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The substitution of carbon by nitrogen was employed to prevent center segregation of carbides during continuous casting of 13%Cr martensitic stainless steel. Characteristics of phase transformation and precipitation behavior have been studied. The substitution of carbon by nitrogen in 13%Cr steel increases its solidus, Ac_1, and Ms temperatures. The nose temperature of TTT diagram of high carbon steel was 675-700℃ while the one of high nitrogen steel was about 725℃. Maximum hardness can be obtained by austenitizing treatment at 1050-1100℃ for the high carbon steel and 1000℃ for the nitrogen added steel. The secondary hardening was maximized by tempering at 500℃. It is due to the precipitation of fine carbide and M_2N nitride. The hardness was drastically decreased as tempered at 550℃ by precipitation of M_23C_6 carbide at prior austenite grain boundaries.

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Microstructure and mechanical properties of 0.2%C steels tempered employing induction heating method were investigated in this study. Effect of alloying elements on tempered microstructure with a variation of tempering temperature was examined. Tensile properties and impact strength of tempered steels were estimated to compare steels with and without alloying elements. The evolution of microstructure of tempered steels revealed the precipitation of acicular carbides with particular orientations and the precipitation and growth of spherical cementite particles with increasing tempering temperature. The process of the microstructural evolution was found to be delayed by addition of alloying elements Cr and Mo. Yield and tensile strength decreased with increasing tempering temperature. Reduction of area and total elongation increased with increasing tempering temperature. Impact strength increased with increasing tempering temperature. All these mechanical properties were closely related to the microstructural evolution of the steels. It was concluded that proper mechanical properties could be obtained for carbon steels without addition of alloying element by controlling tempering temperature using induction heating method.

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The macroscopic texture and r-value (width-to-thickness plastic strain ratio in uniaxial tension) anisotropy in Ti-added high strength IF steels were investigated. In order to consider the effect of carbon on r-value anisotropy in the steels, metallurgical parameters (grain size, precipitates and texture) in the hot-rolled, cold-rolled and annealed specimens were investigated. Initial grain size in the hot band and distribution of precipitates in the annealed specimen were main metallurgical factors influencing the macroscopic annealing texture and r-value anisotropy in the Ti-added high strength IF steels. Predictions of r-value anisotropy were carried out using a full-constraints Taylor polycrystal model. The predicted r-value anisotropy were in good agreement with experimental data.

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The present study aims at interpreting the effect of grain size on fracture toughness in the transition region of Mn-Mo-Ni low-alloy steels. Three kinds of steels with different austenite grain sizes were fabricated, and their microstructures and mechanical properties were examined. Elastic-plastic cleavage fracture toughness,K_Jc, was determined by 3-point bend tests of precracked Charpy V-notch (PCVN) specimens according to ASTM E1921 standard test method. When the austenite grain size decreased, the total number of carbides increased, while the size and the aspect ratio of carbides decreased. Local fracture stresses, estimated from a theoretical stress distribution in front of a crack tip, were found to be mainly determined by the 92%th size of carbides. Cross-sectional areas beneath fracture surfaces were observed to understand microstructural features to affect the cleavage crack propagation behavior. The results showedd that measured cleavage fracture units were smaller than austenite grain sizes, indicating that packet boundaries as well as austenite grain boundaries played an important role in the cleavage crack propagation. Based on the electron back-scatter diffraction (EBSD) results, the cleavage fracture units could also be matched with the effective grain sizes determined by the misorientation tolerance angle of 25˚.

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Ti-47A1-2W alloys, which have been reported as having alloy composition of β-solidification, were directionally solidified by using Bridgman and floating zone type directional solidification(DS) apparatuses. Mechanical properties of DS ingots were evaluated by room temperature and high temperature (800℃) tensile tests. In the results of DS by the floating zone type apparatus at a growth rate of 30 ㎜/h, the lamellar microstructure was well-aligned parallel to the growth direction. Ingots having lamellar orientation parallel to the growth direction showed excellent room temperature tensile ductility. High temperature tensile yield strength was maintained similarly to the yield strength at room temperature. The texture of γ lamellar laths was identified from the results of electron back scattered diffraction pattern (EBSD) analysis of the lamellar orientation of DS ingots, which were manufactured by a floating zone furnace. On the basis of the EBSD results, it was estimated that the preferred growth directions of bcc-β dendrite grown at high temperature near the melting point was in the [001]_β direction at a growth rate of 30 ㎜/h and the [111]_β direction at a growth rate of 90 ㎜/h.

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This paper presents the results from the studies on the surface layers produced by ion nitriding at three different temperatures in order to improve wear resistance, hardness, and to modulate microstructure and chemical composition of surface layers. X-ray diffraction indicated the formation of two phases(TiN, Ti_2N) and the preferred orientations of TiN occured when nitrided at 810℃. Ion nitrided surface had the Vickers hardness of 860 Hv and greatly improved the wear resistance of Ti-6A1-4V. The cytotoxicity was measured with NIH 3T3 fibroblast by SRB method and was compared with that of the FDA approved Ti-6A1-4V. Corrosion resistance and cytotoxicity was similar to untreated Ti-6A1-4V alloy.

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The objective of this study is to investigate the correlation between microstructure and the wear resistance of various ferrous blend coatings applicable to cylinder bores. Seven types of ferrous spray powders, two of which were stainless steel powders and the others were blend powders of ferrous powders mixed with Al_2O_3-ZrO_2 powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coatings showed that iron oxides were formed in the austenitic (or martensitic) matrix by oxidation during spraying, while Al_2O_3-ZrO_2 oxides were mainly formed in the matrix for the blend coatings. The wear test results revealed that the blend coatings showed the better wear resistance than the ferrous coatings because they contained a number of hard Al_2O_3-ZrO_2 oxides. However, they had rough worn surfaces because of the preferential removal of the matrix and the cracking of oxides during wear. The STS 316 coating showed the slightly worse wear resistance than the blend coatings, but they showed the excellent wear resistance to a counterpart material resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In order to improve the overall wear properties with consideration of the wear resistance of a counterpart material, the hardness difference between oxides and matrix should be minimized, while the hardness should be maintained up to a certain level by forming an appropriate amount of oxides.

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An innovative, electrochemical accelerated test has been developed that can predict long-term corrosion behavior. Immersion test in substitute ocean water for 90 days was performed to examine validity for this accelerated aging technique. As a result, the technique induced formation of corrosion product and weight loss within 24 hours that were similar to those obtained naturally from immersion test for 90 days. This indicates that short-term accelerated aging test was in good qualitative and quantitative agreement with long-term immersion test. X-ray diffraction [XRD] analyses revealed that corrosion product formed in substitute ocean water was mainly composed of magnetite[Fe_3O_4]. Based on this experimental result, the lifetime of structural steel submerged in seawater for 60 years was predicted using the accelerated aging technique.

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To investigate the effects of Nb content and annealing parameter on corrosion resistance in Zr-0.8Sn-0.4Fe-0.2Cr-xNb(x=0.1, 0.2, 0.4, 0.8%) alloys, corrosion tests have been carried out under the 400℃ steam condition. The corrosion resistance of the alloys decreased with increasing the Nb content and the accumulated annealing parameter. But the Nb content in precipitates and the hydrogen pick-up fraction in matrix increased when the adding Nb content and the accumulated annealing parameter increased. The fraction of t-ZrO_2 in oxide layers decreased with increasing the accumulated annealing parameter. It was concluded that the corrosion resistance of Zr-0.8Sn-0.4Fe-0.2Cr-xNb alloys could be controlled by the characteristics of (Zr, Nb)(Fe, Cr)_2 type precipitates with the variation of the Nb content and the accumulated annealing parameter.

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Alloys of Ti-47%Al and Ti-47%Al-1%Mn were oxidized isothermally and cyclically between 800 and 1000℃, and their oxidation characteristics were studied. Compared with Ti-47%Al, Ti-47%Al-1%Mn displayed better oxidation resistance at 800℃, slightly better oxidation resistance at 900℃, and worse oxidation resistance at 1000℃, though the scale adherence was better. Manganese tended to present at the lower part of the oxide scale which consisted primarily of an outer TiO_2 layer, and intermediate Al_2O_3-rich layer, and an inner mixed layer of (TiO_2+Al_2O_3).

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