Influence of design parameters in weld joint performance


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Influence of design parameters in weld joint performance

  1. 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME202INFLUENCE OF DESIGN PARAMETERS IN WELD JOINTPERFORMANCEMaqbool Ahmed1, M. Azizuddin 2, B.Balakrishna 3,1Associate Prof RITS, 2Prof & Head Mech Dept RITS,3Associate Prof JNTU Kakinada1. INTRODUCTIONA flange connection used in oil and gas industry failed premature. Investigation wasconducted to analyze the failure causes. Micro/macro structure study , hardness, light andSEM microscopes analysis of the chemistry near and away from the crack suggested that:a) The failure is most probably caused by recent practice of reducing the wall thinness ofthe nipple by grinding to suit to the flange endsb) Welding has caused a brittle micro-structure to develop, making it vulnerable tocrack. Also, sulphur pick – up (either as a result of heat induced by welding or as aresult of ingress from the flowing mediam) near the cracking area shows relativelyhigh concentrations (about twice that of the bulk material about 10 mm away from thecrack line).These findings did emphasize the importance of design factor in accelerating failure. At theend, some recommendations have also been introduced to mitigate the occurrence of suchfailures in the future.2. DEFINITION OF THE PROBLEMFailure of a weld joint of 3/4” flange had resulted in hydrocarbon leak. This 3/4”tapping had been taken from 24” and routed with 1” analyzer line ( new ) with reducer. Thefailure had apparently happened within a time span of about four months.The circumferential crack (65%) was observed in top side HAZ of weld joint.INTERNATIONAL JOURNAL OF ADVANCED RESEARCH INENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online)Volume 4, Issue 3, April 2013, pp. 202-210© IAEME: Impact Factor (2013): 5.8376 (Calculated by GISI)www.jifactor.comIJARET© I A E M E
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME203Figure 1. The cracked-face of the failed flangeFigure 1 shows the appearance of the crack externally. The ¾” flange was directlywelded with the pipe as a butt weld joint.The new 1” analyzer line found rigidly fixed with L- angle and clamps. Also linerouted 90 deg in the direction of 24” header. As the client has stated to us, no pressure gaugeinstalled, it was end blinded before EDM used it as tie-in for analyze sample line. The flu gaswithin the pipe is sour. The design pressure of 24” pipe is 79.8 bar (G) and designtemperature is 85 Deg C. The flange material is a low temperature carbon steel (A350 LF2,¾ inch Sch 80. 3.91mm) and the Nipple material sis also a low temperature carbon steelA333 Gr6.3/4 inch Sch 80. 3.91 mm).1. Examination:The examinations that were carried out on the failed flange were as follows:1.1. Macro-examination1.2. Micro-examination (Metallography, SEM)1.3. Chemical analysis (spark emission)1.4. Mechanical Hardness Test2.1. Macro-examination:Below, we will look at the results obtained from the examinations mentioned above. Figure2 shows the profile of the flange + nipple :Figure 2. Distinguishing the nipple and flange parts in order to define the location of thecrack
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME204Furthermore, The failed flange and nipple were sectioned vertically. This is shown in Figure3a. As seen in this figure, the crack is on a portion (about 70%) of the material of the flange.It is evident from these macro-examination images (Figures 2 and 3a) that the location of thecrack is within the flange area. This is important as this will allow us to concentratemore on the flange area and investigate more deeply on the cause (es) of the crack in thisarea.Figure 3a. The flange and the nipple parts after being cut into two halves.As seen From the half on the right, the crack is very evident. The fracture surfaces were alsoexamined across A-A as shown in Figure 3b.Figure 3b. Fracture surfaces of the failed flange as sectioned through A-A.One of these surface was examined by both macro-and micro-examination. Figures 3cand 3d show the failed surface. As seen in Figure 3c, at least one crack visible with naked eyebeing developed on the fracture surface.A A
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME205Figure 3c. A transverse crack being developed at the cross-section of the fracture surfacealong A-A section from figure 4b. The crack covers about 60% of the length of the crosssection area.Figure 3d: magnified view of Figure 3c (close-up). Some of the “beach- marks” typicl offatigue are shown within the ovalFigure 3e. Beach marks (on the fracture surface) suggesting the likelihood of fatigueIt must be noted that observing bench-marks is one way of suggesting that the failurehas been due to fatigue. In this particular case, there is also another evidence which is themode of the crack (see Figures 10a and 10b). Before and after sectioning the flange + nipple.As seen from Figure 4, there is a reduction of 0.60 mm in the cross section. This trimmingaction will actually reduce the effective cross section to carry the load. This will result inhigher stresses being developedFor a given stress, then, this reduction in size would mean an increase of the localstresses by about 110 %. A possible consequence of developing such stresses is that theymay reach past the yield point of the material, causing plastic deformation by encouraging theformation of internal micro-cracks. If the material, micro-structurally, has also become brittledue to developing of brittle phases, this can enhance the likelihood of crack initiation/propagation especially at structure imperfections.
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME206Figure 4. (Left) the dimension of the flange before removing (right) after cold cuttingAmong the characteristics of the fluid, it had also been mentioned that it contains“impurities” such as dust. The dust particles will cause erosion-corrosion as it is apparentfrom Figure 5.Figure 5. Some signs of erosion-corrosionIt suggests that hard impurities that are accompanying the fluid can also have animpact on accelerating the failure of the piece. These impurities can hit the surface andthrough this physical contact, the effective cross section that may reduceErosion-corrosion is also further enhanced by the impact of improper design as imposed byinappropriate trimming: the difference between the cross sections thus generated is capable ofincreasing the detrimental of the dust micro-particles that are entrained with gas. Figure 6shows how to change in cross section due to design can cause internal deterioration in anequipment.
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME207Figure 6. Schematic presentation of how change in cross section can induce impingementAs it is seen from Figure 6, when the cross section is reduced, because of relativechange in the pressure of the fluid (as a function of fluid velocity), erosion-corrosion can beinduced.Another important matter from Figure 6 is that due to erosion induced as suchgetting a uniformly eroded surface is not likely. As it appears, the inside of the piece will beselectively ploughedThis will create a topography on the surface that will reflect light in different angles. Thisis seen in Figure 5 as bright and dark areas. However, it is not possible to estimate therelative contribution of this factor (erosion-corrosion) to the general set of internal factorsfacilitating corrosion.2.2. Micro-examination:Scanning electron microscope (SEM) of the crack is shown in Figure 7.
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME208Figure 7 shows that in addition to the main crack, some micro-cracks are being developedand start propagating. Another finding that is important from a micro-structural point of viewwas the change observed from a mixed pearlitic-ferritic microstructure to a fully ferriticmicrostructure near the weld zone.There are several processes that can lead to intergranular fracture.1. Micro-void nucleation and coalescence at inclusions or second phase particleslocated along grain boundaries2. Grain boundary crack and cavity formation associated with elevated temperaturestress rupture condition.3. De-cohesion between contiguous grain due to presence of impurity at grainboundaries and in the presence of hydrogen in liquid metals.4. Stress corrosion cracking associated with chemical dissolution along grainboundaries.5. Cyclic loading when the material has insufficient number of independent slipsystems to accommodate plastic deformation between contiguous grain leading tograin boundaries ruptureChemical analysis of sulphur near and away from crack along with other findings in thisinvestigation may suggest that a combination of the mechanisms above could have beenresponsible for observing this crack. However, based on the facts that:a) the part has undergone IG,b) the fluid is a sour gas where there is a relatively high concentration of sulphur near thecrack compared with that of the bulk material, (the impact to be mentioned in section3.3)c) the trimming of the effective load surface that can stimulate conditions of suddenchange in the velocity of the fluid inside, developing (internal) cyclic loading,(explained in section 3.1)Mechanisms 2, 3 and 5 could be the main mechanisms contributing to the failure of the part.2.3. Chemical analysis (spark emision)The main cracked area, as shown below in Figure 10, was also studied for relativeconcentration of sulphur near the cracked area and some 10 mm away from it.The mainreason for selecting sulphur was that the gas was of sour nature, having a relatively highconcentration of sulphur in it. In addition, during welding, a molten pool maintains aconcentration gradient for the alloying elements. The alloying elements will be attracted intothis molten pool and afterwelding the cooling process starts, the alloying elements that nowhave been precipitated at or near the weld line, start to change the mechanical properties ofthe material at that venue.The results of the spectroscopy have been superimposed on the figure that shows thecrack, altogether shown in Figure 12. The dark column represents the sulphur values fromwithin the bulk of the crack whereas the light column shows the relative values near thecrack.
  8. 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME209Figure 10. The change of relative concentration of sulphure (wt%) near the crack and about10 mm away from it within the bulk of the steel.As being evident from Figure10, sulphur is a much higher concentration near the crack,suggesting that either it has been accumulated as a result of welding or as a results of sulphuringress from the sour flu gas flowing inside. At this stage, however, it is impossible todistinguish between these possible two sources of sulphur but the end result is that themicrostructure becomes more vulnerable to cracking.2.4 Hardness TestA section of the failed flange was cut as shown in Figure 11 and the hardness of bothsides of the cut section was tested. Figure 12 shows the change of hardness on both sidesrecorded as HRBW (Rockwell Hardness B Scale Tungsten-Carbide ball Indenter)..Figure 11 . A cross section (AA) of the flange showing the difference in thickness for boththe original pipe width on the base metal and the reduced wall thickness near heat affectedzone (HAZ). Typically the wall thickness in the welded edge has been reduced by 66.90%.Figure12. Change of hardness over the outer and inner surfaces (as from Figure 9)Figure 12 shows that the hardness values (especially near the heat affected zone-HAZ) are far different from those of the parent material, suggesting that the material is toobrittle and susceptible to develop cracks. This matter becomes of importance when weconsider all other pieces of evidence (micro-/ macro- structure) that suggest that one of themain causes of the failure can be linked with the welding.(AA)
  9. 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME2103. CONCLUSIONS AND RECOMMENDATIONSThe failure seems to be a combination of the following factors:1. Wrong welding edge preparation; the trimming has resulted in a thickness too low forthe joint between the nipple and the flange to stand the required mechanical loads.The reduced area has already resulted in creating varying cross sections where thevarying local pressures resulting from varying velocities of the fluid gas alsocontribute more to the vulnerability of the material to failure2. Due to factors such as welding or possible ingress from the flu gas inside the flange,sulphur has been accumulated at the cracking area and most possible around the grainboundaries, giving rise to an intergranular crack,3. Due to the heat induced during welding, the microstructure of the weld zone and HAZhas been transferred into a brittle zone.4. The gas may contain impurities in the form of very tiny dust particles. These particles, entrained by the flow of the gas, are inducing erosion-corrosion resulting in makingthe internal wall even more vulnerable.5. The external factor of fatigue has been accelerated during the last four months ofservice adding already existing internal factors contributing to failure.The following can be recommended to prevent similar cases to happen:a) Avoid any modification in the dimensions of the parts to induce inappropriatelevels of stress as well as the likelihood of getting situations encouragingcavitation,b) Selection of a flange of same internal diameter as that of the nipple to avoid theturbulence in gas flowc) Use low sulphur welding method with more care not to cause too muchsegregation of potentially corrosive alloying elements (such as sulphur) near grainboundaries,d) Reduce the level of impurities (micro-dusts) of the gas to avoid internal erosion-corrosion,e) Observing and controlling the fatigue as induced by excess vibrations.4. REFERENCES1- ASME/ANSI B 16.5:Pipe flanges and flanged fitting ( 1996 ) page 1-2.2- ASME Pressure Vessel and Boiler Code. Section II, Part A, Ferrous materials specifications,Materials: Specifications for carbon steel forging for piping applications. (1999) page 180.3- API specifications 5L; Specifications for line pipe, 42nded, ( 2000 ) page 8.4- Failure analysis of high pressure Butt Weld. F Ahmed,F Hassan and L.Ali. Pak J.Engg &Appl science Vol 3 July 2008 ( P 26-32 )5- Equivalent to assess hardenability of steel and prediction of HAZ Hardness Distribution.Kasuya, T and Hashiba Y.; Nippon Steel Technical Report No 95 January 2007.6- Failure Examination - Faulty Design, Weld Defect-Fracture of a Cross on a Church Steeple.Naumann, Friedrich K; Spies, Ferdinand, PRAKT METALLOGR., 12(5), May 1975, pp.268-271.7- Failure analysis of a Cross country line pipe using CTOD concept - A case study;Sova Bhattacharya, Kannan C,Mohapatra B, Makhija R&D Centre, Indian Oil CorporationLimited, Faridabad, India