500 Materials

Abstract
Two aspects of materials selection for pressure vessels are discussed in this section:
(1) selection for the service conditions, and (2) selection for brittle fracture preven-
tion. The two are usually considered at different stages of the design. Selection for
the service environment is completed first, and materials selection for brittle frac-
ture prevention second. This section also presents typical materials selections, and
discusses the general characteristics of commonly used pressure vessel materials.

Contents Page

510 Selection of Materials for Service Environment 500-2

511 Design Factors
512 Typical Selections
513 Application Criteria for Common Pressure Vessels Materials
514 Summary of Temperature Limitations
520 Selection of Materials for Brittle Fracture Prevention 500-12
521 Definition of Brittle Fracture
522 Examples of Brittle Fracture Experienced by Chevron
523 Design to Prevent Brittle Fracture
524 Recommended Practice for Selecting Steels for New Construction of Pres-
sure Vessels
525 Typical Carbon Steel Selections to Avoid Brittle Fracture in Pressure Vessels
526 Steel Selection for Pressure Vessels Subject to Autorefrigeration
527 Factors Controlling Susceptibility to Brittle Fracture: Additional Technical
Information
530 Guidelines for Preventing Brittle Fracture in Existing Equipment 500-27
531 Determining MPTs

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510 Selection of Materials for Service Environment

This section presents the general principles in the selection of materials to prevent
deterioration in the service environment. It discusses design factors, typical selec-
tions, and characteristics of commonly used materials.

511 Design Factors

Design factors to consider include:
• Operating temperature and pressure
• Service environment
• Cost
• Design life
• Reliability and safety

Operating Temperature and Pressure

Operating temperature and pressure limit the choice of materials and can have a
significant influence on corrosion rates. Temperature can limit materials by
adversely affecting strength, metallurgy, and resistance to corrosion.
For example, carbon steel is limited to a maximum design operating temperature of
800°F. Above 800°F, the strength of carbon steel decreases significantly and the
steel may embrittle because of graphitization.
Corrosion rates frequently increase with temperature. In sour hydrocarbon services,
for example, bare carbon steel is limited to about 550°F because corrosion acceler-
ates at higher temperatures.
Operating pressure can also influence the stability of a material in the service envi-
ronment. Hydrogen attack of steels in high pressure hydrogen at elevated tempera-
tures is an example.

Service Environment
Materials are selected to limit corrosion to acceptable, economic rates in the service
environment. “Service environment” as used here means what the vessel will
contain, its temperature and pressure, any contaminants, physical state, and some-
times flow rate. For a given service environment, materials selection should be
made with consideration for both corrosion rates and other potential deterioration
mechanisms, such as stress corrosion cracking and hydrogen damage.
Information about corrosion rates can be obtained from several sources. Past experi-
ence is the best source if there is a vessel in similar service. A review of the inspec-
tion records for vessels in similar services can indicate whether the materials
selection was correct and what corrosion rates may be expected. The comparison
should also include a review of the similarity of the new and old service environ-
ments.

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Other sources of corrosion data include the Corrosion Prevention and Metallurgy
Manual, laboratory tests, and published data. The Corrosion Prevention and Metal-
lurgy Manual gives general material selection guidelines for several specific plants.
Contact a materials specialist for additional information and specific recommenda-
tions.
Certain environmental conditions may cause other deterioration mechanisms such as
stress corrosion cracking, sulfide stress cracking, and hydrogen attack. The Corro-
sion Prevention Manual describes these mechanisms and, in the chapters dealing
with specific plants, highlights potential deterioration mechanisms to consider.

Cost
The objective is to select the most economical material that will reliably satisfy the
design life of the vessel. This is often achieved by selecting carbon or low alloy
steels in preference to stainless and highly alloyed materials and by specifying
conservative corrosion allowances. See the discussion of design life below.
When stainless steel or a more highly alloyed material is required, it is often prefer-
able to use a carbon or low alloy steel clad with a thin layer of the high alloy mate-
rial. Clad plate is usually less expensive than solid alloy plate unless the thickness of
the vessel is less than 3/8 to 1/2 inch. Clad plate is also preferred because it is less
likely to develop through-wall stress corrosion cracks than solid alloy. Some of the
commonly used cladding materials, such as Types 405 and 410 stainless steel, are
not practical to fabricate for solid wall construction because of the difficulty in
making reliable welds.
For some aqueous services, up to about 200°F, nonmetallic thin film coatings can be
applied to reduce corrosion rates and the need for alloy material.

Design Life
The design life typically used for pressure vessels is 20 years. Exceptions are:
1. Small vessels less than about 400 cubic feet. If the vessels are easily acces-
sible, a design life of 10 years may be appropriate.
2. Large heavy walled vessels, thicker than 2 inches. A 30-year design life is
recommended.
Corrosion allowances are specified to achieve the design life and are based on the
expected corrosion rate. Corrosion allowances are discussed in more detail in the
Corrosion Prevention and Metallurgy Manual, but recommendations for pressure
vessels are summarized in Figure 500-1. If the corrosion allowance required to
achieve the design life is greater than ¼ inch, then a more corrosion-resistant alloy
or a clad vessel is generally economical.

Reliability and Safety

The likelihood and consequences of a failure must be considered in the selection of
pressure vessel materials. Consideration of these factors may lead to conclusions on
materials and corrosion allowances that differ from these minimum guidelines.

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Fig. 500-1 Typical Corrosion Allowances for Pressure Vessels

Type of Vessel Recommended Minimum, Inch
Large Heavy Wall Vessels Made of Carbon and Low 3/16(1)
Alloy Steels
Carbon and Low Alloy Steel Vessels 1/8(2)
Stainless Steel or High Alloy Vessels 1/32(3)
(1) If clad, a 0.10-inch minimum cladding thickness is specified to minimize fabrication problems. In this
case, no additional corrosion allowance is necessary for the carbon or low alloy steel.
(2) 1/8 inch is usually used, unless available corrosion data clearly show a corrosion rate less than 3 mpy.
Water legs on drums normally should have a 3/16-inch minimum corrosion allowance.
(3) Applies to solid alloy equipment only. For cladding, a 0.10-inch minimum cladding thickness is specified.
See Note 1.

Each individual case warrants consideration of these factors, and judgment is then
necessary to choose economical materials.
Various factors to consider include the following:
Likelihood of failure:
1. Past history in same or similar services.
2. Whether onstream inspection can predict failures.
3. Shutdown frequency.
Consequences of failure:
1. Personnel safety: acids, caustic, H2S, HF, etc.
2. Fire hazards: LPG, high pressure H2, proximity to furnace.
3. Lost production (plant profitability).
4. Ease of repair or replacement.
5. Geographic factors: availability of expert craftsmen and replacement material.
6. Will leakage cause catalyst poisoning or affect plant performance?
7. Will plant be shut down, or can equipment be bypassed?
8. Will plant shutdown force related plant shutdowns?
9. Will leakage cause environmental problems such as pollution of navigable
waters?

512 Typical Selections

Figure 500-2 illustrates pressure vessel materials typically selected for common
environments. This table is not suitable for final materials selection, but it may save
time in initial investigation.

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Fig. 500-2 Common Pressure Vessel Materials (1 of 2)

Service Typical Materials Comments Notes
Produced fluids containing water Carbon steel with coating Corrosivity of produced fluids (1)

varies widely
Sweet hydrocarbons, less than 1 Carbon steel May corrode even with trace H2S (1)

ppm H2S above 550°F

Sour hydrocarbons, with more Carbon steel Limited to 550°F Maximum (1)

than 1 ppm H2S below 550°F

Sour hydrocarbons, with more Carbon steel clad with 12% Cr (2)

than 1 ppm H2S above 550°F stainless steel

(1) (3)
Sweet hydrogen, such as in Carbon steel, 1¼ Cr-½ Mo, and Choice depends on temperature ,
catalytic reformers, hydrogen 2¼ Cr-1 Mo steel and hydrogen partial pressure.
and ammonia plants See API Recommended Practice
941 and the Corrosion Preven-
tion and Metallurgy Manual.
(4)
Sour hydrogen; may also contain Carbon steel, 1¼ Cr-½ Mo, and Choice depends on temperature
hydrocarbon. Examples include 2¼ Cr-1 Mo Often clad with Type and on hydrogen and H2S partial
hydroprocessing unit and 321 or 347 stainless steel pressure. See API Recom-
hydrotreated process streams. mended Practice 941 and the
Corrosion Prevention and Metal-
lurgy Manual.
(1)
Steam Carbon steel CO2 corrosion may demand
stainless cladding in a
condensing service
(1)
Amines (MEA, DEA) Carbon steel Stress relieve new pressure
vessels to prevent stress corro-
sion cracking. Stainless steel
cladding is frequently used in
selected areas, such as in regen-
erators, to minimize corrosion.
See the Corrosion Prevention
and Metallurgy Manual and
consult a materials specialist
Caustic (<200°F) Carbon steel To prevent stress corrosion
cracking, stress relief is required
as follows: 1. For caustic with a
concentration less than 30
weight percent, stress relieve in
service above 140°F. 2. For
caustic with a concentration of
30% or greater, stress relieve in
services above 110°F.

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Fig. 500-2 Common Pressure Vessel Materials (2 of 2)

Service Typical Materials Comments Notes
Sulfuric Acid Carbon steel Velocity above about 3 fps and (1)

( ≥ 85% concentration) temperature above 120°F will

result in severe corrosion of
carbon steel. Vessels handling
sulfuric acid and LPG mixes,
such as in alkylation plants,
usually are made of carbon steel.
Sour water Carbon steel Carbon steel may corrode at high
concentrations of NH3 and H2S.
(1) Carbon steel. Grades commonly used for pressure vessel plates are SA 285 Grade C, SA 515 Grade 70 and SA 516 Grade 70. Choice will
be determined by minimum design metal temperature and thickness. See Section 524.
(2) Clad carbon steel. Carbon steel clad with 12% Cr steel is covered by Specification SA 263. We usually designate a base metal plate
(carbon steel per note 1 above) and the cladding as Type 405 or Type 410S. Refer to PVM-MS-1322.
(3) Low alloy steels. 1¼ Cr-½ Mo steel is covered by SA 387 Grade 11 (plate) and SA 336 F11 (forgings). 2¼ Cr-1 Mo steel is covered by SA
387 Grade 22 (plate) and SA 336 F22 (forgings).
(4) Carbon or low alloy steel clad with Type 321 or 347 stainless steel. These plates are covered by SA 264 for roll band cladding. Base
metal plate is designated per notes 1 or 3 above. If forgings are used for shell components or if shell plates are thick, they will be weld
overlay clad rather than roll band clad. Base metal will be designated per notes 1 and 3 above. Refer to PVM-MS-1322.

513 Application Criteria for Common Pressure Vessels Materials

Carbon Steel
Carbon steel with a 1/8- to 1/4-inch corrosion allowance is the economical material
selection for a large percentage of pressure vessels in refinery, chemical plant, and
producing applications. Carbon steels for pressure vessels have a nominal composi-
tion of iron with about 1% manganese and up to 0.35% carbon. Higher carbon
results in poor weldability. In general, carbon steels are readily available and easily
fabricated. A discussion of some limitations follows:
Brittle Fracture. Carbon steels may be susceptible to brittle fracture at normal
ambient temperatures. Refer to Section 520, “Selection of Materials for Brittle Frac-
ture Prevention.”
Hydrogen Attack. Carbon steel will suffer hydrogen attack at elevated temperature
in high pressure hydrogen. Material selection should be based on the “Nelson
Curves.” Refer to the Corrosion Prevention and Metallurgy Manual and American
Petroleum Institute API RP 941 (available in the Corrosion Prevention and Metal-
lurgy Manual).
Graphitization. Welded carbon steel is limited to 800°F maximum to prevent
graphitization. Graphitization is the formation of graphite, primarily in weld heat-
affected zones, from the decomposition of iron carbides. Graphitized steel can fail
under small loads or strains.
Stress Corrosion Cracking (SCC). As-welded or cold-worked carbon steel is
susceptible to stress corrosion cracking in caustic, nitrate, carbonate, amine solu-
tions and in anhydrous ammonia. Stress relief is required to prevent failures. More

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information is given in the Corrosion Prevention and Metallurgy Manual. Consult a

materials specialist for specific applications.
Sulfide Stress Cracking (SSC). High strength steel and hard welds in steel in
aqueous solutions containing H2S are susceptible to sudden nonductile failures.
Controlling maximum strength and hardness is generally sufficient to prevent
cracking. The Company’s pressure vessel specifications limit steel strength and weld
hardness to prevent cracking. Postweld heat treatment may also be beneficial to
prevent cracking. Refer to Section 1000 of the Piping Manual and Section 360 of
the Corrosion Prevention and Metallurgy Manual for more detailed discussion of
material considerations for H2S services.
Hydrogen-Induced Cracking. Some low strength carbon steels may be suscep-
tible to hydrogen-induced cracking (HIC) in wet services containing H2S. Blis-
tering is one example of this type of cracking. Stress Oriented Hydrogen Induced
Cracking (SOHIC) is a specialized type of HIC that has in some cases resulted in
through-wall cracks in carbon steel pressure vessels. Refer to the Corrosion Preven-
tion and Metallurgy Manual for additional details. Steel makers offer steels made
with very low sulfur contents and calcium treated for inclusion shape control to
resist HIC. Standard tests are available to evaluate the HIC resistance of steel plates.
Specification of HIC resistant steels is covered by the supplemental requirements of
PVM-MS-4749 and PVM-MS-4750. Chevron has not traditionally specified these
steels. Postweld heat treatment may also be beneficial to prevent cracking.

Carbon-Moly Steel
Carbon-moly steel is similar to carbon steel but with 0.5% molybdenum added. The
molybdenum improves the steel’s high temperature strength and graphitization resis-
tance. The corrosion resistance is the same as for carbon steel. A discussion of limi-
tations of carbon-moly steels follows:
Brittle Fracture. Unless made to fine-grain practice and normalized, carbon-moly
steels may have poor toughness (increased susceptibility to brittle fracture).
Hydrogen Attack. Experience has indicated that carbon-moly steel cannot be relied
upon to resist hydrogen attack. For new construction, carbon-moly should not be
specified for hydrogen attack resistance. Instead, 1¼ Cr—1½ Mo should be
specified. Refer to the Corrosion Prevention and Metallurgy Manual and API RP
941 for detailed information.
Graphitization. Like carbon steel carbon-moly will graphitize, but carbon-moly is
resistant to a maximum service temperature of 850°F.
Stress Corrosion Cracking. Same as for carbon steel.
Sulfide Stress Cracking. Same as for carbon steel.

Chrome-Moly Steel
Chrome-moly low alloy steels are similar to carbon steel but with chromium and
molybdenum added. Typical grades are 1 Cr-½ Mo, 1¼ Cr-½ Mo, and 2¼ Cr-1 Mo.
The general corrosion resistance of these grades is about equal to that of carbon

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steel, but these grades have better resistance to hydrogen attack and better high
temperature strength. Chrome-moly steels do not graphitize. Chrome-moly steels
are somewhat more difficult to fabricate; they require control of preheat for welding
and postweld heat treatment for all welded construction.
A discussion of limitations of chrome-moly steels follows:
Brittle Fracture. Like carbon steels, chrome-moly steels undergo a ductile-to-
brittle transition at low temperatures and become susceptible to brittle fracture. In
addition, chrome-moly steels in service above about 650°F embrittle in service. The
2¼ Cr-1 Mo steels are particularly susceptible, but 1 Cr-½ Mo and 1¼ Cr-½ Mo
may also be susceptible. The Company's specifications recommend screening tests
on chrome-moly steels to minimize embrittlement.
Hydrogen Attack. Resistance to hydrogen attack is dependent on the chromium
and molybdenum contents in the steel. Resistance improves with increased alloy
content. Refer to the Corrosion Prevention and Metallurgy Manual and API RP 941.
Stress Corrosion Cracking and Sulfide Stress Cracking. Same limitations as for
carbon steel.

Stainless Steel
Stainless steels are alloys of iron and chromium, typically with at least 12% chro-
mium. Additionally, the 300 series stainless steels contain nickel. A term commonly
used for Type 304 stainless steel is 18-8, for 18% chromium-8% nickel. Other
alloying elements such as molybdenum, titanium, and niobium are added for
specific purposes.
Stainless steels are classified as either austenitic, ferritic, martensitic, or duplex
depending on their microstructure.
Austenitic stainless steels have an austenite structure similar to the high tempera-
ture structure of carbon steel. Austenitic stainless steels will not harden with heat
treatment. They are nonmagnetic. Examples are Type 304, 316, 321 and 347. Auste-
nitic stainless steels are readily weldable and are used both for cladding and in solid
wall construction.
Ferritic stainless steels have a ferrite structure similar to the low temperature struc-
ture of carbon steel. Typical examples are Types 405 and 430. Ferritic stainless
steels will not harden with heat treatment. They are magnetic and usually do not
contain nickel. Their use in pressure vessels is primarily as cladding, such as the
Type 405. Solid ferritic stainless construction is limited due to poor weldability.
Martensitic stainless steels can be hardened with heat treatment. They are
magnetic. Type 410 stainless is the most common example. Their use in pressure
vessels is primarily as cladding. Solid martensitic construction is limited due to poor
weldability.
Duplex stainless steels have structures of roughly 50% austenite and 50% ferrite.
They are nonhardenable by heat treatment. The duplex stainless steels are not
currently widely used for pressure vessels but could be considered for both

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cladding and solid wall construction. They have corrosion properties similar to the
austenitics but are higher in strength. They share some of the limitations of both the
ferritics and austenitics.
A discussion of limitations of the stainless steels follows:
Austenitic Stainless Steels in Chloride Solutions. Chloride stress corrosion
cracking of austenitic stainless steels (Types 304, 316, 321, 347, etc.) can occur in
aqueous solutions containing chloride ions. Cracking is most severe where the chlo-
ride ion concentration is high, the solution is hot, the pH is neutral or low, and espe-
cially where evaporation builds up deposits on the stainless steel.
Stainless equipment hydrostatically tested with sea water has failed due to the
residual sodium chloride film left behind. Other failures have been traced to chlo-
rides leaching out of wet insulation. Many failures have resulted from not protecting
stainless equipment from chlorides during shutdowns. There can be an incubation
period of several hours to many weeks before cracking occurs in certain environ-
ments. Cracking can be greatly reduced by stress relieving the stainless equipment
in the 1550°F to 1650°F temperature range. However, complete freedom from chlo-
ride stress corrosion cracking can be assured only by protecting austenitic stainless
steels from any chloride ions or by using the more expensive super stainless grades
with 30% to 45% nickel. Duplex stainless steels have improved resistance to chlo-
ride stress corrosion cracking.
Recommendations to prevent chloride stress corrosion cracking include:
1. Do not select solid wall austenitic stainless steel construction for hot, aqueous
chloride services. If stainless steel is required, use clad construction.
2. Stress relieve vessels made of solid austenitic stainless steel where no other
economical material is available.
Austenitic Stainless Steels in Sulfur-Derived Acids. Sulfur-derived acids can
cause “polythionic acid” stress corrosion cracking of austenitic stainless steels.
Unlike chloride stress corrosion cracking, the austenitic stainless steel must be
sensitized with chromium carbide precipitates along the grain boundaries before
polythionic acid stress corrosion cracking can occur. Sensitization results from
exposure of stainless steel equipment to temperatures in excess of 700°F. If regular
carbon grades such as Types 304 or 316 are used, they may sensitize during
welding.
Neither sulfurous nor polythionic acids are normally found in process units during
operation. However, these acids commonly develop during shutdowns by the oxida-
tion of iron sulfide scale in the presence of moisture and oxygen. They also form in
flue gas condensate.
Freedom from polythionic acid stress corrosion cracking can be assured only by
preventing sensitized austenitic stainless steels from coming in contact with sulfur-
derived acids. Regular grades of austenitic stainless steel (Types 304, 316, etc.)
sensitize easily at temperatures above about 700°F. In fact, the heat of welding is
often enough to sensitize the heat-affected zone. The extra low carbon grades of
stainless steel (Types 304L, 316L, etc.) normally do not sensitize during welding.

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However, they will sensitize with long-term exposure at temperatures above about
700°F. Some austenitic stainless (Types 321 and 347) are chemically stabilized to
minimize sensitization.
Usually polythionic acid cracking is prevented by using the chemically stabilized or
extra low carbon grades of stainless steel and avoiding harmful heat treatments. A
less effective means of prevention is to use regular grades of stainless steel and
require soda ash wash during all shutdowns.
Chromium Stainless Steels in 750°F to 900°F Service. Straight chromium stain-
less steels, such as the ferritic (Types 405 and 430) and martensitic types (Type
410), containing 13% or more chromium can embrittle during exposure to tempera-
tures in the 750°F to 900°F range. This phenomenon is known as 885°F embrittle-
ment. Some of the straight chromium stainless steels are so sensitive to 885°F
embrittlement that even slow cooling through this temperature range will cause
embrittlement. The 885°F embrittlement results in an upward shift in the ductile-to-
brittle transition temperature. Duplex stainless steels are also susceptible to 885°F
embrittlement. Prevent this problem by not using chromium stainless steels for solid
wall construction of pressure vessels.
Stainless Steels Above 1000°F. At elevated temperatures, all stainless steels with
high chromium contents will develop some “sigma phase” which causes embrittle-
ment at lower temperatures. Sigma phase is very hard, nonmagnetic, and brittle. The
composition of sigma phase varies depending on the alloy from which it formed.
Sigma phase normally does not affect the steel's elevated temperature properties but
may make it so brittle at lower temperatures that failures will occur during startup or
shutdown.
The straight chromium ferritic and martensitic stainless steels containing 13% and
more chromium are very susceptible to extensive sigma phase formation at tempera-
tures above about 1000°F. The austenitic stainless steels are not as susceptible
because of their high nickel content, but they can develop damaging amounts of
sigma phase when held between about 1000°F to 1550°F for long periods of time.
Certain highly susceptible austenitic alloys, such as castings and welds, may
develop serious embrittlement in a few hours at temperatures of 1200°F to 1300°F.
Duplex stainless steels are also very susceptible to sigma embrittlement.
Sigma embrittlement is controlled by minimizing ferrite content of stainless steel
welds. Refer to specifications PVM-MS-1322 and PVM-MS-4748. Duplex stain-
less steel is limited to 650°F maximum service temperature to avoid embrittlement.
Sulfide Stress Cracking. The martensitic stainless steels are especially susceptible
to sulfide stress cracking. Welds are difficult to soften with heat treatment and are,
therefore, susceptible to cracking. Low carbon grades, like Type 410S, are used to
limit weld zone hardness.
This cracking is prevented by controlling weld strength and hardness. These
requirements are covered by PVM-MS-4748.

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Other Alloys
Other alloys are not frequently used for pressure vessel construction. Two classes of
alloys occasionally considered are discussed in this section, nickel alloys and tita-
nium alloys.
Nickel Alloys. Examples include Monel, Inconel alloys, Incoloy alloys, and
Hastelloy alloys. Usually very expensive, these alloys are used only for specialized
applications and then usually as cladding. Some nickel alloys have good resistance
to chloride solutions where stainless steels are poor. Fabricating and weldability are
generally good with proper precautions.
Titanium Alloys. These are used infrequently for pressure vessels. Welding is diffi-
cult, requiring very clean conditions. Welding is usually done only in a shop “clean
room,” so field repairs are not practical.

514 Summary of Temperature Limitations

Figure 500-3 summarizes applicable properties and temperature limitations of
commonly used pressure vessel materials.

Fig. 500-3 Maximum Temperature Limits of Common Pressure Vessel Materials, °F

Carbon 18 Cr-8 Ni
Steel C-½ Mo 1¼ Cr-½ Mo 2¼ Cr-1 Mo 12 Cr (410) (304)
Strength(1) (3000 psi) 990 1075 1135 1150 1100 1275
Oxidation (10 mpy 1025 1025 1050 1100 1350 1600
loss)
Graphitization 800 850 N/A N/A N/A N/A
(welded only)
885 Embrittlement N/A N/A N/A N/A 775–950 N/A
Sigma Embrittlement N/A N/A N/A N/A N/A 1100–1700
Hardening on Cooling 1330 1330 1375 1425 1450 N/A
Carbide Precipitation N/A N/A N/A N/A N/A 850–1550
Hydrogen Attack 500 500 1000 1100 N/A N/A
H2pp (750 psi)
Caustic Stress Corro- 140 140 140 140 140 140
sion Cracking
Chloride Stress N/A N/A N/A N/A N/A 140
Corrosion Cracking
Sulfide Stress X X X X X N/A
Cracking
Legend:
X = Susceptible when yield strength exceeds 90 ksi or hardness exceeds Rockwell C22.
N/A = Not applicable

(1) = 100,000 hour stress rupture strength (typical)

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520 Selection of Materials for Brittle Fracture Prevention

521 Definition of Brittle Fracture

Brittle fracture is a sudden, often catastrophic failure which is inherent to “brittle”
materials (discussed below). It involves little or no deformation (yielding), and it
has been experienced in steel structures such as pressure vessels, tanks, pipes, ships,
bridges, beams, and other similar structures, often of welded construction. The most
well known incidents were those occurring to over 200 ships constructed during
World War II, 19 of which completely broke in two. The Company has experienced
two significant cases described below.
The material property “Brittleness” indicates that the material is prone to failure
without deformation. Examples of brittle materials include chalk, brick, glass, and
hardened steels. Brittle structures can, and literally have shattered like glass. Brittle
materials are prone to fracture when they are stressed in the vicinity of a notch or
stress concentration.
The opposite of brittleness is toughness, which for practical purposes can be defined
as a material's ability to resist brittle fracture. Toughness is discussed in more detail
in Section 523. Toughness depends on material strength, thickness, and for steels,
temperature. To resist brittle fracture, higher strength materials and thick materials
require greater toughness than low strength and thin materials. Steels lose tough-
ness as temperature decreases.
Brittle fracture can occur in ferritic steels, such as carbon, carbon-½ moly, chrome-
moly, and 400 series stainless steels, within the normal atmospheric temperature
range. The regular 300 series stainless steels are not susceptible to brittle fracture
until temperatures are below -300°F. However, after exposure above 1100°F, sigma
embrittlement makes 300 series weld metals with large amounts of ferritic phase
susceptible to fracture well above room temperature.
Brittle fractures are infrequent. Most occur during hydrotest rather than in opera-
tion. However, brittle fractures can be catastrophic due to fragmentation of the
structure and fast release of energy. Due primarily to higher quality construction and
maintenance standards, our Company has rarely experienced brittle fractures.
However, to illustrate the importance of this design factor, two incidents of brittle
fracture experienced by Chevron are described below.
Brittle fracture is characterized by a flat fracture surface, and occurs at average
stress levels below those of general yielding. Brittle fracture cracks grow at very
fast speeds (up to 7000 ft/s), so brittle fracture happens quickly and unexpectedly.

522 Examples of Brittle Fracture Experienced by Chevron

1982 Brittle Fracture of Clear Creek LPG Vessel
A 30-year old vessel made of A 212 Grade B carbon steel ruptured unexpectedly on
a winter morning in Wyoming when the temperature was -20°F. The vessel

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contained about 40,000 gallons of LPG. The vessel separated into 13 large pieces,
some of which were thrown 450 feet from the vessel foundation. Fortunately, no one
was injured, and no other plant damage resulted. This vessel’s fabrication practices
were not up to current standards.
Figures 500-4 and 500-5 show two of the pieces of the fractured vessel. The frac-
ture initiated from a preexisting flaw that was about 1.9 inches long by 0.9 inch
deep on the inside of the vessel. The vessel wall thickness was 1 inch. The shell
plate had a Charpy impact toughness of 2 ft-lb at the failure temperature.

Fig. 500-4 Piece of Fractured Clear Creek LPG Vessel: Example 1

Fig. 500-5 Piece of Fractured Clear Creek LPG Vessel: Example 2

1985 Brittle Fracture of Richmond TKC Isomax Steam Generator E-440

The A 182 Grade F-1 carbon-½ moly-steel heat-exchanger channel section frac-
tured during a routine hydrotest after 18 years in service. Metal temperature during
the hydrotest was +50°F. The fracture occurred when the test pressure reached 3400
psig; test pressure was planned for 4500 psig. The steel that the channel was made

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of was purchased without toughness control. At the failure temperature, the steel
had a Charpy impact toughness of 3 ft-lb.
A photograph of the fractured channel is shown in Figure 500-6. An investigation of
the failure showed that a failure of the baffle-to-channel attachment weld occurred
during the hydrotest and initiated the fracture of the channel section. Fortunately, no
one was injured. Plant startup was delayed considerably while the channel was weld
repaired.

Fig. 500-6 Fractured Channel of Heat Exchanger of Richmond TKC Isomax Steam
Generator E-440

523 Design to Prevent Brittle Fracture

Overview
Susceptibility of structures to brittle fracture depends on:
1. Preexisting flaw size
2. Tensile stress level
3. Fracture toughness of the material
Flaw size and stress level are controlled by design, fabrication, and inspection in
accordance with the ASME code. Toughness is controlled by material selection, also
in accordance with the ASME Code.
To prevent brittle fracture, keep flaw sizes small and stress levels low, and use tough
materials. Toughness is a physical property of materials that primarily

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characterizes their resistance to brittle fracture, depending on temperature, loading

rate, and thickness.
Sufficiently tough steels are selected by one of the following:
1. Using materials selection curves, or impact exemption curves, that are in the
ASME Code
2. Using steels that have been Charpy V-Notch (CVN) impact tested to Code
requirements
Using steels selected from the ASME Code impact test exemption curves is highly
preferred. Steel selection is discussed in more detail later in this chapter.

Background: Transition Temperature Approach to Fracture Control

The ferritic steels (carbon, low alloy, and 400 series stainless steels) all undergo a
ductile-to-brittle transition as temperature is lowered. Each of these steels has a
ductile-to-brittle transition temperature range. Above their transition temperature
range these steels are tough; at and below the transition temperature range, they can
fracture in a brittle manner.
Toughness is the ability of a material to absorb energy by yielding (plastic deforma-
tion) prior to failure. Toughness depends on a material’s ductility and strength.
Toughness therefore indicates the material’s ability to resist brittle fracture.
One measure of toughness is the area under a tension stress-strain curve taken to
failure. However, the standard method for pressure vessel applications is an impact
test, which measures the energy to fracture of a specimen under very high strain
rates (sudden impact).
Today the most widely used test for establishing the toughness of low strength steels
is the Charpy V-Notch (CVN) impact test. This test has been chosen from several
available because it correlates well with numerous failures, including World War II
ship fractures. The CVN test is used to make sure the transition temperature of the
steel is below the minimum loading temperature of the vessel. This is known as the
transition temperature approach.
Figure 500-7 illustrates CVN impact test results for a carbon steel. The CVN transi-
tion temperature is defined as the minimum temperature above which the material
requires more than some specified energy to break.
The energy required to establish transition temperature increases with increasing
steel strength. Other definitions of transition temperature, such as those based on
fracture appearance, are not widely used in codes and specifications because of
difficulties in interpretation. Note that the notch toughness of steels shows a temper-
ature transition no matter what test method is used. The actual transition tempera-
ture measured will vary somewhat, depending on the test method employed.
The CVN transition temperature approach was developed empirically after World
War II from analysis of ship failures. Samples from over 100 structural failures in
merchant ships were studied and statistically analyzed. Fracture initiation in those
steels was found to be difficult above a transition temperature corresponding to a

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Fig. 500-7 Illustration of Typical CVN Impact Test Data

Notes:
1. These data illustrate the variation of CVN energy with temperature and with
the orientation of test specimen relative to the direction of principal working.
2. These data must not be considered typical. Wide variation may result even
from specimens from plates of the same specification and thickness.

CVN impact energy of 10 ft-lb. Crack propagation was found to be difficult above a
temperature corresponding to 15 to 25 ft-lb. From these findings, a 15 ft-lb CVN
requirement at the minimum loading temperatures became a widely used fracture
criterion.
In the CVN impact test, notched bars are hit with a swinging pendulum. Specimens
are broken over a range of test temperatures and the energy to break the specimen is
recorded as a function of test temperature. CVN impact test results are in units of
ft-lb (English units). (See ASTM A 370 for more details on CVN testing.)
With the development of the fracture mechanics approach, it became apparent that
CVN requirements to establish transition temperature were dependent on material
yield strength and thickness. Energy requirements to establish the transition temper-
ature increase with yield strength and thickness.
ASME Code, Section VIII, Division 1, requirements for CVN energy for pressure
vessel steels are given graphically in Figure UG-84.1 of the Code. Steels may be
exempted from tests if they meet requirements shown in Figure UCS-66 of this
Code, as discussed later in this chapter.

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ASME Code, Section VIII, Division 2, requirements for CVN energy for pressure
vessel steels are given in Code Table AM-211.1. The current Division 2 require-
ments are less conservative than the Division 1 requirements and do not take into
account the need for higher CVN energy with increasing thickness. Steels may be
exempted from tests if they meet requirements shown in Paragraph AM-218 of this
Code, also discussed later in this chapter.

524 Recommended Practice for Selecting Steels for New Construction of

Pressure Vessels
The recommended practice for selecting steels for new construction of pressure
vessels is to ensure that the vessel temperature is above a minimum temperature
during “loading of the equipment.” This minimum temperature is defined as the
“minimum pressurizing temperature.” Loading includes operation, hydrotest, pres-
sure test, shutdowns, and startups. Specific recommended practices for Division 1
and 2 vessels are described below.
Note that both examples of brittle fracture failure discussed above in Section 522
involved pressurizing (loading) the vessels below the steel's transition temperature.
The LPG vessel steel which fractured at -20°F had a CVN transition temperature of
about +80°F; the steam generator steel which fractured at about +50°F had a CVN
transition temperature of about +200°F.

ASME Code, Section VIII, Division 1, Vessels

Minimum Pressurizing Temperature. Company practice is to specify a minimum
pressurizing temperature (MPT) or minimum design metal temperature (as defined
in Code Paragraph UG-20). The MPT is the lowest temperature at which a pressure
greater than 40% of the maximum allowable working pressure should be applied to
the vessel. Below 40% of the maximum allowable working pressure, stresses are
considered low enough to essentially eliminate the risk of brittle fracture in the
absence of significant other stresses (such as those due to weight and differential
thermal expansion). Due to increases in ASME code allowable stresses for Division
1 vessels built in 1999 and later, MPT is the lowest temperature at which a pressure
>35% of MAWP should be applied to vessels built in 1999 and later. Section 243 of
the Corrosion Prevention and Metallurgy Manual has more detail regarding this
change.
To establish a minimum design metal temperature for new equipment, startup
temperature and reasonably expected abnormal operating temperatures should be
considered, as well as normal operation. (Recommendations for autorefrigeration
conditions are discussed later in this section.) The best available local weather data
should be used to establish startup temperatures if the equipment is not normally
preheated. If local temperature data are not available, the lowest 1-day mean
temperature shown in Figure 2-2 of API-650 can be used. (API-650 is included in
the Tank Manual.)
Materials Selection Requirement. One of two methods is used to assure steels are
used above their transition temperature:

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• Impact test exemption curves

• Charpy V-Notch impact testing
Applying these methods is shown graphically in Figure 500-8.

Fig. 500-8 Simplified Overview of Design for Brittle Fracture Under ASME Code, Section VIII, Division 1.
Courtesy of the ASME

The impact-test exemption curves are preferred to CVN impact testing where prior
data or service experience are available. CVN tests increase materials costs substan-
tially (approximately 2 to 10 cents/pound) and complicate delivery.
Code Figure UCS-66, Impact-Test Exemption Curves, gives the application points
(combinations of thickness and minimum design metal temperature) where prior
data or service experience show specific steels have sufficient toughness for frac-
ture-safe design; i.e., they are above their transition temperature. The application
point is the point corresponding to the thickness and minimum pressurizing or
design metal temperature. A steel has adequate toughness if the application point is
above the steel's curve. Figure 500-9 is a schematic illustration which resembles

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Figure UCS-66. It is for reference only. Do not use Figure 500-9 for design. Figure
UCS-66 first appeared in the 1987 Addenda to the 1986 Code.

Fig. 500-9 Schematic Illustration of Impact Exemption Curves in ASME Code, Section VIII,
Division 1. Do not use this figure for design. Refer to the latest Code Figure
UCS-66. Courtesy of the ASME

To use a steel at an application point below that steel’s curve, CVN impact testing is
required to prove adequate toughness. Requirements for CVN impact testing are
discussed in the next section. The Company’s preference is to use steel with suit-
able inherent toughness rather than requiring impact tests which can cause unneces-
sary delays and expense.
Follow the requirements of Code Paragraph UCS-66 and Figure UCS-66 to select
steels for pressure vessels and to establish the need for impact testing. However, the
following restrictions on the use of UCS-66 are recommended:
1. All grades of SA 285 and SA 515 steels thicker than ¾ inch should be assigned
to Curve A rather than to Curve B. This is more conservative than the require-
ments of Figure UCS-66. SA 285 is often a semikilled steel and SA 515 is

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made to coarse grain practice, so both tend to have poorer impact transition
temperatures (higher) than Curve B indicates.
2. The impact test exemption allowed in Paragraph UG-20(f) should not be
allowed. UG-20(f) eliminates the impact test requirements for carbon steel
1 inch or less in thickness for most pressure vessels. It is better to choose steels
from Figure UCS-66 that do not require testing for the specific application.
Note that Code Paragraph UCS-68(c) allows a 30°F reduction in impact testing
exemption temperature for P-1 materials (carbon steel) that are given postweld heat
treatment (PWHT) and PWHT is not otherwise required by Code. This reduction
should be allowed.
Charpy V-Notch Impact Testing. When a steel is to be used at an application point
below its curve on Figure UCS-66 Impact Test Exemption Curves, CVN impact
testing is required to prove adequate toughness. Requirements for CVN testing are
summarized here.
1. Each plate, forging or pipe used at an application point below its impact test
exemption curve is tested. Usually each plate is tested, while forgings and pipe
are tested in accordance with specifications such as SA 350 and SA 333,
respectively.
2. Three test specimens taken transverse to the major working direction (during
steel making) are tested. It is important that transverse rather than longitudinal
specimens be used because transverse properties are generally poorer. Seam-
less pipe is an exception because its properties do not vary much with orienta-
tion. ASTM A-370 defines transverse and longitudinal CVN specimens.
Specimen orientation is a Company requirement. The Code leaves orientation
optional.
3. The maximum (warmest) allowable CVN test temperature is the minimum
pressurizing or design metal temperature.
4. Minimum CVN energy requirements are in accordance with Code Figure
UG-84.1. Figure UG-84.1 shows CVN energy requirement as a function of
specified minimum yield strength and thickness.
5. When impact testing is required on the parent metals, impact testing of the
heat-affected zone (HAZ) and deposited weld metal is required on the Welding
Procedure Qualification Test Plate (WPQT) or Production Test Plate. See
ASME Code, Section VIII, Division 1, Paragraph UG-84, for definitions of
these terms. See also Code Paragraph UCS-85 concerning heat treatment of test
specimens. Test specimen heat treatment must simulate actual vessel heat treat-
ment.

ASME Code, Section VIII, Division 2, Vessels

Minimum Pressurizing Temperature. ASME Code, Section VIII, Division 2,
Paragraph AD-155, defines the minimum permissible vessel metal temperature for
ferrous metals other than austenitic, that is, for carbon and low alloy steels. The
Code defines both a minimum service temperature and a minimum pressure test

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temperature. For postweld heat-treated vessels, the service temperatures and pres-
sure test temperatures are equal. For as-welded vessels, the pressure test tempera-
ture is 30°F higher than the service temperature for most vessels. At metal
temperatures below the minimum service temperature, pressure applied to the vessel
must be less than 20% of the required test pressure. This will usually be an applied
pressure of 25% of the design pressure.
Materials Selection Requirements. Either impact test exemption curves or CVN
impact testing are used to assure that steels are above their transition temperature.
Requirements are similar to the Code, Section VIII, Division 1, requirements
discussed previously in this section of the manual.
The ASME Code, Section VIII, Division 1, Impact Test Exemption Curves have
been more recently revised than those in Code, Division 2. Paragraph AM-218 spec-
ifies the Code, Division 2, Impact Test Exemptions.
Guidelines for use of the Code, Division 2, Figure AM-218.1, Impact Test Exemp-
tion Curves, are given in Specification PVM-MS-4749, which specifies the use of
Code, Division 2, Figure AM-218.1, with the exemption curve materials assign-
ment given in Figure 500-10.

Fig. 500-10 Recommended Exemption Curve Materials Assignment for Section VIII, Division 2 (Figure AM-218.1)
Plate Forgings Pipe
Curve I
SA 36 (Nonpressure containing attachments only, ≤ ¾" thick)
Curve II
SA 285 SA 105 SA 53
SA 515 SA 181 SA 106
SA 387 (annealed) SA 366 & 182 (annealed) SA 355 (annealed)
Curve III
SA 516 if not normalized
SA 387, Gr. 11 & 12(1) SA 182 or SA 336, Gr. 11 & 12(1) SA 335, Gr. P11 & 12(1)
(1)normalized and tempered

Curve IV
SA 387, Gr. 21 & 22(1) SA 182 or SA 336, Gr. F21 & 22(1) Sa 335, Gr. P21 & 22(1)
(1)normalized and tempered

Curve V
Sa 516 normalized SA 350, LF 1 & 2(1) SA 333, Gr. 1 & 6(1)
SA 537, Cl 1
(1) SA 350 LF 1 & 2 and SA 333 Grades 1 and 6 are acceptable for minimum design temperatures down to -50°F, without additional impact
testing

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These guidelines are slightly more conservative than Code, Division 1, Figure
UCS-66, especially at thickness less than 1½ inches.
For Division 2 vessels, impact tests are required for all carbon and low alloy steels
thicker than 3 inches regardless of minimum pressurizing temperature.
Charpy V-Notch Impact Testing. The ASME Code, Division 2, requirements are
given in AM-204 through AM-218. CVN impact tests are required when a steel is to
be used at a combination of metal temperature and thickness below its exemption
curve.
The impact test energy requirements of Code, Division 1, Figure UG-84.1, are
recommended in place of the Code, Division 2, requirements of Table AM-211.1.
Table AM-211.1 requirements are less conservative than Figure UG-84.1 require-
ments and do not take into account the need for higher CVN energy with increasing
thickness.

525 Typical Carbon Steel Selections to Avoid Brittle Fracture in Pressure

Vessels
Figure 500-11 illustrates typical carbon steel selections for pressure vessels over a
range of thicknesses and minimum pressurizing temperature.

Fig. 500-11 Typical Carbon Steel Selections to Avoid Brittle Fracture in Pressure Vessels
Minimum Shell and Head Plates
Pressurizing
Temp., °F 1 in. Thick 2 in. Thick Nozzle Forgings Pipe
(1),(3) (1),(2),(3)
-50° to 0 Normalized SA 516 or Normalized SA 516 SA 350 LF2 SA 333
SA 537 Class 1 or SA 537 Class 1
0 to 30 Normalized SA 516 or Normalized SA 516 SA 105(3),(4) SA 106(3),(4)
Impact Tested, As-Rolled
SA 516
30 to 60 As-Rolled SA 516, or Normalized SA 516 SA 105(3),(4) SA 106(3),(4)
Impact Tested SA 285
Warmer than 60 SA 285 or As-Rolled Normalized SA 516 SA 105 SA 106
SA 516
(1) Plates may require impact testing; see Code, Division 1, Figure UCS-66.
(2) The SA 516 specification requires plates 1½ inch and thicker to be normalized.
(3) Impact testing of the heat-affected zone (HAZ) and deposited weldmetal is required on the Welding Procedure Qualification Test Plate
when impact testing is required on the parent metal.
(4) Forgings and pipe may require impact testing; see Code, Division 1, Figure UCS-66.

526 Steel Selection for Pressure Vessels Subject to Autorefrigeration

Autorefrigeration should be considered when selecting steels, as described below. In
some liquid services such as LPG, a leak could reduce the pressure and cause a drop
in temperature of a pressure vessel and its contents as the liquid boils off.

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The autorefrigeration of temperature is defined here as the temperature that the

contents of the vessel would reach if the vessel is depressured to 40% of its
maximum allowable working pressure. If the temperature of autorefrigeration is less
than 20°F, then the vessel should be treated as subject to autorefrigeration, and this
used as a design basis to avoid brittle fracture.
(Due to an increase in ASME code allowable stresses in 1999, vessels built in 1999
and later will have autorefrigeration temperatures equal to what the contents would
reach if the vessel is depressured to 35% of its MAWP. Section 243 of the Corro-
sion Prevention and Metallurgy Manual discusses this further. Consult a specialist
to determine if this change applies to your situation.)
Vessels that are subject to autorefrigeration require additional consideration as
follows:
1. Steels from Curve D of Code, Division 1, Figure UCS-66, should be used.
Typically, carbon steel plate steel should be normalized SA 516. Forgings may
be SA 350, Grade LF 2, and pipe may be SA 333, Grades 1 or 6. These steels
have good inherent toughness.
2. Impact testing is not required for autorefrigeration, unless already required at
the normal design temperature. SA 350 and SA 333 materials are, however,
impact tested in accordance with their respective specifications.
Autorefrigeration is not considered equivalent to a cold design or operating temper-
ature due to the lowered pressure. Therefore, the recommended safeguards against
brittle fracture are not as stringent as for a cold operating temperature. The use of
SA 516 steel, and equivalent forging and piping grades, should by itself provide
ample resistance to brittle fracture during autorefrigeration. Impact testing is not
required for autorefrigeration, unless it is required for a cold design temperature
without considering autorefrigeration.

527 Factors Controlling Susceptibility to Brittle Fracture: Additional

Technical Information
General Information
Careful attention to materials selection, design, fabrication, and inspection tech-
niques is necessary to achieve fracture-safe designs. This section of the manual
deals primarily with materials selection. But proper selection of materials alone is
not enough to prevent brittle fracture. Attention in design to avoid stress concentra-
tions, such as notches, and attention to inspection and nondestructive examination
during fabrication to find and eliminate cracks and flaws are necessary to minimize
the risk of brittle fracture. A hydrostatic test to stresses higher than design stresses is
another important factor to reduce the risk of subsequent brittle fracture.
Fracture mechanics is an analytical tool for quantitatively relating the factors
controlling susceptibility to brittle fracture. Fracture mechanics is used only in crit-

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ical applications justifying in-depth engineering analysis. The principal factors to be

considered in fracture mechanics are:
1. Flaw size—The basic theory of fracture mechanics is that brittle fracture
initiates at preexisting flaws (cracks). Welded structures are rarely flaw-free.
Inspection during fabrication is necessary to limit as-built flaw sizes. Fatigue
and stress corrosion cracking can also cause flaws to increase in service, thus
periodic inspections are necessary.
2. Stress—Tensile stress is necessary for brittle fracture to occur. Since stress
concentrations increase nominal stress to cause high local stresses and greater
susceptibility to fracture, they should be minimized. Residual stress from
forming and welding can also affect local stress levels. Postweld heat treatment
(PWHT) lowers residual stress. Thus, PWHT generally lowers the risk of brittle
failure.
3. Fracture toughness—Notch toughness is the ability of a material to deform
plastically in the presence of a notch or crack. Thus, tough materials are resis-
tant to brittle fracture. Fracture toughness is a material property just as strength
is. Notch toughness is a qualitative term, describing a material's resistance to
fracture. Fracture toughness is quantitative and is measured according to the
principles of fracture mechanics.
Relationship of Flaw Crack Size, Stress, and Notch Toughness. Fracture
mechanics uses stress analysis techniques to define a stress intensity factor (KI)
which is proportional to the product of stress and the square root of flaw size. Frac-
ture occurs when the stress intensity factor exceeds a critical value. For a given
material, the critical value is a function of temperature, loading rate, and thickness.
For slow loading rates, the critical value is applicable to static loads and is desig-
nated KIc. For dynamic loading (as in impact tests), the critical value is designated
KId. We are usually interested in the KIc values for essentially static loading condi-
tions. However, we must also consider the possibility of more rapid loading rates,
which can cause much lower KId values (see Figure 500-12).
KIc (or KId) is a material's fracture toughness at a given temperature and loading
rate. It is a material property, as yield and tensile strength are. Steels with a guaran-
teed KIc are not commercially available, so we have to use other methods to specify
and purchase, such as Charpy V-Notch testing.
Figure 500-13 illustrates limits of allowable stress and crack size combinations for
different KIc values. Above the curve for a given fracture toughness, fracture occurs.
Figure 500-13 shows that at a given stress level, a larger flaw is tolerable as tougher
materials are used.

Variables Which Affect Notch Toughness

Temperature. The ferritic steels (carbon, low alloy, and 400 series stainless)
undergo a ductile-to-brittle transition as temperature is lowered. Each of these steels
has a ductile-to-brittle transition temperature range. Above their transition tempera-
ture range these steels are tough; in and below the transition temperature range, they

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Fig. 500-12 Schematic of the Shift in Transition Temperature Due to Loading Rate

can fracture in a brittle manner. Section 523 discusses transition temperature in

more detail.
Factors that affect transition temperature of steels are:
1. Composition—Carbon content has the most effect, and transition temperature
decreases (toughness improves) with decreased carbon content. Increasing
manganese content contributes to a lower transition temperature up to a manga-
nese content of about 2 wt %.
2. Deoxidation practice—Fully killed (fully deoxidized) steels have lower transi-
tion temperatures than semikilled or rimmed steels.
3. Grain size—Fine grained steel gives a lower transition temperature.
4. Heat treatment—Normalized or quenched and tempered steels have lower
transition temperature ranges than as-rolled steels of similar composition. Grain
refinement is a reason.
5. Welding—Welding generally results in a higher transition temperature in the
weld heat-affected zones as compared to the base material. The variables which
improve transition temperature in the base metal, as listed above, also help in

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Fig. 500-13 Relationship of Toughness, Stress, and Crack Size

the heat-affected zone. High carbon content is particularly detrimental because

it causes harder, more brittle heat-affected zones.
Most brittle fractures initiate near welds. Stress concentrations at the weld toe,
weld defects, and residual stresses are more often causes than is poor heat-
affected zone notch toughness. Postweld heat-treated structures generally have
better brittle fracture resistance.
6. Embrittlement phenomena—Certain metallurgical phenomena are damaging
to specific alloys. Temper embrittlement of 2¼ Cr-1 Mo steel after exposure at
650°F to 1000°F is one example. Special guidelines have been developed for
fracture prevention of thick 2¼ Cr-1 Mo hydroprocessing reactors which
operate in the embrittlement range. Some 400 series stainless steels suffer
“885°F embrittlement” in the 650°F to 900°F range. Both temper embrittle-
ment and “885°F embrittlement” cause an unfavorable upward shift of the tran-

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sition temperature, such that the embrittled alloys have poor notch toughness at
atmospheric temperature.
Loading Rate. The toughness of low strength steels decreases with increasing
loading rate. At a given temperature, the toughness measured in an impact test is
lower than the toughness measured in a static test. Figure 500-12 shows a sche-
matic representation of shift in transition temperature due to loading rate. The
magnitude of the shift depends on the yield strength of the material.
Thickness. The fracture toughness of a particular material decreases with increasing
section thickness for two reasons. First, it is metallurgically more difficult to obtain
good toughness properties as thickness increases. Second, thicker sections produce
greater constraint ahead of the notch due to a triaxial state-of-stress. Beyond some
limiting thickness, maximum constraint is obtained (called plane strain), and notch
toughness approaches a minimum value (KIc). Thin
materials have a biaxial state-of-stress (called plane stress), so have less constraint
to plastic flow and act in a more ductile manner.
Due to the severe state-of-stress, thicker sections need to have better fracture tough-
ness in order to have resistance to brittle fracture equivalent to thinner sections.

530 Guidelines for Preventing Brittle Fracture in Existing Equipment

The general rule to follow is to limit a pressure vessel to less than 40% of its
maximum allowable working pressure any time the vessel metal temperature is
below the minimum pressurizing temperature (35% of MAWP for vessels built in
1999 and later). Also, be sure there are no other significant stresses such as those
due to weight and differential thermal expansion. For vessels without a specified
minimum pressurizing temperature, one can be established using the following
guidelines. From a knowledge of the steel types and thicknesses of the vessel, a
minimum temperature for hydrotest, startup, or operation can be established, as
discussed below.

531 Determining MPTs

Guidelines to Determine MPT for Existing Vessels
Note that “minimum pressuring temperature” (MPT) is the same as “minimum
design metal temperature” (MDMT).
Most vessels built since 1969 to Company specifications should already have MPT
on the vessel drawings, or on Safety Instruction Sheets (SISs).
For carbon and low alloy steel vessels without an MPT specified, Figure UCS-66,
along with materials of construction and vessel thickness, can be used to establish
MPT for vessels up to 6 inches thick. For vessels made with materials shown on
Figure UCS-66, the MPT can be established directly from the curves if component
thicknesses are known. For materials not listed in UCS-66, see the next subsection.

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The MPT is determined by the intersection of component thickness with the appro-
priate curve on Figure UCS-66. Consider only welded parts such as shells, heads,
channels, and integrally reinforced nozzles for MPT. The component with the
highest MPT sets the MPT for the vessel.
Reinforcing pad nozzles or small nozzles without reinforcement do not need to be
considered for MPT unless the risks of brittle fracture (i.e., low temperature, autore-
frigeration, adjacent facilities, etc.) warrant extra precaution. Fractures of vessels
with reinforcing pad nozzles generally occur in the shell plate at the reinforcing-
pad-to-shell fillet weld. Integrally reinforced nozzles, however, should be consid-
ered for MPT.
The following guidelines clarify the use of Figure UCS-66 to establish MPT:
1. The thickness of vessel components refers to the thickness at a weld.
2. An MPT does not need to be established for nonwelded parts like cover flanges
or heat exchanger channel covers.
3. To use the curves for normalized material, vessel records must indicate normal-
ized material was used.
4. For P-1 carbon steel vessels that were stress-relieved but were not required to
be stress-relieved by Code, the MPT may be 30°F lower than is given by the
exemption curve on Figure UCS-66. This is consistent with Code Paragraph
UCS-68(c). Normally carbon steel vessels 1¼ inch and less in thickness are not
required to be stress-relieved by Code rules. See ASME Code, Section VIII,
Division 1, Table UCS-23, to determine whether a steel is a P-1 steel.
5. All grades of SA 285 and SA 515 steels thicker than 3/4 inch should be
assigned to Curve A rather than to Curve B.

Guidelines to Determine MPT for Existing Vessels: Assign Obsolete Steel

Specifications to Curve A
The purpose of this section is to identify several obsolete steel specifications used
for the construction of many vessels in the past. All steels listed below, except for
the Code Case steels, should be assigned to Curve A of Figure UCS-66. Any other
steels not listed also should be assigned to Curve A unless sufficient documentation
is available for assignment to a lower curve.
• 1934 API-ASME Code Steels
ASTM A-7 ASTM A-113
ASTM A-10 ASTM A-149
ASTM A-30 ASTM A-150
ASTM A-70

• 1934 ASME Section VIII Steels

ASME S-1 ASME S-26

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ASME S-2 ASME S-27

ASME S-25
• Later Codes
ASTM A 201
ASTM A 212

• Code Case Steels

Code Case 1256 is equivalent to SA 442. (Use Curve B. If normalized, use
Curve D.)
Code Case 1280 is equivalent to SA 516. (Use Curve B. If normalized, use
Curve D.)
• Product Forms Other than Plate
Obsolete specifications for tubes, pipe, forgings, and castings should be
assigned to Curve A unless specific data to the contrary are available.

Past Chevron Practice in Establishing MPTs

An Engineering Department letter dated July 8, 1983, titled “Preventing Brittle
Fracture,” outlined an approach to establishing MPTs for older vessels using
Chevron's impact exemption curves. Chevron developed impact exemption curves
(similar to Figure UCS-66) before they were included in ASME Code, Section VIII,
Division 1.
With Figure UCS-66 now available in the Code, Division 1, Chevron's curves have
been archived. MPTs established on the basis of the 1983 letter are still valid and
need not be changed. Figure UCS-66, when used as recommended above, and
Chevron's former exemption curves give similar MPTs. However, future MPTs
should be based on the latest Code, Figure UCS-66.

API Guide for Prevention of Brittle Fracture of Pressure Vessels

API Guide, API-RP-920, is due to be published in 1990. The Chevron practice
recommended above and based on Code, Figure UCS-66, is a recognized valid
method in the API Guide draft. This API Guide is in the final stages of preparation.

Chevron Corporation 500-29 April 2000