Austempered ductile iron is produced by heat-treating cast
ductile iron to which small amounts of nickel, molybdenum, or
copper have been added to improve hardenability. Specific
properties are determined by the careful choice of heat treating
parameters. Austempering involves the nucleation and growth of
acicular ferrite within austenite, where carbon is rejected into
the austenite. The resulting microstructure of acicular ferrite
in carbon-enriched austenite is called ausferrite. Even though
austenite in austempered ductile iron is thermodynamically
stable, it can undergo strain-induced transformation to
martensite when locally stressed. The result is islands of hard
martensite that enhance wear properties.
Advanced Cast Products uses salt baths for austenitizing,
quenching, and austempering in order to achieve close
dimensional control. Times and temperatures are tightly
controlled throughout the entire process.
Steps in Austenitizing Process
1. Heat castings in a molten salt bath to austenitizing
temperature.
2. Hold at austenitizing temperature to dissolve carbon in
austenite.
3. Quench quickly to avoid pearlite.
4. Hold at austempering temperature in molten salt bath for
isothermal transformation to ausferrite.
Consistent control of times and temperatures throughout the
entire process
1. Initial austenitizing times and temperatures (1550° to 1700°
F.) are controlled to ensure formation of fine grain austenite
and uniform carbon content in the matrix. The precise
temperature is grade dependant.
2. Quench time must be controlled within a few seconds, to avoid
formation of pearlite around the carbon nodules, which would
reduce mechanical properties. Quench temperatures (450° to 750°
F.) must stay above the point of martensite formation (except
for ASTM A 897 Grade 5).
3. In the austempering step which follows austenitizing, the
temperature of the final salt bath must also be closely
controlled. The austempering step is also precisely
time-controlled, to avoid over- or under-processing. By the end
of this step, the desired ADI ausferrite structure has
developed.
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