There are many factors affecting the cutting performance of gray cast iron in actual production, which are generally analyzed from the aspects of raw material composition, melting agent pouring process, machining process and so on.
1. Composition and quality of raw materials
The composition and quality of raw materials have a great influence on the machinability of gray cast iron. When comparing castings at home and abroad, it is found that the composition and quality of raw materials are an important reason why the processability of domestic gray iron castings is inferior to that of foreign castings. The type and content of trace elements in raw materials determine the quality of molten iron. For example, lead in molten iron will seriously deteriorate the graphite morphology; Trace element vanadium can promote the formation of a large amount of sorbite; The affinity of impurity element titanium with nitrogen and carbon in molten iron is strong, forming high hardness titanium nitride or titanium carbide particles, which is harmful to the machinability of gray cast iron. Impurity elements mainly come from raw materials during smelting. Therefore, improving the purification degree of molten iron through technical means is an effective means to improve the processability of gray cast iron.
The methods of purification degree of molten iron mainly include using high-quality raw materials, increasing smelting temperature, filtering molten iron, etc. Higher tapping temperature can improve the mechanical properties and processability of gray cast iron. At present, the tapping temperature of gray iron castings is required to be above 1480 ℃, and the molten iron is allowed to stand for three to five minutes. On the one hand, the increase of tapping temperature can be due to the full floating of impurities and hard spots in molten iron; On the other hand, the coarse graphite in pig iron is dissolved in sufficient high temperature, so that the matrix structure of gray cast iron is more uniform; In addition, high tapping temperature and high-temperature standing of molten iron before pouring can fully eliminate the “heredity” of gray cast iron material. The casting filter screen mainly filters out the high hardness slag inclusion and hard spots in the metal to reduce the wear of these hard inclusions on the tool and improve the machinability of gray cast iron.
The type of graphite and matrix structure in the microstructure of gray cast iron determines its strength. When the matrix strength is basically the same, the shape of cast iron has a great impact on the machinability. This is because the flake graphite in gray cast iron has lubrication and good chip breaking effect on the tool. Therefore, increasing the carbon content will improve the cutting performance of gray cast iron. In addition, the non-uniformity of structure and hardness of gray cast iron seriously affects the cutting performance of the material. When the matrix structure is pearlite and the hardness fluctuates greatly, in the cutting process, the shear deformation resistance of the area with high hardness is large, which increases the cutting force, resulting in greater fluctuation of the cutting force and greater work consumption, which reduces the cutting performance of gray cast iron. Excellent inoculant or alloying can improve the uniformity of microstructure and improve the machinability of materials.
The alloying of gray cast iron samples is mainly through the alloying elements Cu, Cr and so on. After adding alloy elements Cu and Cr into molten iron, Cu and Cr play an important role in promoting the transformation of austenite into pearlite, refining pearlite and generally increasing microhardness. Alloy elements Cu and Cr can effectively inhibit the influence of Si concentration fluctuation on microhardness. In addition, the composite addition of alloy elements Cu and Cr can not only promote the graphitization process of molten iron, but also reduce the white tendency of gray cast iron. Therefore, Cu and Cr alloying can improve the machinability of gray cast iron by improving the uniformity of pearlite structure and microhardness.
Inoculation treatment is a practical method to improve the machinability of gray cast iron. The mechanism of improving the properties of gray cast iron by inoculation is that effective inoculation can refine the matrix structure, reduce the tendency of white cast iron, eliminate the sensitivity of wall thickness and improve the machinability. At present, inoculation technology has been widely used in production, and its variety has developed from single inoculant to multi variety and multi-function. In recent years, scholars at home and abroad have found that rare earth elements can neutralize many impurity elements in gray cast iron, so as to purify molten iron. Especially when rare earth elements react with sulfur, the reaction products can be used as the nucleation core of graphite, improve the morphology and distribution of graphite, improve the mechanical properties of materials and improve the processing properties.
2. Smelting and pouring process
In actual production, it is found that the melting mode of gray cast iron has a certain influence on its processability. For example, the hardness of molten iron melted in electric furnace is lower than that melted in cupola, which is conducive to improving the processing performance. In addition, the cooling rate of castings has a great influence on the graphite morphology and matrix structure of gray cast iron. It is found that too fast cooling speed leads to the increase of carbide content in microstructure, which worsens the machinability of gray cast iron. Therefore, in actual production, changing the type of molding sand, increasing the casting fillet radius, reducing the chilling of casting edges and reducing the unpacking temperature can increase the machinability of gray iron castings.
3. Machining process
When cutting gray cast iron, selecting matching tool materials and reasonable tool geometric parameters and cutting parameters can reduce tool wear and improve the cutting efficiency of gray cast iron. In recent years, scholars at home and abroad analyze and model the cutting process of gray cast iron by using finite element analysis, and simulate the machining process with the help of computer, which provides help for the selection of cutting tools and process in the process of material processing.
4. Tool wear mechanism
Tool wear is mainly caused by the friction contact between tool and chip, tool and workpiece in the cutting process. The mechanism of tool wear mainly includes hard spot wear, bonding wear, diffusion wear, chemical wear and phase change wear. In the process of low-speed cutting, the wear of hard spots is the main cause of tool wear. The mechanical wear caused by the grooves drawn on the tool surface is caused by the impurities in the workpiece material, the hard spots in the matrix structure and the debris of debris accumulation. Bonding wear and diffusion wear mainly refer to the phenomenon of tool wear caused by the increase of molecular force between contact surfaces due to fatigue, thermal stress and defects of tool surface structure. When cutting gray cast iron, the bonding point breaks and takes away the particles on the tool surface, resulting in tool wear. Phase change wear mainly refers to the phenomenon that in high-speed cutting, the cutting temperature of gray cast iron reaches or exceeds the phase change temperature of tool material, resulting in the change of metallographic structure on the tool surface, the decrease of tool hardness and the aggravation of wear.