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ASSIGMENT HOMEWORK 2 HST
Question:
Carburizing and boriding are two processes used for surface
hardening purpose.
Comparing the hardening results from these two processes ta!e a
loo! the
spectrum of hardness obtainable with selected diffusion
processes of steel in "ig. #.
it can be seen that the hardness obtained from the boriding
$1800 %&' is far stronger
than that of carburizing $600 %&'. ()plain the mechanism
that could entail this
phenomenon.
Answer:
Carburizing is a case*hardening process in which carbon is
dissol+ed in the
surface layers of a low-carbon steelpart at a temperature
sufficient $850*
,500C' to render the steel austenitic followed by -uenching and
tempering to
form a martensitic microstructure. he resulting gradient in
carbon content
below the surface of the part causes a gradient in hardness
producing a
strong wear*resistant surface layer on a material usually
low*carbon steel
which is readily fabricated into parts.
/oriding or boronizing is a thermo*chemical surface hardening
process that
can be applied to a wide +ariety of ferrous nonferrous and
cermet materials.
he process in+ol+es heating well*cleaned material in the range
of 00 to
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1000 C $1200 to 1820 "' preferably for 1 to 1# hour in contact
with a
boronaceous solid powder $boronizing compound' paste li-uid or
gaseous
medium. he boriding process results in the formation of either a
single*phaseor double*phase layer of boride with definite
composition. he morphology
growth and phase composition of the boride layers can be
influenced by the
alloying elements in the base material. 3t means formation of
boride layers
depend on the composition of base material. his process usually
applied in
ma!ing tool steel.
nli!e carburizing treatment on ferrous materials where there is
a gradual
decrease in composition from the carbon*rich surface to the
substrate the
boriding of ferrous materials results in the formation of either
a single*phase
or double*phase layer of borides with definite compositions. he
single*phase
boride layer consists of "e#/ while the double*phase layer
consists of an
outer dar!*etching phase of "e/ and an inner bright*etching
phase of "e#/.
he formation of either a single or double phase depends on the
a+ailability of
boron.
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W! "ori#in$ Is Har#er Tan %arburi&in$
he surface material of boriding is harder than carburizing. his
is because of the
acti+ation energy and the atomic radius of boron is different
from carbon. Atomic radius of
/oron $85 pm' is larger than the Carbon $0 pm'. %igher atomic
radius of interstitial
atom in base material ma!es the mo+ement of dislocation is more
difficult. o the
result of surface hardness in boriding is higher than
carburizing.
Atomic radius is also related to the acti+ation energy. he
higher atomic radius the higher
acti+ation energy. /oron atoms need higher acti+ation energy to
dissol+e the atoms to
the substrate than the carbon atoms. 3t ma!es the surface
hardness of boriding is
higher than carburizing and the thic!ness of boride layers is
thic!er than carburizing.
he layers which is resulted by carburizing is different with
boriding. here is a
gradual decrease in composition from the carbon*rich surface to
the substrate with
carburizing while the boriding results in the formation of
either a single*phase or
double*phase layer of borides with definite compositions.
Re'erensi:
ofyan ofri7on. #010. 9ecture+%andout 11 %eat urface reatment:.
e!ni!
;etalurgi dan ;aterial "3< =epo!.
Ad+anced techni-ues for surface engineering: by >olfram
?issler
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