| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 100 === This research is focus on using carbon steel with different carbon content to friction weld with free cutting AlSI303 Cu Austenite stainless steel. Microstructure & mechanical analysis on this friction weld will be analyzed using material science. Plenty of research papers claimed that, the welding ability worsen with the increase in carbon content, Austenite stainless steel typically has heat fissure and its’ corrosive resistance decreases after weld. This research uses different carbon content steel in market to weld with free cutting Austenite stainless steel. Using same friction welding parameters, as well as utilizing different mechanical properties testing, the research discuss about the impact of material structure & mechanical properties of friction welding between different carbon content and free cutting AlSI303 Cu Austenite stainless steel.
Experimental results showed all joints of the friction welding samples have very obvious connecting line. No cracks, weld seam, porosity or other defects were observed near the joints. Flash O.D. (outer diameters) were measured on all samples, and observed that flash O.D. of carbon steel is at 1.03 – 1.17 times of that Austenite stainless steel. The range of HAZ was also measured; the HAZ in carbon steel is larger than Austenite stainless steel, around 2.6 – 3.2 times of Austenite stainless steel. This illustrates the Flash O.D. & HAZ of carbon steel is larger than Austenite stainless steel
Because temperature of friction welding is in the temperature range of sensitization, that causes plenty of carbide at the friction weld area of 303Cu Austenite stainless steel. Therefore the friction weld area is less corrosive resistant than the base matel of 303Cu Austenite stainless steel.
By examining the microstructure at the fractured region friction welded joint for S15C / 303Cu, S45C / 303Cu, there are obvious vortex & sheet like cracks on the tensile test specimen. This is a typical structure of frictional welding. For material SK4 / 303Cu, SK2 / 303Cu, fracture region occurred at the side of the carbon steel friction welded joint. Observation on the microstructure of tensile test specimen, shows no vortex like cracks, instead it has transgranular fracture. The measured hardness on the side is HV806 (SK4), HV510 (SK2) respectively. The friction welded joint surface of 303Cu has a hardness of HV180.
The tensile strength of SK4 / 303Cu welded joint is only 72% of that base carbon steel material. The material analysis result of SK4 shows 1.492% of chromium. Chromium is responsible for material hardness. This will cause microstructure change into quenched Martensite & Austenite residue after SK4 is being friction welded. Quenched Martensite will cause brittleness, and formed transgranular fracture pattern. Therefore, the welded joint strength of SK4 / 303Cu is not desirable.
The tensile strength of S15C / 303Cu and S45C / 303Cu are 83% and 81% of their base matel of carbon steel respectively, failure mode is intergranular fracture, which happened at the friction welded joint. The microstructure of the fracture surface in S15C / 303Cu is fine ferrite, partially cementite and with plenty of micro vacants. For S45C / 303Cu analysis on the grain boundary illustrates ferrite and partially fine pearlite.
The tensile strength of SK2 / 303Cu is 86% of its’ SK2 base matel, the fracture mode of this material is mainly transgranular fracture, its’ fracture surface is on the SK2 side. The friction weld microstructure analysis on grain boundary shows cementite structure, with fine pearlite as the based; its’ highest hardness is HV510. The fine pearlite and cementite SK2 / 303Cu has a better quality in welding.
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