• Journal of the Korean Institute of Gas
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• v.19 no.1
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• pp.64-70
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• 2015

In order to evaluate the degree of hydrogen embrittlement of TRIP steels charged with hydrogen according to varying the current density and the charging time under acid and alkaline electrolyte conditions were tested by small punch test. The results of SP test showed that the degree of hydrogen embrittlement at acid electrolyte condition was more effective factor compared to that of alkaline electrolyte condition. Therefore, all of the charging time and the charging current density were at the condition of acid electrolyte appeared as the main factor of the degree of hydrogen embrittlement in the condition of acid electrolyte. But, it was considered that the charging time compared to the charging current density at the condition of alkaline electrolyte was more effective factor to raise the degree of hydrogen embrittlement.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.6
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• pp.753-760
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• 2013

The hydrogen embrittlement of metallic materials is an important issue from the viewpoint of structural integrity. In this regard, the estimation of mechanical properties and fracture toughness under hydrogen conditions provides very important data. This study provides an experimental validation of the approach for simulating the small punch of Inconel 617 using finite element damage analysis, as recently proposed by the authors, and applies an inverse method for the determination of the constitutive tensile behavior of materials. The mechanical properties obtained from the inverse method are compared with those obtained from the tensile test and validated. The mechanical properties and fracture toughness are predicted by using the inverse method and finite element damage analysis.

• Proceedings of the KSME Conference
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• 2000.11a
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• pp.99-104
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• 2000

This paper describes the high temperature creep characteristics for virgin material of 9Cr1MoVNb steel using small punch creep(SP-Creep) test technique which is developing recently. In addition, the several results of SP-Creep test are compared with that of 2.25Cr- 1Mo steel which is widely used as boiler materials and that of conventional uniaxial creep test. The obtained SP-Creep curves show the creep behaviors of three regimes like that obtained from conventional uniaxial creep test, and SP-Creep properties are definitely depended on applied load and test temperature. The correlation of SP-Creep rate and creep rupture life with applied load has been determined like the correlation between creep rate/rupture life and stress in uniaxial creep test, and also is satisfied with Power law. The creep rupture times of newly 9Cr1MoVNb steel are higher than those of 2.25Cr1Mo steel at the same creep temperature and applied loading condition, and the decrease extent of creep rupture life with loads is very lower compared with 2.25Cr1Mo steel.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.3
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• pp.631-638
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• 1995

In this study, mechanical characteristics test of Functionally Gradient Materia (FGM) was performed by means of Modified Small Punch (MSP) Test with FGM; NiCrAlY-8YSZ and PSZ-Ni. To determine fracture mechanic factor, it was carried out MSP test that has possibility with small specimen (10*10*0.5 mm$^{t}$ ) and AE test to analyze micro fracture mechanism. As a result, fracture behavior became varied from brittle fracture to ductile as the content of Ni(or NiCrAlY) composition was increased and fracture energy was increased too. AE characteristics demonstrated that AE technique can detect the onset of fracture processes and AE energy was suddenly increased in the vicinity of maximum load. Since Young's modulus, fracture stress and fracture toughness was determined by MSP test, it can be known that the composition of NiCrAly 75%/8YSZ25% has the best mechanical property and furthermore this result is supported with fracture surface observation.

• Transactions of Materials Processing
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• v.26 no.5
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• pp.271-276
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• 2017

A tee is the most common pipefitting used to combine or divide fluid flow. Tees can connect pipes of different diameters or change the direction of a pipe run. To manufacture tee type of stainless steel pipe, combinations of punch piercing and burr forming have been widely used in the industry. However, such method is considerably time consuming with regard to performing empirical work necessary to attain process conditions to prevent upper end tearing of the tee product and meet target tee height. Numerous experiments have shown that the piercing profile is the main cause of defects mentioned above. Furthermore, the mold design is formed through trial and error according to pipe diameters and changes in requirements. Thus, the objective of this study was to perform piercing and burring process analysis via finite element analysis using DYNAFORM to resolve problems mentioned above. An optimization design method was used to determine the piercing punch profile. Three radii of the piercing punch (i.e., large, small, and joined radii) were selected as design variables to minimize thinning of a tee pipe. Based on results of correlation and multiple regression analyses, we developed a predictive approximation model to satisfy requirements for both thickness reduction and target height. The new piercing punch profile was then applied to actual tee forming using the developed prediction equation. Model results were found to be in good agreement with experimental results.

• 한국가스학회:학술대회논문집
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• 2006.11a
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• pp.183-186
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• 2006

• Journal of Surface Science and Engineering
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• v.46 no.3
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• pp.126-132
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• 2013

It was investigated that the effects of hydrogen charging on the properties of 590 MPa Dual Phase(DP) steels at the surface layers. The hydrogen-charging time was changed from 5 to 50 hours and current densities from 100, 150, and 200 $mA/cm^2$, respectively. It was found that the hydrogen content in the specimen was increased with as the charging time and the current density. The microvickers hardness of the subsurface zone was increased from 215.3 HV to 239.5 HV due to the increase in current density and charging time. The comparison of the absorbed energies tested by a small-punch (SP) test showed that the absorbed energy of the specimen was greatly reduced from 436 to 283 $kgf-mm^2$ because of hydrogen embrittlement. It was confirmed that bulb aspects of fracture surface became more brittle with increasing hydrogen content.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.22 no.2
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• pp.311-320
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• 1998

The cryogenic fracture behaviors of austenitic stainless steel HN2 developed for nuclear fusion reactor were evaluated quantitatively by using the small punch(SP) test. The electrochemical polarization test was applied to study thermal aging degradation of HN2 steel. The X-ray diffraction(XRD) analysis was conducted to detect carbides and nitrides precipitated on the grain boundary of the heat treated HN2 steel. The mechanical properties of the HN2 steel significantly decreased with increasing time and temperature of heat treatment or with decreasing testing temperature. The integrated charge(Q) obtained from electrochemical polarization test showed a good correlation with the SP energy(ESP) obtained by means of SP tests. From the results observed in the x-ray diffraction and anodic polarization curve, it was known that the material the grain boundary. Combining SP test and electrochemical polarization test, it could be useful tools to non-destructively evaluate the cryogenic fracture behaviors and the aging degradation for cryogenic structural material.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.05c
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• pp.79-84
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• 1996

Small punch test was performed on CF8 duplex stainless steel aged at 370 and 400$^{\circ}C$ up to 5,000 h to evaluate the degree of the thermal aging embrittlement. At room temperature, the SP load-displacement curve was in a similar shape to those of ferritic steels and had a good reproducibility in spite of two-phase structure. The aging heat treatment resulted in a slight increase of the yield strength. As test temperature was lowered, the SP load showed a sudden drop followed by serrations before the SP specimen was fractured, resulting from the cracking of ferrite phase. The extent of thermal embrittlement was assessed in terms of the SP energy. Aging treatment at higher temperature led to a larger shift in the transition temperature and the corresponding change in the fracture mode. The main cause of the degradation was the embrittlement of ferrite phase. Additionally the phase boundary separation profoundly contributed to the degradation of the specimen aged at 400$^{\circ}C$.

• Proceedings of the KIEE Conference
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• 2000.11a
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• pp.265-268
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• 2000

In this study, a small punch creep (SP-Creep) test using miniaturized specimen has been described for the development of the new creep test method for high temperature structural components such as headers and tubes of boiler, turbine casing and rotor, and reactor vessel. The SP-Creep testing technique has been applied to 1Cr-0.5Mo steel used widely as boiler header material and the creep test temperature are varied at $550^{\circ}C{\sim}600^{\circ}C$. From the experimental results, e.g. SP-Creep curve behaviors, the creep rate in steady state and creep rupture life with test temperature and load, the load exponential value(n, m), the activation energy($Q_{spc}$), the Monkman-Grant relation and the creep life assessment equation etc., it can be summarized that the SP-Creep test may be a useful test method to evaluate the creep properties of the heat resisting material such as boiler header.