INVESTIGADORES
MASSONE Juan Miguel
artículos
Título:
Quantification of casting sking in ductile and compacted graphite irons and its effect on tensile properties
Autor/es:
D. STEFANESCU; S. WILLS ; J. MASSONE; F. DUNCAN
Revista:
INTERNATIONAL JOURNAL OF METALCASTING
Editorial:
AMER FOUNDRY SOC INC
Referencias:
Año: 2009 vol. 2 p. 7 - 26
ISSN:
1939-5981
Resumen:
The mechanical properties of ductile (DI)
and compacted graphite (CG) irons are measured and reported on standard
machined specimens (as per ASTM). However, most castings retain most of the
as-cast surface. This surface layer (the casting
skin) includes both surface and subsurface features. Because of the casting
skin, the mechanical properties of the part are typically significantly lower
than those found on standard ASTM
machined specimens.
The technical
objectives of this research were to identify the individual features that
together define skin quality in DI and CGI,
to develop a method for the measurement of skin thickness, and to quantify the
influence of the skin of thin wall (2 to 6 mm) DI castings on its tensile
properties.
The features of the
casting skin include surface (roughness) and subsurface (graphite degradation,
graphite depletion, pearlitic rim) elements. Graphite shape measurements were
used to evaluate graphite degradation. Graphite area measurements were used to
determine the thickness of the graphite depleted layer. Microhardness
measurements are useful when a pearlitic rim occurs. The average thickness of
the skin for thin wall DI castings ranged from 0.15 to 0.45mm,
while for CGI it ranged from 0.7
to 2.5mm.
It was found as
expected that the strength decreased with thicker casting skin. The tensile and
yield strength skin factor (ratio between the strength of as-cast and machined
test plates) was about 0.93. This should be viewed as an upper limit, as only
one of the surfaces of the mechanical properties test plate was as-cast. More
significant reduction in strength should be expected.
Diffusion
calculations confirmed that graphite degradation, graphite depletion and the
pearlitic rim are the result of magnesium and carbon depletion at the
mold/metal interface because of their oxidation. Alternatively, carbon
diffusion from the mold can also result in pearlitic rim formation.