Properties of Metals

Transcription

1 Properties of Metals Alessandro Anzalone, Ph.D. Hillsborough Community College Brandon Campus 1. Mechanical Properties 2. Physical Properties 3. Metallurgical Microscopy 4. Nondestructive Testing 5. References 1

2 Tensile Test The tensile test involves the relationship between tensile stress and strain. Although these terms are sometimes confused, they have separate and distinct meanings. Stress is defined as the resistance of a material to external elements such as force, load, or weight measured in pounds per square inch (PSI). It is calculated by dividing the applied load to a specimen by the cross-sectional area of the specimen. When the stress is applied it will cause the specimen to deform or stretch. Strain is the amount of deformation or stretch that occurs over a standard gage length (usually 2 in.) expressed in inches/inch or as a percentage. This gage length may also be referred to as the original length. 2

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5 The proportional limit is technically the stress value at which the elastic portion of the curve loses its proportionality between stress and strain. Since it is so difficult to determine (when is the line no longer straight?), it is rarely used in a practical manner. As the name implies, the elastic limit is the highest stress (at the limit) at which the specimen will return to its original length. Because this value can be found only by trial and error it is valuable as a concept only. Neither of these values is included among handbook data. 5

6 In Figure 2.6, at the top of the elastic region, the amount of strain increases with very little increase in stress. That is, the metal yields, and this yield point is a very obvious end of the elastic region and is a good value on which to base a design. If the metal was cold-worked or heat-treated, or was a nonferrous metal, it would not have a yield point. In Figure 3.24 the curve for the high-carbon steel does not have a yield point: in that case a line is constructed parallel to the elastic region at a strain value of in. Where that line intersects the stress strain curve is known as the yield strength. The yield point or yield strength, whichever is appropriate, is a handbook value for metals and alloys and is commonly used in design; units are PSI. 6

7 The tensile strength in pounds per square inch is determined by dividing the maximum load (in pounds) by the original crosssection (in square inches) before testing. Elongation, or percent elongation, is defined as the value of strain at failure. It is found by determining the amount the specimen stretched during the test, dividing by the original length, and expressing the result as a percentage. The percent reduction of area is determined in a similar fashion; the reduction in the cross-sectional area of the specimen (at the point where it failed) is divided by the original area and expressed as a percentage. Both these measures are used as indicators of ductility (which is the property that allows a metal to deform permanently when loaded in tension). Any metal that can be drawn into wire is ductile. Soft steel, aluminum, gold, silver, and nickel are ductile metals. Malleability is the ability of metals to be deformed permanently when loaded in compression. Metals that can be rolled, hammered, or pressed into flat pieces or sheets are malleable. Ductile metals are usually also malleable, but there are some exceptions. Lead is very malleable but not very ductile and cannot be drawn into wire very easily, but it can be extruded in wire or bar form. Gold, tin, silver, iron, and copper are malleable metals. Plasticity, a more general property, is the ability of a metal to be permanently deformed without failing. On the stress strain curve of Figure 2.6 it would mean the capability to be worked in the region labeled there as Plastic range. 7

8 Hardness Test Hardness is generally defined as the resistance to penetration or indentation. The greater the hardness, the smaller the indentation with the same penetrator tip and pressure. However, the property of hardness is related to the elastic and plastic properties of metals, and certain hardness tests are based on rebound or elastic hardness. Also, certain scratch tests are used to determine abrasive hardness or resistance to abrasion. Hardness is also related to tensile strength, so it can be assumed that a hard steel is also strong and resistant to wear. 8

9 Impact Test This test is performed to measure the impact strength of a metal; since a specimen with a notch (stress raiser) is used, this test is also said to measure notch toughness. Thus, this test measures the ability of a metal to resist rupture from impact loading (also called mechanical shock) when there is a notch or stress raiser present. The property of toughness is not necessarily related to ductility, hardness, or tensile strength; however, as a rule, a brittle metal with low tensile strength such as cast iron will fail under low shock loads; hardened, tempered tool steels show a high impact strength; and coarse-grained metals have lower shock resistance than fine-grained metals. A notch or groove will lower the shock resistance of a metal. Test specimens are made with given dimensions 9

10 Izod-Charpy Testing Machine Ductile Brittle Transition Temperature Another function of the impact test is to determine the effect of temperature change on various metals. Metals that have the body-centered cubic atom lattice, for example, iron and steel, show a temperature range in which ductility and, more important, toughness drop rapidly. This transition zone can be seen in Figure When the notch-bar specimens show half brittle and half ductile failures, the transition temperature has been reached. 10

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12 Creep Test Creep strength is a metal s ability to resist creep, a continuing plastic flow at a stress below the yield strength of a metal. For the most part, creep is a high-temperature phenomenon that increases as the temperature increases. Table 2.2 gives some stress loads at various temperatures required to cause elongation by creep. Some metals have been designed for high-temperature service such as for aircraft jet turbine engine blades that show no signs of creep at temperatures that would melt many metals. 12

13 Fatigue Test Fatigue in metals can occur when metal parts are subjected to repeated loading and unloading, particularly in cyclic reversals of stress, such as is seen in a rotating shaft having transverse or one-side loading. Fatigue failures can occur at stresses far below the yield strength of a material with no sign of plastic deformation. When machine parts that are subject to cyclic loading are designed, their resistance to fatigue may he more important than yield or ultimate strength. Fatigue is not time dependent and can he initiated by any number of factors such as machining tool marks or welding undercuts and localized stress caused by a welding head. Fatigue can he accelerated by a corrosive atmosphere and by higher frequency stress reversals. 13

14 Modulus of Elasticity The modulus of elasticity is actually an expression of the stiffness of a material. It is also called Young s modulus. If, within the elastic range, the stress is divided by the corresponding strain at any given point, the result will he the modulus of elasticity for that material; thus, 14

15 Conductivity The thermal conductivity and electrical conductivity of metals are both related to the fact that metals have free electrons in their atom lattices. Thus. if a metal such as silver or copper is a good conductor of electricity, it is also a good conductor of heat. Pure metals are better conductors than their alloys. For this reason copper or aluminum used for electrical wiring must be uncontaminated. Electrical copper grades are percent pure. Resistance to the flow of electricity can be altered by several factors: 1. Resistance increases as the temperature increases. 2. it increases as a result of cold-working or heat treating the metal. 3. it increases with the amount of impurities and alloying elements present. 15

16 Thermal Expansion and Contraction Most metals expand when heated and contract when cooled. When heated or cooled each metal expands and contracts at a different rate that is related to its coefficient of thermal. This physical property of metals can cause difficulties in construction and manufacturing. Engineers plan use of expansion joints in paved highways and bridges to avoid buckling due to heating. Reflectivity Some metals have a high degree of reflectivity and are used as light reflectors and in insulation as heat deflectors. Aluminum, nickel, chromium, tin, and bronze are among the metals having high reflectivity, whereas lead has virtually none. In spite of that exception reflectivity is one of the characteristics that distinguishes metals from other substances. 16

17 Ferromagnetism Ferromagnetism is characterized by an attraction between certain metals that have the ability to retain a residual magnetic force. The ferromagnetic metals are iron, cobalt, and nickel. Of these, iron is the most commercially important metal used extensively in electrical machinery. Soft iron does not retain much residual magnetism when removed from a magnetic field. This quality makes it useful for electromagnets and electric motors in which the field is turned on and off or reversed repeatedly. Powerful permanent magnets are made by alloying metals such as cobalt, nickel, iron, and aluminum. Metallurgical Microscopy Metal surfaces are examined by means of metallurgical microscopes and associated techniques of surface preparation. Structures in metals not readily visible to the naked eye may be seen at highpower magnification from 10 times the diameter (lox) to 3000X for light microscopes and many more thousands of diameters with the electron microscope. Techniques of photomicrography in which a camera is attached to the microscope make possible photographic illustrations of specimens. Metallurgical microscopes, unlike those used to see through a specimen, use reflected light from an internal source. Inverted stage microscopes are widely used to study metals. 17

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19 As the name implies, nondestructive testing in no way impairs the part for further use. It does not measure mechanical or physical properties but instead identifies defects such. as voids, inclusions, or cracks that might later lead to failure of the part. Also, improper handling of materials that causes changes in the microstructure can lead to failure. For example, a nick or dent in a highly stressed part such as a spring can cause early failure. At present, even these aberrations can be detected on a production basis. Nondestructive methods can also verify hardness or heat treatment in many cases. Since testing for defects in the early stages of the manufacturing process can substantially reduce rejects, nondestructive testing is closely related to quality control. 19

20 The following are the most common types of nondestructive testing: 1. Magnetic-particle inspection 2. Fluorescent penetrant inspection 3. Dye penetrants 4. Ultrasonic inspection 5. Radiography (X-ray and gamma ray) 6. Eddy-current inspection Magnetic-particle inspection 20

21 Fluorescent penetrant inspection Dye penetrants 21

22 Ultrasonic inspection Radiography (X-ray and gamma ray) 22

23 Eddy-current inspection 1. R Gregg Bruce, William K. Dalton, John E Neely, and Richard R Kibbe,, Modern Materials and Manufacturing Processes, Prentice Hall, 3rd edition, 2003, ISBN:

24 Properties of Metals Alessandro Anzalone, Ph.D. Hillsborough Community College Brandon Campus 24