Showing papers in "Tetsu To Hagane-journal of The Iron and Steel Institute of Japan in 2023"

Review of Oxide Scale in Hot-rolling Process

Abstract: Studies on the oxide scale in hot-rolling process are reviewed. This paper outlines scale formation and the effects of alloying elements on scale formation, scale behaviour in hot-rolling processes such as rolling and descaling, blistering phenomenon, red scale, whisker generation, scale transformation, and Cu-induced hot shortness.

Separation of Phosphorus from Phosphorus-concentrated Steelmaking Slag

Abstract: Since 10 million tons of steelmaking slag, which contains a few percent of phosphorus, are annually produced, the phosphorus amount in the slag is equivalent to the annual import volume of phosphorus rock in Japan. Therefore, the steelmaking slag is attracting attention as a potential phosphorus resources. Phosphorus-concentrated slag obtained by the dephosphorization reaction between high phosphorus hot metal and oxidizing slag at high temperature contains phosphorous comparable to that of phosphorus rock. However, because of high FeO concentration, it is difficult to use for phosphorus resources directly. In this work, the effects of pH, acid type and leaching method on the dissolution behavior of phosphorus from P-concentrated slag were investigated. As a result, phosphorus dissolution progressed at lower pH, and was promoted by the addition of citric acid, which is known as a chelate former. When nylon mill pot stirring with citric acid and alumina mill pot stirring with nitric acid were compared to impeller stirring, respectively. By combining nylon mill pot stirring and citrate leachate, phosphorus dissolution was accelerated, because the slag was pulverized during stirring and a formation of insoluble metal-phosphate was inhibited by the formation of complex ion between leached metal cation and citrate. When the slag was leached with alumina mill pot while controlling pH by nitric acid, the phosphorus dissolution ratio lowered since phosphorus ion and aluminum ion, which is supplied by the dissolution of pot and crushing ball during leaching, constructed secondary products with low solubility along with other dissolved ions.

Diffusion Behavior of Al in Zn Coating Layer of Zn-0.2mass%Al Hot-dip Galvanized Steel Sheets with and without Temper Rolling during Aging after Production

Abstract: It is known that Al added to the Zn coating layer of hot-dip galvanized steel sheets (GI) diffuses to the surface at room temperature and forms Al-based oxides in air. In order to understand the diffusion behavior of Al in Zn coating layer, this study investigated the effect of the aging temperature on the segregation behavior of Al-based oxides in GI with and without temper rolling (skinpass rolling) using a material with a Zn coating weight of about 56 g/m2 with an Al content of approximately 0.20 mass%. The specimens were aged at -15, 5, 20, 38, 100 or 200 ºC in air after production, and the surface and cross sections were observed and analyzed by XRF, SEM-EDX, EBSD and TEM. As a result, up to the aging temperature of 38 ºC, the amount of Al-based oxides increased linearly to the square root of aging time, suggesting that the formation rate is determined by the diffusion of Al in Zn coating layer in this temperature range. However, this linear relationship did not hold at aging temperatures above 100 °C. In addition, in the temper-rolled GI, the formation rate of Al-based oxides is larger than that without temper rolling up to the aging temperature of 38 ºC, and then decreased drastically at aging temperatures above 100 °C. The segregation behavior of Al-based oxides is discussed in view of the diffusion behavior of Al and the changes in the macrostructure of the Zn coating layer during the aging after production.

Carbon Migration Behavior during Rolling Contact in Tempered Martensite and Retained Austenite of Carburized SAE4320 Steel

Abstract: : The changes in the state of carbon in tempered martensite and retained austenite in carburized SAE4320 steel under the rolling contact fatigue (RCF) were investigated using atom probe tomography (APT). In the tempered martensite, the carbons in solid solution and in carbon cluster were readily transferred to the preexisting metastable ( ε ) carbide due to rolling contact, resulting in a localized change from tempered martensite to ferrite accompanied by the growth of carbides. This supports the recently proposed dislocation assisted carbon migration theory. On the other hand, retained austenite with uniformly dis-tributed enriched solute carbon was partially transformed into the very fine deformation-induced martensite due to rolling contact. Furthermore, carbon seemed to be partitioned into retained austenite from the deformation-induced martensite during further rolling contact cycles. This is a new insight into the characteristics of deformation-induced martensite and retained austenite generated by rolling contact. The present study provides a plausible explanation to the phenomenon that the deformation-induced martensitic transformation improves the RCF life.

Quantitative Evaluation of the Relationship between Strain and Color Change in Opal Photonic Crystal Films and Application into Complex Specimen Geometries

Abstract: The color change of opal photonic crystal films (OPCFs) due to deformation was quantitatively evaluated using digital image correlation (DIC) analysis. OPCFs were pasted on specimens of three different gauge geometries, and random patterns were formed on the opposite side of each specimen for DIC analysis. To assess the applicability of using OPCFs-based strain characterization for analyzing steel structural components and associated metallurgical analyses, smooth, width-gradient, and holed specimens were prepared in this study. As deformation increased in the smooth specimen, the color of the OPCFs changed significantly. The color change in the OPCFs could be quantitatively converted into strain values through Hue value analysis. Heterogeneous strain distributions could also be quantitatively analyzed using OPCFs-based analysis at the submillimeter or millimeter scale. When the strain gradient is too high, for example, near a stress concentration site such as a hole, local peeling of the OPCFs away from the specimen surface can occur. Consequently, for quantitative characterization, we must take proper care when measuring this upper limit of the “strain gradient” as well as strain, which would depend on the adhesion and surface condition of the specimen.

Nb添加極低炭素鋼板における冷延過程でのNbCの析出状態変化

Abstract: The change in precipitates during cold rolling of Nb-added interstitial-free (IF) steel was studied by an electrochemical analysis and 3D atom probe (3DAP) measurement. The amount of dissolved Nb and C increased with the rolling reduction ratio. In the 3DAP analysis, the size distribution of NbC was changed by cold rolling, as the extremely coarse precipitates and fine precipitates observed in the sheets before cold-rolling decreased markedly. The C/Nb atomic ratio in NbC near the precipitate/matrix interface was also lowered by cold rolling, which corresponded to a change to a high-energy state, as indicated by thermodynamic calculation. A molecular dynamics calculation suggested that the crystal structure of the precipitate surface was randomized by shear deformation and dislocations increased in the matrix steel around the precipitates. This may be due to the interaction between carbon and dislocations, as reported in the dissolution of cementite under heavy deformation. It is inferred that NbC is not completely dissolved in the matrix phase by cold rolling, but rather that the accumulated energy and dislocation propagation due to cold deformation cause structural collapse near the precipitate surface and heavier deformation of finer precipitates, which is evaluated as the resultant solid solution content by electrochemical analysis.

ボールミリングによる純鉄粉末のミクロ構造変化~x線ラインプロファイル解析およびtem観察~

Abstract: X-ray Line Profile Analysis (XLPA) is a powerful and convenient method to investigate the dislocation density, crystallite size and nature (screw or edge) and arrangement of dislocation in a material. Microstructural observation by Scanning Transmission Electron Microscope (STEM) and XLPA were performed on low-energy ball milled iron powder. It was found that XLPA can be applied to low-energy and short-time ball milled iron powder. Dislocations were found to be concentrated on the surface of the particles by STEM observation, and XLPA was also evaluated on the surface of the particles.

Age-hardening Behavior in γ′-phase Precipitation-hardening Ni-based Superalloy

Abstract: NoDerivatives license (https://creativecommons.org/licenses/by-nc-nd/4.0/). Abstract: Dislocations are often introduced in Ni-based superalloys to impart sufficient strength at both room temperature and high temperatures prior to their use in automobile exhaust gaskets. However, the interaction between the representative γ′ (Ni3(Al, Ti))-phase precipitates and dislocations in high temperature remains unclear. Therefore, this study examined the effect of cold rolling on age-hardening behavior and microstructure evolution, focusing on the formation of γ′-phase Ni3Ti during aging at 700oC for up to 400 h after 60% cold rolling of solution-treated specimens. During the early stage of aging, at 0.03 h, the hardness rapidly increased from 401 HV to 496 HV. Age-hardening continued until 3 h and reached its peak of 536 HV, followed by gradual decrease with aging time. 3D atom probe investigation revealed that the γ′-phase was confirmed after 0.3 h of aging. However, the composition-modulated structure speculated to be caused by spinodal decomposition was observed in the 0.03 h aged specimen. The change in strength with aging time was considered by calculating the contribution of each strengthening mechanism. In the initial stage of aging (0-3 h), dislocation and solid-solution strengthening dominated along with spinodal strengthening. Strengthening by spinodal decomposition in the 0.03 h aged specimen is presumptively accelerated by the introduced dislocations, which is followed by further precipitation strengthening caused by γ′-phase precipitates. In the later stage of aging (3-400 h), precipitation strengthening became dominant and reached its peak at 20 h aging, while dislocation strengthening decreased with aging time.

Solidification Microstructure and Mechanical Properties of B1-type TiC in Fe-Ti-C Ternary Alloys

Abstract: The microstructure of the B1-type TiC formed during solidification and its mechanical properties were investigated using arc-melted Fe-Ti-C ternary alloys. The TiC formed at relatively high temperatures in the liquid as the primary phase exhibited a dendritic shape. With decreasing temperature and/or decreasing Ti and C content in the liquid, its morphology changed to a cubic shape with the {001}TiC habit plane, a plate shape with the {011}TiC habit plane, and a needle shape with the <001>TiC preferential growth direction. The morphology of the TiC was characterized by the anisotropy of its surface energy and its growth rate. The cubic shape with the {001}TiC habit plane was formed as a result of the minimum surface energy of the {001}TiC. However, the plate shape with the {011}TiC habit plane and the needle shape with the <001>TiC preferential growth direction should be formed due to the slowest growth rate of <011>TiC and the fastest growth rate of <001>TiC, respectively. At room temperature, alloy with the dendritic TiC was fractured in the elastic deformation region because the TiC exhibited no plastic deformation. On the other hand, at 800ºC, it was suggested that the TiC has plastic deformability and alloy with the dendritic TiC was also plastically deformed.

Defect Formation in Aluminum-coated Steel by Heat Treatment

Abstract: A carbon tool steel plate was coated with an aluminum foil with a thickness of 100 μm by explosive welding. This aluminum-coated steel was heat-treated in the temperature range of 973−1273 K for up to 3.6 ks in the atmosphere to form a Fe-Al alloy layer on the steel surface. The alloy layer was basically composed of Fe2Al5, two kinds of FeAl (Al-rich FeAl and Fe-rich FeAl), Fe3Al containing carbon (Fe3Al(C)) and ferrite stabilized by aluminum diffusion (α-Fe(Al)). FeAl2 was also detected together with Fe2Al5 at more than 1223 K. At 1273 K, island-shaped Fe3Al(C) was produced in α-Fe(Al), and the Fe-rich FeAl/α-Fe(Al) interface became indistinct. In addition, defects like a crack and a void were observed in the vicinity of Fe-rich FeAl. The purpose of the present study is to investigate the reason for the defect formation. Various heat treatments were conducted for the aluminum-coated steel, and it was found that a temperature-rising step was responsible for the defect formation. This consideration got support from the results of two-step heat treatment, which was a heating process at 1273 K immediately after holding for 600 s near the eutectoid temperature in the Fe-C binary system.

Creep Rupture Strength and Phase Stability of the Austenite Phase for KA-SUS304J1HTB

Abstract: Metallurgical factor causing the heat-to-heat variation in creep rupture strength have been investigated for KA-SUS304J1HTB. In the long-term, there was a maximum difference of 3.5 times in creep rupture time between the heat with low creep strength and the heat with high creep strength. In the heat with low creep rupture strength, most of the creep voids occurred at the matrix/σ phase interface. Moreover, in the heat with low creep rupture strength, the area fraction of σ phase was larger than in the heat with high creep rupture strength. Considering that the difference in the area fraction of σ phase in each heat is related to the difference in phase stability of the austenite phase, the Md value in each heat was evaluated. The Md value is the parameter correlated with phase stability. The creep rupture time of each heat was correlated with the Md value. The smaller the Md value, the longer the creep rupture time. Therefore, the heat-to-heat variation in creep rupture strength is caused by the difference in the phase stability of each heat. In other words, in the heats with low phase stability, creep rupture strength is low because a large amount of σ phase precipitates during the creep test.

光沢剤を含むアルカリジンケート浴から電析したZn-Ni合金皮膜の耐食性

Abstract: Zn–Ni alloys were electroplated on an Fe plate with thicknesses of 40 μm at 500 A·m−2 and 293 K in unagitated zincate solutions. The reaction product of epichlorohydrin and imidazole (IME) was added in the solution as a brightener at the concentration of 0-5 ml/dm−3. The corrosion resistance of the obtained Zn–Ni alloys films was investigated from the polarization curve in 3 mass% NaCl solution before and after the corrosion treatment (formation of corrosion products) in NaCl solution for 48 h. Before the corrosion treatment, the corrosion current density of plated films rarely changed regardless of addition of IME into the zincate solution because the reduction reaction of dissolved oxygen rarely changed. However, in films plated from the solution containing IME, the anode reaction was suppressed and the corrosion potential shifted to noble direction. The suppression of anode reaction with an addition of IME in plating solution is attributed to the increase in γ-phase in plated films. On the other hand, after the corrosion treatment, the morphology of Zn chloride hydroxide of corrosion product was uniformly formed on the surface with increasing the concentration of IME. The reduction reaction of dissolved oxygen was suppressed with increasing the concentration of IME, resulting in decrease in corrosion current density.

Effect of Cu Addition on Solidification Microstructure in High Carbon High Speed Steel Type Cast Alloy

Abstract: Effect of copper addition on solidification microstructure was investigated for high carbon high speed steel type alloys (Fe-2.0%C-5%Cr-5%Mo-5%V-0~4%Cu). The microstructure of all as-cast specimens with different copper content was composed of dendrite of primary γ, MC-γ eutectic and M2C-γ eutectic but it was found X-ray diffraction peaks of α phase by X-ray diffractometry. The volume fraction of each phase in solidification microstructure was approximately constant regardless of copper content. The shapes of dendrite of primary γ changed significantly in response to amount of copper addition: that of specimen with no copper was equiaxed while that of specimen containing copper was columnar which tends to be marked with increasing amount of copper addition. Furthermore, the secondary dendrite arm spacing λ2 decreased with increasing amount of copper addition. As a result, it concluded that the effect of copper addition on solidification microstructure is columnar crystallization and decreasing λ2 of the dendrite with primary γ.

Effect of Pro-eutectoid Cementite on Fatigue Crack Growth Behavior of Pearlitic Steels

Abstract: : The influence of pro-eutectoid cementite on fatigue crack growth behavior was investigated using various pearlitic steels containing from 0.64 to 1.21 mass%C. The fatigue crack growth rates of the hy-po-eutectoid and eutectoid pearlitic steels hardly changed, while that of the 1.21 mass%C steel having a large amount of pro-eutectoid cementite (θ) was accelerated especially in the high stress intensity factor region. Scanning electron microscope (SEM) observation revealed that fatigue fracture surface in the 1.21 mass%C steel more frequently contained islanded brittle fracture surfaces than that in other steels. In the 1.21 mass%C steel, the total area fraction of brittle fracture surface was notably increased with an en-hancement in maximum stress intensity factor ( K max ) due to crack extension. More detailed SEM fractog-raphies were performed comparing between before and after etching in order to identify microstructures beneath the brittle fracture appearances on the fatigue fracture surface of the 1.21 mass%C steel. As a re-sults, it was suggested that the pro-eutectoid θ was involved in the formation of brittle fracture. Based on these investigations, the accelerated fatigue crack growth behavior of hyper-eutectoid steel was discussed in terms of static brittle fracture induced by pro-eutectoid θ near the crack tip.

Flow Visualization and Heat Transfer Characteristics of Droplet Train Obliquely Impinging on a Moving Hot Solid

Abstract: The spray cooling of moving hot solids is widely performed in the steel industry. Understanding flow and heat transfer when droplets impinge on moving hot solids is important. By simultaneous visualization with flash photography and temperature measurement using thermography, the flow and heat transfer of a droplet train obliquely impinging on a moving solid at high temperatures was experimentally investigated. A rectangular test piece (SUS303) was heated to 500 °C at a moving velocity of 0.25–1.5 m/s. The test liquid was water at approximately 25 °C. The pre-impact droplet diameter, impact velocity, and inter-spacing between two successive droplets were 0.69 mm, 2.2 m/s, and 2.23 mm, respectively. The tilt and torsional angles were 50° and -30–60°, respectively. No coalescence of droplets was observed; the droplets deformed independently on the moving solid, even though the torsional angle generated a velocity component along the width of the solid. The surface temperature of solid after droplet impingements depended on the experimental conditions. Wavy temperature profile was obtained when the moving distance of solid was large during two successive collisions. The temperature changed continuously for the small distances. In this regard, a simple model considering droplet movement, collisional deformation behavior, and solid migration can explain this phenomenon by the overlap of the cooling regions of the droplets. Furthermore, experimental and numerical analyses show that the heat removal rate of individual droplets is constant at approximately 12.5 MW/m2 and depends on the total contact time when multiple droplets collide.

人の意思決定が埋め込まれた生産システムのモデル化と分析

Abstract: Most production systems are operated in a human-in-the-loop fashion, and it is sometimes argued that the human decisions involved make it possible, or at least easier, for the systems to cope with various stationary and nonstationary variations. However, it has not been well-studied and understood how this positive contribution of human decisions work, what factors determine its effect, how the function should be supported or fostered, etc. This paper first briefly reviews conventional production systems simulation techniques and discusses why it is difficult for them alone to address aforementioned questions. This next points to some recent attempts, in production systems engineering and related areas, to study human decisions and their effects by complementally using gaming simulation and agent-based simulation and highlights the potential of combining such behavioral and computational scientific approaches. Then, the paper introduces a cognitive framework model composed of interface, interaction, and incentive dimensions. It can be used for formally characterizing the decisions made by an individual facing a problem situation in operating a production system, and functions as a bond connecting behavioral and computational analyses of the decision maker. The paper further presents some example ongoing research projects worked on by the author’s team in this direction and discusses some future perspective.

焼戻しマルテンサイト鋼の水素トラップに及ぼすVおよびMo複合添加の影響

Abstract: The effect of combined addition of V and Mo on hydrogen trapping was investigated using tempered martensitic steels (0.3%C-1.2%Cr-0.5%Mo-0.4%V, 1.5%Mo-0.2%V or 2.0%Mo-0.1%V in mass%), for the purpose of improving hydrogen embrittlement strength. The trapped hydrogen content and the hydrogen trapping energy were evaluated under various tempering conditions and compared with the single addition 0.2%V and 1.0%Mo steels in the previous paper. The trapped hydrogen content and the hydrogen trapping energy of 0.5%Mo-0.4%V and 1.5%Mo-0.2%V steels were higher at 650°C tempering temperature for 1 hour than at 600°C. On the other hand, those values of 0.2%V and 1.0%Mo steels were lower at a tempering temperature of 650°C than at 600°C. This was presumed to be due to the some interaction between V and Mo at a tempering temperature of 650°C. In the 0.5%Mo-0.4%V and 1.5%Mo-0.2%V steels at tempering temperatures of 600°C and 650°C for 1 hour, the plate-shaped Cr enriched cubic (Mo, V)C fine carbides with a width of about 1nm and a length of about 20nm or less were precipitated, and there was no significant difference in the number density. The V/Mo peak ratio of the fine carbides by EDS analysis was higher at a tempering temperature of 650°C than at 600°C and the length of fine carbides was slightly longer. The 2.0%Mo-0.1%V steel was estimated to have less trapped hydrogen content at a tempering temperature of 650°C than at 600°C, similar to single addition steels, because of the smaller V/Mo mole ratio.

Effects of Temperature and Carbon Content in Brick on MgO-C Reaction Behaviour

Abstract: The effects of the ambient temperature, gas flow rate and carbon content in the brick on the behaviour of the MgO-C reaction, which is an inherent phenomenon of MgO-C bricks, were investigated. As a result, the amount of the MgO-C reaction increased as the carbon content in the brick and the temperature increased, but was not significantly changed by increasing the gas flow rate. The apparent activation energies for the following reactions were determined from the results of this study and the results of previous reports.

Effect of Initial Austenite Grain Size on Microstructure Development and Mechanical Properties in a Medium-carbon Steel Treated with One-step Quenching and Partitioning

Abstract: Fe-0.4C-1.2Si-0.8Mn (mass%) alloys austenitized at different temperatures, ranging from 1103 to 1473 K, were subjected to interrupted quenching (IQ) at 473 K and then maintained at that temperature to induce the partitioning of carbon from martensite to austenite (one-step quenching and partitioning (Q&P) process). The initial austenite grain size before the IQ was varied from 20 to 573 μm. As the initial austenite grain size becomes finer, the enrichment of carbon in the untransformed austenite during the partitioning treatment is enhanced, which leads to a greater increase in the volume fraction of retained austenite. The reasons for the increased carbon enrichment were explained by the effective carbon partitioning as well as the promoted bainitic transformation, which were both caused by the increase in the area of the martensite/austenite interface. Tensile tests of the specimens with different initial austenite grain sizes revealed that the mechanical properties of the one-step Q&P specimens improved in both strength and elongation by the refinement of the initial austenite grains.

焼戻マルテンサイト鋼の高c化による粒界破壊強度の向上

Abstract: Effect of carbon (C) content (0.05 mass% to 0.3 mass%) on critical intergranular fracture stress of tempered martensite steel was investigated using 3 mass% manganese (Mn) steel. The critical intergranular fracture stress was obtained by calculating the maximum principal stress at fracture in a circumferential notched specimen tensile test using elastoplastic finite element analysis. As a result, critical intergranular fracture stress of tempered martensite steel increased with increasing C content. Therefore, the dominant factors of critical intergranular fracture stress were examined from the viewpoints of the amount of segregation of each element on prior-austenite grain boundaries and the grain size of the martensite substructure. The first result was found to be the effect of reducing the amount of Mn segregation by increasing C content. This was thought to be because cementite acts as a solid solution site for Mn, and the amount of Mn in solid solution in the matrix phase is reduced by increasing the C content. The second result was found to be the effect of refining the substructure of martensite surrounded by high angle grain boundaries by increasing the C content. This indicates that grain size also affects crack initiation resistance in intergranular fracture by determining the stress concentration at the grain boundaries as the distance of dislocation accumulation.

Hydrogen Embrittlement Mechanism of Ultrafine-grained Iron with Different Grain Sizes

Abstract: To investigate the effect of grain sizes on hydrogen embrittlement of 4N-purity iron, miniature tensile tests were conducted after hydrogen charging for ultrafine-grained specimens produced by high-pressure torsion and subsequent annealing. Hydrogen embrittlement indexes defined from reduction of area were increased with decreasing grain size, and shear-type fracture was occurred with fine dimples on the fracture surface of the specimen with a smaller grain size. The formation and growth of microvoids at triple junctions of grain boundaries ahead of propagated cracks were responsible for such earlier shear-type fracture because necking between adjacent microvoids is more likely and extensively occurred. In the specimens with larger grain sizes or without hydrogen charging, on the other hand, local coalescence and growth of microvoids were predominant due to longer distances between triple junctions, resulting in void coalescence-type fracture with coarser dimple patterns. Therefore, hydrogen atoms introduced by hydrogen charging are considered to enhance the formation of deformation-induced vacancies in ultrafine-grained iron, resulting in shear-type fracture with finer dimple patterns.

Challenges in Materials Integration

Abstract: Materials Integration is the concept of accelerating materials development by linking processing, structure, property, and performance (PSPP) on a computer using any types of models such as theoretical, empirical, numerical-simulation, and machine learning models. In the first and second phases of Cross-ministerial Strategic Innovation Promotion Program in Cabinet Office, Japan, we have developed a system called MInt (Materials Integration for Network Technology), which links PSPP with computational workflows that combine modules implemented, in order to realize the concept of Materials Integration. MInt is equipped with an application programming interface (API) that can be called from various algorithms in the artificial intelligence (AI) field and one can use MInt-API together with the AI algorithms to inversely design materials and processes from desired performance. The target material systems have expanded to steel, aluminum alloys, nickel alloys, and titanium alloys, and the target processes have also expanded to welding, heat treatment, 3D additive manufacturing, and powder metallurgy. MInt is more than just software for materials design; it is designed to serve as a digital platform for industry-academia collaboration. The Materials Integration Consortium has been established with MInt as its core technology, based on the philosophy of sharing tools such as modules and workflows, while competing on how to use them. In materials research and development, which has traditionally been regarded as a competitive area, we hope that a digital collaborative area will be formed and that investment efficiency will be drastically improved.

フローインジェクション－非対称流れ流動場分離(af4)法によるナノ粒子個数濃度の定量分析

Abstract: To determine the number density of fine precipitates in steels by asymmetric flow field-flow fractionation (AF4) with inductively coupled plasma–mass spectrometry (ICP–MS), an analysis method employing flow injection was investigated. For accurate calibration, matrix matching was performed by mixing the standard solution and AF4 carrier solution in front of a nebulizer. Two surfactants were used for AF4 separation; it was found that the appropriate selection of surfactants based on their acidity constant is essential to avoid salt precipitation. In addition, the effect of the AF4 retention time on recovery was investigated. A long retention time led to the adsorption and aggregation of the samples in the AF4 separation channel. Results showed that an AF4 retention time within 20 min facilitated superior recovery. Moreover, five types of AuNPs were analyzed via AF4–ICP–MS and quantified using flow injection analysis. Good analytical performance was achieved for all AuNPs and the recoveries exceeded 93%, and the coefficient of variation was within 5%. The effect of particle size on the recovery was not confirmed.

Effect of Molybdenum Contents on Hardenability and Precipitation Behaviors in B-added Steels Austenitized at High Temperature

Abstract: The effect of molybdenum (Mo) contents on hardenability and precipitation behaviors in Mo-B simultaneously added steels were investigated placing a focus on high austenitizing temperature. The hardenability of 0.5% Mo - 11 ppm B steel austenitized at 1150°C was decreased compared with that austenitized at 950 °C, whereas 1.0% Mo - 10 ppm B and 1.5% Mo - 9 ppm B steels were less affected by high austenitizing temperature than 0.5% Mo - 11 ppm B steel. The Fe23(C, B)6 precipitation by increasing austenitizing temperature was also revealed to be suppressed in 1.5% Mo – 9 ppm B steel. These results indicate that the improved effect of the Mo addition on hardenability by retarding the precipitation of Fe23(C, B)6 still appear in B-added steels austenitized at high temperature. Furthermore, Fe23(C, B)6 precipitation start temperature was increased in Mo-B added steels austenitized at 1150 °C. This result implies that non-equilibrium B segregation mechanism during cooling from high austenitizing temperature enhances the amount of segregated B on grain boundaries leading to the promotion of the borides precipitation at high temperature austenitizing region. However, Mo is presumed to fix a part of thermal vacancies as Mo-V complex resulting in the suppression of non-equilibrium B segregation to grain boundaries during cooling, which is speculated to inhibit the Fe23(C, B)6 precipitation. Thus, the effect of Mo-B combined addition on hardenability was presumably maintained even in high austenitizing temperature region.

Grain Size Dependence of Secondary Creep Rate in Pure Metals and Single-phase Alloys

Abstract: Grain size dependence of secondary creep rate in pure metals and single-phase alloys and the mechanisms/models to interpret the dependence have been reviewed. Two types of the grain size dependence were reported, both of which show a negative dependence approximately below 100 μm. The model proposed by Garofalo in 1960s assumes that the density of dislocations generated at grain boundaries and sub-boundaries determines the secondary creep rate, which is not experimentally supported. Mclean’s model considers preferential subgrain growth near grain boundaries, which might be important in practical steels and alloys with sub-grains such as high Cr ferritic heat resistant steels. Grain boundary sliding (GBS) and its accommodation process in grain is considered as a source of the negative grain size dependence. A finite element modeling performed by Crossman and Ashby, which simulated a deformation process where GBS is accommodated by the power law creep process in the grain interior, indicates that the negative grain size dependence cannot be interpreted by the accommodation process. Based on substructure observation and internal stress measurements, Terada established a “core and mantle” model where the mantle region near grain boundary has no internal stress. This model reasonably interprets the negative grain size dependence.

Microstructures and Tensile Properties of Friction Stir Welded 0.2%C-2%Si-Cr Steels

Abstract: The 0.2C-2Si (mass%) steels with the addition of 0-4 mass% Cr were prepared by hot rolling followed by subsequent annealing for normalization. The steels were subjected to friction stir welding (FSW) conducted above A3 temperature. For all the steels, sound FSWed joints were obtained. Microstructures and tensile properties using small tensile specimens were investigated for both base materials and stir zones. The base materials showed a relatively good balance of strength and ductility when the Cr content is over 3 mass% presumably owing to the relatively fine microstructures of ferrite and martensite. The tensile properties of stir zones were substantially enhanced by FSW, and the stir zone of the 0.2C-2Si-4Cr joint with fully martensitic structure exhibited the surprisingly high tensile strength of 1720 MPa compared with that of the conventional martensitic steel of 0.2 mass%C together with the excellent balance of ductility. This is assumed to be caused by the refinement of block size in the fresh lath martensite and/or the formation of ausformed martensite induced by the dynamically recrystallized fine austenite grains by FSW of the Cr added steels.