Why Primary Graphite is Bad! Author: Rudolf Sillén CEO, NovaCast Foundry Solutions AB Primary graphite an easily our in dutile iron if the ar!on e"uivalent is too high in relation to the ooling rate# Primary graphite an !e the ause of asting defets suh as shrin$ages and ar!on flotation# %et us ompare t&o ases' Case A: Assume A: Assume a hypereuteti alloy &ith C()#* and Si(+# giving CE%(-#./ 0see illustration1# 2uring solidifiation primary graphite &ill preipitate from the li"uidus temperature until the euteti temperature is reahed# 2issolved ar!on in the li"uid is there!y redued !y 3#)/ 0-#. 4 -#)1# 5he dissolved ar!on in the li"uid is thus redued to )#./ 0)#* 4 3#)1# 6f the austenite an dissolve 7#/ ar!on then the amount of ar!on availa!le to reate euteti graphite is 7#/ 0)#. 47#1# 5his euteti graphite e8pands and an redue, or in optimal ases, eliminate shrin$ages# Note that the primary graphite is preipitated too early and is therefore not effetive for reduing shrin$ages that our at a later stage in the solidifiation proess# Case B: Assume B: Assume another alloy &ith &ith C()#* and Si(7#. giving CE%(-#)/# 5hat alloy &ill start to solidify as euteti 0no primary austenite, no primary graphite1# 5herefore the ar!on in the li"uid remains at )#*/ during the euteti solidifiation# 5he amount of euteti graphite is therefore )#* 4 7#(+#7/# 5hus this alloy has almost +3/ more euteti graphite than the hypereuteti alloy &hih is a "uite su!stantial improvement9 6f the amount of euteti graphite and its e8pansion is not suffiient to ompensate for the ontration of the austenite and the remaining li"uid then miro shrin$ages are reated# 5herefore it is very important to avoid primary graphite# Where is the eutectic point? 5he traditional iron4ar!on phase diagram sho&s that the euteti point is around -#)/# :o&ever, that is during e"uili!rium onditions &ith a slo& ooling rate# 5herefore, if the hemial omposition of an alloy is higher than -#) e#g# -#; !ut the ooling rate is high, the solidifiation &ill pro!a!ly NO5 !e hypereuteti as e8peted !ut euteti9 An alloy that is hypereuteti as alulated &ith the formula C
5he diagram a!ove illustrates the appro8imate loation of the euteti point as a funtion of the asting modulus &hih is related to the ooling rate# 5he illustration !elo& sho&s the ourrene of shrin$ages !oth for hypo4 and hypereuteti ompositions# For dutile iron the euteti >point? seems to !e more li$e a plateau#
Expansion of primary graphite 6f the alloy solidifies as hypereuteti then the primary graphite gro&s diretly in the ontat &ith the
li"uid phase# 6ts e8pansion offsets the ontration of the li"uid phase# An alloy &ith ACE%(-#; &ill result in an e8pansion of a!out 3#/ !et&een the li"uidus temperature 0a!out 7+33 C1 and the euteti temperature# 5he ontration of the li"uid phase is a!out 3#@;/ &hih means that the total volume hange from li"uidus to the euteti temperature is more or less ero# 6f the asting uses a feeder then the effet might !e that the feeder does not start to pipe as there is no need for feed metal initially# 6nstead shrin$ages might develop lose to the feeder ne$ &hen feed metal is re"uired later in the solidifiation proess# Belo& is a piture of a real ase &here the feeder did not pipe# 5he miro to the right of the photo sho&s many large nodules &hih proves that the solidifiation &as hypereuteti# hen the solidifiation &as euteti then the feeder piped and the shrin$age disappeared#
6f the alloy solidifies slightly hypoeuteti 0ACE% -#+ -#+;1 then some primary austenite &ill !e the first phase# 5hen there is a ontration in li"uid state and the feeders &ill start piping more easily# Another effet might !e that the graphite spheroids, preipitated at the euteti temperature, are surrounded !y austenite at an earlier stage and their gro&th rate is there!y redued# 5he graphite preipitation pattern might !e more gradual &ith more e8pansion at the end of freeing#
5he piture illustrates the density hanges for a sample &ith hypereuteti omposition during solidifiation# 6nitially the density inreases resulting in a ontration in li"uid state# hen li"uidus is reahed 05%1 then primary graphite is preipitated and e8pands# At the same time the li"uid ontrats# 5he e8pansion and ontration during this phase 0S71 are almost the same &hih means that the density does not hange# hen the lo& euteti is reahed 05Elo&1, euteti graphite i s preipitated &hih auses an e8pansion# 5he first part 0S+1 until 5Ehigh is reahed is the realesene# 5hen the seond phase of euteti freeing ommenes 0S)1, often &ith a slight density inrease espeially lose to the solidus temperature 05S1 also alled end of freeing# 5hen the density inreases further in solid state 0pattern mans shrin$age1# 5he piture !elo& sho&s a typial hypereuteti urve analyed !y A5AS# A5AS#
Conclusion: Avoid preipitation of primary graphite9 For ma8imum euteti graphite adDust the hemial hemial omposition to fit the solidifiation rate in the asting9 sing hemial analysis to alulate the ar!on e"uivalent is not suita!le due to the limited auray 0<=4 3#3; or more1# 5he foundryman might !elieve that the ar!on e"uivalent is &ithin range &hen it is not9 se A5AS A5AS and the ACE% method in order to ahieve the desired target for ACE% &ith high auray 0<=4 3#371#
