Shrinkage cavity and porosity are two common casting defects in large steel castings. They are mainly distributed in the last solidification part of the casting. They are caused by the lack of supplement of shrinkage of liquid metal during cooling and solidification. Shrinkage cavity and porosity casting defects not only reduce the service life of castings, but also cause the scrapping of castings in serious cases. Therefore, avoiding the occurrence of shrinkage cavity and porosity casting defects through process design has become the first consideration of casting enterprises. Therefore, the process design of steel castings requires the casting to realize the sequential solidification process, that is, it is required to produce a positive temperature gradient from the end of the casting to the riser, so as to realize the sequential solidification process.
Niyama criterion method shows that the ratio g / R1 / 2 of the temperature gradient g at the end of solidification to the quadratic root of cooling rate R is the function value that can best reflect the distribution of shrinkage cavity and porosity in the casting. In steel castings, when g / R1 / 2 < 1 ℃ 1 / 2min1 / 2cm-1, shrinkage cavity and shrinkage porosity casting defects will occur in this area. At present, casting simulation software such as magma, FTSolver and ProCAST use niyama criterion method to predict the shrinkage cavity and porosity of castings, which can guide the design of casting process.
The beam is the key component of large-scale cycle crusher. In the actual production process, ZHY Casting often fails the flaw detection on both sides of the beam. Therefore, the niyama criterion method is used to study the niyama function value of different points in the casting from the end to the riser area, and the original casting process is improved according to this value to make the casting meet the flaw detection standard.