Effect of Niobium on the Microstructure and Mechanical Properties of Alloyed Ductile Irons and Austempered Ductile Irons

Mostafa Ahmed,Adel Nofal,Mohamed Soliman,Rüdiger Bähr,Mervat Youssef

doi:10.3390/met11050703

Abstract

In this research, different ductile irons and austempered ductile irons were successfully developed using several alloying contents of nickel, copper and microalloying with niobium. Additionally, special nanocarbon powder was added to the molten iron to enhance the nucleation tendency of spheroidal graphite and compensate for the possible negative effect of Nb addition on the nodule morphology. Metallographic analysis showed that increasing the niobium content in the alloy to 0.1 wt % raises the number of graphite eutectic cells and refines the final structure of the graphite. Moreover, the nodule count of graphite slightly increased, but it concurrently decreased the nodularity when the Nb amount reached 0.1 wt %. SEM micrographs illustrated that nano- to microsized niobium carbides (NbC) particles were dispersed in the matrix of the Nb microalloyed ductile irons. Both optical and SEM micrographs clearly showed that alloying of ductile irons with nickel, copper and microalloying with niobium had a significant effect on defining the final pearlite structure. Coarse, fine, broken and spheroidized pearlite structures were simultaneously observed in all investigated alloys. Dilatometry studies demonstrated that the nano NbC particles acted as nucleation sites for graphite and ferrite needles. Therefore, Nb addition accelerated the formation of ausferrite during the austempering stage. Finally, alloying with Cu, Ni and microalloying with Nb led to developing novel grades of ADI with excellent strength/ductility property combination.

Highlights

Austempered ductile cast iron (ADI) presents an important material in the ductile iron family that exhibits an exceptional combination of mechanical and physical properties, such as an excellent combination of strength and ductility, outstanding fatigue strength and fracture toughness combined with a lower density as well as improved tribological behavior compared to steel
The ADI is produced by applying austempering heat treatment, in which the ductile iron is austenitized at a high temperature within the range of 850–950 ◦ C, it is quenched down to a temperature ranging between 250 and 400 ◦ C and held there for a certain time to allow for the austempering transformation to occur
By increasing niobium content in the alloy to 0.1 wt %, the nodule count (N.C.) of the graphite increased the niobium content in the alloy to 0.1 wt %, the nodule count (N.C.) of the graphite infrom a level around 250 mm−2 to more than 320 mm−2, but it concurrently decreased creased from a level around 250 mm−2 to more than 320 mm−2, but it concurrently dethe nodularity (N) from 90% to around 80% when the Nb amount reaches 0.1 wt %

Summary

Introduction

Austempered ductile cast iron (ADI) presents an important material in the ductile iron family that exhibits an exceptional combination of mechanical and physical properties, such as an excellent combination of strength and ductility, outstanding fatigue strength and fracture toughness combined with a lower density as well as improved tribological behavior compared to steel. The grade with the highest ductility of 9% possesses UTS~900 MPa. The ADI is produced by applying austempering heat treatment, in which the ductile iron is austenitized at a high temperature within the range of 850–950 ◦ C, it is quenched down to a temperature ranging between 250 and 400 ◦ C and held there for a certain time to allow for the austempering transformation to occur. The ADI is produced by applying austempering heat treatment, in which the ductile iron is austenitized at a high temperature within the range of 850–950 ◦ C, it is quenched down to a temperature ranging between 250 and 400 ◦ C and held there for a certain time to allow for the austempering transformation to occur During the latter holding, two consecutive reactions may occur as follows: the first reaction, ausferrite transformation, results in a tailorable microstructure composing of spheroidal graphite in ausferritic matrix (α-ferrite needles + high carbon stabilized austenite, HC-γ) [1,4]. The period between the end of the first reaction and the start of the second reaction is known as the process window, in which the optimum mechanical properties could be achieved [4]

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