PSI - Issue 82

L.F. Guedes et al. / Procedia Structural Integrity 82 (2026) 199–205 Guedes et al. / Structural Integrity Procedia 00 (2026) 000–000

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1. Introduction Supermartensitic stainless steels (SMSS) are corrosion resistant alloys with high mechanical strength and improved toughness in comparison to conventional martensitic stainless steels. These advantages are attributed to the extra-low carbon content, Ni and Mo additions, besides microalloying elements (Ti, V, Nb,…) (Barbosa and Abud, 2013; Taban et al., 2016). The UNS S41426 grade is a Ti-alloyed SMSS containing 12-13%Cr, ~ 5%Ni and ~ 2%Mo (NACE, 2015). It can be purchased as seamless pipes or forged pieces, in two classes of yield strength, 95ksi and 110 ksi (ISO, 2024; NACE, 2015; Tavares et al., 2022). The final mechanical properties are adjusted by tempering parameters, but are also strongly dependent of the C, Mo, Ti and V contents. These elements enhance the secondary hardening during tempering. Toughness is increased by the carbon reduction below 0.03%, and by the proper adjustment of Ti microaddition. Pipes, accessories and mandrels of SMSSs for oil country tubular goods (OCTG) are assembled below the wet Christmas tree. It contains the casing pipes, valves and production column through which the crude oil is extracted. Since sand particles mixed with crude oil, gas, and water are transported in the production column, pipes and accessories are subjected to abrasive wear damage. Nitriding can be used to improve wear resistance of SMSS (Schibicheski Kurelo et al., 2018; Tavares et al., 2022) or as an intermediate process to diamond like carbon (DLC) deposition. In this work, the effects of the plasma ion nitriding process on the tensile properties of a forged UNS S41426 stainless steel were analyzed. Three nitriding thermal cycles were tested and evaluated to determine their influence on surface hardness, strength and ductility of tensile specimens. 2. Experimental The SMSS studied was from a forged piece of UNS S41426, with composition shown in Table 1. Cylindrical subsize tensile specimens with a diameter of 4.0mm, and a gauge length of 16.0mm were machined according to ASTM A370 (ASTM, 2022). The tensile specimens were subjected to plasma ion nitriding in a cylindrical steel reactor connected to a vacuum pump. After evacuation of the chamber, argon, nitrogen and hydrogen gases were introduced with flow rates regulated with three flow meters. The process temperatures were controlled and measured using a type K thermocouple. The specimens were assembled in a holder in order to be equally exposed to the plasma nitriding Three nitriding temperatures were tested, with the thermal cycles shown on Fig. 1. A common pre-treatment at 350 o C for 1h was applied, during which the specimens were subjected to an atmosphere of 50% argon and 50% hydrogen. After that, the gas flow was adjusted, and a gas mixture of 25% N₂ and 75% H₂ was used for nitriding the specimens. The plasma ion nitriding was carried out for 5 hours at three distinct temperatures, 350°C, 400°C, and 570°C, with subsequent gradual inside the plasma reactor. The nitrided samples were named N350, N400 and N570. For comparison, tensile specimens subjected to the thermal cycles, without nitriding, were produced, and named as C350, C400 and C570. The microstructures of the nitride layers were observed, and thicknesses were measured using a scanning electron microscope JEOL JSM 7100F with energy dispersive spectrometer (EDS). The Vickers microhardness profiles in the nitride layer and substrate were determined with load of 10gf with indentations measurements carried out in a scanning electron microscope Hitachi TM4000. Tensile tests were conducted at a slow strain rate (10 -6 s -1 ). After the tests, the fractures and secondary cracks were observed in the light optical (LOM) and scanning electron microscopes (SEM). Table 1. Chemical composition of the UNS S41426 submitted to plasma ion nitriding. C N Cr Ni Mo V Ti Mn Si S P 0.013 0.018 12.46 6.32 1.92 0.060 0.124 0.44 0.26 0.004 0.014