Page 132 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 4.7 Heat Treatment of Ferrous Alloys

broken, the fracture path is along the graphite flakes and has a gray, sooty appear-
ance. These flakes act as stress raisers; as a result, gray iron has negligible ductility
and is weak in tension, although strong in compression, as are other brittle materi-
als. Cn the other hand, the presence of graphite flakes gives this material the capac-
ity to dampen vibrations (by internal friction). This capacity makes gray cast iron a
suitable and commonly used material for constructing machine-tool bases and
machinery structures (Section 253).
Three types of gray cast iron are ferritic, pearlitic, and martensitic. Because of
the different structures, each has different properties and applications. In ferritic
gray iron (also known as fully gray iron), the structure consists of graphite flakes in
an alpha-ferrite matrix. Pearlitic gray iron has a structure of graphite in a matrix of
pearlite; although still brittle, it is stronger than fully gray iron. Martensitic gray
iron is obtained by austenitizing a pearlitic gray iron and then rapidly quenching it,
to produce a structure of graphite in a martensite matrix; as a result, this cast iron is
very hard.
Ductile (Nodular) Iron. In the ductile-iron structure, graphite is in a nodular or
spheroid form (Fig. 4.13b); this shape permits the material to be somewhat ductile
and shock resistant. The shape of graphite flakes is changed into nodules (spheres) by
small additions of magnesium and/or cerium to the molten metal prior to pouring.
Ductile iron can be made ferritic or pearlitic by heat treatment. It can also be heat
treated to obtain, alternatively, a structure of tempered martensite (Section 4.7).

White Cast Iron. The white cast iron structure is very hard, wear resistant, and brit-
tle because of the presence of large amounts of iron carbide (instead of graphite). White
cast iron is obtained either by cooling gray iron rapidly or by adjusting the composition
by keeping the carbon and silicon content low. This type of cast iron is also called white
iron because of the white crystalline appearance of the fracture surface.

Malleable Iron. Malleable iron is obtained by annealing white cast iron in an
atmosphere of carbon monoxide and carbon dioxide, at between 800° and 900°C,
for up to several hours, depending on the size of the part. During this process, the
cementite decomposes (dissociates) into iron and graphite. The graphite exists as
clusters or rosettes (Fig. 4.13c) in a ferrite or pearlite matrix; consequently, mal-
leable iron has a structure similar to that of nodular iron. This structure promotes
ductility, strength, and shock resistance-hence, the term malleable (from the Latin
malleus meaning “it can be hammered”).

Compacted-graphite Iron. The graphite in this structure is in the form of short,
thick, interconnected flakes having undulating surfaces and rounded extremities.
The mechanical and physical properties of this cast iron are intermediate between
those of flake-graphite and nodular-graphite cast irons.

4.7 Heat Treatment of Ferrous Alloys

The various microstructures described thus far can be modified by heat-treatment
techniques-that is, by controlled heating and cooling of the alloys at various rates.
These treatments induce phase transformations that greatly influence such mechan-
ical properties as the strength, hardness, ductility, toughness, and wear resistance of
the alloys. The effects of thermal treatment depend on the particular alloy, its com-
position and microstructure, the degree of prior cold work, and the rates of heating
and cooling during heat treatment. The processes of recovery, recrystallization, and