Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Dislocation wikipedia , lookup
Shape-memory alloy wikipedia , lookup
Crystal structure wikipedia , lookup
Radiation damage wikipedia , lookup
Viscoelasticity wikipedia , lookup
Paleostress inversion wikipedia , lookup
Fatigue (material) wikipedia , lookup
Operator Generic Fundamentals Material Science © Copyright 2017 – Rev 3 Operator Generic Fundamentals 2 Terminal Learning Objectives At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80 percent on the following topics (TLOs): 1. Describe the bonding, structures, and imperfections found in solid materials. 2. Describe the basic microstructure and characteristics of metallic alloys. 3. Describe physical and chemical properties of metals and methods used to modify these properties. 4. Describe common types of corrosion that affect metals. 5. Describe common material failure mechanisms. © Copyright 2017 – Rev 3 TLOs Operator Generic Fundamentals 3 Metallic Bonding and Structures TLO 1 Describe the bonding, structures, and imperfections found in solid materials. 1.1 Describe the types of bonding that occur in materials. 1.2 Describe the following types and features of solids: a. Amorphous b. Crystalline Solids c. Grain Structures 1.3 Describe the following lattice-type structures that occur in metals: a. Body-Centered Cubic (BCC) b. Face-Centered Cubic (FCC) c. Hexagonal Close-Packed (HCP) 1.4 Describe the various imperfections that occur in solid materials. © Copyright 2017 – Rev 3 TLO 1 Operator Generic Fundamentals 4 Metallic Bonding ELO 1.1 - Describe the types of bonding that occur in materials. • Matter exists primarily in three states: – Solid – Liquid – Gas • The atomic and molecular bonding and structures that occur within a substance determines its state • The focus of this lesson is on the solid state of materials © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 5 Types of Bonds • Ionic Bond – One or more electrons wholly transferred from an atom of one element to the atom of another element – Force of attraction due to the opposite polarity of the charge Figure: Ionic Bond for Sodium Chloride © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 6 Types of Bonds • Covalent Bond – Bond formed by shared electrons – When an atom needs electrons to complete its outer shell it shares those electrons with its neighboring atom – Electrons fill both atom’s electron shells Figure: Covalent Bond for Methane © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 7 Types of Bonds • Metallic Bond – Atoms do not share or exchange electrons in order to bond – Many electrons (roughly one for each atom) are more or less free to move throughout the metal – Each electron can interact with many of the fixed atoms Figure: Metallic Bond for Sodium © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 8 Types of Bonds • Molecular bond – A temporary weak charge exists when electrons of neutral atoms spend more time in one region of their orbit than another – The molecule weakly attracts other molecules – Also referred to as a van der Waals bond Figure: Van Der Waals Forces © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 9 Types of Bonds • Hydrogen Bond – Similar to the molecular bond – Occurs due to the ease with which hydrogen atoms are willing to give up an electron to atoms of oxygen, fluorine, or nitrogen Figure: Hydrogen Bond for Ice © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 10 Types of Bonds Examples of Materials and Their Bonds Material Type of Bond Sodium Chloride (Table Salt) Ionic Diamond Covalent Sodium Metallic Solid Hydrogen Molecular Ice (Frozen Water) Hydrogen © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 11 Types of Bonds • The type of bond determines both the material’s tightness and the microscopic properties – Ability to conduct heat or electrical current relates to the freedom of electron movement in the material – Microscopic structure helps predict how that material behaves under certain conditions – Synthetically fabricated materials with a given microscopic structure yield certain desirable properties for specific applications © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 12 Metallic Bonding Knowledge Check This type of bond is characterized by the transference of one or more electrons from one atom to another. A. Covalent B. Ionic C. Molecular D. Electronic Correct answer is B. © Copyright 2017 – Rev 3 ELO 1.1 Operator Generic Fundamentals 13 Solid Material Properties ELO 1.2 - Describe the following types and features of solids: amorphous, crystalline solids, and grain structures. • Through their bonding arrangements, solids have greater bonding attractions than liquids and gases – Many other property variations of solid materials – Depends on inter-atomic bonding – Bonds also dictate the spacing and physical arrangement between atoms in solids – Amorphous or crystalline are classifications used for bonding arrangements © Copyright 2017 – Rev 3 ELO 1.2 Operator Generic Fundamentals 14 Amorphous Materials • Amorphous materials have no regular arrangement of atoms or molecules – Exhibit properties of solids – Have definite shape and volume – Diffuse slowly – Lack sharply defined melting points – Solid but resemble liquids that flow slowly at room temperature – Examples: glass and paraffin © Copyright 2017 – Rev 3 ELO 1.2 Operator Generic Fundamentals 15 Crystalline Solids • In metals and other solids, arrays of atoms in regular patterns create crystal structures – Atoms arranged in a pattern that periodically repeat in a threedimensional geometric lattice • Forces associated with chemical bonding cause this repetition to produce properties such as: – Strength – Ductility – Density – Conductivity – Shape © Copyright 2017 – Rev 3 ELO 1.2 Operator Generic Fundamentals 16 Grain Structure and Boundary • Grain structure refers to the arrangement of the grains in a metal; each grain has a particular crystal or lattice structure Grain Boundary Grain • Each of the light areas is a grain, or crystal, which is the region of space occupied by a continuous crystal lattice • Grain boundaries are the dark lines surrounding the grains © Copyright 2017 – Rev 3 Figure: Grain Structure ELO 1.2 Operator Generic Fundamentals 17 Grain Boundary Minute Individual Crystals • The grain boundary is a region of misfit or interface between grains and is usually the diameter of one to three atoms • Grain boundaries separate arbitrarily oriented crystal regions (polycrystalline) where the crystal structures are identical Figure: Grain Boundaries © Copyright 2017 – Rev 3 ELO 1.2 Operator Generic Fundamentals 18 Grain Size • Average size of grain is part of a metal’s characteristics • Grain size determines the properties of the metal – Smaller grain size increases tensile strength and increases ductility – A larger grain size is preferred for improved high-temperature creep properties © Copyright 2017 – Rev 3 ELO 1.2 Operator Generic Fundamentals 19 Grain Orientation • Another important property of the grains found in metals is their orientation • Random arrangement of the grains such that no one direction within the grains aligns with the external boundaries of the metal sample – Cross rolling the material produces this orientation Figure: Grain Random Arrangement © Copyright 2017 – Rev 3 ELO 1.2 Operator Generic Fundamentals 20 Grain Orientation • Over rolling a metal sample in one direction may develop a grainoriented structure in the rolling direction • Rolling a metal this way gives the grains a preferred orientation • Preferred orientation may be desirable or NOT, depending on the metal’s application Figure: Grain Preferred Arrangement © Copyright 2017 – Rev 3 ELO 1.2 Operator Generic Fundamentals 21 Solid Material Properties Knowledge Check The outside area of a grain that separates it from other grains in a metal is known as _______________. A. grain structure B. crystal boundary C. grain boundary D. crystal structure Correct answer is C. © Copyright 2017 – Rev 3 ELO 1.2 Operator Generic Fundamentals 22 Metallic Lattice Structures ELO 1.3 - Describe the following lattice-type structures that occur in metals: Body-Centered Cubic (BCC), Face-Centered Cubic (FCC), Hexagonal Close-Packed (HCP). • Three most common, basic crystal patterns associated with metals are: – Body-Centered Cubic (BCC) – Face-Centered Cubic (FCC) – Hexagonal Close-Packed (HCP) © Copyright 2017 – Rev 3 ELO 1.3 Operator Generic Fundamentals 23 Body-Centered Cubic • With the BCC atom arrangement, the unit cell consists of eight atoms at the corners of a cube and one atom at the body center of the cube • Example metals: α-iron (Fe) (ferrite), chromium (Cr), vanadium (V), molybdenum (Mo), and tungsten (W) – These metals have two properties in common: high strength and low ductility Figure: Body-Centered Cubic Unit Cell © Copyright 2017 – Rev 3 ELO 1.3 Operator Generic Fundamentals 24 Face-Centered Cubic • Consists of eight atoms at the corners of a cube and one atom at the center of each of the faces of the cube • Example metals: γ-iron (Fe) (austenite), aluminum (Al), copper (Cu), lead (Pb), silver (Ag), gold (Au), nickel (Ni), platinum (Pt), and thorium (Th) – Generally have lower strength and higher ductility than BCC metals Figure: Face-Centered Cubic Unit Cell © Copyright 2017 – Rev 3 ELO 1.3 Operator Generic Fundamentals 25 Hexagonal Close-packed • With the HCP atom arrangement, the unit cell consists of three layers of atoms – Top and bottom layers each contain six atoms at the corners of a hexagon and one atom at the center of each hexagon – Middle layer contains three atoms nestled between the atoms of the top and bottom layers; therefore, the name close-packed • Example metals: – Beryllium (Be) – Magnesium (Mg) – Zinc (Zn) – Cadmium (Cd) – Cobalt (Co) – Thallium (Tl) – Zirconium (Zr) • HCP metals not as ductile as FCC metals Figure: Hexagonal Close-Packed Unit Cell © Copyright 2017 – Rev 3 ELO 1.3 Operator Generic Fundamentals 26 Metallic Lattice Structures Knowledge Check Which of the following basic crystal patterns has the greatest number of atoms per unit cell? A. BCC B. FCC C. HCP D. HCC Correct answer is C. (Hexagonal close-packed) © Copyright 2017 – Rev 3 ELO 1.3 Operator Generic Fundamentals 27 Metallic Imperfections ELO 1.4 - Describe the various imperfections of solid materials. • Previous discussion assumed perfect crystal structures • In reality – Materials are not perfect crystals – Materials have impurities that alter their properties • Even amorphous solids have imperfections and impurities affecting their structure © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 28 Microscopic Imperfection Classifications • Point imperfections – Have atomic dimensions - an atom of a different element replaces an atom of a metal in the metal’s crystalline lattice • Line imperfections or dislocations – Generally many atoms in length • Interfacial imperfections – Larger than line defects – Occur over a two-dimensional area © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 29 Point Imperfections • Divided into three main categories: – Vacancy defects – Substitutional defects – Interstitial defects • Point defects enhance or lessen a material’s usefulness for construction, depending on the intended use Figure: Point Defects © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 30 Point Imperfections - Vacancy Defects • Results from a missing atom in a lattice position • Results from imperfect packing during the crystallization process – may be due to increased thermal vibrations of the atoms from elevated temperatures © Copyright 2017 – Rev 3 Figure: Vacancy Point Defect ELO 1.4 Operator Generic Fundamentals 31 Point Imperfections - Substitutional Defects • Substitutional defects result from an impurity at a lattice position • Caused by an alloying material added to the metal, such as carbon (carbon steel) Figure: Substitutional Point Defect © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 32 Point Imperfections - Interstitial Defects • Interstitial refers to locations between atoms in a lattice structure • Result from an impurity at an interstitial site or one of the lattice atoms in an interstitial position instead of its lattice position • Interstitial impurities called network modifiers act as point defects in amorphous solids © Copyright 2017 – Rev 3 Figure: Interstitial Point Defect ELO 1.4 Operator Generic Fundamentals 33 Line Imperfections • Also called dislocations. Only occur in crystalline materials. Types include: – Edge – Screw – Mixed – Depends on the way they distort the lattice • Dislocations cannot end inside a crystal – Must end at a crystal’s edge or other dislocation, or close back on itself • The ease with which dislocations move through crystals determines their importance © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 34 Line Imperfections – Edge Dislocations • Consist of an extra row or plane of atoms in the crystal’s structure • Imperfection may extend in a straight line through the crystal or follow an irregular path • May be short, extending only a small distance into the crystal and causing a slip of one atomic distance along the glide plane, the direction the edge imperfection is moving © Copyright 2017 – Rev 3 ELO 1.4 Figure: Edge Dislocation Operator Generic Fundamentals 35 Line Imperfections – Edge Dislocations Figure: Slips Along Edge Dislocations © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 36 Line Imperfections – Screw Dislocations • Screw dislocations develop by tearing a crystal parallel to the slip direction – Making a complete circuit shows a slip pattern similar in shape to a screw thread, whether left- or right-handed • To happen, requires some of the atomic bonds to re-form continuously after yielding to return the crystal to its original form © Copyright 2017 – Rev 3 Figure: Screw Dislocation ELO 1.4 Operator Generic Fundamentals 37 Line Imperfections – Mixed Dislocations • The orientation of dislocations varies from pure edge to pure screw • At an intermediate point, dislocations may possess characteristics of each © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 38 Macroscopic (Bulk) Material Defects • Bulk defects are three-dimensional macroscopic material defects • Generally occur on a much larger scale than microscopic defects • Introduced into a material during refinement from its raw state or during the material’s fabrication processes • Most common bulk defects are caused by foreign particles included in the prime material – Called inclusions, they undesirably alter the material’s structural properties – Inclusion examples include oxide particles in a pure metal or a bit of clay in a glass structure © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 39 Macroscopic (Bulk) Material Defects • Other bulk defects include gas pockets or shrinking cavities, usually found in castings – Weaken the material and should be avoided during fabrication • Working and forging of metals causes cracks that act as stress concentrators causing material weakening • Welding or joining defects also classified as a bulk defect © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 40 Metallic Imperfections Knowledge Check Vacancy defects, substitutional defects and interstitial defects are examples of _______________. A. line imperfections B. point imperfections C. interfacial imperfections D. bulk defects Correct answer is B. © Copyright 2017 – Rev 3 ELO 1.4 Operator Generic Fundamentals 41 Metallic Alloys TLO 2 Describe the basic microstructure and characteristics of metallic alloys. 2.1 Describe the common characteristics of alloys. 2.2 Identify the desirable properties of type 304 stainless steel. © Copyright 2017 – Rev 3 TLO 2 Operator Generic Fundamentals 42 Characteristics of Alloys ELO 2.1 - Describe the common characteristics of alloys. • An alloy is a mixture of two or more materials – At least one of which is a metal • Alloy microstructures may consist of: – Solid solutions, where secondary atoms combine as substitutionals or interstitials in a crystal lattice – A crystal with a metallic compound at each lattice point – Secondary crystals imbedded in a primary polycrystalline matrix o Called a composite o Does not imply that component materials are metals © Copyright 2017 – Rev 3 ELO 2.1 Operator Generic Fundamentals 43 Characteristics of Alloys • Characteristics of alloys, generally – Stronger than pure metals – Reduced electrical conductivity – Reduced thermal conductivity • Strength is important for structural materials – used for industrial construction alloys • For example: – Steel alloy – Consists of iron and carbon, and other elements – Aluminum and copper, both are soft and ductile, but alloyed are harder and stronger © Copyright 2017 – Rev 3 ELO 2.1 Operator Generic Fundamentals 44 Characteristics of Alloys • Possible for a material to be composed of several solid phases • The strengths of each is enhanced by creating a solid structure composed of two interspersed phases – Quench the metal from its molten state to form the interspersed phases – Type and rate of quenching determines the final solid structure and its properties © Copyright 2017 – Rev 3 ELO 2.1 Operator Generic Fundamentals 45 Composition of Common Engineering Materials • Varied structures, systems, and components found in industrial applications require different types of materials – Large percentage use a base metal of iron or nickel • Material selection for a specific application requires consideration of the following: – Temperature and pressure – Resistance to specific types of corrosion – Radiation influence – Toughness and hardness (load – creep) – Weight – Other applicable material properties © Copyright 2017 – Rev 3 ELO 2.1 Operator Generic Fundamentals 46 Characteristics of Alloys Knowledge Check Which one of the following is NOT a characteristic of an alloy? A. Usually stronger than pure metals. B. Generally have reduced electrical and thermal conductivity. C. Usually have better ductility than pure metals. D. Usually preferred for industrial construction over pure metals. Correct answer is C. © Copyright 2017 – Rev 3 ELO 2.1 Operator Generic Fundamentals 47 Stainless Steel ELO 2.2 - Identify the desirable properties of type 304 stainless steel. • Stainless steel is a material with many applications in nuclear power plants – Nearly 40 standard types of stainless steel – Some specialized types exist under various trade names • Through variations in the alloying elements, steel (stainless steel or other types) can be adapted to specific applications © Copyright 2017 – Rev 3 ELO 2.2 Operator Generic Fundamentals 48 Stainless Steel • Stainless steel’s primary classifications are austenitic or ferritic, based on lattice structure – Austenitic stainless steels, including types 304 and 316 o have a face-centered cubic structure of iron atoms with the carbon in an interstitial solid solution – Ferritic stainless steels, including type 405 o have a body-centered cubic iron lattice and contain no nickel • Ferritic steels are easier to weld and fabricate and less susceptible to stress corrosion cracking than austenitic stainless steels – Ferritic steels only have moderate resistance to other types of chemical attack © Copyright 2017 – Rev 3 ELO 2.2 Operator Generic Fundamentals 49 Inconel • Another durable metal that has specific applications in some industrial facilities • Resists oxidation and corrosion in extreme environmental service conditions • Well suited for high-temperature applications © Copyright 2017 – Rev 3 ELO 2.2 Operator Generic Fundamentals 50 Alloy Compensation for Stainless Steels and Inconel Alloy Composition of Common Stainless Steels and INCONEL® Alloy Percent Iron (Fe) Percent Carbon (C) Percent Chromium (Cr) Percent Nickel (Ni) Percent Molybdenum (Mo) Percent Manganese (Mn) Percent Silicon (Si) 304 Stainless Steel Balanced 0.08 19.0 10.0 N/A* 2.0 1.0 304 L Stainless Steel Balanced 0.03 18.0 8.0 N/A 2.0 1.0 316 Stainless Steel Balanced 0.08 17.0 12.0 2.5 2.0 1.0 316 L Stainless Steel Balanced 0.03 17.0 12.0 2.5 2.0 N/A 405 Stainless Steel Balanced 0.08 13.0 N/A N/A 1.0 1.0 INCONEL® 8 N/A 15.0 Balanced N/A 1.0 0.5 © Copyright 2017 – Rev 3 ELO 2.2 Operator Generic Fundamentals 51 Type 304 Stainless Steel • Type 304 stainless steel is extremely tough – Contains 18 to 20 percent chromium and 8 to 10.5 percent nickel • Used extensively in applications where corrosion is a concern because it resists most, but not all, types of corrosion © Copyright 2017 – Rev 3 ELO 2.2 Operator Generic Fundamentals 52 Characteristics of Alloys Knowledge Check What are the two desirable characteristics of Type 304 Stainless Steel?___________ and __________ A. high temperature tolerant; toughness B. corrosion resistance; toughness C. cubic iron lattice; corrosion resistance D. corrosion resistance; contains no nickel Correct answer is B. © Copyright 2017 – Rev 3 ELO 2.2 Operator Generic Fundamentals 53 Physical and Chemical Properties of Metals TLO 3 Describe physical and chemical properties of metals and methods used to modify these properties. 3.1 Describe the following terms: a. Strength b. Ultimate tensile strength c. Yield strength d. Ductility e. Malleability f. Toughness g. Hardness 3.2 Describe the following types of treatments used on metals: a. Heat treatment b. Cooling (Quenching) c. Annealing d. Cold working e. Hot working © Copyright 2017 – Rev 3 TLO 3 Operator Generic Fundamentals 54 Characteristics of Material Strength ELO 3.1 - Describe the following terms: strength, ultimate tensile strength, yield strength, ductility, malleability, toughness, and hardness. • Metal properties require multiple terms to describe and quantify their strengths and weakness • An overview of some of these included in earlier lessons is given more detail in this lesson • Strength is the ability of a material to resist deformation • A structure’s strength requirement equals the maximum load that can be borne before failure is apparent © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 55 Determining a Material’s Strength • When under tension, permanent deformation or plastic strain happens in a component before failure • The load-carrying capacity of a material at the instant of failure is less than the maximum load the material can support at a lower strain – Smaller cross-sectional area as deformation of the material occurs • Conversely, when under compression, the load at fracture is the maximum applicable over a significantly enlarged area – compared to the cross-sectional area under no load © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 56 Determining a Material’s Strength • Nominal stress is included when quoting a material’s strength, and qualified by the type of stress applied – For most structural materials, compressive strength equals the tensile strength – This is a safe assumption due to the increase in effective cross section during compression © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 57 Strength and Slip • Grain boundaries in metals prevent slip • The smaller the grain size, the larger the grain boundary area • Decreasing the grain size by cold or hot working the metal: – Retards slip – Increases the metal’s strength © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 58 Ultimate Tensile Strength • The ultimate tensile strength (UTS) is the maximum resistance a material presents to fracture – Equivalent to the maximum load capability of one square inch of cross-sectional area with load applied as simple tension maximum load Pmax UTS = = = psi area of original cross section Ao • On a stress-strain curve, the ultimate tensile strength appears as the stress coordinate value of the highest point on the curve © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 59 Ultimate Tensile Strength • Materials that elongate before breaking have a large reduction in the cross-sectional area – Therefore, the material carries less load in the final stages of the tensile test • Necking is a noticeable decrease in a cross-section prior to failure • Stress creates a narrowed part in the material, similar to a person’s neck © Copyright 2017 – Rev 3 Figure: Ductile Material Stress-Strain Curve ELO 3.1 Operator Generic Fundamentals 60 Yield Strength • Many terms exist for identifying the stress where plastic deformation begins • Yield strength is most commonly used • Yield strength is the stress where a predetermined amount of permanent deformation occurs Figure: Brittle Material Stress-Strain Curve © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 61 Alternate Values Used Instead of Yield Strength • Yield Point – Stress at the point where visible stretch is first observed • Proportional Limit – Stress where the stress-strain curve first deviates from a straight line – Below this limiting value, the material follows Hooke's Law – Infrequently used because it begins very gradually • Maximum Shear Stress – Yield strength criterion is inadequate for components withstanding high pressures, such as pressurized steam generating facilities – To cover these situations, The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code incorporates this measure © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 62 Ductility • Ductility is the ability of a material to deform easily on application of a tensile force • The ability of a material to withstand plastic deformation without rupture – Ductility also considers bendability and crushability • Ductile materials demonstrate great deformation before fracture • Usually, if two materials have the same strength and hardness, the one with the higher ductility is more desirable for engineering applications © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 63 Ductility Determination The percent elongation reported in a tensile test is the: • maximum elongation of the gauge length divided by the original gauge length Figure: Elongation After Failure © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 64 Reduction of Area Determination • Reduction of area is the proportional reduction of the cross-sectional area of a tensile test piece at the plane of fracture measured after fracture final gauge length − initial gauge length Percent elongation = initial gauge length Lx − Lo Percent elongation = = inches per inch × 100 Lo © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 65 Reduction of Area Determination • The reduction of an area has additional information regarding the material’s deformational characteristics • These two characteristics indicate ductility, the ability of a material to elongate in tension • Because elongation is not uniform over the entire gauge length and is greatest at the center of the neck, the percent elongation is not an absolute measure of ductility • The reduction of area, measured at the minimum diameter of the neck, is a better indicator of ductility © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 66 Changes in Ductility • Ductility of many metals changes with altering conditions • Increasing temperature increases ductility – A decrease in temperature causes a decrease in ductility, and potentially a change from ductile to brittle behavior • In nuclear application, irradiation also results in a change in ductility. Material becomes more brittle with greater amounts of radiation exposure • Cold working makes metals less ductile – Performed in a particular temperature region over a specific time interval to obtain plastic deformation without relieving strain hardening © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 67 Changes in Ductility • Heating a cold-worked metal to or above the temperature where metal atoms return to their equilibrium positions increases the ductility of that metal – This process is annealing • Minor additions of impurities to metals can have a marked effect on the change from ductile to brittle behavior © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 68 Advantages of Ductile Material • Ductility is desirable in high-temperature and high-pressure industrial applications – Because of higher metal stresses • High ductility helps prevent failure by brittle fracture © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 69 Malleability • Malleability is a metal’s ability to display large deformation or plastic response when subjected to compressive force Figure: Malleable Deformation of a Cylinder Under Uniform Axial Compression © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 70 Malleability – Deformation by Compression • As compressive force increases, the material contracts axially with the force and expands laterally – Restraint due to friction at the contact faces induces axial tension on the outside of the material • Tensile forces operate around the circumference concurrent with lateral expansion or increasing girth © Copyright 2017 – Rev 3 Figure: Malleable Deformation of a Cylinder Under Uniform Axial Compression ELO 3.1 Operator Generic Fundamentals 71 Malleability – Deformation by Compression • Plastic flow at the material’s center also induces tension • Because of these factors, the criterion of fracture (the limit of plastic deformation) for a ductile material depends on tensile, rather than compressive, stress Figure: Malleable Deformation of a Cylinder Under Uniform Axial Compression © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 72 Changes in Malleability • A material’s temperature change may modify both the plastic flow mode and the fracture mode, resulting in a change in a material’s malleability © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 73 Toughness • Toughness describes the way a material reacts under sudden impacts. • Toughness is the work required to deform one cubic inch of metal until it fractures © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 74 Material Toughness Tests • The Charpy V-Notch Test or the Izod Test measure toughness – Both tests use a notched sample • The location and shape (V-shaped) of the notch are standard – The points of support of the sample, as well as the impact of the hammer, must bear a constant relationship to the location of the notch • These tests mount metal samples in a device like the one shown in the next figure • A pendulum of a known weight is allowed to fall from a set height to strike the sample © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 75 Material Toughness Tests • Energy absorbed by the hammer is measured from the upward swing of the pendulum after it has fractured the material specimen • The greater the amount of energy absorbed by the specimen, the smaller the upward swing of the pendulum and the tougher the material Figure: Charpy V-Notch Test © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 76 Material Toughness Tests Results • Indication of toughness is relative and applies only to cases involving exactly this type of sample and method of loading • A sample of a different shape yields an entirely different result • Notches confine the deformation to a small volume of metal that reduces toughness • The shape of the metal and the material composition determine the material’s toughness © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 77 Hardness • Hardness is the property of a material that enables it to resist plastic deformation, penetration, indentation, and scratching • Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion (steam, oil, water flow, etc.) often increases with hardness © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 78 Hardness Test • Several methods exist for hardness testing. Those most often used are: – Brinell Test – Rockwell Test – Vickers Test – Tukon Test – Sclerscope Test – Files Test © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 79 How Alloys Affect Material Physical Properties • Nickel is an important alloying element – In concentrations of less than 5 percent, nickel increases the toughness and ductility of steel without increasing the hardness – Will not raise the hardness when added in small quantities because it does not form solid carbon compounds (carbides) • Chromium alloyed in steel forms a carbide that hardens the metal – Chromium atoms also occupy locations in the metal’s crystalline lattice, increasing the metal’s hardness without affecting its ductility • Nickel and chromium alloyed together, intensify the effects of chromium, producing steel with increased hardness and ductility – Stainless steels are alloy steels containing at least 12 percent chromium o These steels resist many corrosive conditions © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 80 How Alloys Affect Material Physical Properties • Copper’s effect on steel is similar to nickel – Copper does not form a carbide; however, it increases hardness by retarding dislocation movement within the metal’s crystalline lattice • Molybdenum forms a complex carbide when added to steel – Because of the carbide structure, steel hardens substantially and also minimizes grain enlargement – When alloyed in steel, molybdenum augments the desirable properties of both nickel and chromium © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 81 Physical and Chemical Properties of Metals Knowledge Check _______________ is a material’s maximum resistance to fracture. A. Strength B. Yield strength C. Ductility D. Ultimate tensile strength Correct answer is D. © Copyright 2017 – Rev 3 ELO 3.1 Operator Generic Fundamentals 82 Metal Treatments ELO 3.2 - Describe the following types of treatments used on metals: heat treatment, cooling (quenching), annealing, cold working, and hot working. • Heat treatment and working the metal are metallurgical processes that change the properties of metals • Helpful for understanding how metal treatments yield the metal properties necessary for nuclear plant applications © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 83 Heat Treatment • Large carbon steel components undergo heat treatment that takes advantage of metallic crystalline structures and their effects on the metal to gain certain desirable properties • As hardness and tensile strength increase in heat-treated steel, toughness and ductility decrease – Heat treatment of 304 stainless steel is unsuitable for increasing the metal’s hardness and strength because of its crystalline structure © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 84 Cooling (Quenching) • Varying the rate of cooling (quenching) of the metal, allows control of the metal’s grain size and grain patterns – Generally, the faster a metal cools, the smaller the grain size – Smaller grain size makes the metal harder • The cooling rate used for quenching a metal depends on the method of cooling and the size of the metal • Uniform cooling prevents distortion • Typically, steel components use oil or water for quenching © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 85 Annealing • Another common heat treating process for carbon steel components • During annealing, component heating occurs slowly to an elevated temperature where it is held for a long time, then cooled • The annealing process obtains the following effects: – Softens the steel and improves ductility – Relieves internal stresses caused by previous processes such as heat treatment, welding, or machining – Refines the grain structure © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 86 Cold Working • Plastic deformation in a particular temperature region and over a particular time interval such that the strain (work) hardening is not relieved – Results in decreasing ductility by repeatedly deforming the metal • In the early stages of plastic deformation, slip occurs essentially on primary glide planes and the resulting dislocations form coplanar arrays – As deformation proceeds, cross slip takes place – Structure forms high dislocation density regions that eventually develop into networks • Grain size decreases with strain at low deformation but as deformation continues, the grains reach a fixed size © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 87 Hot Working • The process where metal deformation occurs above re-crystallization temperature, preventing strain hardening from occurring – Usually occurs at elevated temperatures – Metal’s resistance to plastic deformation generally lowers with increasing temperature • Metals display viscous (flow) characteristics at sufficiently high temperatures, and their resistance to flow increases at high forming rates – Because it is a characteristic of viscous substances and the slowed rate of re-crystallization • For this reason, hot working larger massive sections of metal by forging, rolling, or extrusion, is preferred © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 88 Hot Working • Examples of materials for hot working – Lead is hot-worked at room temperature because of its low melting temperature – At the other extreme, molybdenum cold-working occurs when deformed at red-hot temperatures because of its high recrystallization temperatures © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 89 Welding • Welding induces internal stresses that remain in the material after the welding is finished or completed • In stainless steels, such as type 304, the crystal lattice is FCC (austenite) • During high-temperature welding, some surrounding metal may be elevated to between 500°F and 1,000°F • In this temperature region, the austenite transforms into a BCC lattice structure known as bainite • Once the metal cools, regions surrounding the weld contain some original austenite and some newly formed bainite © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 90 Welding • A problem arises because the packing factor (PF = volume of atoms per volume of unit cell) is not the same for FCC crystals as it is for BCC crystals • Bainite occupies more space than the original austenite lattice • This elongation causes residual compressive and tensile stresses in the material – Using heat sink welding minimizes welding stresses from lower metal temperatures – Annealing also reduces welding stress © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 91 Metal Treatments Knowledge Check Varying the rate of cooling of a metal in order to control grain size and grain patterns is _______________. A. heat treating B. annealing C. cold working D. quenching Correct answer is D. © Copyright 2017 – Rev 3 ELO 3.2 Operator Generic Fundamentals 92 Metal Corrosion TLO 4 – Describe common types of corrosion that affect metals. 4.1 Describe the following types of corrosion and methods for controlling: a. General corrosion b. Galvanic corrosion 4.2 Describe the following types of localized corrosion including prevention and control methods: a. Stress corrosion cracking b. Chloride stress corrosion c. Caustic stress corrosion 4.3 Describe hydrogen embrittlement. © Copyright 2017 – Rev 3 TLO 4 Operator Generic Fundamentals 93 General and Galvanic Corrosion ELO 4.1 - Describe the following types of corrosion and methods for controlling: general corrosion and galvanic corrosion. • Corrosion is a major factor when selecting material for use in industrial systems and facilities • The material selected must resist the various types of corrosion caused by the environment and materials and conditions involved in processing at the facility © Copyright 2017 – Rev 3 ELO 4.1 Operator Generic Fundamentals 94 General Corrosion • General corrosion involving water and steel or iron results from chemical action where the steel surface oxidizes, forming iron oxide or rust • Many systems and components in industrial and nuclear plants use iron alloys • Standard methods associated with material selection to protect against general corrosion include: – Using corrosion-resistant materials such as stainless steel, nickel, chromium, and molybdenum alloys – Using protective coatings such as paints and epoxies o Corrosion is electrochemical by nature o Corrosion resistance of the stainless steels occurs from surface oxide films interfering with the electrochemical process © Copyright 2017 – Rev 3 ELO 4.1 Operator Generic Fundamentals 95 General Corrosion • Standard methods to protect against general corrosion include: – Applying metallic and nonmetallic coatings or linings to the surface protects against corrosion, and allows the material to retain its structural strength o For example, a carbon steel pressure vessel lined with stainless steel cladding © Copyright 2017 – Rev 3 ELO 4.1 Operator Generic Fundamentals 96 Galvanic Corrosion • Galvanic corrosion occurs when two dissimilar metals with different electrical potentials are in electrical contact in an electrolyte • May also take place within one metal with heterogeneities or dissimilarities such as: – Impurity inclusions – Difference in grain size – Difference in grain composition – Difference in mechanical stress © Copyright 2017 – Rev 3 ELO 4.1 Operator Generic Fundamentals 97 Galvanic Corrosion Mechanism • In galvanic corrosion, a difference in electrical potential exists between the different metals and serves as the driving force for electrical current flow through the electrolyte – Electrical current results from corrosion of one of the metals • The larger the potential difference, the greater the rate of galvanic corrosion – Results in the deterioration of one of the metals • The less resistant more active metal becomes the anodic or negative corrosion site • Stronger, more noble metal is cathodic or positive and protected • If there were no electrical contact (therefore no current flow), the two metals become uniformly attacked by the corrosive medium (general corrosion) © Copyright 2017 – Rev 3 ELO 4.1 Operator Generic Fundamentals 98 Minimizing Galvanic Corrosion • Material selection is important because different metals may contact each other, forming galvanic cells • Design is important in minimizing system low-flow conditions and resultant areas of corrosion buildup • Loose corrosion products are significant because they transport through systems and deposit in low-flow areas • In nuclear plants, corrosion products exposed to radiation become highly radioactive, further increasing radiation levels and contamination issues © Copyright 2017 – Rev 3 ELO 4.1 Operator Generic Fundamentals 99 Cathodic Protection • Methods exist to reduce galvanic corrosion • For example, when pieces of zinc are attached to the bottom of a steel water tank, the zinc become the anode, and corrodes • The steel in the tank becomes the cathode, and is not affected by the corrosion • The electrical current between the anode (zinc) and the cathode causes the anode to corrode – Passive galvanic cathodic protection • The corroding anode is the sacrificial anode • An external direct current (DC) electrical power source also provides sufficient current to ground and negate corrosion © Copyright 2017 – Rev 3 ELO 4.1 Operator Generic Fundamentals 100 General and Galvanic Corrosion Knowledge Check _______________ is an attack on the entire surface of a metal, where the surface of the metal oxidizes to form rust. A. Chloride stress corrosion B. General corrosion C. Caustic stress corrosion D. Galvanic corrosion Correct answer is B. © Copyright 2017 – Rev 3 ELO 4.1 Operator Generic Fundamentals 101 Characteristics of Localized Corrosion ELO 4.2 - Describe the following types of localized corrosion including prevention and control methods: stress corrosion cracking (SCC), chloride stress corrosion, and caustic stress corrosion. • The previous lesson discussed general and galvanic corrosion • This lesson covers various types of localized corrosion including: – Stress corrosion cracking – Chloride stress corrosion – Caustic stress corrosion © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 102 Localized Corrosion • Localized corrosion is the selective removal of metal by corrosion at small areas or zones on a metal surface in contact with a corrosive environment, usually a liquid • Corrosion attacks small local sites at a much higher rate than the rest of the metal’s surface • Localized corrosion takes place when corrosion combines with other destructive processes such as stress, fatigue, erosion, and other forms of chemical attack – Localized corrosion mechanisms can cause more damage than any individual destructive processes © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 103 Stress-Corrosion Cracking • Stress corrosion cracking is one of the most serious metallurgical problems and a major concern • Stress corrosion cracking is a type of intergranular attack corrosion that occurs at the grain boundaries of a metal under tensile stress • Stress corrosion cracking propagates as stress opens cracks in metal subject to corrosion – Cracks continue to corrode, weakening the metal by further cracking • Cracks follow intergranular or transgranular paths, and there is often a tendency for crack branching © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 104 Causes of Stress Corrosion Cracking • Stresses that cause cracking result from residual stresses from factors such as cold work, welding, grinding, or thermal treatment • Externally applied stress during service is also possible • To propagate, stress corrosion cracking, this stress must be tensile in direction • Stress corrosion cracking occurs in metals exposed to environments where, if stress was not present or was at a much lower level, no damage would happen • If the structure subject to the same stresses was in a non-corrosive environment, there would be no failure © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 105 Preventing Stress Corrosion Cracking • The most effective means of preventing SCC are: – Proper system and component design – Reducing stress – Removing critical environmental species such as hydroxides, chlorides, and oxygen – Avoiding stagnant areas and crevices in heat exchangers where chloride and hydroxide could become concentrated © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 106 Preventing Stress Corrosion Cracking • Low alloy steels are less susceptible than high alloy steels, but are subject to SCC in water containing chloride ions • Chloride or hydroxide ions do not affect nickel-based alloys • Inconel is an example of a nickel-based alloy that is resistant to stress-corrosion cracking • Inconel is composed of 72 percent nickel, 14 to 17 percent chromium, 6 to 10 percent iron, and small amounts of manganese, carbon, and copper © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 107 Chloride Stress Corrosion • Chloride stress corrosion is a concern in the nuclear industry • Intergranular corrosion occurs in austenitic stainless steels under tensile stress in the presence of the following: – Oxygen – Chloride ions – High temperature • Starts with chromium carbide deposits along grain boundaries, leaving the metal open to corrosion • Controlling this form of corrosion involves the following: – Low chloride ion content – Low oxygen content – Use of low carbon steels © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 108 Caustic Stress Corrosion • Carbon steels are susceptible to caustic stress corrosion: – Cracks initiate and grow along the grain boundaries – Similar to other forms of localized corrosion – Extensive crack branching occurs along the grain boundaries • High tensile stress external to the steel or within the steel from fabrication is the driving forces © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 109 Caustic Stress Corrosion • Heat treating Inconel at 620°C to 705°C, depends on prior solution treating temperature and improves its resistance to caustic stress corrosion cracking • Other possible problems found with Inconel include wastage, tube denting, pitting, and intergranular attack © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 110 Characteristics of Localized Corrosion Knowledge Check _______________ is a type of corrosion generally associated with Inconel. A. Chloride stress corrosion B. Caustic stress corrosion C. Galvanic corrosion D. General corrosion Correct answer is B. © Copyright 2017 – Rev 3 ELO 4.2 Operator Generic Fundamentals 111 Hydrogen Embrittlement ELO 4.3 - Describe hydrogen embrittlement. • Personnel awareness of the conditions for hydrogen embrittlement and its formation process are important in understanding when it occurs • This lesson discusses the sources of hydrogen and the characteristics for the formation of hydrogen embrittlement © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 112 Sources of Hydrogen • Hydrogen embrittlement is another form of stress corrosion cracking – Hydrogen embrittlement in high-strength steels has a devastating effect because of the catastrophic nature of the fractures • Steel loses its ductility and strength due to tiny cracks that result from the internal pressure of hydrogen (H2) or methane gas (CH4), which form at the grain boundaries • Significant because of the susceptibility of zirconium alloys to this type of corrosion © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 113 Sources of Hydrogen • Sources of hydrogen causing embrittlement may come from the following: – Steel manufacturing process – Welding – Hydrogen gas in vessels – Byproducts of general corrosion – Corrosion reactions such as rusting, cathodic protection, and electroplating – Byproduct from industrial chemicals © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 114 Hydrogen Embrittlement of Stainless Steel Figure: Hydrogen Embrittlement © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 115 Sources of Hydrogen Figure: Transgranular Cracking © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 116 Sources of Hydrogen Figure: Intergranular Cracking © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 117 Sources of Hydrogen 3Fe + 4H2 O → Fe3 O4 + 4H2 • Hydrogen embrittlement is not a permanent condition • If no cracking occurs and the environmental conditions change so that no hydrogen generates on the metal’s surface, hydrogen rediffuses from the steel, restoring the metal’s ductility © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 118 Minimizing Hydrogen Embrittlement • Industry efforts to address the problem of hydrogen embrittlement include the following: – Controlling the amount of residual hydrogen in steel – Controlling the amount of hydrogen in processing – Developing alloys with improved resistance to hydrogen embrittlement – Developing low or no embrittlement plating or coating processes – Restricting the amount of in-situ (in position) hydrogen introduced during a part’s service life © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 119 Hydrogen Embrittlement Knowledge Check What two conditions are necessary for hydrogen embrittlement in stainless steel to occur? ____________ and ___________. A. Hydrogen B. Oxygen C. Carbon D. Elevated temperature Correct answers are A and C. © Copyright 2017 – Rev 3 ELO 4.3 Operator Generic Fundamentals 120 Material Selection TLO 5 – Describe common material failure mechanisms. 5.1 Describe the following material failure mechanisms: a. Fatigue failure b. Work hardening c. Creep © Copyright 2017 – Rev 3 TLO 5 Operator Generic Fundamentals 121 Material Failure Mechanisms ELO 5.1 - Describe the following material failure mechanisms: fatigue failure, work hardening, and creep. • Material failures in industrial facilities are not limited to brittle fracture • Other failure mechanisms exist, which in time can lead to mechanical component failures • Chief among these are fatigue failure, work hardening, and creep © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 122 Fatigue Failure • Fatigue causes the majority of engineering failures – A material’s tendency to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably below the normal strength • Characterized as brittle, this type of failure may take some time for a fracture to propagate, depending on: – Intensity – Frequency of the stress cycles • Little, if any, warning is given before the failure occurs © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 123 Fatigue Failure • Number of cycles required for fatigue failure at a particular peak stress is generally large, but it decreases as the stress is increased • For some mild steels, an infinite number of cyclical stresses may continue provided – Peak stress (sometimes called fatigue strength) is below the endurance limit value © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 124 Fatigue Failure • Thermal fatigue is the most common fatigue in industrial facilities – Thermal fatigue arises from thermal stresses produced by cyclic changes in temperature • Large thick-walled components, such as steam piping, are subject to cyclic stresses caused by temperature variations during facility startup, normal operation, and shutdown © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 125 Fatigue Failure Mechanism • Caused by the initial formation of a small crack resulting from a defect or microscopic slip in the metal grains • The crack propagates slowly at first and then more rapidly when the local stress increases due to a decrease in the load-bearing cross section – The metal then fractures • Microscopic cracks and notches initiate fatigue failure – Include grinding and machining marks on the surface • Avoid such defects in materials subjected to cyclic stresses (or strains) • These types of defects also favor brittle fracture © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 126 Avoiding Fatigue Failure • For components with low load variations and a high cycle frequency – High fatigue strength – High ultimate tensile strength is desirable • For components with large load variations and low cycle frequencies – High ductility • Plant operations performed in a controlled manner mitigate the effects of cyclic stress by minimizing cyclic stress: – Use of heatup and cooldown limitations – Pressure limitations – Pump operating curves • Keeping records on equipment allows identification of the need for replacement prior to fatigue failure • In high thermal stress piping systems, installed thermal sleeves minimize thermal stresses © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 127 Work (Strain) Hardening • Work hardening occurs when straining a metal beyond the yield point (to the ductile region) • Increasing stress produces additional plastic deformation, causing the metal to become stronger and more difficult to deform © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 128 Work (Strain) Hardening • True stress plotted against true strain shows that the rate of strain hardening (illustrated by the slope of the true stress line) becomes almost constant – The true stress curve almost becomes a straight line – The slope of the true stress line reflects the strain (or work) hardening coefficient or work hardening coefficient Figure: Nominal Versus True Stress-Strain Curve © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 129 Factors Affecting Work (Strain) Hardening • Grain size also influences strain hardening – Small grain size strain hardens more rapidly than the same material with a larger grain size • The effect only applies in the early stages of plastic deformation – Disappears as the structure deforms and grain structure breaks down © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 130 Work (Strain) Hardening Mechanism • Work hardening closely relates to fatigue – For example, bending the thin steel rod becomes more difficult the more the rod is bent – This results from work or strain hardening • Work hardening reduces ductility, increasing the chances of brittle failure • Stronger, but less ductile © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 131 Work (Strain) Hardening as a Material Treatment • Work hardening is useful for treating metal • Prior work hardening (cold working) causes the treated metal to have an apparently higher yield stress • Strengthened metal results © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 132 Creep • At room temperature, structural materials develop the full strain they will exhibit when a load is applied • This is not necessarily the case at high temperatures – For example, stainless steel above 1,000 °F • At elevated temperatures and constant stress or load, many materials continue slowly deforming • This behavior is called creep © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 133 Mechanism of Creep • Rate of creep is constant at constant stress and temperature • After amount of deformation the rate of creep increases and fracture follows • Primary or transient creep (Stage I) – creep rate (slope of the curve) is high at first, but soon decreases • Secondary or steady state creep (Stage II) – creep rate is small and the strain increases slowly with time • Stage III (tertiary or accelerating creep) – creep rate increases more rapidly and strain becomes so large failure results © Copyright 2017 – Rev 3 Figure: Successive Stages of Creep With Increasing Time ELO 5.2 Operator Generic Fundamentals 134 Mechanism of Creep • The rate of creep depends on both stress and temperature • As temperature rises – creep becomes progressively more important and eventually supersedes fatigue as the likely criterion for failure • The temperature where creep becomes important – varies with the material involved • With most industrial alloys used in construction at room temperature or lower, ignoring the small amount of creep strain is permissible • Creep does not become significant until the stress intensity approaches the fracture failure strength © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 135 Limiting Creep • For safe operation, the total deformation due to creep must be below the strain where failure occurs – Stay within acceptable creep rate limits for component lifetime • At normal temperatures in high-pressure component applications – creep limit generally does not pose a limitation • It becomes a concern at extremely high temperatures and pressures © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals 136 Material Failure Mechanisms Knowledge Check Work hardening __________ the ductility of a metal. A. raises B. has no effect on C. lowers D. insufficient information to answer Correct answer is C. © Copyright 2017 – Rev 3 ELO 5.2 Operator Generic Fundamentals