The role of manganese and copper in the eutectoid transformation of spheroidal graphite cast iron

  • PDF / 1,086,352 Bytes
  • 11 Pages / 612 x 792 pts (letter) Page_size
  • 72 Downloads / 249 Views

DOWNLOAD

REPORT


I.

INTRODUCTION

THE microstructure of typical commercial spheroidal graphite (SG) irons in as-cast state or after heat treatment consists of graphite nodules embedded in a ferrite shell and of pearlite. This so-called bull’s-eye structure, according to foundry jargon, is illustrated in Figure 1. The control of this microstructure is of practical importance because it determines the mechanical properties of SG irons.[1] Among other authors, Johnson and Kovacs[2] described how this microstructure evolves during cooling of a casting. The alloy first undergoes the stable eutectoid reaction in which austenite decomposes to give ferrite and graphite. Ferrite nucleates at the graphite/austenite interface and then grows symmetrically around the nodules; this reaction is hereafter called the ferritic reaction. This growth is controlled by diffusion of carbon through the ferrite shell, which makes it quite a slow process. Therefore, the temperature of the metastable eutectoid could be reached before the complete transformation of austenite. Once initiated, the metastable reaction, also named the pearlitic reaction, proceeds quickly because of the cooperative growth of ferrite and cementite. This latter transformation is similar to the pearlitic reaction of steels.[2,3] ALINE BOUDOT and VALE´RIE GERVAL, Graduate Students, and DJAR OQUAB, Research Engineer, JACQUES LACAZE, Research Scientist, are with the Equipe Me´tallurgie Physique, Ecole Nationale Supe´rieure de Chimie de Toulouse, ENSCT, 31077 Toulouse cedex, France. HENRIQUE SANTOS, Professor, is with the Departmento de Engenharia Metalurgia da Universidade do Porto, 4099 Porto codex, Portugal. Manuscript submitted September 19, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A

At high cooling rates, the eutectoid transformation temperature range is depressed, generally resulting in the retardation of ferrite growth and a promotion of pearlite growth.[4,5] On the contrary, a slow cooling rate through the eutectoid transformation and/or a high nodule count promotes the ferritic reaction. Indeed, Askeland and Gupta[6] have shown that the relative amount of ferrite and pearlite is sensitive to changes in nodule count or cooling rate only when the nodule count is low. When the nodule count is high, extremely rapid rates of cooling are required to suppress ferrite growth,[6] as discussed in terms of Fourier’s solute ratio in a previous article.[7] In commercial castings, the cooling rate is determined, in practice, by the section size. Therefore, to control the as-cast structure of a given component, the use of alloying elements promoting or inhibiting the ferritic or pearlitic reactions is an important factor. Tin, Sb, Mn and Cu are among the most usual pearlite promoter elements. Johnson and Kovacs[2] and Kovacs[8] compared the roles of Mn, Sn, and Sb and showed convincingly that Sn and Sb segregate in a thin layer at the surface of graphite, which acts as a barrier to the transfer of carbon atoms to graphite nodules. Venugopalan[3] recently summarized the previous descript

Data Loading...