applications and processing of metal alloys l 1 n.
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Metal Processing

Metal Processing

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Metal Processing

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  1. Applications and Processing of Metal Alloys (L-1) Dr.RabiulHussain School of Material Science & Engineering Jimma Institute of Technolgy, Jimma University E-mail: Ph.No. +251-0966882081 (Ethiopia) +91-9508832510 (India

  2. Applications and Processing of Metal Alloys ISSUES TO BE ADDRESSED: • How are metal alloys classified and how are they used? • What are some of the common fabrication techniques? • How do properties vary throughout a piece of material that has been quenched? • How can properties be modified by post heat treatment? DR.RABIUL HUSSAIN/0966882081

  3. Why metals are Important? • They have properties which satisfy a wide variety of design requirements. • The manufacturing processes by which they are shaped into different products have been developed and refined over many years. • Engineers understand metals. • Their versatile mechanical and physical properties such as: • High stiffness and strength---can be alloyed for high rigidity, strength and hardness. • Toughness-capacity to absorb energy better than other classes of materials • Good electrical conductivity-Metals are conductors • Good thermal conductivity-conduct heat better than ceramics or polymers • Cost-the price of steel is very low as compared with other engineering materials. DR.RABIUL HUSSAIN/0966882081

  4. Applications and Processing of Metal Alloys What is an ALLOY?? • An alloy is a mixture of two elements, one of which is a metal. Alloys often have properties that are different to the metals they contain. This makes them more useful than the pure metals alone. For example, alloys are often harder than the metal they contain. • Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal. DR.RABIUL HUSSAIN/0966882081


  6. TYPES OF METAL ALLOYS • Metal alloys by virtue of its composition are often grouped into two classes: FERROUS ALLOY & NON-FERROUS ALLOY. • Ferrous Alloy: Those alloys which contain iron as the prime component and are produced in larger quantities than any other metal type. • For an engineer, especially, metals are more important owing to ability to carry loads and ease of manufacturing. • Metallic materials are again classified for ease of selection and / or based on their tonnage of usage broadly into two classes–ferrous and non-ferrous. • Ferrous materials–chief constituent is iron(Fe) .E.g.: steel, cast iron. • Metallic materials those are not ferrous are termed as non-ferrous materials. E.g.: Brass , Silver , Aluminium , Titanium. DR.RABIUL HUSSAIN/0966882081

  7. Ferrous Alloys • Ferrous alloys are used extensively because: (1) Iron ores exist in abundantquantities. (2) Economical extraction, refining, and fabrication techniques are available. (3) The alloys may be tailored to have a wide range of properties. Principal disadvantage: • Susceptible to corrosion. • High density i.e . • low specific strength • Low thermal and electrical conductivities DR.RABIUL HUSSAIN/0966882081

  8. Classification of Metal Alloys DR.RABIUL HUSSAIN/0966882081

  9. Classification scheme for various ferrous alloys DR.RABIUL HUSSAIN/0966882081

  10. Ferrous Alloys • Ferrous Alloys are classified broadly into two classes : • Steels • Cast Irons • Disadvantages of Ferrous Alloys: • Relatively high densities • Relatively low electrical conductivities • Generally poor corrosion resistance DR.RABIUL HUSSAIN/0966882081

  11. Steels • Steels are iron-carbon alloys which may contain other alloying elements in appreciable amounts. • Steel contains less than 1.4wt%C and the mechanical properties of steels are sensitive to carbon content. • More than thousands different compositions are possible. • Common steels are classified on the basis of carbon content as follows: • LOW-CARBON STEEL ( <0.25 WT% C) • MEDIUM-CARBON STEELS (0.25- <0.6 WT% C) • HIGH-CARBON STEELS (0.6 - <1.4 WT% C) • Each class of steels are again subdivided into different subclass according to the concentration of other alloying elements. DR.RABIUL HUSSAIN/0966882081

  12. Classification of steels • Plain Carbon Steels- contain only C and a little Mn. • Alloy steels- contain other alloying elements in addition to C and Mn. DR.RABIUL HUSSAIN/0966882081

  13. Low-carbon Steels • Plain low-carbon steels generally contain <0.25 wt% C and are unresponsive to heat treatment. • Strengthening is accomplished by cold work. • Microstructure consist of ferrite and pearlite constituents. • Plain low-carbon steel alloys are relatively soft and weak but have outstanding ductility and toughness. • They typically have a yield strength of 275MPa, tensile strengths between 415-550MPa and a ductility of 25% EL. DR.RABIUL HUSSAIN/0966882081

  14. Low-Carbon Steels • An important class of low-carbon steels are high-strength, low-alloy (HSLA) steels which contain other alloying elements such as Cu,V,Ni and Mo in combined concentration as high as 10% by weight. • HSLA possess higher strength than plain low-carbon steels and may be strengthened by heat treatment. • HSLA steels are more resistant to corrosion than the plain carbon steels. DR.RABIUL HUSSAIN/0966882081

  15. MEDIUM-CARBON STEELS • The medium-carbon steels have carbon concentrations between about 0.25-0.6 wt%. • These alloys may be heat treated by austenitizing, quenching and then tempering to improve their mechanical properties. • Microstructure consists of tempered martensite. • The plain medium-carbon steels have low hardenabilities and can be heat treated with very rapid quenching rate. DR.RABIUL HUSSAIN/0966882081

  16. Medium carbon steels • Heat treatable medium-carbon steels with improved capacity of heat-treatment is made by addition of Cr, Ni and Mo. • These heat-treated alloys are stronger than low-carbon steels, but at a sacrifice of ductility and toughness. DR.RABIUL HUSSAIN/0966882081

  17. High-Carbon Steels • The high carbon steels normally having carbon contents between 0.6 to 1.4 wt% and are the hardest, strongest and yet least ductile of the carbon steels. • They are almost always used in a hardened and tempered condition and as such, are especially wear resistant and capable of holding a sharp cutting edge. • The tool and die steels are high carbon alloys usually containing Cr, V, W and Mo. These alloying elements combine with carbon to form very hard and wear-resistant carbide compounds (e.g., Cr3C6, V4C3 and WC). DR.RABIUL HUSSAIN/0966882081

  18. STAINLESS STEELS • The stainless steels are highly resistant to corrosion (rusting) in a variety of environments. • The predominant alloying element in stainless steel is >11wt%Cr. Corrosion resistant may be further increased by adding Ni and Mo. • Stainless steels are divided into three classes on the basis of the predominant constituent of the microstructure: • Martensitic stainless steels • Ferritic Stainless Steels • Austenitic Stainless Steels DR.RABIUL HUSSAIN/0966882081

  19. Stainless Steels Martensitic Stainless Steels: • From 12 to 17% Cr with amounts ofcarbon (up to 1.1%) necessary to produce martensite. • Matrix: martensitic (after quenching andtempering) with carbides. • FCC→BCT transformation (the α-loop inthe phase diagram is increased). • More expensive due to heat treatment. • General application when corrosion resistance needs to be accompanied with very high strength and hardness. Most common: 440 (17 Cr-0.7+ C); UTS: 1828 MPa, YS: 1690 MPa; δ: 5% DR.RABIUL HUSSAIN/0966882081

  20. Stainless Steels Austenitic Stainless Steels: • From 16 to 25% Cr with Ni (7 to 20%) and very little or no carbon to produce austenitic. • Matrix: austenitic, which allows high formability. • The FCC structure is stabilized at room temperature by Ni. • Most expensive and most corrosion resistant due to high Cr and Ni content. • Very low carbon is needed to avoid intergranular corrosion. • Used when maximum corrosion resistance is needed Most common: 304 (19 Cr – 10 Ni; UTS: 580MPa; YS: 290MPa; δ: 55% ; 316 (18 Cr – 12 Ni; UTS: 500 MPa; YS: 225MPa; δ: 40% DR.RABIUL HUSSAIN/0966882081

  21. Stainless Steels Ferritic Stainless Steels: • From 12 to 30% Cr with small amounts of carbon (0.012-0.20%) is required to get ferritic. • Matrix: ferritic (α-Fe solution) with fine carbide precipitates. • No FCC→BCC transformation • Most inexpensive stainless steel due to no need for Ni. • General application when corrosion resistance is needed. Most common: 430 (17 Cr-0.012 C; UTS: 517 MPa, YS: 345 MPa; δ: 25% 446 (25 Cr-0.20 C; UTS: 552 MPa, YS: 345 MPa; δ: 20% DR.RABIUL HUSSAIN/0966882081

  22. Cast Irons • Cast Irons are ferrous alloys containing >2.14wt% C but most commonly 3.0-4.5 wt% C with some other alloying elements. • They are easily meltable (low melting) and thus relatively easy to cast. • Some cast irons are very brittle. • Cementite (Fe3C) is a metastable compound and under some circumstances it can be made to dissociate or decompose to form -ferrite and graphite, according to the reaction: • Fe3C 3 Fe (α) + C (graphite) DR.RABIUL HUSSAIN/0966882081

  23. T(ºC) 1600 L 1400 Liquid + Graphite g +L 1200 g 1153ºC Austenite 4.2 wt% C 1000  + Graphite a + g 800 740ºC 0.65 600  + Graphite 400 90 0 100 1 2 3 4 C, wt% C (Fe) Fe-C True Equilibrium Diagram • Graphite formation promoted by • Si > 1 wt% • slow cooling • For most cast irons, the carbon exists as graphite, and both microstructure and mechanical behaviour depend on composition and heat treatment. Common cast iron types : gray, nodular, white, malleable and compacted graphite. DR.RABIUL HUSSAIN/0966882081

  24. Types of Cast Irons • Gray Iron: • C and Si content varies between 2.5 & 4.0 wt% 1.0 & 3.0 wt% respectively. • The graphite exists in the form of flakes surrounded by  -ferrite and pearlite matrix. • weak & brittle in tension • Strength and ductility are much higher under compressive loads. • Excellent vibrational dampening • High wear resistant Fig. Optical micrographs of Gray iron DR.RABIUL HUSSAIN/0966882081

  25. Types of Cast Irons • Ductile (or Nodular) iron: • Adding Mg and/or Ce to gray iron results in alloy known as ductile or nodular ironbecause of its microstructure. • Here graphite forms as nodules not flakes • Matrix often pearlite OR ferrite depending on heat treatment. • Ductile irons are stronger and much more ductile than gray iron. Fig. Optical micrograph of ductile iron DR.RABIUL HUSSAIN/0966882081

  26. Types of Cast Irons White Irons: • Cast irons containing less than 1.0wt% Si gives rise to the formation of alloys consisting C as cementite instead of graphite. A fracture surface of this alloy has a white appearance, and thus it is termed as white iron. • White iron is extremely hard but also very brittle. • Its use is limited to applications that require very hard and wear resistant surface such as rollers. Fig. optical micrograph of white iron DR.RABIUL HUSSAIN/0966882081

  27. Types of Cast Irons Malleable Iron: • Heating white iron at temperatures between 800◦C and 900◦C for a prolonged time period and in a neutral atmosphere (to prevent oxidation) causes a decomposition of the cementite, forming graphite, which exists in the form of clusters or rosettes surrounded by a ferrite or pearlite matrix, depending on cooling rate. The resulting cast iron is called malleable iron due to its relatively high strength and appreciable ductility or malleability. Fig. Photomicrograph of Malleable iron DR.RABIUL HUSSAIN/0966882081

  28. Production of Cast Irons Gf, flake graphite; Gr, graphite rosettes; Gn, graphite nodules; P, pearlite; a, ferrite. DR.RABIUL HUSSAIN/0966882081

  29. Type of Cast Irons Compacted Graphite Iron: • Compacted graphite iron (CGI) contains Silicon content ranges between 1.7 and 3.0 wt%, whereas carbon concentration is normally between 3.1 and 4.0 wt%. Silicon is the alloying elements which promotes the formation of graphite. • Microstructurally, the graphite in CGI alloys has a wormlike (or vermicular) shape. this microstructure is intermediate between that of gray iron and ductile (nodular) iron. • The compositions of magnesium, cerium, and other additives must be controlled so as to produce a microstructure that consists of the wormlike graphite particles while at the same time limiting the degree of graphite nodularity, and preventing the formation of graphite flakes. • Depending on heat treatment, the matrix phase will be pearlite and/or ferrite. DR.RABIUL HUSSAIN/0966882081

  30. Compacted Graphite Iron (CGI) • Tensile and yield strengths for compacted graphite irons are comparable to values for ductile and malleable irons, yet are greater than those observed for the higher-strength gray irons. • Ductilitiesfor CGIs are intermediate between values for gray and ductile irons; moduliof elasticity range between 140 and 165 GPa. • Desirable characteristics of CGIs include the following: • Higher thermal conductivity • Better resistance to thermal shock (i.e., fracture resulting from rapid temperature changes) • Lower oxidation at elevated temperatures DR.RABIUL HUSSAIN/0966882081

  31. Compacted Graphite Iron Applications: • Diesel engine blocks, • Exhaust manifolds, • Gearbox housings, • Brake discs • For high-speed trains, and • Flywheels. Fig: Photomicrograph of CGI with ferrite matrix DR.RABIUL HUSSAIN/0966882081

  32. Non-ferrous Alloys • Non-ferrous alloys contain elements other than iron in appreciable amounts. • The metal that is present in maximum amount in the alloy is termed as base metals. • Cast Alloys and Wrought Alloys: Alloys that are so brittle that forming and shaping by appreciable deformation is not possible and must be fabricated by casting are called cast alloys. On the otherhand, those that are amenable to mechanical deformation and can be fabricated by hot or cold working are called wrought alloys. • “Heat treatable”designates an alloy whose mechanical strength is improved by precipitation hardening or a martensitic transformation both of which involve specific heat treating procedure. DR.RABIUL HUSSAIN/0966882081

  33. Non-ferrous materials • Typical advantages of non-ferrous materials over ferrous materials: • high specific strength. • low density. • high electrical and thermal conductivities. • distinct properties thus used for specific purposes. • can be formed with ease. • E.g.: • Al-alloys • Cu-alloys(brass,bronze) • Mg-alloys • Ti-alloys • Noble metals(E.g.:Ag,Au,Pt,Pa) • Refractory metals(E.g.:Nb,Mo,WandTa) DR.RABIUL HUSSAIN/0966882081

  34. Classification of Non-ferrous Alloys DR.RABIUL HUSSAIN/0966882081

  35. Titanium Alloys • Relatively low density, • High melting temperatures, and • High strengths are possible. • Titanium alloys are extremely strong; room-temperature tensile strengths as high as 1400 MPa (200,000 psi) are attainable, yielding remarkable specific strengths. • Titanium alloys are highly ductile and easily forged and machined. • Applications: aircraft structures, space vehicles, and in chemical and petroleum industries. • Disadvantage: Too expensive DR.RABIUL HUSSAIN/0966882081

  36. The Refractory Metals • Metals that have extremely high melting temperatures are classified as refractory metals. Included in this group are niobium (Nb), molybdenum (Mo), tungsten (W),and tantalum (Ta). • Extremely high melting temperatures; • Large elastic moduli, • Hardnesses, and strengths. • Applications: extrusion dies, structural parts in space vehicles, incandescent light filaments, x-ray tubes,and welding electrodes. • Disadvantage: some experience rapid oxidation at elevated temperatures. DR.RABIUL HUSSAIN/0966882081

  37. Super Alloys • The super-alloys have superlative combinations of properties. • These materials are classified according to the predominant metal(s) in the alloy, of which there are three groups—iron–nickel, nickel, and cobalt. Other alloying elements include the refractory metals (Nb, Mo, W, Ta), chromium, and titanium. • Distinctive features: able to withstand high temperatures and oxidizing atmospheres for long time periods. • Applications: aircraft turbines, nuclear reactors, and petrochemical equipment. DR.RABIUL HUSSAIN/0966882081

  38. Noble Metals • The noble or precious metals are a group of eight elements that have some physical characteristics in common. • The noble metals are silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium. • Distinctive features: highly resistant to oxidation, especially at elevated temperatures; soft and ductile. • Limitation: expensive. • Applications: jewelry, dental restoration materials, coins, catalysts, and thermocouples. DR.RABIUL HUSSAIN/0966882081

  39. Nonferrous Alloys • Cu Alloys • Al Alloys Brass:Zn is subst. impurity -low r: 2.7 g/cm3 (costume jewelry, coins, -Cu, Mg, Si, Mn, Zn additions corrosion resistant) -solid sol. or precip. Bronze : Sn, Al, Si, Ni are strengthened (struct. subst. impurities aircraft parts (bushings, landing & packaging) gear) NonFerrous • Mg Alloys Cu-Be : r -very low : 1.7g/cm3 Alloys precip. hardened -ignites easily for strength - aircraft, missiles • Ti Alloys • Refractory metals -relativelylowr:4.5g/cm3 -high melting T’s vs 7.9 for steel • Noble metals -Nb, Mo, W, Ta -Ag, Au, Pt -reactiveathighT’s - oxid./corr. resistant - space applic. Based on discussion and data provided in Section 11.3, Callister & Rethwisch 3e. DR.RABIUL HUSSAIN/0966882081

  40. Next class: fabrication and Processing of metals Any questions???? Thank you for attending the lecture DR.RABIUL HUSSAIN/0966882081