等温淬火球墨铸铁的断裂韧性对等温淬火温度的依赖

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等温淬火球墨铸铁的断裂韧性对等温淬火温度的依赖Dependence of Fracture Toughness of Austempered Ductile
Iron on Austempering Temperature
P. PRASAD RAO and SUSIL K. PUTATUNDA
Ductile cast iron samples were austenitized at 927 7C and subsequently austempered for 30 minutes，
1 hour， and 2 hours at 260 7C， 288 7C， 316 7C， 343 7C， 371 7C， and 399 7C. These were subjected
to a plane strain fracture toughness test. Fracture toughness was found to initially increase with
austempering temperature， reach a maximum， and then decrease with further rise in temperature. The
results of the fracture toughness study and fractographic examination were correlated with micro-
structural features such as bainite morphology， the volume fraction of retained austenite， and its
carbon content. It was found that fracture toughness was maximized when the microstructure con-
sisted of lower bainite with about 30 vol pct retained austenite containing more than 1.8 wt pct
carbon. A theoretical model was developed， which could explain the observed variation in fracture
toughness with austempering temperature in terms of microstructural features such as the width of
the ferrite blades and retained austenite content. A plot of against sy (XgCg)1/2
resulted in a 2
KIC
straight line， as predicted by the model.
I. INTRODUCTION
WHEN ductile iron is subjected to an austempering
treatment， a range of microstructures is obtained depending
on heat treatment parameters such as austenitizing time and
temperature and austempering time and temperature.
[1–5]
This results in austempered ductile irons (ADIs) of different
grades ranging from high-strength–low-ductility types， to
low-strength–high-ductility ones， which have been found to
be economical substitutes for high strength steels in several
applications. The influence of heat treatment parameters on
the microstructure has been extensively studied using op-
tical microscopy， electron microscopy， and X-ray diffrac-
tion. It is now understood that the austempering reaction in
ductile iron is a two-stage process. At the austempering
temperature， ferrite precipitates out of， and grows into， the
austenite. Simultaneously， carbon is rejected from the
growing ferrite plates into the surrounding austenite. Car-
bide precipitation is suppressed because of the high silicon
content. The enrichment of austenite with carbon inhibits
the growth of ferrite and also stabilizes the austenite. This
decomposition of austenite into ferrite and high carbon aus-
tenite is referred to as the stage I reaction. If now quenched
from the austempering temperature， the microstructure will
consist of ferrite platelets in a matrix of stabilized high
carbon austenite. This is the desired microstructure of ADI.
If the iron is held at the austempering temperature for too
long a time， the high carbon austenite will decompose into
ferrite and carbide. This is the stage II reaction and is not
desired， as the carbide precipitation will embrittle the iron.
If the austempering time is too short， the austenite may not
be fully enriched with carbon， and some of it may transform
P. PRASAD RAO， Professor， is with the Department of Metallurgical
and Materials Engineering， Karnatak Regional Engineering College，
Karnatak State， India 574 157. SUSIL K. PUTATUNDA， Associate
Professor， is with the Department of Chemical Engineering and Materials
to martensite on quenching， again leading to embrittlement
of the iron. The optimum austempering time is， therefore，
the period between the end of stage I and the beginning of
stage II. This is called the processing window.
The influence of microstructure on mechanical properties
such as hardness， yield strength， tensile strength， and duc-
tility has been reported by several investigators.
[10–11]
The
important microstructural features of ADI that influence
their mechanical properties are retained austenite content，
carbon content of retained austenite， morphology of ferrite，
precipitated carbide if any， and the presence of any unsta-
bilized austenite that transformed to martensite. The num-
ber of graphite patches， their size， distribution， and
nodularity are also important factors. However， these are
unaffected by heat treatment conditions and are influenced
only by melting and casting practice. Therefore， while
studying the influence of heat treatment parameters on me-
chanical properties， these are generally ignored.
Fracture toughness is an important mechanical property.
An understanding of the influence of microstructure on
fracture toughness is important in order to optimize heat
treatment parameters. Previous investigations have shown
the importance of austempering temperature in controlling
fracture toughness.
[12–15]
These studies have shown that ADI
with lower bainitic microstructure has better fracture tough-
ness than that with upper bainitic microstructure. It has also
been shown that a retained austenite content of around 25
vol pct is desirable and that increasing the carbon content
of the austenite increased fracture toughness. The present
investigation has been undertaken to study further the in-
fluence of microstructure on fracture toughness and to de-
velop a model relating fracture toughness and
microstructural features such as the retained austenite con-
tent， the carbon content of retained austenite， and the width
of the ferrite blade.
II. EXPERIMENTAL WORK