| With the large-scale and lightweight developments of engineering machinery steel, the design of micro-alloying ingredient and appropriate heat treatment process are conmbined to improve the intensity level and meet other performance requiremernts of mechanical engineering steel. There is a great advantage in price of titanium compared with vanadium and niobium. It’s important to achieve the needed performances and reduce production costs for the development of titanium micro-alloying engineering machinery steel.In this paper, for the two kinds of titanium micro-alloying and titanium molybdenum complex micro-alloying engineering machinery steel, continuous cooling experiments with different ways and different tempering experiments were carried out, the Rockwall hardness, optical microscopy, scanning electron microscopy(SEM) transmission electron microscopy(TEM) were used to study the effects of molybdenum on microstructure, precipitates and hardness in different heat treatment conditions of the experiment steels.The result of different cooling experimental showed that bainite was obtained in the conditions of air-cooling and air-blast cooling, and martrnsite was obtained in the condition of water quenching. The obtained granular bainite was first increased and then decreased in the conditioin of air-cooling, air-blast cooling and water quenching. The add of molybdenum is favorable for the granular bainite’ formation and refinemernt; The hardness of experimental steels was increased with the increase of cooling rate and the molybdenum is benefit for the hardness improvement.As can be seen from the results of microstructure at different tempering temperatures, the block ferrite crystal was formed gradually from the decomposition of tempered lath martensite with the increased of tempering temperatures, the microstruction changes were also similar with it in different tempering time experiments. The optimal tempering parameters of the two steels is tempered at 600 ℃ for 1h. The tensile strength is up to 910MPa higher than titanium after tempering at 600 ℃ for 1h of titanium molybdenum steel and with a good toughness..The harden-ability was enhanced and Ms was reduced for the add of molybdenum which not only made the ferrite transformation occurs faster in No.2 steel, but also leaved a higher dislocation density in lath of martensite that transformed by retained austenite in tempering, it’s boundaries between martensite lath or ferrite lath transformed was still clearly visible, the titanium molybdenum steel have a better fine grain strengthening effect.Different tempering temperature precipitates’analysis and results showed that different scales of precipitates were received by the both experimental steels. Composite precipitation was occurred in titanium-molybdenum steel which made the precipitates smaller, a smaller size of 10nm or less (Ti, Mo) C was got. The hardness and obdurability are increased in titanium molybdenum steel.The results of formation energy and binding energy of (Ti1-xMox)C cell model with different Mo content showed that the binding energy’ absolute value of (Ti1-xMox)C was not much decreased, it indicating that the crystal structure stability was weakened to some extent for the add of molybdenum; The formation energy’ absolute value of (Ti1-xMox)C was increased first and later reduced. It’s indicated that the precipitation of TiC was inhibited for the add of molybdenum. As can be see form the results of energy band and image of stats density of (Ti1-xMox)C with different Mo content, the degree of flat band near the Fermi level was first increased and then reduced with the increase of Mo content, and the N(EF) value was first increased and then reduced (Ti0.5Mo0.5) C had the highest number of electrons in the Fermi level, which means the limit Mo concentration in (Til-xMox) C is about 0.5. |
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