• Analytical Science and Technology
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• v.9 no.2
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• pp.179-191
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• 1996

In order to understand the high temperature deformation behavior of superalloy 718, a rotating grade 718 alloy has been compression tested to about 0.7 upset ratio at $927{\sim}1066^{\circ}C$ temperature range and $5{\times}10^{-4}{\sim}5{\times}10^0sec^{-1}$ strain rate. The maximum flow stress was increased with increasing strain rate, and similar behavior was observed with decreasing temperature. At low temperature and high strain rates other than $5{\times}10^{-1}sec^{-1}$, strain softening was occurred mainly by dynamic recovery and deformation twinning processes, while at high temperature and low strain rates strain softening was offseted by dynamic recrystallization. At $5{\times}10^{-1}sec^{-1}$, strain hardening was occurred due to work hardening of the dynamic recrystallized grains. Strain rate sensitivity, m, was varied with strain rates. In the case of lower strain rate tests, m was measured as 0.3 and it was observed that the deformation was mainly controlled by dynamic recrystallization. At higher strain rate, m was lowered to 0.1 and the deformation was controlled by the dynamic recovery and the deformation twinning processes.

• Journal of Welding and Joining
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• v.26 no.5
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• pp.90-96
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• 2008

The objective of this paper is to investigate the effects of the laser power, the material thickness, comer angles, and the loop size on the formation of the comer in the cutting of Inconel 718 super-alloy sheet using high-power CW Nd: YAG laser. In order to investigate the influence of comer angles and loop sizes on the melted area and the formation of comer in the sharp comer, angular cutting tests and loop cutting tests were carried out. The results of the angular cutting tests were shown that the melted area is minimized and the melting mode is changed from nose melting in the thickness direction to the secondary melting induced by the attached dross when the comer angle is $90^{\circ}$. Through the results of loop cutting tests, the variation of the melted area and the comer shape in the sharp comer according to the loop size were examined. In addition, it was shown that a proper loop size is approximately 3 mm. The results of above experiments will be reflected on the knowledge base to generate optimal cutting path of the laser.

• Journal of the Korean Society of Industry Convergence
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• v.22 no.5
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• pp.491-497
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• 2019

Dissimilar friction welding were produced using 15 mm diameter solid bar in superalloy(Alloy718) to chrome molybdenum steel(SCM440) to investigate their mechanical properties. Consequently, optimal welding conditions were n=2000 rpm, HP=60 MPa, UP=120 MPa, HT=10 sec and UT=10 sec when the metal loss(Mo) is 3.5 mm. Acoustic Emission(AE) technique was applied to analyze the dissimilar friction welding of Alloy718 and SCM440. The relationship between the AE parameters and dissimilar friction welding of both material was discussed. In the case of heating time of 6 sec, 10 sec, 14 sec and 20 sec, 5 AE events per 0.5 seconds and energy about $2.7{\times}10^{10}$ were exhibited in heating time. In upsetting time, resulting in various numbers of events per second and very low energy. The frequency range of the signal generated during the heating time was about 200 kHz. However, the upsetting time resulted in a wide range of signals from very low frequency to high frequency of 500 kHz due to rapid plasticity of the material.

• Journal of the Korean Society for Heat Treatment
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• v.35 no.4
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• pp.203-210
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• 2022

In this study, hydrogen embrittlement in Inconel 718 fabricated by selective laser melting (SLM) was investigated. To focus on the effect of the SLM-induced solidification substructure, hydrogen embrittlement behavior of SLM as-built (SLM-AB) sample and that of conventionally produced (Con-S) sample were systematically compared. The detailed microstructural characterization showed that the SLM-AB sample exhibited a solidification substructure including a high density of dislocations and Laves phase, while the Con-S sample showed completely recrystallized grains without any substructure. Although the intrinsic strength in the SLM-AB sample was higher than the Con-S sample, the resistance to hydrogen embrittlement was higher in the SLM-AB sample. Nevertheless, a statistical analysis of the hydrogen-assisted cracks (HACs) revealed that the predominant HAC type of SLM-AB and Con-S samples was similar, i.e., intergranular HAC. The difference in the resistance to hydrogen embrittlement between the SLM-AB and Con-S samples were discussed in terms of the relation between the microstructural feature and its effect on hydrogen accumulation.

• Journal of Advanced Marine Engineering and Technology
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• v.38 no.4
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• pp.409-417
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• 2014

Since the oil price has been significantly jumped for recent some years, the diesel engine of the merchant ship has been mainly used the heavy oil of low quality. Thus, it has been often exposed to severely corrosive environment more and more because temperature of the exhaust gas in a combustion chamber is getting higher and higher with increasing of using the heavy oil of low quality. As a result, wear and corrosion of most parts surrounded with combustion chamber is more serious compared to the other parts of the engine. Therefore, an optimum weldment for these parts is very important to prolong their lifetime in a economical point of view. In this study, four types of filler metals such as Inconel 625, 718, 1.25Cr-0.5Mo and 0.5Mo were welded with SMAW and GTAW methods in the forged steel which would be generally used with piston crown material. And the corrosion properties of weld metal, heat affected zone and base metal were investigated using electrochemical methods such as measurement of corrosion potential, anodic polarization curves, cyclic voltammogram and impedance etc. in 35% H2SO4 solution. The weld metal and base metal exhibited the best and worst corrosion resistance in all cases of filler metals. In particular, the weld metal welded with filler metals of Inconel 718 revealed the best corrosion resistance among the filler metals, and Inconel 625 followed the Inconel 718. Hardness relatively indicated higher value in the weld metal compared to the base metal. Furthermore, Inconel 625 and 718 indicated higher values of hardness compared to 1.25cr-0.5Mo and 0,5Mo filler metals in the weld metal.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.570-576
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• 2002

Application of the aged Inconel 718 in hydrogen environment is seriously restraint by its high hydrogen embrittlement (HE) sensitivity. m previous researches, we have suggested the possibility and applicability of the laser surface annealing (LSA) process in improving the HE resistance of this alloy. Sequentially, a study on the effects of the precipitates in the Inconel 718 on its HE sensitivity was conducted in this research. Firstly, flat bar specimens were heat-treated to obtain various kinds of precipitation microstructures concerning the ${\gamma}$" phase and the 6 phase. Hydrogen was charged into the specimen by a cathodic charging process. The loss in reduction of area (RA) caused by hydrogen charging was used to assess the HE sensitivity. The HE sensitivity of the alloy was lowered with decreasing the volume fraction of ${\gamma}$". Moreover, it was possible to increase the HE resistance of the aged alloy by dissolving the $\delta$ phase, keeping the strength at the same level as that of the common aged alloy. Thus, we concluded that both the $\delta$ phase and the ${\gamma}$" phase affected the HE sensitivity of Inconel 718. Next, two kinds of notch tensile specimens were fabricated, one kind having $\delta$ phase and the other having no $\delta$ phase. All these specimens were aged via the same aging heat treatment process. The LSA process annealed a thin layer of the notch bottom of each specimen. One specimen of each kind was charged with hydrogen by the cathodic hydrogen charging process. Loss in the notch tensile strength (NTS) caused by hydrogen was used to evaluate the HE sensitivity. It was found that while the HE sensitivity of conventionally aged Inconel 718 was decreased by the LSA process, the HE sensitivity of the $\delta$-free aged Inconel 718 could further be decreased. Therefore, for applications in hydrogen environments, it is possible to fabricate alloys with both good HE resistance and high strength by controlling the precipitation conditions, and to improve HE resistance further via applying the LSA process.

• Transactions of Materials Processing
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• v.17 no.7
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• pp.523-527
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• 2008

In this study, mechanical properties of Alloy 718 welded after forgings for jet propulsion component was investigated. Hot-forged and machined work-pieces($230mm\times70mm\times15mm$) which have different grain sizes are welded by electron beam welding technique. After welding, the components were solution heat-treated and aged. Samples were sectioned to analyze the microstructural evolution and formation of micro-crack. It was found that HAZ grain boundary liquation crack generally initiates in the coarse grains rather than the fine grains. Needle-like phases with high Nb contents were found at the outer part near the base metal. Vickers hardness and tensile tests were carried out at room temperature and at $649^{\circ}C$. The tensile properties of electron beam welding specimens exhibited around 100MPa and 10% decrease in strength and elongation, respectively.

• Transactions of Materials Processing
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• v.17 no.3
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• pp.189-196
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• 2008

Alloy718 is the nickel-base super alloy well used as gas turbine components under severe operating conditions because of its high strength at high temperature and excellent creep resistance. In this study, a coupling bolt for the gas turbine component is manufactured by hot heading process instead of whole machining in order to improve the mechanical properties. Die shape for the hot heading has been designed by general design rule of hot forging and also optimal process condition has been investigated by finite element method. The initial billet temperature and the punch speed have been determined by $1150^{\circ}C$ and 600mm/s on the basis of finite element analysis, respectively. The coupling bolt has been manufactured by 200ton screw press and evaluated by experiment in order to investigate the mechanical properties. As a result of experiment, the mechanical properties such as hardness, tensile strength and creep behavior have been superior to those manufactured by machining.

• Journal of the Korean Society for Heat Treatment
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• v.5 no.2
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• pp.85-94
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• 1992

Microstructural evolution of wrought Inconel 718 superalloy with different heat treatment conditions was studied. Heat treatment was performed via conventional(CHT), modified(MHT), Merrick(MeHT) and modified Merrick (MMeHT) methods. The size of ${\gamma}^{\prime}$ and ${\gamma}^{\prime\prime}$ precipitates which are principal strengthening phases in Inconel 718 superalloy increase in order of CHT, MHT, MeHT. For the case of MMeHT, a coexistence of fine ${\gamma}^{\prime\prime}$ precipitate and very coarse particles due to exess growth of ${\gamma}^{\prime\prime}$, which is called bimodal distribution, was observed. CHT gave the finest grain size. (Ti, Nb)C carbide and needle-like ${\delta}$ phase were formed together at grain boundaries for CHT, and were formed both inside and at boundaries of grains for MHT, MeHT and MMeHT. Morphology of partially serrated grain boundaries was developed in all heat treatment conditions except CHT.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2008.11a
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• pp.11-12
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• 2008

Micron-sized Co-alloy T800 powder was coated on Inconel718 (IN718) using high velocity oxygen fuel (HVOF) thermal spraying by the optimal coating process (OCP) determined from the best surface hardness of 16 coatings prepared by Taguchi program. The surface hardness improved 140-160 % from 399 Hv of IN718 to 560-630 Hv by the coating. Porosity of the coating was 1.0-2.7 %, strongly depending on spray parameters. Both friction coefficients (FC) and wear traces (WT) of the coating were smaller than those of IN718 substrate at both $25^{\circ}C$ and $538^{\circ}C$. FC and WT of IN718 and coating decreased with increasing the surface temperature. Tensile bond strength (TBS) and fracture location (FL) of Ti64/T800 were 8,770 psi and near middle of T800 coating respectively. TBS and FL of Ti64/NiCr/T800 were 8,740 psi and near middle of T800 coating respectively. This showed that cohesion of T800 coating was 8,740-8,770 psi, and adhesion of T800 on Ti64 and NiCr was stronger than the cohesion of T800.