Due to the improvement of cast iron performance requirements in various fields, we need to study how to prepare high-performance gray cast iron to meet the needs of industrial development. We know that there are four main factors that have an important impact on the properties of gray cast iron: graphite morphology and size, primary austenite dendrite morphology, pearlite matrix characteristics, and eutectic cluster characteristics.
The shape, size and quantity of graphite will affect the properties of gray cast iron. As for the influence of graphite shape, size and quantity on the mechanical properties of gray cast iron, we know that the finer, curved and passivated graphite at the end, the higher the mechanical properties of gray cast iron. On the contrary, the coarser, straight and sharp graphite at the end, the lower the mechanical properties of gray cast iron. Studies have shown that the more graphite content, the lower the tensile strength and fatigue strength of gray cast iron. Some data show that with the increase of cooling rate, the size of graphite decreases, the tensile strength increases from 182 MPa to 248.5mpa, the elongation decreases, and the tensile strength decreases with the increase of graphite aspect ratio. Therefore, it is also important to control the cooling rate in the solidification process of gray cast iron. The shape of graphite will not only affect the mechanical properties of gray cast iron, but also affect the shock absorption and wear resistance of gray cast iron. It is precisely because of the flake graphite structure of gray cast iron that it has better shock absorption and wear resistance than steel and other types of cast iron.
Because graphite morphology will affect many properties of gray cast iron, we hope to master some factors affecting graphite morphology and size by understanding the formation and growth process of graphite, and affect graphite morphology and size by changing external conditions, so as to improve its properties. Riposan . I et al. Studied the nucleation process of graphite and proposed three stages of graphite nucleation: Stage 1 includes the formation of small oxides, such as Mn, Si, Al, Ti and Zr oxides; Stage 2 includes complex (Mn, x) s nucleation on oxide particles in stage 1; in stage 3, graphite nucleation on (Mn, x) s nucleates on the side with small lattice mismatch with graphite. The three-stage treatment technology of graphite nucleation can obtain higher performance: first, the melt should be overheated to melt the waste slag to remove suspended solids and slag; second, the pretreatment of iron forms elements by adding oxides (Al and Zr) to produce nucleation core; third, inoculation treatment, through CA or Sr element, as the final treatment.
The number of primary austenite dendrites also has a certain impact on the mechanical properties of gray cast iron. The more the number or area ratio of primary austenite dendrites in high-strength gray cast iron, the higher the tensile strength of gray cast iron. A series of Hypereutectic gray cast iron alloys with carbon equivalent between 4.37 and 5.15 were Austempered. The results show that when the austenitic tempering temperature is 360 ℃ and the time is 180 minutes, the microstructure is upper bainite austenitized structure and feathery ferrite, the volume of austenite changes from 16% to 24%, and the tensile strength of alloy gray cast iron is between 188 and 270MPa, The hardness is between 321 and 491vhn.
The strength and hardness of gray cast iron are also affected by the morphological characteristics of pearlite matrix and lamellar spacing. Interlaced pearlite clusters with fine lamellae are more conducive to resist the generation and propagation of cracks and improve the tensile strength of gray cast iron. The effect of eutectic characteristics on the strength of gray cast iron shows that the higher the eutectic density is, the higher the tensile strength of gray cast iron is. The mechanical properties of gray cast iron are mainly determined by the microstructure characteristics, and the microstructure formation of gray cast iron is closely related to the casting temperature. With the increase of casting temperature, the liquidus and eutectic undercooling increase, the eutectic temperature decreases, the Melt Overheating reduces the content of type a graphite, the undercooling depth increases, and the number of eutectic clusters decreases. Therefore, it is necessary to master the appropriate casting temperature.
In order to improve the strength, the traditional method is to increase the number of pearlite by reducing the content of carbon and increasing the content of manganese, so as to improve the strength. However, there are some problems, such as the deterioration of casting process performance and the increase of white mouth tendency. At present, in order to improve the strength of gray cast iron, the common methods are to optimize the carbon equivalent and silicon carbon ratio, adjust the content of manganese and sulfur, alloying, inoculation and modification, etc.