Effect Of Cooling Rate On Microstructure And Properties Of High Nickel Ductile Iron

Owing to its excellent heat,fatigue and oxidation resistance,high nickel ductile iron has been widely applied in heat-resistant parts of automobile exhaust manifold and turbocharger housing.With the rapid development of automobile engine technology,the requirements of heat resistance of high nickel ductile iron get harsh.Meanwhile,the solidification progress plays an important part in casting quality and combination property.Thus,in order to get control of solidification process we need to do some further learning about it.As the physical parameters that constraints metal solidification process,cooling rate has a significant effect on the solidification microstructure of high nickel ductile iron.In this paper,cooling rates of high nickel ductile iron are calculated through numerical simulation.And the calculating samples moduli include 0.375 cm,0.6cm,0.75 cm and 1.5cm.The solidification microstructures of high nickel ductile iron such as nodular graphites,dendrites and carbides,are observed and analyzed with the color etching technology.And the mechanical properties of high nickel ductile iron such as hardness,tensile strength and impact energy are tested also.At last,a rule is established among the cooling rate,microstructure and mechanical properties.The study shows that:The solidification microstructure of high nickel ductile iron consists of morphologically developed austenite dendrites,among which distributes spherical graphite and a small amount of carbides.Austenite dendrites are divided into two types,primary dendrites and shell dendrites.The shape of primary dendrite is elongated,and arranges closely with a clear orientation reversing the heat flow.Shell dendrites grow out from the nodular graphites separated out firstly,and Shell dendrites` shapes are coarser with chaotic growth directions.The nodular graphites precipitated in the late eutectic crystallization mainly locate among the dendrite arms and druses.Small amount of carbides locate in the final solidification zones which are divided into island-like by austenite.The shapes of carbides are irregular.When the sample modulus increases from 0.375 cm to 1.5cm,the cooling rate of casting reduces from 1.58?/s to 0.89?/s,temperature gradients of solidification section are reduced at eutectic crystallization stage,eutectic platform on the cooling curve decreases.As the iron cooling rate increasing,the solidification microstructures of high nickel ductile iron become refined,the number of graphite increase from 93/mm2 to 167/mm2,the size of them reduces from 51?m to 29?m;The morphology of dendrite also become refined,the diameter of dendrite spindle decreases from 43?m to 28?m,and that of primary dendrite arm decreases from 47?m to 17?m,primary dendrite arm space decreases from 38?m to 16?m;The length of carbide decreases from 109?m to 90?m,while the content of carbide reduces from 6.5% to 4.0%.With the increase of cooling rate,mechanical properties tend to promote,the tensile strength increases from 340 Mpa to 355 Mpa,the hardness raises from 143 HBS to 155 HBS,while the impact energy decrease from 12 J to 11 J.