Lanthanum Role in the Graphite Formation in Gray Cast Irons

Eduard Stefan,Mihai Chisamera,Stelian Stan,Iulian Riposan

doi:10.3390/min10121146

Abstract

The present paper reviews original data obtained by the authors from recent separate publications with additional unpublished data, specifically concerning the Lanthanum (La)’s role in the solidification pattern and graphite formation in gray cast irons. Iron melting at 0.018–0.056%S, a 3.7–4.1% carbon equivalent (CE) and less than 0.005%Alresidual are inoculated with La-bearing FeSi alloys at different associations with other inoculating elements. Complex Al-La small inclusions as possible better nucleation sites for (Mn,X)S compounds and La-Ca presence in the body of these sulfides, which possibly provide better nucleation sites for flake graphite, are identified in 0.026%S cast iron. At a lower sulfur content (0.018%S), La,Ca,Al-FeSi alloy still has a high efficiency, but more complex La-bearing alloys are recommended for a higher dendritic austenite amount (LaBaZrTi–FeSi) or for lower eutectic recalescence (LaBaZr–FeSi). La has limited but specific benefits at 0.05–0.06%S irons, including favorable graphitizing factors (a higher amount of graphite precipitated at the end of solidification), lower eutectic recalescence, and a lower value of the first derivative at the end of solidification. When La,Ca,Ba,Al,Zr,S-FeSi treatment (0.035%S base iron) is used, Scanning Electron Microscopy (SEM) analysis finds that the first formed micro-compound is a complex Al-silicate (Zr,La,Ca,Ba presence), which supports the nucleation of the second compound (Mn,Ca,La)S type. At the sulfide-graphite interface, there is a visible thin (nano size) Al-silicate layer (O-Al-Si-Ca-La system), which is more favorable for graphite nucleation (it has better crystallographic compatibility). La is identified in all three important areas of nucleants (the first is formed oxidic nucleus, the second is nucleated Mn-sulfide and the third is a sulfide-graphite interface), thereby increasing the efficiency of graphite nucleation sites.

Highlights

Previous relevant research programs have pointed out that complex manganese sulfide (Mn,X)S type (1–10 μm size), which is usually nucleated on the previous formed oxide type micro-compound (0.1–3 μm size), is the major nucleation site for lamellar graphite formation in commercial gray cast irons, which usually occurs with results of more than 0.025%S [1,2,3]
Microscopy (SEM) analysis finds that the first formed micro-compound is a complex Al-silicate (Zr,La,Ca,Ba presence), which supports the nucleation of the second compound (Mn,Ca,La)S type
The main objective of the first research program (I, Tables 1 and 2) wasto examine the effect of La in addition to Ca and Al in inoculated low S gray cast irons (0.026%S) [12] by using thermal analysis, chill tendency and structure characteristics (Figure 2) and nucleation sites of graphite particles (Figure 3), respectively

Summary

Introduction

Previous relevant research programs have pointed out that complex manganese sulfide (Mn,X)S type (1–10 μm size), which is usually nucleated on the previous formed oxide type micro-compound (0.1–3 μm size), is the major nucleation site for lamellar graphite formation in commercial gray cast irons, which usually occurs with results of more than 0.025%S [1,2,3]. The important role of MnS in graphite nucleation has been noted by other papers [14,15]. For excessively low S-content, nitrides/carbo-nitrides appear to have an important role in graphite nucleation [16,17,18,19].

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