Search Results (15)

An enhancement in fatigue life for ferrite–pearlite low-carbon steel (LCS) at high temperature (HT) has been discovered, where it increased from 190,873 cycles at room temperature (RT) to 10,000,000 cycles at 400 °C under the same stress conditions. To understand the mechanism behind this phenomenon, the evolution of microstructure and dislocation density during fatigue tests was comprehensively investigated. High-power X-ray diffraction (XRD) was employed to analyze the evolution of total dislocation density, while Electron Backscatter Diffraction (EBSD) and High-Resolution EBSD (HR-EBSD) were conducted to reveal the evolutions of kernel average misorientation (KAM), geometrically necessary dislocations (GND) and elastic strains. Results indicate that the enhancement was attributed to the dynamic strain aging (DSA) effect above the upper temperature limit, where serration and jerky flow disappeared but hindrance of dislocations persisted. Due to the DSA effect, periods of increase and decrease in the total dislocations were observed during HT fatigue tests, and the fraction of screw dislocations increased continuously, caused by viscous movement of the screw dislocations. Furthermore, the increased fraction of screw dislocations resulted in a lower energy configuration, reducing slip traces on sample surfaces and preventing fatigue-crack initiation. Full article

Titanium dioxide coatings (TiO₂) were sprayed using a water-stabilized plasma gun (WSP) to form robust self-supporting bodies with the character of a ceramic disc capacitor (CDC). Agglomerated nanometric powder was used as feedstock. Argon was applied for powder feeding as well as coating–cooling to minimize the influence of ambient air. Stainless steel was used as a substrate, and the coatings were released after cooling. A more than three-millimeter-thick self-supporting TiO₂ plate was observed using HR-TEM and SEM. Porosity was studied by image analysis on polished sections. Thermal post-treatment on the coating was conducted at a rather low temperature of 500 °C. The results of the subsequent dielectric measurement showed high permittivity, but this was strongly frequency-dependent and accompanied by a progressively decreasing loss tangent. On the other hand, the plasma-sprayed TiO₂ exhibited persistent DC photoconductivity under and after illumination with a standard bulb. Full article

HR3C steel is an austenitic high-temperature-resistant steel. Because of its good strength and high-temperature performance, it has been widely used in ultra-supercritical power plant boilers. With the increasingly frequent start-up and shutdown of thermal power units, leakages of HR3C steel pipes have occasionally occurred due to the embrittlement of HR3C pipe steel after a long service duration. In this study, the embrittlement mechanisms of HR3C pipe steel are investigated systematically. The mechanical properties of the pipe steel after running for 70,000 h in an ultra-supercritical unit were determined. As a comparison, the pipe steel supplied in the same batch was aged at 700 degrees Celsius for 500 h. The mechanical properties and the micro-precipitation of the aged counterparts were also determined for comparison. The results show that the embrittlement of HR3C pipe steel in service for 70,000 h is obvious. The average impact absorption is only 5.5 J, which is a decrease of 96.7%. It is found that embrittlement of HR3C steel also occurs after 500 h of aging at 700 °C, and the average value of impact absorption energy decreases by 70.4%. The comparison experiment between the in-service pipe steel and the aged pipe steel shows that in the rapid decline stage of the impact toughness of HR3C steel, the M₂₃C₆ carbide in the microstructure has a continuous chain distribution in the grain boundary. There were no other precipitated phases observed. The rapid precipitation and aggregation of M₂₃C₆ carbides leads to the initial embrittlement of HR3C steel at room temperature. The CRFe-type σ phase was found in the transmission electron microscope (TEM) image of the steel pipe after 70 thousand hours of use. The precipitation of the σ phase further induces the embrittlement of HR3C pipe steel after a long service duration. Full article

Advanced high-strength steels (AHSS) have a wide range of applications in equipment safety and lightweight design, and enhancing the strength of AHSS to the ultra-high level of 2 GPa is currently a key focus. In this study, a new process of thermo-mechanical control process followed by direct quenching and partitioning (TMCP-DQP) was developed based on Fe-0.4C-1Mn-0.6Si (wt.%) low-alloy steel, and the effects of microstructure evolution on mechanical properties under TMCP-DQP process and conventional hot rolled quenched and tempered process (HR-QT) were comparatively studied. The results show that the TMCP-DQP process not only shortened the processing steps but also achieved outstanding comprehensive mechanical properties. The TMCP-DQP steel exhibited a tensile strength of 2.23 GPa, accompanied by 11.9% elongation and a Brinell hardness of 624 HBW, with an impact toughness of 28.5 J at −20 °C. In contrast, the HR-QT steel exhibited tensile strengths ranging from 2.16 GPa to 1.7 GPa and elongations between 5.2% and 12.2%. The microstructure of TMCP-DQP steel primarily consisted of lath martensite, containing thin-film retained austenite (RA), nanoscale rod-shaped carbides, and a minor number of nanoscale twins. The volume fraction of RA reached 7.7%, with an average carbon content of 7.1 at.% measured by three-dimensional atom probe tomography (3DAP). Compared with the HR-QT process, the TMCP-DQP process resulted in a finer microstructure, with a prior austenite grain (PAG) size of 11.91 μm, forming packets and blocks with widths of 5.12 μm and 1.63 μm. The TMCP-DQP process achieved the ultra-high strength of low-alloy steel through the synergistic effects of grain refinement, dislocation strengthening, and precipitation strengthening. The dynamic partitioning stage stabilized the RA through carbon enrichment, while the relaxation stage reduced a small portion of the dislocations generated by thermal deformation, and the self-tempering stage eliminated internal stresses, all guaranteeing considerable ductility and toughness. The TMCP-DQP process may offer a means for industries to streamline their manufacturing processes and provide a technological reference for producing 2.2 GPa grade AHSS. Full article

This paper focuses on the effect of rapid annealing on Non-Grain Oriented Electrical Steel (NGO) in terms of microstructure, mechanical properties, and magnetic properties. The Ultra-Fast Heating (UFH) tests were performed by a transversal induction heater on NGO electrical steel samples (cold rolled down to 0.5 mm), varying the heating power (80 kW and 90 kW) and the speed of the strip through the induction heater. This allowed us to exploit heating rates (HR) in the range of 200–300 °C/s and targeting peak temperature (T_peak) up to a maximum of 1250 °C. The comparison between the microstructure as obtained by conventional annealing and the ultra-fast heating process highlights a clear effect in terms of grain size refinement provided by the UFH. In particular, the average grain size as obtained by UFH ranges two/three times lower than by a conventional process. The results show the possibility of applying UFH to NGO steels, targeting mechanical properties such as those obtained by the standard process, combined with the benefits from this innovative heat treatment in terms of green energy and the minimization of CO₂ emissions. Magnetic characterization performed by a single sheet tester (30 × 90 mm) showed that the values of core losses are comparable with conventional NGO grades. Full article

In austenitic steels, the tetragonal Z-phase (NbCrN) has frequently been credited with beneficial strengthening effects during dislocation creep. In the modified Z-phase, niobium is partially substituted by vanadium. The basic objective of this contribution is a detailed characterization of the modified Z-phase in vanadium bearing austenitic AISI 316LN+Nb+V and HR3C steels. Experimental activities were focused on crystallography, thermodynamic and dimensional stability, kinetics of precipitation (TTP diagram) and solvus temperature of the modified Z-phase in the steels examined. Thermodynamic modelling was used for prediction of stable minor phases and solvus temperature of the modified Z-phase. Kinetics of precipitation of the (Nb,V)CrN phase in the AISI 316LN+Nb+V steel was experimentally investigated in the temperature interval of 550–1250 °C. The kinetics of precipitation of the modified Z-phase in austenitic matrix was fast. Results of diffraction studies on particles of the modified Z-phase confirmed the existence of the tetragonal unit cell already after short-term annealing. The solvus temperature of the modified Z-phase in austenitic steels was determined to be lower than that for the NbCrN phase. The decrease in the solvus temperature is dependent on the vanadium content in austenitic steels. Both thermodynamic calculations and experimental results proved that the thermodynamical stability of the modified Z-phase in austenite was high. More data are needed for evaluation of long-term dimensional stability of the (Nb,V)CrN phase in austenitic steels at temperatures for their engineering applications. Full article

Objective: Two kinds of fire-resistant steel with different Nb content (Nb-free and 0.03 wt.%) were prepared for studying the effects of Nb addition on the elevated-temperature strength of fire-resistant steel. Methods: Two stages of heat treatment were carried out on the steels to obtain different microstructures. Typical microstructures, dislocation, and precipitates morphology of steels were observed by SEM and TEM. The dislocation density was calculated by the X-ray data from the microstructures. High temperature and room temperature mechanical properties of steels were determined by tensile testing. Results: The results showed that the YS of N2-HR steel (addition of 0.03 wt.% Nb) at RT and 600 °C was higher than N1-HR steel (Nb-free) by about 81 and 30 MPa, respectively. This indicates that Nb is an alloying element as effective as Mo in increasing the elevated-temperature strength of fire-resistant steel. The dominant strengthening mechanisms of Nb addition on elevated-temperature yield strength are precipitation strengthening and bainite strengthening. Conclusions: Theoretical analysis shows that there are two precipitation strengthening stages in fire-resistant steel: (1) increasing dislocation density during hot rolling, and (2) blocking dislocation movement and recovery in tensile testing. The results also show that the effect of fine grain strengthening is not obvious at high temperature, but is obvious at room temperature. Full article

Nowadays, nickel-saving metastable austenitic stainless steel (MASS) has become the right solution to meeting the growing requirement of higher strength, better corrosion resistance and more cost saving for the automobile industry. Better understanding of the pitting mechanism of the MASS after either cold- or hot-rolled can offer guidance for the producing of high-performance automobile steel. In the current work, for uncovering the pitting mechanism of the cold- and hot-rolled MASS, the microstructural evolution and pitting performance of nickel-saving metastable austenitic stainless (MASS) steel after cold- (CR) and hot-rolling (HR) were researched via electron microscopy technique and electrochemical methods. Austenite composites the main phase of the MASS. Small amounts of martensite film were proven to form in the MASS. The precipitation of Cr-rich M₂₃C₆ carbides was observed in the CR-MASS, while no carbides existed in the HR-MASS. The pitting resistance of the HR-MASS was better than the CR-MASS, which could be attributed to the fact that the stable pits in CR-MASS were initiated near the carbides, whereas the MnS inclusion would serve as the initiation sites for stable pits in HR-MASS. Findings in this work will provide a guidance for developing new generation MASS for automobile industry. Full article

Microstructural morphology is known to have a significant impact on the mechanical properties of dual-phase steels. A fine ferrite grain size and random distribution of small second phase islands are desirable to provide superior isotropic properties compared to the banded second phase distribution that is typical for this type of steel. A rapid alloy prototyping (RAP) facility has been used to investigate three different DP 800 variants by systematically varying the compositions and/or process parameters compared to the ‘standard’ DP800 composition and processing that gives a banded microstructure. For Variant 1, the heating rate during the annealing cycle after cold rolling varied between 0.65 and 30 °C/s for the 45%, 60% and 75% cold reduction samples. It was found that a cold reduction of 75% and heating rate of 15 °C/s resulted in the microstructure that can give the best combination of strength and ductility because of the fine grain size and high martensite volume fraction. For Variant 2, the effect of changing the hot rolled (HR) microstructure (ferrite–pearlite, ferrite–bainite or martensite) on the final microstructure was investigated. Both the ferrite–50% bainite and fully martensite/bainite HR materials for all cold reductions resulted in annealed microstructures with necklace martensite morphology and finer ferrite grains compared to the ferrite–pearlite HR material, which gave a typical banded ferrite–martensite microstructure with a coarser ferrite grain size. For Variant 3, the Mn content was reduced, and increased Nb was used to achieve higher pancaking during the hot rolling stage, which refined ferrite grains in the HR condition with the same hardness. After annealing with the standard parameters only the 45% cold-reduced material produced a finer ferrite grain size than the standard material, whereas the 60% and 75% cold-reduced samples required a higher heating rate to achieve finer ferrite grain sizes due to rapid recrystallisation and growth kinetics. Full article

HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be ${\mathsf{\sigma }}_{{10}^5}^{700}=$ 57.1 MPa and ${\mathsf{\sigma }}_{{10}^5}^{750}=$37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M₂₃C₆, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M₂₃C₆ preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M₂₃C₆ in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large “island” at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture. Full article

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). Full article

Many researchers have studied explosion prevention and fire resistance of high-strength concrete mixed with organic fiber and steel fibers. The fire resistance of high-performance fiber reinforced cement composites is desirable in terms of physical and mechanical properties. However, the use of a polymer as an alternative to organic fiber has not been clearly studied. In this study, a slurry infiltration method was used to obtain slurry-infiltrated fiber-reinforced cementitious composites (SIFRCCs) specimens. Powder polymer was used instead of organic fibers during mixing of the slurry. The compressive and flexural strengths of the specimens after 1 hr of high temperature exposure according to the KS F 2257 (ISO 834) standard fire-temperature curve were measured. The addition of the polymer before and after high temperature (about 945 °C) exposure affected the strength of the SIFRCCs. The compressive and flexural strengths were decreased after exposure to high temperature in comparison with SIFRCCs without polymer because polymer create capillary pores due to melting and burning when exposure to high temperature. This minimizes the vapor pressure inside the concrete model and reduces the failure of the concrete model. The experimental results showed that the flexural strength at a high temperature for 1.0 % polymer content was the highest at 53.8 MPa. The flexural strength was reduced by 40~50% when compared to the flexural strength before high temperature exposure and comparing to SIFRCCs without polymer, the compressive strength in 1.5% polymer is lower, owing to voids that are created in the SIFRCCs after exposure to a high temperature. Full article

The physical metallurgical tests were performed on the test samples made of HR3C steel, taken from a section of a pipeline in the as-received condition and after approximately 26,000 h of service at 550 °C. In the as-received condition, the test material had austenitic microstructure with numerous large primary Z-phase precipitates inside the grains. The service of the test steel mainly contributed to the precipitation processes inside the grains and at the grain boundaries. After service, the following precipitates were identified in the microstructure of the test steel: Z-phase (NbCrN) and M₂₃C₆ carbides. The Z-phase precipitates were observed inside the grains, whereas M₂₃C₆ carbides - at the boundaries where they formed the so-called continuous grid. The service of the test steel contributed to the growth of the strength properties, determined both at room and elevated temperature (550, 600 °C), compared to the as-received condition. Moreover, the creep properties of HR3C steel after service were higher than those of the material in the as-received condition. The increase in the strength properties and creep resistance was connected with the growth of strengthening of the test steel by the precipitation of Z-phase and M₂₃C₆ carbides. Full article

Oxidation behavior of Super304H and HR3C steel in high temperature steam from an ultra-supercritical coal-fired boiler was investigated in this paper. The results showed that the steam oxidized surface of Super304H ware composed of Fe₂O₃, Cr₂O₃ and FeCr₂O₄, the oxide scale had a thickness of 50–70 μm. In addition, the steam oxidized surface of HR3C ware composed of Fe₂O₃, the oxide scale was about 20μm in thickness and contained few pitting. The oxidation product layer of the two samples could be divided into two layers, including outer layer enriched O element and Fe element, and inner layer enriched O element and Cr element. Furthermore, oxide scale spalling was observed on the surface of Super304H sample. Full article

The material of disc cutters is important to full-face tunnel boring machines (TBM). In recent years, disc cutters were optimized and tested by many scholars all around the world. H13(4Cr5MoSiV1) steel is widely used due to its excellent properties, especially in TBM disc cutters. In this paper, H13 steel with optimized composition was prepared and heat treatment. The high temperature compression of H13 steel was conducted at the temperatures ranging from 100 °C to 700 °C, with strain rate at 0.01 s⁻¹. The stress-strain curves, Rockwell hardness and microstructure of H13 steel after compression were obtained and analyzed. The results showed that the compression strength and hardness decreased as the temperature increased; and the compression strength, hardness and ductility decreased rapidly between 600 °C and 700 °C, HR₇₀₀ (the hardness of H13 steel at 700 °C) only reached 33.23 HRC. It is not recommended for TBM disc cutters to work in an environment over 600 °C. Full article