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During the austenite to ferrite transformation in hypoeutectoid steel, and application of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation without the adoption of the equilibrium ferrite fraction is suggested to describe the isothermal phase transformation in the temperature range between Ae_3 and Ae_1. The suggestion is based on the carbon enrichment in the untransformed austenite during transformation, which can prevent the ferrite fraction getting over the equilibrium fraction. Even under isothermal condition, the additivity concept should be used to take the change of the state variables in JMAK equation into account due to the carbon enrichment. The phase transformation kinetics under various isothermal conditions between Ae_3 and Ae_1 was calculated using the method and the calculated results were compared with the experimental data measured by a transformation dilatometer. The phase transformation for non-isothermal conditions was also described on the basis of the JMAK equation. The austenite to ferrite transformation kinetics of the steels could be well described by the suggested method.

keyword : JMAK equation, Phase transformation, Additivity rule, Carbon enrichment

In this study, various types of drop-weight tear test (DWTT) were conducted on a high-toughness line-pipe steel in order to investigate abnormal fracture appearance occurring in the region impacted by a hammer. A pressed notch or a chevron notch was introduced into DWTT specimens, some of which had a back slot designed to decrease the inverse fracture area. After the DWTT, percent shear area, 85% shear appearance transition temperature, area of inverse fracture surface, hardness and work hardening exponent of the hammer-impacted region were measured. The results indicated that the shear area of the pressed notch DWTT specimens was larger than that of chevron notch DWTT specimens. In the hammer-impacted region of all the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and its area fraction was correlated well with Vickers hardness and work hardening exponent(n) measured by an indentation test. The formation of this inverse cleavage fracture raised the fracture propagation transition temperature by decreasing the shear area measured in accordance of the API 5L3 specification, which was confirmed by relating to the Charpy V-notch test data of pre-strained steel specimens.

keyword : High-toughness pipeline steel, Abnormal fracture appearance, Drop-weight tear test, DWTT, API 5L3 specification, Acicular ferrite

A study was made of the effects of alloying elements on microstructural factors and fracture properties of five high speed steel (HSS) rolls. Particular emphasis was placed on the role of hard carbides located along solidification cell boundary and the type of the martensite matrix. Microstructural observation, X-ray diffraction analysis, hardness and fracture toughness measurement, and fractographic observation were conducted on the rolls. Various types of carbides mostly segregated in the cell-boundary regions were dependent upon the type and amount of alloying elements, and played an important role in hardness and fracture toughness due to their high hardness. The effects of alloying elements were analyzed on the basis of the liquidus surface diagram, and indicated that the proper contents of carbon equivalent, tungsten equivalent, and vanadium were 1.9∼2.0%, 10∼11%, and 5∼6%, respectively. The roll containing a small amount of cell-boundary carbides and lath-type tempered martensite matrix had excellent fracture toughness since carbides were well spaced. For a better microstructure and fracture properties of the HSS rolls, therefore, it was suggested that the optimization of alloying elements was required to achieve the homogeneous distribution of carbides.

keyword : High speed steel roll, Alloying element, Thermal fatigue, Fracture toughness, Carbide, Solidification cell, Martensitic matrix

This is a study of thermal fatigue properties in five high speed steel (HSS) rolls. The thermal fatigue mechanism was investigated with focuses on the roll microstructure and the increase in tensile stress which led the specimens to fracture when it reached to the tensile strength. The thermal fatigue test results indicated that the thermal fatigue life decreased with increasing the maximum temperature of the thermal fatigue cycle. The results were then interpreted based on the amount of carbides and the cyclic softening phenomenon associated with the exposure time to elevated temperature. The coarse intercellular carbides on the specimen surface acted as fatigue crack initiation sites as they cleaved at a low stress level to form cracks. The roll having the lowest matrix hardness and the smallest amount of intercellular carbides thus showed better thermal fatigue properties than the other rolls. For the improvement of the thermal fatigue properties of the rolls, this study suggests a homogeneous distribution of carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing the roll compositions.

keyword : High speed steel roll, Thermal fatigue, Cyclic softening, Intercellular carbide, Solidification cell

Phase field modeling is applied to simulate the scale of solidification microstructure of particular reinforced composite material. Dendritic growth in an undercooled liquid containing randomly distributed second particles has been simulated by using phase field modeling for the first time. The particles ahead of advancing solid/liquid interface disturbed the diffusion field and act as a noise, causing the change in growing direction as well as the occurrence of side branching. The composites show finer scale of solidification microstructure (segregation scale) Than the monolithic alloy. The microstructures of the composites are dependent on the relative scales of primary arm spacing(λ_1) in well-developed dendrite, secondary arm spacing(λ_2) near a growing tip and average distance(λ_p) between particles. In case of λ_p≥λ_1, the scale of segregation in solidification microstructure is mainly controlled by λ_1, while it is controlled by λ_2 in case of λ_p＜λ_2. Also fractal patterns appears when dendrite grows in an undercooled liquid containing secondary particles with λ_p＜λ_1.

keyword : Phase field model, Dendritic growth, Composites, Solidification

Fabrication method of aluminum nitride reinforced aluminum composites using gas-melt in-situ reaction was investigated. AA 5083 aluminum alloy was melted in vacuum and nitrogen atmosphere, and then nitrogen gas bearing 12.5% ammonia gas was injected in the molten alloy through alumina and graphite tube for 4 hours at the temperature of 1100℃. The melt was divided into the upper and lower parts. There were many AlN particles in the upper part and no AlN in the lower part. In order to mix the upper and lower part, an impeller was introduced to the melt. There was also two parts with decreased thickness of the upper part and small amount of AlN in the lower part. Formation of AlN has been identified by optical and transmission electron microscopy, X-ray diffraction, and electrolytic extraction method. AlN was evolved in the form of a cluster with hexagonal particles of 1 μm and a rod of about 3 μm length.

keyword : A1N, Aluminum alloys, in-situ reaction, Nitrogen, Ammonia, Gas injection

In this study, the heating unit of direct-heating method was manufactured as being the thermal spray coating of conductive heating material on the surface of heating unit in order to improve the demerits of indirect-heating method. And Cu-Zu ferrite had the properties of thermal conduction and high electrical resistivity was chosen of the conductive heating material. Cu-Zu ferrite composite powders were fabricated by drum type ball milling. The atmospheric plasma spray(APS) process was used to deposit the coatings of Ni-5.5Al-5Mo powders for the bonding layer, Al_2O_3 powders for the electrical insulating layer and Cu-Zn ferrite composite powders for the electrical heating layer onto the Al substrate. The plasma sprayed coatings were performed the heat-treatment at 300℃, 405℃ and 600℃. In order to evaluate the plasma sprayed coatings performance, the coating powder observated microstructure, XRD analysis and EDX mapping analysis and mechanical and electrical properties of plasma sprayed coatings. In the result of observations of microstructures, mechanical properties and electrical properties, the best sprayed coatings with direct heating unit was concluded the sprayed coatings that was heat-treated at 300℃.

keyword : Cu-Zn ferrite, Plasma spray, Electrical resistivity, Heating unit, Composite powder, Ball milling

CERTs (Constant elongation rate tests) using hump type specimens were carried out to investigate the effect of "stress state" in the specimens on the SCC (stress corrosion cracking) behavior of alloy 600 in simulated environment of nuclear power plant (360℃, 200 atm). SCC susceptibility was increased in the hump type specimens compared with usual tensile specimens. Micro-structural observation, hardness measurements and stress analysis around the bended area of hump type specimens were conducted to understand the increase of SCC susceptibility in the specimens. It is considered that the acceleration of SCC in the hump type specimens is attributed to the triaxial stress state and the consequent plastic constraint around bended area in the specimens. CERT using hump type specimen is useful to evaluate the stress corrosion susceptibility of various corrosion system in short testing period.

keyword : Hump specimens, Alloy 600, Stress corrosion, Stress analysis