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Question 1General Principles of Damage Mechanisms and Failure Analysis

In the context of API 571, which statement best describes a damage mechanism?

  1. A chemical, physical, or metallurgical process that degrades the material Correct answer.
  2. A design margin added to compensate for uncertain operating data
  3. An inspection technique used to measure remaining wall thickness
  4. A repair procedure used to restore a damaged pressure boundary

Explanation

A damage mechanism is any process that changes the material's condition or properties and thereby reduces its ability to perform its intended function. Mechanisms are classed broadly as mechanical, metallurgical, chemical, and physical. Inspections, repairs, and design margins are responses to damage, not the mechanisms themselves.

Question 2General Principles of Damage Mechanisms and Failure Analysis

A carbon steel line shows a broad, fairly uniform reduction in wall thickness over a large area of its internal surface. Which damage mechanism is most consistent with this observation?

  1. Chloride stress corrosion cracking
  2. Hydrogen blistering
  3. Cavitation damage
  4. General (uniform) corrosion Correct answer.

Explanation

General corrosion attacks the surface relatively evenly, so metal loss is spread over a wide area rather than concentrated. Localized forms such as cracking, cavitation, and blistering produce isolated pits, cracks, or blisters. Uniform thinning with a broad, smooth profile is the signature of general corrosion.

Question 3General Principles of Damage Mechanisms and Failure Analysis

Which material property is defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain?

  1. Yield strength Correct answer.
  2. Creep strength
  3. Modulus of elasticity
  4. Tensile strength

Explanation

Yield strength marks the end of purely elastic behavior, defined by a specified offset or deviation from the linear stress-strain relationship. Tensile strength is the maximum stress reached before necking and failure. The modulus of elasticity is the slope of the elastic portion of the curve.

Question 4General Principles of Damage Mechanisms and Failure Analysis

In a failure investigation, a hardness survey across a weld and its heat-affected zone is most useful for:

  1. Detecting local softening, hardening, or embrittlement Correct answer.
  2. Estimating the remaining corrosion allowance
  3. Determining the chemical composition of the base metal
  4. Measuring the fracture toughness of the weld metal

Explanation

Hardness is a quick indicator of local strength and microstructure, so a survey across a weld reveals hard zones, soft zones, and improper heat treatment. It is frequently used as a screening tool for embrittlement and for verifying post-weld heat treatment. It does not measure composition, toughness, or wall thickness directly.

Question 5General Principles of Damage Mechanisms and Failure Analysis

A Charpy V-notch impact test is performed primarily to evaluate which material property?

  1. Yield strength at elevated temperature
  2. Notch toughness and the ductile-to-brittle transition Correct answer.
  3. Creep rupture life under sustained load
  4. Resistance to general corrosion

Explanation

The Charpy V-notch test measures the energy absorbed in breaking a notched specimen, which reflects notch toughness and the temperature at which the material transitions from ductile to brittle behavior. It is a key input for assessing brittle fracture risk. It does not measure strength, corrosion resistance, or creep life.

Question 6General Principles of Damage Mechanisms and Failure Analysis

In pressure equipment design, corrosion allowance refers to:

  1. Extra wall thickness provided for the metal loss expected during the design life Correct answer.
  2. The maximum metal loss permitted before the equipment must be rerated
  3. The thickness added to compensate for mill tolerance on plate material
  4. The minimum thickness required by the code at the time of hydrotest

Explanation

Corrosion allowance is thickness added to the calculated minimum required thickness so the component can suffer the expected corrosion over its design life and still be adequate. It is an intentional margin against predictable metal loss. Mill tolerance and hydrotest minima are separate considerations.

Question 7General Principles of Damage Mechanisms and Failure Analysis

API 579-1/ASME FFS-1 is best described as a standard for:

  1. Planning inspections on the basis of risk
  2. Assessing the fitness-for-service of damaged equipment Correct answer.
  3. Qualifying welding procedures for pressure components
  4. Designing cathodic protection systems for buried piping

Explanation

API 579-1/ASME FFS-1 provides assessment procedures for determining whether equipment containing flaws, damage, or metal loss can continue to operate safely. It is applied after damage has been found and quantified. Risk-based inspection planning is the subject of API 580.

Question 8General Principles of Damage Mechanisms and Failure Analysis

API 580 provides guidance for which activity?

  1. Sizing of pressure-relief devices
  2. Risk-based inspection planning Correct answer.
  3. Fitness-for-service assessment of local thin areas
  4. Qualification of nondestructive examination personnel

Explanation

API 580 establishes the principles of risk-based inspection, including how to assess the probability and consequence of failure and use that to set inspection plans. It works together with API 581 for quantitative methods. Flaw assessment belongs to API 579, and personnel qualification to other standards such as ASNT SNT-TC-1A.

Question 9General Principles of Damage Mechanisms and Failure Analysis

Ultrasonic readings taken at the same location on a carbon steel pipe were 0.500 in. five years ago and 0.440 in. today. What is the average corrosion rate?

  1. 0.024 in./year
  2. 0.012 in./year Correct answer.
  3. 0.006 in./year
  4. 0.060 in./year

Explanation

Corrosion rate is the metal loss divided by the elapsed time: (0.500 - 0.440) = 0.060 in. lost over 5 years, giving 0.012 in. per year. Using consistent units and the actual interval between readings is essential. The other values either divide by the wrong interval or repeat the total loss as an annual figure.

Question 10General Principles of Damage Mechanisms and Failure Analysis

A vessel nozzle has a minimum required thickness of 0.200 in. Its current measured thickness is 0.260 in., and the established long-term corrosion rate is 0.010 in./year. What is the remaining life?

  1. 26 years
  2. 2.6 years
  3. 6 years Correct answer.
  4. 20 years

Explanation

Remaining life is the corrosion allowance above the minimum required thickness divided by the corrosion rate: (0.260 - 0.200) / 0.010 = 6 years. The result should then be compared with the intended inspection interval and adjusted for confidence in the rate. Mixing the current thickness with the rate gives the wrong answer.

Question 11General Principles of Damage Mechanisms and Failure Analysis

Which combination of conditions most increases the likelihood of brittle fracture in a carbon steel vessel?

  1. High toughness, thin wall, and a ductile microstructure
  2. Low hardness, fine grain size, and compressive stress
  3. Low temperature, high loading rate, and a sharp flaw Correct answer.
  4. High temperature, low stress, and slow loading

Explanation

Brittle fracture is promoted by conditions that suppress plastic deformation: low temperature relative to the transition temperature, rapid loading, triaxial restraint in thick sections, and sharp flaws or notches that concentrate stress. These factors act together and can cause sudden failure without prior deformation. Conditions favoring plasticity or compressive stress reduce the risk.

Question 12General Principles of Damage Mechanisms and Failure Analysis

How does a corrosive environment typically change the fatigue behavior of carbon steel?

  1. It has no effect on fatigue life until the stress exceeds yield
  2. It removes the endurance limit and shortens fatigue life Correct answer.
  3. It increases ductility and blunts the crack tip
  4. It raises the endurance limit and delays crack initiation

Explanation

In corrosion fatigue, the environment accelerates crack initiation and growth, so the S-N curve is lowered and the true endurance limit disappears. Failure can occur at stress levels well below the in-air fatigue limit. The effect depends on the environment and is not delayed until yield is exceeded.

Question 13General Principles of Damage Mechanisms and Failure Analysis

Creep becomes a design concern for carbon steel at metal temperatures above approximately:

  1. 400 °F (204 °C)
  2. 800 °F (427 °C) Correct answer.
  3. 1200 °F (649 °C)
  4. 150 °F (66 °C)

Explanation

Carbon steel begins to creep at a meaningful rate at roughly 800 °F, which corresponds to about 0.4 of its absolute melting temperature, the usual threshold for creep significance. Below that temperature, time-dependent deformation is generally negligible. Alloy steels raise the threshold but do not eliminate creep.

Question 14General Principles of Damage Mechanisms and Failure Analysis

Which examination is most appropriate for identifying spheroidization of pearlite in a long-term hot-service steel component?

  1. Metallographic examination by field replication or sample removal Correct answer.
  2. Wet fluorescent magnetic particle testing
  3. Acoustic emission monitoring
  4. Ultrasonic thickness measurement

Explanation

Spheroidization is a microstructural change, so it can only be identified by examining the microstructure directly, using field metallographic replication or a removed sample. Surface and volumetric NDE methods detect discontinuities or wall loss, not changes in carbide morphology. Acoustic emission detects active cracking, not microstructure.

Question 15General Principles of Damage Mechanisms and Failure Analysis

Sensitization of austenitic stainless steel occurs when the material is held in which temperature range?

  1. Below ambient temperature
  2. 750–1500 °F (399–816 °C) Correct answer.
  3. 1600–2000 °F (871–1093 °C)
  4. 200–400 °F (93–204 °C)

Explanation

In the 750–1500 °F range, chromium carbides precipitate along grain boundaries and deplete the adjacent metal of chromium, leaving it vulnerable to intergranular corrosion. This is why solution annealing and low-carbon or stabilized grades are used. Outside this range, carbide precipitation is either too slow or the carbides redissolve.

Question 16General Principles of Damage Mechanisms and Failure Analysis

Which condition is generally required for chloride stress corrosion cracking of 300-series stainless steel?

  1. Tensile stress, chlorides, and a metal temperature above about 140 °F Correct answer.
  2. Continuous caustic service above 200 °F
  3. Fully compressive residual stress at weldments
  4. Oxygen-free, dry hydrocarbon service below 100 °F

Explanation

Chloride SCC requires three things together: a susceptible austenitic alloy, a chloride-containing environment, and sustained tensile stress, with elevated temperature strongly accelerating the process. Roughly 140 °F is the commonly cited threshold below which cracking is unlikely. Compressive stress, dryness, or caustic service point to other mechanisms.

Question 17General Principles of Damage Mechanisms and Failure Analysis

Hydrogen blistering and hydrogen-induced cracking in carbon steel are caused by:

  1. Chlorides concentrating in crevices under deposits
  2. Atomic hydrogen diffusing into the steel and recombining at internal defects Correct answer.
  3. Molten sulfur attacking grain boundaries at the surface
  4. Dissimilar metal welds operating above 800 °F

Explanation

Atomic hydrogen generated at the surface, for example by corrosion or cathodic charging, diffuses into the steel and recombines to molecular hydrogen at inclusions, laminations, or hard zones. The resulting pressure produces blisters and stepwise internal cracks. This differs from sulfidation, crevice corrosion, and dissimilar-metal weld degradation.

Question 18General Principles of Damage Mechanisms and Failure Analysis

Which damage mechanism produces deep pitting and gouging on a pump impeller through the collapse of vapor bubbles?

  1. Cavitation Correct answer.
  2. Liquid metal embrittlement
  3. Erosion-corrosion from suspended solids
  4. Microbiologically influenced corrosion

Explanation

Cavitation occurs when vapor bubbles formed in low-pressure regions collapse against a surface, generating intense localized pressure that pits and gouges the metal. Damage is concentrated where pressure recovery occurs, such as impeller discharge areas. Solid-particle erosion produces a smoother, more directional wear pattern.

Question 19General Principles of Damage Mechanisms and Failure Analysis

The heat-affected zone of a weld is best described as:

  1. The area of base metal covered by the weld cap
  2. Weld metal that has been reheated by a subsequent pass
  3. Base metal whose microstructure was altered by the weld thermal cycle without melting Correct answer.
  4. The region where filler metal mixes with the base metal

Explanation

The heat-affected zone is base metal that did not melt but experienced enough heat to change its microstructure and properties. It is often the site of hardening, softening, or embrittlement and is frequently where cracking initiates. The mixed and remelted regions are parts of the weld metal, not the heat-affected zone.

Question 20General Principles of Damage Mechanisms and Failure Analysis

Scanning electron microscopy with energy dispersive spectroscopy is most useful in a failure investigation for:

  1. Measuring wall thickness through insulation
  2. Determining residual stress in a weld
  3. Verifying that post-weld heat treatment was performed
  4. Identifying fracture morphology and the elemental composition of deposits Correct answer.

Explanation

SEM provides high-magnification images of fracture surfaces that reveal whether failure was ductile, brittle, or fatigue-related, while EDS identifies the elements present in corrosion products or deposits. Together they help link the failure to a specific mechanism. They cannot measure thickness, stress, or heat treatment directly.

Question 21General Principles of Damage Mechanisms and Failure Analysis

Sigma phase formation in austenitic or duplex stainless steel is a concern primarily because it:

  1. Increases hardness without affecting corrosion resistance
  2. Raises creep strength in the weld heat-affected zone
  3. Reduces toughness and corrosion resistance Correct answer.
  4. Causes graphitization along the grain boundaries

Explanation

Sigma is a hard, brittle, chromium-rich intermetallic that forms in the approximate range of 1000–1700 °F, especially in cast, welded, or duplex material. Its presence embrittles the steel and ties up chromium, lowering corrosion resistance. It is a degradation, not a strengthening treatment.

Question 22General Principles of Damage Mechanisms and Failure Analysis

Temper embrittlement in 2.25Cr-1Mo steel is best detected by:

  1. A measurable loss in tensile strength at room temperature
  2. A decrease in chromium content near the metal surface
  3. A significant increase in hardness in the heat-affected zone
  4. An increase in the ductile-to-brittle transition temperature with no loss of tensile strength Correct answer.

Explanation

Temper embrittlement is caused by impurity segregation to grain boundaries during slow cooling through roughly 600–1000 °F. It raises the ductile-to-brittle transition temperature and lowers notch toughness while leaving tensile strength and hardness essentially unchanged. Detection therefore requires impact testing or metallurgical evaluation, not hardness or tensile tests.

Question 23General Principles of Damage Mechanisms and Failure Analysis

Graphitization of carbon-0.5Mo steel after long service above about 800 °F results in:

  1. Formation of graphite nodules, mainly in the heat-affected zone, that reduce creep strength Correct answer.
  2. Chromium carbide precipitation at grain boundaries with loss of corrosion resistance
  3. Precipitation of sigma phase accompanied by increased hardness
  4. Coarsening of pearlite lamellae into spheroids

Explanation

In carbon and carbon-molybdenum steels at elevated temperature, carbides decompose into graphite nodules, often concentrated in weld heat-affected zones. The graphite acts as an internal notch and displaces load-bearing metal, reducing creep strength and ductility. Spheroidization and sensitization are different solid-state changes.

Question 24General Principles of Damage Mechanisms and Failure Analysis

885 °F (475 °C) embrittlement affects which group of materials?

  1. Carbon steels exposed to wet hydrogen sulfide
  2. Nickel-based alloys subjected to high-temperature sulfidation
  3. Ferritic, martensitic, and duplex stainless steels Correct answer.
  4. Austenitic stainless steels after sensitization

Explanation

885 °F embrittlement occurs in ferritic, martensitic, and duplex stainless steels held in roughly the 700–1000 °F range, where chromium-rich alpha prime phase precipitates and causes severe loss of toughness. Austenitic grades are not susceptible to this particular mechanism. The damage is metallurgical and is not reversed by ordinary stress relief.

Question 25General Principles of Damage Mechanisms and Failure Analysis

A fracture surface exhibits concentric beach marks leading to a final overload zone. Which primary failure mechanism does this indicate?

  1. Hydrogen embrittlement
  2. Fatigue cracking Correct answer.
  3. Stress corrosion cracking
  4. Creep rupture

Explanation

Beach marks are the macroscopic progression markings produced as a fatigue crack advances incrementally under cyclic loading, ending in a final overload region when the remaining ligament can no longer carry the load. Creep rupture shows cavitation and intergranular damage, while SCC and hydrogen embrittlement produce brittle cracking without this progressive marking pattern.

Question 26General Principles of Damage Mechanisms and Failure Analysis

In API 571, a damage mechanism is best described as:

  1. The planned interval between scheduled inspection turnarounds
  2. The maximum allowable working pressure of a component
  3. A recorded deviation from normal design operating conditions
  4. A physical or chemical process that degrades the material Correct answer.

Explanation

A damage mechanism is the physical or chemical process that causes deterioration of a material or component. Operating deviations, pressure limits, and inspection intervals are operating or design parameters, not mechanisms of degradation.

Question 27General Principles of Damage Mechanisms and Failure Analysis

The bathtub curve used in reliability and failure analysis describes:

  1. Pressure decay during a hydrostatic test
  2. Wall thickness variation around the pipe circumference
  3. Infant mortality, random, and wear-out stages Correct answer.
  4. Corrosion rate plotted as a function of temperature

Explanation

The bathtub curve models equipment life as three periods: early infant-mortality failures, a middle period of random failures, and a final wear-out period. It is a reliability concept, not a corrosion-rate or inspection-measurement plot.

Question 28General Principles of Damage Mechanisms and Failure Analysis

Which statement correctly distinguishes a damage mechanism from a failure mode?

  1. A degradation rate expressed in mpy; a count of applied stress cycles
  2. The process causing degradation; how the component ultimately fails Correct answer.
  3. The visual appearance of the surface; the NDE technique used to find it
  4. A condition that occurs only in welds; a condition that occurs only in base metal

Explanation

A damage mechanism is the process that causes deterioration, such as corrosion or fatigue. The failure mode is the manner in which the component finally loses its function, such as leakage, fracture, or deformation.

Question 29General Principles of Damage Mechanisms and Failure Analysis

A damage mechanism is described as localized rather than general. Which inspection implication follows?

  1. It always occurs uniformly over the entire wetted surface
  2. It can be found only by visual inspection of the exterior
  3. It can cause perforation with little overall wall loss Correct answer.
  4. Uniform grid thickness readings will reliably detect it before leakage

Explanation

Localized corrosion concentrates metal loss in small areas, so a component can perforate while the average wall thickness remains nearly unchanged. Inspection must target likely localized locations rather than rely solely on broad-average thickness readings.

Question 30General Principles of Damage Mechanisms and Failure Analysis

Which statement best distinguishes erosion-corrosion from pure erosion?

  1. Occurs only in stagnant, low-velocity service where flow is minimal
  2. Corrosive fluid plus flow; erosion is mechanical impact Correct answer.
  3. Occurs only at elevated metal temperatures above the design range
  4. Affects only nonmetallic materials such as polymer coatings and linings

Explanation

Erosion-corrosion involves both a corrosive environment and flowing fluid that removes the protective corrosion product film. Pure erosion is mechanical removal of material by impacting particles or droplets without requiring a corrosive fluid.

Question 31General Principles of Damage Mechanisms and Failure Analysis

In a remaining-life calculation based on current thickness, minimum required thickness, and corrosion rate, doubling the corrosion rate will:

  1. Reduce the calculated remaining life by half Correct answer.
  2. Double the calculated remaining life
  3. Increase remaining life until the next inspection
  4. Have no effect on remaining life

Explanation

Remaining life is the thickness margin above the minimum required thickness divided by the corrosion rate. If the corrosion rate doubles while the thickness margin stays the same, the calculated remaining life is halved.

Question 32General Principles of Damage Mechanisms and Failure Analysis

A crack initiates at a weld toe on a component with no corrosion, no elevated-temperature exposure, and no hydrogen service. Which cause category should be evaluated first?

  1. Caustic stress corrosion cracking
  2. High-temperature creep damage
  3. Cyclic mechanical fatigue Correct answer.
  4. Microbiologically influenced corrosion

Explanation

Absence of a corrosive or high-temperature environment points away from environmental and creep mechanisms. A weld toe is a stress concentration, so cyclic mechanical fatigue is the most likely cause category to evaluate first.

Question 33General Principles of Damage Mechanisms and Failure Analysis

Which damage mechanism is characterized by rapid carburization and metal wastage in carbon steel exposed to carbon monoxide/hydrogen gas mixtures at roughly 900–1500 °F?

  1. Metal dusting Correct answer.
  2. Graphitization
  3. Caustic stress corrosion cracking
  4. Sulfidation

Explanation

Metal dusting is a catastrophic carburization mechanism that occurs in carbonaceous gases such as CO/H2 mixtures at roughly 900–1500 °F. It produces rapid metal wastage, often as pits or grooves, rather than simple uniform wall loss.

Question 34General Principles of Damage Mechanisms and Failure Analysis

Why should a damage mechanism review consider upset and startup/shutdown conditions in addition to normal operation?

  1. Upsets always reduce the corrosion rate to negligible, harmless levels
  2. Design codes require that all upset conditions be ignored in assessments
  3. Normal steady-state operation cannot cause any recognized damage mechanism
  4. Some mechanisms are active only during transient conditions Correct answer.

Explanation

Transient conditions can create temperature excursions, condensation, wetting, or stress cycles that are absent during steady operation. Some damage mechanisms, such as certain corrosion or cracking processes, become active only during these transient periods.

Question 35Mechanical and Metallurgical Failure Modes

Spheroidization of the pearlite in carbon steel becomes a concern when metal temperature exceeds approximately which value?

  1. 150°F (66°C)
  2. 850°F (454°C) Correct answer.
  3. 250°F (121°C)
  4. 400°F (204°C)

Explanation

Spheroidization is the gradual change of lamellar pearlite into spheroids of carbide, which begins in carbon steel at roughly 850°F and proceeds faster as temperature rises. It lowers tensile strength and hardness and is a time-at-temperature effect rather than a short-term excursion.

Question 36Mechanical and Metallurgical Failure Modes

Graphitization of carbon steel and carbon-½Mo steel occurs in which metal temperature range?

  1. 400–600°F (204–316°C)
  2. 200–400°F (93–204°C)
  3. 1500–1800°F (816–982°C)
  4. 850–1300°F (454–705°C) Correct answer.

Explanation

Graphitization is the decomposition of iron carbide into graphite plus ferrite, and it takes place in carbon and carbon-molybdenum steels at approximately 850–1300°F. The weld heat-affected zone is usually the most affected region and the graphite nodules act as internal stress raisers that can lead to sudden fracture.

Question 37Mechanical and Metallurgical Failure Modes

Hydrogen embrittlement of high-strength steels is of greatest concern under which condition?

  1. Cyclic loading at cryogenic temperature
  2. Dry hydrogen exposure at 1200°F
  3. Elevated temperature under sustained load
  4. Near-ambient temperature with sustained tensile stress Correct answer.

Explanation

Atomic hydrogen absorbed by a high-strength steel concentrates at regions of triaxial tensile stress and reduces ductility, so cracking occurs under sustained or slowly applied load near ambient temperature. Raising the temperature generally increases hydrogen mobility and reduces the severity of the embrittlement.

Question 38Mechanical and Metallurgical Failure Modes

Gray cast iron that has suffered graphitic corrosion will typically:

  1. Develop branched transgranular cracks
  2. Show uniform wall loss and a polished surface
  3. Lose strength while keeping its shape Correct answer.
  4. Show hydrogen blisters on the surface

Explanation

In graphitic corrosion the iron matrix is selectively leached away and the graphite network is left behind, so the part keeps its original outline while becoming soft, porous and weak. The damage is often invisible until the component fails mechanically.

Question 39Mechanical and Metallurgical Failure Modes

Which conditions are required for chloride stress corrosion cracking of austenitic stainless steel?

  1. Caustic above 50% concentration at 100°F
  2. Chlorides plus stress above 140°F Correct answer.
  3. H2S plus water at ambient temperature
  4. Cyclic thermal stress in dry air

Explanation

Chloride SCC needs three simultaneous conditions: a susceptible austenitic alloy, a source of chloride, and sustained tensile stress at a temperature above roughly 140°F. Unlike carbon steel, annealed austenitic stainless steel is susceptible even without hard welds.

Question 40Mechanical and Metallurgical Failure Modes

Localized erosion-corrosion in a piping system is most likely to be found at which location?

  1. At elbows and after reducers Correct answer.
  2. Straight runs of large-diameter pipe
  3. Dead legs with stagnant flow
  4. Vertical sections with low velocity

Explanation

Erosion-corrosion accelerates where flow direction changes or the stream impinges on the wall, because the protective surface film is repeatedly removed and fresh metal is exposed to the corrodent. Elbows, tees, reducers and downstream of control valves are therefore typical damage sites.

Question 41Mechanical and Metallurgical Failure Modes

Sigma phase forms in austenitic stainless steels after prolonged exposure in which temperature range?

  1. 700–900°F (371–482°C)
  2. 400–600°F (204–316°C)
  3. 1900–2100°F (1038–1149°C)
  4. 1000–1700°F (538–927°C) Correct answer.

Explanation

Sigma is a hard, brittle intermetallic phase that precipitates in austenitic stainless steels between about 1000°F and 1700°F, most readily in alloys containing molybdenum, titanium or niobium and in weld delta ferrite. The result is a marked loss of toughness and ductility at ambient temperature, which can cause cracking during shutdown or startup.

Question 42Mechanical and Metallurgical Failure Modes

885°F embrittlement affects which group of materials?

  1. Carbon steels containing about 0.5% molybdenum
  2. Nickel-base alloys containing niobium
  3. Austenitic stainless steels such as 304 and 316
  4. Ferritic and martensitic stainless steels Correct answer.

Explanation

885°F embrittlement occurs in ferritic and martensitic stainless steels, including the 400-series, when held for long periods in roughly the 700–1000°F range. It produces a loss of room-temperature toughness and ductility that cannot be restored by heat treatment.

Question 43Mechanical and Metallurgical Failure Modes

Temper embrittlement of a 2¼Cr-1Mo reactor shell is most reliably detected by which test?

  1. Ultrasonic thickness measurement
  2. Low-temperature Charpy V-notch testing Correct answer.
  3. Hardness survey with a portable tester
  4. Positive material identification by XRF

Explanation

Temper embrittlement raises the ductile-to-brittle transition temperature without significantly changing strength, hardness or thickness, so it is detected by notch toughness testing at reduced temperature. Charpy impact testing of the embrittled material shows a shift in absorbed energy and fracture appearance.

Question 44Mechanical and Metallurgical Failure Modes

Which method is used to predict the remaining life of equipment operating in the creep range?

  1. Hydrotesting at 1.5 times design pressure
  2. Using the Larson-Miller parameter Correct answer.
  3. Measuring wall thickness at the hottest point
  4. Comparing hardness to the as-new baseline

Explanation

The Larson-Miller parameter correlates stress, temperature and time to rupture so that short-term laboratory rupture data can be extrapolated to long service lives. It is the standard approach for estimating remaining creep life from accelerated stress-rupture tests.

Question 45Mechanical and Metallurgical Failure Modes

Which alloying element provides the greatest resistance to high-temperature sulfidation?

  1. Chromium Correct answer.
  2. Nickel
  3. Molybdenum
  4. Copper

Explanation

Sulfidation resistance increases with chromium content because a protective chromium sulfide/chromium oxide scale slows the inward diffusion of sulfur. This is why 5Cr, 9Cr and higher-chromium alloys are specified as temperature and H2S partial pressure increase.

Question 46Mechanical and Metallurgical Failure Modes

Naphthenic acid corrosion in crude distillation units occurs mainly in which metal temperature range?

  1. 800–1000°F (427–538°C)
  2. 250–400°F (121–204°C)
  3. 150–250°F (66–121°C)
  4. 400–750°F (204–399°C) Correct answer.

Explanation

Naphthenic acids become chemically active in the roughly 400–750°F range, where they attack carbon steel and low-chromium alloys, especially at high velocity in transfer lines and furnace tubes. Attack typically appears as smooth, sharply defined grooves and is controlled by alloying or by blending to reduce total acid number.

Question 47Mechanical and Metallurgical Failure Modes

Metal dusting is best described by which damage appearance?

  1. Uniform thinning with a smooth polished surface
  2. Pitting and grooving with powdery metal debris Correct answer.
  3. Intergranular cracking without metal loss
  4. Blistering with subsurface voids

Explanation

Metal dusting is a catastrophic carburization mechanism in carbon-monoxide-bearing gases at roughly 850–1650°F that produces localized pitting and grooving along with a fine dust of metal, carbide and carbon particles. It can penetrate walls rapidly and is often found at weld heat-affected zones and downstream of the carburizing gas source.

Question 48Mechanical and Metallurgical Failure Modes

Carburization of an austenitic stainless steel tube is most commonly confirmed in the field by which observation?

  1. Higher hardness and magnetic response Correct answer.
  2. A smooth reduction in wall thickness
  3. Intergranular cracks on the outside diameter
  4. A drop in hardness at the inner surface

Explanation

Carbon absorbed at high temperature forms hard carbides and can also promote a magnetic ferrite/martensite response in an otherwise nonmagnetic austenitic alloy. Field screening therefore uses hardness checks and a magnet to identify affected areas before more detailed metallography.

Question 49Mechanical and Metallurgical Failure Modes

Stepwise cracking that follows nonmetallic inclusions in wet H2S service is known as:

  1. Hydrogen blistering
  2. Stress-oriented hydrogen-induced cracking
  3. Sulfide stress cracking
  4. Hydrogen-induced cracking Correct answer.

Explanation

Hydrogen-induced cracking occurs when atomic hydrogen diffuses into the steel, recombines at inclusion-matrix interfaces and generates pressure that links adjacent planar cracks in a stepwise pattern. It does not require applied stress, which distinguishes it from sulfide stress cracking.

Question 50Mechanical and Metallurgical Failure Modes

Sulfide stress cracking of carbon steel is most severe under which condition?

  1. Dry H2S at 900°F
  2. Sour water at 250°F with no applied stress and low hardness
  3. Wet H2S at 400°F
  4. Wet H2S at ambient temperature with hard welds Correct answer.

Explanation

Sulfide stress cracking is a hydrogen-assisted cracking mechanism that is most severe near ambient temperature in the presence of free water, H2S and tensile stress, and it is strongly promoted by high hardness such as that found in unheat-treated welds. Severity generally decreases as temperature rises above about 150°F.