Resistance to Sulfide Stress Cracking (SSC) caused by local hard zones of pipe inner surface has been required in low alloy linepipe steel. In this study, using two samples with different surface hardness, the detailed SSC initiation behavior was clarified by four-point bend (4PB) SSC tests in which immersion time and applied stress were changed in a sour environment containing 0.15 bar hydrogen sulfide (H2S) gas. SSC cracks occurred when the applied stress was higher than 90% actual yield strength (AYS) in higher surface hardness samples over 270 HV0.1. From the fracture surface observation of SSC crack sample, it was found that the mechanism gradually shifted from active path corrosion (APC) to hydrogen embrittlement (HE), and that the influence of APC mechanism remained partially in the process of SSC initiation at the tip of corrosion pit or groove. The polarization measurement in the 4PB SSC test showed that the anodic and cathodic reactions (especially cathodic reactions) were activated when the applied stress was 90% AYS or higher. The FEM coupled analysis simulating the stress and strain concentration at the bottom tip of the corrosion groove and the hydrogen diffusion and accumulation was carried out. The principal stress in the tensile direction showed the maximum value at 0.04-0.06 mm away from the tip of the corrosion groove, and the hydrogen accumulation became the maximum. It was analytically found that the SSC crack initiated and propagated with HE mechanism dominated type when the threshold value of about 0.82 ppm is exceeded.
We have attempted to predict creep rupture time for a wide range of ferritic heat resistant steels with machine learning methods using the NIMS Creep Data Sheet (CDS). The dataset consisted of commercial steel data from 27 sheets in the CDS, covering various grades of carbon steels, low alloy steels, and high Cr steels. The prediction models were constructed using three methods, support vector regression (SVR), random forest, and gradient tree boosting with 5132 training data in order to predict log rupture time from chemical composition (19 elements), test temperature, and stress. Evaluation with 451 test data proved that all three models exhibited high predictivity of creep rupture time; in particular, the performance of the SVR model was the highest with a root mean squared error as low as 0.14 over the log rupture time, which value means that, on average, the prediction error was factor 1.38 (=100.14). The high predictivity achieved with no use of information on microstructure was presumably because the data used was for commercial steels in which there should be a correlation between the composition and the microstructure. We confirmed that the prediction did not work well exceptionally for two heats having the same composition but different microstructures with and without stress relief annealing. The predictivity could be drastically improved by adding the 0.2% proof stress at the creep test temperature as one of the explanatory variables. As a use case of the prediction model, the effect of elements was evaluated for modified 9Cr 1Mo steels.
The present study investigated the microstructure of cutting tool in order to clarify their wear mechanism of cutting tool made of high speed tool steel in the metal cutting process. A special protective oxide surface, which mainly consist of iron, vanadium and oxygen is formed on the surface of the tool during dry cutting wear test. Iron could be diffused from cutting tool and cutting material, and vanadium which alloyed to improve tool life as MC carbide in high speed steel is from cutting tool. During cutting wear test, an amorphous oxide surface seems to exist in a liquid state. At the cutting temperature on the contact point of tool, the surface as so-called “Belag” is melted as a result of eutectic reaction of iron oxide and vanadium oxide. The surface has a role of fluid lubrication between work material and tool. Therefore, the surface is effective in protecting against tool wear at this cutting speed.
The laser-quenching-induced heat-affected zone (HAZ) of carbon steel was nano-mechanically and sub-micro-structurally characterized. Ferrite–pearlite-structured JIS G 4053 SCM440 specimens were laser-irradiated at 275, 260, or 240 W. The specimens were mechanically characterized by nano-indentation, and the micro-structures were observed with scanning electron microscopy (SEM). The HAZ consisted of various phases and micro-structures, including auto-tempered martensite, as-quenched martensite, martensite containing undissolved cementite, and the original ferrite–pearlite. The region and fraction of the HAZ micro-structures depended on the distance from the sample surface and the laser power. The nanohardness of the martensite structures varied widely presumably depending on the thermal history and local carbon content. In particular, the hardness of the martensite containing the undissolved cementite could be interpreted in terms of the solute carbon content estimated based on the area fractions of the undissolved cementite and precipitated carbide, as observed in the binarized SEM images. The thermal history was theoretically simulated to ensure that the micro-structures and associated hardness values were reasonable.
The contact angles between three non-metallic inclusion-type oxide substrates, viz. Al2O3, MgO, and MgO·Al2O3, and molten Fe and molten Fe-based stainless steel (Fe-Cr-Ni alloy) were measured using the sessile drop method in Ar atmosphere at 1873 K. The contact angles between molten Fe and oxide substrates ranged between 111° and 117°, while that between molten Fe-Cr-Ni alloy and substrates ranged between 103° and 105°. The angles between the alloy and each of the substrates were smaller than the corresponding values for Fe, which was attributed to the superior wettability of molten Fe-Cr-Ni alloy on the substrates. The wettability of the molten materials is related to the interfacial tension between the molten metals and each substrate. Thus, the interfacial tension between the molten metals and the non-metallic substrates was quantitatively evaluated using Young’s equation and the measured contact angles; the interfacial tension for molten Fe ranged from 1.862 to 2.781 N·m−1, while that for molten Fe-Cr-Ni alloy ranged from 1.513 to 2.286 N·m−1. Owing to the higher reactivity between molten Fe-Cr-Ni alloy and the substrates, the interfacial tension and energy between them were lower than those between molten Fe and the substrates.
The contact angle between molten Fe-Al alloy with 0.03, 0.3, and 3 mass% Al composition, and Y2O3 matrix oxide substrate with 0.002, 0.32, and 1 SiO2 activity was measured using sessile drop method in Ar atmosphere at 1873 K, and the interfacial tension was evaluated. The contact angle and interfacial tension between the molten Fe-0.3 Al alloy and the Y2Si2O7 + SiO2 (aSiO2 = 1) substrate decreased over time during 60 s after the molten alloy was dropped onto the substrate. The decrease of the contact angle was 20°, and that of the interfacial tension was 628 mN・m−1 Conversely, the other contact angles and the other interfacial energies were almost stable during the same period. The decrease of the contact angles ranged between 0° and 7°, and that of the interfacial tensions ranged 4 and 195 mN・m−1. By observing the wetting behavior for 60 min, it was recognized that the interfacial reaction between the Fe-Al alloy and the oxide substrate was the redox reaction between Al composition in the alloy and SiO2 composition in the substrate, composed of SiO2 decomposition reaction and Al2O3 formation reaction between oxygen absorbed at the interface and Al composition in the alloy. In addition, it was indicated from the interfacial tension dependence on SiO2 activity that the medium SiO2 volume slag for the molten low-Al steel and the low SiO2 volume slag for the molten high-Al steel were effective in preventing the small droplets of molten slag into the molten steel.
Controlling inclusion content in high chromium steel is very important to prevent submerged entry nozzle from clogging in continuous casting and avoid the negative impacts of inclusions on steel properties. Therefore, effects of temperature and content of elements on phase stability diagram should be clarified in chromium bearing steel. However, the effect of chromium content on boundaries of MgO, MgO∙Al2O3 and Al2O3 in phase stability diagram are much different among the researchers. The direction of boundaries shift is affected by chromium content differently. Temperature dependencies of deoxidation equilibrium constants below 1873 K are also scattered. Calcium, which is used to avoid the negative effect of MgO∙Al2O3 inclusion, enlarges liquid region in phase stability diagram. However, the region replaced by liquid oxide is understood differently in low alloyed steel and high chromium steel. In TiOx-Al2O3-MgO system inclusion, commercial thermochemical software predicts that boundaries of Ti2O3, Ti3O5, Al2O3 and TiOx-Al2O3 shift toward lower titanium content in high chromium steel. However, the calculated phase stability diagrams vary among studies even in liquid iron or low alloyed steel. Therefore, equilibrium experiments under various conditions and reliable technique of thermodynamic calculation with high accuracy are desired.