• Journal of Korea Foundry Society
• /
• v.2 no.2
• /
• pp.2-9
• /
• 1982

This paper is concerned with the improvement of impact and tensile Properties of spheroidal graphite cast iron of the following duplex matricess which were heat treated in the eutectic transformation temperature range (that is, $({\alpha}+{\gamma})$ coexisting range) ; ferrite-martensite, ferrite-bainite and ferrite-pearlite. The absorbed energy and maximum load was measured by recording the load-deflection curve with instrumented Charpy impact testing machine in the temperature range from $+100^{\circ}C$ to $-196^{\circ}C$. It was found the ferrite-bainite duplex matrix showed the highest toughness among the above matrices in the room temperature and the low temperature range. Comparison of this matrix to ferrite-pearlite matrix(that is, as cast) showed a lowering of $27^{\circ}C$ in the nil-ductility transition temperature (NDT) and a lowering of $40^{\circ}C$ in the ductile-brittle transition temperature (TrE), Which seems to result from the finner dimple pattern observed using miorofractography.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.17 no.2
• /
• pp.300-312
• /
• 1993

The material deterioration of service-exposed boiler tube steels in fossil power plant was evaluated by using the electrochemical technique namely, modified electrochemical potentiokinetic reactivation(EPR). It was focused that the passivation of Mo$_{6}$C carbide which governs the mechanical properties of Mo alloyed steels did not occur even in the passivity region of steel in sodium molybdate solution and the reactivation peak current (Ip) observed as the result of non-passivation indicating the precipitation of Mo$_{6}$C carbides. To obtain the optimal test conditions for the field test by using the specially designed electrochemical cell, the effects of scan rate, the surface roughness and the pH of electrolyte on Ip value were also investigated. Furthermore, the change of mechanical properties occurred during the long time exposure at high temperature was evlauated quantitatively by small punch(SP) tests and micro hardness test taking account of the metallurgical changes. It is known that reactivation peak current (Ip) has a good relationship with Larson-Miller Parameter(LMP) which represents the information about material deterioration occurred at high temperature environment. In addition it was possible to estimate the ductile-brittle transition temperature (DBTT) by means of the SP test. The Sp test could be, therefore, suggested as a reliable test method for evaluating the material degradation of boiler tube steels. From the good correaltion between the SP DBTT and Ip values shown in this study, it was knows that the change of mechanical properties could be evaluated non-destructively by measurring only Ip values.ues.

• Proceedings of the KSME Conference
• /
• 2000.11a
• /
• pp.105-110
• /
• 2000

The Micromechanics test is new test method which uses comparatively smaller specimen than that required in conventional material tests. There are several methods, such as small-specimen creep test, the continuous indentation test, and small punch(SP) test. Among them, the small punch(SP) test method has been applied to many evaluation fields, such as a ductile-brittle transition temperature, stress corrosion cracking, hydrogen embrittlement, and fracture properties of advanced materials like FGM or MMC. In this study, the small punch(SP) test is performed to evaluate the mechanical properties at high/low temperature from $-196^{\circ}C$ to $650^{\circ}C$ and the material degradation for virgin and aged materials of 9Cr1MoVNb steel which has been recently developed. The ${\Delta}P/{\Delta}{\delta}$ parameter defined a slope in plastic membrane stretching region of SP load-displacement curve decreases according to the increase of specimen temperature, and that of aged materials is higher than the virgin material in all test temperatures. And the material degradation degrees of aged materials with $630^{\circ}C$ -500hrs and $630^{\circ}C$ -1000hrs are $36^{\circ}C$ and $38^{\circ}C$ respectively. These behaviors are good consistent with the results of hardness($H_v$) and maximum displacement(${\delta}_{max}$).

• Korean Journal of Materials Research
• /
• v.4 no.1
• /
• pp.75-80
• /
• 1994

Flexural properties and thermal stability have been studied as a function of blend composition in bifunctional DGERA (diglycidyl ether of hisphenol A)/tetrafunctional TGDDM(tetrag1ycidyl diamino diphenyl methane) cured with DDM(4, 4'-diamino diphenyl methane). The flexural modulus and the glass transition temperature increase with an increase of TGDDM and show discontinuous dependence on blend composition around the composition range of 80/20~60/40(L)GEBA/TGDDM). This can be explained with a structural phase inversion (ductile-to-brittle) in crosslinking networks. With increasing TGDDM, the maximum decomposition temperature(Ts) increases, whereas the activation energy during thermal degradation decreases.

• Nuclear Engineering and Technology
• /
• v.38 no.5
• /
• pp.405-410
• /
• 2006

Microstructural features and their related mechanical property changes in the 309L cladding and the heat affected zone (HAZ) of SA508 cl.3 steel were investigated through the use of TEM, tensile and small punch (SP) tests. The specimens were irradiated at 563 K up to the neutron fluences of $5.79{\times}10^{19}n/cm^2$ (>1MeV). The microstructure of the clad was mainly composed of a fcc ${\gamma}-phase$, a low percentage of bcc ${\delta}-ferrite$, and a brittle ${\sigma}-phase$. Along the weld fusion line there formed a heavy carbide precipitation with a width of $20{\sim}40{\mu}m$, showing preferential cracking during plastic deformation. The yield stress and ductile-to-brittle transition temperature (DBTT) of the irradiated clads increased. The origin of the hardening and the shift of the DBTT are discussed in terms of the irradiation-produced defect clusters of a fine size and brittle ${\sigma}-phase$.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2008.05a
• /
• pp.431-434
• /
• 2008

Effects of microstructure on the toughness of low carbon HSLA steels were investigated. Nickel decreased the ferrite-austenite transformation temperature, resulted in increase of the fraction of bainitic ferrite. However, it was decreased with increasing deformation amount at austenite region. Since fine austenite grains formed by dynamic recrystallization under large strain transformed to acicular ferrite or granular bainite rather than bainitic ferrite. The effective grain size, thus, was decreased by deformation and it resulted in lower ductile-brittle transition temperature (DBTT). The bainitic ferrite was thought to inhibit the fracture crack initiation and to delay the crack propagation by its high dislocation density and hard interlath $2^{nd}$ phase constituents, respectively. Thus, DBTT was also decreased by Ni addition in low carbon HSLA steels.

• Journal of Welding and Joining
• /
• v.10 no.1
• /
• pp.43-51
• /
• 1992

The plastic zone formed around a notch tip is important in analyzing the fracture toughness of structures and particularly weld cracks existed in the weld HAZ (heat affected zone) which produces local plastic deformation at the crack tip. Therefore, in order to analyze the fracture toughness in weld HAZ, it is necessary to investigate the new fracture toughness parameter $K_{c}$ $^{*}$ and critical plastic strain energy $W_{p}$ $^{c}$ according to the shape and size of the plastic zone. 1) If the temperature corresponding to $K_{c}$ $^{*}$=130kg-m $m^{-3}$ 2/ is determined, transition temperature $T_{tr}$ the magnitude of plastic zone size, and heat input change depending on the fracture toughness. The blunted amounts of the parent and weld HAZ show mild linear variation until .delta.=0.4mm and then increase very steeply there after. 2) The relation between the plastic strain energy( $W^{p}$ ) and transition temperature( $T_{*}$tr) in parent metal is more sensitive than that of weld HAZ. However, the plastic strain energy depends on the transition temperature, and thus the yield stress, .sigma.$_{ys}$ becomes an important parameter for plastic strain energy. 3) The critical plastic strain energy( $W_{p}$ $^{c}$ ) absorbed by the plastic zone at the notch tip indicated in case of parent metal: 60J/mm, in case of heat input(20KJ/cm): 75J/mm, in case of heat input(30KJ/cm); 50J/mmJ/mm.

• Geomechanics and Engineering
• /
• v.24 no.2
• /
• pp.157-166
• /
• 2021

This paper aims at the temperature and slenderness ratio effects on physical and mechanical properties of Beishan granite. A series of uniaxial compression tests with various slenderness ratios and temperatures were carried out, and the acoustic emission signal was also collected. As the temperature increases, the fracture aperture of intercrystalline cracks gradually increases, and obvious transcrystalline cracks occurs when T > 600℃. The failure patterns change from tensile failure mode to ductile failure mode with the increasing temperature. The elastic modulus decreases with the temperature and increases with slenderness ratio, then tends to be a constant value when T = 1000℃. However, the peak strain has the opposite evolution as the elastic modulus under the effects of temperature and slenderness ratio. The uniaxial compression strength (UCS) changes a little for the low-temperature specimens of T < 400℃, but a significant decrease happens when T = 400℃ and 800℃ due to phase transitions of mineral. The evolution denotes that the critical brittle-ductile transition temperature increases with slenderness ratio, and the critical slenderness ratio corresponding to the characteristic mechanical behavior tends to be smaller with the increasing temperature. Additionally, the AE quantity also increases with temperature in an exponential function.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.17 no.9
• /
• pp.2271-2279
• /
• 1993

In the case of life prediction on the structures and machines after long service, it is natural to consider a degradation problems. Most of degradation data form practical structures are isolated data obtained at the time of periodical inspection or repair. From such data, it may be difficult to obtain the degradation curve available and necessary for life prediction. In this paper, for the purpose of obtaining a degradation curves, developed the simulate degradation method and fatigue test and Charpy impact test were conducted on the degraded, simulate degraded and recovered materials. Fatigue life prediction were conducted by using the relationship between fracture transition temperature (DBTT : vTrs) obtained from the Charpy impact test through the degradation process and fatigue crack growth constants of m and C obtained from the fatigue test.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.21 no.1
• /
• pp.93-103
• /
• 1997

Charpy V-notch impact tests were performed on the full-, half-and third-size specimens from two ferritic SA 508 Cl. 3 steels for nuclear pressure vessel. New normalization factors were proposed to predict the upper shelf energy(USE) and the ductile-brittle transition temperature(DBTT) of full-size specimens from the measured data on sub-size specimens. The factors for the USE and the DBTT are $(Bb^2/Kt); and; (Bb/R)^1/2/, $ respectively, where B the width, b the ligament size, $K_{t}$ the elastic stress concentration factor, and R the notch root radius. These correlations successfully estimated the USE and DBTT of the full-size specimens based on sub-size specimen data. In addition, the size effects were studied to develop the correlations among absorbed energy, lateral expansion(LE) and displacement. It was also found that the LE was able to be estimated from the displacement obtained by the instrumented impact test, and that the displacement would be used as a criterion for the toughness of the steels corresponding to change in their yield strength.h.