Effect of molybdenum on matrix structure of gray cast iron

(a) Sample S1; (b) Sample S2; (c) Sample S3; (d) Sample S4

According to the different matrix structure, it can be divided into ferrite matrix gray cast iron, pearlite ferrite matrix gray cast iron and pearlite matrix gray cast iron. The strength and hardness of ferrite matrix are poor. Therefore, people prefer to obtain gray cast iron with complete pearlite matrix structure in practical production. However, we can see from Figure 1 that with the increase of molybdenum content, the pearlite content in gray cast iron generally shows a downward trend. Compared with S1 (0.034% Mo) sample, the pearlite content of S3 (0.56% Mo) and S4 (0.77% Mo) sample is significantly reduced and the ferrite content is increased. Mitchell and Á Vila et al. also found that the addition of molybdenum would promote the formation of ferrite and reduce the content of pearlite. For hypoeutectic cast iron, graphite precipitates ahead, the growth of graphite austenite eutectic group begins with the precipitation of graphite, the flake graphite phase leads into the liquid phase first, and the front end of the graphite sheet contacts with the liquid phase, resulting in carbon deficiency in the molten iron around the graphite. The carbon atoms in molten iron are continuously supplemented by carbon atoms through diffusion, so as to always keep the lead. When the carbon concentration of the surrounding liquid phase decreases, austenite crystallizes.

Therefore, the austenite around graphite is always in a carbon poor state, while the addition of molybdenum improves the undercooling. During the formation of pearlite, ferrite is the leading phase and nucleates preferentially [20]. It can also be seen from Fig. 2 that ferrite precipitates before cementite, and when cementite nucleates and grows later, carbon atoms of surrounding austenite are absorbed to make carbon poor austenite area appear around. Therefore, in the last stage of eutectoid reaction, when pearlite grows up in the carbon poor austenite area around graphite, after ferrite nucleation, the lack of carbon atoms in cementite inhibits the formation of pearlite, resulting in ferrite around graphite. It can also be seen from Figure 1 that almost all ferrite grows around graphite. It can be clearly observed from Fig. 3 that molybdenum can refine the pearlite structure and significantly reduce the pearlite lamellar spacing

(a) Sample S1; (b) Sample S2; (c) Sample S3; (d) Sample S4

The smaller the pearlite sheet spacing, the thinner the ferrite and cementite sheets, and the more the phase interface. The dislocations in ferrite are not easy to slide, so the higher the plastic deformation resistance. When the external force is large enough, the dislocation source slides from the ferrite center to the cementite sheet, which is blocked by the cementite sheet. The thinner the cementite and ferrite sheet, the fewer dislocations are blocked, and the smaller the normal stress caused by dislocations in the cementite sheet, the less likely it is to cause cracking, and the higher the tensile strength.

According to the measurement results in Fig. 4, the pearlite layer spacing with 0.77% molybdenum is 166 nm lower than that without molybdenum, because molybdenum increases the eutectoid undercooling. The greater the undercooling, the smaller the layer spacing. Molybdenum dissolved in austenite inhibits and hinders the diffusion of carbon atoms, shortens the migration distance of carbon atoms, delays the decomposition and diffusion process of austenite, and prolongs the incubation period of pearlite. Figure 5 shows the simulation results of J-Mat Pro software (cast iron database). It can be seen that the addition of molybdenum causes the isothermal transformation curve of pearlite to move to the right obviously. The above effects of molybdenum in the eutectoid process of gray cast iron eventually lead to the reduction of pearlite layer spacing, which is helpful to improve the tensile strength of gray cast iron.