Effect of molybdenum on carbide precipitation, microstructure and properties of gray cast iron

Gray cast iron wheel hub is one of the important parts of heavy-duty truck. When heavy-duty truck travels down the rugged mountain road or long-distance downhill continuous braking for a long time, the dynamic load borne by the wheel hub and the temperature difference stress generated by braking are much higher than those of vehicles traveling on flat road. For heavy trucks operating under special conditions for a long time, the service life of the brake hub will be greatly shortened, which poses a serious threat to the life of drivers and the safety of goods.

Therefore, trucks operating under special working conditions for a long time put forward higher requirements for the tensile strength of their hub materials. Generally speaking, gray cast iron has relatively low strength, but it is widely used to manufacture truck wheels because of its good thermal conductivity. The tensile strength of gray cast iron depends on its microstructure, such as the morphology, size, content and distribution of graphite and matrix. Adding alloying elements is the most economical and effective means to improve the microstructure and tensile strength of gray cast iron, in which molybdenum is considered to be one of the most effective elements to improve the tensile strength of gray cast iron.

Bates study shows that when the molybdenum content changes in the range of 0.03% ~ 0.78%, A-type graphite and 100% pearlite matrix structure are produced in gray cast iron; With the increase of molybdenum content, graphite content decreases, and the maximum tensile strength can be increased by about 100 MPa. Fu Li et al. Pointed out that with the increase of molybdenum content from 0.3% to 0.6%, graphite becomes shorter and the tensile strength can be improved by one brand. However, the strengthening effect of molybdenum on pearlite is not obvious. Xu et al. Found that when the molybdenum content is less than 0.22%, the cast iron matrix is 100% pearlite structure. With the increase of molybdenum content, austenite and bainite structure begin to appear in the cast iron. When the molybdenum content exceeds 0.62%, 27.8% austenite and 68.2% bainite structure are produced, which increases the tensile strength by more than 100 MPa. When the molybdenum content reaches 0.95%, Molybdenum carbide will precipitate at the eutectic cell grain boundary, resulting in a slight decrease in tensile strength, but the type, distribution, size and content of graphite do not change significantly with the change of molybdenum content. However, Bihari and Chen believe that molybdenum can improve the tensile strength of gray cast iron by refining pearlite structure and reducing pearlite lamellar spacing.

It can be seen that predecessors have different views on the action mechanism of molybdenum in gray cast iron. Taking the gray cast iron for heavy truck brake hub as the research object, through laboratory test, combined with J-Mat pro and factsage software analysis, the influence law of different molybdenum content on graphite and matrix structure in gray cast iron is studied, and the mechanism of molybdenum improving the tensile strength of gray cast iron for hub is discussed.