Continuum model for predicting microporosity in steel castings

摘要

Using a finite element model for simulating dendritic solidification of multicomponent-alloy castings, the pressure and redistribution of gas-forming elements during solidification and cooling in AISI 8620 steel casting alloy were calculated. The model solves the conservation equations of mass, momentum, energy, each alloy component and gas-forming elements (i.e. hydrogen and nitrogen). By solving for the concentrations of hydrogen, and nitrogen in the intergranular liquid and comparing the sum of their Sievert's pressures with the local pressure within the mushy zone of the alloy, the model predicts regions of possible formation of porosity. The thermal boundary conditions on test-bar castings were deduced from a thermal calculation performed with a commercial code, ProCAST(TM). With these realistic thermal boundary conditions, our porosity-simulations were carried out for many cases with combinations of different initial contents of the gas-forming elements: hydrogen in the range of 3-7 ppm and nitrogen in the range of 0-100 ppm. The calculated results are summarized in a plot that separates castings expected to have porosity from those with no porosity. The effect of adding titanium to form TiN inclusions and inhibit the development of porosity during solidification was also investigated. [References: 51]