Search Results (14)

In the present work, chemical and ion beam surface treatments were performed in order to modify the electrochemical behavior of industrial austenitic–martensitic steel VNS-5 in 3.5 wt. % NaCl. Immersion for 140 h in a solution containing 0.05 M potassium dichromate and 10% phosphoric acid promotes formation of chromium hydroxides in the outer surface layer. By means of a new type of ion source, based on a high-current pulsed magnetron discharge with injection of electrons from vacuum arc plasma, ion implantation with Ar⁺ and Cr⁺ ions of the VNS-5 steel was performed. It has been found that the ion implantation leads to formation of an Fe- and Cr-bearing oxide layer with advanced passivation ability. Moreover, the ion beam-treated steel exhibits a lower corrosion rate (by ~7.8 times) and higher charge transfer resistance in comparison with an initial (mechanically polished) substrate. Comprehensive electrochemical and XPS analysis has shown that a Cr₂O₃-rich oxide film is able to provide an improved corrosion performance of the steel, while the chromium hydroxides may increase the specific conductivity of the surface layer. A scheme of a charge transfer between the microgalvanic elements was proposed. Full article

Austenitic stainless steels are used widely in many fields due to their good mechanical properties and high resistance to corrosion. This work focuses on the reconstruction of the passive film after scratching. The purpose of the study was to compare changes in the rate of passive layer reconstruction and to discuss the effect of both the type of material and its electrochemical treatment on the reconstruction of the passive layer for two types of stainless steel: 304 and 316. The XPS tests performed indicate a significantly higher Cr/Fe ratio for the samples after the electropolishing process of 1.41–1.88 compared to the as-received samples of 0.82–0.86. After 2–3 min of sputtering the surface with Ar+ ions, a decrease in chromium content can be observed, with a simultaneous increase in nickel content, visible especially for the electropolished samples. A new approach in the conducted research was to scratch the test samples under controlled conditions, then evaluate the dynamics of the passive layer reconstruction using the AFM method, and then confront the obtained results with XPS measurements for the corresponding samples. For the as-received samples (2B finish) and those after surface treatment, regardless of the level of contamination of the electropolishing process bath, the reconstruction time was similar, which was approximately 2 h, although certain differences in the process dynamics were noticeable. Full article

Stainless steel is widely used in various industrial fields due to its excellent corrosion resistance and mechanical properties. The key to this corrosion resistance is the thin passive film that naturally forms on the metal surface. Passive films are characterized by oxide film theory and adsorption theory, each uniquely explaining the structure and mechanism of the protective film on the metal surface. Research on the semiconductive properties of passive films on stainless steel offers diverse viewpoints, classifying theories into the point defect model and the bipolar fixed charge-induced passivity. Specific changes in passive film attributes that lead to degradation, however, are not fully understood. In this study, we analyzed the inner and outer layers of the passive film on super austenitic stainless steel SR-50A under various conditions in acidic and alkaline chloride environments. The interpretations of these results were based on the point defect model and the bipolar model for the passivation mechanism, and correlations between p-type and n-type semiconductor properties and passivation behavior were examined. The surface of the stainless steel forms a passive film comprising two layers with p-type and n-type semiconductive properties, independent of the pH of the solutions. The corrosion resistance increases as the p-type and n-type semiconductive tendencies become more balanced, consequently enhancing the properties of the passive film. Full article

This research examines the behavior of electrochemical passivation and the chemistry of surface films on 654SMO super austenitic stainless steel and C276 nickel-based alloy in simulated condensates from flue gas desulfurization in power plant chimneys. The findings indicate that the resistance to polarization of the protective film on both materials initially rises and then falls with either time spent in the solution or the potential of anodic polarization. Comparatively, 654SMO exhibits greater polarization resistance than C276, indicating its potential suitability as a chimney lining material. Mott–Schottky analysis demonstrates that the density of donors in the passive film formed on 654SMO exceeds that on C276, potentially due to the abundance of Fe oxide in the passive film, which exhibits the characteristics of an n-type semiconductor. The primary components of the passive films on both materials are Fe oxides and Cr oxides. The formation of a thin passive film on C276 in the simulated condensates is a result of the low Gibbs free energy of nickel oxide and low Cr content. The slower diffusion coefficient of point defects leads to the development of a thicker and more compact passive film on the surface of 654SMO. Full article

As a type of ultra-high strength steel, AerMet100 steel is used in the aerospace and military industries. Due to the fact that AerMet100 steel is difficult to machine, people have been exploring the process of additive manufacturing to fabricate AerMet100 steel. In this study, AerMet100 steel was produced using an in situ rolling hybrid with wire arc additive manufacturing. Microstructure, tensile properties, and fracture toughness of as-deposited and heat-treated AerMet100 steel were evaluated in different directions. The results reveal that the manufacturing process leads to grain fragmentation and obvious microstructural refinement of the AerMet100 steel, and weakens the anisotropy of the mechanical properties. After heat treatment, the microstructure of the AerMet100 steel is mainly composed of lath martensite and reversed austenite. Alloy carbides are precipitated within the martensitic matrix, and a high density of dislocations is the primary strengthening mechanism. The existence of film-like austenite among the martensite matrix enhances the toughness of AerMet100 steel, which coordinates stress distribution and restrains crack propagation, resulting in an excellent balance between strength and toughness. The AerMet100 steel with in situ rolling is isotropy and achieves the following values: an average ultimate strength of 1747.7 ± 16.3 MPa, yield strength of 1615 ± 40.6 MPa, elongation of 8.3 ± 0.2% in deposition direction, and corresponding values in the building direction are 1821.3 ± 22.1 MPa, 1624 ± 84.5 MPa, and 7.6 ± 1.7%, and the K_IC value up to 70.6 MPa/m^0.5. Full article

This work aims to explore the impact of side loads, drill-pipe tool-joint (DP-TJ) speed (rpm), and mud type on the austenitic stainless steel SM2535-110 casing wear characteristics. Actual field drill pipe tool joints, casings, and drilling muds are used in this study. The results of the study show that under both types of lubrication, the wear volume increased with radial load and DP-TJ speed. SM2535-110 casing specimens tested under oil-based mud (OBM) lubrication had higher casing wear volumes than those obtained under water-based mud (WBM) lubrication. This unexpected behavior is mainly due to the increase in the surface hardness of the casing specimens tested under WBM. The results also show that the specific wear rate or wear factor (K) (which is defined as the volume loss per unit load per unit distance sliding) values of specimens tested under WBM are in general two to four times higher than those obtained under OBM. While K values under WBM increase with both the side load and rpm, those under OBM show a sharp decrease with rpm. This behavior under OBM is due to this lubricant’s higher viscosity and the change of lubrication regime from thin film to thick film lubrication at higher rpm. Scanning electron microscopy (SEM) and the digital microscopic imaging (DMI) of SM235-110 casing specimens show that an aggressive combination of adhesive, abrasive, and plastic deformation was observed under WBM, while the dominant wear mechanism under OBM is abrasive wear. Full article

The present paper describes a study case of the failure investigation of duplex stainless steel (UNS S31803) on the tube and tube sheet sections of BEM TEMA-type shell and tube heat exchanger with seawater as the cooling medium. The heat exchanger’s shell design pressure was 22.6 MPa at 422 K, and the tube design pressure was 1 MPa at 339 K. Although UNS S31803 offers high strength, high resistance to chloride-induced SCC, and high resistance to pitting attack in chloride environments, the heat exchanger in this study experienced some material degradation after 28 months of use; 102 out of 270 tubes failed, 26 tubes leaked and were plugged on both sides, and scale plugged 76 tubes. The examination in this study case revealed the formation of white-colored biofilm inside the tubes; XRD examination revealed that the film contained CaCO₃. Using microstructural examination on the inner surface of the tube, the austenite grains were shown to have been preferentially attacked; this phenomenon is typical in duplex stainless steel which fails due to crevice corrosion. According to the examination result, the failure in this case was caused by crevice corrosion between the substrate and surface deposits that was enhanced by microbiological-induced corrosion (MIC). Recommendations to avoid similar failures are also suggested in this paper. Full article

The study systematically compares the low-cycle fatigue (LCF) behaviors of novel martensitic steel (22MnSi2CrMoNi) and maraging steel (00Ni18Co9Mo4Ti). Results show that the two types of tested steel have a similar cyclic deformation behavior. The cyclic softening resistance of 22MnSi2CrMoNi steel is slightly inferior to that of 00Ni18Co9Mo4Ti steel at a low total strain amplitude. However, the gap gradually disappears with the increase of the total strain amplitude. At the same plastic strain amplitude, the LCF lifetime of 22MnSi2CrMoNi steel is higher than that of 00Ni18Co9Mo4Ti steel. The retained austenite film between martensite lath and the existence of precipitated phase in matrix can effectively improve the fatigue lifetime of the two types of tested steel. Full article

The electrochemical behavior of 304 austenitic stainless steel (304ASS) was studied by different methods such as potentiodynamic polarization, EIS, SEM, and Raman spectroscopy. Potentiodynamic polarization data suggest that 304 ASS could be more susceptible to corrosion due to the presence of H₂S. The coexistence of H₂S and Cl⁻-type ionic species in 304 ASS lead to a decrease in the corrosion resistance as compared to the H₂S-free condition. It is seen that CO₂ helps form a passive layer on the metallic surface, which eventually decreases its corrosion rate. Raman spectroscopy analysis shows that the passive layer developed under different condition consists of FeCO₃, FeS₂, Fe₂O₃, Fe(OH)₂, etc. SEM images further confirm that elemental S⁻ and Cl⁻ can infiltrate the passive film and cause the passive film to deteriorate. Full article

Implantable devices fabricated from austenitic type 316L stainless steel have been employed significantly in medicine, principally because the material displays excellent mechanical characteristics and corrosion resistance. It is well known, however, that interaction of exposure of such a material to blood can initiate platelet adhesion and blood coagulation, leading to a harmful medical condition. In order to prevent undesirable surface platelet adhesion on biomaterials employed in procedures such as renal dialysis, we developed an ultrathin anti-thrombogenic covalently attached monolayer based on monoethylene glycol silane chemistry. This functions by forming an interstitial hydration layer which displays restricted mobility in the prevention of surface fouling. In the present work, the promising anti-thrombogenic properties of this film are examined with respect to platelet aggregation on 316L austenitic stainless steel exposed to whole human blood. Prior to exposure with blood, all major surface modification steps were examined by X-ray photoelectron spectroscopic analysis and surface free-angle measurement by contact angle goniometry. End-stage anti-thrombogenicity detection after 20 min of blood exposure at 100 s⁻¹, 300 s⁻¹, 600 s⁻¹, 750 s⁻¹, and 900 s⁻¹ shear rates revealed that a significant reduction (>90%) of platelet adhesion and aggregation was achieved for surface-modified steel, compared with untreated material. This result is confirmed by experiments conducted in real time for 60-minute exposure to blood at 100 s⁻¹, 600 s⁻¹, and 900 s⁻¹ shear rates. Full article

Because of their excellent combination of strength and ductility, quenching and partitioning (Q & P) steels have a great chance of being added to the third generation of advanced high strength steels. The large ductility of Q & P steels arises from the presence of 10% to 15% of retained austenite which postpones necking due to the transformation induced plasticity (TRIP) effect. Moreover, Q & P steels show promising forming properties with favourable Lankford coefficients, while their planar anisotropy is low due to a weak texture. The stability of the metastable austenite is the key to obtain tailored properties for these steels. To become part of the newest generation of advanced high strength steels, Q & P steels have to preserve their mechanical properties at dynamic strain rates and over a wide range of temperatures. Therefore, in the present study, a low-Si Q & P steel was tested at temperatures from −40 °C to 80 °C and strain rates from 0.001 s⁻¹ to 500 s⁻¹. Results show that the mechanical properties are well-preserved at the lowest temperatures. Indeed, at −40 °C and room temperature, no significant loss of the deformation capacity is observed even at dynamic strain rates. This is attributed to the presence of a large fraction of austenite that is so (thermally) stable that it does not transform in the absence of deformation. In addition, the high stability of the austenite decreases the elongation at high test temperatures (80 °C). The additional adiabatic heating in the dynamic tests causes the largest reduction of the uniform strain for the samples tested at 80 °C. Quantification of the retained austenite fraction in the samples after testing confirmed that, at the highest temperature and strain rate, the TRIP effect is suppressed. Full article

The research work and related tests aimed to identify the effect of filler metal-free laser beam welding on the structure and properties of butt joints made of steel 700MC subjected to the TMCP (thermo-mechanically controlled processed) process. The tests involved 10-mm thick welded joints and a welding linear energy of 4 kJ/mm and 5 kJ/mm. The inert gas shielded welding process was performed in the flat position (PA) and horizontal position (PC). Non-destructive testing enabled classification of the tested welded joints as representing the quality level B in accordance with the requirements set out in standard 13919-1. Destructive tests revealed that the tensile strength of the joints was 5% lower than S700MC steel. The results of tensile tests and changes in structure were referred to joints made using the MAG (Metal Active Gas) method. The tests of thin films performed using a high-resolution scanning transmission electron microscope revealed that, during laser beam welding, an increase in dilution was accompanied by an increase in the content of alloying microadditions titanium and niobium, particularly in the fusion area. A significant content of hardening phases in the welded joint during cooling led to significant precipitation hardening by fine-dispersive (Ti,Nb)(C,N) type precipitates being of several nanometres in size, which, in turn, resulted in the reduction of plastic properties. An increase in the concentration of elements responsible for steel hardening, i.e., Ti and Nb, also contributed to reducing the weld toughness below the acceptable value, which amounts to 25 J/cm². In cases of S700MC, the analysis of the phase transformation of austenite exposed to welding thermal cycles and the value of carbon equivalent cannot be the only factors taken into consideration when assessing weldability. Full article

In the present work, the tensile deformation characteristics of the high performance multiphase steel with complex microstructures are investigated. A mixture of ferrite, bainite, and 14.4 vol% retained austenite (RA) with an average grain size of less than 3 μm of the matrix is obtained after specific heat treatment. Tensile tests are performed with increasing strain, i.e., 0%, 5%, 10%, 15%, and 20%. Then X-ray diffraction, transmission electron microscope and electron backscatter diffraction are utilized to analyze the deformation-transformation behaviors of the complex microstructures. Phase transformation of the RA, which is controlled by its morphology and distribution, contributes to high strain hardening capacity of the steel. The blocky-type RA that locates in ferrite grain boundaries shows less stability and transforms easily at early deformation stage, while the film-like RA that distributes between bainitic ferrite shows higher stability and transforms continuously throughout plastic deformation. Moreover, the substructure formation by dislocation configuration in ferrite grains begins with randomly distributed dislocations and ends up with cellular structures, resulting in ferrite subdivision during deformation and also grain refinement strengthening. As a result, the experimental steel is reinforced not only by the martensite transformation of RA, but also ferrite refinement. Full article

The pitting corrosion resistance and passive behavior of type 304 borated stainless steels (Fe_balance–18Cr–12Ni–1.5Mn–(0.19, 0.78, and 1.76 wt %)B) manufactured through conventional ingot metallurgy were investigated. The alloys were composed of an austenitic matrix and Cr₂B phase, and the volume fraction of Cr₂B increased from 1.68 to 22.66 vol % as the B content increased from 0.19 to 1.76 wt %. Potentiodynamic polarization tests measured in aqueous NaCl solutions revealed that the pitting corrosion resistance was reduced as the B content increased and the pits were initiated at the matrix adjacent to the Cr₂B phase. It was found that the reduced resistance to pitting corrosion by B addition was due to the formation of more defective and thinner passive film and increased pit initiation sites in the matrix. Full article