Visual Examination Assisted; RCA of Cracks In Thin Walled Duplex Stainless Steel Welds

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1 More info about this article: Abstract: Visual Examination Assisted; RCA of Cracks In Thin Walled Duplex Stainless Steel Welds Kuldip KUMBEPHALKAR,P B PATIL, Bhagwan SHINDE PRAJ Industries Limited, Gat No. 745 & 748 Sanaswadi Pune , Maharashtra, India. kuldipkumbephalkar@praj.net Evaporator used for evaporation are shell and tube type heat exchangers. Shell side vapors temperature 70 C and tube side liquid temperature 59 C. This temp. difference lead to heat transfer and evaporation take place. TDS of liquid is 35% in tube side. Tube side comprises of bonnet shell, head and flange. Flange of this bonnet shell is bolted to tube sheet of main shell. Material of construction of the bonnet shell is Duplex Stainless Steel thickness 3mm. After few years of operation it was observed that the long seam weld developed longitudinal cracks and the process fluid started leaking through the cracks. The cracks were repaired by welding and the equipment was put into operation. After couple of weeks again it cracked and leak started. Due to this repeated cracking phenomena RCA was done. The bonnets shell were subjected to thorough Visual Examination which included Characterization of flaws viz. Size, shape and orientation of flaws. It was also subjected to metallographic study and corrosion tests as per G48 and were analyzed for RCA. This paper describes root cause analysis of repeated failure of weld seams in bonnet shell and the corrective/preventive actions identified and implemented for subsequent repairs. 1. Background: 1.1 PRAJ Industries Limited supplied multiple effect evaporation plant in Year 2013 and was commissioned in December The plant consisted of various equipment such as Vapor Liquid separators (VLS-1,2 & 3), evaporators 1,2 & 3 for effect I,II,III. Material of construction (MOC) of VLS and channel shells of evaporators is duplex stainless steels. Heaters and few other equipment are manufactured with MOC as 304L. Vapor liquid separators are vessels & evaporators are shell and tube type heat exchangers with channels shells at top and bottom. The MOC used for construction of these channel shells is duplex stainless steel in accordance with SA 240 UNS : 2205 material. The overall dimensions of these channel shells are 710 mm ID X 3 mm thick X 550 mm long. 1.2 For channel shell long seam welds of evaporator for effect-3. The severity of aqueous chloride environment (concentration) increases as it moves from effect -1 to effect 3 viz. evaporator for effect -3 is subjected to more severe corrosive condition compared to 1 & 2. Cracks were observed at the centre of the long seam welds of bottom and top channel shell of evaporator for VLS effect 3 1

2 1.3 It is observed that leakage through the same repaired long seam joints, hence it was decide to cut the piece of cracked long seam and subject it to investigations. The rest of joints were again repaired with the same welding procedure to put the plant in operation on temporary basis, mean time fabricate new channel shells and supply it to Customer free of cost. For manufacturing new channel shells it was necessary to identify root causes so that the same will not get repeated in new channel shells and new order being executed by PRAJ. 2 The results of the investigations are as below: 2.1 Design parameters : In view of failure only on channel shell of evaporator vis-à-vis no failure in VLS although both of them are constructed with same MOC (duplex material); Design parameters such as design pressure, operating temperatures and process fluid of vapor liquid separator and channel shells of evaporators were reviewed and it appears that these parameters cannot bear any significant effect on the failure of the weld joints. Designer may study the effect of vacuum generated on top side leading to stresses on long seam welds- Stiffeners from outside are not seen in the drawing. 2.2 Weld geometry considerations: It has been observed that WFD drawings indicate single side 60 0 V-groove joint with the requirement of back chipping and re-welding for VLS which is 4 mm thick and for channel shells of evaporators indicate square butt joint for shell and dish-end weld seams without back chipping, these are 3 mm thick. The study indicates that square butt weld joints result into higher heat input and dilution of weld and heat affected zone in the material which will affect ferrite and austenite phase balance in DSS. Also the root side being purged with pure argon will assist in dissipating the heat quickly (resulting into quenching) and may lead to formation of high ferrite (> 60%) at root side. This high ferrite will lead to decrease in corrosion resistance. 2.3 Investigations with respect to Manufacturing at shops and site: Some unclean conditions and square butt welding of shell long seams without gap may also have contributed to ferrite formation more than desired, leading to decrease in corrosion resistance. At site it was also noted that long seams were welded only from one side and circum seams were observed to be welded from both sides as it is a normal practice to perform fit of dish end to shell with V-edge preparation leading to inconsistent welding methodology adopted. Welding procedure used for welding these joints at manufacturing locations were reviewed. Heat input control was not indicated in the WPS used for welding of these joints on the shop floor, however interpass control temperature (<150 0 C) is adequately defined in the welding procedure. 3 Investigations based on study of failed sample drawn from evaporator long seam for effect Visual inspection : Visual inspection of long seam weld joint reveals heavy pitting primarily originating from heat affected zone and then spreading on the weld to the extent that it has resulted into physical separation of the material (crack). Visual examination of circum-seam weld 2

3 joint reveals no abnormality with respect to pitting or cracking in weld as well as heat affected zone. 3.2 Chemical analysis : The chemical analysis study is indicated in table below : Plate Material Chemical Analysis Chemistry C Mn Si Ni Cr Mo P S N Min required as per Code Max required as per Code Actual for 3 mm thk Actual for 3 mm thk Weld Chemical Analysis Chemistry C Mn Si Ni Cr Mo P S N Cu Min required as per Max required as per Actual for 3 mm thk Actual for 3 mm thk Parent metal chemistry meets the requirement of Codes, Weld chemistry of long seam does not meet the requirements and indicates lower nickel percentage which is austenite stabilizer. This was also verified by PMI for confirmation. This lower nickel percentage in welds is due to dilution with the base material and is mainly attributed to square butt joint design. 3.3 Hardness : Required V/S Actual Sr Vickers Required Actual Hardness Test BG/ (C/S) BG/ (L/S) Weld HAZ

4 Hardness readings indicated above are acceptable both in weld and heat affected zone hence it can be concluded that there is no effect of cold working on the base material; also it is obvious in view of low thickness. 3.4 Microstructure: Micro structure study was carried out to determine phase balance in duplex stainless steels and to verify presence of any detrimental intermetallic phases. Required proportion is : 50:50-Ferrite and Austenite, however it is practically difficult to achieve this ideal requirements without heat treatment hence recommended phase balance in as welded condition is < 60% ferrite and austenite > 40 % BG/ (L/S) Weld Metal SID:BG Frame Area Ferrite (%) Austenite (%) Weld (mm²) Frame Frame Frame Frame Frame Frame Frame Frame Frame Frame Average

5 3.4.2 BG/ (L/S) HAZ SID:BG Frame Area Ferrite (%) Austenite (%) (L/S) HAZ (mm²) Frame Frame Frame Frame Frame Frame Frame Frame Frame Frame Average

6 3.4.3 BG/ (C/S) Weld Metal SID:BG Frame Area Ferrite (%) Austenite (%) Weld (mm²) Frame Frame Frame Frame Frame Frame Frame Frame Frame Frame Average

7 3.4.4 BG/ (C/S) HAZ SID:BG Frame Area Ferrite (%) Austenite (%) (C/S) HAZ (mm²) Frame Frame Frame Frame Frame Frame Frame Frame Frame Frame Average This microstructure study reveals that ferrite and austenite balance in heat affected Zone of longseam welds is not balanced; observed is Ferrite 63% and austenite 37%. Rest of the microstructure is within specified limits. The lab report also specifies absence of other detrimental intermetallic phases. 4. Conclusion (Root causes): 4.1 Leakage through welds: From Visual examination results and results of microstructure it indicates that the failure is a result of excess ferrite formed in the HAZ of the long seam welds due to higher dilution resulting from inadequate weld joint configuration; possible use of square butt with zero gap, inadequate controls on heat input and environmental conditions such as cleanliness, 7

8 inadequate inter-pass cleaning. Welding procedure was not validated for phase balance and other appropriate corrosion tests were not performed prior to its use. 5. Corrective/Preventive actions: Based on above conclusion, following corrective actions are decided for manufacturing of channel shells and duplex stainless steel vessels and exchangers. 5.1 Weld joint configuration single 60 0 V with root gap as 1.5 to 2.0mm. 5.2 Welding procedure addressed control of heat input and cleaning requirements. 5.3 Filler material base chemistry verified and it contain nickel and molybdenum on higher side of the specified range. 5.4 Used Pure argon and 2% nitrogen as shielding gas during welding. 5.5 Welding procedure is validated by testing for phase balance, and ferric chloride test. 5.6 On job verification of ferrite from face and root side, include in QA plan. 8