Kuppan T. Heat Exchanger Design Handbook
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664
Chapter
12
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Poole, J.
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Borenstien,
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Swindeman,
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P., Corrosion control of industrial cooling water systems,
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Beynon, E., Cooper,
N.
R.,
and Haningan, H. J., Cooling System Corrosion
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and Materials, Engine Coolant Testing: State of the Art, ASTM STP 705, (W.
H.
Ailor,
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the Ferritic Stainless Steels,
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A. Lula, American Society for Metals,
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E.
A. D.,
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and
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and Tofield, B. C., Effective corrosion monitoring,
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and Schwarzenbach, A., Power Generating Equipment, in
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ber 1992, Lukens Steel Company, Coatesville, PA.
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13
Material Selection and Fabrication
1
MATERIAL SELECTION PRINCIPLES
In engineering practice the selection of materials of construction depends on the equipment
being designed and the service requirements imposed on them. Selection
of
materials involves
the thorough understanding of their availability, source, lead time, product forms, and size. In
simple terms, the following factors are to be considered while selecting the materials for heat
exchanger and pressure vessel components:
1.
Compatibility of the materials with the process fluids.
2.
Compatibility of the materials with the other component materials.
3.
Ease of manufacture and fabrication by using standard methods like machining, rolling,
forging, forming, and metal joining methods such as welding, brazing, and soldering.
4.
Strength and ability to withstand operating temperature and pressure.
5.
cost.
6.
Availability.
While selecting materials one has to balance
many
requirements; these requirements in-
clude the following
[
11:
Expected total life
of
plant or process.
Expected service life of the material.
Reliability (safety, hazard, and economic consequences of failure).
Material costs.
Fabrication costs.
Maintenance and inspection costs.
Availability in required size, shape, thickness, and
so
on, and delivery time.
Return on investment.
667
668
Chapter
I3
Materials are selected based on past experience, corrosion tests, the literature, and the
recommendations of material suppliers. The direct measure of success in materials selection
and fabrication is reflected in the behavior of the equipment in service. In order to assure safe,
reliable and uninterrupted service with monetary benefits, proper attention must be given
to
materials selection commencing from the design stage and must continue through fabrication,
installation, and maintenance. Once onstream, the equipment should be monitored for specified
performance. With this background information, the logical sequence of material selection for
the construction of pressure vessels and heat exchangers is discussed next.
1.1
Materials Selection
While selecting materials for the construction of heat exchangers and pressure vessels, the
following points may be considered for the desired performance and the life of the equipment
PI:
1. Review of operating process.
2.
Review of design.
3.
Selection of material.
4.
Evaluation of material.
5.
Specification.
Finally the material should be cost-effective. Do cost-benefit analysis for an optimum material
selection.
1.2
Review
of
Operating Process
The foremost step in the material selection is the thorough review
of
the process environment
and equipment operating conditions like operating temperature, pressure, and phases
of
the
fluids. The following operating data are required by the design engineer
[3]:
1. Environment-nature and composition
of
fluid handled, water chemistry, steam quality,
constituentskoncentration of a solution, conductivity, pH, aeration, impurities, and
so
on,
2.
Pressure-average and range, constant or cyclic, internal and external loadings.
3.
Temperature-average and range, constant or variable, thermal gradients, and thermal
shock.
4.
Velocity-flow rate, linear velocity, nominal and range, degree of agitation, turbulence,
etc.
In addition to operating temperature and pressure, other factors such as the startups and shut-
downs, intermittent operation, transients and pressure surges, and momentary failure of the
system must be considered for the satisfactory performance of a material.
1.3
Review
of
Design
After the review
of
the operating process, the type or design of the equipment and various
assemblies and subassemblies, size, complexity, and criticality in service should be considered.
Applicability of various fabrication techniques like forging, machining, welding, soldering,
brazing, etc.,
is
considered.
1.4
Selection
of
Material
The selection of proper materials for wetted and nonwetted parts, pressure and nonpressure
parts, and equipment support is an important design step. Material Standards like ASTM, DIN,
BS,
ISO,
and JIS give data on a large number of ferrous and nonferrous materials. Many of
669
Material Selection and Fabrication
these materials have been adopted in the Pressure Vessel Codes by the country of origin in the
Material Standards. Data from these material standards are generally restricted to the physical
properties, such as Young’s modulus, yield strength, minimum tensile strength, maximum per-
missible operating temperature, elongation, coefficient of expansion, etc. But codes and stan-
dards do not explicitly express the suitability of the materials for the intended service environ-
ment, either internal or external. These details must be provided by the corrosion engineer or
many sources are to be referred.
ASME Code Material Requirements
Pressure Parts.
All materials used for pressure-retaining parts must meet the ASME Code
(ASME Boiler and Pressure Vessel Code, Section 11-Material Specifications, Part A-Ferrous
Materials,
Part
B-Nonferrous Materials, Part C-Welding Rods, Electrodes, and Filler Met-
als, and Part D-Material Properties, 1992, American Society
of
Mechanical Engineers, New
York) general requirements for materials. Only code-specified material shall be used in the
vessel construction or repair. The manufacturer should deliver the material after thorough in-
spection, with the chemical composition and metallurgy and properties as stipulated in the
relevant specification. The allowable stresses for the different code-permitted materials should
not be exceed.
Nonpressure Parts.
Materials used for nonpressure parts shall be compatible with the process
fluids, ambient environment, andor shall be of weldable quality.
Functional Requirements of Materials
Function of equipment has frequently been equated with long, trouble-free service
[4].
Selected
material has certain functional requirements, which ultimately reflect on the function of the
equipment. Those functional requirements are given in Table
1.
Functional requirements of
materials given in Ref.
5
have been enlarged to include additional information and presented
in Table 1.
With reference to functional requirements, the following featuredparameters are discussed.
Table
1
Functional Requirements of Materials
Characteristics Functional requirements
Strength
Tensile, torsion, bending, compression, shear, buckling, fatigue, creep, rup-
ture strength, etc.
Physical properties
Design may require certain properties peculiar to the materials (thermal,
electrical, acoustic, magnetic, etc.), coefficient of thermal expansion,
thermal conductivity, modulus
of
elasticity, specific gravity (strength to
weight ratio).
Temperature resistance
Low
temperature: subzero and cryogenic.
Intermediate temperature.
Elevated temperature.
Geometric stability
Stiffness of the structure, effect of sustained loading, etc.
Toughness
Ability to cope with a sudden load, impact, low-temperature operation,
etc.
Fabricabili ty
Ability to be formed, forged, machined, joined, ease of heat treatment, etc.
Corrosion resistance
Compatibility
of
the materials with the process fluids, with the other
component materials in contact; resistance to ambient environment;
passivity; hydrogen attack.
Wear and abrasion
Loss
of surface oxide film (passive films), which is helpful in corrosion
resistance.
670
Chapter
I3
1. Strength.
2.
Fatigue strength.
3.
Brittle fracture.
4.
Toughness.
5.
Creep.
6.
Resistance
to
temperature.
7.
Heat and corrosion.
8.
Corrosion resistance.
9.
Hydrogen attack.
10.
Fabricability
.
Strength.
While the term strength applies
to
many mechanical properties, it most commonly
refers
to
tensile strength and yield strength. These strengths are best understood by examining
a stress-strain curve. Other related strengths are compression, bending (flexural), shear. and
torsional strength.
Its
is better
to
use high strength materials
to
keep the thickness of parts less.
Fatigiie Strength.
Fatigue is the failure of a metal by fracture when it is subjected
to
cyclic
stress. The usual case involves rapidly fluctuating stresses that may be well below the tensile
strength.
As
stress is increased, the number of cycles required to cause failure decreases.
A
stress-number of cycles
(S-N)
curve is conventionally plotted for a metal as shown
in
Fig. 1.
The stress is plotted against the logarithm
of
the number
of
cycles. There is usually a stress
level below which no failure will occur, for a specified large number of cycles, and this
is
called the endurance limit. Since the stress limits that are required
to
prevent distortion and
bursting keep the nominal stresses at or below the endurance limit, the possibility of fatigue
failure is almost always associated with discontinuities, notches, shocks, residual stresses, etc.
Fatigue failures are most likely
to
occur at welded joints, which, by virtue of their design,
give rise to significant stress concentration effects. Some of the material and design characteris-
tics favorable to fatigue resistance are as follows
[6]:
increased tensile strength via alloying or
heat treatment, minimum workpiece thickness, polished surfaces., low design stress, smaller
grain size, absence of metallurgical discontinuities and residual stresses, presence of surface
compressive stress, minimize environments conducive
to
corrosion and fatigue, and avoidance
cn
v)
h,
L-
Y
v)
Figure
1
Stress-cycle
(S-N)
curve.
671
Material Selection and Fabrication
of metallic coating, such as Cr, Ni, or Cd plating, which can reduce fatigue strength due to
microcracks in the plating. For pressure vessels, hydrostatic pressures that exceed the yield
strength of the metal can relieve residual stresses and can improve fatigue strength
[7].
Brittle Fracture.
Brittle fracture is the term given to describe failure by rapid crack propaga-
tion at nominal stress below the yield strength. Brittle fracture has been mostly observed in
bcc crystal structure metals, notably ferritic steels, in which complete rupture takes place under
certain conditions at stress levels not only below the ultimate tensile strength but even lower
than yield strength. The conditions associated with brittle fracture of steels include the follow-
ing
[8,
91:
1.
A
preexisting defect.
2.
The presence of residual andor applied tensile stress across the defect, which is more
serious if triaxial.
3.
A
sufficiently low temperature.
4.
Several metallurgical factors-deoxidation practice, composition, rolling practice, and
subsequent heat treatment [8].
The subject of brittle fracture is of interest to the designers of long-life pressure vessels for the
following reasons [9]:
1.
Even if the vessel is to operate at an elevated temperature, where the brittle fracture is not
to be expected, there is a danger of failure of the vessel during the preservice cold hydro-
static test; another case of practical importance is the temper embrittlement of Cr-Mo
steels, a condition caused by elevated temperature service after a long time
in
refinery
service.
2.
Cyclic variations of load and temperature during long service may enlarge small initial
defects to the critical size and result in brittle fracture during startup, shutdown, or cold
periodic tests.
3.
In the special case of nuclear vessels, neutron bombardment can produce large increase in
the transition temperature.
In welded structures, brittle fractures invariably initiate from weld defects. Weldment fail-
ure due to brittle fracture is dependent on through-thickness toughness, the critical defect/
notch size, residual welding strains, mechanical constraints, grain size, inclusion content, and
temperature
[
101. It must not be overlooked that a normally ductile material can fail in a brittle
manner at locations where complex and triaxial stress fields exist
[
1
11.
Measures to Minimize Brittle Fracture.
Brittle fracture may be prevented by selecting a
notch-tough material, and avoiding notches. The following design practises will help to mini-
mize brittle fracture
[
121:
1. Eliminate all defects, which is impracticable.
2.
Employ design stress less than required for propagation, but this may not be justified
economically.
3.
Select materials whose transition temperature is below operating temperature.
4.
If the structures are not stress relieved, residual stresses may be superimposed on the
applied stress, and will cause local yielding in the vicinity of a defect and thus initiate a
brittle crack.
Some simple practices to minimize the chance of nonservice failure have been suggested
by Bravenec
[
131.
These recommendations include
[
131:
672
Chapter
13
1.
Sheared and torched edges should be conditioned by grinding prior to forming.
2.
Warm
forming should be used whenever the ambient temperature is less than
75°F.
3.
Material selection should meet end use requirements and code requirements; however, it
must be recognized that Code stipulates only a minimum standard.
Toughness.
A
material’s toughness is its ability to absorb energy and deform plastically be-
fore fracture, and the amount of energy absorbed during both the deformation and fracture is
the measure of toughness. Toughness rather than strength is the most important low-tempera-
ture mechanical property. Notch toughness is influenced by chemical composition, microstruc-
ture, grain size, grain flow pattern, section size, hot and cold working temperature, method
of
fabrication, and surface conditions like carburization and decarburization
[6].
Basic Tests.
The most common toughness tests use notched specimens and measure
either notch toughness, usually by an impact test, or fracture toughness, which is measured at
a relatively low strain rate. Impact tests reveal low-temperature strength, lack of ductility, and
brittleness, and these tests are being particularly relevant to ferritic materials, which may show
a transition from ductile-to-brittle behavior. Two commonly used methods are the Charpy and
Izod tests.
Charpy V-Notch, CVN (ASTM
E23).
The Charpy V-notch test is considered the most
appropriate because a part or structure will generally fail due to a notch or other stress concen-
tration. This test is used to obtain absorbed energy, lateral expansion, and fracture appearance
from a
10
mm
x
10
mm
x
55
mm piece of a material subjected to impact loading. The speci-
men has a 0.254-mm (0.010-in) radius notch. Test results given in foot-pounds measure the
capacity to absorb energy and thus signify the material ability to resist failure at points of local
stress concentration.
Precrucked Charpy V-Notch.
This is a test often used as a correlative measure of fracture
toughness.
Nil-Ductilio Transition Temperature, NDlT (ASTM
E208).
This test defines the temper-
ature at which a small flaw will initiate when subjected to yield loads. It is used with a Charpy
V-notch test to establish the reference fracture toughness in
ASME
Code.
Fracture Toughness.
The notch toughness approach does not work well with high-
strength steels and nonferrous metals because of the gradual transition from ductile to brittle
behavior
[6].
Also, it is felt that
CVN
test results alone are not reliable enough to predict
whether an existing crack of a certain size will grow slowly, causing leakage before failure, or
fail catastrophically under the very slow strain rates likely to be encountered in LNG contain-
ment vessels
[
141. In these situations, fracture toughness value is used to assess metals tough-
ness, and the approach is known as fracture mechanics approach. Plane-strain fracture tough-
ness, or
K,,,
values are used to calculate the critical crack size in a stressed component that
can result
in
an
abrupt failure.
K,,
values, unlike notch toughness values, are independent of
test specimen shape
[6].
Fracture Toughness (ASTM
E399
and
E813).
These are the only two tests that yield
“ASTM
valid” fracture toughness values.
E399
is for linear-elastic fracture toughness
KIL;
and
E813
is for the “J-Integral,” which can be converted to
Klc.
Creep.
Creep is defined as slow deformation of material with time, with the deformation
taking place at elevated temperatures without increase in stress. Creep curve is shown in Fig.
2.
At room temperature creep is nonexistant in steel. Many pressure vessel steels have to
operate in creep conditions and the general basis
of
design is based on data from test specimens
and involves the definition of stress below which failure will not occur within the expected
life
of
the component. The phenomenon of creep is well understood, the basis of design is
673
C
Material Selection and Fabrication
Primary
Secondary
tertiary
creep
111
acture
.-.
E!
c.
U,
Figure
2
Creep curve.
clear, and no special problems need be encountered. It is worth, however, sounding two notes
of warning
[
151:
1.
If the design is poor
so
that unforseen high local stresses occur, then the local creep
deformation may exceed the creep ductility and lead to premature failure.
2.
If the structures are not stress relieved, residual stresses may be superimposed on the
applied stress, and again lead to premature creep failure. This may well occur in welds
around nozzles. It is thus important to remember that vessels operating under creep condi-
tions should be stress relieved prior to service.
Creep Test. Tests for resistance of a material to elevated temperature generally fall into
three classes: short-time tests (tension, compression, bending, shear, torsion, and impact) run
at elevated temperature, short-time tests run at room temperature after elevated-temperature
exposure for various lengths of time, and long-time tests run at room temperature. Long-time
strength tests, in turn, can be classified as constant load (creep, stress-rupture, and many fatigue
tests) or constant deflection (stress relaxation and some fatigue tests) tests [6].
The standard test followed to determine creep is ASTM
E139,
"Standard practice for
conducting creep, creep-rupture, and stress-rupture tests of metallic materials." The Larsen-
Miller parameter is considered reliable for using data from high-temperature tests to predict
creep-rupture time. The Larson-Miller parameter used for predicting stress-rupture data for
low alloy steels is given by
P=
1.87(k
+
log
t)
x
10-3
where
T
is temperature
(K),
t
is hours, and
k
is a constant
(=20)
for low alloy steels. For
temperature in
OR,
eliminate the constant
1.8
in the equation.
Temperature Resistance.
Temperature resistance of construction materials is discussed with
reference to various temperature of operation, since basic materials behavior is intimately re-
lated to operating temperature. Accordingly, the temperature of opera
ion is class
fied into
these four ranges:
1. Subzero and cryogenic operation.
2.
Low-temperature operation (up to
200°C).
3.
Intermediate-temperature operation
(200"C--650"C).
4.
High-temperature operation (>65OoC).