발간논문

Home > KJMM 논문 > 발간논문

The experimental results for the fabrication of B₄C powder by Self-propagating high temperature synthesis are summerized as follows : (1) Even if theoretical adiabatic temperature of this reaction is a high, the self-combustion on the system was impossible. But the combustion was possible by another combustion process, a chemical furnace. (2) The reaction parameter to influence greatly in synthesis of B₄C powder is mole ratio of each elements. Experimental results show that the optimum ratio of B₂O₃, Mg and C is 2.0, 6.0∼7.0 and 1.0∼1.4 respectively. (3) The produced B₄C powder have a spherical shape and the size of production particle depends on the applied pressure. The higher applied pressure becomes, the smaller particle size is. (4) Since the impurity is evaporated by high temperature reaction, the purity of production powder is higher than that of reactants.

keyword :

The tempered martensite embrittlement (TME) has been analyzed in terms of impact toughness, fracture behavior, and microstructures in the 4140 steel and the 4340 steel with the Ni addition. On isothermal tempering, THE was detected by the continuous decreases in impact toughness, at 200 and 250℃ for 1-400 h. On isochronal tempering for 1 h, the THE troughs with the minimum at 300℃ in the 4140 steel and at 350℃ in the 4340 steel were observed. The presence of coarse carbides at the impurity-weakened grain boundaries led to the intergranular TME. As the impact toughness decreased in the TME region, the intergranular area extended. The intergranular fracture was mostly observed at the low level near or below about 5 J. THE in the 4340 steel was presented by a slower process, as compared to the 4140 steel. This may be caused by the delay of the formation of coarse boundary-carbides and by the high intrinsic toughness with the Ni-addition, requiring the coarser boundary-carbides for the activation of brittle intergranular fracture.

keyword :

The squeeze casting process is well known to yield superior cast products to those from gravity casting. New alloy systems of Al-Si-Cu-Mg which possess good castability and excellent mechanical properties have been examined to optimize cast alloy compositions. The alloy composition of Al-12wt%Si-3wt%Cu-0.7wt%Mg with and without Sr, Ti and B modifier was investigated in this study using brinell hardness, tensile strength and elongation. Tensile strength of 39 to 41 ㎏/㎟ and elongation of 1.7% to 2.5% was obtained from this squeeze cast alloy under T6 heat treament condition. These properties represent 10∼20% improvement over those from gravity cast products. Modification with Sr, Ti and B on this alloy resulted in 40% increase of elongation, although no significant change in wear resistance was found. Compared to AC8A and AC4CH alloys, Wear losses of this newly designed Al-Si-Cu-Mg alloy were reduced to 1/2 to 1/3, Abrasive wear tracks were mainly observed in this alloy, whereas both abrasive and adhesive wear tracks were found in conventional cast alloys.

keyword :

In order to study the effect of alloying elements on the spheroidization of lamellar pearlite, quantitative microscopic investigation has been undergone with the four kinds of steel (Fe-C, Fe-C-0.3wt%M; M=Ni, Mn, Cr) by annealing at 700℃. It was found that the rate of spheroidization in described by a cubic law;(1/S)³-(1/S_o)³=K_s·t, where S is the total area of the interface between ferrite and cementite per unit volume, S_o is the initial value and K_s is the rate constant. The K_s is approximated by K_s=K_o/(3f)³, were K_o is the rate constant in Ostwald ripening of cementite particles and f is the volume fraction of cementite. In addition, the fraction of spheroidization X can be described by X=(S_o―S)/S_o for the effect of alloying elements, it was shown that the small additions of Cr, Mn remarkably retared the spheroidization mainly due to the large distribution coefficient between the ferrite and cementite.

keyword :

The solidification behavior of steel in continuous casting billet mold was examined by plant test, i. e. tracer(FeS) test in mold and solidification modelling. From the above test, it has been found that solid shell grows linearly with time regardless of steel grades in billet mold. Solidification constant k was found to be 32-37㎜/min for low carbon (C%=0.07-0.19) steel, 41-44㎜/min for high carbon(C%=0.40-0.73) steel, respectively. Unevenness of solid shell along the transversal and longitudinal direction for the low carbon steel grade which solidifies as δ ferrite initially was greater than that of high carbon steel grade. When applied an in-mold electro-magnetic stirring(MEMS), solid shell formed uniformly in thickness direction. Finally, measured solid shell profiles in mold were found to agree well with the calculated results from the solidification modelling.

keyword :

In load-controlled tensile tests at 4K, an abrupt and large discontinuous yielding behavior occurs, which remarkably differs from the discontinuous yielding observed in displacement-controlled tests. The tensile properties under a load control mode may, therefore, be different from that obtained under a displacement control mode. A series of tensile tests has been performed in this regard using 304 and 316 metastable austenitic stainless steels at 4K under a load control mode to examine the effect of specimen size and loading rate on the discontinuous yielding behavior. Four different stress rates ranging from 0.154 to 154 MPa/s were used, while the diameters of tensile specimens were 4.5, 6.35 and 8㎜. Both the stress rate and the specimen diameter were found not to affect the yield strength. However, the stress and strain at the onset of discontinuous yielding decreased as the stress rate and specimen diameter increased. The discontinuous yielding behavior under a load control mode was then compared with that under a displacement control mode.

keyword :