The Structure of Metals Mechanical Behavior, Testing and Manufacturing Properties of Materials Physical Properties of Materials Metal Alloys: Structure and Strengthening by Heat Treatment Ferrous Metals and Alloys: Production, General Properties and Applications

6. Nonferrous Metals and Alloys: Production, General Properties and Applications 7. Polymers: Structure, General Properties and Applications 8. Ceramics, Graphite and Diamond: Structure, General Properties and Applications 9. Composite Materials: Structure, General Properties, and Applications

Chapter Objectives Introduction Structure of Alloys Phase Diagrams The Iron-Carbon System The Iron-Iron Carbide Phase Diagram and the Development of Microstructures in Steels Cast Irons Heat Treatment of Ferrous Alloys Hardenability of Ferrous Alloys Heat Treatment of Nonferrous Alloys and Stainless Steels Case Hardening Annealing Heat-Treating Furnaces and Equipment Design Considerations for Heat Treating

Various phases in alloys and their significance in manufacturing. Chapter Outline Microstructure of ferrous alloys and characteristics of phase diagrams. Various phases in alloys and their significance in manufacturing. Mechanisms by which properties are altered. Procedures used for heat-treating ferrous metals to enhance their properties. Microstructures of nonferrous alloys and their significance. Procedures used for heat-treating nonferrous alloys. Characteristics of heat-treating furnaces. Design considerations for successful heat treating.

The most common example of property improvement is heat treatment. 4.1 Introduction The most common example of property improvement is heat treatment. Heat treatment modifies microstructures and, thereby, produces a variety of mechanical properties that are important in manufacturing, such as improved formability and machinability.

4.1 Introduction Fig. 4.1 shows the cross-section of gear teeth showing induction-hardened surfaces. This chapter follows the outline shown in Fig. 4.2, beginning with the effects of various alloying elements, and followed by the solubility of one element in another, phases, equilibrium phase diagrams, and the influences of composition, temperature, and time.

4.1 Introduction

4.1 Introduction

These metals are known as pure metals. 4.2 Structure of Alloys Atoms are all of the same type, except for the presence of rare impurity atoms. These metals are known as pure metals. Commercially pure metals are used for various purposes. Pure metals have somewhat limited properties; these properties can be enhanced and modified by alloying. An alloy is composed of two or more chemical elements, at least one of which is a metal.

4.2 Structure of Alloys Alloying consists of two basic forms: solid solutions and intermetallic compounds.

Two terms are essential in describing alloys: solute and solvent. 4.2.1 Solid Solutions Two terms are essential in describing alloys: solute and solvent. Solute is the minor element that is added to the solvent, which is the major element. When the particular crystal structure of the solvent is maintained during alloying, the alloy is called a solid solution.

Substitutional solid solutions If the size of the solute atom is similar to that of the solvent atom, the solute atoms can replace solvent atoms and form a substitutional solid solution. Two conditions generally are required to form complete substitutional solid solutions: 1. The two metals must have similar crystal structures. 2. The difference in their atomic radii should be less than 15%.

Interstitial solid solutions If the size of the solute atom is much smaller than that of the solvent atom, each solute atom can occupy an interstitial position; such a process forms an interstitial solid solution.

Interstitial solid solutions If the size of the solute atom is much smaller than that of the solvent atom, each solute atom can occupy an interstitial position; such a process forms an interstitial solid solution. There are two conditions necessary for forming interstitial solutions: 1. The solvent atom must have more than one valence. 2. The atomic radius of the solute atom must be less than 59% of the atomic radius for the solvent atom.

Interstitial solid solutions An important family of interstitial solid solutions is steel, an alloy of iron and carbon, where carbon atoms are present in interstitial positions between iron atoms.

4.2.2 Intermetallic compounds Intermetallic compounds are complex structures consisting of two metals in which solute atoms are present among solvent atoms in certain proportions.

4.2.3 Two-phase systems Most alloys consist of two or more solid phases and may be regarded as mechanical mixtures; such a system with two solid phases is known as a two-phase system. A phase is defined as a physically distinct and homogeneous portion in a material; each phase is a homogeneous part of the total mass and has its own characteristics and properties.

4.2.3 Two-phase systems Fig 4.3(a) shows a schematic illustration of grains, grain boundaries, and particles dispersed throughout the structure of a two-phase system, such as a lead-copper alloy. The grains represent lead in solid solution in copper, and the particles are lead as a second phase. Fig 4.3(b) shows a schematic illustration of a two-phase system consisting of two sets of grains: dark and light. The dark and the light grains have separate compositions and properties.

4.2.3 Two-phase systems Alloying with finely dispersed particles (second-phase particles) is an important method of strengthening alloys and controlling their properties.

4.3 Phase Diagram Pure metals have clearly defined melting or freezing points, and solidification takes place at a constant temperature. Fig 4.4(a) shows a cooling curve for the solidification of pure metals. Note that freezing takes place at a constant temperature; during freezing, the latent heat of solidification is given off. Fig 4.4(b) shows the change in density during the cooling of pure metals. When the temperature of the molten metal is reduced to the freezing point, the energy of the latent heat of solidification is given off while the temperature remains constant.

4.3 Phase Diagram

4.3 Phase Diagram A phase diagram, also called an equilibrium or constitutional diagram, shows the relationships among the temperature, the composition, and the phases present in a particular alloy system under equilibrium conditions. Equilibrium means that the state of a system remains constant over an indefinite period of time.

The second circle shows the formation of dendrites. 4.3 Phase Diagram Fig 4.5 shows a phase diagram for nickel-copper alloy system obtained at a slow rate of solidification. Note that pure nickel and pure copper each has one freezing or melting temperature. The top circle on the right depicts the nucleation of crystals. The second circle shows the formation of dendrites. The bottom circle shows the solidified alloy with grain boundaries.

4.3 Phase Diagram

Fig 4.7 shows the lead-tin phase diagram. Fig 4.6 shows the mechanical properties of copper-nickel and copper-zinc alloys as a function of their composition. The curves for zinc are short, because zinc has a maximum solid solubility of 40% in copper. Fig 4.7 shows the lead-tin phase diagram. Note that the composition of the eutectic point for this alloy is 61.9% Sn-38.1% Pb. A composition either lower or higher than this ratio will have a higher liquidus temperature.

4.3 Phase Diagram

4.3 Phase Diagram Eutectic point is the point at which the liquid solution decomposes into the components alpha and beta. Eutectic points are important in applications, such as soldering, where low temperatures are desirable to prevent thermal damage to parts during joining.

4.4 The Iron-Carbon System Fig 4.8 shows a iron-iron carbide phase diagram. Because of the importance of steel as an engineering material, this diagram is one of the most important of all phase diagrams.

4.4 The Iron-Carbon System Ferrite Alpha ferrite, or simply ferrite, is a solid solution of body-centered cubic iron; it has a maximum solid solubility of 0.022% C at a temperature of 727°C. Ferrite is relatively soft and ductile; it is magnetic from room temperature to 768°C—the Curie temperature.

4.4 The Iron-Carbon System Austenite Within a certain temperature range, iron undergoes a polymorphic transformation from the bcc to an fcc structure, becoming gamma iron or (more commonly) austenite. In Fig 4.9, The unit cells for (a) austenite, (b) ferrite, and (c) martensite. The effect of percentage of carbon (by weight) on the lattice dimensions for martensite is shown in (d). Note the interstitial position of the carbon atoms. Also note, the increase in dimension c with increasing carbon content: this effect causes the unit cell of martensite to be in the shape of a rectangular prism.

4.4 The Iron-Carbon System

4.4 The Iron-Carbon System Cementite The right boundary of Fig. 4.8 represents cementite, which is 100% iron carbide having a carbon content of 6.67%. Cementite, from the Latin caementum (meaning stone chips), is also called carbide.

4.5 The Iron-Iron Carbide Phase Diagram and the Development of Microstructures in Steel Fig 4.10 shows the schematic illustration of the microstructures for an iron-carbon alloy of eutectoid composition (0.77% carbon) above and below the eutectoid temperature of 727°C.

The reason for slow cooling is to maintain equilibrium. 4.5 The Iron-Iron Carbide Phase Diagram and the Development of Microstructures in Steel The reason for slow cooling is to maintain equilibrium. Eutectoid reaction means that at a certain temperature, a single, solid phase (austenite) is transformed into two other solid phases (ferrite and cementite). The structure of eutectoid steel is called pearlite. Fig 4.11 shows a microstructure of pearlite in 1080 steel formed from austenite of a eutectoid composition. In this lamellar structure, the lighter regions are ferrite, and the darker regions are carbide.

4.5 The Iron-Iron Carbide Phase Diagram and the Development of Microstructures in Steel

4.5 The Iron-Iron Carbide Phase Diagram and the Development of Microstructures in Steel The ferrite in the pearlite is called eutectoid ferrite, and the ferrite phase is called proeutectoid ferrite. The cementite in the pearlite is called eutectoid cementite, and the cementite phase is called proeutectoid cementite.

4.5.1 Effects of Alloying Elements in Iron The eutectoid temperature may be raised or lowered from 727°C, depending on the particular alloying element. Lowering the eutectoid temperature means increasing the austenite range. As result, an alloying element (such as nickel) is known as an austenite former. Chromium and molybdenum have a bcc structure, thus favoring the bcc structure of ferrite. These elements are known as ferrite stabilizers.

a. Gray cast iron, or gray iron 4.6 Cast Irons The term cast iron refers to a family of ferrous alloys composed of iron, carbon (ranging from 2.11% to about 4.5%), and silicon (up to about 3.5%). Cast irons are classified according to their solidification morphology from the eutectic temperature, as follows: a. Gray cast iron, or gray iron b. Ductile cast iron, nodular cast iron, or spheroidal graphite cast iron c. White cast iron d. Malleable iron e. Compacted graphite iron

4.6 Cast Irons Cast irons also are classified by their structure: ferritic, pearlitic, quenched and tempered, or austempered. Fig 4.12 shows the phase diagram for the iron-carbon system with graphite (instead of cementite) as the stable phase. Cementite is not completely stable; it is metastable, with an extremely low rate of decomposition.

4.6 Cast Irons

Graphite exists largely in the form of flakes. 4.6 Cast Irons Grey Cast Iron Graphite exists largely in the form of flakes. It is called gray cast iron or gray iron, because when it is broken, the fracture path is along the graphite flakes and has a gray, sooty appearance. Fig 4.13 (a) shows ferritic gray iron with graphite flakes. (b) Ferritic ductile iron (nodular iron) with graphite in nodular form. (c) Ferritic malleable iron. This cast iron solidified as white cast iron with the carbon present as cementite and was heat treated to graphitize the carbon.

4.6 Cast Irons

Ductile (Nodular) Iron 4.6 Cast Irons Ductile (Nodular) Iron In the ductile-iron structure, graphite is in a nodular or spheroid form. White Cast Iron The white-cast-iron structure is very hard, wear-resistant, and brittle because of the presence of large amounts of iron carbide (instead of graphite). White cast iron is obtained either by cooling gray iron rapidly or by adjusting the composition by keeping the carbon and silicon content low.

Compact-graphite Iron 4.6 Cast Irons Malleable Iron Malleable iron is obtained by annealing white cast iron in an atmosphere of carbon monoxide and carbon dioxide, at between 800° and 900°C, for up to several hours, depending on the size of the part. Compact-graphite Iron The graphite in this structure is in the form of short, thick, and interconnected flakes having undulating surfaces and rounded extremities. The mechanical and physical properties of this cast iron are intermediate between those of flake-graphite and nodular-graphite cast irons.

4.7 Heat Treatment of Ferrous Alloys The various microstructures described thus far can be modified by heat-treatment techniques—that is, by controlled heating and cooling of the alloys at various rates. These treatments induce phase transformations that greatly influence such mechanical properties as the strength, hardness, ductility, toughness, and wear resistance of the alloys.

4.7 Heat Treatment of Ferrous Alloys Pearlite If the ferrite and cementite lamellae in the pearlite structure of the eutectoid steel, they are thin and closely packed, the microstructure is called fine pearlite; if they are thick and widely spaced, it is called coarse pearlite.

4.7 Heat Treatment of Ferrous Alloys Spheroidite When pearlite is heated to just below the eutectoid temperature and then held at that temperature for a period of time (subcritical annealing). Fig 4.14 shows the microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700°C. Unlike the lamellar shapes of cementite (which act as stress raisers) spheroidites (spherical particles) are less conducive to stress concentration because of their rounded shapes.

4.7 Heat Treatment of Ferrous Alloys

4.7 Heat Treatment of Ferrous Alloys Bainite Visible only by using electron microscopy, bainite is a very fine microstructure consisting of ferrite and cementite—somewhat like a pearlitic but having a different morphology. A product called bainitic steel as it is generally stronger and more ductile than pearlitic steels at the same hardness level.

4.7 Heat Treatment of Ferrous Alloys Martensite When austenite is cooled at a high rate (such as by quenching it in water), its fcc structure is transformed into a body-centered tetragonal (bct) structure. This structure can be described as a body-centered rectangular prism which is elongated slightly along one of its principal axes . This microstructure is called martensite

4.7 Heat Treatment of Ferrous Alloys Martensite Fig 4.15 (a) shows the hardness of martensite as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray plate-like regions are martensite; they have the same composition as the original austenite (white regions). The expansions of materials can cause parts to undergo distortion or to even crack during heat treatment; quench cracking of steels is caused by rapid cooling during quenching.

4.7 Heat Treatment of Ferrous Alloys

4.7 Heat Treatment of Ferrous Alloys Retained austenite If the temperature to which the alloy is quenched is not sufficiently low, only a portion of the structure is transformed to martensite. The rest is retained austenite. Retained austenite can cause dimensional instability and cracking, and it lowers the hardness and strength of the alloy.

4.7 Heat Treatment of Ferrous Alloys Tempered Martensite Martensite is tempered in order to improve its mechanical properties. Tempering is a heating process by which hardness is reduced and toughness is improved. Fig 4.16 shows the hardness of tempered martensite as a function of tempering time for 1080 steel quenched to 65 HRC. Hardness decreases because the carbide particles coalesce and grow in size, thereby increasing the interparticle distance of the softer ferrite.

4.7 Heat Treatment of Ferrous Alloys

4.7.1 Time-temperature-transformation diagrams Fig 4.17 (a) shows Austenite-to-pearlite transformation of iron-carbon alloy as a function of time and temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature of 675°C. (c) Microstructures obtained for a eutectoid iron-carbon alloy as a function of cooling rate. Fig (b) and (c) diagrams are called isothermal transformation (IT) diagrams, or time-temperature-transformation (TTT) diagrams.

4.7.1 Time-temperature-transformation diagrams

4.7.1 Time-temperature-transformation diagrams The differences in the hardness and the toughness of the various structures obtained are shown in Fig 4.18. Fig (a) and (b) Hardness and (c) toughness for annealed plain-carbon steels as a function of carbide shape. Carbides in the pearlite are lamellar. Fine pearlite is obtained by increasing the cooling rate. The spheroidite structure has sphere-like carbide particles.

4.7.1 Time-temperature-transformation diagrams

4.7.1 Time-temperature-transformation diagrams Fig 4.19 shows the mechanical properties of annealed steels as a function of composition and microstructure. Note in (a) the increase in hardness and strength and in (b) the decrease in ductility and toughness with increasing amounts of pearlite and iron carbide.

4.7.1 Time-temperature-transformation diagrams