Microsegregation as a Function of Interdendritic Distance Using Thermal Analysis Applied to a Peritectic Steel Alloy

Abstract

Abstract Microsegregation of carbon in a 36NiCrMo16/AISI 4340 peritectic steel alloy is investigated using thermal analysis of temperature–time data obtained from experimental castings. The thermal analysis allows the calculation of the solid fraction evolution using Newtonian heat transfer analysis (single thermocouple), with both the Newtonian baseline (NBL) method and the dynamic baseline (DBL) method explored for the calculation of the zero-reaction curve. As is commonly reported, the microsegregation of carbon is determined as a function of solid fraction evolution. In this study, this is extended to determine the microsegregation over the actual half-interdendritic distance. This is achieved by calculating the evolution of the secondary dendrite arm spacing $${\lambda }_{2}^{f}(t)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi>λ</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mi>f</mml:mi> </mml:msubsup> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>t</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> and then mapping the solid fraction evolution $${g}_{s}(t)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>g</mml:mi> <mml:mi>s</mml:mi> </mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>t</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> over the half-interdendritic distance $$0.5{\lambda }_{2}^{f}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>0.5</mml:mn> <mml:msubsup> <mml:mi>λ</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mi>f</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> . This result presents a spatial account of the extent and severity of microsegregation, which can be used directly in the modelling of homogenization heat treatment. It further shows that the relationship between solid fraction evolution and coarsening is not linear, as assumed in current homogenization heat treatment models.