Machine tools play an important role in the modernization of national economy. Machine tool refers to the manufacturing machine, conventionally referred to as machine tool. Generally divided into metal cutting machine tools, forging machine tools and woodworking machines. There are many ways to process mechanical parts in modern machinery manufacturing: in addition to cutting, there are casting, forging, welding, stamping, extrusion, etc. generally, the parts with higher requirements for metal precision and finer surface roughness need to be processed by cutting on the machine tool. The base plate is often used as the worktable of CNC machine tools, so the quality of the base plate of machine tools will directly affect the machining accuracy of large machine tools, especially the base plate of large machine tools. Because of its large volume, different degrees of shrinkage defects are often found in the hole of the base plate after drilling, which affects the overall accuracy and stability of the machine tools and can not meet the needs of its work. In addition, the weight of the base plate of large machine tool is generally 20 ~ 50 tons. If it becomes a scrap because of unqualified quality, it will not only cause great economic losses, but also seriously affect the production progress of the workshop. Therefore, it is of great significance to optimize and improve the casting process for the base plate of large machine tool to fundamentally solve the problem of unstable casting quality.
1 original casting process
The base plate of large-scale press is generally made of high-strength cast iron ht200-300, of which 2500 × four thousand and six hundred × 340 backing plate as an example.
The original casting process is shown in Figure 1. The upper and lower parts of the casting are divided. The important processing surface is located at the bottom of the lower box. Resin sand molding is adopted and the casting is carried out unilaterally. The pouring temperature is 1470 ~ 1480 ℃, and the carbon equivalent of hot metal is controlled at 3.60 ~ 3.70%. After drilling the casting, it is found that there are different degrees of shrinkage defects in the base plate hole, as shown in Figure 2.
After rough drilling, holes with different areas were found on the inner wall of the hole. Generally, the holes appear below 70mm on the upper surface of the casting, distributed in the center of the casting, and the hole wall is dendritic. Through the analysis, it can be determined that the casting defect is the shrinkage cavity defect in the heavy ductile iron.
2 Process improvement
According to the types of base plate defects, corresponding measures were taken in the aspects of gating system, riser, composition control of molten iron, pouring temperature and chilling of castings, including increasing riser, changing single side pouring to double side pouring, increasing carbon equivalent, reducing pouring temperature, heel impact point, changing graphite block placement method, etc.. By taking the above measures, the internal shrinkage defects of base plate castings can be reduced and eliminated. The improved casting process is shown in Figure 3, and the specific process measures are as follows:
2.1 increase the carbon equivalent of large ductile iron with equal thickness of backing plate
The carbon equivalent of the hot metal is increased from 4.30% eutectic to 4.55% hypereutectic to reduce the shrinkage tendency of the hot metal. The molten iron near eutectic composition has narrow crystallization temperature range and belongs to layer by layer solidification mode, which is easy to produce concentrated shrinkage cavity; Secondly, low melting point, easy to melt, at the same temperature, high superheat, good fluidity; In addition, the Fe3C phase is reduced, and the splitting effect on the collective is reduced.
2.2 adjust and control the pouring temperature of hot metal
Under the present process condition, the discharging temperature of base plate and other castings is reduced from 1470 ~ 1480 ℃ to 1450 ~ 1460 ℃, and the amount of nodularizing agent pig iron cover is increased from 2.0% to 2.0 ~ 2.5%. The pouring temperature can be controlled between 1320 ℃ and 1340 ℃ by controlling the furnace outlet temperature and nodularizing agent capping process, which can not only meet the pouring temperature requirements of castings, but also reduce the shrinkage tendency caused by high pouring temperature.
2.3 improving gating system
The original single side pouring is changed to double side pouring. Double side pouring can reduce the flow resistance of molten iron, shorten the time of filling the mold cavity, which is conducive to the filling of large castings, and improve the effect of punching point feeding.
2.4 improvement of pouring process
The punching point process of the backing plate was improved, and the punching point of the backing plate was changed from the original ladle punching point to another hot metal punching point. Because the temperature of the other hot metal was high, the hot metal kept high temperature for a long time, and the punching time was prolonged. The flushing time was extended from 20 minutes to 30-40 minutes. According to the feeding principle of riser, the solidification time of riser should be greater than or equal to the solidification time of casting; There should be enough liquid metal in the riser to supplement the liquid shrinkage and solidification shrinkage of castings; During solidification, there should be a smooth feeding channel between riser and feeding part. For large castings, such as the base plate of the press, the riser also plays the role of slag discharge to realize the sequential solidification of the castings. Therefore, the filling point on the basis of the original riser is more powerful for the feeding of large castings, thus reducing the tendency of casting shrinkage.
2.5 effect verification
After the improvement of the process, several types of press base plates were cast. In the past year, the qualified rate of the finished products of the base plate has been greatly improved, and the scrap rate is zero. It can be seen that after the process improvement, the quality effect of large castings is very obvious, and it also proves that the reasonable composition design, gating system and gating process have a very important impact on the yield of castings. Figure 4 is the drilling diagram of the base plate after the process improvement, and figure 5 is the physical diagram of the base plate after the process improvement.
In the casting production of large-scale castings, adopting reasonable casting process, increasing carbon equivalent of molten iron, improving gating system, controlling pouring temperature of molten iron, reasonably setting feeding riser, etc., effectively feeding the parts prone to defects, improving and controlling solidification conditions of molten iron can obtain high-quality and dense castings and improve the qualified rate of casting products, The material is saved and the production cost is reduced.