API-571 API Exam Dumps & Practice Test Questions

Question 1:

What type of damage is most commonly caused by the formation of sigma phase in metals?

A. Corrosion
B. Increased hardness
C. Cracking
D. Loss of ductility

Correct Answer: C

Explanation:

The sigma phase is a brittle intermetallic compound that tends to form in certain high-alloy steels, especially stainless steels with high chromium and molybdenum content, such as duplex or austenitic-ferritic stainless steels. This phase typically precipitates during prolonged exposure to intermediate temperature ranges, roughly between 600°C and 950°C, such as in the heat-affected zones during welding or during improper heat treatment.

The presence of sigma phase has a detrimental effect on the mechanical properties of metals. While it is a hard phase, its most damaging characteristic is its inherent brittleness, which disrupts the ductile metal matrix. The result is a significant reduction in toughness and an increase in susceptibility to brittle fracture.

Now, considering the possible damage types:

• Corrosion (A): While sigma phase can indirectly reduce corrosion resistance by depleting chromium in the surrounding matrix, corrosion itself is not the direct or primary damage caused by sigma phase precipitation.

• Increased hardness (B): Sigma phase is indeed harder than the surrounding metal, but hardness alone is not a form of damage. Hardness increase accompanies brittleness, which leads to failure modes such as cracking rather than hardness itself causing damage.

• Cracking (C): This is the principal damage caused by sigma phase. The brittle nature of sigma phase leads to intergranular cracking, where cracks propagate along grain boundaries weakened by sigma precipitation. This cracking severely compromises the load-bearing capacity of the material and can result in premature or sudden failure under service conditions.

• Loss of ductility (D): Sigma phase reduces ductility by making the metal more brittle, but this loss in ductility is a symptom rather than the direct form of damage. The actual mechanical failure manifests as cracking due to brittle fracture.

To summarize, while sigma phase affects several material properties, its most significant and direct damage is brittle cracking. This makes option C the correct answer, as it accurately represents the primary consequence of sigma phase embrittlement in metals.

Question 2:

Which component increases the severity of high-temperature sulfide corrosion in H₂S-containing streams at temperatures above roughly 500°F?

A. Amine
B. Hydrogen
C. Sulfides
D. All of the above

Correct Answer: B

Explanation:

High-temperature sulfide corrosion, commonly referred to as sulfidation, is a major degradation mechanism in industries such as oil refining and petrochemicals, where equipment is exposed to hydrogen sulfide (H₂S) at elevated temperatures (above ~500°F or 260°C). This corrosive process involves the chemical reaction of H₂S with metal surfaces, leading to the formation of metal sulfides and subsequent material loss.

The severity of this corrosion is influenced by several factors, but one key aggravator is hydrogen.

Hydrogen plays a crucial role in enhancing sulfide corrosion for several reasons:

• Hydrogen permeation: Hydrogen atoms can diffuse into steel, weakening the metal's microstructure and reducing its resistance to sulfidation. This creates a synergy between H₂S and hydrogen, accelerating the corrosion process.

• Hydrogen attack: At high pressures and temperatures, hydrogen can cause additional damage known as high-temperature hydrogen attack (HTHA), where hydrogen reacts internally with carbides in the steel, forming methane gas bubbles that cause cracking and blistering.

Examining the other options:

• Amine (A): Amines are used in gas treating processes to remove H₂S and CO₂. While degradation products from amines can contribute to corrosion under some conditions, they do not directly increase the severity of high-temperature sulfide corrosion.

• Sulfides (C): Sulfides form as a result of sulfidation and are part of the corrosion mechanism, but they are not an external factor that exacerbates the severity. The question seeks a factor that increases severity beyond the baseline sulfidation caused by H₂S.

• All of the above (D): Since neither amines nor sulfides directly increase the severity of sulfidation beyond the role of hydrogen, this option is incorrect.

In summary, the presence of hydrogen in H₂S-containing streams at elevated temperatures profoundly increases the aggressiveness of sulfidation corrosion by altering the protective sulfide scales on metal surfaces and facilitating deeper material degradation. Therefore, option B is the correct answer.

Question 3:

In vessels and piping systems, where does creep cracking typically develop?

A. Under high pressures
B. At locations with stress concentrations
C. Where fluid velocities are highest
D. None of the above

Correct Answer: B

Explanation:

Creep cracking is a critical failure mechanism in metals exposed to high temperatures over prolonged periods. It occurs due to the material’s tendency to deform slowly and permanently under sustained stress, a process known as creep deformation. This phenomenon is particularly relevant in components like pressure vessels, piping, and boilers that operate at elevated temperatures for years.

To fully grasp where creep cracking happens, two primary factors must be considered:

1. Elevated Metal Temperatures:
   Creep effects become significant when metals are heated to a substantial fraction of their melting point, typically around 40% to 50% of their absolute melting temperature (in Kelvin). For common materials such as steel, this means temperatures exceeding roughly 400°C (752°F). At these high temperatures, atomic movement within the metal lattice increases, enabling gradual, time-dependent deformation even if the applied stress is below the yield strength. This sustained strain leads to internal damage accumulation and eventual cracking.
   
   

2. Stress Concentrations:
   Creep cracking tends to initiate in areas where localized stresses are higher than the surrounding regions. These stress concentrations arise due to:
   
   

• Geometric discontinuities such as sharp corners, notches, or weld toes

• Heat-affected zones from welding processes

• Corrosion pits or other material imperfections

Because these localized stresses amplify the strain on the metal, they accelerate creep damage by promoting void formation and micro-cracks that can grow into significant cracks over time.

Now, evaluating the provided options:

• A. Pressures: While pressure inside vessels contributes to overall stress, it acts more uniformly and is not the main cause of creep cracking. The problem lies in local stress amplification rather than the general pressure.

• B. Stress concentrations: This is the correct answer. Creep cracking is strongly linked to these localized high-stress areas, combined with elevated temperature, which together create the conditions necessary for creep damage and crack propagation.

• C. Velocities: Flow velocity might influence erosion or corrosion but has no direct effect on creep, which is a thermomechanical failure mode unrelated to fluid dynamics.

• D. None of the above: Incorrect because option B accurately describes the cause.

In conclusion, creep cracking occurs primarily where elevated temperatures coincide with stress concentrations. These localized stress points intensify the creep deformation process, leading to crack initiation and growth, making B the correct answer.

Question 4:

What substance is commonly found filling thermal fatigue cracks?

A. Chlorides
B. Hydroslime
C. Oxides
D. Sulfides

Correct Answer: C

Explanation:

Thermal fatigue cracks develop in materials subjected to repeated heating and cooling cycles. These cycles cause the material to expand and contract, producing fluctuating stresses that gradually initiate and propagate cracks, especially near stress concentrators like welds, notches, or surface flaws.

One notable characteristic of thermal fatigue cracks is that they often become filled with oxides. This happens due to several interrelated processes:

Oxidation at Elevated Temperatures:
Most metals react with oxygen when exposed to high temperatures, forming metal oxides on their surfaces. Within a thermal fatigue crack, fresh metal surfaces are continuously exposed as the crack opens and closes during thermal cycling. This continuous exposure allows oxygen to penetrate deeply into the crack, leading to oxidation along the crack walls.

Crack Pumping Mechanism:
As the material heats and cools, the crack repeatedly opens and closes, acting like a pump that draws oxygen and other gases inside. This mechanism promotes oxidation within the crack, resulting in the gradual accumulation of oxide debris that fills the crack interior.

Diagnostic Significance:
The presence of oxide deposits inside cracks is a classic indicator used by failure analysts to identify thermal fatigue. Oxide-filled cracks differ from other cracking types, such as stress corrosion cracking or hydrogen embrittlement, which typically do not show this feature.

Considering the other options:

A. Chlorides: These ions are commonly involved in stress corrosion cracking, particularly in stainless steels, but they are not typical fillers of thermal fatigue cracks.

B. Hydroslime: This is not a recognized metallurgical term or known crack-filling substance and thus is invalid.

D. Sulfides: While sulfides exist as inclusions in some steels, they do not form as deposits inside thermal fatigue cracks. Sulfides may contribute to certain corrosion-related failures but are not characteristic of thermal fatigue crack filling.

In summary, the filling of thermal fatigue cracks by oxides results from the cyclic exposure of fresh metal surfaces to oxygen at elevated temperatures. This oxidation process inside the crack is a hallmark of thermal fatigue damage, making C the correct answer.

Question 5:

How does the thickness of a material section affect its resistance to brittle fracture, considering the constraint and triaxial stresses at the crack tip?

A. Thinner, Lower
B. Thicker, Lower
C. Thinner, Higher
D. Thicker, Higher

Correct Answer: B

Explanation:

This question addresses the relationship between material thickness, constraint at the crack tip, and the material’s ability to resist brittle fracture. Understanding this relationship is fundamental in fracture mechanics and materials science, especially when assessing structural integrity under stress.

In fracture mechanics, constraint refers to the limitation on plastic deformation near a crack tip. When the constraint is high, the material surrounding the crack tip cannot easily deform plastically, which leads to higher triaxial stresses—stress components acting in three dimensions—that promote brittle fracture.

Thicker sections inherently impose greater constraint at the crack tip. Because thicker materials provide less room for plastic deformation, the crack tip remains more constrained, which raises the triaxial stresses locally. This increased stress state intensifies the driving force for brittle fracture, thereby reducing the material’s resistance to fracture.

Conversely, thinner sections offer more freedom for plastic deformation around the crack tip. This “relaxation” helps blunt the crack tip, which lowers triaxial stresses and raises the material’s resistance to brittle fracture.

Examining the options:

• A (Thinner, Lower): Incorrect. Thinner materials generally have higher resistance to brittle fracture due to lower constraint, not lower resistance.

• B (Thicker, Lower): Correct. Thicker sections have higher constraint, increasing triaxial stresses at the crack tip, which results in lower resistance to brittle fracture.

• C (Thinner, Higher): Incorrect. While thinner materials have higher resistance, this is due to lower constraint, not higher.

• D (Thicker, Higher): Incorrect. Thicker materials do not have higher resistance; increased constraint reduces their resistance to brittle fracture.

Thus, the correct answer is B, which accurately represents how thicker material sections, by increasing crack tip constraint and triaxial stress, lead to lower resistance to brittle fracture.

Question 6:

What visual pattern typically characterizes stress corrosion cracks, making them identifiable on a material surface?

A. Tree shaped
B. Craze-cracked
C. Multiple crack
D. None of the above

Correct Answer: A

Explanation:

Stress corrosion cracking (SCC) is a serious failure mechanism that occurs when a material under tensile stress is exposed to a corrosive environment. It leads to crack formation that often develops suddenly and can cause catastrophic failures in critical components like pipelines, pressure vessels, and aircraft parts.

A key feature of SCC is its characteristic crack morphology. Rather than appearing as a single clean crack, SCC often shows a branched pattern where multiple subsidiary cracks extend from a main crack. This branching pattern resembles the limbs of a tree, which gives the cracks a distinctive “tree shaped” or dendritic appearance.

This tree-like morphology arises because corrosive agents penetrate the material along grain boundaries or weak planes, while tensile stress propagates the cracks outward in multiple directions. This results in the interconnected network of cracks spreading from the main crack, visually similar to branches stemming from a trunk.

Reviewing the options:

• A (Tree shaped): Correct. This term precisely captures the multi-branched pattern of SCC and is commonly used in materials failure analysis to describe the visual appearance of SCC cracks.

• B (Craze-cracked): Incorrect. Craze cracking refers mainly to fine networks of superficial cracks seen in polymers, which look like spider webs and differ significantly from SCC patterns in metals.

• C (Multiple crack): Incorrect. Although SCC involves multiple cracks, this phrase is vague and does not describe the distinctive interconnected branching.

• D (None of the above): Incorrect because "tree shaped" is a widely accepted and accurate descriptor.

In summary, the tree shaped description best represents the complex, branched crack patterns characteristic of stress corrosion cracking, aiding early detection and failure prevention. Therefore, the correct choice is A.

Question 7:

At what temperature should Post Weld Heat Treatment (PWHT) be performed to effectively prevent caustic embrittlement cracking?

A. 1100º F
B. 1150º F
C. 1200º F
D. 1250º F

Correct Answer: C

Explanation:

Caustic embrittlement is a specific type of stress corrosion cracking that typically occurs in carbon steel components exposed to highly alkaline environments, such as boilers or chemical process equipment. It arises when tensile stresses, elevated temperatures, and concentrated caustic solutions (high pH) combine, causing cracks along grain boundaries and ultimately compromising the material’s integrity.

One of the key methods to prevent this damaging phenomenon is Post Weld Heat Treatment (PWHT). Welding inherently introduces residual stresses into the material because of uneven heating and cooling cycles. These residual tensile stresses become focal points for crack initiation when the material is exposed to caustic substances. PWHT helps by heating the welded material to a specific temperature, holding it there for a set duration, and then cooling it gradually. This process relieves residual stresses and modifies the microstructure to increase resistance against cracking.

Selecting the correct PWHT temperature is crucial for effectively mitigating caustic embrittlement. Generally, PWHT temperatures for carbon steel range from 1100ºF to 1250ºF. However, for preventing caustic embrittlement specifically, the optimum temperature is widely accepted to be around 1200ºF.

Evaluating the options:

• 1100ºF: While this temperature falls within the general range, it may not provide sufficient stress relief for environments where caustic embrittlement is a concern. It’s often viewed as the lower threshold of the effective range.

• 1150ºF: Closer to the ideal range but might still leave some residual stresses unreleased, especially in severe service conditions.

• 1200ºF: This temperature strikes the right balance. It effectively reduces residual stresses without causing undesirable microstructural changes or grain growth, thereby optimizing resistance to caustic cracking.

• 1250ºF: Though this temperature may relieve stresses well, it carries risks like excessive grain coarsening or reduced mechanical properties if not tightly controlled.

In conclusion, PWHT performed at approximately 1200ºF is the most effective and commonly recommended temperature for preventing caustic embrittlement cracking in welded carbon steel components. Thus, C is the correct answer.

Question 8:

When a liquid’s temperature nears its boiling point, how does this affect the likelihood of cavitation caused by vapor bubble formation?

A. Less likely
B. More likely
C. Not likely
D. None of the above

Correct Answer: B

Explanation:

Cavitation is a damaging process in fluid systems, caused by the rapid formation and violent collapse of vapor bubbles within a liquid. This phenomenon usually occurs when the pressure in the liquid falls below its vapor pressure at a given temperature. When vapor bubbles collapse near surfaces such as pump impellers or propellers, they release intense energy that erodes metal and reduces equipment life.

To understand why cavitation becomes more probable as a liquid’s temperature approaches its boiling point, it’s essential to consider the relationship between temperature, vapor pressure, and bubble formation.

Every liquid has a characteristic vapor pressure — the pressure at which molecules escape the liquid phase and form vapor. This vapor pressure rises as the liquid’s temperature increases. At the boiling point, the vapor pressure equals the ambient pressure, causing the liquid to boil and form bubbles freely.

When a liquid is near its boiling temperature, its vapor pressure is high. This means it requires only a slight drop in pressure for vapor bubbles to form within the fluid. For example, water at 100°C has a vapor pressure equal to atmospheric pressure, so bubbles form easily. At lower temperatures, the vapor pressure is lower, so a more significant pressure drop is needed to initiate cavitation.

In practical terms, this makes hot fluids much more prone to cavitation. Pumps or turbines operating with fluids close to their boiling points can experience cavitation even with minor local pressure dips. This leads to faster wear, noise, vibration, and performance issues.

For instance, a pump moving water at 95°C might suffer cavitation damage more readily than the same pump moving cooler water at 50°C, where vapor bubble formation is harder due to the lower vapor pressure.

Therefore, operating conditions where the fluid temperature is near boiling must be carefully managed to prevent cavitation—usually by maintaining adequate net positive suction head (NPSH) or controlling fluid velocities.

In summary, the closer the fluid temperature is to its boiling point, the more likely vapor bubbles will form, making cavitation damage more probable. Hence, the correct answer is B. More likely.

Question 9:

A flanged line made of A106 Grade B carbon steel, carrying caustic wash water at 200°F, shows signs of atmospheric corrosion. 

Which of the following factors is most likely to have accelerated this corrosion?

A. Sulfides
B. Fly ash
C. Caustic
D. None of the above

Answer: B

Explanation:

To understand the cause of atmospheric corrosion on an A106 Grade B carbon steel pipe carrying caustic wash water at elevated temperature, it is essential to consider the material characteristics and the environmental conditions that promote corrosion.

A106 Grade B is a common carbon steel widely used in piping for high-temperature industrial applications, including chemical plants and refineries. While it has good mechanical strength and thermal resistance, it does not inherently resist corrosion, particularly when exposed to atmospheric contaminants.

Atmospheric corrosion occurs when the external surface of the pipe reacts with environmental elements such as moisture, oxygen, and airborne pollutants. This type of corrosion can be accelerated by the presence of deposits or particulate matter that trap moisture and create localized acidic or corrosive microenvironments.

Let's analyze each option:

• A. Sulfides: Sulfides, such as hydrogen sulfide (H₂S) or sulfur compounds, can cause corrosion, especially in internal or process environments through sulfidation. However, atmospheric corrosion due to sulfides typically requires the presence of sulfur gases in the air. Without evidence of a sulfur-rich atmosphere, sulfides are less likely the main cause here.

• B. Fly ash: Fly ash consists of fine particulate residues from combustion processes, especially coal-fired power plants. It contains oxides of silicon, aluminum, iron, and other metals, along with unburned carbon particles. When fly ash settles on exposed surfaces, it can trap moisture and create acidic conditions when combined with atmospheric water vapor. This trapped moisture and the chemical nature of fly ash can accelerate localized atmospheric corrosion, especially at joints or flanged connections where moisture can persist. Therefore, fly ash is a plausible and common contributor to external corrosion in industrial areas.

• C. Caustic: Caustic substances like sodium hydroxide are highly corrosive but typically cause corrosion internally where the liquid contacts the metal. Caustic corrosion on external surfaces is unlikely unless there is a leak or spillage exposing the outside pipe surface. Since the question focuses on atmospheric corrosion rather than internal or chemical process corrosion, caustic is an improbable cause here.

• D. None of the above: This option is incorrect because fly ash is a well-known cause of accelerated atmospheric corrosion in industrial environments.

In conclusion, the presence of fly ash deposits on the pipe’s external surfaces can trap moisture and create conditions conducive to corrosion. This makes fly ash the most likely factor accelerating atmospheric corrosion on the flanged carbon steel pipe carrying caustic wash water at 200°F.

Question 10:

What term describes the microstructural change in carbon steels exposed to temperatures between 850°F and 1400°F, where the carbides become unstable and transform from their usual plate-like shape into rounded particles?

A. Carburization
B. Spheroidization
C. Graphiding
D. 885º Embrittlement

Answer: B

Explanation:

The microstructural phenomenon where carbon steel carbides transform when held within a temperature range of approximately 850°F to 1400°F is called spheroidization. This transformation fundamentally changes the morphology of carbides, which greatly influences the mechanical properties of the steel.

Carbon steels typically contain iron carbide (cementite, Fe₃C) distributed in pearlitic structures as thin, plate-like lamellae. When steel is exposed to elevated subcritical temperatures—especially below the eutectoid temperature (~1333°F or 723°C)—for extended periods, these lamellar carbides become thermodynamically unstable. Over time, the elongated plate-like carbides tend to break down and agglomerate into spherical or globular forms. This shape change minimizes the surface energy of the carbide phase, making the steel microstructure more stable under the given thermal conditions.

This spheroidization process has a significant effect on the steel’s mechanical behavior. Rounded carbides reduce internal stresses and allow the material to deform more easily, which leads to increased ductility and toughness but reduced hardness and strength. Consequently, spheroidization is often employed intentionally in heat treatments to improve machinability of high-carbon and tool steels by softening the material.

Now let’s briefly consider the other options:

• A. Carburization: This process involves adding carbon to the surface of a steel to harden it. It is a surface modification treatment, not a microstructural transformation of existing carbides within the bulk material.

• C. Graphiding: This refers to the formation of free graphite in steel, typically seen after prolonged high-temperature exposure in certain alloy systems. This transformation results in carbide decomposition into graphite and is different from spheroidization of carbides.

• D. 885º Embrittlement: This embrittlement phenomenon affects ferritic stainless steels exposed near 885°F due to precipitation of brittle phases like sigma phase. It is unrelated to carbide morphology changes in carbon steels.

In summary, spheroidization is a temperature-dependent microstructural change in carbon steels where the carbide phase transforms from plates into more stable, rounded shapes. This transformation enhances ductility and machinability, which is critical for processing steels that require softening before further mechanical shaping or finishing.