High strength ductile iron is an important engineering structural material. Although it came out late, it has good machinability and low manufacturing cost. Now it has been widely used to manufacture parts with complex stress and high requirements of strength, toughness and wear resistance, such as automobile rear axle, gear, crankshaft, connecting rod, locomotive wheel, roller and grinding ball. The existence of carbide is an important factor affecting the wear resistance and impact toughness of cast iron. Niobium is a strong carbide forming element, which can be limited solid solution in austenite, which is helpful to improve the stability and hardenability of undercooled austenite. The results show that the addition of niobium can refine the grain size and improve the mechanical properties of cast iron; The results show that the interlaminar spacing of pearlite decreases obviously, the graphite is refined, and the tensile strength, hardness and impact toughness of gray cast iron are improved; In addition, the wear resistance, strength and impact toughness of high chromium cast iron are improved by adding niobium.
The solidification cooling curve of Mn Cu Mo nodular cast iron was measured by thermal analysis method, and the as cast structure was quantitatively analyzed by image analysis software. The effect of niobium on eutectic temperature and as cast structure of nodular cast iron was studied, so as to provide theoretical basis for improving the properties of nodular cast iron.
(1) With the increase of Nb content, the eutectic temperature of nodular cast iron increases from 1133 ℃ to 1137 ℃, and then decreases to 1132 ℃, with little change, that is, Nb has little effect on the eutectic temperature of nodular cast iron.
(2) With the increase of Nb content, the graphite area fraction of nodular cast iron increases from 15% to 16% and then decreases to 14%, which means that Nb has little effect on the graphitization of nodular cast iron. However, with the increase of Nb content, the amount of spheroidal graphite in nodular cast iron decreases and the morphology of graphite deteriorates. When the niobium content is less than 0.42%, the spheroidization rate of graphite is 92% ~ 89%; When the niobium content increases to 0.69%, the spheroidization rate decreases to 84%, that is to say, the higher the niobium content, the greater the influence on the spheroidization rate.
(3) The addition of niobium reduces the ferrite content and refines the pearlite in the matrix of as cast ductile iron.