Download Operator Generic Fundamentals Material Science

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.
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TLOs
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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Figure: Grain Structure
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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
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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
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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
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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
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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.
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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)
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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
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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
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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
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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)
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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
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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
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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
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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
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Figure: Vacancy Point Defect
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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
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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
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Figure: Interstitial Point Defect
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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
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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
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Figure: Edge Dislocation
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Line Imperfections – Edge Dislocations
Figure: Slips Along Edge Dislocations
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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
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Figure: Screw Dislocation
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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Figure: Ductile Material Stress-Strain Curve
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Figure: Malleable Deformation of a
Cylinder Under Uniform Axial Compression
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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)
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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Hydrogen Embrittlement of Stainless
Steel
Figure: Hydrogen Embrittlement
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Sources of Hydrogen
Figure: Transgranular Cracking
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Sources of Hydrogen
Figure: Intergranular Cracking
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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