metals

Article A Study on the Failure of AISI 304 Stainless Tubes in a Gas Heater Unit

Abbas Bahrami 1 and Peyman Taheri 2,*

1 Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran 2 Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands * Correspondence: [email protected]; Tel.: +31-15-278-2275

 Received: 7 August 2019; Accepted: 31 August 2019; Published: 3 September 2019 

Abstract: This paper investigates a failure in convection section tubes of a gas heater unit in a petrochemical plant. Tubes are made of AISI 304 . The failure is reported after 5 years of service at working temperature 500 ◦C. The failure is in the form of circumferential cracks in the vicinity of the weld. Various characterization techniques, including optical and electron microscopes as well as energy-dispersive X-ray spectroscopy (EDS), were used to study the failure. Results showed that that the damage has initiated in the heat affected zone (HAZ) area parallel to the weld/base metal interface. Cracks have propagated alongside grain boundaries, resulting in an intergranular fracture. The main cause of failure was concluded to be attributable to the grain boundary sensitization and intergranular grain boundary attack due to improper and long time exposure of tubes to high temperature. Possible mitigation strategies to minimize similar failures will be discussed.

Keywords: convection tubes; AISI 304 stainless steel; failure analysis; sensitization

1. Introduction and Case Background This paper investigates the root cause analysis of a failure in convection section tubes in a gas heater unit in a petrochemical plant. This unit is used to heat up the reformed gas from a reformer unit. The gas composition is up to 70% H2, 20%CO, and the rest is a mixture of CH4, CO2,H2O, and N2. The gas enters into the heating unit at 250 ◦C and exits the unit at 570 ◦C. The flue gas at the fireside is from coal firing. The temperature of the flue gas at the inlet of the gas heating unit is 1050 ◦C. This temperature decreases to 450 ◦C at the outlet of the unit. Tubes at the first four rows are made of heat resistant HP40 alloy. Tubes at the 5th and 6th rows, where the failure is observed, are made of AISI 304 stainless . The temperature at 5th and 6th rows is approximately 500 ◦C. Austenitic stainless steel AISI 304 is known to have an excellent combination of high temperature mechanical properties, high temperature resistance, and superior structural stability [1–6], making it a popular choice for convection tubes for temperature range 500 to 700 ◦C. Tubes in similar working conditions are reported to be failed due to oxidation, erosion, creep, thermal fatigue and stress relaxation cracking [7–17]. Depending on the working condition, either of these mechanisms or a combination of them can control the damage. In this case, the installation is designed to continuously work. There is a shutdown in the system every 3 months for cleaning and maintenance. In one of the shutdowns after 5 years of service, a temperature abnormality was reported, which is normally taken as an indication for leakage. None-destructive testing (NDT) evaluation confirmed the leakage in the vicinity of weldments. This paper investigates root cause of observed failure in this failure. Results will be used to propose mitigation strategies. Mitigation measures are effective only when there is a good understanding of correlation between microstructure, mechanical properties, environmental parameters, and service conditions.

Metals 2019, 9, 969; doi:10.3390/met9090969 www.mdpi.com/journal/metals Metals 2019, 9, x FOR PEER REVIEW 2 of 8 good understanding of correlation between microstructure, mechanical properties, environmental parameters, and service conditions.

2. ExperimentalMetals 2019, 9, 969 Methods 2 of 7 Samples were taken from damaged tubes. According to the specification, the alloy is AISI 304. Inductively2. Experimental-coupled Methods plasma atomic energy spectroscopy (ICPAES) was used to make sure that the alloy has standard chemical composition. Optical and Scanning Electron Microscopes (SEM Philips Samples were taken from damaged tubes. According to the specification, the alloy is AISI 304. XL30Inductively-coupled, Philips, Eindhoven, plasma The atomic Netherlands energy spectroscopy) together with (ICPAES) energy was-dispersive used to make X-ray sure spectroscopy that the (EDSalloy, Ametek has standard, PA, USA chemical) were composition. used to study Optical the microstructure and Scanning Electron of failed Microscopes specimens. (SEM Macro Philips images wereXL30, taken Philips, by Stereo Eindhoven, Microscope The Netherlands) Nikon SMZ together 800 (Nikon with, energy-dispersive Tokyo, Japan). For X-ray metallography, spectroscopy (EDS, samples wereAmetek, prepared PA, by USA) grinding were usedthe surface to study with the microstructure180 to 1200 grinding of failed papers, specimens. followed Macro by images polishing were with diamondtaken paste by Stereo (3 and Microscope 1 micron). Nikon Microstructure SMZ 800 (Nikon, of samples Tokyo, Japan).were studied For metallography, in both as-polished samples and afterwere etching. prepared The byformer grinding is usefu the surfacel when with it comes 180 to to 1200 analyzing grinding sensitized papers, followed grain boundaries by polishing. withTo make sure diamondthat there paste is a (3minimum and 1 micron). trace of Microstructure polishing particles of samples on the were surface, studied samples in both as-polished were washed and with acetone.after Etching etching. was The formerperformed is useful with when Nitric it comes acid 60% to analyzing (Voltage sensitized was 3 V grainand samples boundaries. were To kept make for 45 s in etchant)sure that. thereThe fracture is a minimum surface trace was of studied polishing by particles means onof thea scanning surface, sampleselectron weremicroscope washed (SEM) with . acetone. Etching was performed with Nitric acid 60% (Voltage was 3 V and samples were kept for 45 s in etchant). The fracture surface was studied by means of a scanning electron microscope (SEM). 3. Results and Discussion 3. Results and Discussion 3.1. Visual Examinations 3.1. Visual Examinations Figure 1 shows an example of a failed tube together with stereomicroscopic image of crack. As can be seen,Figure the1 showscrack has an example propagated of a failedcircumferentially tube together parallel with stereomicroscopic to the welding imageline. The of crack. crack has As can be seen, the crack has propagated circumferentially parallel to the welding line. The crack has propagated in HAZ area in the vicinity of the weld. stereomicroscopic image shows that the crack propagated in HAZ area in the vicinity of the weld. stereomicroscopic image shows that the crack has has propagated alongside grain boundaries. There is no defect in the form of crack or cavities inside propagated alongside grain boundaries. There is no defect in the form of crack or cavities inside the the weld,weld, inferring that that the the weld weld itself itself has has a perfect a perfect quality. quality. In some In some areas areas there arethere some are deposits some deposits on the on the surface.surface. Other thanthanthese these mentioned mentioned features, features, there there is hardly is hardly any otherany other abnormality abnormality in the formin the of form of bulging,bulging, ththinning,inning, or or localized localized deformation deformation in any in form. any The form. fact The that there fact that is no there bulging is or no localized bulging or localizeddeformation deformation infers that infers creep that is notcreep a major is not issue. a major issue.

FigureFigure 1. 1.MacroMacro and and stereomicroscopic stereomicroscopic images images of of failed failed specimen. specimen.

3.2. Chemical Analysis

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Metals 2019, 9, x FOR PEER REVIEW 3 of 8 3.2. Chemical Analysis The chemical composition of the alloy was checked with ICPAES to make sure that the chemical composition of the alloy is within the standard range. Table Table 11 compares compares thethe chemicalchemical compositioncomposition ofof the alloy with the standard values,values, showingshowing thatthat thethe alloyalloy hashas aa standardstandard chemicalchemical composition.composition.

TableTable 1. Chemical composition of the alloy, compared with standard valuesvalues (in(in wt%).wt%).

AISIAISI 304 304 C C Si Si Mn Mn P PS SCr Cr NiNi FeFe MeasuredMeasured -- 0.7 0.7 1.40 1.40 0.011 0.011 0.006 0.006 18.10 18.10 10.0010.00 Bal.Bal. StandardStandard - -0.75 0.75 2.00 2.00 0.045 0.045 0.030 0.030 18.0018.00–20.00–20.00 8.00–10.508.00–10.50 Bal.Bal.

3.3. Analysis of Surface Deposits/Structure Deposits/Structure Figure 22 showsshows graingrain structurestructure andand depositsdeposits onon thethe surfacesurface ofof thethe tubetube nearnear thethe damageddamaged area.area. Two mainmain features areare noteworthynoteworthy toto mention.mention. The first first noticeable observation is the so so-called-called “grain falling-out”falling-out” phenomenon (see Figure 22a).a). ItIt appearsappears thatthat graingrain boundariesboundaries atat thethe surfacesurface areare heavilyheavily oxidized, resulting in the weakening of grain boundary and detachment of a whole grain from the surface. This This is is also also confirmed confirmed by by the the SEM SEM image image (Figure (Figure 2b).2b). It Itis isalso also noticeable noticeable that that some some areas areas on onthe thesurface surface are are covered covered a black a black deposit. deposit. The The EDS EDS analysis analysis of of these these deposits deposits are are given given in in Figure Figure 22c,c, showing that elements like C, Cl, Mg, Na, and Ca Ca are present in the composition of of these these deposits. deposits. Given that the outer surface of the tube is the the fire fire-side,-side, one can conclu concludede that these dep depositsosits are typical flyfly ashes, coming from the fuel. In this case fly fly ash corrosion appears to have a significant significant attribution to the observed observed failure. Fly Fly ash ash is is a a residue residue generated due due to coal combustion and contains finefine particles. In this casecase ashash isisbrought brought to to the the system system via via flue flue gas. gas. The The composition composition of of fly fly ash ash depends depends on theon the coal coal and and the combustionthe combustion condition. condition. In most In most cases, cases, fly ash fly contains ash contains ferric oxide,ferric oxide, aluminum aluminum oxide, siliconoxide, silicon dioxide, dioxide and calcium, and calcium oxide [oxide18]. Fly [18] ash. Fly particles ash particles are normally are normally collected collected by filter by bags filter and bags/or electrostaticand/or electrostatic filter. Yet, filter. there Yet, is there a chance is a chance that very that small very residues small residues are not are captured not captured by filters. by filters. These residuesThese residues are then are precipitated then precipitated on the outeron the surface outer surface of the tube. of the Fly tube. ash Fly residues ash residues can accelerate can accelerate surface corrosions.surface corro Elementssions. Elements like Ca and like Na Ca can and di ff Nause can into diffuse grain boundaries, into grain resulting boundaries, in the resulting weakening in the of grainweakening boundaries of grain and boundaries ultimately and grain ultimately detachment grain on detachment the surface. on Needless the surface. to mention Needless that to elementsmention likethat Clelements can also like severely Cl can corrodealso severely stainless corrode steels. stainless steels.

Figure 2. Cont.

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Figure 2. (a) Optical microscope image of grain falling-out at the surface, (b) SEM image of grain falling-out at the surface, and (c) SEM/energy-dispersive X-ray spectroscopy (EDS) analyses of deposits Figure 2. (a) Optical microscope image of grain falling-out at the surface, (b) SEM image of grain on the surface. falling-out at the surface, and (c) SEM/energy-dispersive X-ray spectroscopy (EDS) analyses of 3.4. Microstructuredeposits on the Analysis surface. Figure3 shows optical microscope and SEM images of HAZ structure in the as-polished condition. 3.4. Microstructure Analysis Figure3a,b show that grain boundaries are mostly covered with a rather thick black phase layer. FigureFigure3c,d depict 3 shows that optical the thickness microscope of the and grain SEM boundary images phaseof HAZ layer structure can be as in high the as as-polished 10 µm. Thecondition grain. boundaryFigure 3a,b phase showappears that grain to boundaries be rather brittle. are mostly This covered is clearly with seen a rather in Figure thick3c black in which phase thelayer. grain Figure boundary 3c,d depict phase that is the broken. thickness The of EDS the grain analyses boundary of grain pha boundary,se layer can given be as in high Figure as 103c,d, µm. showThe grain that boundary the grain phase boundary appears phase to be is richrather in brittle. oxygen, This , is clearly and seen . in Figure It3c appears in which that the graingrain boundariesboundary phase are sensitized is broken. and The areEDS heavily analyses oxidized. of grain Grain boundary, boundary given sensitization in Figure 3c,d, takes show place that whenthe grain chromium boundary phase precipitates is rich in oxygen, at grain carbon boundaries., and chromium. Chromium It appears carbide precipitationthat grain boundaries at grain boundaryare sensitized creates and a chromium-depletedare heavily oxidized zone. Grain in the boundary vicinity ofsensitization sensitized graintakes boundaries.place when Givenchromium that chromiumcarbide precipitates is the major at element grain boundaries. when it comes Chromium to the corrosion carbide resistanceprecipitation in stainless at grain steels, boundary this narrowcreates chromium-depleteda chromium-depleted region zone in adjacent the vicinity to the of grainsensitized boundaries grain boundaries. makes grain Given boundaries that chromium preferential is the corrosionmajor element spots. when A continuous it comes networkto the corrosion of resistance on grain in boundariesstainless steels, and intergranularthis narrow chromium corrosion- makesdepleted grain region boundaries adjacent weak to the and grain brittle. boundaries Cracks can makes therefore grain boundariesmore easily preferential propagate alongside corrosion sensitizedspots. A continuous grain boundaries. network Sensitization of carbides of on grain grain boundaries boundaries of austenitic and intergranular stainless steels corrosion takes makes place grain boundaries weak and brittle. Cracks can therefore more easily propagate alongside sensitized in temperature range 510 to 780 ◦C[19]. Any thermal exposure into this temperature range during weldinggrain boundaries. or service could Sensitization potentially of resultgrain in boundaries the sensitization of austenitic of grain stainless boundaries steels in stainlesstakes place steels. in Thetemperature temperature range and 510 time to required780 °C [19 to]. cause Any sensitizationthermal exposure and intergranular into this temperature corrosion range depends during on thewelding chemistry or service of the could alloy, potentially and more result specifically in the onsensitiz the carbonation of content grain boundaries of the alloy. in Obviously,stainless steel thes. higherThe temperature the carbon contentand time of therequired alloy, theto cause higher sensitization is the risk of and sensitization. intergranular For this corrosion reason, depend low on stainlessthe chemistry steel grades of the suchalloy as, and AISI more 304L specifically and AISI 316L on alloysthe carbon are known content to of have the an alloy. excellent Obviously, resistance the againsthigher sensitizationthe carbon content during of welding. the alloy, In higherthe higher alloyed is the stainless risk of steels sensitization. such as alloys For AISIthis reason, 904L, grain low boundarycarbon stainless susceptibility steel grades to sensitization such as AISI is also 304L essentially and AISI not 316L a concernalloys are [19 known]. In this to case, have the an observedexcellent sensitizationresistance against has possibly sensitization its root during in the weldingwelding. practice. In higher The alloyed service stainless temperature steelsis such on the as alloys lower sideAISI of904L, the sensitizationgrain boundary temperature susceptibility range to and sensitization service exposure is also hasessentially possibly not not a theconcern major [19 contribution]. In this case, to thethe sensitization.observed sensitization If sensitization has possibly was due its to root the in service the welding exposure, pract oneice. should The service have seentemperature sensitization is on inthe the lower base side metal of asthe well, sensitization which obviously temperature is not range the case. and service exposure has possibly not the major contribution to the sensitization. If sensitization was due to the service exposure, one should have seen sensitization in the base metal as well, which obviously is not the case.

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Figure 3. Optical microscope images of grain structure in the heat affected zone (HAZ) area of failed specimens;Figure ( a3., bOptical) an overall microscope view, andimages (c,d of) SEMgrain/EDS structure analyses in the of heat grain affected boundary zone phase(HAZ) and area inside of failed grains. specimens; (a,b) an overall view, and (c,d) SEM/EDS analyses of grain boundary phase and inside 3.5. Assessmentgrains. of Grain Boundary Sensitization ASTM A262 [20] is a standard assessment method that can be used 3.5. Assessment of Grain Boundary Sensitization to evaluate grain boundary sensitization in austenitic stainless steels. The ASTM A262 standard contains fiveASTM diff A262erent [20 practices.] is a standard When intergranular stainless steels corrosion are exposed assessment tohigh method temperature that can be service used to for a long time,evaluate carbide grain precipitates boundary sensitization on grain boundaries. in austenitic This stainless creates steels. Cr-depleted The ASTM areas A262 in the standard vicinity of graincontains boundaries, five different making practices. the alloy When susceptible stainless to steels intergranular are exposed corrosion. to high temperatur This standarde service is a for popular a long time, carbide precipitates on grain boundaries. This creates Cr-depleted areas in the vicinity of assessment method for interganular corrosion susceptibility of stainless steels. This standard contains grain boundaries, making the alloy susceptible to intergranular corrosion. This standard is a popular five diassessmentfferent practices, method whichfor interganular each being corrosion useful susceptibility for different conditionsof stainless steels. and materials. This standard Practice contains A is the simplestfive method,different whichpractices, is basedwhich each on the being etching useful of for alloys different in Oxalic conditions acid. and Obtained materials. etch Practice structures A is can be categorizedthe simplest into method, the following which is based groups on [the20]: etching of alloys in Oxalic acid. Obtained etch structures can be categorized into the following groups [20]: Step Structure: steps are formed between grains, no ditches at grain boundaries; Dual Structure:Step Structure: Some ditches steps are at formed grain boundaries between grains, in addition no ditches to at steps; grain and boundaries; Dual Structure: Some ditches at grain boundaries in addition to steps; and Ditch Structure: One or more grains completely surrounded by ditches. Ditch Structure: One or more grains completely surrounded by ditches. The formation of the latter etch structure is an indication of grain boundary sensitization. Figure4 shows the etch structure of AISI 304 stainless steel in the base metal (Figure4a) and in the HAZ (Figure4b). A clear di fference is observed between these structures. While the former is a typical step Metals 2019, 9, x FOR PEER REVIEW 6 of 8

The formation of the latter etch structure is an indication of grain boundary sensitization. Figure

4 showsMetals the2019 etch, 9, 969 structure of AISI 304 stainless steel in the base metal (Figure 4a) and in6 ofthe 7 HAZ (Figure 4b). A clear difference is observed between these structures. While the former is a typical step structure, the latter is a clear ditch structure, inferring that HAZ has become severely sensitized structure, the latter is a clear ditch structure, inferring that HAZ has become severely sensitized during duringwelding welding and and service service exposure. exposure.

Figure 4. Optical microscope images of grain structure in (a) base metal and (b) HAZ area of failed Figurespecimen, 4. Optical etched microscope according images to standard of grain ASTM structure A262, Practice in (a A.) base metal and (b) HAZ area of failed specimen, etched according to standard ASTM A262, Practice A. 4. Concluding Remarks and Preventive Measures 4. ConcludingThis paperRemarks investigates and Preventive a failure in convectionMeasures section tubes of a gas heater unit in a petrochemical plant. Tubes are made of AISI 304 stainless steel. The failure is reported after 5 years of service at This paper investigates a failure in convection section tubes of a gas heater unit in a working temperature 500 ◦C. The failure is in the form of circumferential cracks in the vicinity of the petrochemicalweld. The plant. following Tubes conclusions are made can of be AISI drawn, 304 based stainless on the steel. obtained The results: failure is reported after 5 years of service at working temperature 500 °C. The failure is in the form of circumferential cracks in the Damage has initiated at HAZ area in the vicinity of the weld. Cracks have propagated alongside vicinity• of the weld. The following conclusions can be drawn, based on the obtained results: grain boundaries.  DamageGrain has boundaries initiated inat theHAZ HAZ area area in are the covered vicinity with of a the rather weld. thick Cracks grain boundary have propaga phase.ted Further alongside • grain fromboundaries. the HAZ area in the base metal, there is hardly any indication of such grain boundary phase. Results show that grain boundaries are rich in chromium and are sensitized. Sensitization of grain  Grain• boundaries in the HAZ area are covered with a rather thick grain boundary phase. Further from theboundaries HAZ area is associated in the withbase the metal, formation there of ais narrow hardly Cr-depleted any indication region in of the such vicinity grain of grain boundary boundaries. This makes grain boundaries susceptible against corrosion. phase. The test method, proposed in ASTM A262, confirms the susceptibility of grain boundaries in the  • ResultsHAZ show area that to intergranular grain boundaries attack. are rich in chromium and are sensitized. Sensitization of grain Someboundaries deposits is on associated the outer surface with the of tubes formation are observed. of a narrow These deposits Cr-depleted are rich region in C, Cl, in Mg, the Na, vicinity • of grainSi, Al,boundaries. and Ca, indicating This makes that these grain residues boundaries on the surfacesusceptible are fly against ashes. corrosion.  The testCarbon method, content proposed has a very in decisiveASTM influenceA262, confirms on the susceptibility the susceptibility of grain of boundaries. grain boundaries Superior in the • HAZ arearesistance to intergranular to grain boundary attack. susceptibility can be achieved by reducing the carbon content values  Some lessdeposits than 0.03 on wt%.the outer It is highly surface recommended of tubes are to use observed. AISI 304L These or other deposits low-carbon are stainlessrich in steelC, Cl, Mg, Na, Si,grades Al, and instead Ca, ofindicating AISI 304. that these residues on the surface are fly ashes. Stabilizing elements like Nb and Ti can also enhance the resistance to sensitization. These elements  Carbon• content has a very decisive influence on the susceptibility of grain boundaries. Superior form stable carbides at high temperature. This way less carbon is available for the formation resistanceof chromium to grain carbides boundary at grain susceptibility boundaries. Commonly can be achieved used stabilized by reducing austenitic the stainless content valuesgrades less thanare AISI 0.03 321 wt%. and AISI It is 347. highly Replacement recommended of currently to used use AISI AISI 304 304L alloy or with other stabilized low-carbon stainlessgrades steel is alsogrades highly instead recommended. of AISI 304.  Stabilizing elements like Nb and Ti can also enhance the resistance to sensitization. These elementsAuthor Contributions: form stableConceptualization, carbides at high A.B.; methodology, temperature. A.B.; This formal way analysis, less A.B.; carbon investigation, is available A.B. and for the P.T.; writing—original draft preparation, A.B.; writing—review and editing, P.T.; visualization, A.B.; supervision, formationP.T.; project administration,of chromium P.T. carbides at grain boundaries. Commonly used stabilized austenitic stainless steel grades are AISI 321 and AISI 347. Replacement of currently used AISI 304 alloy Funding: This research received no external funding. with stabilized grades is also highly recommended.

Author Contributions: Conceptualization, A.B.; methodology, A.B.; formal analysis, A.B.; investigation, A.B. and P.T.; writing—original draft preparation, A.B.; writing—review and editing, P.T.; visualization, A.B.; supervision, P.T.; project administration, P.T.

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Acknowledgments: Authors would like to thank Delft University of Technology for support in publishing this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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