• Journal of Welding and Joining
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• 제11권4호
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• pp.79-90
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• 1993

Shielded metal arc welding, one-ploe and two-pole submerged arc welding were accomplished to investigate microstructure changes on phase transformation behavior in $60kg/mm^2$ quenched and tempered high strength steel. Microstructures were examined by optical micrograph and TEM. In shielded metal arc welding (oxygen 250ppm), the inclusions were small size (0.3-0.5$\mu\textrm{m}$)and small in number. In submerged arc welding (oxygen 430-529ppm), the inclusions were larger(0.7-2$\mu\textrm{m}$) than that of shielded metal arc welding and large in number. Microstructure mainly depends on number and distribution of inclusions in fusion zone of weld metal. It was noticed that a limited number of inclusions favors the formation of acicular ferrite.

• 열처리공학회지
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• 제4권3호
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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.

• 열처리공학회지
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• 제14권5호
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• pp.267-274
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• 2001

Microalloyed steels, which substituted by conventional quenched and tempered steels, have been used in a wide variety of structural and engineering application. The main driving force for preference of MA steels is a cost reduction which can be achieved by an omission of heat treatment. In this study, low carbon martensitic MA steels in 0.18C-0.30(0.60)Si-2.00(1.80)Mn-0.05S-1.5Cr-0.05(0.10)V-0.015Ti(wt%) were investigated to know the effects of cooling method on the mechanical properties and microstructures of Si, Mn, V added microalloyed steel at different reheating temperature. Microstructure of oil quenched steels which were comprised lath martensite, auto-tempered martensite and retained austenite, had more various structure than that of air cooled steel made of mainly bainite. Therefore, oil quenched steels, which had more various microstructure, had better strength-toughness balance compare to air cooled steels. In the impact test, fracture mode of oil quenched steels, which showed good mechanical properties, were dimple, but that of air cooled steels were cleavage.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2009년도 추계학술대회 논문집
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• pp.117-118
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• 2009

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2004년도 춘계학술대회 논문집
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• pp.395-399
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• 2004

Four microalloyed steels containing B were investigated in terms of hardenability, mechanical properties and microstructure depending upon the cooling rates in order to develop the steel grade for the cold forged fasners. The alloy with the largest DI value among 4 alloys, which contains $0.12\%\;C,\;1.54\%\;Mn,\;0.65\%\;Cr,\;0.11\%V,\;0.040\%Ti\;and\;0.0033\%B$, showed the larest shift to the right hand side in the TTT diagram, implying the wide allowable cooling rate range subsequent to hot rolling in long bar processing, Mechanical tests indicated that yield strength are dependent upon the DI value in water quenched specimens but other properties showed almost the same values. In the same grade of steel, the increase in cooling rates causes the decrease in elongation but the increase in strength, reduction of area and Charpy impact values. Microstructural examination in steel grade with the larest DI values revealed martensitic structure In the water quenched state, a mixture of martensite and bainite in the oil quenched, and ferrite + pearlite in the air cooled and the forced air cooled but the latter showed finer microstructure.

• 대한조선학회지
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• 제21권1호
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• pp.35-42
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• 1984

In the present study, rotating bending fatigue tests have been carried out in three kinds of carbon steel specimens; an annealed low carbon steel, an annealed high carbon steel and quenched-tempered high carbon steel; with a small artificial surface defect that might exist in real structures. Fatigue crack lengths have been observed by a method of replication in order to investigate the growth characteristic of fatigue crack in the viewpoints of strength of materials and fracture mechanics. The main results obtained are as follows: 1) The effect of a small surface defect upon the reduction of fatigue limit is considerably large, and the rate of fatigue limit reduction grows in the following order; annealed low carbon steel(mild steel), annealed high carbon steel, quenched-tempered high carbon steel. 2) When the growth rate of surface crack length(2a) was investigated in the viewpoints of fracture mechanics based upon $ ${\Delta}K_{\varepsilon}$, the dependence of stress level and of surface defect size disappear, and there exists a linear relationships between d(2a)/dN and ${\Delta}K_{{\varepsilon}t},\;\Delta_{{\varepsilon}t}\sqrt{{\pi}a}$, on log. plot, i.e, $d(2a)/dN={C{\cdot}{\Delta}K_{\varepsilon}}^3_t$, where ${\Delta}_{{\varepsilon}t}\sqrt{{\pi}a}$ a is the cyclic total strain intensity factor range.

• Journal of Welding and Joining
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• 제33권1호
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• pp.41-48
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• 2015

In this study, the effect of heat input on weld metal microstructure and the effects of dissimilar weld heat affected zone in quenched and tempered ASTM A517 on the stainless steel AISI 316L is investigated through the optimization of welding parameters. For this purpose, two welding techniques are used, tungsten-conventional gas and pulsed gas with weld wire ER 309MoL with Diameter 2.4 mm. Research showed that the grain size of the heat affected zone in pulsed welding is less compared with conventional welding; weld metal structure is fully austenitic, it has a finer structure in the pulsed method. Additionally, the growth of weld metal adjacent steel A517 is different from steel 316L. Further, investigation showed that the rate of dilution is less in the pulsed method and the impact energy is increased in each three regions of the weld metal and heat affected zones in the pulsed method; the fracture in the weld metal and heat affected zone of steel 316L is quite soft and it is semi-crispy in the heat affected zone of steel A517.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
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• pp.570-571
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• 2006

The mixed atomized iron powders with 0.3% graphite and 1% Ni powders were cold pressed and sintered at $1200^{\circ}C$ for 30 min under pure Ar gas atmosphere. Some of the sintered specimens were intercritically annealed at $760^{\circ}C$ and quenched in water (single quenching). The other sintered specimens were first fully austenized at $890^{\circ}C$ and water quenched. These specimens were then intercritically annealed at $760^{\circ}C$ and re-quenched in water. The experimental results showed that the wear coefficient effectively decreased in the double quenched specimen.

• 소성∙가공
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• 제20권8호
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• pp.588-594
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• 2011

A new manufacturing process is presented for automotive drive plate using a boron-containing carbon steel sheet, which is hot-formed and press quenched. Particular attention was given to the capability of the process in minimizing dimensional change.

• Journal of Welding and Joining
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• 제6권3호
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• pp.27-36
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• 1988

In fabrication of various welded structures made of high strength steels, the occurence of hydrogen assisted cracking and embrittlement in HAZ is prime importance. The present work was carried out to clarify the effect of chemical compositions, especially B and/or Ti addition on the cold cracking susceptibility and HAZ embrittlement in low crabon equivalent steel. Tests results showed that the addtio of optimum boron content in steel with low Pem value i.e., below 0.20 % was the best way to improve the weldability as well as the mechanicla properties of $60kg/mm^2$ grade quenched and tempered high strength steels.