CALCULATION OF OPTIMAL TEMPERATURE INTERVALS OF MATRIX AUSTEMPERING WITH NODULAR GRAPHITE CAST IRONS

Abstract. Purpose. In austempering temperature range 260...420 °C of bainitic cast irons with nodular graphite (ADI) the changing of the behavior of functions of mechanical properties is observed adequately to the structural condition. Objective of present work was an investigation of temperature influence of isothermal holding on microstructure formation of metal matrix and mechanical properties of cast irons with nodular graphite specimens and to calculate the optimal temperature intervals of their matrix austempering. Methodology. Investigated globular graphite cast irons were thermally hardened by isothermal quenching on industrial equipment according to enterprise ?Amsted-Rail? technology. Thermally treated samples were subject of metallographic analysis and mechanical tests according to standard methods. Mathematical processing carried out using the program ?Excel 2003?. Approximation of experimental data was done with software ?Curve Expert?. Further analysis of the model concerning the presence of extreme points and inflection points of the curve dependencies were realized using program ?MathCAD 14?. Findings. Temperature ranges of bainite structures forming in ADI metal matrix have been defined in the present work: lower bainite (260...280 °C), mixed bainite structure (300...360 °C), upper bainite (360...420 °C). Study of ADI mechanical properties demonstrated intense monotonous decreasing of ultimate tensile strengths in studied temperature range, naturally the opposite direction but identical regarding the behavior increased plastic properties and step ? type, according to the structural condition, the nature of the changes in hardness. Consideration of the hardness functional dependence vs. austempering temperature obtained using experimental data approximation. Model analysis in terms of finding extreme points and inflection points was carried out. Originality. Mathematical processing of functional dependence of structurally sensitive properties (hardness) vs. austempering temperature determined the presence of point of corresponding polynomial curve inflection and its position. Practical value. Calculated quantitative values of ADI bainite matrix formation temperature intervals and appropriate behaviors of their products property, allow for forecasting prescribe industrial modes of thermal hardening for details of the specific conditions of operation.