Surface Layer Of Austenitic Steel Research Articles

INVESTIGATION OF THE MICROSTRUCTURE OF AISI 321 STAINLESS STEEL AFTER LASER SURFACE MELTING

The aim of the present paper is to investigate the microstructure of laser-melted surface layers of austenitic steel for biomedical applications. The surface of prismatic specimens from AISI 321 stainless steel was treated by continuous CO2 laser. Three modes of laser processing were used, ensuring surface melting. The microstructure was observed by OM and SEM, while the chemical composition was investigated by EDX analysis. It was found that the microstructure of as-delivered steel was two-phase and relatively inhomogeneous in morphology and chemical composition. It consisted of austenite with grain sizes between 20 - 150 μm, relatively large amount of striped δ-ferrite and spherical carbides along the grain boundaries. After laser melting, the microstructure remained two-phase (δ-ferrite and austenite), but became more homogeneous in morphology and composition. Different dendrites morphology in the particular regions of the molten layer was confirmed - fine equiaxed dendrites on the top surface and columnar at the bottom of the molten pool. Delta-ferrite is located in the interdendritic areas and in larger amounts in the transition zone between the molten.

Statistical Characteristics of Microplastic Strains of the Surface Layer of Austenitic Steel Under Monotonic and High-Cycle Loadings

The mechanical state of the surface layer of the studied metal is controlled by correlation between the ray energy of a coherent light source reflected from the surface of the metal polycrystal and the amplitude of microplastic strains of the elastically deformed surface, based on the analysis of statistical characteristics of the discrete brightness distribution of the image elements of the reflected ray. The regularities of the effect of mechanical loading on the statistical characteristics of the discrete strains distribution under elastic deformation of the surface layer of austenitic steel specimens are defined. An analysis of the sequence of correlation characteristics of the surface deformation defects allows one to reveal a linear dependence of the distribution of the discrete properties of the deformed surface relief on the load intensity factor. The obtained correlation characteristics show that with an increased load the brightness amplitude of the speckle pattern image elements drops due to the increase in the mean-square relief amplitude of the surface deformation defects. By analyzing statistical characteristics of the deformed surface of austenitic steel specimens, quantitative parameters of the descriptive statistics of scattered damage are defined. The regularities of the elastoplastic strain effect on the distribution of discrete surface deformations are obtained. A nonparametric evaluation of the loading intensity factor effect on the sequence of terms in series of mean-square sampled values of damage correlation characteristics under cyclic and static loadings, as well as the determination of strain amplitudes and loading durations as damaging factors during elastoplastic deformation of austenitic steel specimens, are performed.

Microconstituents of the Modified Surface Layer of Austenitic Steel With Nanofibres of Aluminium Oxyhydroxide

In the paper the authors provide the results of experimental study of the effect caused by introduction of nanostructured fibres of aluminium oxyhydroxide into the surface layer of austenitic steel upon its microconstituents. The authors show that, due to introduction of given fibres dendrite size is reduced and equilibrium structure is formed.

Testing of Ni‐Plated Ferritic Steel Interconnect in SOFC Stacks

AbstractStack tests were run at 850 °C for periods from 80 hours to 1,150 hours to develop contacting procedures and at the same time evaluate the performance of a 5 μm electroplated nickel coating on a ferritic Fe22Cr interconnect. The metallic nickel coating reacted relatively quickly during the initial heating to 1,030 °C. During this time, 20–70 μm thick surface layers of austenitic steel were formed, which were covered by a 1–4 μm chromia layer on the anode side and by a layer of mixed Cr‐Fe‐Ni‐spinels over a 1–4 μm chromia layer on the cathode side. The microstructure and composition of the protective scale on the cathode side was susceptible to pitting‐type corrosion patterns, which may limit the life expectancy to less than 2,000 hours for the 200 μm thick interconnect tested. The initial area‐specific resistances (ASR) at the interconnect/cathode current collector interface and the interconnect/anode current collector interface were very low (2 and 10 mΩ cm2, respectively). The cathode side interface resistance increased over the 1,150 hours by ∼7 mΩ cm2/103�h, but the anode side interface resistance decreased during the first 600 hours of the experiment before it started to show a slight increase, < 1 mΩ cm2/103�h, maintaining values below 1 mΩ cm2.