발간논문

Home > KJMM 논문 > 발간논문

This study examined the degree and the avoidable conditions of the intergranular sensitization on the commercial stainless steel(STS 304, 304L, 316, 316L) in the temperature range of 450℃ to 850℃. The electrochemical potentiokinetic reactivation test was used in detecting the degree of sensitization of STS 304, 304, 316 and 316L in an electrolyte which contained sulfuric acid with KSCN as an additive. On the isothermal sensitization process, the chief variables choosed the sensitizing temperatures (580, 650, 750, 800℃) and time of the isothermal hold (0.5, 1.0, 10, 24, 72 hr). In the case of isothermal heat treatment, the degree of sensitization was increased with the heat treating time as 580℃ and 650℃. but the degree of sensitization was derreased by long heat treating time at 730℃ and 800℃, because of a self-healing of the chromiume depleted zone. The degree of sensitization was reduced as the carbon content in the austenitic stainless steel decreased. The molybdenum in the austenitic stainless s eel increased the intergranular corrosion resistance. As continuously cooled from 1050℃ to 450℃, STS 304, 304L, and 316 were not sensitized at cooling rates faster than 5, 0.4, and 0.25℃/sec, respectively. as continuously cooled from different annealing temperature under constant cooling rate, the degree of sensitization was extremely served at 900℃ but decreased above 950℃.

keyword :

A mathematical model is proposed to describe time dependent pressure, relative density and temperature relations of metal powders during hot compaction. Experimental data are obtained from hot compaction of iron powder. The model is based partly on micromechanical (hollow sphere) analysis and partly on phenomenological analysis. At constant temperature, the pressure and densification measure relations for the instantaneous and equilibrium responses are described by linear response laws. A time dependent densification of metal powders under constant temperature and pressure is described by a power law creep response. The experimental data support the assumed linear response laws and the power law creep response. Agreement between theoretical curves and experimental data is very good.

keyword :

Submicron iron powders were produced by reducing ferric chloride(FeCl₃) with hydrogen gas. The dependence of reduction rate on the various parameters were inves-tigated by measuring the conductivity of water through which the reaction gas was passed out. The reduction rate increased with increasing the temperature up to 900℃ and increas-ing both the FeCl₃ partial pressure and H₂: Ar gas ratio up to 3 : 1 The particle size and distribution were affected by all of the above three parameters.

keyword :

The response of a martensitic stainless steel, AISI 403, to stress corrosion cracking (SCC) under Mode Ⅰ and Mode Ⅲ loading has been investigated. SCC tests were conducted in 3.5 % NaCl solution using precracked specimens, tempered at 390℃, 520℃ and 610℃. Average crack propagation rates at various stress intensity levels were measured and compared for each mode. Experimental evidence indicates that Mode Ⅰloading exhibits higher crack propagation rate or stronger susceptibility to SCC than Mode Ⅲ loading under all the tempering conditions, which suggests that hydrogen embrittlement is involved in SCC. Furthermore, fractographic study reveals that SCC under both of the loading mode occurs on the notch plane with a predominant mode of intergranular fracture in all of the aforementioned tempering conditions.

keyword :

The effects of Nb addition and annealing on the magnetic properties were in vestigated for (Fe_(0.82)Ni_(0.18-x)Nb_x)_(83)Si_5B_(12) amorphous alloys which have law metalloid content. In this study, the amorphous nature of alloys prepared by single roll method was confirmed by X-ray diffraction method. Magnetic field annealing was performed at various temperatures below the crystalliztion temperature for 20 minutes in N₂gas atmosphere. With increasing Nb content, the thermal stability and both AC and DC magnetic properties were improved. Longitudinal magnetic field annealing also greatly improved the DC and 60 Hz characteristics. These improvements were attributed to the reduction of domain width by Nb addition, the induction of uniaxial magnetic anisotropy, and disappearance of Maze domains by magnetic field annealing. Transverse magnetic field annealing found to be very effective for the improvement of high frequency magnetic properties.

keyword :

Mechanica1 properties and iodine-stress-corrosion-cracking resistance of Zircaloy-4 sheet processed by cross rolling and heat treatment were studied to improve the materials quality for nuclear fuel cladding application. The stress-corrosion-cracking tests were conducted at 300℃ under gaseous iodine atmosphere in which the rate of crack propagation as a function of stress intensity factor was determined. The room temperature yield strength, elongation and the critical stress intensity factor under the 300℃ iodine atmosphere varied sensitively with the methods of rolling and heat treatment, which was related to the distribution characteristies of basal poles. The density of basal poles along the sheet normal increased by a cross rolling followed by beta-phase heat treatment and furnace cooling, which resulted in an improvement in the stress-corrosion-cracking resistance of the alloy. The appearance of stress-corrosion -cracking fracture surface was predominantly brittle, in which transgranular fracture as dominant when a beta-phase heat treatment followed by furnace cooling was conducted, whereas intergranular fracture was dominant when the finishing process was either alpha-phase recrystallization heat treatment or cross rolling.

keyword :

This study investigated the effects of various factors on the oxidation reactions of arsenic trioxide in the nitric acid solution. The oxidation rate of As₂O₃was increased with the addition of hydrochloric acid as a catalyst and was not affected by stirring of the solution. The most favorable concentration of HNO₃for oxidation of As₂O₃was found to be 12N, and no oxidation reaction occurred below 4N. The acidity of this reaction involves both homogeneous and heterogenous reactions, and the initial reaction rate was obtained as a following empirical equation at 7N HNO₃, 0.4g/L HCI and 75℃. (수식)

keyword :

The effect of heat treatments and cold reduction on the martensitic transformation and pseudoelasticity behavior in Ti-Ni-Al alloys has been investigated by means of electrical resistance measurement, tensile test, optical and transmission electron micros copy. The transformation temperatures have linearly decreased with the increase of Al content and R-phases have been formed along the grain boundary. The thermal hysteresis of R-phase transition has been hardly affected by cold reduction, but that of martensitic transformation has been remarkably increased. The critical stress for the first stags in Ti_(50)Ni_(49)Al_(1.0) alloy is almost costant (40∼50 Mpa) irrespective of cold reduction, but that of stress induced martensite is considerably increased.

keyword :

The purpose of this study is to clarify the critical factor controlling the HAZ toughness of high strength steel. The HAZ toughness has been known to be directly associated with the characteristics of local brittle zone (LBZ) having lowest toughness within the coarse grained HAZ. The microstructural sources for LBZ, identified in this study as the intercritically reheated coarse grained HAZ, were confirmed to be the presence of MA constituent formed in the coarse grained microstructure. The amount of the M-A constituent, which controls the HAZ toughness. is strong1y dependent on both the conditions of weld thermal cycle and the chemical composition of base metal. The amount of the MA constituent formed in the TMCP steel decreases significantly with increasing the cooling time, while that in the normalized steel does not. This fact can explain the effect of welding heat input on the HAZ toughness of TMCP steel and normalized steel reported in Part l.

keyword :

Effects of intercritical annealing on tensile properties of Fe-C-1.5 Si steels were investigated ; changes of the properties were interpreted refer to the microstructural changes due to microalloying of 0.5Mo-0.015Ti and 0.069Al and isothermal bainite transformation at 400-440℃. Increases of carbon content and the bainitic transformation temperature increased the fractional retained austenite and the volume fraction of martensite, respectively. Best tensile properties were obtained when the steels with 0.069 % Al were isothermally transformed at 420℃ after intercritical annealing at 770℃.

keyword :