• Transactions of Materials Processing
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• v.30 no.2
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• pp.91-98
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• 2021

The present study deals with the microstructure and mechanical properties of 700 MPa-grade high-strength seismic resistant reinforced steel bars fabricated by various TempCore process conditions. For the steel bars, in the surface region tempered martensite was formed by water cooling and subsequent self-tempering during TempCore process, while in the center region there was ferrite-pearlite or bainite microstructure. The steel bar fabricated by the highest water flow and the lowest equalizing temperature had the highest hardness in all regions due to the relatively fine microstructure of tempered martensite and bainite. In addition, the steel bar having finer microstructures as well as the high fraction of tempered martensite in the surface region showed the highest yield and tensile strengths. The presence of vanadium precipitates and the high fraction of ferrite contributed to the improvement of seismic resistance such as high tensile-to-yield strength ratio and high uniform elongation.

• Korean Journal of Materials Research
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• v.32 no.11
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• pp.466-473
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• 2022

The effect of Cr and Mo contents on the hydrogen embrittlement of tempered martensitic steels was investigated in this study. After the steels with different Cr and Mo contents were austenitized at 820 ℃ for 90 min, they were tempered at 630 ℃ for 120 min. The steels were composed of fully tempered martensite with a lath-type microstructure, but the characteristics of the carbides were dependent on the Cr and Mo contents. As the Cr and Mo contents increased, the volume fraction of film-like cementite and prior austenite grain size decreased. After hydrogen was introduced into tensile specimens by electrochemical charging, a slow strain-rate test (SSRT) was conducted to investigate hydrogen embrittlement behavior. The SSRT results revealed that the steel with lower Cr or lower Mo content showed relatively poor hydrogen embrittlement resistance. The hydrogen embrittlement resistance of the tempered martensitic steels increased with increasing Mo content, because the reduction in the film-like cementite and prior austenite grain size plays an important role in improving hydrogen embrittlement resistance. The results indicate that controlling the Cr and Mo contents is essential to achieving a tempered martensitic steel with a combination of high strength and excellent hydrogen embrittlement resistance.

• Korean Journal of Materials Research
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• v.25 no.11
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• pp.607-611
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• 2015

The microstructural evolution of Grade 91 tempered martensite ferritic steels heat treated at $760{\sim}1000^{\circ}C$ for two hours was investigated using scanning electron microscopy(SEM), energy disperse spectroscopy(EDS), electron backscattered diffraction (EBSD), and transmission electron microscopy(TEM); a microhardness tester was also employed, with a focus on the grain and precipitate evolution process as well as on the main hardening element. It was found that an evolution of tempered martensite to ferrite($760{\sim}850^{\circ}C$), and to fresh martensite($900{\sim}1000^{\circ}C$), occurred with the increase of temperature. Simultaneously, the parabolic evolution characteristics of the low angle grain boundary(LAGB) increased with the increase of the heating temperature(highest fraction of LAGB at $925^{\circ}C$), indicating grain recovery upon intercritical heating. The main precipitate, $M_{23}C_6$, was found to be coarsened slightly at $760{\sim}850^{\circ}C$; it then dissolved at $850{\sim}1000^{\circ}C$. Besides this, $M_3C$ cementite was formed at $900{\sim}1000^{\circ}C$. Finally, the experimental results show that the hardness of the steel depended largely on the matrix structure, rather than on the precipitates, with the fresh martensite showing the highest hardness value.

• Journal of the Korean Society for Heat Treatment
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• v.4 no.3
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• pp.54-62
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• 1991

The effect of tempering temperature on the ultrasonic propagation velocity at SCM 440 steel quenched from $870^{\circ}C$ and $1000^{\circ}C$ has been studied by metallurgical and crystallographical observation. The measurements of ultrasonic velocity were made on the specimen by appling an immersion ultrasonic pulse-echo technique with a constant frequency of 10 MHz. The quenched microstructure of this steel was a lath martensite. As the tempering temperature was increased, the martensite was transformed into the tempered martensite composed of cementite and carbide. The ultrasonic velocity increased with increasing the tempering temperature. It was thought that these were resulted from the microstructural transformation. The change of ultrasonic propagation velocity with quenching and tempering heat treatment was resulted from microstrain due to the change of internal stress. Considering these results concerning to the change of ultrasonic propagation velocity. the phenomena of microstructural transformation were estimated. Consequently, it was thought that the degree of quenching and tempered heat treatment of steel could be nondestructively evaluated with the change of ultrasonic propagation velocity.

• Journal of the Korean Society for Heat Treatment
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• v.20 no.1
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• pp.38-44
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• 2007

• Transactions of Materials Processing
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• v.27 no.5
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• pp.281-288
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• 2018

This study deals with the microstructure and tensile properties of 600 and 700 MPa-grade high-strength seismic reinforced steel bars. High-strength seismic resistant reinforced steel bars (SD 600S and SD 700S) were fabricated by TempCore process, especially the SD 700S specimen was more rapid cooled than the SD 600S specimen during the TempCore process. Although two specimens had microstructure of tempered martensite in the surface region, the SD 600S specimen had ferrite-degenerated pearlite in the center region, whereas the SD 700S specimen had bainite-ferrite-degenerated pearlite in the center region. Therefore, their hardness was highest in the surface region and revealed a tendency to decrease from the surface region to the center region because tempered martensite has higher hardness than ferrite-degenerated pearlite or bainite. The SD 700S specimen revealed higher hardness in the center region than SD 600S specimen because it contained a larger amount of bainite as well as ferrite-degenerated pearlite. On the other hand, tensile test results indicated the SD 600S and SD 700S specimens revealed continuous yielding behavior because of formation of degenerated pearlite or bainite in the center region. The SD 600S specimen had a little higher tensile-to-yield ratio because the presence of ferrite and degenerated pearlite in the center region and the lower fraction of tempered martensite enhance work hardening.

• Korean Journal of Materials Research
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• v.28 no.7
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• pp.391-397
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• 2018

This study deals with the microstructure and tensile properties of 700 MPa-grade high-strength and seismic reinforced steel bars. The high-strength reinforced steel bars (600 D13, 600 D16 and 700 D13 specimens) are fabricated by a TempCore process, while the seismic reinforced steel bar (600S D16 specimen) is fabricated by air cooling after hot rolling. For specimens fabricated by the TempCore process, the 600 D13 and 600 D16 specimens have a microstructure of tempered martensite in the surface region and ferrite-pearlite in the center region, while the 700 D13 specimen has a microstructure of tempered martensite in the surface region and bainite in the center region. Therefore, their hardness is the highest in the surface region and shows a tendency to decrease from the surface region to the center region because tempered martensite has a higher hardness than ferrite-pearlite or bainite. However, the hardness of the 600S D16 specimen, which is composed of fully ferrite-pearlite, increases from the surface region to the center region because the pearlite volume fraction increases from the surface region to the center region. On the other hand, the tensile test results indicate that only the 700 D13 specimen with a higher carbon content exhibits continuous yielding behavior due to the formation of bainite in the center region. The 600S D16 specimen has the highest tensile-to-yield ratio because the presence of ferrite-pearlite and precipitates caused by vanadium addition largely enhances work hardening.

• Journal of the Korean Society for Nondestructive Testing
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• v.18 no.4
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• pp.292-303
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• 1998

Recent study has demonstrated that some magnetic properties are sensitive to the microstructural state of material. The ASTM A 508 Gr. 3 reactor pressure vessel steel has various microstructural changes including martensitic and bainitic phases, and various sizes of grain and precipitates in the weld heat-affected zone (HAZ). To correlate the microstructural state with Barkhausen noise (BN), specimens were prepared through simulating various weld thermal cycles using a thermal simulator. The conventional magnetic properties, i.e. coercive force, remanence and maximum induction, did not change significantly, whereas the BN amplitude and energy during a magnetization cycle changed markedly with microstructural state. The BN increased with increasing grain and carbide sizes, and the tempered bainite structure showed higher BN parameter than tempered martensite.

• Journal of the Korean Society for Heat Treatment
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• v.34 no.1
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• pp.1-9
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• 2021

The martensitic stainless steel has excellent corrosion resistance and higher strength by quenching and tempering heat treatment. It has been widely used as blade material due to these properties. The hardness and impact toughness of martensitic stainless steel depended strongly on tempering temperatures. The 12Cr martensite stainless steel (SS 410) tempered about 540℃ showed temper embrittlement. To know cause of temper embrittlement in terms of phase identification, a detailed analysis of electron diffraction patterns during TEM observations has been carried out on the <110>α-Fe and <113>α-Fe zone axes for temper embrittlement specimen. The double electron diffraction spots at 1/3(211) and 2/3(211) positions were observed. The lattice space between individual diffraction spots was about 3.5 Å and this value coincide with three times to α-bcc lattice space (1.17 Å). The area which found double diffraction spots was judged metastable "zone" similar to the omega phase and induced embrittlement of SS410 material.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1861-1868
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• 1992

Laser beam hardening of 12%-Cr steel has been evaluated by using a continuous wave 3kW co$_{2}$ laser with a hardening mirror set. Experiment was performed on the hardening condition with a laser power of 2.85kW and travel speed of 1.0 and 1.5m/min. Multi passes have been also tried to find the hardening characteristics of partly overlapped zone. The black paint to use at high temperature was adopted to increase the absorptivity of laser beam energy with the wavelength of 10.6.mu.m at the surface of base metal. The microstructure of the hardened layers was observed by using a light microscopy, SEM and TEM. A fine Lamellar martensite formed in the hardened zones exhibits very high Vickers microhardness of 600Hv, whereas the tempered martensite distributes in the base metal with Vickers microhardness of 240Hv.It has been found that laser hardening with multi passes showed no significant drop of the hardness between adjacent passes.