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

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A Study on the Failure of AISI 304 Stainless Steel Tubes in a Gas Heater Unit metals Article A Study on the Failure of AISI 304 Stainless Steel 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 stainless steel. 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 welding 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 steels. 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 corrosion 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, th thinning,inning, or or localized localized deformation deformation in any in form. any Theform. fact The that therefact that is no there bulging is orno 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 Metals 2019, 9, 969 3 of 7 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.
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