• 열처리공학회지
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• 제22권2호
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• pp.67-74
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• 2009

440A martensitic stainless steels which were modified with reduced carbon content (${\sim}$0.5%) and addition of small amount of nickel, vanadium, tungsten and molybdenum were manufactured. Effects of alloying elements and heat treatment on the pitting corrosion in 3.5% NaCl were investigated through the electrochemical polarization tests. The lowest pitting potential, $E_p$, was obtained when austenitizing temperature was $1250^{\circ}C$ and this is because of the grain coarsening. When austenitized at $1050^{\circ}C$ and tempered at $350{\sim}750^{\circ}C$, the highest $E_p$ was obtained at $350^{\circ}C$, while the lowest at $450^{\circ}C$ and $550^{\circ}C$ regardless of alloying elements added. But $E_p$ was increased a little at the tempering temperature of $450^{\circ}C$ and $550^{\circ}C$ when 0.4 wt.% of tungsten was added. More pitting was observed at $450{\sim}550^{\circ}C$, and pitting was formed at regions where Cr concentration is low or grain boundaries are intersecting and showed irregular shape.

• Journal of Welding and Joining
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• 제18권6호
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• pp.48-54
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• 2000

The corrosion properties of Ni-Cr-W-Mo-B alloy sprayed by the high velocity oxy-fuel spraying (HVOF) was studied as a function of heat treatment by using both potentiodynamic polarization and immersion tests in the H₂SO₄ solution. The mechanical property was also evaluated by a microhardness tester. Microstructural characteristics of te as-sprayed and annealed coatings at 550, 750 and 950℃ have been analyzed by means of OM, XRD, SEM and TEM. The results showed that the corrosion resistance was improved by increasing the annealing temperature. As-sprayed coating had metastable and heterogeneous phases such as amorphous, nanocrystalline and very refined grain and precipitates, which induced a localized corrosion. The localized corrosion occurred preferentially at the unmelted particles which were composed of Ni matrix and Cr, W and Mo riched phase segregated in the boundaries. As annealing temperature was increased, the microstructure had shown some changes - reduction of porosity and s[plat boundary decomposition and crystallization of amorphous/nanocrystalline phases, grain coarsening,, formation and growth of precipitates such as {TEX}$M_{23}C_{6}${/TEX} and {TEX}$M_{7}C_{3}${/TEX}. In addition, the compositional difference between matrix and boundary phases gradually disappeared, which changed the corrosion type from localized corrosion to general corrosion and thus enhanced corrosion resistance.

• 한국분말재료학회지
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• 제17권6호
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• pp.477-481
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• 2010

Cu-Sn based alloys were manufactured by gas atomization spray casting route in order to achieve a fine scale microstructure and a high tensile strength. The spray cast Cu-10Sn-2Ni-0.2Si alloy had an equiaxed grain microstructure, with no formation of brittle ${\delta}-Cu_{41}Sn_{11}$ phase. Aging treatment promoted the precipitation of finely distributed particles corresponding to ${\delta}-Ni_2Si$ intermetallic phase throughout the $\alpha$-(CuSn) matrix. The cold-rolled Cu-Sn-Ni-Si alloy had a very high tensile strength of 1200 MPa and an elongation of 5%. Subsequent aging treatment at $450^{\circ}C$ for 1h slightly reduced the tensile strength to 700 MPa and remarkably increased the elongation up to 30%. This result has been explained by coarsening the precipitates due to over aging and reducing the dislocation density due to annealing effects.

• 대한금속재료학회지
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• 제49권2호
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• pp.104-111
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• 2011

An ultrafine grained Mg-9Al-1Zn magnesium alloy with the mean grain size less than $1{\mu}m$ was produced by using high-ratio differential speed rolling. The processed alloy exhibited excellent superplasticity at relatively low temperatures. The micro-forming tests were carried out using a micro-forging apparatus with micro V-grooved shaped dies made of silicon and the micro-formability was evaluated by means of micro-formability index, $R_f$ ($=A_f/A_g$, $A_f$: formed and inflowed area into the V-groove, $A_g$: area of the V-groove). The $R_f$ value increased with temperature up to $280^{\circ}C$ and then decreased beyond $300^{\circ}C$. The decrease of the $R_f$ value at $300^{\circ}C$ was attributed to the accelerated grain coarsening. Increasing the micro-forging pressure increased the $R_f$ values. At a given die geometry, die filling ability decreased as the die position moved away from the die center to the end. FEM simulation predicted this behavior and a method of improving this problem was proposed.

• Nuclear Engineering and Technology
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• 제51권5호
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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.

• 열처리공학회지
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• 제32권3호
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• pp.124-130
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• 2019

Friction stir welding is one of the interesting welding methods for titanium and its alloy which proceeds with plastic flow due to thermo-mechanical stirring and friction heat. Solid-state welding can solve severe problems such as high-temperature oxidation, interstitial oxygen diffusion and grain coarsening by liquid-state welding. Dynamic recrystallization and grain refinement can vary significantly with the plunging load and rotational speed of tool during friction stir welding, and suitable process conditions must be optimized to obtain microstructure and better mechanical characteristics. Suitable FSW conditions were 1000 kg of plunging load and 200 rpm of rotational speed and it showed YS 270 MPa, UTS 332.1 MPa, and El 17.3%, which were very similar to those of wrought titanium sheet.

• 한국표면공학회지
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• 제26권2호
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• pp.87-107
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• 1993

The grain size of the final products of WC-Co and WC-Ni composite powders is dependent on the size of the starting material and the conditions employed for the reduction and carburization. APT-Co and -Ni com-plex salts were prepared by the substitution reaction between ammonium ions in APT and the metal ions in Co(NO3)2 and Ni(NO3)2 solutions of different concentrations(0.1 to 0.7M) at $50^{\circ}C$ and the grain sizes of the com-plex salts was $0.54~0.76\mu\textrm{m}$. The complex which calcined the complex salts at $700^{\circ}$~80$0^{\circ}C$ for 60min. were 0.2~0.5$\mu\textrm{m}$. W-Co($5.92^{\circ}C$) and -Ni(6.95%) powders which reduced the complex oxides with H2d atmo-sphere(flow rate;600cc/min.) at $700^{\circ}$~$800^{\circ}C$ for 60min. were $0.5~0.6\mu\textrm{m}$. The mean grain sizes of WC-Co and WC-Ni composite powders which carburized both complex metals of W-Co and W-Ni at $800^{\circ}C$ for 60min. were $0.5~0.6\mu\textrm{m}$, and take place the coarsening of the grain above $800^{\circ}C$ and the optmium ratio of C3H8 and H2 was 0.2 for the control of the free carbon. The effect of Co contents on the particle sizes decreased from 0.4 to $0.25\mu\textrm{m}$ with increasing the content from 2.0 to 7.6w%. The activation energies on the reductions of oxides and the formations of carbides were as follows ; W-Co : Q = 8.7 kcal/mole, W-Ni : Q = 8.1 kcal/mole, WC-Co pow-der : Q = 17.8 kcal/mole, WC-Ni powder : Q = 16.6 kcal/mole.

• 한국분말재료학회지
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• 제20권2호
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• pp.85-99
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• 2013

Powder metallurgy applied rapid heating to sintering starting year 1900. Since 1970 the study has intensified. Now rapid sintering concepts embrace a spectrum of options ranging from dunk cycles to microwave, induction, exothermic, electric field, and spark approaches. Most of the efforts are targeting reduced microstructure coarsening during sintering, although reduced material decomposition is another common goal. The efforts are impressive for simple shapes and success metrics such a small grain size after densification. Several barriers need to be removed prior to application in powder metallurgy commercial sintering. Rapid heating research needs to focus on significant property gains, accurate product dimensions, and lower costs. So far each property gain obtained with rapid heating is matched by traditional sintering and composition changes. Several examples are cited to show the goals for the next round of innovations.

• 한국해양공학회지
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• 제11권2호
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• pp.57-69
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• 1997

The objetive of this study is to compare the mechanical proprties of 6000 series Al-Mg-Si aluminum alloy (AC120) with 5000 series Al-Mg aluminum alloy (TG25), and to investigate the influence of corrosion solution for fatigue life. Comparing of TG25 and AC120 alloy sheets, TG25 alloy sheets showing higher plastic ratio and total elongation have better formability than AC120 alloy sheets. The hardness of nugget area was a little higher than that of base metal area. Also, grain coarsening was observed in HAZ(Heat Affected Zone). In a corrosion fatigue experiment, the fatigue life decreased as concentration increased, when a dipping time was constant. The life decreased as dippling time increased, when a concentration was constant.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2001년도 춘계학술강연 및 발표대회
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• pp.14-14
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• 2001

WC-Co 계의 입성장 억제는 현재 초경합금 분야에서 공학적으로 가장 중요한 이슈들 중의 하나이다 VC를 비롯한 입방정 탄화물이나 $Cr_3C_2$ 등의 여러 가지 탄화 물이 혼합되어 입성장 억제에 이용되는데 입성장 억제의 효과는 대략적으로 용해되는 탄화물의 양에 의존하고 있는 것으로 추정된다. 보다 효율적으로 입성장 억제를 실현하려면 입성장 기구를 명확히 할 필요가 있다. 최 등[1]은 VC가 WC 입자 표면 에서의 edge energy를 증가시켜서 2차원 핵생성의 에너지 장벽을 올리게 하고 이에 따라 입성장이 억제된다는 모델을 제안하였다. 이러한 모텔을 입증하기 위해서는 이론적으로만 예측이 가능한 edge energy보다 좀 더 명확한 물리적인 변수가 제시되어야 할 것이다. 여기서는 또 다른 계인 NbC- TiC-Co 계에서 NbC와 TiC의 버에 따른 입성장 거동과 입자의 형상간의 관계로부터 업성장과 edge energy 그리 고 edge energy와 입자 형상간의 관계를 알아보고 이로부터 좀 더 구체적인 의미 에서의 입성장 모텔과 입성장 억제기구를 제시하고자 한다.