Steel and steel alloys

These comprises of vital knowledge on Steels and Alloys.

1. STEEL AND STEEL ALLOYS Name : Abdulmalik Abdulsalaam Ojochegbe University : NKE Department : Environmental Engineering

2. CONTENTS Types of Irons and Steels Properties of Structural Steels Heat Treatment and Hardening of Steels Effects of Grain Size Steel Alloys Welding Ferrous Materials Effects of Steel Production Methods Effects of Punching and Shearing Corrosion of Iron and Steel

3. What is steel? Steel is an alloy of iron and other elements, including carbon, nickel and chromium. Steel is stronger than pure iron and can be used for everything from sauce pans, bridges, etc.

4. Types of steel Different types of steel are classified by how much carbon they contain. Varying the amount of carbon gives steel different properties. For example, a higher carbon content makes a hard steel. low carbon steel(Structural) contains less than 0.25% carbon high carbon steel contains more than 0.5% carbon stainless steel – an alloy of iron that contains at least 11% chromium and smaller amounts of nickel and carbon titanium steel – an alloy of iron and titanium

5. What is Iron? Iron is a strong magnetic and silvery-grey metal, much used as a material for construction and manufacturing, especially in the form of steel. Iron is a chemical element with symbol Fe, Its atomic number is 26 and atomic mass is 55.85.

6. Types of iron Iron as an element is not in common use. However, the term iron is used for different materials which contain a high percentage of iron in elemental form. C ategorized as : pure iron, wrought iron, cast iron, pig iron, and direct reduced iron.

7. Pure iron is a term used to describe new iron produced in an electric arc furnace where temperatures sufficient to melt the iron can be achieved. Wrought iron is a form of commercial iron which has very low carbon content (less than 0.10 %), less than 0.25 % of impurities consisting of Sulphur, phosphorus, silicon and manganese. Cast iron represents a large family of ferrous alloys. Cast irons are multi-component ferrous alloys, which solidify with a eutectic. The major elements of cast irons are iron, carbon (2 % or more), silicon (1 % to 3 %), minor elements (less than 0.1 %), and often alloying elements (less than 0.1%). Pig iron is an intermediate product of a steel plant which is cast in a pig casting machine from hot metal (liquid iron) produced during smelting of iron ore in a blast furnace or in a smelting reduction furnace. Direct reduced iron ( DRI) is produced by the reduction of iron ore (in the form of lumps or pellets) by either non-coking coal or a reducing gas produced by reforming of natural gas. The reducing gas can also be produced by the gasification of coal.

8. Structural Steel This is a category of steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section.

9. Types of Structural steel Carbon steel is a special type of steel that, as the name suggests, has a higher concentration of carbon than other types of steel. Most types of steel have a relatively low carbon content of about 0.05% to 0.3%. High strength low alloy steels is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. They have a carbon content between 0.05 and 0.25% to retain formability and weldability. High strength low alloy (HSLA) steels are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition. Forged steel is a material resulting from alloying iron and carbon under extremely high pressure. Forged steel has less surface porosity, a finer grain structure, more tensile and fatigue strength, and greater ductility than any other processed steel. Heat Treated Constructional Alloy Steels are low carbon alloy steels that have been heat treated by conventional liquid quenching and tempering to a strength level substantially higher than that of the high strength low alloy grades.

10. Properties of Structural Steels 1. Density of a material is defined as mass per unit volume. Structural steel has density of 7.75 to 8.1 g/cm3. 2. Elastic Modulus Elastic modulus or modulus of elasticity is the measurement of tendency of an object to be deformed when force or stress is applied to it. Typical values for structural steel range from 190-210 gigapascals. 3. Poisson's Ratio It is the ratio between contraction and elongation of the material. Lower the value, lesser the object will shrink in thickness when stretched. Acceptable values for structural steel are 0.27 to 0.3. 4. Tensile Strength of an object is the determination of limit up to which an object can be stretched without breaking. Fracture point is the point at which an object breaks after application of stress. Structural steel has high tensile strength so is preferred over other materials for construction. 5. Yield Strength Yield strength or yield point is the stress at which an object deforms permanently. It cannot return to its original shape when stress is removed. Structural steel made of carbon has yield strengths of 187 to 758 megapascals. Structural steel made of alloys has values from 366 to 1793 megapascals. 6. Melting Point There is no defined value for melting point due to the wide variations in types of structural steel. Melting point is the temperature at which object starts to melt when heated. 7. Specific Heat Specific heat or heat capacity is the amount of heat which needs to be applied to the object to raise its temperature by a given amount. A higher value of specific heat denotes greater insulation ability of the object. Structural steel made of carbon has values from 450 to 2081 and that made from alloys has values ranging from 452 to 1499. 8. Hardness is the resistance of an object to shape change when force is applied. There are 3 types of hardness measurements. Scratch, indentation and rebound. Structural steel made by using alloys has hardness value between 149-627 Kg. Structural steels made of carbon has value of 86 to 388 Kg.

11. Heat Treatment and Hardening of Steels Heat Treatment of Hardening steels is the heating and cooling of metals to change their physical and mechanical properties, without letting it change its shape. Heat treatment could be said to be a method for strengthening materials but could also be used to alter some mechanical properties such as improving formability, machining, etc. The most common application is metallurgical but heat treatment of metals can also be used in the manufacture of glass, aluminum, steel and many more materials.

12. Effects of Grain Size When a low-carbon steel is heated above the A3 temperature line, for example, to hot rolling and forging temperatures, the steel may grow coarse grains. For some applications, this structure may be desirable; for example, it permits relatively deep hardening, and if the steel is to be used in elevated-temperature service, it will have higher load-carrying capacity and higher creep strength than if the steel had fine grains. Fine grains, however, enhance many steel properties: notch toughness, bendability, and ductility. In quenched and tempered steels, higher yield strengths are obtained. Furthermore, fine-grain, heat-treated steels have less distortion, less quench cracking, and smaller internal stresses. Steel Alloys Alloy steel is steel that includes about 5% alloying elements in its composition. These alloying elements can include manganese, chromium, vanadium, nickel, and tungsten. The addition of alloying elements increases overall machinability and corrosion resistance. it is most commonly used to manufacture pipes, especially pipes for energy-related applications. It’s also used in the manufacturing of heating elements in appliances like toasters, silverware, pots and pans, and corrosion-resistant containers.

13. Welding Ferrous Materials General welding characteristics of the various types of ferrous metals are as follows: Wrought iron is ideally forged but may be welded by other methods if the base metal is thoroughly fused. Slag melts first and may confuse unwary operators. Low-carbon iron and steels (0.30%C or less) are readily welded and require no preheating or subsequent annealing unless residual stresses are to be removed. Medium-carbon steels (0.30 to 0.50%C) can be welded by the various fusion processes. In some cases, especially in steel with more than 0.40% carbon, preheating and subsequent heat treatment may be necessary. High-carbon steels (0.50 to 0.90%C) are more difficult to weld and, especially in arc welding, may have to be preheated to at least 500F and subsequently heated between 1200 and 1450F. For gas welding, a carburizing flame is often used. Care must be taken not to destroy the heat treatment to which high-carbon steels may have been subjected. Tool steels (0.80 to 1.50%C) are difficult to weld. Preheating, post annealing, heat treatment, special welding rods, and great care are necessary for successful welding. Structural steels may be welded by shielded metal arc, submerged arc, gas metal arc, flux-cored arc, electroslag, electro gas, or stud-welding processes. Types Of Welding

14. Shielded-metal-arc welding fuses parts to be joined by the heat of an electric arc struck between a coated metal electrode and the material being joined, or base metal. The electrode supplies filler material for making the weld, gas for shielding the molten metal from the air, and flux for refining this metal. Submerged-arc welding fuses the parts to be joined by the heat of an electric arc struck between a bare metal electrode and base metal. The weld is shielded from the air by flux. The electrode or a supplementary welding rod supplies metal filler for making the weld. Gas-metal-arc welding produces fusion by the heat of an electric arc struck between a filler-metal electrode and base metal, while the molten metal is shielded by a gas or mixture of gas and flux. For structural steels, the gas may be argon, argon with oxygen, or carbon dioxide. Electroslag welding uses a molten slag to melt filler metal and surfaces of the base metal and thus make a weld. The slag, electrically conductive, is maintained molten by its resistance to an electric current that flows between an electrode and the base metal. The process is suitable only for welding in the vertical position. Moving, water-cooled shoes are used to contain and shape the weld surface. The slag shields the molten metal. Electro gas welding is similar to the electroslag process. The electro gas process, however, maintains an electric arc continuously, uses an inert gas for shielding, and the electrode provides flux. Stud welding is used to fuse metal studs or similar parts to other steel parts by the heat of an electric arc. A welding gun is usually used to establish and control the arc, and to apply pressure to the parts to be joined. At the end to be welded, the stud is equipped with a ceramic ferrule, which contains flux and which also partly shields the weld when molten

15. Effects of Steel Production The iron and steel industry is the world’s biggest energy consuming manufacturing industry with the largest share in the world’s economy. In this framework, steel companies are becoming increasingly aware about the sustainability challenges, in order to satisfy such requirements and to increase their competitiveness through an adequate management of resource and energy. Steel production is in fact characterized by an energy-intensive activity, since the largest part of the production process takes place at high temperatures. Besides, iron ore is converted into metallic iron by using carbon as reducing agent. As global warming due to CO2 emissions, steel production is considered one of today’s main environmental problem and environmental impact of steel production mainly focus on reduction of energy use. This reduction of energy use is mainly achieved by important process measures and the reduction of material losses in the different production steps (also by looking at the use of steel slag as an opportunity to save natural resources) as well as of good housekeeping practices. Next to CO2, large industrial steelworks also emit pollutants addressing the discussion to “how and where” treat them.

16. Effects of Punching and Shearing Punching holes and shearing during fabrication are cold-working operations that can cause brittle failure. Bolt holes, for example, may be formed by drilling, punching, or punching followed by reaming. Drilling is preferable to punching, because punching drastically cold-works the material at the edge of a hole. This makes the steel less ductile and raises the transition temperature. The degree of embrittlement depends on type of steel and plate thickness. Furthermore, there is a possibility that punching can produce short cracks extending radially from the hole. Consequently, brittle failure can be initiated at the hole when the member is stressed. Should the material around the hole become heated, an additional risk of failure is introduced. Heat, for example, may be supplied by an adjacent welding operation. If the temperature should rise to the 400 to 850F range, strain aging will occur in material susceptible to it. The result will be a loss in ductility. Reaming a hole after punching can eliminate the short radial cracks and the risks of embrittlement. For the purpose, the hole diameter should be increased by 1⁄16 to 1⁄4 in by reaming, depending on material thickness and hole diameter. Shearing has about the same effects as punching. If sheared edges are to be left exposed, 1⁄16 in or more material, depending on thickness, should be trimmed by gas cutting. Note also that rough machining, for example, with edge planers making a deep cut, can produce the same effects as shearing or punching.

17. Corrosion of Iron and Steel Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide. It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Rusting is the common term for corrosion of elemental iron and its alloys such as steel. Many other metals undergo similar corrosion, but the resulting oxides are not commonly called "rust".

18. How to Prevent corrosion Deaeration: If oxygen is removed from water, corrosion stops. In hot-water heating systems, therefore, no fresh water should be added. Boiler feedwater is sometimes deaerated to retard corrosion. Coatings and painting: Rust formation can be controlled with coatings, such as paint, lacquer, varnish, or wax tapes that isolate the iron from the environment. Large structures with enclosed box sections, such as ships and modern automobiles, often have a wax-based product (technically a "slushing oil") injected into these sections. Bluing: Bluing is a technique that can provide limited resistance to rusting for small steel items, such as firearms; for it to be successful, a water-displacing oil is rubbed onto the blued steel and other steel. Galvanization: Galvanization consists of an application on the object to be protected of a layer of metallic zinc by either hot-dip galvanizing or electroplating. Zinc is traditionally used because it is cheap, adheres well to steel, and provides cathodic protection to the steel surface in case of damage of the zinc layer. In more corrosive environments (such as salt water), cadmium plating is preferred. Cathodic protection: Cathodic protection is a technique used to inhibit corrosion on buried or immersed structures by supplying an electrical charge that suppresses the electrochemical reaction. If correctly applied, corrosion can be stopped completely. In its simplest form, it is achieved by attaching a sacrificial anode, thereby making the iron or steel the cathode in the cell formed.

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