The following criteria which are intended to assess susceptibility to the occurrence of a specific failure mode are based on the pattern set below describing each failure mode.
Key factors
Causes: What are the root causes of this failure mode? Which materials are more susceptible? Which mechanical, thermal and chemical loads initiate this failure? Which manufacturing processes or procedures contribute to this failure?
Type of Damage: Which type of damage is caused by this failure mode? When can it occur? Where does it occur? What is the propagation rate? What are the possible consequences if damage occurs?
Criterion for Assessing Susceptibility
Criterion for Minimizing Risk: What is the tolerance limit for this damage? Which inspection techniques should be used to detect the damage? Which are the corrective actions to be taken for minimizing risk?
Table 3 Determination of a Failure Mode
Material Screening questions Failure modes
Austenitic stainless steels
Is the material exposed to chlorides and water in the 35–150C temperature range?
Chloride stress Is the material exposed to temperatures
between 600C and 900C?
Creep Ferritic stainless
steels
Is the material a high chromium (>13%) ferritic steel and the operating temperature between 350C and 550C?
475C Embrittlement
Cr–Mo steels Is the material 114Cr–1=2 Mo,
21=4 Cr–1=2 Mo, 3Cr–1Mo steel and the operating temperature
between 345C and 565C?
Temper embrittlement
C and low-alloy steels
Is the operating temperature >200C, and the operating pressure >0.5 MPa, and the partial pressure of hydrogen high?
Hydrogen attack
High temperature superalloys
Is the operating temperature >650C and is the material exposed to sulfur and sodium, and=or potassium compounds?
Oxidation–sulfidation phenomenon (hot corrosion)
This pattern can be used to describe other failure modes, which have not been included here for space reasons. The following mechanisms will be analyzed:
(A) thinning mechanisms, (B) cracking mechanisms:
(a) high temperature hydrogen attack, (b) chloride stress corrosion cracking, and (c) high temperature hydrogen attack, (C) metallurgical changes:
(a) brittle fracture:
(i) temper embrittlement,
(ii) sigma-phase embrittlement, and (iii) 475C (885F) embrittlement, (D) material processing:
(a) suitability of general structural steels for fusion welding:
(i) tendency to brittle fracture, (ii) tendency to age,
(iii) tendency to harden,
(iv) hydrogen assisted cold cracking, HACC (v) tendency to segregate,
(vi) tendency to lamellar tearing (anisotropy), (vii) weld metal solidification cracking, (viii) HAZ liquation cracking, and
(xi) tendency to lamellar tearing.
A. Thinning Mechanisms 1. Causes
There exist several damage modes that result in the type of damage characterized as material thinning. If the rate of thinning is greater than expected, thinning will eventually result in failure due to ductile overload. There are two types of damage: general thinning and localized thinning. General thinning is defined as a relatively uniform metal loss over a significant area of the equipment. This type of damage includes uniform corrosion, such as ammine corrosion, sour water corrosion, hydrofluoric acid corrosion, sulfuric acid (H2SO4) corrosion, high temperature (H2S=H2) corrosion, high temperature sulfi-dic=naphthenic acid corrosion, hydrochloric (HCl) corrosion, and high temperature oxidation.
2. Type of Damage
An important step in design is to determine the locations where general or localized thin-ning is potentially active. Localized thinthin-ning includes pitting corrosion resulting in numer-ous surface cavities, crevice corrosion resulting from the concentration of aggressive chemical species, selective galvanic corrosion in the region between two electrochemically different metals, erosion–corrosion, corrosion-fatigue, wear. Wear is usually defined as the
underside removal of material from contacting surfaces through mechanical action. Wear includes abrasive wear, erosion wear, grinding wear, gouging wear, adhesive wear, fretting wear, contact stress fatigue, cavitation fatigue, etc. The damage mechanism must be con-sidered during the review of the material selection, and the material surface treatment spe-cification. Environmental features, such as flow velocity and critical concentration of corroded agents must be considered.
3. Criterion for Assessing Susceptibility
The first step is to identify which damage modes are potentially active and to determine the estimated rate of thinning. The corrosion rate can be predicted by standard corrosion curves. The fraction of wall loss due to thinning can be calculated on the basis of the design life, thinning rate and thickness.
4. Criterion for Minimizing Risk
As discussed previously, uniform corrosion can be avoided through adequate material selection and corrosion allowance. To minimize wear, the type of material, chemical com-position, microstructures, and hardness must be considered in the material specification.
The susceptibility of the selected material shall be assessed by corrosion or wear test included in the material specification during the detail design stage. The designer can recommend that the equipment be assessed for thinning during service. In-service inspec-tion includes intrusive and non-intrusive inspecinspec-tion with visual examinainspec-tion, ultrasonic thickness measurements or profile radiography; in the case of localized corrosion, the areas to be examined shall be specified by a designer.
B. Cracking Mechanisms
1. High Temperature Hydrogen Attack
a. Causes. Hydrogen attack occurs on carbon and low-alloy steels when they are exposed to temperatures above 200C (390F) and subjected to a high partial hydrogen pressure. Hydrogen attack can cause catastrophic fractures on half-pressure units, such as catalytic reformers and becomes a high risk factor for them due to thermal, mechanical, and chemical loads, associated with this type of reactors. This kind of degradation is caused by the presence of hydrogen during service. Hydrogen can dissolve in metal as atomic hydrogen reacting with iron carbides so as to produce methane. This methane can-not diffuse out of the metal. If the methane pressure is sufficient, cracks can arise in the steel. Internal decarburization may occur at high temperatures. The accumulation of methane in discontinuities, usually in grain boundaries, causes high stresses and micro-voids, which can lead to cracking or blister formation in the case of steels with a high con-tent of inclusions or laminations. Internal cracking is more frequent in carbon, C–12Mo and Cr–Mo steels when the partial hydrogen pressure is high, whereas decarburization is more common in Cr–Mo steels exposed to high temperatures and subjected to low par-tial hydrogen pressures. In an advanced stage of damage, we can obtain decarburized cracked steel whose strength is lower than half its original strength and its ductility is reduced to zero. Nelson’s curves (API RP 941) [21] enable the assessment of susceptibility to this type of damage for specific operating conditions and material composition. Steels containing only Fe3C carbides are the most susceptible ones, while alloyed steels contain-ing more stable carbides are more resistant. Base metal and postweld heat treatments affect the material resistance to hydrogen attack. A heat treatment at temperatures above
649C (1200F) and below the lowest critical temperature is recommended for Cr–Mo steels. The degree of a material cold deformation affects hydrogen service. Hydrogen attack is accelerated with cold deformation.
b. Type of Damage. In the initial stage, the hydrogen attack damage mode causes microvoids in grain boundaries, which collapse forming intergranular microcracks. These interconnected microcracks cause macroscopic cracks that may be connected with the sur-face. The weld heat-affected zone can be easily attacked, and this attack may result in the catastrophic collapse of the component. Since hydrogen attack is controlled by diffusion, the time for it to occur and the operation temperature are closely related. In some cases, the resulting microcracks are not easily distinguishable from those resulting from the creep mechanism. The presence of tramp elements, such as As, Sb, Sn, and P increases the sus-ceptibility to this phenomenon.
c. Criterion for Assessing Susceptibility. The susceptibility to this phenomenon can be assessed through the parameter
Pv¼ logðPH2Þ þ 3:09 104ðTÞðlogðtÞ þ 14Þ
where PH2is the partial hydrogen pressure (Kgf=cm2), T the temperature (K), and t is the time (hours). With the operation temperature and the estimated partial hydrogen pressure, and considering 200,000 hr of service, we can calculate the Pv parameter; in the case of carbon steel, if Pv¼ 4.53, the material exposed to these conditions is considered to be not susceptible to this damage mode [6].
d. Criterion for Minimizing Risk. This type of damage can be detected through ‘‘in situ’’ metallography, and advanced ultrasonic backscatter technique (AUBT). When the damage is advanced and connected with the surface, the magnetic particles or dyed pene-trant tests are effective. If operating conditions are characterized by a Pv> 4.40, an ade-quate selection of the material using Nelson’s curves is recommended. In addition, Cr–Mo steels should be specified in accordance with the criteria required for avoiding temper embrittlement. Stress relief heat treatments are also recommended after both welding pro-cesses and those propro-cesses involving cold deformation.
2. Chlorides Stress Corrosion Cracking (SCC)
a. Causes. Stress corrosion cracking may arise when a susceptible material, such as austenitic stainless steel containing 8–10% nickel, is simultaneously combined with certain levels of tensile stresses and chloride ion concentrations in an aqueous environment and within a 35C (95F)–200C (390F) temperature range. Tensile residual stresses, resulting from manufacturing processes like welds, contribute in causing this type of damage. Also, cold plastic deformation causes hazardous residual stresses.
b. Type of Damage. Chloride stress corrosion cracking causes surface cracks, which are generally transgranular and branched, and initiate on the surface propagating in a direction perpendicular to tensile stresses. The cracking surface is similar to that of brittle fracture, with little or null plastic deformation. Damage can occur during service or shut-down periods in the inner side of a container due to aggressive content conditions, as well as on its external surface under insulations. Hydraulic test conditions should also be con-sidered. Those zones under high mechanical strength are more susceptible. Pitting corro-sion can contribute to concentrate chlorides and initiate stress corrocorro-sion. It is difficult to estimate a damage rate for stress corrosion cracking, since it can be fast enough, and dif-ficult to notice through visual inspection, to cause significant losses or the brittle collapse of the component.
c. Criterion for Assessing Susceptibility. This failure mode can be operative in the case of an austenitic stainless, 304=316 steel, subjected to tensile stresses arising from the manufacturing process or service loading conditions (tensile stresses>80 MPa are enough, provided that the chloride content is higher than 1 ppm and the temperature is above 35C (95F). High susceptibility is associated with aqueous solutions with pH<10, temperatures above 65C (150F) and a chloride content> 10 ppm. Chloride contents over 1000 ppm can cause stress corrosion cracking even with pH> 10, provided the temperature is above 90C (195F). If austenitic stainless steels covered by insulation operate in a continuous or intermittent manner at temperatures ranging between 35C (95F) and 150C (300F), they are susceptible to this damage mode.
d. Criterion for Minimizing Risk. The likelihood that this failure mode will become operative must be avoided during design due to the severe consequences of the associated type of damage mode. Tensile stresses must be reduced by controlling manufacturing pro-cesses and design. Postweld stress relief heat treatments can be used to minimize suscept-ibility to stress corrosion cracking, attempting to avoid stainless steel sensitization by using stabilized or low carbon grades. If corrosive environments causing stress corrosion on the selected material cannot be avoided, the selection of another material should be evaluated.
The materials to be used can be those with a high Ni content, such as superaustenitic stain-less steels or Ni basis superalloys, or alloys with a low Ni content such as duplex stainstain-less steels or ferritic stainless steels of high purity (extra low interstitials). Some titanium alloys can also be selected.
C. Metallurgical Changes 1. Brittle Fracture
a. Causes. Brittle fracture is the sudden failure of a structural component, which usually initiates in a pre-existent flaw. They can be highly hazardous due to the cost result-ing from equipment replacement, as well as possible damages to the personnel, environ-ment and facilities. In order to minimize risks associated with brittle fracture, the materials are required to have adequate toughness when they are being used, even when they do not operate during service at low temperatures. Material toughness is the materi-al’s ability to absorb energy as plastic deformation without breaking. Brittle fracture depends on the following factors: (a) Stress multi-axiality: high loading levels and their spatial distribution in three directions increase the tendency to brittle fracture. If the loads are too low, brittle fracture does not occur. (b) Residual stresses associated with a weld increase the tendency to brittle fracture. Postweld heat treatments are beneficial. (c) Large thickness is more susceptible. (d) Load application rate: a sudden application of loads may contribute to brittle fracture. (e) Material properties: fine grain structures, such as tem-pered martensite, with low contents of impurities, are associated with a high degree of toughness. Other microstructural elements, such as precipitates, second-phase particles, dislocations, and solutes in a solid solution contribute to increase the sybut deteriorate toughness. (f) Temperature: a wide range of materials, ferritic steels, in particular, present a transition temperature between ductile behavior at high temperatures and brittle beha-vior below these temperatures. The minimum metal temperature should always be above the material transition temperature. The metal temperature considered is the lowest tem-perature among the operation, upsets, and the hydraulic test temtem-peratures; in the case of pressurized liquids, the boiling point of the liquid at atmospheric pressure should be con-sidered. The brittle fracture is not very common, since design stresses are low enough to prevent it from occurring. In general, this failure mode is not operative at temperatures
above 150C (300F). Those flaws that may initiate this fracture mechanism are difficult to detect through conventional non-destructive tests; therefore, it is not possible to develop a strategy for minimizing the risk of brittle fracture in the elimination of crack-like flaws exclusively. In ferritic steels, a ductile-to-brittle transition occurs as the temperature decreases. High-strength steels can undergo a less abrupt ductile-to-brittle transition, although they may register lower values of absorbed energy within the ductile range.
Titanium alloys, aluminum, as well as austenitic stainless steels register no transition between ductile fracture and brittle fracture modes depending on the temperature. Frac-ture toughness decreases with an increase in the strain rate, and increases with the temperature in a manner similar to the variation of absorbed energy with the temperature in the Charpy curve.
b. Type of Damage. Design should be intended to avoid susceptibility to the initia-tion and propagainitia-tion of cracks. At low temperatures, the fracture occurs due to comple-tely brittle cleavage mechanisms, absorbing very little energy; thus cracks may initiate and propagate easily. At high temperatures, on the other hand, the fracture occurs due to duc-tile dimple mechanisms, with a significant absorption of energy. Cracks do not easily initiate and propagate in the upper shelf region of the curve; while in the ductile–brittle transition zone, though they initiate easily, they propagate with difficulty. The damage associated with fracture toughness may occur when the operation temperature is low, or during the equipment shutdown and start-up periods. Stress concentrators prove to be critical regions for the initiation of brittle fractures. A subcritical crack growth rate may result from different failure modes, such as creep, low-cycle fatigue, hydrogen embrittle-ment, and stress corrosion cracking. When the crack exceeds the critical size, it can pro-pagate in a sudden way causing the catastrophic fracture of the component.
c. Criterion for Assessing Susceptibility. There are several methodologies used to determine a structural component’s susceptibility to brittle fracture, such as those pre-sented in the ASME Boiler and Pressure Vessel Code API 581 (6,16) and API RP 579 [20]. These design criteria are intended to prevent brittle fracture due to low temperatures or low toughness, or when taking into consideration the applied loads, the material spe-cification (often qualified for use by performing impact tests), the minimum temperature, residual stresses, postweld heat treatments, and thickness.
d. Criterion for Minimizing Risk. Once a component has proven to be susceptible to brittle fracture, the material with adequate fracture toughness properties should be speci-fied and the performance of mechanical tests, such as the Charpy test and more complex fractomechanical tests should be required. The Charpy V-notch impact test is widely used to characterize the ductile-to-brittle transition in steels. In general, in low strength steels, this test is used as a quality control criterion for structural steels, establishing a minimum level of absorbed energy in order to determine ductile-to-brittle transition temperature, thus steels can be compared. The Charpy test is of a relative value for design, since the notch does not really represent possible present flaws, the test piece size does not match the real thickness, and the loading conditions are not real, either. Other tests (CAT: crack arrest transition temperature, FATT: fracture–appearance, and FTE: fracture-transition elastic) enable the designer to determine not only the ductile-to-brittle transition ture but also the critical stresses for the propagation of cracks depending on the tempera-ture, thus allowing one to define the temperature above which propagation is difficult. In order to analyze whether a certain flaw (characterized by its size and shape) is critical for the present stress levels on a structure, the Fracture Mechanics approach is used [22].
According to the LEFM (Linear Elastic Fracture Mechanics) approach, the crack propa-gates when the stress intensification factor, K, which depends on the geometry, mode and
magnitude of the applied loads, and on the size, shape and location of defects, reaches a critical Kcvalue. This approach is valid for those materials in which plastic deformation is restricted to the tip of a crack and the analyzed defects are crack-like defects with zero radius at the tip of the crack. Fracture toughness is determined by means of the ASTM Standard E 399 [23]. To assess those materials whose behavior involves a large plastic deformation around the tip of a crack, the EPFM (Elastic Plastic Fracture Mechanics) has been developed. The concept used is that of the J-integral, which leads to a parameter for fracture toughness known as JIC. The method used to determine the JICparameter is defined in the ASTM 813 [24]. ASME Section VIII Division 2 rules specify toughness in terms of a minimum value of absorbed energy (27 J (20 ft-lb)) at the minimum allowable temperature. Other criteria determine 54 J (40 ft-lb) at 10C (50F) or at the minimum operation temperature. This value corresponds to approximately KIc¼ 137 MPa m1=2. In order to meet the specified requirements for toughness in service, the materials should be manufactured through secondary-metallurgy practices so as to ensure an adequate degree of homogeneity, the control of impurities, inclusion shape and size, and sulfur and oxygen contents, and applying fine grain practice and thermal treatments, such as normalizing and tempering or quenching and tempering. In the case of steels, this is accomplished by means of microalloying, thermo-mechanical treatments, and postweld stress relief heat treatments. With respect to impact loads, it is recommended to design the component as an energy absorbing system, with high resilience modulus (sy2=2E), with sufficient ductility to release stresses in high concentration zones and adequate fatigue resistance. Stress concentrators must be reduced to the minimum, avoiding abrupt sec-tion changes. If possible, the material should be used, so that impact load direcsec-tions
magnitude of the applied loads, and on the size, shape and location of defects, reaches a critical Kcvalue. This approach is valid for those materials in which plastic deformation is restricted to the tip of a crack and the analyzed defects are crack-like defects with zero radius at the tip of the crack. Fracture toughness is determined by means of the ASTM Standard E 399 [23]. To assess those materials whose behavior involves a large plastic deformation around the tip of a crack, the EPFM (Elastic Plastic Fracture Mechanics) has been developed. The concept used is that of the J-integral, which leads to a parameter for fracture toughness known as JIC. The method used to determine the JICparameter is defined in the ASTM 813 [24]. ASME Section VIII Division 2 rules specify toughness in terms of a minimum value of absorbed energy (27 J (20 ft-lb)) at the minimum allowable temperature. Other criteria determine 54 J (40 ft-lb) at 10C (50F) or at the minimum operation temperature. This value corresponds to approximately KIc¼ 137 MPa m1=2. In order to meet the specified requirements for toughness in service, the materials should be manufactured through secondary-metallurgy practices so as to ensure an adequate degree of homogeneity, the control of impurities, inclusion shape and size, and sulfur and oxygen contents, and applying fine grain practice and thermal treatments, such as normalizing and tempering or quenching and tempering. In the case of steels, this is accomplished by means of microalloying, thermo-mechanical treatments, and postweld stress relief heat treatments. With respect to impact loads, it is recommended to design the component as an energy absorbing system, with high resilience modulus (sy2=2E), with sufficient ductility to release stresses in high concentration zones and adequate fatigue resistance. Stress concentrators must be reduced to the minimum, avoiding abrupt sec-tion changes. If possible, the material should be used, so that impact load direcsec-tions