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Chevron Corporation 500-1 April 2000 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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Page 1: CHEVRON Pressure Vessel -  Materials

500 Materials

AbstractTwo 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 EnvironmentThis 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 FactorsDesign factors to consider include:

• Operating temperature and pressure• Service environment• Cost• Design life• Reliability and safety

Operating Temperature and PressureOperating temperature and pressure limit the choice of materials and can havesignificant 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 temperatu800°F. Above 800°F, the strength of carbon steel decreases significantly and thsteel may embrittle because of graphitization.

Corrosion rates frequently increase with temperature. In sour hydrocarbon servfor example, bare carbon steel is limited to about 550°F because corrosion accates at higher temperatures.

Operating pressure can also influence the stability of a material in the service eronment. Hydrogen attack of steels in high pressure hydrogen at elevated temptures is an example.

Service EnvironmentMaterials are selected to limit corrosion to acceptable, economic rates in the seenvironment. “Service environment” as used here means what the vessel will contain, its temperature and pressure, any contaminants, physical state, and sotimes flow rate. For a given service environment, materials selection should bemade with consideration for both corrosion rates and other potential deterioratiomechanisms, such as stress corrosion cracking and hydrogen damage.

Information about corrosion rates can be obtained from several sources. Past eence is the best source if there is a vessel in similar service. A review of the instion records for vessels in similar services can indicate whether the materials selection was correct and what corrosion rates may be expected. The comparisshould also include a review of the similarity of the new and old service environments.

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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.

CostThe 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 capplied to reduce corrosion rates and the need for alloy material.

Design LifeThe 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 accesible, 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 onexpected corrosion rate. Corrosion allowances are discussed in more detail in tCorrosion Prevention and Metallurgy Manual, but recommendations for pressure vessels are summarized in Figure 500-1. If the corrosion allowance required toachieve the design life is greater than ¼ inch, then a more corrosion-resistant aor a clad vessel is generally economical.

Reliability and SafetyThe likelihood and consequences of a failure must be considered in the selectiopressure vessel materials. Consideration of these factors may lead to conclusiomaterials and corrosion allowances that differ from these minimum guidelines.

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500 Materials Pressure Vessel Manual

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 SelectionsFigure 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.

Fig. 500-1 Typical Corrosion Allowances for Pressure Vessels

Type of Vessel Recommended Minimum, Inch

Large Heavy Wall Vessels Made of Carbon and Low Alloy Steels

3/16(1)

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.

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Pressure Vessel Manual 500 Materials

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 varies widely

(1)

Sweet hydrocarbons, less than 1 ppm H2S

Carbon steel May corrode even with trace H2S above 550°F

(1)

Sour hydrocarbons, with more than 1 ppm H2S below 550°F

Carbon steel Limited to 550°F Maximum (1)

Sour hydrocarbons, with more than 1 ppm H2S above 550°F

Carbon steel clad with 12% Cr stainless steel

(2)

Sweet hydrogen, such as in catalytic reformers, hydrogen and ammonia plants

Carbon steel, 1¼ Cr-½ Mo, and 2¼ Cr-1 Mo steel

Choice depends on temperature and hydrogen partial pressure. See API Recommended Practice 941 and the Corrosion Preven-tion and Metallurgy Manual.

(1), (3)

Sour hydrogen; may also contain hydrocarbon. Examples include hydroprocessing unit and hydrotreated process streams.

Carbon steel, 1¼ Cr-½ Mo, and 2¼ Cr-1 Mo Often clad with Type 321 or 347 stainless steel

Choice depends on temperature and on hydrogen and H2S partial pressure. See API Recom-mended Practice 941 and the Corrosion Prevention and Metal-lurgy Manual.

(4)

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

(1)

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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513 Application Criteria for Common Pressure Vessels Materials

Carbon SteelCarbon 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 Fture Prevention.”

Hydrogen Attack. Carbon steel will suffer hydrogen attack at elevated temperatin 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 heataffected zones, from the decomposition of iron carbides. Graphitized steel can 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 sotions and in anhydrous ammonia. Stress relief is required to prevent failures. M

Sulfuric Acid ( ≥ 85% concentration)

Carbon steel Velocity above about 3 fps and 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.

(1)

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.

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

Service Typical Materials Comments Notes

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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 SteelCarbon-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 isresistant 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 SteelChrome-moly low alloy steels are similar to carbon steel but with chromium andmolybdenum added. Typical grades are 1 Cr-½ Mo, 1¼ Cr-½ Mo, and 2¼ Cr-1 The general corrosion resistance of these grades is about equal to that of carbo

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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.2¼ Cr-1 Mo steels are particularly susceptible, but 1 Cr-½ Mo and 1¼ Cr-½ Momay also be susceptible. The Company's specifications recommend screening on chrome-moly steels to minimize embrittlement.

Hydrogen Attack. Resistance to hydrogen attack is dependent on the chromiumand molybdenum contents in the steel. Resistance improves with increased allocontent. 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 SteelStainless steels are alloys of iron and chromium, typically with at least 12% chrmium. Additionally, the 300 series stainless steels contain nickel. A term commused 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 duplexdepending on their microstructure.

Austenitic stainless steels have an austenite structure similar to the high tempeture structure of carbon steel. Austenitic stainless steels will not harden with hetreatment. They are nonmagnetic. Examples are Type 304, 316, 321 and 347. Anitic stainless steels are readily weldable and are used both for cladding and inwall construction.

Ferritic stainless steels have a ferrite structure similar to the low temperature sture of carbon steel. Typical examples are Types 405 and 430. Ferritic stainlesssteels will not harden with heat treatment. They are magnetic and usually do nocontain nickel. Their use in pressure vessels is primarily as cladding, such as thType 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 pressuvessels is primarily as cladding. Solid martensitic construction is limited due to pweldability.

Duplex stainless steels have structures of roughly 50% austenite and 50% ferritThey are nonhardenable by heat treatment. The duplex stainless steels are notcurrently 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 ride stress corrosion cracking can be assured only by protecting austenitic stainsteels from any chloride ions or by using the more expensive super stainless grwith 30% to 45% nickel. Duplex stainless steels have improved resistance to chride stress corrosion cracking.

Recommendations to prevent chloride stress corrosion cracking include:

1. Do not select solid wall austenitic stainless steel construction for hot, aquechloride services. If stainless steel is required, use clad construction.

2. Stress relieve vessels made of solid austenitic stainless steel where no otheconomical 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 besensitized with chromium carbide precipitates along the grain boundaries beforpolythionic acid stress corrosion cracking can occur. Sensitization results from exposure of stainless steel equipment to temperatures in excess of 700°F. If recarbon 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 durinoperation. However, these acids commonly develop during shutdowns by the otion of iron sulfide scale in the presence of moisture and oxygen. They also formflue gas condensate.

Freedom from polythionic acid stress corrosion cracking can be assured only bpreventing sensitized austenitic stainless steels from coming in contact with suderived acids. Regular grades of austenitic stainless steel (Types 304, 316, etcsensitize easily at temperatures above about 700°F. In fact, the heat of weldingoften enough to sensitize the heat-affected zone. The extra low carbon grades stainless steel (Types 304L, 316L, etc.) normally do not sensitize during weldin

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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 stabilizedminimize sensitization.

Usually polythionic acid cracking is prevented by using the chemically stabilizedextra low carbon grades of stainless steel and avoiding harmful heat treatmentsless effective means of prevention is to use regular grades of stainless steel anrequire 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 embritment. Some of the straight chromium stainless steels are so sensitive to 885°Fembrittlement that even slow cooling through this temperature range will causeembrittlement. The 885°F embrittlement results in an upward shift in the ductilebrittle transition temperature. Duplex stainless steels are also susceptible to 88embrittlement. Prevent this problem by not using chromium stainless steels for soliwall 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 embritment at lower temperatures. Sigma phase is very hard, nonmagnetic, and brittlecomposition of sigma phase varies depending on the alloy from which it formed

Sigma phase normally does not affect the steel's elevated temperature propertimay make it so brittle at lower temperatures that failures will occur during startushutdown.

The straight chromium ferritic and martensitic stainless steels containing 13% amore chromium are very susceptible to extensive sigma phase formation at temtures above about 1000°F. The austenitic stainless steels are not as susceptiblbecause of their high nickel content, but they can develop damaging amounts osigma phase when held between about 1000°F to 1550°F for long periods of timCertain highly susceptible austenitic alloys, such as castings and welds, may develop serious embrittlement in a few hours at temperatures of 1200°F to 130Duplex stainless steels are also very susceptible to sigma embrittlement.

Sigma embrittlement is controlled by minimizing ferrite content of stainless steewelds. Refer to specifications PVM-MS-1322 and PVM-MS-4748. Duplex stainless steel is limited to 650°F maximum service temperature to avoid embrittlem

Sulfide Stress Cracking. The martensitic stainless steels are especially susceptito sulfide stress cracking. Welds are difficult to soften with heat treatment and atherefore, susceptible to cracking. Low carbon grades, like Type 410S, are uselimit 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 AlloysOther 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 “cleroom,” so field repairs are not practical.

514 Summary of Temperature LimitationsFigure 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 Steel C-½ Mo 1¼ Cr-½ Mo 2¼ Cr-1 Mo 12 Cr (410)

18 Cr-8 Ni (304)

Strength(1) (3000 psi) 990 1075 1135 1150 1100 1275

Oxidation (10 mpy loss)

1025 1025 1050 1100 1350 1600

Graphitization (welded only)

800 850 N/A N/A N/A N/A

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 H2pp (750 psi)

500 500 1000 1100 N/A N/A

Caustic Stress Corro-sion Cracking

140 140 140 140 140 140

Chloride Stress Corrosion Cracking

N/A N/A N/A N/A N/A 140

Sulfide Stress Cracking

X X X X X N/A

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 FractureBrittle fracture is a sudden, often catastrophic failure which is inherent to “brittlematerials (discussed below). It involves little or no deformation (yielding), and ithas been experienced in steel structures such as pressure vessels, tanks, pipebridges, beams, and other similar structures, often of welded construction. Thewell known incidents were those occurring to over 200 ships constructed duringWorld War II, 19 of which completely broke in two. The Company has experienctwo significant cases described below.

The material property “Brittleness” indicates that the material is prone to failurewithout deformation. Examples of brittle materials include chalk, brick, glass, anhardened steels. Brittle structures can, and literally have shattered like glass. Bmaterials are prone to fracture when they are stressed in the vicinity of a notch stress concentration.

The opposite of brittleness is toughness, which for practical purposes can be deas a material's ability to resist brittle fracture. Toughness is discussed in more din Section 523. Toughness depends on material strength, thickness, and for stetemperature. To resist brittle fracture, higher strength materials and thick materrequire greater toughness than low strength and thin materials. Steels lose tougness as temperature decreases.

Brittle fracture can occur in ferritic steels, such as carbon, carbon-½ moly, chromoly, and 400 series stainless steels, within the normal atmospheric temperaturange. The regular 300 series stainless steels are not susceptible to brittle fractuntil temperatures are below -300°F. However, after exposure above 1100°F, sembrittlement makes 300 series weld metals with large amounts of ferritic phassusceptible to fracture well above room temperature.

Brittle fractures are infrequent. Most occur during hydrotest rather than in operation. However, brittle fractures can be catastrophic due to fragmentation of the structure and fast release of energy. Due primarily to higher quality constructionmaintenance standards, our Company has rarely experienced brittle fractures. However, to illustrate the importance of this design factor, two incidents of brittlfracture experienced by Chevron are described below.

Brittle fracture is characterized by a flat fracture surface, and occurs at averagestress levels below those of general yielding. Brittle fracture cracks grow at verfast speeds (up to 7000 ft/s), so brittle fracture happens quickly and unexpecte

522 Examples of Brittle Fracture Experienced by Chevron

1982 Brittle Fracture of Clear Creek LPG VesselA 30-year old vessel made of A 212 Grade B carbon steel ruptured unexpecteda 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.

1985 Brittle Fracture of Richmond TKC Isomax Steam Generator E-440The 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 duthe hydrotest was +50°F. The fracture occurred when the test pressure reachedpsig; test pressure was planned for 4500 psig. The steel that the channel was m

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

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

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500 Materials Pressure Vessel Manual

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.

523 Design to Prevent Brittle Fracture

OverviewSusceptibility 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

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

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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 ControlThe 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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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.

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.

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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, prsure test, shutdowns, and startups. Specific recommended practices for Divisioand 2 vessels are described below.

Note that both examples of brittle fracture failure discussed above in Section 52involved pressurizing (loading) the vessels below the steel's transition temperaThe LPG vessel steel which fractured at -20°F had a CVN transition temperatuabout +80°F; the steam generator steel which fractured at about +50°F had a Ctransition temperature of about +200°F.

ASME Code, Section VIII, Division 1, Vessels

Minimum Pressurizing Temperature. Company practice is to specify a minimumpressurizing temperature (MPT) or minimum design metal temperature (as definin Code Paragraph UG-20). The MPT is the lowest temperature at which a presgreater than 40% of the maximum allowable working pressure should be appliethe vessel. Below 40% of the maximum allowable working pressure, stresses aconsidered low enough to essentially eliminate the risk of brittle fracture in the absence of significant other stresses (such as those due to weight and differenthermal expansion). Due to increases in ASME code allowable stresses for Div1 vessels built in 1999 and later, MPT is the lowest temperature at which a pres>35% of MAWP should be applied to vessels built in 1999 and later. Section 24the 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 shouldconsidered, as well as normal operation. (Recommendations for autorefrigeraticonditions are discussed later in this section.) The best available local weathershould be used to establish startup temperatures if the equipment is not normapreheated. 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 the Tank Manual.)

Materials Selection Requirement. One of two methods is used to assure steels aused 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.

The impact-test exemption curves are preferred to CVN impact testing where pdata or service experience are available. CVN tests increase materials costs sutially (approximately 2 to 10 cents/pound) and complicate delivery.

Code Figure UCS-66, Impact-Test Exemption Curves, gives the application poi(combinations of thickness and minimum design metal temperature) where priodata or service experience show specific steels have sufficient toughness for frture-safe design; i.e., they are above their transition temperature. The applicatipoint is the point corresponding to the thickness and minimum pressurizing or design metal temperature. A steel has adequate toughness if the application poabove the steel's curve. Figure 500-9 is a schematic illustration which resemble

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

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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.

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 assignedto Curve A rather than to Curve B. This is more conservative than the requments of Figure UCS-66. SA 285 is often a semikilled steel and SA 515 is

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

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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 testingexemption temperature for P-1 materials (carbon steel) that are given postweldtreatment (PWHT) and PWHT is not otherwise required by Code. This reductioshould be allowed.

Charpy V-Notch Impact Testing. When a steel is to be used at an application pobelow its curve on Figure UCS-66 Impact Test Exemption Curves, CVN impacttesting is required to prove adequate toughness. Requirements for CVN testingsummarized here.

1. Each plate, forging or pipe used at an application point below its impact tesexemption curve is tested. Usually each plate is tested, while forgings and 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 (duristeel making) are tested. It is important that transverse rather than longitudspecimens be used because transverse properties are generally poorer. Seless pipe is an exception because its properties do not vary much with orietion. ASTM A-370 defines transverse and longitudinal CVN specimens. Specimen orientation is a Company requirement. The Code leaves orientaoptional.

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 theheat-affected zone (HAZ) and deposited weld metal is required on the WeldProcedure 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 treatmentspecimens. Test specimen heat treatment must simulate actual vessel heament.

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 temperatureferrous metals other than austenitic, that is, for carbon and low alloy steels. TheCode defines both a minimum service temperature and a minimum pressure te

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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 must be less than 20% of the required test pressure. This will usually be an apppressure 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 temperaRequirements 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 ifies the Code, Division 2, Impact Test Exemptions.

Guidelines for use of the Code, Division 2, Figure AM-218.1, Impact Test Exemtion Curves, are given in Specification PVM-MS-4749, which specifies the use 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 stethicker than 3 inches regardless of minimum pressurizing temperature.

Charpy V-Notch Impact Testing. The ASME Code, Division 2, requirements aregiven in AM-204 through AM-218. CVN impact tests are required when a steel ibe used at a combination of metal temperature and thickness below its exemptcurve.

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-211.1Table AM-211.1 requirements are less conservative than Figure UG-84.1 requiments and do not take into account the need for higher CVN energy with increathickness.

525 Typical Carbon Steel Selections to Avoid Brittle Fracture in Pressure Vessels

Figure 500-11 illustrates typical carbon steel selections for pressure vessels ovrange of thicknesses and minimum pressurizing temperature.

526 Steel Selection for Pressure Vessels Subject to AutorefrigerationAutorefrigeration should be considered when selecting steels, as described belsome liquid services such as LPG, a leak could reduce the pressure and causein temperature of a pressure vessel and its contents as the liquid boils off.

Fig. 500-11 Typical Carbon Steel Selections to Avoid Brittle Fracture in Pressure Vessels

Minimum Pressurizing

Temp., °F

Shell and Head Plates

Nozzle Forgings Pipe1 in. Thick 2 in. Thick

-50° to 0 Normalized SA 516(1),(3) or SA 537 Class 1

Normalized(1),(2),(3) SA 516 or SA 537 Class 1

SA 350 LF2 SA 333

0 to 30 Normalized SA 516 or Impact Tested, As-Rolled SA 516

Normalized SA 516 SA 105(3),(4) SA 106(3),(4)

30 to 60 As-Rolled SA 516, or Impact Tested SA 285

Normalized SA 516 SA 105(3),(4) SA 106(3),(4)

Warmer than 60 SA 285 or As-Rolled SA 516

Normalized SA 516 SA 105 SA 106

(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.

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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, andused as a design basis to avoid brittle fracture.

(Due to an increase in ASME code allowable stresses in 1999, vessels built in and later will have autorefrigeration temperatures equal to what the contents wreach 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 specialistto determine if this change applies to your situation.)

Vessels that are subject to autorefrigeration require additional consideration asfollows:

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 mbe SA 350, Grade LF 2, and pipe may be SA 333, Grades 1 or 6. These stehave good inherent toughness.

2. Impact testing is not required for autorefrigeration, unless already requiredthe normal design temperature. SA 350 and SA 333 materials are, howeveimpact tested in accordance with their respective specifications.

Autorefrigeration is not considered equivalent to a cold design or operating temature due to the lowered pressure. Therefore, the recommended safeguards agbrittle fracture are not as stringent as for a cold operating temperature. The useSA 516 steel, and equivalent forging and piping grades, should by itself provideample resistance to brittle fracture during autorefrigeration. Impact testing is norequired for autorefrigeration, unless it is required for a cold design temperaturewithout considering autorefrigeration.

527 Factors Controlling Susceptibility to Brittle Fracture: Additional Technical Information

General InformationCareful attention to materials selection, design, fabrication, and inspection techniques is necessary to achieve fracture-safe designs. This section of the manudeals primarily with materials selection. But proper selection of materials alonenot enough to prevent brittle fracture. Attention in design to avoid stress concentions, such as notches, and attention to inspection and nondestructive examinaduring fabrication to find and eliminate cracks and flaws are necessary to minimthe risk of brittle fracture. A hydrostatic test to stresses higher than design stresanother 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 c

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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-freeInspection during fabrication is necessary to limit as-built flaw sizes. Fatiguand stress corrosion cracking can also cause flaws to increase in service, tperiodic inspections are necessary.

2. Stress—Tensile stress is necessary for brittle fracture to occur. Since stressconcentrations increase nominal stress to cause high local stresses and grsusceptibility to fracture, they should be minimized. Residual stress from forming and welding can also affect local stress levels. Postweld heat treat(PWHT) lowers residual stress. Thus, PWHT generally lowers the risk of brfailure.

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 retant to brittle fracture. Fracture toughness is a material property just as streis. Notch toughness is a qualitative term, describing a material's resistancefracture. Fracture toughness is quantitative and is measured according to tprinciples 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. Fture occurs when the stress intensity factor exceeds a critical value. For a givenmaterial, the critical value is a function of temperature, loading rate, and thickne

For slow loading rates, the critical value is applicable to static loads and is desinated KIc. For dynamic loading (as in impact tests), the critical value is designatKId. 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 ratewhich can cause much lower KId values (see Figure 500-12).

KIc (or KId) is a material's fracture toughness at a given temperature and loadinrate. It is a material property, as yield and tensile strength are. Steels with a guteed KIc are not commercially available, so we have to use other methods to spand purchase, such as Charpy V-Notch testing.

Figure 500-13 illustrates limits of allowable stress and crack size combinations different KIc values. Above the curve for a given fracture toughness, fracture occFigure 500-13 shows that at a given stress level, a larger flaw is tolerable as tomaterials 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 shas a ductile-to-brittle transition temperature range. Above their transition tempture range these steels are tough; in and below the transition temperature rang

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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 temperatudecreases (toughness improves) with decreased carbon content. Increasinmanganese content contributes to a lower transition temperature up to a mnese content of about 2 wt %.

2. Deoxidation practice—Fully killed (fully deoxidized) steels have lower transition 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 lowertransition temperature ranges than as-rolled steels of similar composition. Grefinement is a reason.

5. Welding—Welding generally results in a higher transition temperature in theweld heat-affected zones as compared to the base material. The variables improve transition temperature in the base metal, as listed above, also help

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

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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 damaginto specific alloys. Temper embrittlement of 2¼ Cr-1 Mo steel after exposure650°F to 1000°F is one example. Special guidelines have been developed 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 t

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

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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 EquipmentThe 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 VesselsNote 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 Mon the vessel drawings, or on Safety Instruction Sheets (SISs).

For carbon and low alloy steel vessels without an MPT specified, Figure UCS-6along with materials of construction and vessel thickness, can be used to estabMPT for vessels up to 6 inches thick. For vessels made with materials shown oFigure UCS-66, the MPT can be established directly from the curves if componthicknesses are known. For materials not listed in UCS-66, see the next subsec

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ed or ther ation

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 thexemption curve on Figure UCS-66. This is consistent with Code ParagrapUCS-68(c). Normally carbon steel vessels 1¼ inch and less in thickness arrequired to be stress-relieved by Code rules. See ASME Code, Section VIIDivision 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 AThe purpose of this section is to identify several obsolete steel specifications usfor the construction of many vessels in the past. All steels listed below, except fthe Code Case steels, should be assigned to Curve A of Figure UCS-66. Any osteels not listed also should be assigned to Curve A unless sufficient documentis available for assignment to a lower curve.

• 1934 API-ASME Code Steels

• 1934 ASME Section VIII Steels

ASTM A-7 ASTM A-113

ASTM A-10 ASTM A-149

ASTM A-30 ASTM A-150

ASTM A-70

ASME S-1 ASME S-26

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• Later Codes

• Code Case Steels

Code Case 1256 is equivalent to SA 442. (Use Curve B. If normalized, useCurve D.)

Code Case 1280 is equivalent to SA 516. (Use Curve B. If normalized, useCurve 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 MPTsAn 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 cur(similar to Figure UCS-66) before they were included in ASME Code, Section VDivision 1.

With Figure UCS-66 now available in the Code, Division 1, Chevron's curves habeen archived. MPTs established on the basis of the 1983 letter are still valid aneed not be changed. Figure UCS-66, when used as recommended above, anChevron'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 VesselsAPI 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 validmethod in the API Guide draft. This API Guide is in the final stages of preparati

ASME S-2 ASME S-27

ASME S-25

ASTM A 201

ASTM A 212

Chevron Corporation 500-29 April 2000