High-nickel austenitic cast iron is smelted in a medium frequency induction heating furnace. What should be paid attention to in pre-furnace treatment?

①Preparation of charge.

Austenitic cast iron is sensitive to gases, shrinks greatly, and is prone to shrinkage and porosity, and must be delivered and accepted according to its composition. When using an intermediate frequency heating furnace for smelting, special attention should be paid to the accuracy and cleanness of the ingredients when selecting the furnace materials, and they must not be overly oxidized or contain harmful impurity elements. In the process of mass production of Ni20 type austenitic flake iron castings, because most of the scrap steel used in a certain furnace was severely corroded thin plates, the liquid iron was oxidized, the riser of the castings rose, and the tensile strength of the sample dropped from the conventional 200MPa to 145MPa. , In addition, it is important to keep the amount of recycled materials relatively stable in the ingredients. As the amount of recycled materials increases, shrinkage and shrinkage holes in the castings will increase significantly.

②Smelting.

When using a medium frequency induction heating furnace for melting, the melting speed should be as fast as possible, and strict precautions should be taken to prevent bridging of the furnace materials, excessive melting temperature and excessive heat preservation time. The molten iron must be well deoxidized before and during the process. Commonly used deoxidizers such as silicon, manganese, and aluminum can be used as deoxidizers. Pay attention to the effect of deoxidizer on the final composition of molten iron.

③Spheroidization treatment.

Foreign production of austenitic ductile iron usually uses nickel-magnesium, nickel-copper-magnesium alloy or pure magnesium. It is believed that rare earths and calcium are detrimental to the spheroidization of austenitic cast iron. Experience shows that when producing thin and medium-sized austenitic ductile iron parts, satisfactory results can be achieved by using the low-rare magnesium alloy nodulizing agent currently available in the domestic market. The use of high and medium rare earth rare earth magnesium alloy spheroidizing agents can easily produce excessive carbides in castings, resulting in irregular shapes of graphite balls and a decrease in the toughness and processing performance of castings. When producing large or thick parts, the spheroidizing agent should not contain light rare earths, because when the casting solidifies slowly, cerium will cause the graphite spheroidization to decline and fragments of graphite will easily appear. The suitable residual magnesium content in austenitic ductile iron is slightly higher than that of ordinary ductile iron, generally the minimum is 0.035%. Magnesium in ductile iron is detrimental to welding performance. From the perspective of repair weldability, the lower the residual magnesium, the better. When Ni15Cu6 type austenitic cast iron is treated with magnesium, the free copper phase will precipitate. However, when treated with magnesium-free rare earths, the problem of free copper does not occur. Under strict control, rare earths can also be used as spheroidizing agents to produce Ni15Cu6 austenitic ductile iron.