For a long time, the material of machine tool casting in China is gray cast iron, and the standard of gray cast iron has been established. Why should the machine tool industry formulate the industry standard of the machine tool casting? This is because the machine tool as the working master has special requirements for the casting: high tensile strength, high elastic modulus, good wear resistance of the guide rail, excellent dimensional stability, good vibration absorption, good processability, high dimensional accuracy and surface finish. These properties of machine tool casting directly affect the quality of machine tool. The quality of machine tools in China still lags far behind the world level, which can be reflected from the import and export of machine tools in China. Although China’s machine tool production quantity is the first in the world, it can’t stop the import. At present, China’s machine tool import accounts for 23% of the world’s machine tool import, that is to say, the machine tool import is also the first in the world, especially in high-end CNC machine tools, the import quantity is very large. In terms of machine tool export, there is a larger gap. The proportion of machine tool export between the main machine tool producing countries and the total machine tool production is more than 50%. Such as Germany, the United States, Japan, Italy, Taiwan, Switzerland, etc. Seven countries with a proportion of less than 50% are Russia, Mexico, South Korea, Brazil, Canada, China and India, with China ranking second from the bottom. As the largest machine tool output country in the world, China’s machine tool import and export situation shows that there is still a big gap between China’s machine tools and advanced countries in terms of quality and performance level.
There are many factors that affect the quality of machine tools. The quality of machine tool casting, which accounts for 70% ~ 80% of the weight of machine tools, is one of the important factors. The investigation of 11 influential domestic machine tool factories shows that with the development of CNC machine tools in China, the quality requirements for machine tool castings are increasing. The main problems and gaps of domestic machine tool casting are as follows.
(1) Casting strength. The improvement of casting strength of domestic machine tool is achieved by greatly reducing carbon equivalent. Therefore, in the production of HT300 and ht350 with low carbon equivalent, the shrinkage porosity caused by the increase of shrinkage, the deformation caused by the increase of casting stress, and the deterioration of processing performance caused by the increase of hardness are the common problems in the production of high-grade machine tool castings.
In the survey, shrinkage and deformation are listed as enterprises with production difficulties, accounting for 36% ~ 45%. Some enterprises regard shrinkage, deformation and dimensional accuracy stability as one of the most important problems to be solved. In grey cast iron, the mechanical properties and casting properties of cast iron are contradictory. High carbon equivalent and high strength are the development direction of machine tool casting, and there is still a big gap in China. The problems listed below, such as thin wall, dimensional stability, shock absorption, machinability and so on, are all related to the low carbon equivalent of domestic machine tool castings. To obtain high strength under low carbon equivalent is the primary gap of machine tool castings in China.
(2) Thin wall of casting. The weight of machine tools in China is about 8% ~ 10% heavier than that in foreign countries, and the main wall thickness of machine tool castings is also larger than that in foreign countries. The main wall thickness of medium-sized machine tool castings in foreign countries has reached 14-25 mm, and that of small machine tools has reached 8-12 mm. Low carbon equivalent high strength gray iron is an important obstacle to realize thin wall and machine tool lightweight. In the investigation, 72% of the enterprises filled in “thin wall” in the column of “what aspects of the requirements of the development of modern machine tools for machine tool castings have been improved”.
(3) Dimensional accuracy stability. Low carbon equivalent and high strength gray cast iron lead to the increase of internal stress. The lower the carbon equivalent, the higher the internal stress and the larger the deformation. In the survey, 81% of enterprises think that the contradiction between high strength, high rigidity and low stress of machine tool casting is a problem to be solved urgently. Low carbon equivalent and low Si / C ratio is one of the important reasons for the increase of internal stress.
(4) Shock absorption. In the past 50 years, the machining accuracy of machine tools has been increased by 100 times. The machine tools in developed countries have entered the level of submicron and nanometer, which has higher requirements for dimensional accuracy stability and vibration reduction of machine tool castings. Under the same strength, the high carbon equivalent gray iron is superior to the low carbon equivalent in vibration reduction, because the micro plastic deformation and dislocation of flake graphite under stress accelerate the vibration attenuation, and more graphite is conducive to the improvement of vibration reduction.
(5) Processability. Low carbon equivalent high strength gray cast iron deteriorates the machining performance of machine tool castings. 45% of the enterprises in the survey think that the coordination of high strength and cutting performance is an urgent problem to be solved. At the same strength, it is desirable to have a lower hardness, but it is difficult to achieve at low carbon equivalent and high strength.
(6) Large scale. The development of heavy-duty machine tools makes the casting of machine tools develop towards large-scale. The production of heavy-duty castings of 40-150 t is increasing day by day. 72% of the enterprises in the survey think that the requirements of modern machine tools for large-scale machine tool castings have been improved [2]. Due to the production of large section and large tonnage castings, the problems of low strength, low hardness, poor section sensitivity and uneven structure occur from time to time. Especially in the production of heavy-duty gray cast iron castings, the reduction of carbon equivalent leads to the increase of shrinkage porosity, internal stress, the increase of cementite and cracks in the thin wall. The performance of the test bar of large castings is far from that of the body, and the test bar is often qualified. However, the graphite of the body casting is coarse, the performance is reduced, and the structure of the guide rail is uneven, resulting in uneven hardness and microcracks after quenching. Large castings are generally not easily scrapped, so the workload of welding repair and repair is increased, the number of returned products is increased, and the quality is reduced.
(7) Aging treatment to eliminate casting stress. Among the investigated enterprises, 70% are thermal aging enterprises, 20% are vibration aging enterprises and 10% are natural aging enterprises [2]. In the aging process, aging is not in place, aging is only in form, and even there is the phenomenon of increasing new stress and fracture after thermal aging. The effect of eliminating casting stress by thermal aging is closely related to the heating rate, holding temperature and cooling rate. If it is not carried out properly, it will be twice as effective as before. The temperature difference in the annealing furnace shall be ± 20 ℃, and the excessive temperature difference will cause the increase of casting stress. The placement mode and position of workpiece in the annealing furnace are also closely related to the stress relief. The implementation is not in place, and the effect is far from satisfactory [8].
(8) Ductile iron. With the development of large-scale CNC machine tools, the number of nodular cast iron machine tools has increased dramatically. This material not only increases the strength and rigidity of the machine tool casting, but also reduces the weight of the machine tool. However, due to the particularity of ductile iron material manufacturing, it also brings a series of problems such as shrinkage porosity, slag inclusion, poor spheroidization, etc. The machine tool nodular iron castings are usually large ones, and there are also various problems that are easy to occur in large section nodular iron. At present, there are more and more 40 ~ 150 t ductile iron machine tools. At present, there are many large tonnage and large section ductile iron machine tools in China. The problems of nodulation decline, abnormal structure, reduction of graphite number, shape distortion and nodulation decline often occur in large section ductile iron castings.
(9) Modulus of elasticity. Domestic enterprises are not included in the internal control indicators. Elastic modulus is an important performance index of machine tool casting material, but in recent investigation, no domestic enterprise has tested it.
