Strain calculations, pitting resistance, and chloride stress corrosion cracking testing are currently used as the key indicators to delimit the minimum bend radius for 22% chrome duplex stainless steels without heat treatment to be 3.3 times the tube diameter for u-bend heat exchanger tubing. However, existing data does not address the limitations of this alloy, in the as cold worked condition, for sour services in the refining industry. This study evaluates the sulfide stress corrosion cracking resistance of as-bent and integrally finned 22% Cr duplex stainless steel UNS-S32205 tubing for refinery sour services by presenting hardness data and corrosion testing per ASTM G48 and NACE TM0177 of tight u-bend specimens with bend radius up to 1.5 times the tube diameter as well as integrally finned tubes. As a follow up from a previous study, the corrosion resistance of as finned 25% Cr super duplex stainless steel will also be presented.
The use of Duplex Stainless Steels (DSS) in refinery sour environments is governed by ANSI/NACE MR0103/ISO 17945NACE "Metallic materials resistant to sulfide stress cracking in corrosive petroleum refining environments" which limits DSS base materials to be used in Hydrogen Sulfide (H2S) services to a maximum hardness of 28 HRC for materials with a PREN ≤ 40 and to a maximum hardness of 32 HRC for those materials with PREN > 40.1 These hardness values are in line with the hardness requirements of solution annealed as produced straight tubes, but when the heat exchanger design requires the use of integral finning or u-bend tubes, these are subject to significant work hardening that results in as bent and as finned heat exchanger tubes with hardness measurement as high as 418 HV0.5 or 35.6 HRC which clearly exceeds the allowable limits stated above.
For 22% Cr DSS integrally finned tubes to meet the hardness values as required by NACE MR0103, some end users have had to develop alternate manufacturing routes such as re-solution annealing the full length of integrally finned tubes to reestablish the hardness values to the acceptable levels.2 In the case of u-bend tubes, other end users have adopted the recommendation of limiting the bend radius to 3.3D which is the equivalent to a 15% induced strain. This is done in an effort to avoid the post bend heat treatment which, when done incorrectly, can alter the duplex microstructure promoting the formation of intermetallic precipitates and affecting the corrosion resistance of the alloy.3