Heat Exchanger World Magazine April 2025 | Page 29

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» Table 1. Physical and mechanical properties of SS316 L, Alloy 2205 & Alloy 25072

Materials

SS 316L 16.5 x 10-6 186 17 485 170 Alloy 2507 13.5 x 10-6 186 16 795 550 Alloy 2205 13.5 x 10-6 186 17 655 450

temperatures may also result in a reduction in the overall strength of DSS. Expansion joints are known to compensate for the thermal expansion-induced stresses. However, the expansion joints were made from the SS 316L, which itself has a 22 % higher thermal expansion coefficient than alloy 2507( refer to Table 1), which may not compensate for the compressive stresses( due to thermal elongation). There were no engineering reviews to check the suitability of existing( i. e., SS 316L) expansion joints with Alloy 2507 tubes, which provided no clarity on the stress value / stress field around the tube – tube sheet weld joints. Since DSS and austenitic SS don’ t follow any endurance limit,( unlike carbon steel, low alloy steel, and titanium) they manifest fatigue failures even below yield point. This implies that even at the lower thermal stresses( i. e., well below the tensile strength) the cracking may still happen depending on the temperature swing and the number of cycles. 1

Metallurgical outlook The welding between the tubes and tube sheet was performed using the approved and well-practiced welding procedure specification( WPS) deploying 1 / 16”( or 1.6 mm) thick filler wire ER 2209. The inter-pass temperature was kept at 320 F( or 160 ° C) to avoid any metallurgical defects( sensitisation, sigma phase embrittlement, etc.) as well as thermal distortions. 1 NDEs and pressure tests were performed at maximum allowable working pressure( MAWP) for the bundle with upgraded tube metallurgy. Considering the inter-pass and operating temperatures, the risk of oxidations and cracking due to ϭ-phase and 885 F( or 475 ° C) embrittlement was remote. API practices recommend a maximum inter-pass temperature of < 150 ° C for the standard DSS and up to 120 ° C for super DSS. 2 The higher inter-pass temperature than the recommended limits( for super DSS) may trigger some change in the weldment behaviour. However, the filler wire in this case was made from standard DSS( i. e., ER 2209) and the tubes were from super DSS, so there were two governing( as well as conflicting) inter-pass temperature limits. This may lead to a heterogenous nature with the weldment( though not fully investigated in this study). Since the tube sheet had a significantly higher thickness( 45 mm) than the tubes and the weldment, this may have resulted in the rapid cooling of the weldment. A review of the WPS revealed that it specified an inter-pass temperature of 160 ° C for the tube thickness ranging from 10 mm to unlimited thickness. This may have implications in terms of higher cooling rate( s) of weldments, especially at the tube-tube sheet welds. 2 The rapid cooling deprives the weldment from forming the austenite, which is the key reason behind the fracture toughness in a DSS microstructure. 2 From the review of the metallurgical compositions, it was evident that Alloy 2205, which is a standard grade DSS, has a nitrogen content between 0.14- 0.20 whereas Alloy 2507, which is a super DSS, has a higher nitrogen content( i. e., 0.24- 0.2) and a

DETAIL- I

See detail- I Heat flux

Figure 2. Schematic of heat dissipation in the tube-tube sheet welds

SS 316L tube-sheet ER 2209 weldment Alloy 2507 tube www. heat-exchanger-world. com Heat Exchanger World April 2025

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