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      • CO_2 레이저 表面硬化 處理된 AISI 4140 鋼의 疲勞特性에 관한 硏究

        박근웅,이상윤 동아대학교 공과대학부설 생산기술연구소 1996 生産技術硏究所硏究論文集 Vol.1 No.-

        This study has been performed to investigate into some effects of the power density and traverse speed of laser beam on the optical microstructure, hardness and fatigue resistance of AISI 4140 steel treated by laser surface hardening technique. Optical micrograph has shown that the coarsening of lath martensite and the reduction of usdissolved complex carbides occur with increasing the power density at a given traverse speed. Hardness measurements have revealed that ad the power density increase, hardness value of outermost surface layer increase from H_v=630 to H_v=685 in case of traverse speed of 2.0m/min and hardening depth also increases considerably, showing that hardness values significantly decrease at the boundary between the surface hardened layer and the core. Fatigue test has exhibited that the fatigue strength of laser surface hardened specimen is superier compared to that of untreated specimen, showing that values for the fatigue strength at Nf=10^7 AISI 4140 steel laser-surface-hardened at a low power density of 4076w/㎠ and a high power density of 8153w/㎠ under the condition of a given traverse speed of 2.0m/min are 39kgf/㎟ and 48kgf/㎟, respectively, whereas the fatigue strength of untreated specimen is 26kgf/㎟. Under high stress condition it has been shown that a rapid cleavage fracture appears on the surface hardened layer with a fatigue fracture start point of surface at the tension-compression value of R=-1 and some dimples of ductile fracture are formed in the core. Under low stress condition small amount of oxides appears at the outermost area of fracture surface due mainly to mutual friction occurring during fatigue test and the beach mark of typical fatigue fracture is shown in the vivinity of fatigue fracture start point.

      • CO₂ 레이저 분산빔에 의한 표면경화가 구상흑연주철에 피로특성에 미치는 영향

        박근웅,이상윤,한유희 한국레이저가공학회 1999 한국레이저가공학회지 Vol.2 No.2

        This study has been performed to investigate into some effects of the output power and traverse speed of laser beam on the microstructures, hardness and fatigue resistance of the ductile iron surface-hardened by CO₂ laser defocussed beam. Optical micrographs have shown that with increasing the output power and decreasing the traverse speed, the martensite was coarsened and some retained austenite were appeared in ductile iron. The microstructures of hardening zone were composed of bull's eye and some nodular graphite dissolved structures by the effect of self quenching. Fatigue fracture characteristics of ductile iron have appeared in the high stress and low stress ranges. The fracture initiated at nodular graphites in the surface hardened layer due to the stress concentration caused by a notch effect. The interior graphite nodules were broken away or popped out during crack propagation. Fatigue test has shown that values of fatigue strength considerably increased with increasing output power at a given traverse speed.

      • KCI등재

        Co2레이저 비임을 이용한 표면경화 처리중 형성된 AISI 4140의 잔류응력에 관한 연구

        이상윤,한유희,이준범,박근웅 한국열처리공학회 1996 熱處理工學會誌 Vol.9 No.4

        This study has been performed to investigate into some effects of power density and traverse speed of laser beam on the microstructure, hardness and residual stress of AISI 4140 treated by laser surface hardening technique. Optical micrograph has shown that large martensite and a small amount of retained austenite in outermost surface layer and fine lath martensite in inner surface hardened layer are formed under the condition of a given power density and traverse speed. Hardness measurements have revealed that as the power density increases at a given 2.0m/min of the traverse speed, the maximum hardness values of outermost surface hardened layer is increased from Hv=635 to Hv=670. X-ray analysis for residual stress has exhibited that low compressive residual stress values are obtained in center point of the cress section of surface hardened layer with in mid point between the edge and the center point, about 1.5㎜ from the center point, due simply to a difference in self-quenching rate. It has been shown that the higher the power density at a given traverse speed and the olwer the traverse speed at a given power density, the more the compressive residual stress values are increased due to an increase in the input heat of laser beam.

      • KCI등재

        레이저 표면경화처리된 회주철의 내마모특성에 관한 연구

        이상윤,한유희,박근웅 한국열처리공학회 1996 熱處理工學會誌 Vol.9 No.4

        This study has been performed to investigate into some effects of power density and traverse speed of laser beam on optical microstructure, hardness and wear characteristics of gray cast iron treated by laser surface hardening technique. Optical micrograph has shown that large martensite and a small amount of retained austenite appear in outermost surface layer with fine martensite in inside hardened zone. Hardness measurements have revealed that the range of maximun hardness value is Hv=650±15 and as the power density increases and the traverse speed decreses, the depth of hardened zone increases due to as increase in input power density. Wear test has exhibited that wear rasistance of laser surface hardened specimen is superier compared to that of untreated specimen under the condition of same load at a given sliding distance, showing that absorption results of an wxidized substance due to a heavy abrasion appear in untreated specimen. The amount of weight loss of laser surface hardened specimen with respect to sliding distance at a given load decreses with increasing traverse speed at a given power density and with increasing power density at a given traverse speed.

      • KCI등재

        CO2레이저 표면경화 처리된 회주철의 피로특성에 관한 연구

        이상윤,한유희,박근웅 한국열처리공학회 1995 熱處理工學會誌 Vol.8 No.3

        This study has been performed to investigate some effects of the power density and traverse speed of laser beam on the optical microstructure, hardness and fatigue resistance of gray cast iron treated by laser surface hardening technique. Optical micrograph has shown that the dissolution of graphite flakes and the coarsening of lath martensite tend to increase with a small amount of retained austenite as the power density increases under the condition of a given traverse speed. Hardness measurements have revealed that as the power density increases, hardness values of outermost surface layer increases from Hv=620 to Hv=647 in case of traverse speed of 2.0m/min at gray cast iron. Fatigue test has exhibited that the fatigue strength of laser surface hardened specimen is superier compared to that of untreated specimen, showing that values for the fatigue strength at N_f=10^7 of gray cast iron laser-surface-hardened at a low power density of 4076w/㎠ and a high power density of 8153w/㎠ under the condition of a given traverse speed of 2.0m/min are 15㎏/㎟ and 20㎏_f/㎟, respectively, whereas the fatigue strength of untreated specimen is 11㎏_f/㎟. Under high stress-low cycle condition a noraml brittleness fracture appears, whereas a ductile fracture with beach mark is observed in the specimen tested under low stress-high cycle condition.

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