Papers by Keyword: Austenitic Stainless Steel

Abstract: Final turning, which is a finishing process for obtaining components with specific precise parameters, affects the integrity of the surface and its properties, whether hardness or surface residual stresses. The synergistic effect of these factors affects the susceptibility of the material, to stress corrosion cracking. In this work, 2 types of austenitic stainless steel, namely AISI 304 and AISI 321, were turned. Tool with positive cutting geometry was used for turning. The cutting parameters that varied were the cutting speed (100 and 250 m.min⁻¹) and the tool feed (0.12, 0.2 and 0.3 mm·rev⁻¹). The depth of cut was the same for all turnings (0.8 mm). Subsequently, the prepared samples were exposed in MgCl₂ solution based on the ASTM G36 for 96 hours. After this time, the samples were analysed using SEM, where the density of surface cracks was monitored. When comparing the crack density, an increase in density was visible for AISI 304 compared to AISI 321. It was shown that with increasing cutting speed, the density of cracks increased significantly, as well as with increasing tool feed. On the cross-sections the depth and length of the cracks were analysed. Crack depth and length increased with increasing feed too.

Abstract: Austenitic stainless steels are characterised by excellent corrosion resistance and good formability, but their low hardness and fatigue life are limitations in demanding applications. The aim of this study was to analyze the effect of solution annealing and plasma nitriding on the microstructure, hardness and fatigue properties of AISI 304 steel. The experimental material was examined in three states: initial, after solution annealing and after plasma nitriding. Solution annealing resulted in the removal of deformation martensite, giving a homogeneous austenitic structure with a decrease in hardness. On the contrary, plasma nitriding produced a hard nitride layer (1291 HV0.01), while no martensite retransformation took place. The results of the fatigue tests showed that the specimens after plasma nitriding reached the highest fatigue limit (878 MPa), while the specimens in the initial condition had the highest number of cycles to fracture. Fractographic analysis revealed typical fatigue failure characteristics in all conditions. The study highlights the possibility of optimising the fatigue properties of austenitic steels through an appropriate combination of thermal and chemical-heat treatments.

Abstract: The purpose of this study is to clarify the mechanical properties of the expanded austenite (S phase) formed in austenitic stainless steel (ASS). A small thin rolled plate of SUS304 with 0.5 mm thickness was used as test sample. The test sample was nitrided by active screen plasma nitriding (ASPN) at low processing temperature of 400 °C and 450 °C during 4 hrs. processing time. S phase was formed on the surface of the test sample. The surface hardness of ASPN sample was higher than that of untreated sample. Furthermore, tensile tests and fracture surface observations revealed that the tensile strength was also improved compared to untreated samples.

Abstract: Austenitic stainless steels (ASSs) are characteristic with a combination of good mechanical and corrosion properties. Therefore, they are used in the primary circuits of nuclear power plants. Under the influence of a corrosive environment containing chloride ions and mechanical loading, the phenomenon of stress corrosion cracking occurs in ASSs. SCC can also be initiated by the surface condition of ASSs. Machining is usually the last stage of production, during which a significant deformed zone with high residual tensile stresses can be created, which can accelerate the initiation of stress corrosion cracking. Research is focused on analyzing the influence of final turning on microstructural changes of the surface-machined layer caused by various turning parameters (e.g.: cutting speed, feed, depth of cut, cutting tool geometry). No significant microstructure changes were observed between the samples by light microscopy, so we focused on transmission electron microscopy (TEM) on thin lamellas prepared using the focus ion beam (FIB) technique. TEM observation confirmed the presence of a deformed zone and a passivation layer. In the case of the sample that was turned with a higher feed and cutting speed, the passivation layer was discontinuous. Such a microstructural change can significantly affect the corrosion resistance of ASS.

Abstract: The paper summarizes the results of the analysis made on the ruptured tube of the reactor made of austenitic stainless steel in which the conversion of aniline to diphenylamine occurs at elevated temperature and pressure and in the presence of a catalyst. The tube wall perforation occurred in the piping used to measure the pressure in the reactor and a fire occurred after an aniline, ammonia and catalyst vapour leak. The material analyses carried out clearly showed that the thinning and subsequent perforation of the tube wall was due to a specific corrosion attack, so-called metal dusting, which occurs at elevated temperatures and in the presence of a carburizing atmosphere.

Abstract: Austenitic stainless steels are strong candidates for cryogenic applications such as liquid hydrogen storage (20 K) and nuclear fusion technology (4 K) but suffer from low yield strength. In this study, austenitic stainless steels with varying nitrogen contents were evaluated. TMCP-processed and solution-annealed plates were manufactured using a pilot-scale rolling mill, and their microstructures were characterised. Tensile tests were performed from room temperature down to-180°C to assess the cryogenic yield strength of the plates. At all temperatures TMCP significantly increased the yield strength e.g. by a factor of 2 at room temperature, with the effect being mainly due to substantial dislocation hardening. The thermally activated component of yield strength depended mainly on nitrogen content via dislocation-nitrogen interactions, which was found to be much weaker in TMCP plates. Solution annealed plates therefore presented remarkable yield strength at cryogenic temperatures for the highest nitrogen level investigated.

Abstract: Welding joint Austenitic stainless steels can undergo intergranular corrosion attack in some service conditions, such as oil refineries, petrochemical plants, and industrial furnaces operating at high temperatures (between 500°C and 675°C). This research focuses on fabricating dissimilar metal welds to avoid failure due to sensitization and investigate stress-relieved carbon steel by Post-weld heat treatment (PWHT). This work studies a dissimilar weld joint (DWJ) of ASTM 335 grade P11 joint to ASTM 304L and PWHT after welding. These welding processes are used in multi-pass gas tungsten arc welding (GTAW) using filler metal R309L. PWHT at temperatures of 550 °C with holding times of 40, 50, and 60 minutes and then slow cooling in atmospheric air temperature, It was used to compare the results holding time carbon steel ASTM A335 Gr. P11 while processing. Holding time at 50 minutes has the maximum result of PWHT with 182 HV and has a good distribution of perlite with fine grain and increasing holding time also increases formed carbide on the grain boundary of stainless steel 304L.

Abstract: Synchrotron X-rays have the advantage of being a micro beam, allowing us to create detailed stress maps. However, due to a dendritic structure, measuring the residual stresses of a welded part is difficult. This challenge is caused by the difference in the positions of the diffracted crystal grains. To address this problem, we proposed a double exposure method. In this presentation, the double exposure method was applied to measure the residual stress of the plate that was cut from the welded pipe. Detailed strain maps under a plane stress state were obtained. Conversely, the residual stress distributions of the welded pipe under a triaxial stress state were measured using neutrons. From these results, the detailed stress maps of the root part of the butt-welded pipe were made up by the complimentary use of the synchrotron X-rays and neutrons. The results can significantly explain theinitiation and propagation of stress-corrosion cracking. We name this analysis method the quantum beam hybrid stress analysis.

Abstract: The addition of hydrogen in shielding gas has been found to contribute to increasing the productivity of gas tungsten arc (GTA) welding compared to the processes using argon-nitrogen shielding gas. In this report, the GTA welding joints of AISI 304 stainless steel with Ar-N₂ and Ar-N₂-H₂ addition were fabricated. The microstructure and pitting corrosion resistance were studied in this work. The corrosion characterisation of welds metal was investigated by a potentiodynamic method in NaCl solution. It was found that H₂ addition also had effects on delta-ferrite microstructures and corrosion behaviour. Increasing hydrogen in argon-nitrogen shielding gas increased the delta-ferrite content and improved pitting corrosion resistance.

Abstract: The present work deals with the causes of the damage to the distribution wheel of the main circulation pump of the nuclear power plant. During the solution of the problem, a significant influence of the final machining on the formation of cracks was found. Coarse surface machining can cause strain-induced martensite on the distribution wheel surface. Small pitting, which was observed on the surface of the distribution wheel, can be caused by the presence of martensite with less corrosion resistance. Thermal stress and residual stress after coarse final surface machining caused the growth of the cracks which initiated from the pitting.