Abstract
The precipitation of (Cr,Fe)23C6 carbide could significantly degrade the mechanical properties of Nb-bearing cast austenitic heat-resistant steels, designed for exhaust components of automotive gasoline engines at 1000 °C. In the current research, the precipitation behavior of (Cr,Fe)23C6 carbide in these alloys, with great variations in N/C (Nitrogen/Carbon) ratio, was investigated through the liquid metal cooling directional solidification method, combined with thermodynamic calculations. Microstructural characterization suggested that the (Cr,Fe)23C6 carbide formed in the steady-state zone and the competitive zone, upon cooling to room temperature, after the solidification ended. It grew in the colony of the δ-ferrite, through the eutectoid reaction and showed different concentrations of C and Si from the δ-ferrite. Its precipitation temperature decreased significantly with increasing the N/C ratio, thereby retarding its precipitation. Therefore, the quantity of (Cr,Fe)23C6 carbide could be limited though increasing the N/C ratio of this type of alloys.
Highlights
The technology of turbocharger has been used increasingly to improve the automotive engine power and reduce the exhaust emissions, since more stringent environmental regulations were implemented worldwide [1]
TEM and the corresponding selected area diffraction pattern (SADP) analyses, identified that the cellular precipitate was (Cr,Fe)23 C6 carbide, while the large, vermicular precipitate was the residual δ-ferrite, which dissolved into the electrolyte when extracting the precipitates from the as-cast alloys
The precipitation behavior of (Cr,Fe)23 C6 carbide in a series of novel Nb-bearing cast austenitic heat-resistant steels, with variations in N/C ratio, were investigated in the current study
Summary
The technology of turbocharger has been used increasingly to improve the automotive engine power and reduce the exhaust emissions, since more stringent environmental regulations were implemented worldwide [1]. The existing high-performance alloys are capable of operating at this temperature, they are not suitable for exhaust component applications (i.e., exhaust manifold and turbocharger housing) due to their much higher costs [5]. The automotive industries have interest to develop novel and economic alloys, which are durable against such ultra-high temperature. Austenitic heat-resistant steels are attractive candidates, owing to their excellent mechanical properties and oxidation resistance at high temperatures [6,7]. A series of novel cast austenitic heat-resistant steels were developed for automotive applications, based on the CALPHAD (CALculation of PHAse Diagrams) and experimental methods [8,9,10,11]. It is urgent to control the quantity of (Cr,Fe) C6 carbide
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