Engineering Alloys PDF

CHAPTER 9 Engineering Alloys 1  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Engineering Alloys Pure or nearly pure metals are not useful in most engineering a...

CHAPTER 9 Engineering Alloys 1  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Engineering Alloys Pure or nearly pure metals are not useful in most engineering applications (except for electrical applications). In most applications, especially, structural, metal alloys are used. Alloys are made of combination of metals and sometimes nonmetals. Alloys are classified in two major groups: 1- ferrous alloys and 2- nonferrous alloys. Ferrous alloys contains iron as the main ingredient (90% of world's production of all metals is ferrous alloys). Nonferrous alloys contain little or no iron. All steels are ferrous alloys. Titanium and aluminum alloys are examples of nonferrous alloys. 2 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Metal Cost Metals are selected for applications Based on properties and relative cost. Class Discussion Topic: If a titanium and a steel alloy are equally suitable for your structural applications, what other factors would you consider in selecting your metal? 3 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Production of Iron and Steel Iron ore (iron oxide) is mixed with coke inside a blast furnace to produce pig iron. Pig iron is used to make steel through the basic oxygen process. 4 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Steel Making – Oxygen Process Pig iron and 30% steel scrap are fed into refractory furnace to which an oxygen lance is inserted. Ø Oxygen reacts with the liquid bath to form iron oxide. Ø Next carbon reacts with iron oxide to form carbon monoxide. FeO + C Fe + CO Slag forming fluxes are added next. Carbon content and other impurities are lowered/controlled. Molten steel is continuously cast, cast into ingots, or formed directly into useful shapes. Half of the raw steel is produced by recycling old steel (junk cars 5 or old appliances) Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The Iron-Carbon Binary Alloy System Plain carbon steel contains 0.03% to 1.2% C; 0.25 to 1.0% Mn and minor amounts of other impurities. Ferrite phase, α : Very low solubility of carbon. Max 0.02 % at 727oC and 0.005% at 0oC. Austenite phase, γ: Solubility of C is 2.08% at 1147oC and 0.8% at 0oC. Ferrite phase, δ: exists above 1394oC and below melting (limited value), Cementite, Fe3C: Intermetallic compound. 6.67% C and 93.3% Fe. α, γ, δ are all interstitial solid solutions of carbon in iron in BCC (α), FCC (γ), and BCC (δ) structures respectively. 6 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Invariant Reactions in the Fe – Fe3C Phase Diagram Peritectic reaction: 1495oC Liquid (0.53%C) + δ (0.09% C) γ (0.17% C) Eutectic reaction: o Liquid (4.3% C) 1147 C γ austenite (2.08%C) + Fe3C ( 6.67%C) Eutectoid reaction: o γ Austenite (0.8%C) 727 C α Ferrite(0.02%C) + Fe3C ( 6.67%C) Hypoeutectoid less than 0.8% 0.8% C more than 0.8% Hypereutectoid Steel Eutectoid Steel Steel 7 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Slow Cooling of Plain Carbon Steel A steel alloy containing 0.77% C is called the eutectoid plain carbon steel. If eutectoid steel is heated slightly above 727oC and held there for sufficient time, the structure will become homogeneous austenite, γ. This process is called austenitizing. If cooled below eutectoid temp, a lamellar microstructure of α ferrite (light) and cementite (dark) is formed called pearlite. 8 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Slow Cooling of Plain Carbon Steel, Cont. A steel alloy containing greater 0.02% and less than 0.77% C is called the hypoeutectoid plain carbon steel. If a sample of 0.4% C steel alloy (hypoeutectoid alloy) is austenitized (heated up to point “a ”) and then slowly cooled to point “b”, proeutectoid α (or primary α) is formed. Further cooling to point “d ”produces pearlite which consists of eutectoid α and cementite, Fe3C. Pearlite Proeutectoid or primary α 9 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Slow Cooling of Plain Carbon Steel, Cont. A steel alloy containing greater 0.77% is called the hypereutectoid plain carbon steel. If a sample of 1.2% C steel alloy (hypereutectoid alloy) is austenitized (heated up to point “a ”) and then slowly cooled to point “b”, proeutectoid cementite (or primary cementite) is formed. Further cooling to point “d ”produces pearlite which consists of eutectoid α and eutectoid cementite, Fe3C. Pearlite Proeutectoid or primary cementite 10 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Heat Treatment of Plain Carbon Steels By heating steel alloys to the γ region (austenitizing) more C atoms can be absorbed by the FCC crystal (bottom left figure). This followed by cooling the alloy at different rates, produces alloys of different mechanical properties. If after austenitizing, the alloy is cooled very fast or quenched, martensite is formed. Martensite is a metastable phase consisting of super saturated interstitial solid solution of C in BCC or BCC tetragonal iron (bottom right figure). Extensive amount of carbon is trapped in the martensite microstructure due to quenching. 11 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Microstructure of Martensite If the steel alloy contains less than 0.6% C, the resulting martensite will consists of domains of lath of different orientation called lath martensite. If the steel alloy contains greater than 1.0% C, the resulting martensite will consists of fine structure of parallel twins called plate martensite. Between 0.6% and 1.0% C, the resulting martensite will be mixed. Lath type Plate type 12 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Martensite, Cont. Upon quenching, the transformation from austenite, γ, to martensite is diffusionless – there is no time for diffusion. No change of relative position of carbon atoms after the transformation. Martensite is much stronger and harder than pearlite with the same carbon content. These properties are further enhanced with increases in carbon content. Enhanced Strength is due to high dislocation concentration and interstitial solid solution strengthening. 13 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Isothermal Decomposition of Austenite Isothermal decomposition experiments are used to investigate microstructural changes in eutectoid steel. Steel samples are first austenitized, then rapidly cooled to a specific temperatures below the eutectoid temperature in a salt bath. Finally, the samples are quenched in water at various time intervals. Each sample’s microstructure is then analyzed. 14 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Isothermal Decomposition of Austenite, Cont. The microstructural changes during isothermal decomposition of eutectoid plain carbon steel are shown below. Samples are hot quenched to 705oC in a salt bath and kept at that temperature for various time intervals and subsequently quenched to room temperature. Note, after 5.8 minutes at 705oC, coarse pearlite appears in the austenitic sample and after 66.7 minutes all of the austenite has transformed to pearlite. 15 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Isothermal Decomposition of Austenite, Cont. If the isothermal decomposition process is repeated by cooling to progressively lower temperatures, an isothermal transformation (IT) diagram may be developed; the S-shaped blue lines in the figure. The first S-shaped curve indicates the time needed to for the transformation of austenite to begin and the second curve indicates the time required to complete it. Hot quenching to different temperatures, represented by red lines, produces different microstructures including coarse pearlite, fine pearlite, bainite, and martensite. 16 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Isothermal Decomposition of Austenite, Cont. If hot quenching temperature is between 5500C to 2500C, an intermediate structure bainite is produced. Bainite contain nonlamellar eutectoid structure of α ferrite and cementite. Upper bainite Between 5500C and 3500C Lower bainite Between 3500C and 2500C Upper Bainite Lower Bainite 17 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display IT Diagrams for Non-eutectoid Steels The S curves of IT diagrams for non-eutectoid steels are shifted to left (see hypoeutectoid steel in the figure – 0.47% C). Note the transformation-begin-curve is discontinuous. Therefore, not possible to hot- quench from austenitic region to produce 100% martensite The extra transformation line (top black line) indicates start and formation of proeutectoid ferrite. IT diagrams are used only for understanding the transformation process; in industry, isothermal transformation or heat treatments is not used. 18 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Continuous Cooling Transformation Diagram In industrial heat treatment, continuous cooling from the austenitic state to room temperature is performed in one step. For plain carbon steel, the transformation from austenite to other phases takes place over a range of temperatures (as opposed to Isothermal Transformation). Note all dotted lines indicate the path of transformation at various cooling rates. The solid blue region indicates the region of transformation. Note, the start and finish lines shifted to the right compared to IT. No bainite transformation path. 19 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Continuous Cooling Transformation Diagram In continuous cooling of eutectoid steel at various cooling rates, different microstructures may be produced. Water quenching produces a rapid cooling rate to 100% martensite. The blue path is the critical or lowest possible cooling rate to produce 100% martensite. Oil quenching produces a lower cooling rate leading to a mixture of martensite and pearlite. If the slower cooling paths do not intersect the red line, no martensite will be produced. Such slow cooling rates produces fine an coarse pearlite. 20 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Annealing and Normalizing A plain carbon steel sample is fully annealed when it is heated to temperatures 40oC above austenite ferrite boundary, held for necessary time, and cooled slowly to room temperature. A plain carbon steel sample is process annealed (or stress relief) when it is heated to temperatures below eutectoid temperature and cooled slowly to room temperature. Process annealing is normallyApplied to hypoeutectoid steel. A plain carbon steel sample is normalized fully when it is heated to temperatures 40oC above austenite ferrite and austenite cementite boundaries, and cooled slowly to room 21temperature. Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Annealing and Normalizing, Cont. The full anneal heat treatment softens the steel to its lowest strength and grows large equiaxed grains. The microstructure mostly consists of proeutectoid ferrite/cementite (depending on carbon content) and pearlite. The process anneal heat treatment relieves the internal residual stresses through realignment of dislocations. The normalization heat treatment process refines the grain structure, produces a stronger steel compared to fully annealed steel, and creates a more uniform composition. The micro- structure consists of proeutectoid ferrite and pearlite. 22 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Tempering of Plain Carbon Steel Quenching the austenitized plain carbon steel results in formation of a strong yet brittle martensitic steel. Ductility of the martensitic steel may be improved through a heat treatment process called tempering. After quenching, the formed martensite is heated at temperatures below eutectoid for a period of time. The dark line in the figure shows the tempering path. The tempering temperature influences, the strength, ductility, and microstructure of the steel. For instance tempering between 400oC and 700oC produces a spherodite microstructure. 23 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Martempering and Austempering Martempering (Marquenching) refers to the process of austenitizing; quenching at around Ms, holding in quenching media until temperature is uniform; removing before Bainite forms; and cooling at a moderate rate. Austempering refers to a similar process as martempering but held at quenching media till austenite to Bainite transformation takes place. The table shows the impact of various tempering processes on hardness, toughness, and ductility of 1095 steel. 24 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Calssification of Plain Carbon Steel Four digit AISI-SAE code. If the first two digits are 10,this indicate a plain carbon steel. Last two digits indicate carbon content in 100th wt%. Example: 1030 steel indicate plain carbon steel containing 0.30 wt% carbon. In addition to carbon, other impurities such as Mn may also exist in plain carbon steel but in minute amounts. The table shows the impact of carbon Content and heat treatment on various Plain carbon steels. 25 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Low Alloy Steels Limitations of plain carbon steels: Ø Cannot be strengthened beyond 690 MPa without loosing ductility and impact strength. Ø Not deep hardenable. Ø Low corrosion resistance Ø Rapid quenching leads to crack and distortion. Ø Poor impact resistance at low temperature. To overcome the above deficiencies of plain carbon steels, alloy steels are developed that contain significant amount of impurities other than carbon such as manganese, nickel, chromium, molybdenum and tungsten These steels are often used in automotive and construction. 26 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Classification of Alloy Steels First two digits indicate principal alloying element. Last two digits indicate %carbon. 27 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Distribution of Alloying Elements Distribution depends upon compound and carbide forming tendency of each element. 28 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Effects of Alloying Element on Eutectoid Temperature Mn and Ni lower eutectoid temperature. They act as austenite stabilizing element. Tungsten, molybdenum and titanium raise eutectic temperature. They are called ferrite stabilizing elements. 29 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Hardenability Hardenability determines the depth and distribution of hardness induced by quenching. Hardenability depends on Ø Composition Ø Austenitic grain size Ø Structure before quenching Jominy hardenability test: Ø Cylindrical bar (1 inch dia and 4 inch length with 1/16 in flange at one end is austenitized and one end is quenched. Ø Rockwell C hardness is measured up to 2.5 inch from quenched end. 30 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Hardenability, Cont. For 1080 plain carbon steel, the hardness value at quenched end is 65 HRC while it is 50 HRC at 3/16 inch from quenched end. Alloy steel 4340 has high hardenability and has hardness of 40 HRC 2 inches from quenched end. In alloy steel, decomposition of austenite to ferrite is delayed. Cooling rate depends on bar dia, quenching media and bar cross section. 31 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mechanical Properties of Low Alloy Steels 32 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Aluminum Alloys Precipitation strengthening heat treatment creates fine dispersion of precipitated particles in the metal which hinder dislocation movements. Basic steps : Ø Solution heat treatment: Alloy sample heated to a temperature between solvus and solidus and soaked at that temperature. Ø Quenching: Sample then quenched to room temperature in water. Ø Aging: Solutionized and quenched sample is then aged to form finely dispersed particles. 33 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Decomposition Products Created by Aging Super saturated solid solution is in unstable condition. Alloy tends to seek a lower energy state by decomposing into metastable or equilibrium phase. Supersaturated solid solution as highest energy state. Equilibrium precipitate has lowest energy state. 34 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Effects of Aging on Strength Aging curve: Plot of strength or hardness versus aging time. As aging time increases alloy becomes stronger harder and less ductile. Overaging decreases strength and hardness. 35 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Example - Al 4% Cu Alloy Al -4% Cu is solutionized at about 5150C Alloy is rapidly cooled in water. Alloy is artificially aged in 130 – 1900C Structures formed : Ø GP1 Zone: At lower aging temperature, copper atom is segregated in supersaturated solid solution. Ø GP2 Zone: Tetragonal structure, 10-100 nm diameter. Ø θ’ Phase: Nucleates heterogeneously on dislocation. Ø θ Phase: Equilibrium phase, incoherent (CuAl2). 36 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Correlation of Structure and Hardness GP1 and GP2 Zones increase hardness by stopping dislocation movement. At 130oC when θ’ forms, hardness is maximum. After θ’ forms, GP2 zones are dissolved and θ’ gets coarsened reducing hardness. 37 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display General Properties of Aluminum Low density, corrosion resistance. High alloy strength (about 690 MPa) Nontoxic and good electrical properties. Aluminum Hot Sodium Production: + Aluminate Ore (Bauxite) NaOH Aluminum hydroxide is precipitated from aluminum solution. Aluminum hydroxide is thickened and calcined to Al2O3 which is dissolve in cryolite and electrolyzed. Metallic aluminum sinks to bottom and is tapped out. 38 Foundations of Materials Science and Engineering, 6th Edn. Smith and Hashemi