# Influence of Manganese Content on the Dissolution of theFe3C Iron Carbide Phase (Cementite) during a Graphitization Anneal of Spheroidal Graphite Cast Iron

Authors: Jalel Briki; S. Ben Slima
Journal: Journal of Metallurgy
 (2012)

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## Abstract

The transformation of iron carbide cementite was investigated on spheroidal graphite containing different contents of manganese. The isothermal maintain were carried out at 600, 650, 700, and 750°C for periods of up to one hour. It was observed that the kinetics of graphitization is slowed even more than cast iron rich in manganese.


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## Body

## 1. Introduction

Graphitization heat treatment aims mainly at softening the structure by transforming the ferrite matrix. The corresponding treatment in the case of cast iron is usually time consuming and costly. This explains the number of studies devoted to the influence of the addition of alloying elements on the processes involved in the graphitization [1].Copper, nickel, and silicon are commonly considered part graphitization. The effect of the molybdenum and/or manganese differs because of their power stabilizing against the carbides [2–4]. Thus, these elements cause a slowing of the kinetics of graphitization. The influence of manganese after the graphitization process of spheroidal graphite cast iron will be specifically studied.


## 2. Materials

In Table1 are collected the composition of the studied spheroidal graphite cast iron and hardness in the state of reception. Values of carbon content—measured using a LECO CS 244 type dosing—show that it is a hypoeutectic cast iron (1.7 < C < 4.3).Table 1
Composition in weight percent of the spheroidal graphite cast iron studied.
CSiMnMgSPCuHV300 N3.342.572.150.040.0080.020.042903.122.000.900.040.0130.0200.02220In their “as received state” the samples under study have a ferrito-perlitic structure. Thus, the perlitic cementite leads to the formation of graphite, according to the following decomposition reaction (R):RFe3C⟶3Fe+C.
This graphitization—similar to the bainitic or martensitic structure—is called secondary (as opposed to primary graphitization developed, either in the liquid metal or in the austenite supersaturated in carbon).The results obtained confirm the retarding effect exerted by manganese on the kinetics of graphitization of spheroidal graphite cast iron? In fact, we have the following.(i)
As shown in Figures1(a) and 1(b), the magnitude of the dilatometric expansion corresponding to the reaction (R) decreases as the manganese content increases from 0.9 to 2.15 wt%.(ii)
The dilatometric curves recorded during isothermal heat treatment applied to the cast iron containing 2.15 wt% (Figure1(a)) show the importance of the temperature parameter (θg) on the process of graphitization. Indeed, we note that cementite graphitized even faster than θg is close to Ac1. In addition, early maintenance develops a contraction whose amplitude and duration increase when θg decreases: this contraction is related to the coalescence of pearlite, in which the cementite lamellae tend to fragment then gather in small globules.Figure 1
Dilatometric behavior while maintaining materials at different temperaturesθg: (a) material with 2.15% Mn; (b) material with 0.7% Mn.After treatment for one hour at different temperaturesθg, the microscopic examination shows (Figure 2) that the amount of ferrite formed is more consistent when temperature θg was high. Furthermore, pearlite preferentially occupies the remaining internodules areas, which are substantially richer in manganese. Note that, before decomposing, pearlite tends to take a globular form at the interfaces ferrite/pearlite. The same dilatometric test—and micrographic examinations carried out on the cast iron with 0.9 wt% Mn of content—clearly shows that lower manganese content allows the graphitization to progress more rapidly. In particular, the dilatometric curves of Figure 1(b) confirm that with temperature θg and holding time given, the amplitudes of expansions observed are higher in the case of 0.9% Mn cast iron.Micrographic controls after holding for one hour at different temperatures of the cast iron with 2.15% Mn.
(a)(b)(c)Similarly, the micrographic controls carried out after one hour of isothermal heat treatment (Figure3) shows that if the manganese content is 0.9 wt%, it forms more ferrite, while the amount of pearlite, not yet graphitized, is less important.Micrographic controls after holding for one hour at different temperatures of the cast iron with 0.9% Mn.
(a)(b)(c)The comparison of hardness measurements carried out at the end of such isothermal treatment (Figure4) for both levels of manganese considered shows that it produces a continuous decrease of this characteristic when the temperature θg increases. Note that the curve on the ductile cast iron containing 0.9% Mn is from 40 to 50 Vickers points below the one containing 2.15% Mn. What is more, the curve reflecting the evolution of the hardness of the cast iron with 2.15% Mn shows an increase from θg = 700°C. This results in the formation of some amount of austenite, which changes into martensite during the final cooling: hence the hardening at the end found after return to the ambient temperature. This transformation is confirmed by the fact there is a martensite starting point Ms in dilatometry (Figure 1(a) inset).Figure 4
Variation of hardness after one hour of graphitization with temperatureθg.


## 3. Discussion and Conclusion

With the increases of the content of manganese in cementite, it becomes more stable: therefore its decomposition kinetics (in iron and graphite) is slowed down. When the proportion of iron atoms substituted by manganese atoms increases, the Gibbs energy of formation of complex cementite decreases steadily (Figure5).Figure 5
Relationship between the Gibbs energy of formation and manganese content of cementite complex.Indeed, manganese is likely to fit into the primary carbide. This is because the physical characteristics of manganese are very similar to those of iron (Table2).Table 2Atomic numberDensity (g/cm3)Electronic structureMn257.43(A) 3d54s2Fe267.85(A) 3d54s2Furthermore, the lattice carbide Mn3C is isomorphic to that of the carbide Fe3C and their lattice parameters are very similar (Table 3).Table 3SymmetryParametres (Å)Mn3COrthorhombica = 4.530; b = 5.080; c = 6.772Fe3COrthorhombica = 4.523; b = 5.088; c = 6.743In conclusion, all the results confirm that the kinetics of graphitization of the cementite is further slowed when the melt is rich in manganese, while, also, recent studies show that the structure of the matrix can play a role in this development [5].



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*Source: 841901-2012-05-20.xml*