발간논문

Home > KJMM 논문 > 발간논문

Rolling contact fatigue of high-carbon chromium bearing steel, STB 2 (AISI 52100 or JIS SUJ 2), with matrix of lower bainite has been studied in comparison with that of the steel with conventional matrix of tempered martensite. Rolling contact fatigue test was conducted in the lubricant at a shaft speed of 500 rpm under maximum Hertzian shear stress of 557Kg/㎟, using thrust-type rolling contact fatigue tester. It was observed that spherical carbides dispersed in the matrix of lower bainite formed by partial austenitizing at 840℃ for 30 min and isothermal holding at 250℃ for 2 hrs. Local softening occurred between 300 and 600㎛ below the raceway in the matrix of tempered martensite due to rolling contact fatigue. On the other hand, microhardness in the matrix of lower bainite was markedly increased within the region 400㎛ in depth below the raceway. Propagation of subsurface crack in the lower bainite was retarded by deflection, creating plastic zone along the front of crack. The fatigue life of the specimen with the lower bainite was improved in comparison with that of the specimen with the tempered martensite.

keyword :

This Paper describes the results on the fabrication of Fe-Cr powder using the waste pickling Liquor of steel which contains 9.1wt% of Fe ion and the waste sludge of Cr-plating which contains 2.9wt% Cr for the purpose of the making a high quality material out of the elements from the wastes and the solution about problem of enviromental pollution by the wastes. The results obtained from experiment are as in the following. 1) The above wastes were precipitated under conditions of pH 7 with NaOH after mixing-solution under conditions of pH 1.0-1.5. A precipitate became Fe, Cr oxides particles by a oxidation in the air. The character of the oxides was that a reflectance was 12.5% in wavelength 700nm and its color was dark-brown which heat-resistance came to 1000℃. The particle size was under 0.3㎛ and mean particle size was 0.1㎛ as intergrative 50%. 2) Homogeneous Fe-Cr was obtained by mixed 20wt% starch with Fe, Cr oxides and then H₂gas reduction at 1200℃. The particle size was 5-10mm and come out about 0.46% C carburization. 3) The Fe-Cr powder under 23㎛ was obtained by grinding after nitriding at conditions of 590℃, H₂/ NH₃mixed gas.

keyword :

Deformation response and densification behavior of sintered porous alloy steel preform under uniaxial compression were investigated at room and high temperatures, with various initial relative densities and strain rates. Matrix material response of porous alloy steel preform was also investigated by using fully dense alloy steel cylinder. Theoretical analyses with several yield functions from the literature and matrix material response did not agree with experimental data for porous alloy steel cylinder. A new yield function based on Kim`s yield function and experimental data was proposed. Agreement between theoretical analysis with the proposed yield function and experimental data for porous alloy steel cylinder was good. Finally, displacement-relative density and displacement-load relations, deformed geometry and density distribution of porous alloy steel ring under uniaxial compression at high temperature were compared with the finite element simulations using the proposed yield function and Shima and Oyane`s yield function, respectively.

keyword :

The effect of the distribution of non-metallic inclusions on fatigue life in bearing steels was investigated. Most oxide inclusions observed in this work were globular oxides. The increase in the oxygen content has an effect of increasing the amount of oxides inclusions and decreasing fatigue life. But the direct relationship between the distribution of oxide inclusions and fatigue life could not be formulated. The increase in sulfur content results in the decrease in fatigue life. The phenomenon that the exis tence of MnS does not hinder the crack propagation during rolling contanct fatigue test was observed. And also the occurence of cracks and cavities at the MnS inclusion was observed.

keyword :

Precipitation structure, physical and mechanical properties have been examined in Al-Cu-Li-Mg-Ag-Zr(Cl) and Al-Li-Cu-Zr(2090) alloys using hardness and density measurement, tensile test, transmission electron microscopy. And, in an attempt to assess the composition of precipitate, energy dispersive X-rey microanalysis was performed using the scanning transmission electron microscopy technique. The tensile strength 630MPa, density 2.69g /㎤, hardness number H_RB 96 and specific strength extremely exceeding that of 2090 and 7075-T6 alloy are achived. In the near peak-aged condition, finer and homogeneous distribution of θ` and T₁, phases than that of an 2090 alloy were precipitated and due to small amount of Mg addition, S` phase in Al-Cu-Mg alloy was precipitated. Energy dispersive X-ray spectra microanalysis was performed on the relatively coarse black particle obserbed in the matrix and matrix phase. The result of EDXS microanalysis, the X-ray spectrum from the matrix reveals Al and Cu peak but black particle reveals Al, Cu and Ag peak which does not reveal from the matrix.

keyword :

Abstract An investigation was conducted into the effects of loading rate on the fracture initiation of 2124 Al composite reinforced with SiC whiskers. Dynamic void initiation tests were carried out at room temperature using round notch specimens loaded in tension with a strain rate of about 10³/sec, and were compared with those for quasi-static loading. Detailed fractographic observations and cross-sectional microstructure analysis were also made to identify local micromechanical processes of void or crack initiation. One of the important results is that the void initiation sites of the composite are strongly influenced by the loading rate: voids initiate at whisker ends under quasi-static loading and at whisker sides under dynamic loading. Futhermore, the favored direction of crack growth is also dependent upon the loading rate, i.e, approximately 45 degrees under quasi-static loading rate and perpendicular to the tensile axis under dynamic loading rate. Such different local fracture processes with the different Loading rates are interpreted in terms of the role of SiC whiskers on the load carrier in the 2124 Al-SiC_w composite.

keyword :