Characteristics and Transformation Mechanism of Nonmetallic Inclusions in 304 Stainless Steel during Heat Treatment at 1250 °C

Evolutions of two typical types of nonmetallic inclusions, i.e., inclusions based on CaO-SiO₂-Al₂O₃ and MnO-SiO₂-Al₂O₃ of 304 stainless steel were investigated in laboratory-scale experiments under isothermal heat treatment at 1250 °C for 0, 30, 60 and 120 min. Results show inclusion population density increases at the first stage and then decreases while their average size decreases and then increases. Moreover, almost no Cr₂O₃ content within the inclusion before the heat treatment, but Cr₂O₃ content increases gradually along with increasing heat treatment time. Furthermore, the increasing of Cr₂O₃ content in the inclusions would increase their melting points and reduce their plasticities. The experimental results and thermodynamic analysis indicate that there are three steps for inclusion evolution during the heat treatment process, in which Ostwald ripening plays an important role in inclusion evolution, i.e., inclusions grow by absorbing the newly formed small-size MnO-Cr₂O₃ inclusions.     

Effects of Sodium Phosphate and Sodium Nitrite on the Pitting Corrosion Process of X70 Carbon Steel in Sodium Chloride Solution

In this paper, effects of sodium phosphate (Na₃PO₄) and sodium nitrite (NaNO₂) on the pitting corrosion of X70 carbon steel in 0.10 mol/L NaCl solution were investigated by potentiodynamic polarization technique, electrochemical impedance spectroscopy (EIS) method, scanning electron microscope (SEM) and scanning electrochemical microscope (SECM). The SECM equipment was used to observe the dynamic processes of the pitting corrosion in situ. Na₃PO₄ or NaNO₂ in the sodium chloride solution decreased the local anodic dissolution and increased the pitting resistance of the specimen. By analysis and comparison, it can be concluded that the inhibition effect of Na₃PO₄ is mainly due to the formation of a salt film, while the corrosion inhibition of NaNO₂ is principally attributed to a protective oxide film on the electrode surface.     

Evaluation of Hybrid Melamine and Steel Fiber Reinforced Geopolymers Composites

This study investigated the influence of the steel and melamine fibers hybridization on the flexural and compressive strength of a fly ash-based geopolymer. The applied reinforcement reduced the geopolymer brittleness. Currently, there are several types of polymer fibers available on the market. However, the authors did not come across information on the use of melamine fibers in geopolymer composites. Two systems of reinforcement for the composites were investigated in this work. Reinforcement with a single type of fiber and a hybrid system, i.e., two types of fibers. Both systems strengthened the base material. The research results showed the addition of melamine fibers as well as steel fibers increased the compressive and flexural strength in comparison to the plain matrix. In the case of a hybrid system, the achieved results showed a synergistic effect of the introduced fibers, which provided better strength results in relation to composites reinforced with a single type of fiber in the same amount by weight.     

Fracture Models and Effect of Fibers on Fracture Properties of Cementitious Composites—A Review

Cementitious composites have good ductility and pseudo-crack control. However, in practical applications of these composites, the external load and environmental erosion eventually form a large crack in the matrix, resulting in matrix fracture. The fracture of cementitious composite materials causes not only structural insufficiency, but also economic losses associated with the maintenance and reinforcement of cementitious composite components. Therefore, it is necessary to study the fracture properties of cementitious composites for preventing the fracture of the matrix. In this paper, a multi-crack cracking model, fictitious crack model, crack band model, pseudo-strain hardening model, and double-K fracture model for cementitious composites are presented, and their advantages and disadvantages are analyzed. The multi-crack cracking model can determine the optimal mixing amount of fibers in the matrix. The fictitious crack model and crack band model are stress softening models describing the cohesion in the fracture process area. The pseudo-strain hardening model is mainly applied to ductile materials. The double-K fracture model mainly describes the fracture process of concrete. Additionally, the effects of polyvinyl alcohol (PVA) fibers and steel fibers (SFs) on the fracture properties of the matrix are analyzed. The fracture properties of cementitious composite can be greatly improved by adding 1.5–2% PVA fiber or 4% steel fiber (SF). The fracture property of cementitious composite can also be improved by adding 1.5% steel fiber and 1% PVA fiber. However, there are many problems to be solved for the application of cementitious composites in actual engineering. Therefore, further research is needed to solve the fracture problems frequently encountered in engineering.     

Studies on the Oxidation Behavior and Microstructural Evolution of Two Nb-Modified HR3C Austenitic Steels under Pure Water Vapor at 650 °C

The steam oxidation behavior of three heterogeneous HR3C alloys was investigated at 650 °C comparatively. After a long-term oxidation process for 1000 h, the results demonstrated that the commercial HR3C alloy already exhibited a high oxidation resistance. However, the spallation resistance of the oxide scale was low during the initial oxidation period. The addition of a moderate amount of Nb into the alloy (1#HR3C) increased the oxidation resistance of the alloy. In addition, the improvement of the microstructural stability was substantially influenced by solid solution strengthening and fine grain strengthening. However, the addition of excessive Nb could significantly affect the growth model of the oxide scale and negatively affect the oxidation performance and microstructural evolution of the alloy (2#HR3C).     

Influence of Nb Content on Precipitation,Grain Microstructure,Texture and Magnetic Properties of Grain-Oriented Silicon Steel

The effects of Nb content on precipitation, microstructure, texture and magnetic properties of primary recrystallized grain-oriented silicon steel were investigated by various methods. The results show that the precipitates in primary recrystallized sheets are mainly MnS, Nb(C,N), composite precipitates of MnS and AlN, and composite precipitates of Nb(C,N) and AlN. Adding niobium could refine the primary recrystallized microstructure. The steel with 0.009 wt% Nb possesses the finest and the most dispersed precipitates, which contributes to the finest primary recrystallized microstructure due to the strong pinning force. Adding niobium is beneficial to obtain large volume fraction favorable texture for grain-oriented silicon steel, and the effect of Nb addition is not obvious when the content is higher than 0.009 wt%. After final annealing, the steel with 0.009 wt% Nb shows the best magnetic properties, B₈₀₀ = 1.872 T, P_1.7/50 = 1.25 W/kg.     

Comparison of Properties of Hardfaced Layers Made by a Metal-Core-Covered Tubular Electrode with a Special Chemical Composition

In this article, the results of research on the metal-mineral-type abrasive wear of a wear-resistant plate made by a tubular electrode with a metallic core and an innovative chemical composition using the manual metal arc hardfacing process were presented. The properties of the new layer were compared to the results of eleven wear plates manufactured by global suppliers, including flux-cored arc welding gas-shielded (FCAW-GS, Deposition Process Reference Number: 138), flux-cored arc welding self-shielded (FCAW-SS, Deposition Process Reference Number: 114), automated hardfacing, and manual metal arc welding (MMAW, Deposition Process Reference Number: 111) hardfacing T Fe15 and T Fe16 alloys, according to EN 14700:2014. Characterization of the hardfaced layers was achieved by using hardness tests, optical microscopy, confocal microscopy, scanning electron microscopy, and EDS (Energy Dispersive Spectroscopy) and X-ray diffraction analyses. Based on wear resistance tests in laboratory conditions, in accordance with ASTM G65-00: Procedure A, and surface layer hardness tests, in accordance with PN-EN ISO 6508-1, the wear plates most suitable for use in metal-mineral conditions were chosen. The results demonstrated the high metal-mineral abrasive wear resistance of the deposit weld metal produced by the new covered tubular electrode. The tubular electrode demonstrated a high linear correlation between the surface wear resistance and the hardness of the metal matrix of the tested abrasive wear plates. In addition to hardness, size, shape, the dispersion of strengthening phases, and the base metal content, depending on hardfacing technology and technological parameters, impact wear resistance is represented by volumetric loss caused by effect-free or constrained dry abrasive medium contact. The presented results can be used in machine part material selection and wear planning for applications in inspection, conservation, and regeneration interval determination. The obtained results will be applied in a real-time wear rate prediction system based on the measurement of the working parameters.     

Modelling of the Guillotine Cutting Process by Means of a Symmetrical Blade with the Defined Geometry

This paper modelled the cutting process of a bundle consisted of ultra-thin cold-rolled steel sheets using a guillotine. The geometry of a cutting tool with given dimensions was assumed. A bundle of sheets being cut was modelled as deformable, the cutting tool was rigid, and the finite element method along with computer system LS-DYNA was employed. Numerical simulations of the complex state of stress and of the corresponding complex state of strain were carried out. Cutting processes belong to fast changing physical phenomena, and therefore, highly nonlinear dynamical algorithms were applied in order to solve this particular problem. Experimental investigations were also conducted by means of the scanning electron microscopy. It was found that the fracture region consisted of two distinct zones: brittle and ductile separated from each other by the interfacial transition. Morphological features of the brittle, ductile, and the transition regions were identified. The ductile and brittle zones were separated at the depth of ca. 1/5 thickness of the cut steel sheet. Finally, the numerical results obtained by usage of the finite element method as well as experimental ones in the form of microscopic images were compared, showing quite good agreement.     

Influence of Chloride Concentration on Stress Corrosion Cracking and Crevice Corrosion of Austenitic Stainless Steel in Saline Environments

Stainless steels are used as canister materials for interim storage of spent fuel. Crevice corrosion has proved to be a safety concern of 304L stainless steel spent fuel canisters, when exposed to the saline environments of coastal sites. To study the effects of chloride concentration and test duration on the crevice corrosion behavior, and the effect of relative humidity on the initiation of discrete SCC cracks, a test program was conducted on the 304L steel specimens sprayed with synthetic sea water of 3.5 wt.%. The salt-deposited specimens, wrapped up with a crevice former to form a crevice configuration, were then exposed to an environment at 45 °C with a pre-set 45%, 55%, and 70% relative humidity (RH), for 400 h and 10,000 h, respectively. The surface features and crack morphology of the tested 304L stainless-steel specimens were examined by energy-dispersive spectrometry (EDS) and electron back scatter diffraction (EBSD). For the specimens deposited with a chloride concentration of 1 g/m², no cracks were found in the corroded regions after 400-h exposure, whereas SCC cracks were observed with the specimens tested for 10,000 h at all three pre-set relative humidity. The specimens tested at the pre-set relative humidity 45% are characterized with discrete SCC cracks, but, on the other hand, those exposed to the environments of 55% and 70% relative humidity show SCC cracks of distinct features. From the results of 10,000-h tests, it is inferred that the chloride concentration threshold for SCC initiation of 304L stainless steel at 45 °C is between 0.1 g/m² and 1 g/m².     

Comparison of Long-Term Strength Development of Steel Fiber Shotcrete with Cast Concrete Based on Accelerator Type

This study analyzed the effect of accelerating agents, such as aluminate, cement mineral, and alkali-free accelerators, on the long-term performance of steel-fiber-reinforced shotcrete. The shotcrete performance was studied based on the type and amount of steel fiber added. Performance tests were performed to identify the accelerator providing better long-term performance to the steel-fiber-reinforced shotcrete. Changes in strength and flexural performance over time were investigated. The compressive strength and flexural strength tests on 1-, 3-, 6-, 12-, and 24-month-old test specimens were performed, wherein 37 kg of steel fiber was added to the cement mineral and aluminate mixes, and 40 kg of steel fiber was added to the alkali-free mix. The 1-month compressive strength result of all the test variables satisfied the Korea Expressway Corporation standard. The compressive strength of the cast concrete and shotcrete specimens increased with age, demonstrating a strength reduction, particularly in the 24-month-old shotcrete specimens. Thus, the shotcrete performance may deteriorate in the long-term. In the 24-month-old specimen, substantial flexural strength reduction was observed, particularly in the aluminate and alkali-free specimens. The relative strength of the specimens was compared with that of the cast concrete mold specimens. The results suggest the use of alkali-free accelerators, considering the long-term performance of tunnels and safety of workers. Moreover, increasing the steel fiber performance rather than the amount of low-performance steel fiber must be considered.