Materials better ability to form austenite than standard DSS Alloy 2209. The API practices and ASME Section-II Part 3( A8.49 & A8.52) recommend super DSS grade filler wire( i. e., ER 2594) for the welding of super DSS grades such as Alloy 2507. The ferrite number of ER 2209( which is a standard DSS) is 54-60, whereas for the super DSS grade ER 2594 it’ s 45-55. This implies the higher propensity of ER 2209 to retain the dominant ferritic microstructure( instead of transition to austenite) that generally results from the presence of nitrogen during the slow cooling of weldment( s). 2 In summary, the higher nitrogen content and lower ferrite in the ER 2594 super DSS filler wire( than standard DSS) will have enhanced ability to form the austenite and hence the increased toughness of weldment than ER 2209. Figure 2 shows a schematic of the weld between tube and tube sheet. So, the failure of the weldment( via cracking) resulted from several factors( or their combined effects) as below:
• High tensile stresses from thermal elongation of tubes, since the SS 316L expansion joints could be incompatible with the Alloy 2507 due to the 22 % higher coefficient of thermal expansion
• The reduced toughness of the weldment due to the reduced austenite caused by the rapid cooling of the weldment
• Internal stresses due to dissimilar welds may have contributed to dissimilar metal weld( DMW) cracking 1
• Fatigue failure due to thermal cycling caused by multiple start-up and shutdowns
Summary Under heat transfer conditions, corrosion rates from formic acid are higher than in the non-heat transfer condition. This factor was not accounted for during the material selection for the twin pass heat exchanger in the first place. The cracking that occurred even after the tubes were upgraded to Alloy 2507 stemmed from high tensile stress at the weld-tube sheet junction due to the tube expansion force( due to dissimilar thermal expansion coefficient). In addition, internal stresses due to dissimilar welds may have also contributed to dissimilar metal weld( DMW) cracking. Finally, fatigue due to thermal cycling was caused by multiple start-ups and shutdowns. The repairs involving dissimilar metals and corrosive service should be accompanied by Engineering reviews.
References:
1. API RP 571,“ Damage Mechanisms Affecting Fixed Equipment in the Refining Industry”( Washington, DC: API, 2020).
2. Immad Ovais Rohela, Ahmad Raza Khan Rana, Yasar Irshad,“ Case Study On The Failure Of Duplex Stainless Steel Heat Exchanger From Thermal Shock,” AMPP CORROSION 2024( Paper No. 20923).
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