| Summary: | 碩士 === 大同大學 === 材料工程研究所 === 90 === Abstract
The properties of elongation and tensile strength of malleable iron are similar to low carbon steel. Before heat —treating the as cast condition is white iron. The microstructure of white iron is full of carbide and pearlite. The properties of carbide hard, and brittle. Besides high hardness and nice wear resistant, the white iron has no ductile almost and low impact value. With those poor mechanical properties, the application of white iron was limited. Malleable heat-treatment need very long time, so that is uneconomic. With ADI develop well in recently, ductile iron after austempered heat-treatment improve in strength, toughness, hardness, wear resistance, and fatigue resistance.
Although the microstructure of malleable iron is similar to ductile iron, but the papers to discuss malleable iron with austempered are poor, and the research of white iron with austempered has never seen. So this experiment try to do austempered heat treatment on white iron, hope to get ausferrite matrix, then discuss the properties and compare with malleable iron and ADI. According the result to evaluate the value of austempered heat-treatment of white iron. We hope to reduce the heat- treatment time and provide the research results to industry for application.
As the result showed: Both the higher of austenizing temperature and longer of keeping time are useful to eliminate carbide, and add copper element also the same result, and to eliminate carbide can reduce austenizing time. Change austempered temperature can produce other austempered structure, which could improve mechanical properties. Analysis microstructure discover the shape of temper carbon with AWI is not good than ADI, but the matrix is proportion for austenitizing that because produce very fine ausferrite to promote fracture toughness of white iron and other mechanical properties. Calculate retain austenite, that discover the high fracture toughness value (KIC) have higher retain austenite, upper ausferrite on fracture toughness are better than lower ausferrite. Although AWI fracture toughness is higher than as-cast, but see the fracture geography of SEM find still is brittle fracture. The last, compare with ADI and Malleable Iron discover, AWI fracture toughness is lower than ADI, and hardness of AWI is higher than both (ADI and Malleable Iron).
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