Surface Of Austenitic Stainless Steel Research Articles

Characterisation of TiC layers deposited using an electrical discharge coating process

Electrical discharge machining (EDM) is a non-conventional, high-accuracy machining process for the manufacture of complex shapes, regardless of hardness of the workpiece. There is interest to develop the EDM technique for coating or surface modification by using a powder metallurgy (PM) tool electrode and/or added powder suspended within the dielectric fluid. We report on the EDM deposition of TiC coatings onto stainless steel, using either Cu or TiC electrodes, with and without Ti powder in the working oil. EDM processed layers exhibited hardness values ∼ 3-4 times higher than the substrate, emphasising the ability of EDM to impart improved mechanical performance to the surface of austenitic stainless steel.

The effect of process conditions in heat-assisted boronizing treatment on the tensile and bending strength characteristics of the AISI-304 austenitic stainless steel

In this study, AISI 304 austenitic stainless steel surface was boronized with nanoboron and ekabor-III powders at 950 and 1000°C for 2 and 4 hours period by solid-state box boronizing method. Then, behaviors of the boronized specimen in the microstructure, three-point bending, and tensile strength characteristics were investigated. As a result of the boriding process, the boride layer thickness in the range of 23–67 µm and microhardness value in the range of 1020–2200 HV have been obtained according to the increase in processing time and temperature and to the particle size of the boron source (0, 1). The coating layer on boronized specimens did not exhibit any sign of reaction caused by the tensile strength applied until the yield point was in both tests. Although the particle size of the boron agents was more effective on the boronized specimen’s bending and tensile strength behaviors, it was observed that processing temperature and its duration are effective as well.

Molecular dynamics and density functional theory study on the corrosion inhibition of austenitic stainless steel in hydrochloric acid by two pyrimidine compounds

Quantum chemical calculations based on density functional theory method were performed on two pyrimidine derivatives which may be used as corrosion inhibitors for austenitic stainless steel. The quantum chemical properties of the two pyrimidine derivatives that are most relevant to their potential action as corrosion inhibitors have been calculated. To explain the inhibition performance of the pyrimidine derivatives, their local reactivities were analyzed through Fukui functions. The binding energies of the inhibitors with the surface of austenitic stainless steels were studied. A model has been suggested to calculate the approximate inhibition efficiencies of the pyrimidine derivatives. All calculations were carried out in both gas and liquid phases.

DFT study of the effects of interstitial impurities on the resistance of Cr-doped γ-Fe(111) surface dissolution corrosion.

Using density-functional calculations, we studied the interaction between interstitial impurities (N, C) and γ-Fe(111) surfaces doped, or not, with Cr, as well as the effect of Cr doping on the dissolution corrosion resistance of the γ-Fe(111) surface. The elementary processes studied afforded microscopic insights into the formation of a Cr-depleted zone, a phenomenon that leads to local corrosion of the stainless steel surface. The aim of this work was to study, at the atomic scale, the effects of N and C on the segregation behavior of Cr and the synergetic effect between co-doped atoms on the resistance to dissolution corrosion of austenitic stainless steel surfaces. The results showed that interstitial impurities prefer to be trapped at near-surface sites, which can impact the segregation behavior of Cr such that it shifts from the surface to the subsurface. Electrode potential calculations and density of states analysis demonstrated that doping with Cr or inserting interstitial impurities into the solid solution can improve the surface corrosion resistance of an fcc Fe substrate, but detrimental effects on the surface corrosion resistance are induced by interactions between Cr and interstitial impurity atoms in co-doped surfaces. The formation of near-surface Cr carbides and nitrides (speculated to be Cr2N and Cr23C6 due to the results obtained for particular co-doped surfaces) was also noted. The results of our theoretical calculations explain some of the experimental results observed at the atomic scale.

Active screen plasma surface co-alloying of 316 austenitic stainless steel with both nitrogen and niobium for the application of bipolar plates in proton exchange membrane fuel cells

Austenitic stainless steel has been researched as a promising candidate material for bipolar plates in proton exchange membrane fuel cells. However, its interfacial contact resistance (ICR) is about 16 times higher that of the Department of Energy (DOE) target (10 mΩ cm2), which leads to undesirable fuel cell performance. In this work, a new hybrid plasma surface engineering process, based on active screen plasma co-alloying, has been developed to simultaneously alloy 316 austenitic stainless steel (316 SS) surfaces with both nitrogen and niobium. The results demonstrated that the layer structure of the modified surfaces can be tailored by adjusting the treatment conditions. All the plasma treated 316 SS samples exhibited significantly reduced ICR below the DOE target of 10 mΩ cm2. The corrosion resistance of the N/Nb co-alloyed 316 SS was much better than active screen plasma nitrided and marginally better than the untreated material.

Preventing hydrogen embrittlement in stainless steel by means of compressive stress induced by cavitation peening

In this paper, it has been demonstrated that compressive residual stress induced by cavitation peening which is one of the mechanical surface modification techniques can reduce invasion of the surface of austenitic stainless steel by hydrogen. Cavitation peening was done with employing a cavitating jet in air. The specimens were prepared with different processing time of cavitation peening. Then, stress measurement was performed using an X-ray diffraction analysis. After that, the surface was charged with hydrogen employing a cathodic charging method. Hydrogen content was evaluated by a thermal desorption analysis using a gas chromatography. From the obtained results, hydrogen content was reduced along with increase in compressive residual stress at surface. In particular hydrogen content became to 15% at processing time of 2 s/mm introducing compressive residual stress of 378 MPa. In short, cavitation peening can drastically prevent invasion by hydrogen.

Effects of molybdenum on the composition and nanoscale morphology of passivated austenitic stainless steel surfaces.

Surface analysis by time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy and scanning tunnelling microscopy has been applied to provide new insight on Mo effects on the composition and nanostructure of the passive films grown in sulfuric acid on well-controlled Fe-17Cr-14.5Ni-2.3Mo(100) austenitic stainless steel single crystal surfaces. A duplex hydroxylated oxide matrix, 1.8-1.9 nm thick, is formed with a strong partition between Cr(iii) and Fe(iii) in the inner and outer layers, respectively. Cr(iii) is increasingly enriched by preferential iron oxide dissolution upon passivation and ageing. Ni, only present as oxide traces in the film, is enriched in the alloy underneath. Mo, mostly present as Mo(iv) in the Cr-rich inner layer prior to anodic polarisation, becomes increasingly enriched (up to 16% of cations) mostly as Mo(vi) in the Fe-rich outer layer of the passive film, with ageing promoting this effect. Metallic Mo is not significantly enriched below the passive film produced from the native oxide covered surface. Mo does not markedly impact the nanogranular morphology of the native oxide film nor its local thickness variations assigned to substrate site effects on Cr(iii) enrichment. Site specific preferential passivation still takes place at the (native) oxide-covered step edges of the alloy surface, and transient dissolution remains preferentially located on the terraces. Nanostructures, possibly Mo-containing, and healing local depressions formed by transient dissolution during passivation, appear as a specific effect of the Mo presence. Another Mo effect, observed even after 20 h of passivation, is to prevent crystallisation at least in the Fe-rich outer part of the passive film where it is concentrated mostly as Mo(vi) (i.e. molybdate) species.

Thermodynamic Simulation of Low Temperature Colossal Carburization of Austenitic Stainless Steel

Low temperature colossal carburization (LTCC), as a new and more efficient surface strengthening technology, is generally used to enhance the wear and fatigue resistance of austenitic stainless steel, and not harmful to excellent corrosion resistance. In the present paper, a thermodynamic model was developed by means of diffusion-controlled transformations (DICTRA) method, which can investigate carbon concentration distribution and growth regularity of carburized layer on austenitic stainless steel surface treated by LTCC. Meanwhile, an experimental investigation on carburization layer evolution was performed for 316L stainless steel to verify the validity of the model. The results show that the simulated carbon concentration distribution is well consistent with experimental data. With the assistance of the thermodynamic model, the key parameters such as carburizing temperature and time, carbon activity of carburizing gas and alloying element content may be analyzed for optimizing of LTCC.

Effect of Austenite Stability on Pack Aluminizing of Austenitic Stainless Steels

Aluminide coatings were applied to the surfaces of several austenitic stainless steels—UNS S30300, S30400, S30900, S31000, and S31600 (Type 303, 304, 309, 310, and 316)—by the halide activated pack cementation process. The coating compositions, microstructures, and hardness were determined for the different steels coated at 850°C for 25 h. The stability of the austenite phase for each type of steel was calculated by determining the ratio of the nickel to the chromium equivalents based on their nominal compositions. The thickness of the inner diffusion zone in the coating was shown to be inversely related to the austenite stability of the steels. Microhardness measurements were obtained across the coating thickness and into the substrate. The hardness values followed the same trends as the aluminum composition profile into the substrate.

Surface Characteristics of Silicon Nitride Compounds Deposited on Plasma Nitrided Austenitic Stainless Steels 316L

Silicon nitride compound deposited by plasma enhanced chemical vapor deposition (PECVD) on plasma nitrided stainless steel 316L using tetraethylorthosilicate (TEOS):H2:N2 mixtures. Plasma nitriding is extensively used to improve the hardness and wear performance of steels. It is also known that silicon nitride compounds can have interesting properties, such as low friction coefficient, high hardness and wear resistance. Therefore the deposition processes were carried out at optimized gas compositions and temperatures in order to obtain a combination of the best mechanical and enhanced tribological properties. The composition and structure of the surface layers were characterized using XRD, Glancing angle X-ray diffraction (GAXRD), optical and EDX built in electron microscopy, Microhardness and pin-on-disc wear tests. The compound of α-Si3N4 was found on the top surfaces of PECVD and plasma nitrided austenitic stainless steel 316 L. The nitride compounds were composed of Fe2-3N, Fe4N in addition to chromium nitride. PECVD treatments substantially improved the hardness and wear resistance of plasma nitrided SS 316 L and reduced the friction coefficient and wear rate of the samples. The investigation showed that the combination of PECVD of organosilicon compounds and plasma nitriding results in superior hi gh hardness, low friction and high wear resistance of treated surfaces if compared to those of conventional plasma nitrided surfaces.

Influence of waterjet peening and smoothing on the material surface and properties of stainless steel 304

The present study investigates a combined effect of waterjet peening and smoothing on the surface of austenitic stainless steel 304. An analysis of surface finish and the change in hardness were evaluated. The waterjet treatment on the surface was conducted in steps with multiple passes. Initially, the surface was treated with a higher energy and later with a lower energy of the water droplets. The surfaces treated with multiple steps of decreasing energy produced a smoother surface with lower peak heights and a slightly higher increase in the hardness than the surface treated with only a single step. The hardening layer was also maintained during the later step treatment. The combined action of surface hardening and smoothing using multiple steps in waterjet treatment is useful in increasing the hardness and reducing the roughness of the surface.

Self-organised nanoporous anodic films on superaustenitic stainless steel

Self-organised nanoporous anodic films were fabricated on 904L superaustenitic stainless steel by the electrochemical treatment in ethylene glycol electrolyte containing perchloric acid. The effects of experimental conditions, such as electrolyte temperature and applied voltage on nanoporous morphology, were studied by the way of scanning electron microscopy. The results show that lower solution temperature promotes formation of orderly nanopores. Nanoporous anodic films cannot be formed, and pitting will appear on the stainless steel when temperature is above 40°C. The sizes of pores gradually increase with increasing applied voltage when the applied voltage is lower than 30 V. Porous morphology will change from roughly circular to polygonal, and the size will change from uniform to uneven when the applied voltage is higher than 30 V. It is difficult to form orderly nanoporous anodic films when the applied voltage is too high. Difference of element content in stainless steel is also an important factor for the porous morphology. When the applied voltage is 50 V, orderly pores are formed on 316 austenitic stainless steel surface. However, under the same conditions, this phenomenon does not occur on 904L superaustenitic stainless steel surface. From the analysis of X-ray photoelectron spectroscopy, the films are composed of various metal oxides.

Concurrent multi-response optimization of austenitic stainless steel surface roughness driven by embedded lean and green indicators

Abstract International ecodesign initiatives encourage drilling process improvements that are also attentive to energy and peripheral materials consumption. A stainless-steel heat-exchanger component is optimized for surface quality during drilling while taking into account lean-and-green improvements. A non-linear four-factor fractional factorial scheme has been utilized to investigate the surface roughness performance due to: 1) the feed rate, 2) the spindle speed, 3) the twist drill type and 4) the coolant concentration. We address simultaneously the cumulative effect of multiple consecutive drillings on the surface quality of the last machined hole of a tube-sheet test plate. The concurrent optimization effort regulated the weighted surface quality performance of the drilling process by suppressing consumptions for: 1) energy – monitored at the final drilling and 2) coolant fluid. The weighted distribution-free optimization scheme was repeated to compound information from a key lean process indicator, the drilling processing time. It was found that the feed rate (at 70 mm/min) and the spindle speed (at 350 rpm) commonly influence non-linearly the surface roughness response (predicted at 1.05 μm) in both multi-response scenarios. The drill type (Guhring No. 8520) was also a statistically important effect when excluding optimality with respect to improved lean and green process performance. In such a case, the spindle speed needed to be decreased to 300 rpm for a predicted surface roughness of 0.86 μm (confirmed within 3%).

Effect of Hydrogen on the Micro- and Macro-Strain near the Surface of Austenitic Stainless Steel

The objective of this study is to evaluate the effect of hydrogen on the micro-and macro-strain of austenitic stainless steel using X-ray diffraction. When hydrogen is trapped in lattice sites, it can affect both the micro-and macro-strain. The micro-strain was evaluated through fitting profiles to measured X-ray diffraction profile using a fundamental parameter method. The macro-strain, i.e., the residual stress, was evaluated by a 2D method using a two-dimensional PSPC. The experimental samples were charged with hydrogen by a cathodic charging method. The results revealed that the induced residual stress was equi-biaxial and compressive, and that the micro-strain increased. Both of these varied rapidly with increasing hydrogen charging time. Saturation occurred at a compressive stress of around 130 MPa. On reaching saturation, the hydrogen charging was terminated and desorption of hydrogen began at room temperature. Then, the strains decreased and the compressive stress reverted, ultimately, to a tensile stress of 180 MPa. Martensitic transformation occurred due to hydrogen charging and this had a significant effect on the X-ray diffraction profile.

Increase in the local yield stress near surface of austenitic stainless steel due to invasion by hydrogen

In order to determine the effect of hydrogen on the local yield stress near the surface of austenitic stainless steel, an indentation test combined with inverse problem analysis was employed. For austenitic stainless steel, the indentation test is an effective method since the hydrogen is distributed near to the surface because of its high solubility and low diffusion coefficient. Although uniaxial tensile tests can also provide useful data, greater variations in the mechanical properties due to the presence of hydrogen can be detected through indentation tests. In this study, Secondary Ion Mass Spectrometry (SIMS) was used to measure hydrogen depth profiles in order to establish the relationships between the hydrogen absorption depth and the effects due to hydrogen evaluated using the indentation test. The results showed that the yield stress doubled due to hydrogen absorption and then reverted to its initial state due to hydrogen desorption at room temperature. Also, hardening due to the presence of hydrogen, which was determined using an indentation test, was found to be dependent on the relationship between the plastic deformation depth and the hydrogen absorption depth.

Structural Details of Laser Treated SS 304 and SS 316L(N) Plates

Laser Shock Peening (LSP) is adopted to induce compressive residual stresses on to the surface of austenitic stainless steels (SS) 304 and 316L(N). The SS plates were subjected to LSP using different laser energies in the direct X-ray diffraction studies have been performed on the laser treated samples in order to understand the variations in precise lattice parameter and also to understand the strain induced in these materials. In this paper correlations has been made between laser energies applied, resultant microstructural features and induced residual stresses for the above mentioned structural materials for fusion reactor vessels.

Cathodic cage plasma nitriding of austenitic stainless steel (AISI 316): influence of the working pressure on the nitrided layers properties

In this work, the influence of working pressure on the characteristics of the layers produced by the nitriding treatment on the AISI 316 austenitic stainless steel surface using the Cathodic Cage Plasma Nitriding technique (CCPN) is assessed. The treatments were carried out at a temperature of 723 K for 5 hours under working pressures of 120, 250 and 500 Pa. The morphology, microstructure and corrosion resistance were studied through optical microscopy, x-ray diffraction, and electrochemical potential curves. We analyzed the effects of working pressure on the layer thickness, microhardness and corrosion resistance. The E×i curves and the electrochemical impedance of polarized samples were measured in order to investigate the effect of the ion transfer inside the produced film placed into the anodic solution. The electrochemical impedance of the films were measured, and showed an apparent capacitive behavior. For treatment under a pressure of 120 Pa we found the nitrides Fe3N, Fe4N and CrN. For treatment at a pressure of 250 Pa, the CrN disappears and the observed phases are the nitrides Fe3N, Fe4N and the S-phase. For a treatment pressure of 500 Pa there is an increase in the corrosion resistance of nitrided layers, confirmed by the predominance of S-Phase in the x-ray diffraction pattern. On the other hand, the sample nitrided under the pressure of 250 Pa showed very unstable impedance behavior when immersed in NaCl solution, indicating that the treatment at this specific working pressure has a negative effect on the corrosion resistance of plasma nitrided AISI 316 stainless steel surfaces.

An Investigation of Carburized Layers on Austenitic Stainless Steels

In this study, low temperature plasma carburizing of austenitic stainless steels has been studied, with the aim to improve the hardness and wear resistance of austenite layers. Austenitic stainless steels were plasma carburized at 450°C, using C3H8 as carbon carrier gas. Depth profiles of hardness, hardness distribution and the friction coefficient of carburized layer were measured. The results show that carburized layers range from 10 to 20μm, causing the improvement of the hardness and wear resistance of the surface of austenitic stainless steels.

Character and chemical-wear response of high alloy austenitic stainless steel (Ortron 90) surface engineered with magnetron sputtered Cr–B–N ternary alloy coatings

Many transition metal borides, in particular CrB2, show high inherent hardness, corrosion resistance and chemical stability. Appropriate additions of nitrogen to the Cr–B material system could in principle allow the synthesis of multi-phase materials containing hard, corrosion resistant (Cr–B, Cr–N) and lubricious (h-BN) phases providing a combination of good corrosion–wear resistance and in-situ lubrication. This factor has stimulated interest in the development of Cr–B–(N) magnetron sputtered thin tribological coatings. In the present reported work, several Cr–B–(N) coatings were produced by reactively sputtering a CrB2 target in an argon atmosphere containing increasing amounts of nitrogen. They were mostly applied to Ortron 90 (Fe–20Cr–10Ni–2Mo–0.4N) austenitic stainless steel substrates. The coating structure changed from crystalline to largely amorphous with increasing nitrogen content, this was accompanied by a fall in nano-indentation hardness from 40GPa at 0at.% to 18GPa at 21at.%. N. The corrosion–wear performance of Cr–B–(N) coated Ortron 90 stainless steel when sliding against aluminium oxide in 0.9% saline solution, was worse than uncoated Ortron 90, due to their propensity to undergo chemical (dissolution) based wear. In these tests, no friction reduction was noted for the Cr–B–N coated materials. However, the CrB1.87 coated Ortron 90 displayed superior corrosion–wear resistance and a slightly lower friction coefficient.

Surface coating with a high resistance to hydrogen entry under high-pressure hydrogen-gas environment

Development of a surface coating with high resistance to hydrogen entry under a high-pressure hydrogen-gas environment is presented. Two aluminum-based coatings were developed on the basis of preliminary tests: two-layer (alumina/Fe–Al) and three-layer (alumina/aluminum/Fe–Al) coatings, deposited onto cylindrical and pipe (Type 304 austenitic stainless steel) surfaces by immersion into a specially blended molten aluminum alloy. The coated specimens were exposed to hydrogen gas at 10–100 MPa at 270 °C for 200 h. Specimen hydrogen content was measured by thermal desorption analysis; hydrogen distributions were analyzed by secondary ion mass spectroscopy. Both coatings showed high hydrogen-entry resistance at 10 MPa. However, resistance of the two-layer coating clearly decreased with an increase in pressure. In contrast, the three-layer coating showed excellent hydrogen-entry resistance at a wide pressure range (10–100 MPa), achieved by the combined effect of alumina, aluminum, and Fe–Al layers.