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Chapter 3 Changing the Properties of Metals

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Metals

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Ferrous- From the Latin word ferrum, meaning iron. Describes an alloy containing a significant amount of iron.

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Cementite- Fe3C. Also known as iron carbide, a compound of iron and carbon.

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Anneal- In general, the heating of metal to soften it or reduce its strength.

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Figure 1.14 Schematic illustration of the effects of recovery, recrystallization, and grain growth on mechanical properties and on the shape and size of grains. Note the formation of small new grains during recrystallization. Source: G. Sachs. Annealing

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Allotropy- The ability of a material to exist in several crystalline forms.

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Austenite- A solid solution of iron and carbon and sometimes other elements in which gamma iron, characterized by a face- centered crystal structure, is the solvent.

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Ferrite- A magnetic form of iron. A solid solution in which alpha iron is the solvent, characterized by a body- centered cubic crystal structure.

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Figure 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 (see Fig. 1.9). Note, also, 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. Austenite, Ferrite, and Martensite

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Curie temperature- The temperature above which metals are no longer magnetic. For iron this temperature is 1414F. (It is named for Pierre Curie, husband of Marie Curie.)

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Phase- A portion of an alloy, physically homogeneous throughout, that is separated from the rest of the alloy by distinct boundary surfaces. The following phases occur in the iron-carbon alloy: molten alloy austenite ferrite cementite graphite

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Figure 4.3 (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. (b) 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. Two-Phase System

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Phase change- The transformation of a substance from one distinct, separate form to another. When a molten metal solidifies it has changed phase, and depending on the alloy system, it may solidify as one phase (a single phase) or as two phases (a mixture).

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Phase diagram- For binary (two metals) alloy systems, a diagram of temperature versus percent composition, with 100 percent of one metal on one axis and 100 percent of the second on the other. Lines on the diagram separate single phases from mixtures of phases.

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Phase diagram- Additional time is required to change from one phase to another. In the construction of a phase diagram all the time necessary for phase changes to occur is assumed available; it is thus often termed an equilibrium phase diagram.

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Figure 4.5 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 (see Section 10.2). The bottom circle shows the solidified alloy, with grain boundaries. Nickel-Copper Alloy Phase Diagram

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Figure 4.7 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. Lead-Tin Phase Diagram

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Figure 4.8 The 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. Iron-Iron Carbide Phase Diagram

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Figure 4.12 Phase diagram for the iron-carbon system with graphite (instead of cementite) as the stable phase. Note that this figure is an extended version of Fig. 4.8. Extended Iron-Carbon Phase Diagram

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Aluminum-Copper Phase Diagram Figure 4.21 (a) Phase diagram for the aluminum-copper alloy system. (b) Various micro- structures obtained during the age-hardening process. Source: L. H. Van Vlack; Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.

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Figure 3.4 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. Cooling Curve

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Liquidus- The temperature at which freezing begins during cooling and ends during heating under equilibrium conditions, represented by a line on a two-phase diagram.

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Solidus- Seen as a line on a two-phase diagram, it represents the temperatures at which freezing ends when cooling, or melting begins when heating under equilibrium conditions.

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Eutectic- The alloy composition that freezes at the lowest constant temperature, causing a discrete mixture to form in definite proportions.

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Solubility- The degree to which one substance will dissolve in another.

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Eutectoid- The alloy composition that transforms from a high temperature solid into new phases at the lowest constant temperature. In binary (double) alloy systems, it is a mechanical mixture of two phases that forms simultaneously from a solid solution as it cools through the eutectoid (A1/A3,1 in steels) temperature.

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Eutectoid Steel Microstructure Figure 4.17 Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700 C (1292 F). Magnification: 1000X. Source: Courtesy of USX Corporation.

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Pearlite- The lamellar mixture of ferrite and cementite in slowly cooled iron-carbon alloys as found in steel and cast iron.

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Pearlite Microstructure Figure 4.11 Microstructure of pearlite in 1080 steel, formed from austenite of eutectoid composition. In this lamellar structure, the lighter regions are ferrite, and the darker regions are carbide. Magnification: 2500X. Source: Courtesy of USX Corporation.

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Hypoeutectoid- Used to identify those metallic alloys that have a composition less than that of the eutectoid composition.

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Hypereutectoid- Used to identify metallic alloys that have a composition greater than that of the eutectoid composition.

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Quenching- The process of rapid cooling of metal alloys for the purpose of hardening. Quenching media include air, oil, water, molten metals, and fused salts.

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Time-temperature transformation, or TTT, or IT- A diagram that shows the transformation of austenite at one temperature when the time for transformation is taken into account. Because the transformation occurs at one temperature it is also termed an IT or isothermal transformation diagram.

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Martensite- Iron phase supersaturated in carbon that is a nonequilibrium product of austenite transformation.

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Martensite (b) Figure 4.18 (a) Hardness of martensite, as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same composition as the original austenite (white regions). Magnification: 1000X. Source: Courtesy of USX Corporation.

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Bainite- An austenitic transformation found in some steels and cast irons. The microstructure consists of ferrite and a fine dispersion of cementite that has the needlelike appearance of martensite.

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Induction-Hardened Surface Figure 4.1 Cross-section of gear teeth showing induction-hardened surfaces. Source: TOCCO Div., Park-Ohio Industries, Inc.

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Outline of Heat Treatment Processes for Surface Hardening

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Figure 4.23 Heat-treating temperature ranges for plain-carbon steels, as indicated on the iron-iron carbide phase diagram. Source: ASM International. Figure 4.24 Hardness of steels in the quenched and normalized conditions, as a function of carbon content. Heat Treatment Processes

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Surface hardening- Usually refers to the hardening of the surface of steel; this can be accomplished two ways: using a steel with sufficient carbon to achieve the hardness desired heat treating just the surface, or raising the carbon content of the surface and heat treating the whole part Surface peening will also harden the surface of most metals.

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Precipitation hardening- A process of hardening an alloy by heat treatment in which a constituent precipitates from a supersaturated solid solution while at room temperature or at some slightly elevated temperature.

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Plastic deformation- Deformation that occurs when so much stress is applied to a solid that it does not return to its original condition.

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Work hardening- Also called strain hardening and cold working, in which the grains become distorted and elongated in the direction of working (e.g., rolling).

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Full annealing- Heating a metal to an elevated temperature for a long enough time that any evidence of prior cold working or heat treating is removed, and then cooling at a slow rate; the cooling is often accomplished by letting the metal cool in the furnace with the burners off. Under these conditions the metal will approximate the condition predicted by the phase diagram.

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Figure 1.14 Schematic illustration of the effects of recovery, recrystallization, and grain growth on mechanical properties and on the shape and size of grains. Note the formation of small new grains during recrystallization. Source: G. Sachs. Annealing

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Stress relief anneal- The reduction of residual stress in a metal part by heating it to a given temperature and holding it there for a suitable length of time. This treatment is used to relieve elastic stresses caused by welding, cold working, machining, casting, and quenching.

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Process anneal- An annealing process used within a sequence of cold-working operations that causes the metal to recrystallize, so that additional cold working can be done.

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Normalize- To homogenize and produce a uniform structure in alloy steels by heating above the transformation range and cooling in air.

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Spheroidizing- Consists of holding carbon steel for a period of time at just under the transformation temperature. An aggregate of globular carbide is formed from other microstructure such as pearlite.

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Recrystallization- A process in which the distorted grain structure of metals that are subjected to mechanical deformation is replaced by a new strain-free grain structure during annealing.

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Residual stress- Stresses induced within the structure of a metal by cold working, machining, and heat treatments and remaining in the metal after the treatment is completed.