• Nuclear Engineering and Technology
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• v.51 no.5
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• pp.1365-1372
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• 2019

The effects of Y and Ti on the microstructure stability and tensile properties of the reduced activation ferritic/martensitic steel have been investigated. The addition of Y and Ti affected the prior austenite grain size due to the pinning of the inclusions. Ti addition of 0.008 wt% to the steel was intended to promote the precipitation of nano-sized carbides with a high resistance to coarsening. 8Ti14Y exhibited a higher yield strength and a lower DBTT than the other alloys due to the fine grain size and additional precipitation hardening by (Ti, Ta)-rich MX. After thermal exposure at $550^{\circ}C$ for 1500 h, yield strength was dropped significantly in exposed 0Ti13Y. On the contrary, a lower reduction of YS was observed in 8Ti14Y. The $M_{23}C_6$ in 0Ti13Y and 8Ti14Y and MX in 25Ti14Y and 39Ti15Y coarsened seriously during ageing, which could be responsible for the reduction of the tensile properties of alloys.

• Journal of Advanced Marine Engineering and Technology
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• v.35 no.4
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• pp.451-459
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• 2011

Austenitic stainless steels of 300 series are widely used as the structural material due to excellent their cryogenic mechanical properties at cryogenic temperature. There are 316 steel which molybdenum is added to improve the austenitic stability, 316L which carbon contents is reduced to decrease the grain boundary precipitation during welding process, and 316LN which nitrogen is added to improve the austenitic stability and the mechanical strength. But material researches for the welding conditions and mechanical properties at the cryogenic temperature were insufficient so far. In this paper, the characteristics of mechanical properties considering the effect of welding conditions and cryogenic temperature are studied.

• Journal of Korea Foundry Society
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• v.32 no.6
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• pp.284-288
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• 2012

The effects of Si content on the variation of microstructure and processing window of austempered ductile cast iron were investigated. Four different Si contents between 2.42 and 3.37 wt.% were used. The influence of silicon on the microstructure and processing window of these materials were studied. Austenitizing was performed at $900^{\circ}C$ for 60min and austempering temperature were both $340^{\circ}C$ and $360^{\circ}C$ and austempering time were for 4min upto 119min and for 5min upto 160min respectively. After heat treatment, the evolution of stage I and stage II were performed by optical metallography, XRD, hardness test. The results showed that $t_2$ was delayed as Si contents was increased due to the fact that Si retarded the formation of cementite ($Fe_3C$). The high silicon content promoted the stability of the metastable two-phase combination of austenite and ausferrite.

• Journal of the Korean Society for Heat Treatment
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• v.29 no.4
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• pp.176-180
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• 2016

In this study, a microstructural investigation was conducted on the cracking phenonmenon occurring during hot rolling of Fe-23Mn high-manganese steels with different aluminium and carbon contents. Particular emphasis was placed on the phase stability of austenite and ferrite dependent on the chemical composition. An increase in the aluminum content promoted the formation of ferrite band structures which were easily deformed or cracked. In the steels containing high carbon contents of 0.4 wt.% or higher, on the other hand, the volume fraction and thickness of ferrite bands decreased and thus the cracking frequency was significantly reduced. Based on these findings, it is said that the microstructural evolution occurring during hot rolling of high-manganese steels with different aluminium and carbon contents plays an important role in the cracking phenomenon. To prevent the cracking, therefore, the formation of second phases such as ferrite should be minimized during the hot rolling by the appropriate control of the chemical composition and process parameters

• Journal of the Korean Society for Heat Treatment
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• v.10 no.2
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• pp.128-137
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• 1997

The formation processes of the retained austenite(${\gamma}_R$) in SHCP100 steel sheets were investigated in order to improve the transformation induced plasticity(TRIP) effect of ${\gamma}_R$. An excellent combination of elongation about 23% and high strength over 830 MPa was achieved by processing of intercritical annealing and isothermal holding. The mechanical properties of TRIP-aided dual phase steel was found to depend on the volume ratio of each phase and the volume fraction of ${\gamma}_R$. It was also noted that the proper mechanical stability of ${\gamma}_R$ improved the mechanical properties. In this work, the best balance of strength-ductility was obtained by holding the steel at $420^{\circ}C$ for 500sec. after annealing at $730^{\circ}C$ for 300 sec.

• Korean Journal of Materials Research
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• v.28 no.3
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• pp.129-134
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• 2018

This study was carried out to investigate the effect of grain size on the damping capacity of the Fe-26Mn-4Co-2Al damping alloy. ${\alpha}^{\prime}$ and ${\varepsilon}-martensite$ were formed by cold working, and these martensites were formed with a specific direction and surface relief. With an increase in grain size, the volume fraction of ${\alpha}^{\prime}$ and ${\varepsilon}-martensite$ increased by decrement the austenite phase stability. This volume fraction more rapidly increased in cold-rolled specimen than in the specimen that was not cold-rolled. The damping capacity also increased more with the augmentation an increased grain size and more rapidly increased in cold-rolled specimen than in the specimen that was not cold rolled. The effect of grain size on the damping capacity was larger in the cold-rolled specimen than the specimen that was not cold-rolled. Damping capacity linearly increased with an increase in volume fraction of ${\varepsilon}-martensite$. Thus, the damping capacity was affected by the ${\varepsilon}-martensite$.

• Journal of Welding and Joining
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• v.28 no.2
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• pp.79-83
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• 2010

In this study, newly developed welding consumables for EGW were welded in EH 36 TM steel plates and their welded joints were evaluated in point of mechanical properties and microstructures compared with imported consumables. Newly developed welding consumables were evaluated as good arc stability and slag fluidity, substantially the same with imported products. The tensile strength of all welded joints were sufficient to meet the requirements specified in a ship’s classification(490~640MPa) and all areas of fracture were heat affected zone(HAZ). Charpy absorbed energy values of all EG welded metals were sufficient to meet the requirements of classification(min. 34J) and those of newly developed wires were evaluated to be better than those of imported wires. As a result observing microstructures of single and tandem EG welded metals through optical and scanning electron microscope (OM&SEM), no grain boundary ferrite(PF(G)) were created in a prior austenite grain boundary and a volume fraction of a fine acicular ferrite were observed very high.

• Corrosion Science and Technology
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• v.13 no.2
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• pp.70-80
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• 2014

Duplex stainless steels with nearly equal fraction of the ferrite(${\alpha}$) phase and austenite(${\gamma}$) phase have been increasingly used for various applications such as power plants, desalination facilities due to their high resistance to corrosion, good weldability, and excellent mechanical properties. Hyper duplex stainless steel (HDSS) is defined as the future duplex stainless steel with a pitting resistance equivalent (PRE=wt.%Cr+3.3(wt.%Mo+0.5wt.%W)+30wt.%N) of above 50. However, when HDSS is welded with gas tungsten arc (GTA), incorporation of nitrogen in the Ar shielding gas are very important because the volume fraction of ${\alpha}$-phase and ${\gamma}$-phase is changed and harmful secondary phases can be formed in the welded zone. In other words, the balance of corrosion resistance between two phases and reduction of $Cr_2N$ are the key points of this study. The primary results of this study are as follows. The addition of $N_2$ to the Ar shielding gas provides phase balance under weld-cooling conditions and increases the transformation temperature of the ${\alpha}$-phase to ${\gamma}$-phase, increasing the fraction of ${\gamma}$-phase as well as decreasing the precipitation of $Cr_2N$. In the anodic polarization test, the addition of nitrogen gas in the Ar shielding gas improved values of the electrochemical parameters, compared to the Pure Ar. Also, in the erosion-corrosion test, the HDSS welded with shielding gas containing $N_2$ decreased the weight loss, compared to HDSS welded with the Ar pure gas. This result showed the resistance of erosion-corrosion was increased due to increasing the fraction of ${\gamma}$-phase and the stability of passive film according to the addition $N_2$ gas to the Ar shielding gas. As a result, the addition of nitrogen gas to the shielding gas improved the resistance of erosion-corrosion.

• Journal of the Korean Institute of Gas
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• v.2 no.1
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• pp.7-13
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• 1998

In this study, for the safety performance of LNG storage tank, the fracture toughness in X-grooved weld HAZ(heat-affected zone) of $9\%$ Ni steel was evaluated qualitatively and quantitatively, and the relation with the change in microstructure was analyzed. The toughness assessment was peformed through the modified CTOD test proposed for thick weldment with X-groove. Additionally, microstructures of HAZ were evaluated by OM, SEM and XRD. From the results, HAZ toughness of SMA(shielded metal arc)-welded $9\%$ Ni steel decreased as the evaluated region approached the fusion line. The decrease in toughness was apparently caused by the increase in the fraction of coarse-grained zone within HAZ. On the other hand, toughness drop with decreasing test temperature in F.L.(fusion line) ${\~}$F.L.+3mm was larger than that in F.L.+5mm${\~}$F.L.+7mm region due to the fact that in the former regions, retained austenite had poor stability.