Effect Of Nitrogen, Titanium And Niobium On Microstructure And Properties Of Gray Cast Iron

Currently, the high strength gray cast iron is obtained by adding alloy elements, such as chromium, molybdenum, copper and nickel. However, the production cost grows with the increasing price of alloys. In order to reduce the production costs, micro-alloy elements, like nitrogen (N), titanium (Ti) and niobium (Nb) were added based on the composition of HT250 in this paper. The effects of N, Ti and Nb on microstructure and properties of gray cast iron were studied by means of optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), tensile and hardness test.The results indicate that microstructure of the samples containing 0.0055%-0.013%N and 1.0%-1.36%Mn was composed of A-type graphite, fine lamellar pearlite and a little ferrite. With the increase of N and Mn, the length of flake graphite descend, the width increased slightly, and graphite became more blunt, which reduced the adverse influence of graphite on the matrix; the fine lamellar pearlite content increased slightly, and the pearlite interlamellar spacings decreased. The tensile strength and hardness of samples increased gradually with the increase of N and Mn. The gray cast iron containing 0.012%N and 1.24%Mn, both the tensile strength and hardness of the specimens reached their maximum values, which were 395MPa and 260HBW respectively. Pores began appearing under casting surface when N content exceeded 0.011%.When N content was about 0.0080%, and the Ti content ranged between 0.055% and 0.149%, the microstructure of samples was A-type mixed with D-type graphite, and the matrix was pearlite and ferrite. With the rise of Ti content:the proportion of A-type graphite decreased while D-type graphite increased; the ferrite content in the samples increased and pearlite content decreased moderately. The tensile strength declined with the increase of Ti content. When the content of Ti was 0.149%, the tensile strength of the samples dropped to 230MPa whilst the hardness increased slightly to 219HBW. In gray cast iron containing N, the majority of Ti formed Ti-rich triangler, quadrangular and irregular carbonitrides, which embedded dispersedly in the matrix; the rest Ti dissolved in the matrix uniformly and formed solid solution.When N content was about 0.0080%, and the Nb was between 0.004%and 0.177%, the micro structure of samples was consisted of A-type graphite and pearlite. When the content of Nb exceeded 0.051%, D-type and E-type graphite appeared in the samples. With the rise of Nb content, the tensile strength and hardness of samples increased gradually. Both the tensile strength and hardness reached the highest point at 0.177%Nb, the corresponding values were 360MPa and 226HBW respectively. Most of Nb formed as Nb-rich carbonitrides, which were embedded in the matrix with several morphologies such as quadrate, diamond, irregular strip and bar; and a small amount of Nb dissolved in the matrix uniformly and formed solid solution.In summary, the tensile strength and hardness of gray cast iron with micro-alloying elements such as Mn-N and Nb-N were improved significantly, however, the tensile strength fell with Ti and N addition. The tensile strength and hardness of the samples, with 0.0085%N and 0.124%Mn content, reached 307MPa and 237HBW respectively, and there were no pores under the casting surface. Meanwhile, the tensile strength and hardness of the sample containing 0.0079%N and 0.177%Nb arrived at 360MPa and 226HBW, respectively.