• Journal of the Korean Society for Heat Treatment
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• v.28 no.3
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• pp.139-145
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• 2015

Effects of austenitizing, cryogenic treatment and tempering conditions on the phase change, microstructure and Vickers hardness value have been studied in STD11 steel for tool and die. The volume fraction of retained austenite increased with a rise in austenitizing temperature, while the volume fraction of eutectoid $M_7C_3$ carbides decreased. The retained austenite could be reduced by cryogenic treatment i.e., maintaining at $LN_2$ temperature ($-196^{\circ}C$) for 12hrs but a little amount of retained austenite did not transform to martensite further although holding time increased to 24 hrs or more. The microstructure of the quenched and then cryogenictreated specimen showed nano-sized and needle-shaped carbides in matrix due to the decomposition of martensite by tempering, but that of the one without cryogenic treatment still revealed retained austenite by tempering even at $500^{\circ}C$.

• Journal of the Korean Society for Heat Treatment
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• v.33 no.2
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• pp.57-64
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• 2020

In the present study, we investigated whether the surface oxidation of C-bearing TWIP steel ℃curs in the air during specimen delivery from an annealing furnace to a water bath and how the microstructure and tensile properties are influenced by surface oxidation. A cold-rolled Fe-18Mn-0.6 (wt%) steel was exposed in the air for 5 s after annealing at various temperatures (750℃, 850℃ and 1000℃) for 10 min in a vacuum, and then water-quenched. For comparison, another specimen, which had been quartz-sealed in a vacuum, was annealed at 1000℃ for 10 min and immediately water-quenched without exposure to air. The 750℃ and 850℃-annealed specimens and the quartz-sealed specimen showed a γ-austenite single phase in the entire specimen due to negligible surface oxidation. However, the 1000℃-annealed specimen exhibited a dual-phase microstructure consisting of ε-martensite and γ-austenite at the sub-surface due to decarburization. Whereas the specimens without decarburization revealed high elongations of 70-80%, the decarburized specimen exhibited a low elongation of ~40%, indicating premature failure due to cracking inside the decarburized layer with ε-martensite and γ-austenite.

• Journal of Welding and Joining
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• v.16 no.1
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• pp.114-124
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• 1998

The effect of matrix phase (austenite, pearlite, martensite) on the low stress abrasion resistance in the chromium-carbide-type high Cr white iorn hardfacing weld deposites has been investigated. In order to examine matrix phase, a series of alloys with different matrix phase by changing the ratio of Cr/C system by heat treatment were employed. The alloys were deposited twice on a mild steel plate using self-shielding flux cored arc welding process. The low stress abrasion resistance of the alloys against sands was measured by the Dry Sand/Rubber Wheel Abrasion Test(RWAT). Even though formation of pearlite phase in the matrix showed higher hardness than that of austenite, there was no observable difference in wear resistance between the pearlite and austenite phase for the same amount of chromium-carbide in the matrix. On the other hand, the formation of martensitic phase,, from heat treated austenitic alloys (high content of Cr), enhanced wear resistance due to its fine secondary precipitates.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.2
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• pp.260-266
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• 2002

DLC films were deposited on Si wafer by RF plasma assisted CVD using CH4 gas. Tribological tests were conducted using rotating type ball on disk friction tester in dry air. Four kinds of mating balls were used. The mating balls were made with stainless steel but apply different annealing conditions to achieve different hardness conditions. Testing results in all load conditions showed that the harder the mating materials, the lower the friction coefficient among the three kind of martensite mating balls. In case of austenite balls, the friction coefficients were lower than fully annealed martensite ball. The high friction coefficient in soft martensite balls seems to be caused by the larger contact area between DLC film and ball. The wear tracks of DLC films and mating balls could have proven that effect. Measuring the wear track of both DLC films and mating balls have similar tendency comparing to the results of friction coefficients. Wear rate of austenite balls were also smaller than that of fully annealed martensite ball. The results of effect of applying load showed, the friction coefficients were become decrease when the applying loads exceed critical load conditions. The wear track of mating balls showed that some material transfer occurs from DLC film to mating ball during the high friction process. Raman spectra analysis showed that transferred material was a kind of graphite and contact surface of DLC film seems to undergo phase transition from carbon to graphite during the high friction process.

• Journal of the Korean Society for Heat Treatment
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• v.17 no.6
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• pp.346-355
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• 2004

The flame quenching process has been employed to modify the surfaces of commercial marine propeller material, aluminum bronze alloy (Cu-8.8Al-5Ni-5Fe), and the microstructure, hardness and wear properties of the flame-quenched layers have been studied. The thermal history was accurately monitored during the process with respect to both the designed maximum surface temperature and holding time. The XRD and EDX analyses have shown that at temperatures above $T_{\beta}$, the microstructure consisting of ${\alpha}+{\kappa}$ phases changed into the ${\alpha}+{\beta}^{\prime}$ martensite due to an eutectoid reaction of ${\alpha}+{\kappa}{\rightarrow}{\beta}$ and a martensitic transformation of ${\beta}{\rightarrow}{\beta}^{\prime}$. The ${\beta}^{\prime}$ martensite phase formed showed a face-centered cubic (FCC) crystal structure with the typical twinned structure. The hardness of the flame-quenched layer having the ${\alpha}+{\beta}^{\prime}$ structure was similar to that of the ${\alpha}+{\kappa}$ structure and depended sensitively on the size and distribution of hard ${\kappa}$ and ${\beta}^{\prime}$ phases with depth from the surface. As a result of the sliding wear test, the wear resistance of the flame-quenched layer was markedly enhanced with the formation of the ${\beta}^{\prime}$ martensite.

• Journal of Welding and Joining
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• v.26 no.3
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• pp.51-60
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• 2008

This paper is to evaluate fracture characteristics of API 2W Gr.50 TMCP steel weldment typically applied for offshore structures, with the focus on the influence of heat input arising from flux cored arc welding. Based on the results and insights developed from this study, it is found that the toughness for both CTOD and impact exhibits a tendency to decrease as the weld heat input increases. The reheated zone of weldmetal exhibit lower hardness than solidified zone and microstructure that are liable to affect the toughness are acicular ferrite and martensite-austenite constituents (M-A). In particular, M-A is a more effective micro-phase for CTOD toughness than impact toughness.

• Journal of the Korean Society for Heat Treatment
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• v.27 no.2
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• pp.79-89
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• 2014

The effect of carbon on the microstructure and texture of low carbon steels was investigated in a series of 1.6 Mn-0.3Cr-0.2Mo-0.001B steels with carbon ranging from 0.021 to 0.048%. Intensity of {111} orientation increased with decreasing the carbon content, resulting in the increase in $r_m$ value. The highest $r_m$ value of 1.30 was obtained in 0.021%C steel annealed at $820{\sim}850^{\circ}C$ according to the typical galvannealing heat cycle. Martensite volume fraction was not substantially affected by the annealing temperature. It was found that the fine and uniformly distributed martensite particles which were present in amounts of about 5% volume fraction were desirable for the highest $r_m$ value. The other factor affecting the high $r_m$ value was the preferred epitaxial growth of retained ferrite with {111} orientation into austenite during cooling.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.178-181
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• 2004

Heat treatment is one of important manufacturing process that determine the quality of the products. Because of a difference of mechanical property by heat treatment, It is necessary to This papers presents flow stress and yield point through tensile test. The goal of this study is to obtain the data of flow stress and yield point at martensite, bainite, ferrite/pearlite phase structure using SCM420, SCr420. The result of tensile test is satisfied and is expected to develop an available FEM analysis.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2003.11a
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• pp.50-55
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• 2003

Tribological testing of DLC films was conducted using a rotating type ball on a disk friction tester in a dry chamber. This study made use of four kinds of mating balls that were made with stainless steel but subjected to diverse annealing conditions in order to achieve different levels of hardness. In all load conditions using martensite mating balls, the test results demonstrated that the friction coefficient was lower when the mating materials were harder. The high friction coefficient found in soft martensite balls appeared to be caused by the larger contact areas. The wear track on the mating balls indicated that a certain amount of material transfer occurs from the DLC film to the mating ball during the high friction process. Raman Spectra analysis showed that the transferred materials were a kind of graphite and that the contact surface of the DLC film seemed to undergo a phase transition from carbon to graphite during the high friction process.

• Corrosion Science and Technology
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• v.17 no.3
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• pp.129-137
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• 2018

The resistance to corrosion of additive manufactured (3D printing) Ti-6Al-4V alloys was investigated using micro-electrochemical tests. In terms of corrosion resistance, the acicular martensitic ${\alpha}^{\prime}$ phase in such additive manufactured Ti-6Al-4V was the focus of attention, and its behavior was distinct from that of conventional subtractive manufactured Ti-6Al-4V. To order to identify ${\alpha}^{\prime}$ phase, XRD tests were performed and micro Vickers hardness was measured for different grains (bright and dark grains) in the additive manufactured Ti-6Al-4V alloy. Micro-electrochemical tests were performed to measure corrosion resistance of bright and dark grains in the additive manufactured Ti-6Al-4V alloy with specially designed electrochemical micro-droplet cell. Critical pitting temperature (CPT) measurement was performed to evaluate the resistance to pitting corrosion of additive manufactured Ti-6Al-4V alloys with different volumes of ${\alpha}^{\prime}$ phase and subtractive manufactured Ti-6Al-4V alloy. The dark grains of the laminated Ti-6Al-4V alloy distributed broader than the bright grains measured with low microhardness. The dark grains of the Ti-6Al-4V alloy, which was rich in martensite ${\alpha}^{\prime}$, had lower general corrosion and pitting resistance than bright grains. As the fraction of martensite ${\alpha}^{\prime}$ phase increased, the resistance to the pitting corrosion decreased.