• Korean Journal of Metals and Materials
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• v.50 no.7
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• pp.487-493
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• 2012

To analyze the factors on stretch-flangeability for 980 MPa-grade hot-rolled steels, two types of steels (Fe-Cr and Fe-Mo) were manufactured by hot-rolling. Manufactured steels at the low coiling temperature, such as 400 and $500^{\circ}C$, had poor stretch-flangeability due to un-uniformly distributed carbides and a large deviation of interphase hardness. However, when the coiling temperature was set at $650^{\circ}C$ with Fe-Cr steel, 998 MPa of ultimate tensile strength, 19% of total elongation and 65% of the hole expanding ratio were achieved by microstructural constituents of polygonal ferrite (PF) and granular ferrite (GF) dispersed with fine carbides (<50 nm). Therefore, the material to attain 980 MPa with superior formability was the Fe-Cr steel that was precipitation-hardened in polygonal ferrite and granular ferrite at the coiling temperature $650^{\circ}C$.

• Korean Journal of Materials Research
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• v.31 no.7
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• pp.409-419
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• 2021

The effect of solidification rate on micro-segregation in investment casting of IN738LC superalloy was studied. In Ni-based superalloys, the micro-segregation of solute atoms is formed due to limited diffusion during cast and solidification. The microstructure of cast Ni-based superalloys is largely divided into dendrite core of initial solidification and interdendrite of final solidification. In particular, mosaic shaped eutectic γ/γ' and carbides are formed in the interdendrite of the final solidification region in some cases. The micro-segregation phenomena formed in regions of dendrite core and interdendrite including eutectic γ/γ' and carbides were analyzed using OM, SEM/EDS and micro Vickers hardness. As a result of analysis, the lack of (Cr, W) and the accumulation of Ti were measured in the eutectic γ/γ', and the accumulation of (Cr, Mo) and the lack of Ti were measured in the interdendrite between dendrite and eutectic. Carbides formed in interdendritic region were composed of (Ti, W, Mo, C). The segregation applied to each microstructure is mainly due to the formation of γ' with Ni₃(Al,Ti) composition. The Ni accumulation accompanied by Cr depletion, and the Ti accumulated in the eutectic region as a γ' forming elements. The Mo tends to diffuse out from the dendrite core to the interdendrite, and the W diffuse out from the interdendrite to the dendrite core. Therefore, the accumulation of Mo in the interdendrite and the deficiency of W occur in the eutectic region located in the interdendrite. Heat treatment makes the degree of the micro-segregation decrease due to the diffusion during solid solution. This study could be applied to the heat treatment technology for the micro-segregation control in cast Ni-based superalloys.

• Journal of the Korean Society for Heat Treatment
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• v.24 no.6
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• pp.318-326
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• 2011

This study has been performed to investigate the effect of Al addition on High Temperature Gas Nitriding (HTGN) in 13%Cr-0.16%C stainless steel with different Al contents of 0.54%, 1.76% and 2.36%, respectively. HTGN treatment was carried out at $1100^{\circ}C$ for 1 hr, 5 hrs and 10 hrs. Nitrogen-permeated surface layers showed round type carbides of $Cr_{23}C_6$ and needle type nitrides of AlN in the matrix of martensite, representing 600~700 Hv. And the thickness of the surface layer increased with increasing Al content and HTGN treatment time. The inner region that was not permeated nitrogen showed chromium carbides in the mixed phase of martensite and ferrite for the 0.53% Al alloyed steel, however chromium carbides in the matrix of ferrite single phase were shown for the steels with the addition of 1.76%Al and 2.36%Al, representing the hardness of ~200 Hv. During nitrogen permeation from surface to the interior, substitutional elements of Cr, Al and Si moved toward the surface and interstitial element of carbon also moved from interior to the surface. This movement of alloying elements leads high concentration of these elements at the outmost surface, subsequently the lowest peak of substitutional elements were shown in the vicinity of near surface. After showing the lowest peak, the high concentration region of Al and C were formed due to the continuous movement of Al toward the surface. The long discontinuous precipitates of $Cr_{23}C_6$ and AlN were formed along the outmost surface owing to the high concentration of these alloying elements.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.22 no.1
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• pp.177-189
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• 1998

The contents of this paper include a non-destructive technique for evaluating the degradation of the boiler superheater tube in a fossil power plant through an electrochemical polarization test. Correlation between Ip of polarization parameter and SP-DBTT for the superheater tubes in long-term use was obtained. 1.0Cr-0.5Mo steel was degraded by softening, and the degree of degradation was dependent upon carbides with Cr and Mo elements. Since brittle fracture at low temperature and ductile fracture mode at high temperature were shown, similarity between standard Charpy and small punch tests could be found. In addition, SP-DBTT showing the degree of degradation was higher, as the time-in use of the materials got longer. Electrolyte including picric acid of 1.3 g in distilled water of 100ml at 25.deg. C temperature and sodium tridecylbenzene sulfonate with 1g could be applied to evaluate the degradation of 1.0Cr-0.5Mo steel by means of the electrochemical polarization test. Ip and Ipa values measured through the electrochemical test are the appropriate parameters for representing the degradation of the superheater tube(1.0Cr-0.5Mo steel) for the fossil power plant. It is poassible to evaluate the degradation of materials with different time histories electrochemically, by Ip value only, at field test.

• Journal of the Korean Society for Heat Treatment
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• v.27 no.5
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• pp.242-252
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• 2014

A comparative study was performed on the microstructures and the mechanical properties of STD11 and 8Cr steel. The specimens were quenched from $1030^{\circ}C$ and tempered at $240^{\circ}C$ and $520^{\circ}C$. Vickers hardness, impact toughness and tensile tests were conducted at various tempering temperatures. Microstructural characterization to measure grain size, volume fraction of retained austenite and distribution of carbides was carried out by using SEM, EBSD, TEM and X-ray diffraction techniques. Due to finer $M_7C_3$ carbides dispersed, 8Cr steel showed larger impact toughness and plasticity than STD11 irrespective of the tempering temperature. While 8Cr steel had lower hardness in as-quenched state and after tempering at $240^{\circ}C$ owing to smaller carbide content and more retained austenite, it was harder after tempering at $520^{\circ}C$ due to larger precipitation hardening from finer $M_{23}C_6$.

• Journal of Power System Engineering
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• v.15 no.2
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• pp.49-54
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• 2011

Effect of tempering treatment on the mechanical properties of 12Cr heat resistant steel with ferrite phase was investigate in this study. As time and temperature of tempering treatment were increase, C and Cr contents in matrix structure were decreased. Due to increase of the amount of Cr26C6 type carbides. It was confirmed in mechanical properties experimental that tensile strength and hardness were decreased, while elongation and impact value were, increased with increasing the time and temperature.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.211-216
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• 2002

To investigate transition region in welded overlay relating to disbonding crack, the effect of vanadium addition on disbonding of Cr-Mo steels overlay welded with austenitic stainless steel was studied. V modified Cr-Mo steels have a higher resistance to disbonding than V free Cr-Mo steel. One reason is due to the fact that fine vanadium carbide precipated in base metal traps hydrogen and thus decreases the susceptibility to the disbonding. The second is related to the higher stability of the vanadium and stable carbides formed during PWHT, in which the carbon diffusion to the interface is lower than for V free Cr-Mo steel. Decreasing the carbon content at the interface of the weld overlay shows good resistance to the disbonding. Hence, it is important to control the carbon content at the interface of the weld overlay.

• Korean Journal of Materials Research
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• v.22 no.1
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• pp.46-53
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• 2012

In this study, mechanical tests and microstructural analyses including TEM analyses with EDX of precipitates in modified 9Cr-1Mo steel were carried out to determine the cause of embrittlement observed after heat-treatment, which limits the usage of the alloy for power plants. Mod. 9Cr-1Mo steel specimens at austenite temperature were quenched to the molten salt baths at $760^{\circ}C$ and $700^{\circ}C$, in which the specimens were kept for 10 min ~ 10 hr with subsequent air-cooling. Impact tests showed that the impact value dropped abruptly when the specimens were kept longer than 30 min at $\sim760^{\circ}C$ reaching to minima in about 1 hr, and then increasing at further retention. The tensile strength of the specimens reached the minimum value without much change afterward, whereas the values of elongation showed the same trend as that of the impact value. The isothermally heat-treated steel at $700^{\circ}C$ also showed a minimum impact value in about 1 hr. These results suggest that the isothermal heattreatment at 760 and $700^{\circ}C$ for about 1 hr induces temporal embrittlement in Mod. 9Cr-1Mo steel. The microstructural examination of all the specimens with extraction replica of the carbides revealed that the specimens with temporal embrittlement had $Cr_2C$, indicating that the cause of the embrittlement was the precipitation of the $Cr_2C$. In addition, TEM/EDX results showed that the Fe/Cr ratio was 0.033 to 0.055 for $Cr_2C$, whereas it was 0.48 to 0.75 for $Cr_{23}C_6$, making the distinction of the $Cr_2C$ and $Cr_{23}C_6$ possible even without direct electron diffraction analyses.

• Nuclear Engineering and Technology
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• v.31 no.6
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• pp.561-571
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• 1999

Microstructure and mechanical properties of five Cr-Mo steels for nuclear industry applications have been investigated. Transmission electron microscopy, energy dispersive spectrometer, differential scanning calorimeter, hardness, tensile, and impact test were used to evaluate the Cr and W effect on the microstructure and mechanical properties. Microstructures of Cr-Mo steels after tempering are classified into three types : bainitic 2.25Cr-lMo steel, martensitic Mod.9Cr-lMo, HT9M, and HT9W steels, and dual phase HT9 steel. The majority of the precipitates were found to be M$_{23}$C$_{6}$ carbides. As minor phases, fine needle-like V(C,N), spherical NbC, fine needle-like Cr-rich Cr$_2$N, and Cr-rich M$_{7}$C$_3$were also found. Addition of 2wt.% W in Cr-Mo steels retarded the formation of subgrain and dissolution of Cr$_2$N precipitates. Hardness and ultimate tensile strength increased with increasing Cr content. Though Cr content of HT9W steel was lower than that of HT9 steel, the hardness of HT9W was higher due to the higher W content. W added HT9W steel had the highest ultimate tensile strength above $600^{\circ}C$. But impact toughness of W added steel (HT9W) and high Cr steel (HT9) was low.w.w.

• Journal of the Korean Society for Heat Treatment
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• v.26 no.2
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• pp.72-79
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• 2013

The effects of homogenization, hot-forging, and annealing condition on microstructure and hardness of a modified STD61 hot-work tool steel were investigated. The ingot specimen had a dendritic structure consisting of bainite and martensite. Spherical VC particles of approximately 50 nm and cuboidal (V,Ti)C particles of about 100 nm were observed in the ingot specimen. After homogenization, the dendritic structure was blurred, and the difference in hardness between martensite and bainite became narrow, resulting in the more homogeneous microstructure. Needle-shaped non-equilibrium $(Fe,Cr)_3C$ particles were additionally observed in the homogenized specimen. The hot-forged specimen had bainite single phase with spherical VC, cuboidal (V,Ti)C, and needle-shaped $(Fe,Cr)_3C$ particles. After annealing at $860^{\circ}C$, the microstructures of specimens were ferrite single phase with various carbides such as VC, $(Fe,Cr)_7C_3$, and $(Fe,Cr)_{23}C_6$ because of relatively slow cooling rates. The size of carbides in annealed specimens decreased with increasing cooling rate, resulting in the increase of hardness.