Engineering Materials and Metallurgy PDF

Downloaded From : www.EasyEngineering.net CONTENTS S.NO TOPIC PAGE NO UNIT 1 CONSTITUTION OF ALLOYS AND PHASE DIAGRAMS 1.1 Classification of materials...

Downloaded From : www.EasyEngineering.net CONTENTS S.NO TOPIC PAGE NO UNIT 1 CONSTITUTION OF ALLOYS AND PHASE DIAGRAMS 1.1 Classification of materials 4 1.2 Type of bonding 4 1.3 Crystal structures 5 1.4 Imperfection in solids 5 1.5 Introduction to phase diagram 6 1.6 Solid solution 6 1.7 Iron Carbon Diagram 7 1.8 Metal types 13 ww 1.8.1 1.8.2 1.8.3 Ferrous metals Alloy steels Non ferrous metals 13 16 17 w.E UNIT 2 HEAT TREATMENT PROCESS 2.1 2.2 Hardening asy Basic principles of heat treatment 20 20 2.3 2.4 Annealing Normalizing En 22 25 2.5 2.6 Hardening process Thermo chemical process gin 25 26 2.7 2.8 Tempering Martempering and Austempering eer 27 27 UNIT 3 FERROUS AND NON FERROUS METALS ing 3.1 3.2 Effect of alloying elements on steel properties Characteristics of alloying elements.ne 29 30 3.3 3.4 3.5 Maraging steels Heat treatment cycle Classificaion of copper and its alloys 30 31 32 t 3.5.1 Brasses 33 3.5.2 Bronze 33 3.5.3 Tool and die steel 34 3.6 Effects of alloying elements on steel 35 UNIT 4 NON METALLIC MATERIALS 4.1 Polymers 36 4.2 Historical development 37 4.3 Polymer synthesis 40 4.4 Modification of Natural polymer 41 4.5 Polymer Architecture 42 4.6 Polymer morphology 43 Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net 4.7 Mechanical properties 45 4.8 Phase behavior 46 4.9 Polymer materials 52 4.9.1 Elstomer 52 4.9.2 Thermoplastics 52 4.9.3 Thermoets 52 4.10 Polmer structure 53 4.11 Polyisoperene 56 4.12 Isobutene-isoperene 56 4.13 Ethylene propylene 57 4.14 Silicone 58 4.15 Thermoplasts 59 4.16 Fundamentals of ceramics 61 4.17 Effect of microstructure on tribological properties of 67 ceramics ww UNIT V MECHANICAL PROPERTIES AND DEFORMATION MECHANISM 5.1 5.2 w.E Plastic deformation Brinell hardness est 71 72 5.3 5.4 asy Vickers hardness test Rockwell hardness test 73 74 5.5 5.6 Charpy impact test Fatigue test En 75 79 5.7 Creep test gin 82 Question bank University questions eer 86 91 ing.ne t Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME8491 ENGINEERING MATERIALS AND METALLURGY LTPC 3003 OBJECTIVES: To impart knowledge on the structure, properties, treatment, testing and applications of metals and non-metallic materials so as to identify and select suitable materials for various engineering applications. UNIT I ALLOYS AND PHASE DIAGRAMS 9 Constitution of alloys – Solid solutions, substitutional and interstitial – phase diagrams,Isomorphous, eutectic, eutectoid, peritectic, and peritectoid reactions, Iron – carbon equilibrium diagram. Classification of steel and cast Iron microstructure, properties and application. UNIT II HEAT TREATMENT 10 Definition – Full annealing, stress relief, recrystallisation and spheroidising – normalising, hardening and Tempering of steel. Isothermal transformation diagrams – cooling curves superimposed on I.T. diagram CCR – Hardenability, Jominy end quench test - Austempering, martempering – case hardening, carburizing, Nitriding, cyaniding, ww carbonitriding – Flame and Induction hardening – Vacuum and Plasma hardening. UNIT III FERROUS AND NON-FERROUS METALS 9 w.E Effect of alloying additions on steel- α and β stabilisers– stainless and tool steels – HSLA, Maraging steels – Cast Iron - Grey, white, malleable, spheroidal – alloy cast irons, Copper and copper alloys – Brass, Bronze and Cupronickel – Aluminium and Al-Cu – precipitation asy strengthening treatment – Bearing alloys, Mg-alloys, Ni-based super alloys and Titanium alloys. UNIT IV NON-METALLIC MATERIALS En Polymers – types of polymer, commodity and engineering polymers – Properties and 9 gin applications of various thermosetting and thermoplastic polymers PP, PS, PVC, PMMA, PET,PC, PA, ABS, PI, PAI, PPO, PPS, PEEK, PTFE, Polymers – Urea and Phenol eer formaldehydes)- Engineering Ceramics – Properties and applications of Al2O3, SiC, Si3N4, PSZ and SIALON –Composites-Classifications- Metal Matrix and FRP - Applications of Composites. UNIT V MECHANICAL PROPERTIES AND DEFORMATION MECHANISMS ing Mechanisms of plastic deformation, slip and twinning – Types of fracture – Testing of 8.ne materials under tension, compression and shear loads – Hardness tests Brinell, Vickers and Rockwell, hardness tests, Impact test lzod and charpy, fatigue and creep failure mechanisms. OUTCOMES: TOTAL : 45 PERIODS Upon completion of this course, the students can able to apply the different materials, their t processing, heat treatments in suitable application in mechanical engineering fields. TEXT BOOKS: 1. Avner,, S.H., “Introduction to Physical Metallurgy”, McGraw Hill Book Company,1994. 2. Williams D Callister, “Material Science and Engineering” Wiley India Pvt Ltd, Revised Indian Edition 2007 REFERENCES: 1. Raghavan.V, “Materials Science and Engineering”, Prentice Hall of India Pvt. Ltd., 1999. 2. Kenneth G.Budinski and Michael K. Budinski, “Engineering Materials”, Prentice Hall of India Private Limited, 4th Indian Reprint 2002. 3. Upadhyay. G.S. and Anish Upadhyay, “Materials Science and Engineering”, Viva Books Pvt. Ltd., New Delhi, 2006. 4. U.C.Jindal : Material Science and Metallurgy, "Engineering Materials and Mettalurgy", First Edition, Dorling Kindersley, 2012 Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY COURSE OBJECTIVE  Primary objective is to present the basic fundamentals of materials science and engineering.  Expose the students to different classes of materials, their properties, structures and imperfections present in them.  Help understand the subject with ease by presenting the content in a simplified and logical sequence at a level appropriate for students.  Aid the teaching learning process through relevant illustrations, animations and web content and practical examples. ww Highlight important concepts for each topic covered in the subject w.E  Provide opportunity of self-evaluation on the understanding of the subject matter. Historical Perspective: asy Materials are so important in the development of civilization that En we associate Ages with them. In the origin of human life on Earth, the Stone Age, people used only natural materials, like stone, clay, skins, and wood. When gin people found copper and how to make it harder by alloying, the Bronze Age started about 3000 BC. The use of iron and steel, a stronger material that ee gave advantage in wars started at about 1200 BC. The next big step was the discovery of a cheap process to make steel around 1850, which enabled the rin railroads and the building of the modern infrastructure of the industrial world. g.n et SCE 1 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY Quest for newer materials: The driving force for the progress- Stone Age to IT age ww w.E Quest for more advanced materials to meet the growing needs as the civilization 300000 BC - Stone age asy progressed.A look at the history of materials chronologically clearly reveals this People living in caves and hunting with stone-made weapons En 200,000 BC Discovery of fire – Said to be the most significant discovery in human gin civilization. However, till the time the fire was controlled to contain and utilize the heat, it was not significant. ee Containing the fire – Was not possible without materials. Started with rin clay a ceramic material pots and now we have all kinds of means to control and contain fire. Introduction of metals 5500 BC First metals to be discovered – Copper and Gold g.n fire and hammering in early days et 5000 BC Material processing - Annealing and Shaping. Throwing copper into camp 4000 BC Melting and casting of metals. Melting of Gold to give it different shapes 3500 BC Reduction of copper from its ore – Nile Valley The dawn of metallurgy. Perhaps discovered by chance much before by early potters Discovery of Alloy - Metal Combinations 3000 BC The discovery of alloy – combination of metals Mixing of Tin with Copper – Bronze Copper ore invariably contains some Tin – Mixing of different ores having different Tin content produced the first Bronzes. Iron and Steel – Building blocks of human civilization 1450 BC Iron wheels – discovery of iron making. Revolution in warfare and cultivation 1500 BC Invention of Blast furnace – Production of pig iron from ores SCE 2 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY 1855 AD Sir Henry Bessemer 1813 -1898) Bessemer steel making patent 20TH CENTURY Many other steel making processes – LD, Electric Arc, VAR for making high quality steels Early 20th Century – The golden era 1886 AD Hall process- Electrochemical process forextraction of Aluminium from Alumina A l2O3 1890-1910AD Revolution in Transportation – Discovery of automobiles and Aero plane 1939 AD Process for making Nylon – Introduction of plastics AND SO ON….. ww Materials Science and Engineering: w.E The combination of physics, chemistry, and the focus on the relationship between the properties of a material and its microstructure is the domain of Materials Science. The development of this science allowed designing materials and provided a knowledge base for asy the engineering applications Materials Engineering. En Advantages of Studying Materials Science and Engineering and performance. gin  To be able to select a material for a given use based on considerations of cost  To understand the limits of materials ee  To change the material properties based on the use. rin  To be able to create a new material that will have some desirable properties g.n et SCE 3 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY UNIT 1.CONSTITUTION OF ALLOYS AND PHASE DIAGRAMS 1.1 CLASSIFICATION OF MATERIALS 1.1.1Metals Valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Metals are usually strong, conduct electricity and heat well and are opaque to light shiny if polished. Examples: aluminum, steel, brass, gold. 1.1.2 Semiconductors The bonding is covalent e lectrons are shared between atoms. Their electrical ww properties depend extremely strongly on minute proportions of contaminants. They are opaque to visible light but transparent to the infrared. Examples: Si, Ge, GaAs. w.E 1.1.3Ceramics Atoms behave mostly like either positive or negative ions, and are bound by Coulomb forces between them. They are usually combinations of metals or asy semiconductors with oxygen, nitrogen or carbon oxides, nitrides, and carbides. Examples: glass, porcelain, many minerals. 1.1.4.Polymers En gin Are bound by covalent forces and also by weak van der Waals forces, and usually based on H, C and other non-metallic elements. They decompose at ee moderate temperatures 100 - 400 C , and are lightweight. Other properties vary greatly. Examples: plastics nylon, Teflon, polyester and rubber. rin 1.2. TYPES OF BONDING 1.2.1 Ionic Bonding g.n et This is the bond when one of the atoms is negative h as an extra electron) and another is positive h as lost an electron. Then there is a strong, direct Coulomb attraction. An example is NaCl. In the molecule, there are more electrons around Cl, forming Cl - and less around Na, forming Na+. Ionic bonds are the strongest bonds. 1.2.2 Covalent Bonding In covalent bonding, electrons are shared between the molecules, to saturate the valency. The simplest example is the H2 molecule, where the electrons spend more time in between the nuclei than outside, thus producing bonding. 1.2.3 Metallic Bonding In the metallic bond encountered in pure metals and metallic alloys, the atoms contribute their outer-shell electrons to a generally shared electron cloud for the whole block of SCE 4 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY metal.  Secondary Bonding V an der Waals  Fluctuating Induced Dipole Bonds Polar  Molecule-Induced Dipole Bonds  Permanent Dipole Bonds 1.3.CRYSTAL STRUCTURES Atoms self-organize in crystals, most of the time. The crystalline lattice is a periodic array of the atoms. When the solid is not crystalline, it is called amorphous. Examples of crystalline solids are metals, diamond and other precious stones, ice, graphite. Examples of amorphous solids are glass, amorphous carbon a -C , amorphous Si, most plastics ww 1.3.1.Unit Cells w.E The unit cell is the smallest structure that repeats itself by translation through the crystal. The most common types of unit cells are the faced centered cubic F CC , the body-centered cubic F CC and the hexagonal close-packed HCP. Other types exist, particularly among minerals. asy 1.3.2.Polymorphism and Allotropy En Some materials may exist in more than one crystal structure, this is called polymorphism. gin If the material is an elemental solid, it is called allotropy. An example of allotropy is carbon, which can exist as diamond, graphite, and amorphous carbon. 1.3.3.Polycrystalline Materials ee rin A solid can be composed of many crystalline grains, not aligned with each other. It is called g.n polycrystalline. The grains can be more or less aligned with respect to each other. Where they meet is called a grain boundary. 1.4.IMPERFECTION IN SOLIDS Materials are not stronger when they have defects. et The study of defects is divided according to their dimension: 0D zero dimension - point defects: vacancies and interstitials Impurities. 1D - linear defects: dislocations edge, screw, mixed 2D - grain boundaries, surfaces. 3D - extended defects: pores, cracks SCE 5 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY 1.5 Introduction to phase diagram Component Pure metal or compound e.g., Cu, Zn in Cu-Zn alloy, sugar, water, in syrup. Solvent Host or major component in solution. Solute Dissolved, minor component in solution. System Set of possible alloys from same component e.g., iron-carbon system. ww Solubility Limit Maximum temperature. solute concentration that can be dissolved at a given w.E Phase asy Part with homogeneous physical and chemical characteristics 1.6.Solid Solutions En A solid solution occurs when we alloy two metals and they are completely soluble in each other. If a solid solution alloy is viewed under a microscope only one type of crystal can be seen gin just like a pure metal. Solid solution alloys have similar properties to pure metals but with greater strength but are not as good as electrical conductors. The common types of solid solutions are ee 1 Substitutional solid solution rin 2 Interstitial solid solutions Substitution solid solution g.n et The name of this solid solution tells you exactly what happens as atoms of the parent metal or solvent metal are replaced or substituted by atoms of the alloying metal solute metal In this case, the atoms of the two metals in the alloy, are of similar size. Interstitial solid solutions: In interstitial solid solutions the atoms of the parent or solvent metal are bigger than the atoms of the alloying or solute metal. In this case, the smaller atoms fit into interstices i.e spaces between the larger atoms. Phases One-phase systems are homogeneous. Systems with two or more phases are heterogeneous, or mixtures. This is the case of most metallic alloys, but also happens in ceramics and polymers. A two-component alloy is called binary. One with three components is called ternary. SCE 6 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY Microstructure The properties of an alloy do not depend only on concentration of the phases but how they are arranged structurally at the microscopy level. Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space. A binary alloy may be A single solid solution Two separated essentially pure components. Two separated solid solutions. A chemical compound, together with a solid solution. Phase diagram: A graph showing the phase or phases present for a given composition as a function of temperature. ww Poly phase material: A material in which two or more phases are present. w.E Eutectoid: Transforming from a solid phase to two other solid phases upon cooling. Peritectoid: asy En Transforming from two solid phases to a third solid phase upon cooling. Peritectoid reaction: gin A reaction in which a solid goes to a new solid plus a liquid on heating, and reverse occurs on cooling. and control of properties. ee Iron-Iron Carbon diagram is essential to understand the basic differences among iron alloys rin Iron is allotropic; at room temperature pure iron exists in the Body Centered Cubic crystal g.n form but on heating transforms to a Face Centered Cubic crystal. The temperature that this first transformation takes place is known as a critical point and it occurs at 910 degrees Celsius. et This change in crystal structure is accompanied by shrinkage in volume, sine the atoms in the face centered crystal are more densely packed together than in the body centered cubic crystal. At the second critical point the F.C.C crystal changes back to a B.C.C crystal and this change occurs at 1390 degrees Celsius.  Iron above 1390 degrees is known as delta iron  Iron between 1390 and 910 degrees is known as gamma iron, Iron below 910 degrees is known as alpha iron. 1.7.IRON CARBON DIAGRAM Iron-carbon phase diagram Iron-carbon phase diagram describes the iron-carbon system of alloys containing up to 6.67% of carbon, discloses the phases compositions SCE 7 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY and their transformations occurring with the alloys during their cooling or heating. Carbon content 6.67% corresponds to the fixed composition of the iron carbide Fe3C. The diagram is presented in the picture: ww w.E asy En The following phases are involved in the transformation, occurring with iron- carbon alloys: gin L - Liquid solution of carbon in iron; ee δ-ferrite - Solid solution of carbon in iron. Maximum concentration of carbon in δ-ferrite is 0.09% at 2719 ºF rin 1493ºC - temperature of the peritectic transformation. g.n The crystal structure of δ-ferrite is BCC cubic body centered. Austenite - interstitial solid solution of carbon in γ-iron. solubility of carbon - up to 2.06% at 2097 ºF 1147 ºC. et Austenite has FCC cubic face centered crystal structure, permitting high Austenite does not exist below 1333 ºF 723ºC and maximum carbon concentration at this temperature is 0.83%. α-ferrite - solid solution of carbon in α-iron. α-ferrite has BCC crystal structure and low solubility of carbon - up to 0.25% at 1333 ºF 723ºC. α-ferrite exists at room temperature. Cementite - iron carbide, intermetallic compound, having fixed composition Fe3C. Cementite is a hard and brittle substance, influencing on the properties of steels and cast irons. The following phase transformations occur with iron-carbon alloys: Alloys, containing up to 0.51% of carbon, start solidification with formation of crystals of δ-ferrite. Carbon content in δ-ferrite increases up to 0.09% in course solidification, and at 2719 ºF 1493ºC remaining liquid phase and δ- ferrite perform peritectic transformation, resulting in formation of austenite. SCE 8 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY Alloys, containing carbon more than 0.51%, but less than 2.06%, form primary austenite crystals in the beginning of solidification and when the temperature reaches the curve ACM primary cementite stars to form. Iron-carbon alloys, containing up to 2.06% of carbon, are called steels. Alloys, containing from 2.06 to 6.67% of carbon, experience eutectic transformation at 2097 ºF 1147 ºC. The eutectic concentration of carbon is 4.3%. In practice only hypoeutectic alloys are used. These alloys carbon content from 2.06% to 4.3% are called cast irons. When temperature of an alloy from this range reaches 2097 ºF 1147 ºC, it contains primary austenite crystals and some amount of the liquid phase. The latter decomposes by eutectic ww mechanism to a fine mixture of austenite and cementite, called ledeburite. All iron-carbon alloys steels and cast irons experience eutectoid transformation at w.E 1333 ºF 723ºC. The eutectoid concentration of carbon is 0.83%. When the temperature of an alloy reaches 1333 ºF 733ºC , austenite transforms to asy pearlite fine ferrite-cementite structure, forming as a result of decomposition of austenite at slow cooling conditions. Critical temperatures En gin Upper critical temperature point A3 is the temperature, below which ferrite starts to ee form as a result of ejection from austenite in the hypoeutectoid alloys. Upper critical temperature point ACM is the temperature, below which cementite rin starts to form as a result of ejection from austenite in the hypereutectoid alloys. Lower critical temperature point A1 is the temperature of the austeniteto-pearlite eutectoid transformation. Below this temperature austenite does not exist. g.n Magnetic transformation temperature A2 is the temperature below which α-ferrite is ferromagnetic. et Phase compositions of the iron-carbon alloys at room temperature o Hypoeutectoid steels car bon content from 0 to 0.83% consist of primary proeutectoid) ferrite ac cording to the curve A3 and pearlite. o Eutectoid steel ca rbon content 0.83% entirely consists of pearlite. o Hypereutectoid steels carbon content from 0.83 to 2.06% consist of primary p roeutectoid cementite according to the curve ACM and pearlite. o Cast irons c arbon content from 2.06% to 4.3% consist of proeutectoid cementite C2 ejected from austenite according to the curve ACM , pearlite and transformed ledeburite ledeburite in which austenite transformed to pearlite. SCE 9 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY ww w.E asy En gin ee rin g.n hence forming Eutectic reaction. L ↔ γ + Fe3C et At 4.3% carbon composition, on cooling Liquid phase is converted in to two solids Eutectoid: 0.76 wt%C, 727 °C γ 0.76 wt% C ↔ α 0.022 wt% C + Fe3C Shown below is the steel part of the iron carbon diagram containing up to 2% Carbon. At the eutectoid point 0.83% Carbon, Austenite which is in a solid solution changes directly into a solid known as Pearlite which is a layered structure consisting of layers of Ferrite and Cementite SCE 10 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY ww w.E asy En gin ee rin g.n et SCE 11 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY ww w.E asy En gin ee rin g.n et SCE 12 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY ww w.E asy En gin 1.8. METAL TYPES ee rin The metals that Steelworkers work with are divided into two general classifications: ferrous and nonferrous. Ferrous metals are those composed primarily of iron and iron alloys. Nonferrous metals are those composed g.n primarily of some element or elements other than iron. Nonferrous metals or et alloys sometimes contain a small amount of iron as an alloying element or as an impurity. 1.8.1.FERROUS METALS Ferrous metals include all forms of iron and steel alloys. A few examples include wrought iron, cast iron, carbon steels, alloy steels, and tool steels. Ferrous metals are iron- base alloys with small percentages of carbon and other elements added to achieve desirable properties. Normally, ferrous metals are magnetic and nonferrous metals are nonmagnetic. IRON Pure iron rarely exists outside of the laboratory. Iron is produced by reducing iron ore to pig iron through the use of a blast furnace. From pig iron many other types of iron and steel are produced by the addition or deletion of carbon and alloys. The following paragraphs discuss the different types of iron and steel that can be made from iron ore. SCE 13 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY PIG IRON Pig iron is composed of about 93% iron, from 3% to 5% carbon, and various amounts of other elements. Pig iron is comparatively weak and brittle; therefore, it has a limited use and approximately ninety percent produced is refined to produce steel. Cast-iron pipe and some fittings and valves are manufactured from pig iron. WROUGHT IRON Wrought iron is made from pig iron with some slag mixed in during manufacture. Almost pure iron; the presence of slag enables wrought iron to resist corrosion and oxidation. The chemical analyses of wrought iron and mild steel are just about the same. The difference comes from the properties controlled during the manufacturing process. Wrought iron can be gas and arc welded, machined, plated, and easily formed; however, it has a low ww hardness and low-fatigue strength. CAST IRON w.E Cast iron is any iron containing greater than 2% carbon alloy. Cast iron has a high- compressive strength and good wear resistance; however, it lacks ductility, malleability, and impact strength. Alloying it with nickel, chromium, molybdenum, asy silicon, or vanadium improves toughness, tensile strength, and hardness. A malleable cast iron is produced through a easily as the low-carbon steels. They are used for crane prolonged annealing En process. hooks, axles, shafts, setscrews, and so on. INGOT IRON gin Ingot iron is a commercially pure iron 99.85% iron) that is easily formed and ee possesses good ductility and corrosion resistance. The chemical analysis and properties of this iron and the lowest carbon steel are practically the same. The lowest carbon steel, known as rin dead- soft, has about 0.06% more carbon than ingot iron. In iron the carbon content is considered an impurity and in steel it is considered an alloying element. The primary use for ingot iron is for galvanized and enameled sheet. g.n STEEL et All the different metals and materials that we use in our trade, steel is by far the most important. When steel was developed, it revolutionized the American iron industry. With it came skyscrapers, stronger and longer bridges, and railroad tracks that did not collapse. Steel is manufactured from pig iron by decreasing the amount of carbon and other impurities and adding specific amounts of alloying elements. Do not confuse steel with the two general classes of iron: cast iron greater than 2% carbon and pure iron less than 0.15% carbon. In steel manufacturing, controlled amounts of alloying elements are added during the molten stage to produce the desired composition. The composition of a steel is determined by its application and the specifications that were developed by the following: American Society for Testing and Materials ASTM , the American Society of Mechanical Engineers ASME , the Society of Automotive Engineers SAE , and the American Iron and Steel Institute AISI. Carbon steel is a term applied to a broad range of steel that SCE 14 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY falls between the commercially pure ingot iron and the cast irons. This range of carbon steel may be classified into four groups: HIGH-CARBON STEEL/VERY HIGH-CARBON STEEL Steel in these classes respond well to heat treatment and can be welded. When welding, special electrodes must be used along with preheating and stress- relieving procedures to prevent cracks in the weld areas. These steels are used for dies, cutting tools, milltools, railroad car wheels, chisels, knives, and so on. LOW-ALLOY, HIGH-STRENGTH, TEMPERED STRUCTURAL STEEL A special lowcarbon steel, containing specific small amounts of ww alloying elements, that is quenched and tempered to get a yield strength of greater than 50,000 psi and tensile strengths of 70,000 to 120,000 psi. Structural members made from these high-strength steels may have smaller cross- w.E sectional areas than common structural steels and still have equal or greater strength. Additionally, these steels are normally more corrosion- and abrasionresistant. High-strength steels are covered by ASTM specifications. NOTE: This type asy of steel is much tougher than low-carbon steels. Shearing machines for this type of steel must have twice the capacity than that required for low-carbon steels STAINLESS STEEL En gin This type of steel is classified by the American Iron and Steel Institute AISI into two general series named the 200-300 series and 400 series. Each series includes several types of AUSTENITIC. ee steel with different characteristics. The 200-300 series of stainless steel is known as This type of steel is very tough and ductile rin in the as-welded g.n condition; therefore, it is ideal for welding and requires no annealing under normal atmospheric conditions. The most well-known types of steel in this series are the 302 and 304. They are commonly called 18-8 because they are composed of 18% chromium and 8% nickel. The chromium nickel steels Low-Carbon Steel... et 0.05% to 0.30% carbon are the most widely used and are normally nonmagnetic. Medium-Carbon Steel... 0.30% to 0.45% carbon High-Carbon Steel... 0.45% to0.75% carbon their crystalline structure into two general groups. One Very High-Carbon Steel... 0.75% to 1.70% carbon group is known as FERRITIC CHROMIUM and the other group as MARTENSITIC CHROMIUM. SCE 15 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY 1.8.2.ALLOY STEELS Steels that derive their properties primarily from the presence of alloying element other than carbon are called ALLOYS or ALLOY STEELS. Note, however, that alloy steels always contain traces of other elements. Among the more common alloying elements are nickel, chromium, vanadium, silicon, and tungsten. One or more of these elements may be added to the steel during the manufacturing process to produce the desired characteristics. Alloy steels may be produced in structural sections, sheets, plates, and bars for use in the as rolled condition. Better physical properties are obtained with these steels than are possible with hot. These alloys are used in structures where the strength of material is especially important. Bridge members, railroad cars, dump bodies, dozer blades, and crane booms are made from alloy steel. Some of the common alloy steels are briefly described in the paragraphs below. ww NICKEL STEELS These steels contain from 3.5% nickel to 5% nickel. The nickel increases w.E the strength and toughness of these steels. Nickel steel containing more than 5% nickel has an increased resistance to corrosion and scale. Nickel steel is used in the manufacture of aircraft parts, such as propellers and airframe support members. CHROMIUM STEELS asy These steels have chromium En added to improve wear resistance, and strength. These steels contain between 0.20% hardening ability, to 0.75% chromium gin and 0.45% carbon or more. Some of these steels are so wear that they are used for the races and balls in highly resistant to antifriction bearings. CHROME VANADIUM STEEL ee Chromium steels are highly resistant to corrosion and to scale. rin This steel has the maximum amount of strength with the least amount g.n of weight. Steels of this type contain from 0.15% to 0.25% vanadium, 0.6% to 1.5% chromium, and 0.1% to 0.6% carbon. Common uses are for crankshafts, gears, axles, and quality hand tools, such as wrenches and sockets. TUNGSTEN STEEL et other items that require high strength. This steel is also used in the manufacture of high- This is a special alloy that has the property of red hardness. This is the ability to continue to cut after it becomes red-hot. A good grade of this steel contains from 13% to 19% tungsten, 1% to 2% vanadium, 3% to 5% chromium, and 0.6% to 0.8% carbon. Because this alloy is expensive to produce, its use is largely restricted to the manufacture of drills, lathe tools, milling cutters, and similar cutting tools. MOLYBDENUM This is often used as an alloying agent for steel in combination with chromium and nickel. The molybdenum adds toughness to the steel. It can be used in place of tungsten to make the cheaper grades of high-speed steel and in carbon molybdenum high- pressure tubing. SCE 16 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY MANGANESE STEELS The amount of manganese used depends upon the properties desired in the finished product. Small amounts of manganese produce strong, free-achgining steels. Larger amounts b etween 2% and 10% produce a somewhat brittle steel, while still larger amounts 11% to 14% p roduce a steel that is tough and very resistant to wear after proper heat treatment. 1.8.3. NONFERROUS METALS Nonferrous metals contain either no iron or only insignificant amounts used as an alloy. Some of the more common nonferrous metals Steelworkers work with are as follows: copper, brass, bronze, copper-nickel alloys, lead, zinc, tin, aluminum, and Duralumin. ww NOTE: These metals are nonmagnetic. COPPER This metal and its alloys have many desirable properties. Among w.E the commercial metals, it is one of the most popular. Copper is ductile, malleable, hard, tough, strong, wear resistant, machinable, weld able, and corrosion resistant. It also has high-tensile strength, fatigue strength, and work with but be asy thermal and electrical conductivity. Copper is one of the easier metals to careful because it easily becomes work-hardened; This process, called annealing, En however, this condition can be remedied by heating it to a cherry red and then letting it cool. restores it to a softened condition. Annealing and softening are the gin only heat-treating procedures that apply to copper. Seams in copper are joined by riveting, silver brazing, bronze brazing, soft soldering, gas welding, or electrical arc welding. Copper is frequently used to give a protective coating to CARBON STEELS ee sheets and rods and to make ball floats, containers, and soldering coppers. rin Carbon steels are iron-carbon alloys containing up to 2.06% of carbon, up to1.65% of manganese, up to 0.5% of silicon and sulfur and phosphorus as impurities. Carbon content in carbon steel determines its strength and ductility. The g.n higher carbon content, the higher steel strength and According to the steels classification there are following groups of carbon steels: the et lower its ductility.  Low carbon steels C < 0.25%  Medium carbon steels C =0.25% to 0.55%  High carbon steels C > 0.55%  Tool carbon steels C>0.8% Designation system of carbon steels Chemical compositions of some carbon steels Properties of some carbon steels Low carbon steels C < 0.25% Properties: good formability and weldability, low strength, low cost. Applications: deep drawing parts, chain, pipe, wire, nails, some machine parts. SCE 17 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY Medium carbon steels C =0.25% to 0.55% Properties: good toughness and ductility, relatively good strength, may be hardened by quenching Applications: rolls, axles, screws, cylinders, crankshafts, heat treated machine parts. High carbon steels C > 0.55% Properties: high strength, hardness and wear resistance, moderate ductility. Applications: rolling mills, rope wire, screw drivers, hammers, wrenches, band saws. Tool carbon steels C>0.8% - subgroup of high carbon steels ww Properties: very high strength, hardness and wear resistance, poor weldability, low ductility. Applications: punches, shear blades, springs, milling cutters, knives, razors. Designation system of carbon steels w.E American Iron and Steel Institute AIS I together with Society of Automotive Engineers SAE) have established four-digit with additional letter prefixes designation system: asy LOW-ALLOY, HIGH-STRENGTH, TEMPERED STRUCTURAL STEEL A special lowcarbon steel, containing specific small amounts of En alloying elements, that is quenched and tempered to get a yield strength of greater than 50,000 psi and tensile strengths of 70,000 to 120,000 psi. Structural gin members made from these high-strength steels may have smaller cross- sectional areas than common structural steels and still have equal or greater ee strength. Additionally, these steels are normally more corrosion- and abrasionresistant. High-strength steels are covered by ASTM specifications. NOTE: This type rin of steel is much tougher than low-carbon steels. Shearing machines for this type of steel must have twice the capacity than that required for low-carbon steels g.n STAINLESS STEEL et This type of steel is classified by the American Iron and Steel Institute AISI into two general series named the 200-300 series and 400 series. Each series includes several types of steel with different characteristics. The 200-300 series of stainless steel is known as austenitic. AUSTENITIC This type of steel is very tough and ductile in the as-welded condition; therefore, it is ideal for welding and requires no annealing under normal atmospheric conditions. The most well-known types of steel in this series are the 302 and 304. They are commonly called 18-8 because they are composed of 18% chromium and 8% nickel. The chromium nickel steels Low-Carbon SCE 18 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY SAE 1XXX First digit 1 indicates carbon steel 2-9 are used for alloy steels ; Second digit indicates modification of the steel. 0 - Plain carbon, non-modified 1 - Resulfurized 2 - Resulfurized and rephosphorized 5 - Non-resulfurized, Mn over 1.0% Last two digits indicate carbon concentration in 0.01%. ww Example: SAE 1030 means non modified carbon steel, containing 0.30% of carbon. A letter prefix before the four-digit number indicates the steel making technology: w.E A - Alloy, basic open hearth asy B - Carbon, acid Bessemer C - Carbon, basic open hearth D - Carbon, acid open hearth En E - Electric furnace gin Example: AISI B1020 means non modified carbon steel, produced in acid Bessemer and containing 0.20% of carbon. SAE/AISI grade 1006 C, % 0.08 max Mn,% ee Chemical compositions of some carbon steels 0.35 max P,% max 0.04 rin S,% max 0.05 1010 1020 0.08-0.13 0.17-0.23 0.30-0.60 0.30-0.60 0.04 0.04 g.n 0.05 0.05 1030 1045 1070 0.27-0.34 0.42-0.50 0.65-0.76 0.60-0.90 0.60-0.90 0.60-0.90 0.04 0.04 0.04 0.05 0.05 0.05 et 1090 0.85-0.98 0.60-0.90 0.04 0.05 1117 0.14-0.20 1.10-1.30 0.04 0.08-0.13 1547 0.43-0.51 1.35-1.65 0.04 0.05 SCE 19 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY UNIT II HEAT TREATMENT PROCESS 2.1.BASIC PRINCIPLES OF HEAT TREATMENT Heat treatment of a metal or alloy is a technological procedure, including controlled heating and cooling operations, conducted for the purpose of changing the alloy microstructure and resulting in achieving required properties. There are two general objectives of heat treatment: hardening and annealing. 2.2.HARDENING Hardening is a process of increasing the metal hardness, strength, toughness, fatigue resistance. ww Strain hardening (work hardening – strengthening by cold work (cold deformation) w.E Cold plastic deformation causes increase of concentration of dislocations, which mutually entangle one another, making further dislocation motion difficult and therefore resisting the deformation or increasing the metal strength.  asy Grain size strengthening h ardening - strengthening by grain refining. En Grain boundaries serve as barriers to dislocations, raising the stress required to cause plastic deformation.  gin Solid solution hardening- strengthening by dissolving an alloying element. ee rin Atoms of solute element distort the crystal lattice, resisting the dislocations motion. Interstitial elements are more effective in solid solution hardening, than substitution elements. g.n  et Dispersion strengthening – strengthening by adding second phase into metal matrix. The second phase boundaries resist the dislocations motions, increasing the material strength. The strengthening effect may be significant if fine hard particles are added to a soft ductile matrix composite materials. Hardening by result of Spinodal decomposition. Spinodal structure is characterized by strains on the coherent boundaries between the Spinodal phases causing hardening of the alloy. SCE 20 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY ww w.E asy En  gin Precipitation hardening age hardenin g - strengthening by precipitation of fine particles ee of a second phase from a supersaturated solid solution. rin The second phase boundaries resist the dislocations motions, increasing the material strength. The age hardening mechanism in Al-Cu alloys may be illustrated by the phase g.n diagram of Al-Cu system. When an alloy Al-3%Cu is heated up to the temperature TM, all CuAl2 particles are dissolved and the alloy exists in form of single phase solid solution α-phase. This operation is called solution treatment. et Slow cooling of the alloy will cause formation of relatively coarse particles of CuAl2 intermetallic phase, starting from the temperature TN.However if the the cooling rate is high qu enching, sol id solution will retain even at room temperature T F. Solid solution in this non-equilibrium state is called supersaturated solid solution. Obtaining of supersaturated solid solution is possible when cooling is considerably faster, than diffusion processes. As the diffusion coefficient is strongly dependent on the temperature, the precipitation of CuAl2 from supersaturated solution is much faster at elevated temperatures lo wer than TN.T his process is called artificial aging. It takes usually a time from several hours to one day. When the aging is conducted at the room temperature, it is called natural aging. Natural aging takes several days or more. SCE 21 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY Precipitation from supersaturated solid solution occurred in several steps:  Segregation of Cu atoms into plane clusters. These clusters are called called Guinier- Preston1 zones G-P1 zones.  Diffusion of Cu atoms to the G -P1 zones and formation larger clusters, called GP2 zones or θ” phase. This phase is coherent with the matrix.  Formation of ‘θ’ phase which is partially coherent with the matrix. This phase provides maximum hardening. 2.3.ANNEALING Annealing is a heat treatment procedure involving heating the alloy and holding it at a certain temperature a nnealing temperature, followed by controlled cooling. Annealing results in relief of internal stresses, softening, ww chemical homogenizing and transformation of the grain structure into more stable state. w.E Annealing stages: - a relatively low temperature process of reducing internal asy mechanical stresses, caused by cold-work, casting or welding. During this process atoms move to more stable positions in the crystal lattice. Vacancies En some dislocations are annihilated. and interstitial defects are eliminated and gin Recovery heat treatment is used mainly for preventing stress-corrosion cracking and decreasing distortions, caused by internal stresses. - ee alteration of the grain structure of the metal. rin If the alloy reaches a particular temperature recrystallization or annealing temperature g.n new grains start to grow from the nuclei formed in the cold worked metal. The new grains absorb imperfections and distortions caused by cold deformation. The grains are equi- axed and independent to the old grain structure. et As a result of recrystallization mechanical properties strength, ductility of the alloy return to the pre-cold-work level. The annealing temperature and the new grains size are dependent on the degree of cold-work which has been conducted. The more the cold-work degree, the lower the annealing temperature and the fine recrystallization grain structure. Low degrees of cold- work less than 5% may cause formation of large grains.Usually the annealing temperature of metals is between one-third to one-half of the freezing point measured in Kelvin absolute temperature scale. ov er-annealing, secondary recrystallization) - growth of the new grains at the expense of their neighbors, occurring at temperature, above the recrystallization temperature. This process results in coarsening grain structure and is undesirable. SCE 22 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY THE SOFTENING PROCESSES Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is sometimes done inadvertently due to manufacturing processes that either heat or cool the metal such as welding or forming. Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics. Steels are particularly suitable for heat treatment, since they ww respond well to heat treatment and the commercial use of steels exceeds that of any other material. Steels are heat treated for one of the following reasons: w.E Softening Softening is done to reduce strength or hardness, remove residual stresses, improve toughnesss, restore ductility, refine grain size or change the electromagnetic properties of the steel. asy Restoring ductility or removing residual stresses is a necessary operation when a En large amount of cold working is to be performed, such as in a cold- rolling operation or wiredrawing. gin Annealing — full Process, spheroidizing, normalizing and tempering— austempering, martempering are the principal ways by which steel is softened. Hardening: ee Hardening of steels is done to increase the strength rin and wear properties. One of the pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content then the steel can be directly hardened. Otherwise t h e surface of the part g.n has to be Carbon enriched techniques. Material properties using some Modification: Heat diffusion treatment of materials in addition to hardening and treatment is used etto hardening modify softening. These processes modify the behavior of the steels in a beneficial manner to maximize service life, e.g., stress relieving, or strength properties, e.g., cryogenic treatment, or some other desirable properties, e.g., spring aging. Annealing Used variously to soften, relieve internal stresses, improve machinability and to develop particular mechanical and physical properties.In special silicon steels used for transformer laminations annealing develops the particular microstructure that confers the unique electrical properties.Annealing requires heating to above the As temperature, holding for sufficient time for temperature equalisation followed by slow cooling. See Curve 2 in Figure.1 SCE 23 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY ww Figure 1. An idealised TTT curve for plain carbon steel. w.E Stress from the forming operations can affect both rimfire and centerfire cartridge cases. For many cases, especially those with bottlenecks, asy the stresses are so great that high-temperature annealing must be used.After forming, a bottleneck case may appear perfectly serviceable. However, En massive stresses are likely to remain in these areas. If the ammunition is loaded and stored without addressing these stresses, cracks can appear in the bottleneck area. gin ee rin g.n et SCE 24 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY Case bottlenecks are normally flame-annealed by the following process: set of gas burners that rapidly heat the neck and shoulder area to glowing. he heated area of the case is immediately tipped into a water bath to quench the case, establishing the large grain size. area. In commercial ammunition, this dark area may be polished out for cosmetic reasons; in U.S. military ammunition, the discoloration remains visible. ww gradually, from small grains in the head area to large ones at the case mouth, determines case hardness. w.E All high pressure cases must have variable metallurgical properties depending on the part of the case, as follows: asy - must be tough and relatively unyielding, small brass grains contribute to the toughness. 2.4.NORMALISING En gin Also used to soften and relieve internal stresses after cold work and to ee refine the grain size and metallurgical structure. It may be used to break up the dendritic a s cast structure of castings to improve their machinability and future heat treatment response or to mitigate banding in rolled rin steel. This requires heating to above the As temperature, holding for g.n sufficient time to allow temperature equalization followed by air cooling. It is therefore similar to annealing but with a faster cooling rate. Curve 3 in Figure I would give a normalized structure. 2.5.THE HARDENING PROCESSES et Hardening In this process steels which contain sufficient carbon, and perhaps other alloying elements, are cooled quenched sufficiently rapidly from above the transformation temperature to produce Martensite, the hard phase already described, see Curve 1 in Figure 1.There is a range of quenching media of varying severity, water or brine being the most severe, through oil and synthetic products to air which is the least severe. Tempering After quenching the steel is hard, brittle and internally stressed. Before use, it is usually necessary to reduce these stresses and increase toughness by 'tempering'. There will also be a reduction in hardness and the selection of tempering temperature dictates the final properties. Tempering curves, SCE 25 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY which are plots of hardness against tempering temperature. exist for all commercial steels and are used to select the correct tempering temperature. As a rule of thumb, within the tempering range for a particular steel, the higher the tempering temperature the lower the final hardness but the greater the toughness. It should be noted that not all steels will respond to all heat treatment processes, Table 1 summaries the response, or otherwise, to the different processes. ww w.E asy En gin ee rin Boronised substrates will often require heat treatment to restore mechanical properties. g.n As borides degrade in atmospheres which contain oxygen, even when combined as CO or C02, they must be heat treated in vacuum, nitrogen or nitrogen/hydrogen atmospheres. PROCESSING METHODS In the past the thermochemical processes were carried out by pack et cementation or salt bath processes. These are now largely replaced, on product quality and environmental grounds, by gas and plasma techniques. The exception is boronising, for which a safe production scale gaseous route has yet to be developed and pack cementation is likely to remain the only viable route for the for some time to come. The gas processes are usually carried out in the now almost universal seal quench furnace, and any subsequent heat treatment is readily carried out immediately without taking the work out of the furnace. This reduced handling is a cost and quality benefit. TECHNIQUES AND PRACTICE As we have already seen this requires heating to above the As temperature, holding to equalise the temperature and then slow cooling. If this is done in air there is a real SCE 26 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY risk of damage to the part by decarburisation and of course oxidation. It is increasingly common to avoid this by ‗bright„ or ‗close„ annealing using protective atmospheres. The particular atmosphere chosen will depend upon the type of steel. NORMALISING In common with annealing there is a risk of surface degradation but as air cooling is common practice this process is most often used as an intermediate stage to be followed by machining, acid pickling or cold working to restore surface integrity. HARDENING With many components, hardening is virtually the final process and great care must taken to protect the surface from degradation and decarburisation. The ‗seal quench„ furnace is ww now an industry standard tool for carbon, low and medium alloy steels. The work is protected at each stage by a specially generated atmosphere. w.E Some tool steels benefit from vacuum hardening and tempering; salt baths were widely used but are now losing favour on environmental grounds. 2.7.TEMPERING asy Tempering is essential after most hardening operations to restore some toughness to En the structure. It is frequently performed as an integral part of the cycle in a seal quench furnace, with the parts fully protected against oxidation and decarburisation throughout the gin process. Generally tempering is conducted in the temperature range 150 to 700°C, depending on the type of steel and is time dependent as the microstructural changes occur relatively slowly. ee Caution: Tempering c a n ,in some circumstances, m a k e the steel brittle which is the opposite of what it is intended to achieve. There are two forms of this brittleness rin Temper Brittleness which affects both carbon and low alloy steels g.n when either, they are cooled too slowly from above 575°C, or are held for excessive times in the range above 575°C and rapidly cooling. et 375 to 575°C. The embrittlement can be reversed by heating to Blue Brittleness affects carbon and some alloy steels after tempering in the range 230 to 370°C The effect is not reversible and susceptible steels should not be employed in applications in which they sustain shock loads. If there is any doubt consult with the heat treater or in house metallurgical department about the suitability of the steel type and the necessary heat treatment for any application. 2.8.MARTEMPERING AND AUSTEMPERING It will be readily appreciated that the quenching operation used in hardening introduces internal stresses into the steel. These can be sufficiently large to distort or even crack the steel. Martempering is applied to steels of sufficient hardenability and involves an isothermal hold in the quenching operation. This allows temperature equalisation across the section of the part and more uniform cooling and structure, hence lower stresses. The steel SCE 27 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY can then be tempered in the usual way. Austempering also involves an isothermal hold in the quenching operation, but the structure formed, whilst hard and tough, does not require further tempering. The process is mostly applied to high carbon steels in relatively thin sections f o r s p r i n g s o r s i m i l a r p a r t s. These processes are shown schematically in the TTT Curves, f igures 2a and 2b. there is sufficient heat sink in the part and an external quench is not needed. There is a much lower risk of distortion associated with this practice, and it can be highly automated and it is very reproducible ww w.E asy En - the case walls must combine flexibility and strength to gin contribute to the obturation system. - must be softer larger brass grains to prevent cracks from the strain of holding a bullet. ee rin g.n et SCE 28 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY UNIT 3 FERROUS AND NON FERROUS METALS 3.1.EFFECT OF ALLOYING ELEMENTS ON STEEL PROPERTIES Alloying is changing chemical composition of steel by adding elements with purpose to improve its properties as compared to the plane carbon steel. The properties, which may be improved Stabilizing austenite - increasing the temperature range, in which austenite exists. The elements, having the same crystal structure as that of austenite cubic face centered - FCC, raise the A4 point the temperature of formation of austenite from liquid phase and decrease the A3 temperature. These elements are nickel Ni, manganese Mn , cobalt Co and c opper ww Cu. Examples of austenitic steels: austenitic stainless steels, Hadfield w.E steel 1%C, 13%Mn, 1.2%Cr. Stabilizing ferrite - decreasing the temperature range, in which austenite exists. asy The elements, having the same crystal structure as that of ferrite cubic body centered - BCC , lower the A4 point and increase the A3 temperature. En gin These elements lower the solubility of carbon in austenite, causing increase of amount of carbides in the steel. The following elements have ferrite stabilizing effect: chromium Cr , Si. ee tungsten W , Molybdenum Mo , vanadium V , aluminum Al and silicon rin Examples of ferritic steels:transformer sheets steel 3%Si , F-Cr alloy Carbide forming - elements forming hard carbides in steels. g.n The elements like chromium Cr , tungsten W , molybdenum Mo ,vanadium V , titanium Ti, niobium Nb , tantalum Ta ,zirconium Zr fo rm hard o ften complex et carbides, increasing steel hardness and strength.Examples of steels containing relatively high concentration of carbides: hot work tool steels, high speed steels. Carbide forming elements also form nitrides reacting with Nitrogen in steels. Graphitizing - decreasing stability of carbides, promoting their breaking and formation of free Graphite. The following elements have graphitizing effect: silicon Si, nickel Ni, cobalt Co , aluminum Al. Decrease of the eutectoid concentration. The following elements lower eutectoid concentration of carbon: titanium Ti, mo lybdenum Mo), tungsten W, silicon Si, chromium Cr , nickel Ni. SCE 29 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY Increase of corrosion resistance. Aluminum Al, silicon Si, and chromium Cr form thin an strong oxide film on the steel surface, protecting it from chemical attacks. 3.2.CHARACTERISTICS OF ALLOYING ELEMENTS Manganese Mn - improves hardenability, ductility and wear resistance. Mn eliminates formation of harmful iron sulfides, increasing strength at high temperatures. Nickel Ni - increases strength, impact strength and toughness, impart corrosion resistance in combination with other elements. Chromium Cr - improves hardenability, strength and wear resistance, sharply increases corrosion resistance at high concentrations > 12%. Tungsten W - increases hardness particularly at elevated temperatures due to stable carbides, refines grain size. ww Vanadium V - increases strength, hardness, creep resistance and impact resistance due to formation of hard vanadium carbides, limits grain size. w.E Molybdenum Mo) - increases hardenability and strength particularly at high temperatures and under dynamic conditions. Silicon Si - improves strength, elasticity, acid resistance and promotes large grain sizes, asy which cause increasing magnetic permeability. Titanium Ti - improves strength and corrosion resistance, limits austenite grain size. En Cobalt Co) - improves strength at high temperatures and magnetic permeability. Zirconium Zr - increases strength and limits grain sizes. gin Boron B - highly effective hardenability agent, improves deformability and machinability. Copper Cu - improves corrosion resistance. Aluminum Al - deoxidizer, limits austenite grains growth. 3.3.MARAGING STEELS ee Maraging steels fr om martensitic and aging rin are steels iron alloys although they cannot hold a good cutting edge. Aging refers to the extended heat-treatment process. These steels are a special class of low- carbon g.n which are known for possessing superior strength and toughness without losing malleability, ultra- high-strength steels which derive their strength not from precipitation of inter-metallic compounds. The principal alloying element is 15 to 25% nickel. Secondary alloying elements are added et from carbon, but to produce intermetallic precipitates, which include cobalt, molybdenum, and titanium. Original development was carried out on 20 and 25% Ni steels to which small additions of Al, Ti, and Nb were made. The common, non-stainless grades contain 17-19% nickel, 8-12% cobalt,3-5% molybdenum, and 0.2-1.6% titanium. Addition of chromium produces stainless grades resistant to corrosion. This also indirectly increases hardenability as they require less nickel: high-chromium, high-nickel steels are generally austenitic and unable to transform to martensite when heat treated, while lower-nickel steels can transform to martensite. SCE 30 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY Properties Due to the low carbon content maraging steels have good machinability. Prior t o aging, they may also be cold rolled to as much as 80- 90% without cracking. Maraging steels offer good weldability, but must be aged afterward to restore the properties of heat affected zone. When heat-treated the alloy has very little dimensional change, so it is often machined to its final dimensions. Due to the high alloy content maraging steels have a high hardenability. Since ductile FeNi martensites are formed upon cooling, cracks are non-existent or negligible. The steels can be nitrided to increase case hardness, and polished to a fine surface finish. Non-stainless varieties of maraging steel are moderately corrosion- resistant, and resist stress corrosion and hydrogen embrittlement. Corrosion- ww resistance can be increased by cadmium plating or phosphating. w.E 3.4.HEAT TREATMENT CYCLE The steel is first annealed at approximately 820 °C 1,510 °F for 15- 30 minutes for thin sections and for 1 hour per 25 mm thickness for heavy asy sections, to ensure formation of a fully austenitized structure. This is followed by air cooling to room temperature to form a soft, heavily- dislocated iron-nickel lath untwinned) martensite. Subsequent aging precipitation hardening temperature of 480 to En of the more common alloys for approximately 3 hours at a 500 °C produces a fine dispersion of Ni3X,Y intermetallic where X phases and Y along are gin dislocations solute left elements by added martensitic transformation, for such precipitation. coherent semi-coherent precipitates, leading Laves phases ee Overaging leads to a reduction in stability of the primary, metastable, to such their as dissolution Fe2Ni/Fe2Mo. rin and replacement with Further excessive heat- g.n treatment brings about the decomposition of the martensite and reversion to austenite. Newer compositions of maraging steels have revealed other intermetallic stoichiometries and crystallographic relationships with the parent martensite, including rhombohedral and massive complex Ni50 X,Y,Z 50 Ni50M50 in simplified notation. Uses et Maraging steel's strength and malleability in the pre-aged stage allows it to be formed into thinner rocket and missile skins than other steels, reducing weight for a given strength. Maraging steels have very stable properties, and, even after overaging due to excessive temperature, only soften slightly. These alloys retain their properties at mildly elevated operating temperatures and have maximum service temperatures of over 400 °C 752 °F They are suitable for engine components, such as crankshafts and gears, and the firing pins of automatic weapons that cycle from hot to cool repeatedly while under substantial load. Their uniform expansion and easy machinability before aging make maraging steel useful in high-wear SCE 31 Department of Mechanical Engineering Downloaded From : www.EasyEngineering.net Downloaded From : www.EasyEngineering.net ME 8491 ENGINEERING MATERIALS AND METALLURGY components of assembly lines and dies. Other ultra-high-strength steels, such as Aermet alloys, are not as machinable because of their carbide content. In the sport of fencing, blades used in competitions run under the auspices of the Fédération Internationale d'Escrime are often made with maraging steel. Maraging blades are required in foil and épée because crack propagation in maraging steel is 10 times slower than in carbon steel, resulting in less blade breakage and fewer injuries. The notion that such blades break flat is a fencing urban legend: testing has shown that the blade-breakage patterns in carbon steel and maraging steel blades are identical. Stainless maraging steel is used in bicycle frames and golf club heads. It is also use