Investigation of Structural Cast Iron Hardness for Castings of Automobile Industry on the Basis of Construction and Analysis of Regression Equation in the Factor Space «Carbon (C) - Carbon Equivalent (Ceq)»

The object of research is structural iron with lamellar graphite, in which the carbon equivalent (Ceq) is in the range (4.2–4.4) %, and the carbon content (C) in the range (3.42–3.57) %. The aim of research is description of the distribution of the hardness value of structural cast iron of serial meltings in the C–Ceq factor space at fixed values of the Cr–Ni–Cu–Ti alloy content in narrow intervals. It is shown that a polynomial regression equation of the form HB=HB(C, Ceq) can be used to obtain a workable analytical description. It is shown that such structure of the equation and those obtained by the method of least squares corresponding coefficient estimates provide 92 % accuracy of the forecast even with a small sample of data.On the basis of the canonical transformation of the response surface and its ridge analysis, it is established that it is possible in principle to satisfy different requirements for hardness. So, if the range of the planning area C=(3.42–3.57) % and Ceq=(4.2–4.4) % is chosen as the imposed constraint, then several suboptimal solutions are possible. This is the case if the task of minimizing hardness is not set and the range HB=180–250 satisfies the quality requirements specified by the production conditions. If the priority is hardness minimization, then the suboptimal solution is one and it is like the point of intersection of the constraint line (r=1.414) and the lower ridge line y=y(r). On the basis of this, it is concluded that the suboptimal solutions are multivariant, depending on the requirements of production. A nomogram has been constructed, which makes it possible to select in a rational way the technological regimes of out-of-furnace treatment in the part concerning the adjustment of the chemical composition of the alloy