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- 저자 : Steel, R. (검색결과 61 건)
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Effect of PCBN tool grade on joint strength and tool life in friction stir spot welded DP 980 steel
Hartman, T.,Miles, M.P.,Hong, S.-T.,Steel, R.,Kelly, S. Elsevier 2015 Wear: An international journal on the science and Vol.328 No.-
<P><B>Abstract</B></P> <P>Friction stir spot welding of advanced high strength steel, like dual phase (DP) 980, is a very demanding application in terms of tool wear. The best candidate material currently being tested is polycrystalline cubic boron nitride (PCBN). The purpose of the current work is to study the effect of PCBN material grade, as specified primarily by CBN grain size, on weld quality and tool life, for the spot welding of DP 980 steel. With a nominal composition of 90% CBN, three tool grades were tested. Grade A had CBN grain sizes of 3–6μm, grade B had grain sizes of 12–15μm, and grade C had a multimodal distribution, with grain sizes ranging from 4 to 40μm. The best performance in terms of joint strength and tool life was achieved with grade A. The effect of fine CBN grain size was less adhesion of DP 980 on the tool surface over time, less abrasive wear, and higher lap shear failure loads of the welds that were produced, compared to the other grades. A-type tools were the most consistent in both the number of welds per tool, and the number of welds that reached acceptable lap shear failure loads. B-type tools, performed slightly better than C-type tools in terms of wear, but neither of them was able to achieve consistent, acceptable lap shear failure load values after the first 200 welds. In fact only one out of five C-type tools was able to produce acceptable lap shear failure loads after the first 100 welds. Based on the experimental evidence, tool wear was likely dominated by grain pullout. This is consistent with the correlation showing that larger CBN grain size was related to shorter tool life.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We monitored wear resistance of PCBN tools over thousands of friction stir spot welds in DP 980 steel. </LI> <LI> CBN grain size was found to have a significant impact on tool life and on joint strength. </LI> <LI> Grain pullout was postulated to be the primary wear mechanism based on the evidence. </LI> </UL> </P>
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Cr-Ni 耐熱鋼의 Al-Si-Mn에 의한 脫酸生成物의 擧動
金營三 동아대학교 공과대학 부설 한국자원개발연구소 1979 硏究報告 Vol.4 No.1
This study shows some variations on the deoxidation behavior of products formed by adding Al, Si and Mn element in 28Cr-13Ni heat resiting steel. The favorable condition for the decrease in inclusion content has been found to be obtained when six minutes or more measured after adding the Al-Si-Mn deoxidizer into the melt have elapsed. The crystal structure of the slags and the inclusions extracted by Br-Methanol solution has been found to be amorphous. However, when the inclusions extracted by Br-Methanol solution are heated for 20 minutes at 800℃, the crystal substance of α-Fe₂O₃ can be obtained.
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Cho, H.H.,Hong, S.T.,Roh, J.H.,Choi, H.S.,Kang, S.H.,Steel, R.J.,Han, H.N. Elsevier Science 2013 ACTA MATERIALIA Vol.61 No.7
A three-dimensional thermomechanical simulation of friction stir welding (FSW) processes is carried out for ferritic stainless steel by utilizing an Eulerian finite volume method under the steady state condition, and the simulation result is compared directly with both the measured temperature histories during FSW and the microstructural changes after FSW. Based on a viscoplastic self-consistent approach for polycrystal, the texture development in the FSWed material is determined from the velocity gradients along the streamlines in the material flow field. The simulation results show that the heat is generated mainly near the interface between the tool and the workpiece, and that the viscosity changes drastically in the vicinity of the boundary between the stir zone and the thermomechanically affected zone. From the predicted streamlines, it can be indicated that the strong material flow mainly develops on the retreating side of the tool. Also, the simulation results show that the shear deformation texture is significantly developed in the FSWed region. The measured temperatures and microstructural characteristics agree fairly well with the predicted data.
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Microstructural analysis of friction stir welded ferritic stainless steel
Cho, H.H.,Han, H.N.,Hong, S.T.,Park, J.H.,Kwon, Y.J.,Kim, S.H.,Steel, R.J. Elsevier Sequoia 2011 Materials science & engineering. properties, micro Vol.528 No.6
High-quality, defect-free welds were successfully produced in 409 ferritic stainless steel by friction stir welding. A remarkably fine-grained microstructure was observed in the stir zone, and the fraction of low angle grain boundary in the stir zone significantly increased as compared to that in the base material. An increase in plunging depth led to an increase of the fraction of low angle grain boundary, a decrease in grain size, and an increase in hardness in the stir zone.
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Joint Strength in High Speed Friction Stir Spot Welded DP 980 Steel
Nathan Saunders,Michael Miles,Trent Hartman,Yuri Hovanski,Sung-Tae Hong,Russell Steel 한국정밀공학회 2014 International Journal of Precision Engineering and Vol. No.
High speed friction stir spot welding was applied to 1.2 mm thick DP 980 steel sheets under different welding conditions, using PCBNtools. The range of vertical feed rates used during welding was 2.5~102 mm per minute, while the range of spindle speeds was 2500~6000 rpm. Extended testing was carried out for five different sets of welding conditions, until tool failure. These welding conditionsresulted in vertical welding loads of 3.6~8.2 kN and lap shear tension failure loads of 8.9~11.1 kN. PCBN tools were shown, in thebest case, to provide lap shear tension failure loads at or above 9 kN for 900 spot welds, after which tool failure caused a rapid dropin joint strength. Joint strength was shown to be strongly correlated to bond area, which was measured from weld cross sections. Failure modes of the tested joints were a function of bond area and softening that occurred in the heat-affected zone.
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Current Mechanistic Approaches to the Chemoprevention of Cancer
Steele, Vernon E. Korean Society for Biochemistry and Molecular Biol 2003 Journal of biochemistry and molecular biology Vol.36 No.1
The prevention of cancer is one of the most important public health and medical practices of the $21^{st}$ century. We have made much progress in this new emerging field, but so much remains to be accomplished before widespread use and practice become common place. Cancer chemoprevention encompasses the concepts of inhibition, reversal, and retardation of the cancer process. This process, called carcinogenesis, requires 20-40 years to reach the endpoint called invasive cancer. It typically follows multiple, diverse and complex pathways in a stochastic process of clonal evolution. These pathways appear amenable to inhibition, reversal or retardation at various points. We must therefore identify key pathways in the evolution of the cancer cell that can be exploited to prevent this carcinogenesis process. Basic research is identifying many genetic lesions and epigenetic processes associated with the progression of precancer to invasive disease. Many of these early precancerous lesions favor cell division over quiescence and protect cells against apoptosis when signals are present. Many oncogenes are active during early development and are reactivated in adulthood by aberrant gene promoting errors. Normal regulatory genes are mutated, making them insensitive to normal regulatory signals. Tumor suppressor genes are deleted or mutated rendering them inactive. Thus there is a wide range of defects in cellular machinery which can lead to evolution of the cancer phenotype. Mistakes may not have to appear in a certain order for cells to progress along the cancer pathway. To conquer this diverse disease, we must attack multiple key pathways at once for a predetermined period of time. Thus, agent combination prevention strategies are essential to decrease cancer morbidity. Furthermore, each cancer type may require custom combination of prevention strategies to be successful.
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Giant Electron-Phonon Coupling and Deep Conduction Band Resonance in Metal Halide Double Perovskite
Steele, Julian A.,Puech, Pascal,Keshavarz, Masoumeh,Yang, Ruoxi,Banerjee, Subhasree,Debroye, Elke,Kim, Cheol Woong,Yuan, Haifeng,Heo, Nam Ho,Vanacken, Johan,Walsh, Aron,Hofkens, Johan,Roeffaers, Maart American Chemical Society 2018 ACS NANO Vol.12 No.8
<P>The room-temperature charge carrier mobility and excitation-emission properties of metal halide perovskites are governed by their electronic band structures and intrinsic lattice phonon scattering mechanisms. Establishing how charge carriers interact within this scenario will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications. Herein we evaluate the charge carrier scattering properties and conduction band environment of the double perovskite Cs<SUB>2</SUB>AgBiBr<SUB>6</SUB><I>via</I> a combinatorial approach; single crystal X-ray diffraction, optical excitation and temperature-dependent emission spectroscopy, resonant and nonresonant Raman scattering, further supported by first-principles calculations. We identify deep conduction band energy levels and that scattering from longitudinal optical phonons-<I>via</I> the Fröhlich interaction-dominates electron scattering at room temperature, manifesting within the nominally nonresonant Raman spectrum as multiphonon processes up to the fourth order. A Fröhlich coupling constant nearing 230 meV is inferred from a temperature-dependent emission line width analysis and is found to be extremely large compared to popular lead halide perovskites (between 40 and 60 meV), highlighting the fundamentally different nature of the two “single” and “double” perovskite materials branches.</P> [FIG OMISSION]</BR>
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