High-chromium (22-34 per cent) Cast Iron Alloys and Their

Jo urn al o f Scienti fic & Industri al Research
Vol. 62 , June 2003, pp 583-588
High-chromium (22-34 per cent) Cast Iron Alloys and Their Simulated Behaviour
at the Sugar Industry
E Zumelzu *, 0 Opitz and C Cabezas
Instituto de Materiales y Procesos Termomeca ni cos. Uni vers id ad Austral de C hil e, Casill a 567 , Valdivia, C hil c
A Parada and L Goyos
Facultad de Ingeni e rfa Mecani ca. ISPJAE. La Habana, C uba
Received: 17 Janu a ry 2003; rev ised received: 06 March 2003; accep ted: 25 Marc h 2003
The good result s ob tai ned by cast iron wi th C r a nd Cr- Ni investigated in th e maki ng of part for pumping equipmen t in
suga r cane industry are g iven. Cas t iron all oy co upo ns were prepared in o rd er to stu dy th eir ha rd ness, res ilience, co rrosio n
res istance. a nd wear-out behav io ur by means of simul ating the co ndition s found at th e suga r ind ustry processes.
Mi cros tru ctural characterisati on of all oys was mad e by scanning elec tron mi croscopy (SEM) . and th e electrochemica l tria ls in
o rde r to assess corrosion susceptibil ity were pe rformed with a specific sugar soluti o n. The behavio ur of all oys to wea r-o ut a nd
co rros io n de pe nd s mainl y o n the chemical co mposition and effect of the all oying ele me nt s the form ati on of ca rbid es during
so liditi cati o n, and the presence o f impuriti es and in clusions at a surface level. The res ult s ob tained in th is study mad e possib le
the selecti o n of better all oys, which can be e mpl oyed at the sugar ind ustry such as 0.78 C-33 , 5 Cr-7 .7 Ni which have a highchromium co nte nt. This mate ri al impli es a considerable economic saving for sugar indu stry.
Keywords : High chromium cas t iron , Simul ation , Sugar industry, Scanning Electron Mi croscopy
material s
c harac teris tics suc h as res ili ence, wear and corrosion
res istance for severe service application s ' · .
T he development of new method s and thermal
treatment for cast iron all oys all ow new mate rial s to
perform better mechanically and again st co rros ion
when unde r aggress ive industri al environments 3.4 .
White c hrome cast irons are curre ntly leading several
fi e ld s of applications, be ing mainl y emp loyed in
mechanical parts su bmitted to severe wearing-away.
In addition , they are used whe n hi gh corrosion
resistance is required . Therma l treatments app li ed to
these all oys are concerned with two large fields
which invo lve standard quench in g a nd annealing
treatme nts, and maintenance treatments' under c riti ca l
te mperature . The latter has been app li ed to dec rease
costs invo lved with stand ard treatments and overco me
*Co rrespo ndin g au thor
technological difficulties impli ed in the treat me nt of
large parts. In the evaluati on of new all oys , des igned
to operate in aggress ive medi a, it becomes essential to
know the transformation s undergo ne durin g the rmal
treatments and the ir influ e nce on th e ex pected
mechanical and corros ion properties'.
For the preparation of sampl es, various factors
a nd aspects related to thi s type of all oys were
considered . Thus, e g, chrome addit ion strong ly
of carbides
dur in g
so lidification and pearlite- like matrix formati on
during e utecto id trans formation 6 .
At 9.5- 15 per cent c hrome, carb ides of type
(Cr,Fe)7C3 appea r, and at a hi gh 30 pe r ce nt c hrome,
carbide type (C r,FebC(, so lidifi es . O n the oth e r
hand , an increase in th e c hrome co ntent durin g
casti ng, reduces the equilibrium te mperature of
eutec ti c tran sformation (TO> accordin g to th e stab le
Fe-C system a nd tn creases th e equilibrium
temperature of eutec ti c tra nsfo rmati on (T ill)
acco rdin g to the un stab le system . Greater levels of
C r initiat e formation of an ox id e- impe ne trabl e laye r
o n th e surface of cas t p ieces, which adhere very well
to the metal. Due to thi s, chrome iro ns a re resistant
to th e action of hea t a nd various co rrodin g media .
Above 16 per cent Cr, an oxide la ye r ty pe FcO.Cr20 ,
for ms o n the iro n s urface, which stron g ly increases
th e resistance to hea t, makin g opera ti o ns poss ibl e at
tc mpe ratures of th e o rd e r of I , I 00
On th e other
hand, it is we ll known that c hro me in c reases
hardness and te ns il e strength of white a nd g rey
irons . Furthermore, lite rature shows that hi g hchrome alloys ex hibit good eros io n-co rro s ive a nd
eros ion-ab ras ive behaviours'>· 10 . Also, no n- sta nd a rd
al loys of minut e scattered carbides a nd matrix alloys
(ex: ) present good res po nse to eros io n. For eros ionco rros io n res istant alloys, matrices are sugges ted to
be sta inl ess with limited parti c ipatio n of carbides,
and e rosio n-abras io n res istant alloys req uire hi g h
participation of carbides, makin g poss ible a greater
hard e nin g of th e st ructure; as a res ult of coo lin g and
the sca tte rin g of ca rbides th e structure is capable of
s ustainin g th e de te rioration mec hani sms as a
co nseq ue nce of defo rmation and c uttin g by abras ive
. Ies II . 12 .
Howeve r, addition of nicke l in hi gh-chrome
a ll oys faci I itates fo rmation of pea rl ite, increases
hardness and te ns il e strength of iron. Furthermore th e
presence of Ni reduces volume of carbides in the
a ll oy. Therefore the action of Ni is of a co mpl ex
nature, stabil is ing austenite which presents a greate r
ca pac ity of carbon disso luti on than ferrite, and
decreasing carbides precipitation. The prese nce of Ni
in c ry stal I ine matrix , du e to its smaller atomic
diamete r, ca n counterwe ig h effects of ch rome and
molibdene, obtaining, the re fore, a more uniform
stress fie ld. Thi s causes a reduction in the di stortion
e ffect of the lattice w hi c h favours th e ex it of
ca rbidin g e lements '. Th e prec ipitation of carbides is
also reduced in thi s situation.
The re fore the purpose of the present work was
the preparation and manufacture of high-chrome (2334 pe r cent) a ll oys by determining the adequate
combinations of chemical components through tests
invol vin g thermal treatme nts , in order to e valuate the
influ e nce on a ll oys so as to warrant th e best
mec hanical behav iour in wear-out co nd itions and
co rros ion res istance in s ugar media as th ose found in
indu strial processes.
Th e preparation of hi g h-c hrome iro n a ll oys that
co mbine res istance to wear-out and co rros ion allows
a greate r be ha vioural knowl e dge o f these materials,
mainly fo r their use in application s in suga r indu stry
where s uc h deterio ration s take place; and with a n
inc reased knowledge of th e practi a l and theoretical
a lloyin g e le me nt limitati o ns.
Materials and Methods
Several hi g h-c hro me alloys wcre prepa red in an
inductio n furnace of 1.2 kg capac ity. high-frequency,
with me lt in in ves tme nt casting, in flat coupons 70 x
70 x 5 mm represen tin g the ave rage thi ck ness of cast
Iro n walls III sugar process ing and handling
equipme nts .
The criteria used to select a part icu la r c hem ical
co mposition were based mainl y o n hi g h carbon
contents in order to obtain a good parti c ipati on of
carbides. Chrome was used in am oun ts above 18 per
cent (22-34 per ce nt), as it con tributes to th e
formation of stru ctures with hi g h parti c ipati o n of hard
chrome carbides thu s ach iev ing a good co rrosio neros ion res istance. Ni c ke l was used at 0.5-7.7 pe r
cent, as it improves th e formation of a passivation
layer in corros ion processes; as up LO 8 pe r cent Ni is
reco mmend ed in alloys used in sugar appli ca tion s. On
th e other hand , ni cke l at about 2 pe r cent leve l
provides be tte r mec hanical propert ies a nd causes
auto-quenching in structures of hardened matri ces
most adequate for erosion wear-o ut. Additionally,
small propo rtion s of other a lloyi ng e le me nts we re
added to cast iron alloys as g iven in Table I.
Furthermore, standard control sa mpl es of
regul ar al loy were prepared , w hi ch conta in ed
different proportion of alloying e lements , in order to
eva lu ate cha nges In the properti es of these
ex perime ntal a ll oys. The resultin g tes ted alloy
sa mples were characterised by scanning elect ro n
mi croscopy (SEM) throu g h EDAX. Hardn ess of a ll oy
samples was mechani ca ll y measured th rough standard
procedures by means of a Roc kwell C T es ting Dev ice
so as to dete rmine res ilience, to kn ow the ca pac ity o f
the mate rial to absorb e ne rgy during e lasti c
defor mation .
All wear-out trials were pe rformed in situ at a
Su gar Processing Plant ; in that conn ec tion, a bypass
on the juice-mixing lin e wa s made . Th e medium
conditions we re as foll o ws : pH 5.2, co rrespondin g to
organic acids at 30 °C; tangen tial acce le ration of 15-
5 85
Tabl e 1-
C hemical composi tion of all oys
C hemi cal Composit ion, per cen t by Illass
0 .20
< 0.05
0 .56
< 0.05
0 .1 6
0 .80
1. 73
0. 12
0 . 15
33 .50
< 0.05
< 0.05
0 .1 8
2 1. 30
< 0.05
0 .35
< 0.05
Nb < 0 .3; and Co < 0. 1
20 mis, in th e presence o f hard solids .(Si0 2) at 0.61.5 per cent by we ig ht. Th e tri a ls we re pe rfo rmed for
200 h in a 45-c m drum , in which all a ll oy sampl es
were pl aced s imul a ting blades spinning at 1,700 rpm .
The wear-out was de te rmined by weight loss
w he n a ll oy samples were co mpared with a control
alloy (85 Cu-5 Sn-5 Zn-5 Pb).
To study the poss ibl e improvements in the wearo ut res istance properties of cast alloy sampl es,
the rmal treatme nt was carri ed out which co ns isted of
the austenisation at 975°C for 1 h, coo lin g by movin g
a ir d own to 300 °C, a nd keeping a ll oy sampl es for
8 h. The results of cast alloys were compa red with
th ose of the rmal treatme nts to determine the ex iste nce
of a decrease in the wear-out prope rti es of hi ghc hrome alloys.
In o rde r to eva lu a te the corrosio n performance,
alloy lO x 10 mm sampl es were prepared . A
standardi sed e lectroc he mi cal trial with pote nti osta ti c
measurements was carri ed out to determine the
corros ion c urre nt den s ity (ieorr), measured at a n
overpotential of 10 mY (ESC) afte r a time spa n of 30
min . In thi s the iron a ll oy sampl es were used as
e lectrodes; a platinum counter e lectrode, and a
ca lo me l re fe rence e lectrode were also e mpl oyed . The
e lectro lyte used as a corrosive medium, which
s imulates the indu stri a l sugar juice, is desc ribed in
Tabl e 2 .
The set of trials carri ed out with hi g h-c hrome
iron alloys and the respective mi c rostructura l
c harac te ri sati ons, allowed de te rmination of sampl es
which performed bes t and co rrelated these res ults
w ith a ppli cation s in suga r process in g under wear-o ut
and co rros ion conditi o ns.
Table 2-Solutio n simulating the indu strial suga r j ui ce
Concen trati o n
Phos phoric ac id
6.0 g
140.0 g
Sac harose
Sodium ch lo ride
400 ppm
Sili co n
I per cent
840 IllL
Citric acid
3.0 g
-.. 3
a +-----,----1
----fr- WITH
Fig ure I -
Evaluation of wea r resistance versus CrlC rati o of
cast iron all oys
Results and Discussion
Th e th e rmal treatme nt appl ied to the all oy
sampl es obtained from casting red uced th e average
wear-out of the tested alloys , as s how n in Figure I .
A ll oy sample I, used as co nt ro l; was a lowca rbon and high-c hrome a ll oy w hic h rep resent s the
standard compos iti on of c hro me a ll oys de ve lo ped fo r
use In ex treme wear-out cond iti o ns. Wear-out
res istan ce, un der tes ted cond iti o ns, decreased
accord in g to th e CrlC ratio. T he C propo rti on
influ e nced suc h dec rease th rough the fo rmati o n o f
c hrome ca rb ides a nd lead to th e co nseque nt c hrome
loss fro m th e matrix whi c h caused the a ll oy stru cture
to be susceptibl e to corros io n. Ho we ve r, hi g he r
c hrome pro porti ons a ll owed a hi g h-c hrome stainl ess
matri x.
W he n a therm a l treatme nt was appli ed to a lloy
sa mp les 4 a nd 5, th ey sho wed be tte r wear-out
res ista nce th a n th e co nt ro l sa mpl e I . Sa mpl es with a
hi ghe r c hro me pro po rti o n in c reased the wea r-o ut
resistance, whi c h was ev ide nt with a hi ghe r Ni
con ten t; th e refo re, therma l treatme nt of a ll oys a nd
in co rpo rati o n o f Ni at 2 pe r cent leve l pro ved
benefic ia l in in c reas in g resis tance.
'~ 0.5
o +------,------,------,-----,------,
30. 9
Figure 3 -- Resi lience variation versus CrlC ratio of cast iron
all oys
'"E 1CXXl
Greate r hardness occ urred in sa mpl es 2 and 5,
whi c h were over 10 points in the a + (Cr, Fe)7C6
phase whe n co mpared to the contro l. T he inc reas ing
carbo n co nte nt favoured formation of a greate r
num ber of c hro me carb ides at nic ke l concentrati o n of
1.5 pe r ce nt.
Co ncernin g ha rdness, a decreas in g tre nd was
shown as the CrlC rati o inc reased (Fi gure 2) .
In the studi ed a ll oys, the mec hani ca l pro pe rty,
the res ili e nce first in c reased with CrlC ratio up to
CrlC 30 .9, and the n w ith furth e r inc rease in CrIC ,
fro m 30. 9 to 71 .6, it dec reased in a lmost a linear
manne r (F ig ure 3).
Alloy sarrple t-k>.
Fi gure 4 -- Evalu ati on of co rros ion suscept ibili ty versus
cast iro n alloys
All oy sampl es 3 a nd 4 exceeded the co ntro l
sampl e I in res il ience. In these high-c hro me iron
alloy s, nicke l was sig nifi cant in su ch an in c rease,
s ince g reate r res ili ence va lu es were o bt ained at a Ni
conte nt of 7 per cent.
Th e results of the corros io n suscept ibility tri a ls
fo r hi g h chrome iron a ll oys tested in suga r med ia are
shown in Fi gure 4 .
( 3)
:r: 2)
~ ;:o
~ 15
:r: 1O
0 +------.-------.------,------,-------
Figure 2 -- Evaluation of hard ness as a function of the CrlC ra tio
of cast iron alloys
All oy sa mpl es 2 a nd 4 presented th e best
corrosion res istance. Sampl e 5 was hi gh in carbo n
and low in ni c ke l, a nd during its manu fac turing a
passivati o n layer was achi eved w hi c h protected the
al loy surface aga in st th e sugar medi um . O n the o th e r
hand , sa mp le 3 wi th propo rti o ns 24 C r-7 Ni , a lso
provi ded good corros io n res ista nce. A hi gh ch ro me
conte nt, ma inta ini ng th e N i ra ti o, d id no t lead to good
results aga in st corros ion. The re fo re, ca re mu st be
ta ke n in the rati os of these alloy in g e lemen ts.
The sa mpl es show ing the lowest susceptibility
to corrosio n were sa mpl es 2 and 4, with ioni c
clisso lut io ns of Fe, Cr, and Ni under 0.05 mg/L. The
SEM of sa mpl e 4 showed no ev idence of surface
damage (Figure 5) on the all oy ca used by th e
elec trolyte used, and this exh ibited a homogeneous
morp hology, without defects. The sampl e 2 also did
not show any evidence of deterioration (Figure 6) ,
however, small and incipient pits could be
apprec iated (top upper and left corners).
On the co ntrary, the SEM of samp le 5 (Figure 7)
showed intergranular co rrosio n wit h ioni c dissolutioll
(b lack line) caused by chro me separati on from the
alloy. The Figure 8 with the hi ghes t magnifi cati on
showed the damage morphology by the sugar medium
betwee n the ferrite grains and corros ion products
(li ght areas) in sa mpless
Figure 6 - Incipienl pits on the surfacc of sample 2 (SEM. X 500)
Of the res ults obtained under the ex perimental
co nditions of thi s stud y the following co nc lu sions
cou ld be drawn :
• The chemical compos iti on has a stro ng influence
on th e properties of all oys, i. e., the characteri stics
01 carb ides rega rdin g th eir proportion , size and
shape; and the type of matrix formed.
• With rega rd to the structural characteri sti cs the
amount of chrome ill the matri x of tested alloys
proved to be signifi cant and determinant of the
proporti on, size, and di stance of carb ides which
cluste r th emse lves formi ng small discontinuous
co lonies on grain edges, with distances that va ry
from 3 to 6 ~l1n .
Fi gure 7 -
Surface morphology of sample 5 with intcrgranular
corros ion (SEM, X 500)
Figure 8 - Effcct of corrosion on the micrnst ru clUre of
sample 5, damage morphology (SEM, 4, 26ll)
Figure 5-S urface morphol ogy of samp le 4 without attack
(SEM. X 6,400)
• Three attack mechanisms of co rros ion on the
alloys under study were obse rved; I' i~., ga lva ni c
action , pitting mi crocorros ion aro und impurities,
All th e observations were carried out, to
eval uate the potential perfomance of cast iron alloys
under study, i.e., the corrosion resistance, electron
mechanica l properties, comply with the requirements
for discrimination and adequate information to be
incorporated into a total quality control, and also in
this manner, to study necessary strategies for the
se lecti on of better alloys.
and inte rgranular corrosion
detac hme nt from the matrix.
The authors gratefu ll y acknowledge the
C hile/CITMA proj ec t W 1998 02 159; and the
Laboratory for Iron and Stee lmarking, Universiteit
Gent, Belgium.
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