Mechanical and microstructural characterization of Nodular Cast Iron (NCI) with Niobium Additions

Tânia Nogueira Fonseca Souza,Francis José Saldanha Franco,Maria Teresa Paulino Aguilar,Rogéria Alves Perilo Sousa Nogueira,Paulo Roberto Cetlin

doi:10.1590/1516-1439.249413

Tânia Nogueira Fonseca Souza, Francis José Saldanha Franco + Show 3 more

Open Access

https://doi.org/10.1590/1516-1439.249413

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Abstract

Nodular cast irons (NCIs) involve the inoculation and nodulization of 0.03 to 0.06wt% Magnesium, as well as the addition of Molybdenum, Chromium, Nickel, Copper and Vanadium, although the presence of Niobium is not common. The literature reports that Niobium leads to better mechanical properties of the NCIs, and recent results indicate that 0.5wt% Niobium increases the strength and the Charpy notch toughness of NCIs, but decreases its hardness, in relation to a similar NCI without Niobium. The present report studies the addition of 0.23, 0.47, 0.67, and 0.85wt% Niobium to a NCI covering the effects on the microstructures, yield and tensile strengths, tensile elongations and Charpy toughness. The NCIs were prepared in a 120 kg medium frequency induction furnace, and samples were obtained following the ASTM A842 standard; the various test specimens were extracted from these samples according to specific standards for each test. Fracture analysis was performed in fractures specimens after the Charpy tests. It was shown that the volume fraction of pearlite increases as the Niobium content is raised, leading to increments in the yield and tensile strengths and hardness of these materials. Modest increment of tensile ductility and Charpy toughness are observed only for an addition of 0.23% Niobium. Fracture analysis revealed typical brittle cleavage surfaces and decohesion between grains and grain - graphite nodules interfaces, and very low porosity

Highlights

Nodular Cast Iron is an engineering material displaying high ductility, elastic modulus, mechanical strength and corrosion resistance; in addition, it has low cost and is easy to produce and machine[1], and is widely used as a structural material
The presence of Niobium in Nodular cast irons (NCIs) leads to the formation of polygonal and eutectic Niobium carbides, whose morphology must be controlled, since their agglomeration and the presence of coarse carbides compromise the mechanical properties of the material[11,15]
The present study investigates the influence of a range of Niobium contents (0.23 to 0.85 wt%) on the microstructure, mechanical strength, Charpy toughness and ductility of NCIs

Summary

Introduction

Nodular Cast Iron (hereafter called “NCI”) is an engineering material displaying high ductility, elastic modulus, mechanical strength and corrosion resistance; in addition, it has low cost and is easy to produce and machine[1], and is widely used as a structural material. During the last years the industrial use of NCIs iron has been increasing and many investigations on the characterization and improvement of its mechanical properties have been developed[2]. The dissolution of Niobium in NCIs seems to be difficult and studies indicate that this is linked to the low working temperature and to the high Carbon content of cast irons[15,16]. The present study investigates the influence of a range of Niobium contents (0.23 to 0.85 wt%) on the microstructure, mechanical strength, Charpy toughness and ductility of NCIs. A fracture analysis is presented. Hardness tests (24 measurements for each sample) were performed in a semi-automatic Brinell durometer (Heckert - 3000D, load of 1838,7 N and a spherical indenter with diameter 2.5mm) according to ASTM standards[17,18,19] and Charpy impact testing was performed according to the ASTM-A327 standard[20]

Experimental Procedures

Results and Discussion

Conclusions