• 한국안전학회지
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• 제16권2호
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• pp.44-50
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• 2001

The rotated bending fatigue test was conducted in air md in 3.5% NaCl salt solution to investigate the fatigue fracture behaviour of raw material and F.E.M dual phase steel made from raw material(SS41) by a suitable heat treatment. This study has compared the initial microcrack creation of material by tensile test with that by fatigue test. And the rotated bending test of cantilever type under the condition of 3.5% NaCl salt solution and air has investigated the corrosion fatigue fracture behaviour with the variation of stress concentration factor determined by each of notch shapes. The initial microcrack have been developed in fragile grainboundary with general corrosion occurring in raw material : in the pits built up by corrosion in F.E.M. dual phase steel because pits bring out stress concentration. It is small that the degree of decrease in corrosion fatigue life for F.E.M. dual phase steel compared with raw material because the notch sensitivity of F.E.M. dual phase steel is lower than raw material in reason of characteristics with two-phase construction.

• Journal of Welding and Joining
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• 제7권3호
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• pp.19-27
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• 1989

Both the SS41 steel and the M.E.F(martensite encapsulated islands of frrite) dual phase steel made of SS41 steel by heat treatment were welded by friction welding, and then manufactured machinemade Vnotch standard Charpy impact specimens and precracked with a fatigue system at BM(base metal), HAZ(heat affected zone) and WZ(weld interface Zone). The impact test of them was performed with an instrumented impact test machine at a number of temperatures in constant loading velocity and the dynamic fracture characteristics were studied on bases of the absorbed energy, dynamic fracture toughness and fractography from the test. The results obtained are as follows; At the room temperature, the absorbed energy is HAZ.geq.WZ.geq.BM in case of the M.E.F. dual phase steel: BM.geq.HAZ.geq.WZ in case of the SS41 steel, HAZ.geq.BM.geq.WZ at the low temperature. The absorbed energy is decreased markedly with the temperature lowering; it is highly dependent on the temperature. The dynamic fracture toughness of the M.E.F. dual phase steel is HAZ.geq.WZ.geq.BM at the room temperature; BM.geq.WZ.geq.HAZ below-60.deg. C. Therefore the reliability of friction welding is uncertain at the low temperature(below-60.deg. C). The dynamic fracture toughness of the SS41 steel; HZA.geq.WZ.geq.BM at overall temperature region. The flaw formed by rotational upsetting pressure was shown y SEM; in this region. The absorbed energy per unit area and dynamic fracture toughness were low relative to other region.

• 비파괴검사학회지
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• 제16권1호
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• pp.10-18
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• 1996

본 연구에서는 SS41 불림재와 M.E.F. 복합 조직강에 대해서 파괴 역학 파라미터와 X-선 파라미터 사이의 관계를 규명하였다. 피로 균열 진전 실험이 수행되었고, X-선 회절 방법이 피로 파단면에 적용되었다. 피로 파단면을 순차적으로 전해 연마하면서 깊이에 따른 X-선 파라미터(잔류응력, 반가폭)의 변화를 관찰하였다. 최대 소성역 깊이 $w_y$는 SS41 불림재에서는 반가폭의 분포로, M.E.F. 복합 조직강에 대해서는 잔류 응력의 분포로 결정할 수 있었다. 피로 파단면의 깊이 방향에 따른 X-선 파라미터의 분포로부터 $K_{max}$과 $w_y$의 관계식이 실험 데이타로부터 얻어지고, 이 식들으로부터 $K_{max}$은 $w_y$의 측정에 의하여 추정되어질 수 있다.

• 한국해양공학회지
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• 제5권2호
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• pp.58-66
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• 1991

Not only difference of fatigue crack growth and propagation behavior resulted from the grain size, the hardness ratio and volume fraction in M.E.F. dual phase steel composed of martensite in hard phase and ferrite in soft phase, but also the effects of the plastic constraint were investigated by fracture mechanics and microstructural method. The main results obtained are as follows: 1) The fatigue endurance of M.E.F. steel increases with decreasing the grain size, increasing the ratio of hardness and volume fraction. 2) The initiation of slip and crack occures faster as the stress level goes higher. These phenomena result from the plastic constraint effect of the second phase. 3) The crack propagation rate in the constant stress level is faster as the grain size gets larger, the ratio of hardness lower and volume fraction smaller.

• 한국안전학회지
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• 제18권3호
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• pp.46-53
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• 2003

For the tensile tests of the F.E.M., microvoids are created by the boundary separation process at the martensite boundary or neighborhood and at inclusions within the fracture. to grow to the ductile dimple fracture. For the case of the M.E.F., microvoids created at the discontinuities of the martensite phase which exists at the grain boundary of the primary ferrite are grown to coalescence with the cleavage cracks induced at the interior of the ferrite, which as a result show the discontinuous brittle fracture behavior. In spite of their similar tensile strengths, the fatigue limit and the notch sensitivity of the M. E.F. is superior to those of the F.E.M., The M.E.F. is much more insensitive to notch than F.E.M. from the stress concentration factor($\alpha$).

• 한국해양공학회지
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• 제5권2호
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• pp.198-198
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• 1991

Not only difference of fatigue crack growth and propagation behavior resulted from the grain size, the hardness ratio and volume fraction in M.E.F. dual phase steel composed of martensite in hard phase and ferrite in soft phase, but also the effects of the plastic constraint were investigated by fracture mechanics and microstructural method. The main results obtained are as follows: 1) The fatigue endurance of M.E.F. steel increases with decreasing the grain size, increasing the ratio of hardness and volume fraction. 2) The initiation of slip and crack occures faster as the stress level goes higher. These phenomena result from the plastic constraint effect of the second phase. 3) The crack propagation rate in the constant stress level is faster as the grain size gets larger, the ratio of hardness lower and volume fraction smaller.

• 한국해양공학회지
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• 제2권2호
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• pp.78-86
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• 1988

In order to investigate the corrosion pit occurrence and growth characteristic of M.E.F.(martensite encapsulated islands of ferrite) dual phase steel was made with a suitable heat treatment of raw material(SS41), a corrosion fatigue test was performed under rotary bending in the salt solution having a concentration from 0.01 wt percent to 3.5 wt percent. The fatigue strength of dual phase steel was remarkably decreased with an increase in concentration of salt solution; approximately from 63% to 80% in case of dual phase steel and from 40% to 71% in case of raw material. Corrosion pit occurred in the martensite phase and fatigue cracks from corrosion pits were selectively propagated in martensite phases. In the observation of corrosion pits at the origin of fatigue cracks, it had been found that corrosion pits were grown into hemispherical pits and a/c(the surface diameter, 2c and the depth, a of corrosion pit)was about 1.0-1.5regardless of the variation of salt solution concentration. The difference of corrosion pit depth growth rate was increased with an increase in concentration of salt solution according to an increase in stress level.

• 한국해양공학회지
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• 제9권2호
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• pp.141-148
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• 1995

Fatigue tests were carried out by a rotary bending testing machine of cantilever type. M.E.F.(ferrite encapsulated islands of martensite) materials were made by a series of heat treatment from a low carbon steel(SM 20 C). The fatigue tests were conducted at stress levels of 302 MPa and with frequencies of 25Hz, 2.5 Hz and 0.5 Hz in 3% NaCl solution. The fatigue strength increased with frequency got higher. The microcracks and corrosion pits were generated at the boundary between the matrix and the 2nd phase. The cracks generated by the corrosion pits were coalesced with the pits around the notch and became the initial crack. The $N_i/N_f$ ratio increased as the frequency and stress level decreased. The interference phenomenon increased with stress level and frequency gots higher. The crack propagation rate was delayed as the stress level lowers and the frequency gets higher, however, the range of the stress intensity factor depended only on a stress level.

• 대한기계학회논문집
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• 제11권6호
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• pp.867-876
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• 1987

본 연구에서는 보통강재인 SS 41 강을 열처리하여 얻은 M.E.F.복합조직강의 모상입경변화와 3.5% NaCl수용액의 pH변화 조건하에서 반복굽힘피로실험을 하여 부식 피로파괴에 미치는 영향에 대해 고찰하였다.

• 한국재료학회지
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• 제19권6호
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• pp.293-299
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• 2009

The microstructures and mechanical properties of Dual Phase (DP), Transformation-Induced Plasticity (TRIP), and Quenching & Partitioning (Q&P) steels were investigated in order to define the strengthening mechanism of 0.2 C steel. An intercritical annealing between Ac1 and Ac3 was conducted to produce DP and TRIP steel, followed by quenching the DP and TRIP steel being quenched at to room temperature and by the TRIP steel being austemperingaustempered-air cooling cooled the steel toat room temperature, respectively. The Q&P steel was produced from full austenization, followed by quenching to the temperature between $M_s$ and $M_f$, and then enriching the carbon to stabilize the austenite throughout the heat treatment. For the DP and TRIP steels, as the intercritical annealing temperature increased, the tensile strength increased and the elongation decreased. The strength variation was due to the amount of hard phases, i.e., martensite and bainite, respectively in the DP and TRIP steels. It was also found that the elongation also decreased with the amount of soft ferrite in the DP and TRIP steels and with the amount of the that was retained in the austenite phasein the TRIP steel, respectively for the DP and TRIP steels. For the Q&P steel, as the partitioning time increased, the elongation and the tensile strength increased slightly. This was due to the stabilized austenite that was enriched with carbon, even when the amount of retained austenite decreased as the partitioning time increased from 30 seconds to 100 seconds.