Work Cited Cottrell,Alan An Introduction to Metallurgy London Universities Press 2000 Rollason,E.C. Metallurgy for Engineers USA Butterworth-Heinemann 1987 The Basic Oxygen Steelmaking (BOS) Process By John Stubbles, Steel Industry Consultant available at http://www.steel.org/Making%20Steel/How%20Its%20Made/Processes/Processes%20Info/The%20Basic%20Oxygen%20Steelmaking%20Process.aspx From STEEL WORKS the Online Resource for Steel Metals Handbook 8th edition, Vol 8,American Society of Metals,Ohio 1973 Read More