873∼1273 K에서 열화된 강화흑연강의 기계적 특성 및 비파괴적 평가에 관한 연구

Abstract

Compacted graphite cast iron (CGI), also known as vermicular graphite iron, is a metal which is gaining popularity in applications that require either greater strength, or lower weight, than cast iron. The graphite in compacted graphite cast iron differs in structure from that in gray iron, because the graphite particles are shorter and thicker. This results in stronger adhesion between the graphite and the iron, giving the material a greater tensile strength. The first commercial application for compacted graphite cast iron was for the brake discs for high speed rail trains. More recently, compacted graphite iron has been used for diesel engine blocks. It has proven to be useful in the manufacture of V topology diesel engines, where the loading on the block is very high between the cylinder banks, and for heavy goods vehicles, which use diesel engines with high combustion pressures. It is also used for turbo housings and exhaust manifolds, in the latter case to reduce corrosion. The properties of CGI340 did not yet provide more detailed information on the mechanical and physical properties of CGI340 as a function of the graphite nodularity. Heat treatment was conducted on CGI340 from 873 to 1273 K. CGI340 without heat treatment has 396 MPa, max tensile strength, 2.45 %, elongation, 163 Hv, Vickers hardness. CGI340 with the pencil lead fracture test has 160 ∼ 200 kHz frequency area, and 2500∼2700 km/sec extensional mode speed. The microstructure of CGI340 showed spheroidal graphite and vermicular particles (worm-shaped). In the result of acoustic emission analysis according to tensile test, AE signal of the high frequency area from the yield point was obtained. CGI340 was evaluated for the relationship between the sound velocity, attenuation coefficient and tensile strength. The obtained results are as follows. The signal strength of the C scan image decreased with the increase of heat treatment temperature and time. The amplitude of A scan and B scan were also low. The graphite that was grown was of vermicular type, and the grain boundary with much ultrasound scattering was increased. The sound velocity was dependent on the heat treatment temperature and time, and the attenuation coefficient did not have correlations. The higher the heat treatment temperature, the smaller the tensile strength and the sound velocity. However, the tensile strength was proportional to the sound velocity. The sound velocity was faster, in accordance with the high tensile strength. CGI340 was evaluated for wear characteristics, according to DLC coating and DLC synthesis. The obtained results are as follows: The microstructure of CGI340 shows both spheroidal graphite and partial disposition graphite. The mechanical properties of CGI340 were tensile strength of 411.5 MPa, yield strength of 313.8 MPa, elongation of 2.7% and micro Vickers hardness of 318 Hv. We could identify DLC by micro-RAMAN, scratch test and nano-indentation. The friction coefficient shows excellent value of under 0.2 in all specimens. A DLC coating of 90 min was the best condition.