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Time-temperature-precipitation (TTP) characteristics of high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N stainless steel (all in weight percent) were investigated in terms of precipitation behavior, and crystallographic features of the second phases. The precipitation reactions can be categorized into three stages, i.e. (i) high temperature region (above 950℃): mainly coarse grain boundary (intergranular) Cr2N; (ii) nose temperature region: intergranular Cr2N→cellular Cr2N→intragranular Cr2N+sigma(σ); (iii) low temperature region (below 750℃): intergranular Cr2N→cellular Cr2N→intragranular Cr2N+σ+chi(χ)+M7C3 carbide, respectively. Based on the analyses of selected area diffraction (SAD) patterns, the crystallographic features of the second phases were determined: (i) Cr2N (trigonal, P31m)-[001]y∥[1100]Cr2N,(111)y∥(0001)Cr2N; (ii)χ(b.c.c., I43m)-K-S orientation relationship; (iii) M7C3 (orthorhombic, Pnma)-[001]γ∥[101]M7C3,(111)y∥(131)M7c3.

keyword : time-temperature-precipitation, TTP, high nitrogen steel, HNS, Cr2N, M7C3 carbide and crystallography

The distribution and prediction interval for the misorientation angle of grain boundary at which Cr2N was precipitated during heating at 900℃ for 10(4) sec were estimated. The misorientation angles had a lognormal distribution with parameters μ=3.680, σ=0.09817. The prediction interval of the misorientation angle could be obtained by the median rank regression method and the mathematical method. The prediction interval by the median rank regression had larger values than that by the mathematical method.

keyword : Cr2N, misorientation angle, mathematical method, median rank regression method

The recently devlopoped ML(Mechano-Luminescence) technique enables macro-scale visualization of crack propagation during catastrophic failure, together with transformation zone (crack wake) in both crack front and side areas of Mg-PSZ in realistic time frame. The ML made it possible to precisely detect a relatively fast crack propagating in the speed range from 5 to 15 m/sec, thereby realizing so-called quasi-dynamic R-curve. A systematic observation of growth in wake height, as well as wake length, was made successfully in quasi-dynamic condition. Effective-toughening applied-stress intensity factor increased up to 22 MPa(m)(1/2). The h0 values obtained from the ML observation deviated significantly from that predicted by Evans-McMeeking model(J. Am. Ceram. Soc. 65,242(1982)); rahter it supports Marshall et al.`s results by interference microscopy and Raman spectroscopy (J Am. Ceram. Soc. 73,2659(1990)) in the quasi-static crack propagation. It was also mentioned that conventional R-curves spanning over several millimeters range reported for long-crack systems such as DCB(Double Cantilever Beam), CT(Compact Tension), SENB(Single Edged Notched Bar), and etc. can not be explained by Evans-McMeeking model.

keyword : Mechano-Luminescence, M-L, Paint, Crack Propagation, Transformation (Crack Wake) Zone, Mg-PSZ

Cermet coating by thermal spraying has been a feasible application for the wear resistant coating on the various machine components. Tungsten carbide-based coatings are generally used for the abrasive, sliding, and erosive wear resistance of turbine engine part applications. Carbide coatings are predominantly deposited by high velocity oxy-fuel (HVOF) process in order to avoid the decarburization of the carbides during thermal spraying. Although the HVOF process has much lower temperature and less in-flight time compared with plasma spraying process, decarburization of WC occurs during the HVOF spraying and it causes degradation of wear properties. In this study, in order to minimize the decarburization of WC for WC-10Co-4Cr coatings, HVOF process parameters were optimized using Taguchi`s method for design and analysis of experiment. HVOF process parameters for optimization were the flow rates of gases (oxygen and fuel), powder feeding rate and spraying distance. Decarburization of WC was estimated by comparing the relative height of the main peaks of W2C and WC (I(W2C)/I(WC)). After optimization, decarburization of WC was diminished without considerable deterioration of microstructure.

keyword : HVOF, Taguchi`s method, WC-Co-Cr coating, decarburization

Using electrodeposition, the influence of Re on thermal stability of Ni-P films was studied. Ni-P and Ni-Re-P films were electrodeposited on Cu plates in a sulfamate bath respectively. Electrodeposition was carried out at 1A/dm2 and the film`s thickness was 170nm. The concentration of P and Re in the films was analyzed by WDXRF(Wave Dispersive X-ray Fluorescence Spectrometers) and the concentration was 6wt.% and 10wt.% respectively. DSC(Differential Scanning Calorimetry) and XRD(X-ray diffraction) showed that the crystallization temperature of Ni-Re-P films was 76℃ higher than that of Ni-P films. The results indicated that the thermal stability of Ni-Re-P films was superior to that of Ni-P films.

keyword : diffusion barrier, Cu diffusion, thermal stability, Ni-P electrodeposition, Ni-Re-P electrodeposition

The resistance to pitting and crevice corrosion of various stainless steels was evaluated by measuring critical pitting temperature (CPT), critical crevice temperature (CCT) and critical crevice potential (CCP). CPT values obtained from immersion test were compared with those done by electrochemical test methods. The relationship between CPT and CCT was investigated in terms of PREN (pitting resistance equivalent index). The effect of types of crevice formers on CCT was examined. CCP values for different stainless steels were measured and compared with their CCT values. The properties of passive film obtained by capacitance and abrading electrode measurements were compared with the resistance to corrosion. Results show that CCP and CCT increased with increasing PREN, indicating the linear relationship between CPT and CCT values. Independent of types of crevice former such as block or washer, CCT showed the same values in both cases. The correlation between CCT and CCP showed that CCT linearly increased with increasing PREN while CCP increased up to 40 of PREN, approached the constant values above 40. The higher PREN values resulted in the lower donor density and the closer repassivation rate parameter to 1.

keyword : pitting corrosion, crevice corrosion, critical pitting temperature, CPT, critical crevice temperature, CCT, critical crevice potential, CCP, PREN, capacitance, repassivation

The melting range, electrical conductivity, and microstructure of Cu-Ca alloys and thixoforming process design of developed Cu-Ca alloy were investigated. High purity calcium was added to molten copper up to 1.4% by weight. SIMA(Strain induced melting activated) process has been carried out to modify the copper dendrites to spherical shapes. The semisolid range, grain size and electrical conductivity of Cu-Ca alloy were significantly influenced by the content of calcium and prestrain. Thixoforming experiment of developed Cu-Ca alloy were performed. Efficiency of rotor was negative influenced by internal defects. Thixoformed Cu-Ca rotor shows high efficiency to that of die cast Al rotor by optimization of forming process.

keyword : Thixoforming, STMA process, Cu-Ca alloy, electrical conductivity, rotor

Following the trend of industrial high strength and toughness structural steel manufacturing, it is evolving and increasing needs of basic research and development efforts. One of the examples is, due to the high cost of vanadium, the replacement of Fe-V steel to Fe-Nb steel. However, there still exist difficulties for commercial production of Fe-Nb steel because of poor impact toughness of this steel comparing with Fe-V steel. In this study, a fundamental investigation was carried out to analyze the microstructure, tensile properties and impact toughness of Fe-Nb steel as a function of the rolling conditions with no post heat treatment. The steel samples were prepared with three different compositions, i.e., Fe-V steel (Fe-0.05V-0.001Nb), Fe-V-Nb steel (Fe-0.014V-0.03Nb), Fe-Nb steel(Fe-0.003V-0.033Nb). The steels were deformed with controlled rolling conditions, namely start rolling temperature(SRT) 1150℃ for the all experiment, but two different finish rolling temperature(FRT); 950℃ and 860℃. Ferrite grain size decreased and thus impact toughness was measured higher at FRT of 860℃ than at FRT 950℃ treated steels. Even though the impact toughness of Fe-Nb steel was quite low due to inhomogeneous distribution of ferrite grain size, Fe-V-Nb steel exhibited impact toughness improved with controlled rolling at FRT 860℃. Application of the controlled rolling process showed one of the possibilities to replace Fe-V steel with Fe-V-Nb and/or Fe-Nb steel.

keyword : controlled rolling, Fe-V-Nb steels, hot deformation simulation, microstructure, tensile properties, impact toughness