API571-Part3

4.2 Mechanical & Metallurgical Failure Mechanisms

4.2 Mechanical and Metallurgical Failure Mechanisms

4.2.1 Graphitization.4.2.2 Softening (Spheroidization).4.2.3 Temper Embrittlement.4.2.4 Strain Aging.4.2.5 885F (475C) Embrittlement.4.2.6 Sigma Phase Embrittlement .4.2.7 Brittle Fracture.4.2.8 Creep and Stress Rupture.4.2.9 Thermal Fatigue .4.2.10 Short Term Overheating Stress Rupture.4.2.11 Steam Blanketing.4.2.12 Dissimilar Metal Weld (DMW) Cracking.4.2.13 Thermal Shock.4.2.14 Erosion/Erosion Corrosion.4.2.15 Cavitation.4.2.16 Mechanical Fatigue.4.2.17 Vibration-Induced Fatigue.4.2.18 Refractory Degradation.4.2.19 Reheat Cracking.

Par. 3. - Definitions 4.2.3 Temper Embrittlement 4.2.7 Brittle Fracture 4.2.9 Thermal Fatigue 4.2.14 Erosion/Erosion-Corrosion 4.2.16 Mechanical Failure 4.3.2 Atmospheric Corrosion 4.3.3 Corrosion Under Insulation (CUI) 4.3.4 Cooling Water Corrosion 4.3.5 Boiler Water Condensate Corrosion 4.3.10 Caustic Corrosion 4.4.2 Sulfidation 4.5.1 Chloride Stress Corrosion Cracking (Cl-SCC) 4.5.2 Corrosion Fatigue 4.5.3 Caustic Stress Corrosion Cracking 5.1.2.3 Wet H2S Damage (Blister/HIC/SOHIC/SCC) 5.1.3.1 High Temperature Hydrogen Attack (HTHA)

Par. 3 Definitions 4.2.7 Brittle Fracture 4.2.9 Thermal Fatigue 4.2.14 Erosion/Erosion Corrosion 4.2.16 Mechanical Fatigue 4.2.17 Vibration-Induced Fatigue 4.3.1 Galvanic Corrosion 4.3.2 Atmospheric Corrosion 4.3.3 Corrosion Under Insulation (CUI) 4.3.5 Boiler Water Condensate Corrosion 4.3.7 Flue Gas Dew Point Corrosion 4.3.8 Microbiological Induced Corrosion (MIC) 4.3.9 Soil Corrosion 4.4.2 Sulfidation 4.5.1 Chloride Stress Corrosion Cracking (Cl-SCC) 4.5.3 Caustic Stress corrosion Cracking 5.1.3.1 High Temperature Hydrogen Attack (HTTA)

2013-API510 Examination2013- API570 Examination

300, 400 & Duplex SS containing ferrite phases

600 F~ 1000 F885F embrittlement

510

Exam Affected materialsTemperaturesDamage Mechanism

Ferritic, austenitic & duplex SS.Sigma forms most rapidly from the ferrite phase that exists in 300 Series SS and duplex SS weld deposits. It can also form in the 300 Series SS base metal (austenite phase) but usually more slowly.

1000 F~ 1700 FSigma-Phase Embrittlement

Pre-1980s C-steels with a large grain size and C- Mo

Intermediate temperatureStrain Aging

2 Cr-1Mo low alloy steel, 3Cr-1Mo (lesser extent), & HSLA Cr-Mo-V rotor steels

650 F~ 1070 FTempered Embrittlement

Low alloy steel up to 9% Cr850 F ~ 1400 FSpheroidisation

Plain carbon steelC, C Mo

800F~1100F for C Steel875F for C Mo Steel

Graphitisation

All metals and alloys.Cold liquid impinge on hot surface

Thermal Shock

510/570

510/570


Carbon steel / 300 SS junction

Operating temperatureDissimilar Metal Weld (DMW) Cracking

Carbon steel and low alloy steels

>1000 FSteam Blanketing

All fired heater tube materials and common materials of construction

>1000 FShort Term Overheating Stress Rupture

All materials of constructionOperating temperatureThermal fatigues

All metals and alloys700 F ~ 1000 FCreep & stress ruptureC, C- Mo, 400 SSBelow DTBTTBrittle Fracture

AllService temperatureErosion/Erosion Corrosion

570/510

AllService temperatureMechanical fatigue570/510

AllService temperatureVibration-Induced Fatigue

570


4.2.1 Graphitization(API510/570)

Prolong Exposure800F ~ 1100F for C Steel>875F for C Mo Steel

4.2.1 Graphitization 4.2.1.1 Description of Damage

a) Graphitization is a change in the microstructure of certain carbon steels and 0.5Mo steels after long-term operation in the 800F to 1100F (427C to 593C) range that may cause a loss in strength, ductility, and/or creep resistance.800F 1100F

b) At elevated temperatures, the carbide phases in these steels are unstable and may decompose into graphite nodules. This decomposition is known as graphitization./C - Mo, ,

4.2.1.2 Affected Materials

Some grades of carbon steel and 0.5Mo steels. (/0.5)

/C - Mo, ,

Graphitization Location:Areas with tubes containing carbon steel and C-Mo. Most likely in the weld heat-affected zones and high residual stress areas. : C- Mo ,

Probable cause:Prolonged exposure to above 800F (425C) for carbon steels and greater than 875F (470C) for the carbon- molybdenum alloys. In graphitized boiler components, the nucleation of graphite likely starts by the precipitation of carbon from super-saturated ferrite, an aging phenomenon. This nucleation is enhanced by strain, in effect a strain aging. The preferential formation of graphite within the heat-affected zone is dependent on the balance of the structure being nearly strain free. Thus the more unstable heat-affected zone microstructure will decompose into ferrite and graphite before the annealed ferrite and pearlite of the normalized structure will. If the base metal is cold-worked, the annealing of the weld will slow the nucleation of graphite, and the strained tube will graphitize before the heat-affected zone.

,(),,.

4.2.1.3 Critical Factors

a) The most important factors that affect graphitization are the chemistry, stress, temperature, and time of exposure.

b) In general, graphitization is not commonly observed. Some steels are much more susceptible to graphitization than others, but exactly what causes some steels to graphitize while others are resistant is not well understood. It was originally thought that silicon and aluminum content played a major role but it has been shown that they have negligible influence on graphitization.

c) Graphitization has been found in low alloy C-Mo steels with up to 1% Mo. The addition of about 0.7% chromium has been found to eliminate graphitization.

d) Temperature has an important effect on the rate of graphitization. Below 800F (427C), the rate is extremely slow. The rate increases with increasing temperature.

e) There are two general types of graphitization. First is random graphitization in which the graphite nodules are distributed randomly throughout the steel. While this type of graphitization may lower the room-temperature tensile strength, it does not usually lower the creep resistance.

f) The second and more damaging type of graphitization results in chains or local planes of concentrated graphite nodules.

Weld heat-affected zone graphitization is most frequently found in the heat-affected zone adjacent to welds in a narrow band, is called eyebrow, graphitization.

Non-weld graphitization is a form of localized graphitization that sometimes occurs along planes of localized yielding in steel. It also occurs in a chain-like manner in regions that have experienced significant plastic deformation as a result of cold working operations or bending.

Weld heat affected zone graphitization

is most frequently found in the heat-affected zone adjacent to welds in a narrow band, corresponding to the low temperature edge of the heat affected zone. In multi-pass welded butt joints, these zones overlap each other, covering the entire cross-section. Graphite nodules can form at the low temperature edge of these heat affected zones, resulting in a band of weak graphite extending across the section. Because of its appearance, this graphite formation within heat affected zones is called eyebrow graphitization.

Type 2: HAZ graphite nodules

eyebrow! Type2 Graphitization

eyebrow! Graphitization

,..NMD

eyebrow! Type 2 Graphitization

Non-weld graphitization is a form of localized graphitization that sometimes occurs along planes of localized yielding in steel. It also occurs in a chain-like manner in regions that have experienced significant plastic deformation as a result of cold working operations or bending. , .

Type 2: Non Weld Chains or local planes of concentrated graphite nodules

4.2.1.4 Affected Units or Equipment

a) Primarily hot-wall piping and equipment in the FCC, catalytic reforming and coker units.

b) Bainitic grades are less susceptible than coarse pearlitic grades.c) Few failures directly attributable to graphitization have been reported in the

refining industry. However, graphitization has been found where failure resulted primarily from other causes.

d) Several serious cases of graphitization have occurred in the reactors and piping of fluid catalytic cracking units, as well as with carbon steel furnace tubes in a thermal cracking unit and the failure of seal welds at the bottom tube sheet of a vertical waste heat boiler in a fluid catalytic cracker. A graphitization failure was reported in the long seam weld of a C 0.5Mo catalytic reformer reactor/inter-heater line.

e) Where concentrated eyebrow graphitization occurs along heat-affected zones, the creep rupture strength may be drastically lowered. Slight to moderate amounts of graphite along the heat-affected zones do not appear to significantly lower room or high-temperature properties.

f) Graphitization seldom occurs on boiling surface tubing but did occur in low alloy C-0.5Mo tubes and headers during the 1940s. Economizer tubing, steam piping and other equipment that operates in the range of temperatures of 850F to 1025F (441C to 552C) is more likely to suffer graphitization.

4.2.1.5 Appearance or Morphology of Damage

1. Damage due to graphitization is not visible or readily apparent and can only be observed by metallographic examination (Figure 4-1 and Figure 4-2).

2. Advanced stages of damage related to loss in creep strength may include micro-fissuring / microvoid formation, subsurface cracking or surface connected cracking.

0.5m

Figure 4-1 High magnification photomicrograph of metallographic sample showing graphite nodules. Compare to normal microstructure shown in Figure 4-2.

Figure 4-2 High magnification photomicrograph of metallographic sample showing typical ferrite-pearlite structure of carbon steel.

4.2.1.6 Prevention / Mitigation /Graphitization can be prevented by using chromium containing low alloy steels for long-term operation above 800F (427C).

The addition of about 0.7% chromium has been found to eliminate graphitization.

0.7% chromium.

4.2.1.7 Inspection and Monitoring

a) Evidence of graphitization is most effectively evaluated through removal of full thickness samples for examination using metallographic techniques. Damage may occur mid-wall so that field replicas may be inadequate.

b) Advanced stages of damage related to loss in strength include surface breaking cracks or creep deformation that may be difficult to detect.

4.2.1.8 Related Mechanisms

Spheroidization (see 4.2.2) and graphitization are competing mechanisms that occur at overlapping temperature ranges. Spheroidization tends to occur preferentially above 1025F (551C), while graphitization predominates below this temperature.

Spheroidization (see 4.2.2) and graphitization are competing mechanisms that occur at overlapping temperature ranges. Spheroidization tends to occur preferentially above 1025F (551C), while graphitization predominates below this temperature.

Graphitization can be prevented by using chromium containing low alloy steels for long-term operation above 800F (427C).

Affected Materials: Some grades of carbon steel and 0.5Mo steels. Graphitization has been found in low alloy C-Mo steels with up to 1% Mo. The addition of about 0.7% chromium has been found to eliminate graphitization.

Creep Type: Microvoid formation & jointing of ligament between voids

Typical ferrite-pearlite structure of carbon steel.

Typical martensitic structure of carbon steel.

Figure 13: Lower bainite generated by isothermal transformation of 52100 steel at 230C for 10h

http://www.msm.cam.ac.uk/phase-trans/2011/Bearings/index.htm l

Random graphitization

http://davidnfrench.com/Graphitization.html

Chain graphitization

Graphitization of A Cast Iron Main

,:1. : 800F to 1100F2. : (), Mo3. : (1) () (2) HAZ/

(,)4. : Mo,, 875F 800F5. 0.7%Cr . 6. API 510/570.

http://www.chasealloys.co.uk/steel/alloying-elements-in-steel/#chromium

4.2.2 Carbide Spheroidization(API510/570)

SpheroidizationProlong Exposure850F ~ 1400F

4.2.2 Softening (Spheroidization) 4.2.2.1 Description of DamageSpheroidization is a change in the microstructure of steels after exposure in the 850F to 1400F (440C to 760C) range, where the carbide phases in carbon steels are unstable and may agglomerate from their normal plate-like form to a spheroidal form, or from small, finely dispersed carbides in low alloy steels like 1Cr-0.5Mo to large agglomerated carbides. Spheroidization may cause a loss in strength and/or creep resistance.

4.2.2.2 Affected MaterialsAll commonly used grades of carbon steel and low alloy steels including C-0.5Mo, 1Cr-0.5Mo,1.25Cr-0.5Mo, 2.25Cr-1Mo, 3Cr -1Mo, 5Cr-0.5Mo, and 9Cr-1Mo steels.

: ,., 0.5Mo, ,

Figure 9: The microstructures near the OD surface were mostly decarburized. The remaining carbides were highly spheroidized and agglomerated along the ferrite grain boundaries. The microstructure 90 from the rupture and at the tube end is shown.(Nital etch, Mag. 500X)

Figure 10: The microstructures near the ID surface consisted of partially and highly spheroidized and agglomerated carbides along the ferrite grain boundaries. The microstructure 90 from the rupture and at the tube end is shown. (Nital etch, Mag. 500X)

http://www.met-tech.com/short-term-overheat-rupture-of-t11-superheater-tube.html

Differences

Softening (Spheroidization), at prolong exposure to high temperature carbide phases in carbon steels are unstable and may agglomeratefrom their normal plate-like form to a spheroidal form, or from small, finely dispersed carbides in low alloy steels like 1Cr-0.5Mo to large agglomerated carbides. ,,, .

Graphitisation, the carbide phases in carbon/Molydenum steels are unstable and may decompose into graphite nodules.,/0.5,.

;(1) /(2) ,

Spheroidization in physical metallurgy, a process consisting in the transition of excess-phase crystals into a globular (spheroidal) form. The transition occurs at relatively high temperatures and is associated with a decrease in the interfacial energy. Of particular importance is the spheroidization of the cementite plates contained in pearlite. In this process, the lamellar pearlite is converted into granular pearlite. As a result, the hardness and the strength of the metal are significantly decreased, but the ductility is increased.

Carbide Spheroidization

Figure 2. Etched sample of a section of non-corroded material (200X Original Magnification with Nital Etch)

Figure 1. Corroded sheath exterior (0.85X Original Magnification)

http://www.matter.org.uk/steelmatter/forming/4_5.html

Graphitisation and spheroidization both were high temperature phenomenon.

Graphitisation affect normal carbon steel. It is a break down of carbides into ferrite and free graphite (carbon) nodules.,/.

Spheroidization affect Cr-Mo low carbon steel up to 9%Cr. It is a agglomeration of carbides forming spheroidal carbides. ,

Spheroidization affect Cr-Mo low carbon steel up to 9% Cr. It is a agglomeration of carbides forming spheroidal carbides 9%Cr ,, ().

9% Chromium .

Residual stress & cold works accelerated graphitization..

For spheroidisation coarse-grained steels are more resistant than fine-grained. Fine grained silicon-killed steels are more resistant than aluminum killed.

. .

Susceptibility to Spheroidization Annealed steels are more resistant to spheroidization than

normalized steels.

Coarse-grained steels are more resistant than fine-grained.

Fine grained silicon-killed steels are more resistant than aluminum-killed.

The loss in strength may be as high as about 30% but failure is not likely to occur except under very high applied stresses.

Plain carbon steelC, C Mo


GraphitisationNO

Low alloy steel up to 9% Cr.850F ~ 1400FSpheroidisationNO Some grades of carbon steel and 0.5Mo steels.

Exam Affected materialsTemperaturesDM

All commonly used grades of carbon steel and low alloy steels including C-0.5Mo, 1Cr-0.5Mo,1.25Cr-0.5Mo, 2.25Cr-1Mo, 3Cr-1Mo, 5Cr-0.5Mo, and 9Cr-1Mo steels.

Spheroidization (see 4.2.2) and graphitization are competing mechanisms that occur at overlapping temperature ranges. Spheroidization tends to occur preferentially above 1025F (551C), while graphitization predominates below this temperature.Discussion: Graphitization occurs on some carbon steel and 0.5Mo steels only.

Spheroidization is a change in the microstructure of steels after exposure in the 850F to 1400F (440C to 760C) range, where the carbide phases in carbon steels are unstable and may agglomerate from their normal plate-like form to a spheroidal form.

:1. 850F to 1400F,2. : ,,

(),3. :, 0.5Mo, 9Cr1Mo,4. ,5. ,6. ,,7. API 510/570API 510/570

4.2.3 Temper Embrittlement

API510-Exam

650oF~ 1070oF

API510-Exam


650F~ 1070FTempered Embrittlement

0.5Mo Steel, Low alloy steel up to 9 % Cr

850F ~ 1400FSpheroidization

Plain carbon steel / 0.5Mo Steel


Graphitization

API510-Exam

4.2.3 Temper Embrittlement 4.2.3.1 Description of Damage

Temper embrittlement is the reduction in toughness due to a metallurgical change that can occur in some low alloy steels as a result of long term exposure in the temperature range of about 650F to 1100F (343C to 593C) . This change causes an upward shift in the ductile-to-brittle transition temperature as measured by Charpy impact testing. Although the loss of toughness is not evident at operating temperature, equipment that is temper embrittled may be susceptible to brittle fracture during start-up and shutdown.

2 Cr-1Mo ~ 3Cr-1Mo650F to 1100F,, . ,,,,.

API510-Exam

Temper embrittlement is the reduction in toughness due to a metallurgical change that can occur in some low alloy steels as a result of long term exposure in the temperature range of about 650F to 1100F (343C to 593C) . 2 Cr-1Mo~ 3Cr-1Mo650F to 1100F ,,.

API510-Exam


a) Primarily 2 Cr-1Mo (P5A) low alloy steel, 3Cr-1Mo (P5A) (to a lesser extent), and the high-strength low alloy Cr-Mo-V (P5C) rotor steels.

b) Older generation 2 Cr-1Mo materials manufactured prior to 1972 may be particularly susceptible. Some high strength low alloy steels are also susceptible.

c) The C- Mo (P3) and 1 Cr- Mo (P4) alloy steels are not significantly affected by temper embrittlement. However, other high temperature damage mechanisms promote metallurgical changes that can alter the toughness or high temperature ductility of these materials.

2 Cr-1Mo~ 3Cr-1Mo, Cr-Mo-V.

API510-Exam

3Cr-1Mo (to a lesser extent) The C-0.5Mo, 1Cr-

0.5Mo and 1.25Cr-0.5Mo alloy steels are not significantly affected.

Primarily 2.25Cr-1Mo low alloy steel. and the high-strength low alloy Cr-Mo-V rotor steels.

Temper Embrittlement

API510-Exam

API510-Exam

[Embrittlement temperature 650F~1070F]

API510-Exam

API510-Exam

.,.

SEM fractographs of tempered embrittled material show primarily intergranular cracking due to impurity segregation at grain boundaries ,

API510-Exam

http://www.twi.co.uk/news-events/bulletin/archive/1999/january-february/welding-and-fabrication-of-high-temperature-components-for-advanced-power-plant-part-1/

API510-Exam

4.2.3.3 Critical Factors a) Alloy steel composition, thermal history, metal temperature and exposure

time are critical factors. ,b) Susceptibility to temper embrittlement is largely determined by the

presence of the alloying elements manganese and silicon, and the tramp elements phosphorus, tin, antimony, and arsenic. The strength level and heat treatment/fabrication history should also be considered. /(//).

c) Temper embrittlement of 2.25Cr-1Mo steels develops more quickly at 900F (482C) than in the 800F to 850F (427C to 440C) range, but the damage is more severe after long-term exposure at 850F (440C). ,.

API510-Exam

d) Some embrittlement can occur during fabrication heat treatments, but most of the damage occurs over many years of service in the embrittling temperature range. ,.

e) This form of damage will significantly reduce the structural integrity of a component containing a crack-like flaw. An evaluation of the materials toughness may be required depending on the flaw type, the severity of the environment, and the operating conditions, particularly in hydrogen service. . ,, ,.

API510-Exam


a) Temper embrittlement occurs in a variety of process units after long term exposure to temperatures above 650F (343C). It should be noted that there have been very few industry failures related directly to temper embrittlement.

b) Equipment susceptible to temper embrittlement is most often found in hydroprocessing units, particularly reactors, hot feed/effluent exchanger components, and hot HP separators. Other units with the potential for temper embrittlement include catalytic reforming units (reactors and exchangers), FCC reactors, coker and visbreaking units.

c) Welds in these alloys are often more susceptible than the base metal and should be evaluated.

,,,,,,. ,.

API510-Exam

API510-Exam

http://www.twi-global.com/technical-knowledge/job-knowledge/defects-

imperfections-in-welds-reheat-cracking-048/

http://en.wikipedia.org/wiki/Welding_defect

API510-Exam


a) Temper embrittlement is a metallurgical change that is not readily apparent and can be confirmed through impact testing. Damage due to temper embrittlement may result in catastrophic brittle fracture. ,.

b) Temper embrittlement can be identified by an upward shift in the ductile-to-brittle transition temperature measured in a Charpy V-notch impact test, as compared to the non-embrittled or de-embrittled material (Figure 4-5). Another important characteristic of temper embrittlement is that there is no effect on the upper shelf energy.V- .,.

c) SEM fractographs of severely temper embrittled material show primarily intergranular cracking due to impurity segregation at grain boundaries.

API510-Exam

API510-Exam

Intergranular CrackingAPI510-Exam

Intergranular Cracking

API510-Exam

Cr5C2/ Cr3C2 precipitated

Low Cr grain boundary

Progressive crack

Metal surface

Crack initiation & growth

API510-Exam

Tensile stress

Grain boundary decohesion-Crack initiation pt.

Cr depleted grain boundaryPPT as Cr5C2/Cr3C2

Embrittlement Mechanism

Grain Boundary

4.2.3.6 Prevention / Mitigation

a) Existing Materials

1. Temper embrittlement cannot be prevented if the material contains critical levels of the embrittling impurity elements and is exposed in the embrittling temperature range.

2. To minimize the possibility of brittle fracture during startup and shutdown, many refiners use a pressurization sequence to limit system pressure to about 25 percent of the maximum design pressure for temperatures below a Minimum Pressurization Temperature (MPT). Note that MPT is not a single point but rather a pressure temperature envelope which defines safe operating conditions to minimize the likelihood of brittle fracture.

API510-Exam

3. MPTs generally range from 350F (171C) for the earliest, most highly temper embrittled steels, down to 125F (52C) or lower for newer, temper embrittlement resistant steels (as required to also minimize effects of hydrogen embrittlement).

4. If weld repairs are required, the effects of temper embrittlement can be temporarily reversed (de-embrittled) by heating at 1150F (620C)[compared: embrittlement temperature 650F~1070F] for two hours per inch of thickness, and rapidly cooling to room temperature. It is important to note that re-embrittlement will occur over time if the material is re-exposed to the embrittling temperature range.

API510-Exam

API510-Exam

Heating at 1150F (620C) [compared: embrittlement temperature 650F~1070F (343C to 593C) ] for two hours per inch of thickness, and rapidly cooling to room temperature.

Existing Material: De-embrittlement treatment.

b) New Materials

The best way to minimize the likelihood and extent of temper embrittlement is to limit the acceptance levels of manganese, silicon, phosphorus, tin, antimony, and arsenic in the base metal and welding consumables. In addition, strength levels and PWHT procedures should be specified and carefully controlled. /,,,,,.

Acceptance Level of Mn, Si, P, Sn, Sb, As.

API510-Exam

Susceptibility to temper embrittlement

A common way to minimize temper embrittlement is to limit the "J*" Factor for base metal and the "X" Factor for weld metal, based on material composition as follows:

J* = (Si + Mn) x (P + Sn) x 104 {elements in wt%}X = (10P + 5Sb + 4Sn + As)/100 {elements in ppm}

API510-Exam

Typical J* and X factors used for 2.25 Cr steel are a maximum of 100 and 15, respectively. Studies have also shown that limiting the (P + Sn) to less than 0.01% is sufficient to minimize temper embrittlement because (Si + Mn) control the rate of embrittlement.

J* : 100 Max. (Base metal)X : 15 Max. (Weld metal)

API510-Exam

API510-Exam


a) a) A common method of monitoring is to install blocks of original heats of the alloy steel material inside the reactor. Samples are periodically removed from these blocks for impact testing to monitor/establish the ductile-brittle transition temperature. The test blocks should be strategically located near the top and bottom of the reactor to make sure that the test material is exposed to both inlet and outlet conditions.

b) Process conditions should be monitored to ensure that a proper pressurization sequence is followed to help prevent brittle fracture due to temper embrittlement.

4.2.3.8 Related Mechanisms

Not applicable.

API510-Exam

Figure 4-5 Plot of CVN toughness as a function of temperature showing a shift in the 40-ft-lb transition temperature.

API510-Exam

Temper embrittlement is inherent in many steels and can be characterized by reduced impact toughness. The state of temper embrittlement has practically no effect on other mechanical properties at room temperature. Figure 1 shows schematically the effect of temperature on impact toughness of alloy steel which is strongly liable to temper embrittlement. Many alloy steels have two temperature intervals of temper embrittlement. For instance, irreversible temper brittleness may appear within the interval of 250-400C and reversible temper brittleness, within 450C-650C.http://www.keytometals.com/Articles/Art102.htm

API510-Exam

Metallurgy of Mo in alloy steel & ironTemper embrittlement may occur when steels are slowly cooled after tempering through the temperature range between 450 and 550C. This is due to the segregation of impurities such as phosphorus, arsenic, antimony and tin on the grain boundaries. The molybdenum atom is very large relative to other alloying elements and impurities. It effectively impedes the migration of those elements and thereby provides resistance to temper embrittlement.http://www.imoa.info/molybdenum_uses/moly_grade_alloy_steels_irons/tempering.php

API510-Exam

Other/ Other reference: http://www.twi.co.uk/technical-knowledge/faqs/material-faqs/faq-what-is-temper-embrittlement-and-how-can-it-be-controlled/

:1. : 650F to 1100F,2. : ,3. :, 2.25Cr1Mo ~ 3Cr1Mo,,4. /,,,,,,5. : (?),6. .7. ,.8. API 510/570API 510/570

4.2.4 Strain Aging(API510/570)

Intermediatetemperature


600F~ 1000F885oF embrittlement

pre-1980s carbon steels with a large grain size and C-0.5 Mo




Plain carbon + Low alloy steel up to 9% Cr

850oF ~ 1400oFSpheroidization

Plain carbon steel800F for C Steel875F for C Mo Steel

Graphitisation

0.5Mo .

4.2.4 Strain Aging 4.2.4.1 Description of DamageStrain aging is a form of damage found mostly in older vintage carbon steels and C-0.5 Mo low alloy steels under the combined effects of deformation and aging at an intermediate temperature. This results in an increase in hardness and strength with a reduction in ductility and toughness.

4.2.4.2 Affected MaterialsMostly older (pre-1980s) carbon steels with a large grain size and C-0.5 Mo low alloy steel.

When susceptible materials are plastically deformed and exposed to intermediate temperatures, the zone of deformed material may become hardened and less ductile.

1980( / C- Mo), ,

http://link.springer.com/article/10.1007%2Fs11668-006-5014-3#page-1http://link.springer.com/article/10.1007%2FBF02715166#page-1http://matperso.mines-paristech.fr/Donnees/data03/386-belotteau09.pdf

Most of the effects of cold work on the strength and ductility of structural steels can be eliminated by thermal treatment, such as stress relieving, normalizing, or annealing. However, such treatment is not often necessary..

AISC- Guide to Design Criteria for Bolted and Riveted Joints

4.2.4.3 Critical Factorsa) Steel composition and manufacturing process determine steel susceptibility.b) Steels manufactured by the Bessemer or open hearth process contain

higher levels of critical impurity elements than newer steels manufactured by the Basic Oxygen Furnace (BOF) process.

c) In general, steels made by BOF and fully killed with aluminum will not be susceptible. The effect is found in rimmed and capped steels with higher levels of nitrogen and carbon, but not in the modern fully killed carbon steels manufactured to a fine grain practice.

: (), //: , ,.

d) Strain aging effects are observed in materials that have been cold worked and placed into service at intermediate temperatures without stress relieving. ().

e) Strain aging is a major concern for equipment that contains cracks. If susceptible materials are plastically deformed and exposed to intermediate temperatures, the zone of deformed material may become hardened and less ductile. This phenomenon has been associated with several vessels that have failed by brittle fracture. , ,.

f) The pressurization sequence versus temperature is a critical issue to prevent brittle fracture of susceptible materials..

g) Strain aging can also occur when welding in the vicinity of cracks and notches in a susceptible material. .

Bessemer Process

Open hearth Process

Basic Oxygen Process

http://metallurgyfordummies.com/steelmaking-technology/

Metallurgy for Dummies

Capped Steel,:

It has characteristics similar to those of rimmed steels but to a degree intermediate between those of rimmed and semi-killed steels.

A deoxidizer may be added to effect a controlled running action when the steel is cast. the gas entrapped during solidification is in excess of that needed to counteract normal shrinkage, resulting in a tendency for the steel to rise in the mould.

The capping operation caused the steel to solidify faster, thereby limiting the time of gas evolution, and prevents the formation of an excessive number of gas voids within the ingot.

Capped steel is generally cast in bottle-top moulds using a heavy metal cap.

Capped steel may also be cast in open-top moulds, by adding aluminum or ferro-silicon on the top of molten steel, to cause the steel on the surface to lie quietly and solidify rapidly.

4) Rimmed Steel ,: In rimmed steel, the aim is to produce a clean surface low in carbon

content. Rimmed steel is also known as drawing quality steel. The typical structure results for a marked gas evolution during solidification

of outer rim. They exhibit greatest difference in chemical composition across sections

and from top to bottom of the ingot. They have an outer rim that is lower in carbon, phosphorus, and sulphur

than the average composition of the whole ingot and an inner portion or core that is higher the average in those elements.

In rimming, the steel is partially deoxidized. Carbon content is less than 0.25% and manganese content is less than 0.6%.

They do not retain any significant percentage of highly oxidizable elements such as Aluminum, silicon or titanium.

A wide variety of steels for deep drawing is made by the rimming process, especially where ease of forming and surface finish are major considerations.

These steel are, therefore ideal for rolling, large number of applications, and is adapted to cold-bending, cold-forming and cold header applications.

:1. ?F to ?F,2. :,

..3. : .4. ,,.5. .6. API 510/570API 510/570

4.2.5 885F (475C) embrittlement885F -/(API510/570)

885F (475C) embrittlement 600F~ 1000F

300*, 400 & Duplex SS containing ferrite phases

600F~ 1000F885F embrittlement

pre-1980s carbon steels with a large grain size and C-0.5 Mo




Plain carbon + Low alloy steel up to 9% Cr

850oF ~ 1400oFSpheroidization

Plain carbon steel800F for C Steel875F for C Mo Steel

Graphitisation

:.* .

4.2.5 885F (475C) Embrittlement4.2.5.1 Description of Damage

885F (475C) embrittlement is a loss in toughness due to a metallurgical change that can occur in stainless steel containing a ferrite phase, as a result of exposure in the temperature range 600F~1000F (316C to 540C).


a) 400 Series SS- ferritic & martensitic (e.g., 405, 409, 410, 410S, 430, and 446).

b) Duplex stainless steels such as Alloys 2205, 2304, and 2507.c) Wrought and cast 300 Series SS containing ferrite, particularly welds and

weld overlay.

: 600F to 1000F (///-).

http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&LN=CN&NM=102

885F (475C) embrittlementEmbrittlement of stainless steels containing ferrite phase upon extended exposure to temperatures between 730F and 930F (400C and 510C ). This type of embrittlement is caused by fine, chromium-rich precipitates that segregate at grain boundaries: time at temperature directly influences the amount of segregation. Grain-boundary segregation of the chromium-rich precipitates increases strength and hardness, decreases ductility and toughness, and changes corrosion resistance. This type of embrittlement can be reversed by heating above the precipitation range.

885F ,.,()..

Most refining companies limit the use of ferritic stainless steels to non-pressure boundary applications because of this damage mechanism.

885F embrittlement is a metallurgical change that is not readily apparent with metallography but can be confirmed through bend or impact testing (Figure 4-6).

The existence of 885F embrittlement can be identified by an increase in hardness in affected areas. Failure during bend testing or impact testing of samples removed from service is the most positive indicator of 885F embrittlement.

885F embrittlement is reversible by heat treatment to dissolve precipitates, followed by rapid cooling. The de-embrittling heat treatment temperature is typically 1100F (593C) or higher and may not be practical for many equipment items. If the de-embrittled component is exposed to the same service conditions it will re-embrittle faster than it did initially.

4.2.5.3 Critical Factors a) The alloy composition, particularly chromium content, amount of ferrite

phase, and operating temperature are critical factors.,

b) Increasing amounts of ferrite phase increase susceptibility to damage when operating in the high temperature range of concern. A dramatic increase in the ductile-to-brittle transition temperature will occur.()

c) A primary consideration is operating time at temperature within the critical temperature range. Damage is cumulative and results from the precipitation of an embrittling intermetallic phase that occurs most readily at approximately 885F (475C). Additional time is required to reach maximum embrittlement at temperatures above or below 885F (475C). For example, many thousands of hours may be required to cause embrittlement at 600F (316C). ,(intermetallic phase)/.

d) Since 885F embrittlement can occur in a relatively short period of time, it is often assumed that susceptible materials that have been exposed to temperatures in the 700F to 1000F (371C to 538C) range are affected.700F1000F

e) The effect on toughness is not pronounced at the operating temperature, but is significant at lower temperatures experienced during plant shutdowns, startups or upsets.,.

f) Embrittlement can result from tempering at higher temperatures or by holding within or cooling through the transformation range. ()885F.

Fig. 2Microstructure of solution-annealed 304LN stainless steel

Fig. 3Oxalic acid etched microstructures of 304LN stainless steel sensitized for (a) 1 h, (b) 25 h, (c) 50 h, and (d) 100 h.


a) Impact or bend testing of samples removed from service is the most positive indicator of a problem.

b) Most cases of embrittlement are found in the form of cracking during turnarounds, or during startup or shutdown when the material is below about 200F (93C) and the effects of embrittlement are most detrimental.

c) An increase in hardness is another method of evaluating 885F embrittlement.

Impact energy and brinell hardness as function of time exposure qt 475C475oC Embrittlement in a Duplex Stainless Steel UNS S31803

475oC Embrittlement in a Duplex Stainless Steel UNS S31803http://www.scielo.br/scielo.php?pid=S1516-14392001000400003&script=sci_arttext

http://www.sciencedirect.com/science/article/pii/S0921509309000197

885F (475C) Embrittlement of stainless steels in alloys containing a ferrite phase (Ferritic/Martensitic/Duplex stainless steel and ferrite phases in austenitic stainless steel e.g. weldareas) :

Grain-boundary segregation of the chromium-rich precipitates increases strength and hardness, decreases ductility and toughness, and changes corrosion resistance (lower). (intermetallic).

This type of embrittlement can be reversed by heating above the precipitation range.

Impact testing/bend test & hardness testing used to evaluate susceptibility. ,,.

Restrict the used of ferritic steel to non-pressure boundary application. .

4.2.6 Sigma-Phase Embrittlement

(API510/570)

Sigma-Phase Embrittlement1000oF~1700oF


1000F~ 1700FSigma phase embrittlement

300*, 400 & Duplex SS containing ferrite phase

600F~ 1000F885F embrittlement

:, *

4.2.6 Sigma Phase Embrittlement4.2.6.1 Description of Damage

Formation of a metallurgical phase known as sigma phase can result in a loss of fracture toughness in some stainless steels as a result of high temperature exposure.


a) 300 Series SS wrought metals, weld metal, and castings. Cast 300 Series SS including the HK and HP alloys are especially susceptible to sigma formation because of their high (10-40%) ferrite content.

b) The 400 Series SS and other ferritic and martensitic SS with 17% Cr or more are also susceptible (e.g., Types 430 and 440).

c) Duplex stainless steels.

: ,,.: ,,.

http://www.hindawi.com/journals/isrn.metallurgy/2012/732471/


Sigma-Phase Embrittlement Description: Embrittlement of iron-chromium alloys caused by precipitation at grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 565oC and 980oC (1050oF and 1800oF). Sigma phase embrittlement results in severe loss in toughness and ductility and can make the embrittled material structure susceptible to intergranular corrosion.

565oC ~ 980oC (1050oF ~ 1800oF),(), .

4.2.6.3 Critical Factors a) Alloy composition, time and temperature are the critical factors.

, , .b) In susceptible alloys, the primary factor that affects sigma phase

formation is the time of exposure at elevated temperature. ;.

c) Sigma phase occurs in ferritic (Fe-Cr), martensitic (Fe-Cr), austenitic (Fe-Cr-Ni) and duplex stainless steels when exposed to temperatures in the range of 1000F to 1700F (538C to 927C). Embrittlement can result by holding within or cooling through the transformation range. ,,,,1000F to 1700F ,.

d) Sigma forms most rapidly from the ferrite phase that exists in 300 Series SS and duplex SS weld deposits. It can also form in the 300 Series SS base metal (austenite phase) but usually more slowly. ,.

e) The 300 Series SS can exhibit about 10% to 15% sigma phase. Castaustenitic stainless steels can develop considerably more sigma. 10%~15%,.

f) Formation of sigma phase in austenitic stainless steels can also occur in a few hours, as evidenced by the known tendency for sigma to form if an austenitic stainless steel is subjected to a post weld heat treatment at 1275F (690C). ,1275F,.

g) The tensile and yield strength of sigmatized stainless steels increases slightly compared with solution annealed material. This increase in strength is accompanied by a reduction in ductility (measured by percent elongation and reduction in area) and a slight increase in hardness.,., ,.

http://www.intechopen.com/books/metallurgy-advances-in-materials-and-processes/homogenization-heat-treatment-to-reduce-the-failure-of-heat-resistant-steel-castings

()phase in austenitic matrix

()phase in austenitic matrix

h) Stainless steels with sigma can normally withstand normal operating stresses, but upon cooling to temperatures below about 500F (260C) may show a complete lack of fracture toughness as measured in a Charpy impact test. Laboratory tests of embrittled weld metal have shown a complete lack of fracture toughness below 1000F (538C),500F(1000F).

i) The metallurgical change is actually the precipitation of a hard, brittle intermetallic compound that can also render the material more susceptible to intergranular corrosion. The precipitation rate increases with increasing chromium and molybdenum content..,.

Cr% & Mo%

The precipitation rate increases

- (intermetallic-sigma phase).,.


a) Common examples include stainless steel cyclones, piping ductwork and valves in high temperature FCC (fluidized catalytic cracking )Regenerator service.

b) 300 Series SS weld overlays and tube-to-tubesheets attachment welds can be embrittled during PWHT treatment of the underlying CrMo base metal.

c) Stainless steel heater tubes are susceptible and can be embrittled.

FCC (fluidized catalytic cracking )Regenerator service

http://www.phxequip.com/plant.73/fluid-catalytic-cracker-unit.aspx


Stainless steel heater tubes

tube-to-tubesheets attachment

4.2.6.5 Appearance or Morphology of Damage a) Sigma phase embrittlement is a metallurgical change that is not readily

apparent, and can only be confirmed through metallographic examination and impact testing. (Tables 4-1 and 4-2),

b) Damage due to sigma phase embrittlement appears in the form of cracking, particularly at welds or in areas of high restraint. .

c) Tests performed on sigmatized 300 Series SS (304H) samples from FCC regenerator internals have shown that even with 10% sigma formation, the Charpy impact toughness was 39 ft-lbs (53 J) at 1200F (649C).: 304H: 10%/: 649C / 53J

:: 304H: 10%/: 649C / 53J

:SPECIFICATION FOR HEAT-RESISTINGCHROMIUM AND CHROMIUM-NICKEL STAINLESSSTEEL PLATE, SHEET, AND STRIP FORPRESSURE VESSELSASTM SA-240

SPECIFICATION FOR HEAT-RESISTING CHROMIUM AND CHROMIUM-NICKEL STAINLESS STEEL PLATE, SHEET, AND STRIP FOR PRESSURE VESSELS ASTM SA-240

d) For the 10% sigmatized specimen, the values ranged from 0% ductility at room temperature to 100% at 1200F (649C). Thus, although the impact toughness is reduced at high temperature, the specimens broke in a 100% ductile fashion, indicating that the wrought material is still suitable at operating temperatures. See Figures 4-7 to 4-11. .1200F.

e) Cast austenitic stainless steels typically have high ferrite/sigma content (up to 40%) and may have very poor high temperature ductility.40%,/.

Evaluation of Sigma Phase Embrittlement of a Stainless Steel 304H Fluid Catalyst Cracking Unit Regenerator Cyclone 304H.Authors: Ali Y. Al-Kawaie and Abdelhak KermadABSTRACTTesting was performed on a 304H stainless steel sample removed from a Fluid Catalyst Cracking Unit (FCCU) regenerator cyclone after 25 years of service to check for sigma phase formation. Sigma phase is a nonmagnetic inter-metallic phase composed mainly of iron and chromium (Fe-Cr), which forms in ferritic and austenitic stainless steels during exposure at the temperature range 1,050 F to 1,800 F (560 C to 980 C), causing loss of ductility and toughness. Cracking may also occur if the component was impact-loaded or excessively stressed during shutdown or maintenance work. This article discusses the effect of sigma phase embrittlement on the FCCU regenerator cyclone after extended high temperature service.

http://www.saudiaramco.com/content/dam/Publications/Journal%20of%20Technology/Spring2011/Art%2012%20-%20JOT%20Internet.pdf

Table 2. Impact testing (Test Method: ASTM E23)

Table 3. Micro-hardness testing. Test load: 200 g,Calibration Block Hardness: 256 + 10 HV, Measured Hardness of the calibration block: 258 VHN.

Fig. 1. Cyclone sample, as received.

Fig. 2. Micrograph showing carburized layer at the outer (top) surface, 100x (As received).

Fig. 3. Micrograph showing the microstructure at the outer (top) surface, 100x (Heat treaded).

Note: solution annealing at 1,066 C for four hours, followed by a water quench before testing.

Fig. 4. Micrograph showing sigma formation at the center of the sample. Estimated volume fraction 7%, 100x (As received).

Fig. 5. Micrograph showing the microstructure at the center of the heat treated sample, 100x (Heat treated).


Fig. 6. Micrograph showing sigma phase at the inner (bottom) surface of the original sample, 100x (As received).

Fig. 7. Micrograph showing the microstructure at the inner surface of the heat treated sample, 100x (Heat treated).


Fig. 8a. SEM fractography showing the brittle fracture surface (Top - As received)

Fig. 8b. SEM fractography showing the ductile fracture (Bottom - Heat treated) of the impact tested samples.

4.2.6.6 Prevention / Mitigation

a) The best way to prevent sigma phase embrittlement is to use alloys that are resistant to sigma formation or to avoid exposing the material to the embrittling range. (IOW)

b) The lack of fracture ductility at room temperature indicates that care should be taken to avoid application of high stresses to sigmatized materials during shutdown, as a brittle fracture could result. . ,; ().*

c) The 300 Series SS can be de-sigmatized by solution annealing at 1950F (1066C) for four hours followed by a water quench. However, this is not practical for most equipment. (1950F). .

Note* .

d) Sigma phase in welds can be minimized by controlling ferrite in the range of 5% to 9% for Type 347 and somewhat less ferrite for Type 304. The weld metal ferrite content should be limited to the stated maximum to minimize sigma formation during service or fabrication, and must meet the stated minimum in order to minimize hot short cracking during welding. .

e) For stainless steel weld overlay clad Cr-Mo components, the exposure time to PWHT temperatures should be limited wherever possible..

What causes knife-line attack? For stabilized stainless steels and alloys, carbon is bonded with stabilizers (TiC or NbC) and no weld decay occurs in the heat affected zone during welding. In the event of a subsequent heat treatment or welding (above 1200oC), however, first the TiC / NbC may dissociated into free Ti, Nb and C, on cooling precipitation of chromium carbide Cr23C6 is possible and this leaves the narrow band adjacent to the fusion line susceptible to intergranular corrosion.

What causes weld decay? As in the case of intergranular corrosion, grain boundary precipitation, notably chromium carbides in non-stabilized stainless steels, is a well recognized and accepted mechanism of weld decay. In this case, the precipitation of chromium carbides is induced by the welding operation when the heat affected zone (HAZ) experiences a particular temperature range (550oC~850oC). The precipitation of chromium carbides consumed the alloying element - chromium from a narrow band along the grain boundary and this makes the zone anodic to the unaffected grains. The chromium depleted zone becomes the preferential path for corrosion attack or crack propagation if under tensile stress.

Anodic site

4.2.6.7 Inspection and Monitoring a) Physical testing of samples removed from service is the most positive

indicator of a problem. .

b) Most cases of embrittlement are found in the form of cracking in both wrought and cast (welded) metals during turnarounds, or during startup or shutdown when the material is below about 500F (260C) and the effects of embrittlement are most pronounced. 500F (260C) ,,.

4.2.6.8 Related Mechanisms Not applicable.

Figure 10: Shaeffler diagram showing the embrittlement region of thephase [33].http://www.hindawi.com/journals/isrn.metallurgy/2012/732471/

http://www.metalconsult.com/failure-analysis-furnace-tubes.html

http://www.metallograf.de/start-eng.htm?/untersuchungen-eng/sigmaphase/sigmaphase.htm

http://www.industrialheating.com/articles/90371-sigma-phase-embrittlement

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1517-70762009000300017

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1517-70762009000300017

Table 4: Chemical composition of ferrite austenite and sigma phases at 900oC

Polishing markings

Improperly etched specimen showing little or no sign of sigma phase

NaOH etched

Oxalic acid etched-Duplex SS

Sigma-phase embrittlement

:1000oF~1700oF : :: , ,

. : (1950F).

. API 510/570

4.2.7 Brittle FractureAPI 510/570

API510/570-Exam

Brittle FractureBelow DTBTTDTBT: Ductile to brittle transition temperature.

API510/570-Exam

No shear lip, little micro void coalescence, little deformation

API510/570-Exam

4.2.7 Brittle Fracture 4.2.7.1 Description of Damage Brittle fracture is the sudden rapid fracture under stress (residual or applied) where the material exhibits little or no evidence of ductility or plastic deformation. - ()., .4.2.7.2 Affected Materials Carbon steels and low alloy steels are of prime concern, particularly older steels. 400 Series SS are also susceptible. ; , /Affected Materials Ferritic/Martensitic STEELS /

API510/570-Exam

4.2.7.3 Critical Factors a) When the critical combination of three factors is reached, brittle fracture can

occur:1. The materials fracture toughness (resistance to crack like flaws) as

measured in a Charpy impact test;2. The size, shape and stress concentration effect of a flaw;

3. The amount of residual and applied stresses on the flaw.

b) Susceptibility to brittle fracture may be increased by the presence of

embrittling phases. c) Steel cleanliness and grain size have a significant influence on toughness

and resistance to brittle fracture..

API510/570-Exam

d) Thicker material sections also have a lower resistance to brittle fracture due to higher constraint which increases triaxial stresses at the crack tip.,;

d) In most cases, brittle fracture occurs only at temperatures below the Charpy impact transition temperature (or ductile-to-brittle transition temperature), the point at which the toughness of the material drops off sharply. .

API510/570-Exam

Definition of Brittle Fracture

a steel member may experience a brittle fracture. Three basic factors contribute to a brittle-cleavage type of fracture. They are;

a triaxial state of stress, a low temperature, and a high strain rate or rapid rate of loading.

All these factors need not be present. Crack often propagates by cleavage breaking of atomic bonds along specific crystallographic planes (cleavage planes), propagate rapidly without further increase in applied stress (applied or residual) with little indication of plastic deformation.

In contrast, a ductile fracture occurs mainly by shear, usually preceded by considerable plastic deformation.

API510/570-Exam

API510/570-Exam

API510/570-Exam

Figure 7: The fracture examination using a SEM on C1 and C2 revealed features typical of transgranular fracture (left and middle) and signatures of intergranular cracking (left and right). The presence of both intergranular and transgranular features indicates a mixed-mode fracture morphology.http://www.drillingcontractor.org/tubular-fracturing-pinpointing-the-cause-14544

API510/570-Exam

In Case 2 from Oklahoma, the pin connection twisted off while making up the pin connection of a saver sub.http://www.drillingcontractor.org/tubular-fracturing-pinpointing-the-cause-14544

API510/570-Exam

Figure 4: The fracture on the Case 1 sub showed a grainy texture and chevron marks that point toward the initiation site, which is typical morphology for brittle cracking

API510/570-Exam

Figure 5: The fracture on C3 exhibited a small fatigue region that was followed by brittle fracture. The fracture surface had a grainy appearance and presented a minuscule shear lip, which is also typical of a brittle fracture

API510/570-Exam

Various stages during ductile fracture are schematically shown in above figure. (a) Necking, (b) Cavity formation (microvoid), (c) Cavity coalescence to form a crack (microvoid coalescence), (d) Crack propagation, (e) Fracture (shear fracture). ()

API510/570-Exam

API510/570-Exam

Brittle fracture of a shaft caused by a small fatigue crack close to the keyway. The fatigue would be expected to start at the keyway root but actually began at a surface defect.http://www.surescreen.com/scientifics/library-of-failures.php

API510/570-Exam

API510/570-Exam

http://www.wermac.org/misc/pressuretestingfailure2.html

4.2.7.4 Affected Units or Equipment a) Equipment manufactured to the ASME Boiler and Pressure Vessel Code,

Section VIII, Division 1, prior to the December 1987 Addenda, were made with limited restrictions on notch toughness for vessels operating at cold temperatures. However, this does not mean that all vessels fabricated prior to this date will be subject to brittle fracture. Many designers specified supplemental impact tests on equipment that was intended to be in cold service. ASME198712,,. ,,.

b) Equipment made to the same code after this date were subject to the requirements of UCS 66 (impact exemption curves). ASME VIII DIV1 UCS 66 .

API510/570-Exam

c) Most processes run at elevated temperature so the main concern is for brittle fracture during startup, shutdown, or hydrotest/tightness testing. Thick wall equipment on any unit should be considered. ,,

d) Brittle fracture can also occur during an auto-refrigeration event in units processing light hydrocarbons such as methane, ethane/ethylene, propane/propylene, or butane. This includes alkylation units, olefin units and polymer plants (polyethylene and polypropylene). Storage bullets/spheres for light hydrocarbons may also be susceptible. .

e) Brittle fracture can occur during ambient temperature hydrotesting due to high stresses and low toughness at the testing temperature.

API510/570-Exam

ASME VIII Div.1- Charlie Chong/ Fion Zhang

UCS-66 MATERIALS.

UCS-66

The main material property that API 510 / ASME VIII is concernedwith is that of resistance to brittle fracture. The fundamental issue is therefore whether a material is suitable for the minimum design metal temperature (MDMTdesign) for which a vessel is designed. This topic is covered by clause UCS-66 of ASME VIII.

ASME VIII Div.1- Charlie Chong/ Fion Zhang UCS-66

Steps:

1. UG-20 for exemption on impact testing.2. UCS-66.

Identified material Group A,B,C,D. Figure UCS-66 to determine the allowable MDMT. Figure UCS-66.1 to determine the reduction in MDMT based on

coincident ratio.

3. UCS-68(c) to determine on further reduction in MDMT.


Figure UCS 66.1 Coincident Ratio

The Coincident Ratio is based on a vessels extra thickness due to its design calculations which were based on its Maximum Temperature.Meaning that; As metals temperature increases its strength decreases, hotter means weaker, therefore the allowable stress is decreased during calculations resulting in vessel that requires thicker walls when hot than when it is operating at its coldest temperature, the MDMT.

This ratio takes credit for the extra wall thickness that is present, but not needed to resist pressure at the MDMT. The following graphic will help. Usually when there is a drop in temperature there is also a drop in the pressure. The two operating conditions are calculated and the Ratio is determined. This Ratio is given on the exam and you need only use the table to apply this rule.


How to use FIG. UCS-66 & FIG. UCS-66.1 to determine allowable impact test value (MDMTallowable)


Steps:1. Determine material group.2. Determine MDMT allowable on

the graph.3. If Design MDMT higher than

MDMT allowable, no test requires.4. If MDMT allowable is higher than

design MDMT goto FIG. UCS-66.1


Steps:5. If the coincident ratio is 0.70

reduction of 30oF from the MDMT allowable. The revised MDMT allowable = 59oF.

6. If the revised MDMTallowable is higher than the design MDMT, check on item 7.

7. If material is P1, UCS-68(c) If postweld heat treating is performed when it is not otherwise a requirement of this Division, a reduction of 30oF. The resulting MDMT allowable may be colder than -55oF.


Once the MDMT allowable had been ascertained, further reductions are possible by within -55F capping with exception

1. Low coincident stress ratio

2. postweld heat treating is performed when it is not otherwise a requirement of this Division on P1 materials.

- 55oF


UCS-66 (b2) For minimum design metal temperatures colder than -55F (-48C), impact testing is required for all materials, except as allowed in (b)(3) below and in UCS-68(c).

UCS-66 (b3) When the minimum design metal temperature is colder than -55F (-48C) and no colder than -155F (-105C), and the coincident ratio defined in Fig. UCS-66.1 is less than or equal to 0.35, impact testing is not required.


UCS-66 (b2) For minimum design metal temperatures colder than -55F (-48C), impact testing is required for all materials, except as allowed in (b)(3) below and in UCS-68(c).

UCS-68(c) If postweld heat treating is performed when it is not otherwise a requirement of this Division, a 30F (17C) reduction in impact testing exemption temperature may be given to the minimum permissible temperature from Fig. UCS-66 for P-No. 1 materials. The resulting exemption temperature may be colder than -55F (-48C).



a) Cracks will typically be straight, non-branching, and largely devoid of any associated plastic deformation (no shear lip or localized necking around the crack) (Figure 4-6 to Figure 4-7).:,,.

b) Microscopically, the fracture surface will be composed largely of cleavage, with limited intergranular cracking and very little microvoid coalescence.: (?)

API510/570-Exam

API510/570-Exam

Crack propagation (cleavage) in brittle materials occurs through planar sectioning of the atomic bonds between the atoms at the crack tip.

API510/570-Exam

API510/570-Exam

API510/570-Exam

4.2.7.6 Prevention / Mitigation /a) For new equipment, brittle fracture is best prevented by using materials

specifically designed for low temperature operation including upset and auto-refrigeration events. Materials with controlled chemical composition, special heat treatment and impact test verification may be required. Refer to UCS 66 in Section VIII of the ASME BPV Code.

b) Brittle fracture is an event driven damage mechanism. For existing materials, where the right combination of stress, material toughness and flaw size govern the probability of the event, an engineering study can be performed in accordance with API 579-1/ASME FFS-1 , Section 3, Level 1 or 2. (:, )FFS-1,.

c) Preventative measures to minimize the potential for brittle fracture in existing equipment are limited to controlling the operating conditions (pressure, temperature), minimizing pressure at ambient temperatures during startup and shutdown, and periodic inspection at high stress locations. IOW ,,.

API510/570-Exam

d) Some reduction in the likelihood of a brittle fracture may be achieved by: ;

1. Performing a post weld heat treatment (PWHT) on the vessel if it was not originally done during manufacturing; or if the vessel has been weld repaired/modified while in service without the subsequent PWHT.

2. Perform a warm pre-stress hydrotest followed by a lower temperature hydrotest to extend the Minimum Safe Operating Temperature (MSOT) envelope. /.

API510/570-Exam

4.2.7.7 Inspection and Monitoring a) Inspection is not normally used to mitigate brittle fracture.

b) Susceptible vessels should be inspected for pre-existing flaws/defects.

4.2.7.8 Related Mechanisms Temper embrittlement (see 4.2.3), strain age embrittlement (see 4.2.4), 885oF (475oC) embrittlement (see4.2.5), titanium hydriding (see 5.1.3.2) and sigma embrittlement (see 4.2.6).

, ,

Temper embrittlement

API510/570-Exam

API510/570-Exam

Microvoid due to plastid yielding & ductile fracture (non creep type)

API510/570-Exam

Microvoid due to plastid yielding & ductile fracture (non creep type)

Further reading:

http://www.sut.ac.th/engineering/metal/pdf/MechMet/14_Brittle%20fracture%20and%20impact%20testing.pdf

http://lecture.civilengineeringx.com/structural-analysis/structural-steel/brittle-fracture/

http://www.keytometals.com/articles/art136.htm

http://people.virginia.edu/~lz2n/mse209/Chapter8.pdf


http://www.techtransfer.com/resources/wiki/entry/3645/

API510/570-Exam

API510/570-Exam

Brittle Fracture

:/400oF, :/, , (MSOT), FFS-1,, ; (1) , (2) ,;

,,885oF,,.

API571-Part3

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