Papers by Keyword: Stainless Steel

Abstract: Modified stainless steel grade X45CrNiW 18-9 has been developed through replacement of nickel by nitrogen. The modified steels have been produced in induction furnace under nitrogen pressure and were cast into round ingots with 100 mm diameter. The cast modified stainless steels were forged, followed by air cooling. The nitrogen contents were determined and compared with those calculated using Institute of Metal Science (IMS) equation: The results showed that there are great deviations between the actual nitrogen contents and predicted values through IMS equation. So, an equation has been derived based on chemical composition, pressure and temperature at 1600 °C. [N%] = 0.0078 + 0.0406*X, where X is function of chemical composition and nitrogen pressure. The derived equation has been used to calculate the nitrogen content of different stainless steels using published data. The results reveal the difficulty of deriving a general equation for the prediction of nitrogen content covering different steel compositions. So it is necessary to use narrow composition range. The phases of modified stainless steels have been investigated using Schaeffler diagram, dilatometer and microstructure observations. Both partial and total replacement of nickel using 0.33-0.50% nitrogen, are effective to produce a modified fully austenitic stainless steel grade X45CrNiW 18-9.

Abstract: The acidic environment and polarization in proton exchange membrane fuel cells (PEMFC) result in severe electrochemical corrosion issues for the bipolar plates of metal-based fuel cells. AISI 304 stainless steel is studied in this paper as the bipolar substrate and treated with plasma nitriding to improve its corrosion resistance performance. The influence of process parameters on the growth pattern of nitrided layers was discussed, and the microstructure and properties of modified layers were systematically studied. Results show that an expanded austenite nitride layer is obtained when the nitriding temperature is under 450°C, which has good corrosion resistance. When CrN precipitates at higher nitriding temperatures, the corrosion resistance of the nitride layer sharply decreases. The presence of valence states of N and Cr atoms in the nitrided layer determines the corrosion resistance of nitrided stainless steel samples.

Abstract: Electrochemical analysis of corrosion in molten nitrate salt of stainless steel grade SS 430, SS 2205, SS 2507 and SS 304 is directly performed to evaluate corrosion resistance. Stainless steels are exposed to molten nitrate salt at 600 °C for two hours. This is done in a furnace equipped with a working electrode terminal, reference and counter electrodes. According to this, electrochemical impedance spectroscopy and potentiodynamic polarization techniques are applied in situ to measure corrosion rates and corrosion resistance. Microstructures of stainless steel reveal the ferrite phase for SS 430, the austenite phase for SS 304 and the duplex phase for SS 2205 and SS 2507. In this study, the existence of an austenite phase promotes corrosion resistance in molten nitrate salt.

Abstract: Compressor valve spring failure and rapid fracture occurred in a petrochemical enterprise. To find the cause of the failure, and to ensure the safe, stable, and continuous operation of the device, the failure of the spring is analyzed. In this paper, through the macro inspection, chemical composition analysis, metallographic analysis, scanning electron microscopy analysis, energy spectrum analysis, hardness analysis, and other tests, it is concluded that the main reason for the spring fracture is the fatigue fracture caused by unqualified materials. Suggestions are given to avoid similar problems in the future, and it is hoped that this failure analysis will provide valuable experience for similar failure problems in petrochemical enterprises.

Abstract: A wide range of applications request components that work in an environment where they are subject to high temperatures, combined with a corrosive action of the same environment. The components with a complex geometry can be obtained only using some assembling processes, among which are welding and brazing. One of the most important advantages of these processes is the possibility to obtain a sealed joint, that guarantees it to be waterproof. In comparison with welding process, brazing process has also an important technological advantage: it is a process that is able to produce a quality joint in a very small, narrow and tight places., where is very difficult or almost impossible to reach with other welding processes (e.g. GMAW/MIG – Gas Metal Arc Welding, GTAW/TIG – Gas Tungsten Arc Welding, SMAW – Shield Metal Arc Welding, FCAW - Flux Cored Arc Welding, SAW – Submerged Arc Welding). The research in this direction carried out in University Politehnica Timisoara, was focused on using brazing as a joining process to obtain a complex geometry part working in these environments. The brazed joint will create a dissimilar joint, putting in contact stainless steel with a Ag-Cu brazing alloy, which creates diffusion processes at microstructural level as well as phase transformations (due to thermal cycle and diffusion) which have a large impact on operating behavior of these joints. This paper presents some results on investigation phase and microstructural constituents transformations that took place in these brazed joints.

Abstract: This study provides a comprehensive investigation into the microstructure, hardness, tensile strength, and bending fatigue behavior of a Wire Arc Additively Manufactured (WAAM) component composed of dissimilar materials—Carbon Steel (CS) and 316L stainless steel. Microscopic analysis reveals distinct microstructural characteristics, such as equiaxed ferrite grains in WAAM CS and a coarse columnar structure with delta-ferrite phases in WAAM 316L. A macroscopic phase map indicates a predominantly Body-Centered Cubic (BCC) structure near the interphase, suggesting element migration between CS and 316L due to high heat input. Higher magnification scans highlight martensitic structures on both sides of the interphase, with the CS side exhibiting larger grain sizes. Hardness assessment along the built direction shows a peak hardness of 407 HV near the interphase on the 316L side, contrasting with the CS side's average interphase hardness of 316 HV due to larger grain sizes. The yield strength of both WAAM CS and WAAM dissimilar material was consistently measured at 392 MPa. In comparison, WAAM 316L exhibited a slightly lower yield strength of 359 MPa. Notably, WAAM 316L demonstrated the highest tensile strength among the materials, reaching 656 MPa. Meanwhile, WAAM CS displayed a robust tensile strength of 503 MPa, and the WAAM dissimilar material exhibited a yield strength of 520 MPa. In terms of elongation, WAAM CS and WAAM 316L showcased values of 44.9% and 49.6%, respectively. On the other hand, WAAM dissimilar material exhibited a somewhat lower elongation of 20.4%, suggesting a different mechanical behavior in terms of ductility. Bending fatigue tests on WAAM 316L, WAAM CS, and the dissimilar material reveal a fatigue limit of approximately 225 MPa for WAAM 316L, 210 MPa for WAAM CS, and approximately 210 MPa for the dissimilar material. In the low-cycle and medium-cycle regimes, the dissimilar material exhibits slightly superior fatigue strength, potentially due to its marginally higher static strength. Notably, consistent fractures on the CS side during fatigue tests underscore a recurring behavior in the dissimilar material.

Abstract: This study investigated the impact from nitrogen content in backing gases on the microstructure and corrosion resistance of food grade stainless steel weld metal. Three types of backing gases were employed: 100%Ar, 85%Ar+15%N₂, and 100%N₂. Statistical analysis using ANOVA revealed a significant effect from nitrogen content on the ferrite phase fraction within the weld metal microstructures (p-value = 3.5E-05), indicating a reduction in the ferrite phase with increasing nitrogen content. Moreover, increasing nitrogen content positively shifted the pitting corrosion potential, indicating enhanced corrosion resistance. Optical microscopy confirmed lower pit density in samples with nitrogen backing gas as compared with samples with argon backing gas. These findings underscore the crucial role of nitrogen content in backing gases at influencing microstructure and corrosion resistance in stainless steel weld metal, with higher nitrogen levels correlated with improved corrosion resistance.

Abstract: TIG (Tungsten Inert Gas) welding has more and more application options. To always create new ways of optimization, a more in-depth study of the effects on the components subject to joining is necessary. The paper includes a study on the hardness resulting in the specific areas of the welded joint, using TIG welding with high frequency pulsed arc, applied to join some stainless-steel plates. The relative variation of the hardness was determined and analysed, in correlation with the process parameters. The study is part of the research on obtaining a good energy efficiency in the welding process, based on the requirements imposed on the quality of the joints.

Abstract: Copper and stainless steel possess distinct properties that make them suitable for different applications (i.e. e-mobility, nuclear power plants, etc.). However, the high thermal conductivity of copper presents a significant challenge in welding. In fact, researchers have explored various fusion welding processes for joining copper to steel and concluded that fusion welding is generally difficult or even unsuitable for obtaining sound and defects free joints. The present study investigated the feasibility of dissimilar lap joint between copper and stainless-steel thin plates using Cold Gas Tungsten Arc Welding (CGTAW) without a filler material and with no significant geometrical distortion of welded plates. The weld was created by consecutive partially overlapped spots, whose welding time varied between 100 and 150 ms, in upgraded conventional TIG machine equipped with cold TIG welding function. Samples made with 150 ms welding time showed a near-uniform distribution of equiaxed copper grain microstructure, while those obtained with 100 ms exhibited significant differences in grain size with the presence of steel inclusions in globule and vortex shapes. The joints demonstrated exceptional flexibility, allowing it to be bent up to a 180º angle without any visible damage. The maximum tensile strength of sample obtained with a welding time of 150 ms was 220 MPa with a fracture located in the heat-affected zone. The sample welded with a welding time of 100 ms exhibited 171 MPa of tensile strength with the fracture along the melting spot area due to pronounce mixing of welded materials. All the samples showed ductile behavior in the fracture zone. Eventually, the application of CGTAW resulted in promising at obtaining joint with good mechanical properties.

Abstract: 80Ni20Cr coatings produced by high-velocity oxygen fuel (HVOF) thermal spray technique are novel and widely used to improve the corrosion and wear resistance of metal and steel components in many applications, especially in coal-fired power plants. The present study investigated the effects of nano-Al₂O₃ at 0.5 wt.% on the microstructure of HVOF-sprayed 80Ni20Cr coating deposited on AISI 304L steels corresponding to its coating hardness. The coating was successfully sprayed with a thickness of 150 – 180 µm. The microstructure and phase formed by the coating were analyzed by a field emission electron microscope (FE-SEM) and an X-ray diffractometer (XRD). Synchrotron X-ray fluorescence spectroscopy (SRXRF) was used to confirm the Cr solid solution in the Ni-based coating. The presence of the nano-Al₂O₃ phase in the 80Ni20Cr coating was characterized by electron backscattered diffraction (EBSD). The nano-Al₂O₃ particles were homogenously distributed in the coating layers. The incorporation of nano-Al₂O₃ into 80Ni20Cr enhanced coating characteristics by decreasing surface roughness by 23% and increasing coating hardness by around 4%.