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
and
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
Introduction
It
is
necessary
to
improve
material s
c harac teris tics suc h as res ili ence, wear and corrosion
2
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
iron
favours
formation
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)
J SCI IND RES VOL 62 JU E 2003
584
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
7
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
s
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 .
partlc
0c:
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
l
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
ZUMELZU el al.: HIGH CHROMIUM CAST IRO N ALLOYS
Tabl e 1-
C hemical composi tion of all oys
C hemi cal Composit ion, per cen t by Illass
Alloy
C
Cr
Ni
Mo
Mn
Cu
Si
Ti
V
0.50
35.80
0 .20
< 0.05
0 .56
< 0.05
0.72
0 .1 6
0.02
2
0 .80
23.50
1.50
0.083
0.29
0.04
1. 73
0. 12
0.01
3
0.78
24.15
7.00
0.12
0.30
0.05
1.74
0 . 15
0.03
4
0.78
33 .50
7.68
< 0.05
0.40
< 0.05
0.94
0 .1 8
0.03
5
1.48
2 1. 30
1.10
< 0.05
0 .35
< 0.05
1.96
0.12
0.03
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
Component
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
Water
840 IllL
Citric acid
3.0 g
6
N
b
,...
5
~4
N
E
1:.
-.. 3
Cl
'i::'2
C\)
CI)
:::
a +-----,----1
14.4
----fr- WITH
TREATMEN
T
CrlC
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 .
586
J SC I IND RES VOL 62 JUNE 2003
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.
2.5
~
'"'E
u
E
en
~
CIl
U
c:
,!!:!
'~ 0.5
a:
o +------,------,------,-----,------,
14.4
29.4
30. 9
42.9
71.6
eriC
Figure 3 -- Resi lience variation versus CrlC ratio of cast iron
all oys
1cxxx)
~
'"E 1CXXl
u
100
<t
.s.
...
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.
0
4J
1.5
---
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).
2
...0
10
1
0.1
0.01
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 .
:Ii
( 3)
a:
:r: 2)
~ ;:o
Q)
~ 15
ell
:r: 1O
5
0 +------.-------.------,------,-------
144
::9.4
3)9
429
71.6
OC
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.
ZUMELZU e/ al.: HI GH CHROMI UM CAST IRON ALLOY S
587
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)
Conclusions
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,
J SCIIND RES VOL 62 JUNE 2003
588
chrome
3
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
microscopy
observations,
microanalyses,
and
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.
4
and inte rgranular corrosion
detac hme nt from the matrix.
due
to
Acknowledgements
The authors gratefu ll y acknowledge the
financial
support
provided
by
CONICYT
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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