EXPERIMENTAL INVESTIGATION ON THE EFFECT OF COPPER UPON EUTECTOID TRANSFORMATION OF AS-CAST AND AUSTENITIZED SPHEROIDAL GRAPHITE CAST IRON
J. Sertucha and P. Larrañaga AZTERLAN, Durango (Bizkaia) Spain J. Lacaze
Université de Toulouse, CIRIMAT, Toulouse, France M. Insausti
Universidad del País Vasco, Bilbao (Bizkaia), Spain Copyright © 2010 American Foundry Society Abstract
Copper is known as a pearlite promoter in cast iron and has been used as such for a long time, most often together with low amounts of manganese. Literature data, however, has shown that these two elements act differently on the ferritic and pearlitic transformations. In order to provide more insight in the role of this element on the solid-state transformation of spheroidal graphite (SG) cast iron, this paper investigates the effect of adding copper in small step increments, from 0.11 to 1 wt. % Cu, to SG irons containing about 0.15 wt. % Mn.
The characteristic temperatures for the stable and metastable Introduction
The use of selective additions for promoting pearlite for- mation in cast irons has become common practice a long time ago, with emphasis on low-cost elements such as cop- per, manganese, arsenic and tin.1
eutectoid transformations as recorded during cooling after solidification in standard cups and after austenitizing are presented together with microstructure information. It is found that a copper content of about 0.6 wt. % is the upper limit over which only small amounts of ferrite could be obtained except at very low cooling rates. This is tentatively related to the lowering of the temperature for ferrite and the associated decreased kinetics for austenite decomposition in the stable system.
Keywords: pearlite, eutectoid, metastable, ferrite, transformation temperature
phase field in the isopleth sections corresponding to the alloy composition5,6
Conversely, the presence
of such species in foundry returns and scraps may lead to difficulties in achieving fully ferritic matrices. The need for appropriate control of melt composition before casting has thus triggered extensive experimental studies.1,2
While tin
and arsenic become effective at very low levels, they are associated with environmental hazards so that metallic ele- ments such as manganese and copper are preferred.
The way copper and manganese act on the eutectoid trans- formation has lead to several, potentially contradictory, explanations as reviewed by Pan et al.2
It was claimed in a
more recent study that the major effect of metallic alloying additions is to change the relative position of the stable and metastable eutectoid transformation temperature ranges.3 More precisely, it has been suggested that ferrite and pearlite do inherit the composition of the parent austenite in substi- tutional elements because there is not time enough for their long range redistribution.4,5
Accordingly, the temperature at
which the transformation may start upon cooling at a finite rate was shown to be the lowest temperature of the three-
International Journal of Metalcasting/Winter 10 Tp (°C)=727+21.6wSi +0.023(wSi )2 -21.0wCu -25.0wMn with eventually additional terms for Mo, Cr, Ni.7 Equation 2 This analy-
sis has been verified with some success to literature data7 concerning the temperature for the start of the stable and metastable transformations. Also, it could be demonstrated using literature data1,2
significantly affected by additions of Cu, Mn, As or Sn at the levels generally used.8
Further, it has been shown that low level addition of copper to a Mn-free cast iron does not promote pearlite3,5
and this
. The effect of manganese is just the oppo- site, hence its strong pearlite promoter effect. Accordingly,
may be understood by observing that copper decreases Tα much less than Tp
that the growth rate of pearlite is not
for the metastable one. These temperatures could be calculated by means of a thermodynamic software and ap- propriate databank and expressed as:
perature will be denoted as Tα and Tp
Tα (°C)=739+18.4wSi +2.0(wSi )2 -14.0wCu -45.0wMn Equation 1
, as shown in Figure 1. This reference tem- for the stable transformation
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