Study Of Niobium Micro-alloying On Gray Cast Iron Used In6K Cylinder Heads

6K cylinder head was one of the most important parts of a6K diesel engine. In order to increase the service life of the head, gray cast iron with some trace alloying element is probably an effective way in obtaining suitable materials. This topic purpose is:based on gray cast iron which was made in a domestic company, by adding trace Nb element, improve the mechanical properties of6K cylinder head,but not significantly increase cost, make it meet application requirements.This article, by testing metallographic structure and mechanical properties of gray cast iron samples, analyzed the form of Nb on gray cast iron,the influence law and mechanism of Nb in gray cast iron, and based on this, discussed the influence on the structure and properties of6K cylinder head, determined the most appropriate adding amount of Nb. Finally, developed a kind of Nb microalloyed high strength gray cast iron cylinder head, meet the application requirements.Research shows that there are three forms of Nb on in gray cast iron:(1) before the solidification of gray cast iron, Nb and C combined to form small NbC particles in liquid iron, these particles may act as nucleation site resulting in refining of graphite and eutectic cell;(2) dissolved in the matrix to solid solution strengthening;(3) formed niobium-rich phases embedded in the boundary of eutectic cells, which is the main form of Nb. Niobium-rich phase was formed at the end of the eutectic reaction, it was carbide containing Nb, Mo and Ti, bar shaped (including Y type and V type) with lengths ranging from2-30μm.By adding Nb, pearlite transformation temperature was reducedsaturated, degree of eutectoid undercooling was increased, and the drag effect of dissolved Nb prevented the diffusion of carbon atoms, so the pearlite spacing was reduced. Bar shaped niobium-rich phases embedded in the boundary of eutectic cells precipitation that consumed parts of carbon element and prevented the formation of bulk grain boundary carbides and steadites, so carbides and steadites were refined. By increasing niobium content, the lamellar spacing of pearlite was reduced; the graphite, eutectic cell, carbide and steadite were refined. Therefore, properties including tensile strength, toughness, fatigue property and thermal fatigue property were improved.When the Nb content was0.20%, the tensile strength and the impact energy were maximum, and fatigue life and thermal fatigue life were significantly improved. For plates which simulated cylinder head, the strength increased to342MPa and maturity got to1.20, which indicated0.20%Nb gray cast iron had high metallurgical quality. Therefore, the0.20%was determined as the most suitable addition amount of Nb in cylinder head.In the the original HT250, by adding0.20%Nb elements and pouring cylinder head body, we have successfully developed a Nb micro-alloyed high strength gray iron cylinder head. Body strength of the cylinder head was more than300MPa and achieved the requirement of HT300brand. The cylinder head had passed the1000hours thermal shock durability bench test and2000hours "11work conditions" durability bench test, which met the application requirements.