Journal o f Scienti fic & Industrial 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 Termomecanicos. Uni vers idad Austral de Chile, Casill a 567 , Valdivia, C hil c

and

A Parada and L Goyos

Facultad de Ingenierfa Mecani ca. ISPJAE. La Habana, Cuba

Received: 17 January 2003; revised received: 06 March 2003; accep ted: 25 March 2003

The good results obtai ned by cast iron wi th Cr and Cr- Ni investigated in the maki ng of part for pumping equipment in sugar cane industry are given. Cast iron a lloy coupons were prepared in order to study their hard ness, res ilience, corrosion resistance. and wear-out behav iour by means of simul ating the conditions found at the sugar industry processes. Mi cros tructural characterisati on of all oys was made by scanning elec tron microscopy (SEM). and the electrochemica l tria ls in order to assess corrosion susceptibil ity were performed with a specific sugar solution. The behaviour of a ll oys to wea r-out and corros ion depends mainl y on the chemical co mposition and effect of the a lloying elements the formati on of carbides during so liditi cati on, and the presence o f impuriti es and inclusions at a surface level. The results obtained in th is study made possib le the selecti on of better all oys, which can be employed at the sugar industry such as 0.78 C-33 , 5 Cr-7 .7 Ni which have a high­chromium content. This materi al impli es a considerable economic saving for sugar indu stry.

Keywords : High chromium cas t iron , Simulation , Sugar industry, Scanning Electron Mi croscopy

Introduction

It is necessary to improve material s characteris tics suc h as res ilience, wear and corrosion res istance for severe service applications '·

2.

T he development of new methods and thermal treatment for cast iron alloys allow new material s to perform better mechanically and against corros ion when under aggress ive industri al environments3

.4 .

White chrome cast irons are currently leading several fi e lds of applications, be ing mainly employed in mechanical parts submitted to severe wearing-away. In addition , they are used when high corrosion resistance is required . Therma l treatments app lied to these a ll oys are concerned with two large fields which involve standard quench ing and annealing treatments, and maintenance treatments' under criti cal temperature . The latter has been app li ed to decrease costs invo lved with standard treatments and overcome

*Corresponding au thor

technological difficulties implied in the treatment 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 transformations undergone during thermal treatments and the ir influence on the expected mechanical and corros ion properties'.

For the preparation of samples, various factors and aspects related to thi s type of all oys were considered . Thus, e g, chrome addit ion strongly favours formation of carbides dur ing iron solidification and pearlite- like matrix formati on during eutectoid transformation 6

.

At 9.5- 15 per cent c hrome, carb ides of type (Cr,Fe)7C3 appear, and at a hi gh 30 pe r cent chrome, carbide type (Cr,FebC(, so lidifi es . O n the other hand , an increase in the c hrome content during casti ng, reduces the equilibrium te mperature of eutec tic tran sformation (TO> accordin g to the stab le Fe-C system and tncreases the equilibrium temperature of eutec ti c transformati on (T ill)

584 J SCI IND RES VOL 62 JU E 2003

acco rding to the unstab le system. Greater levels of C r initiate formation of an ox ide- impe ne trable laye r on 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 and various corrodin g media7

.

Above 16 per cent Cr, an oxide laye r type FcO.Cr20 , for ms on the iron surface, which s trong ly increases the resistance to hea t, makin g opera ti o ns poss ibl e at tc mperatures of the o rder of I , I 00 0c: On the other hand, it is we ll known that c hro me increases hardness and te ns il e strength of white and grey ironss. Furthermore, lite rature shows that hig h­chrome alloys ex hibit good eros ion-co rros ive and eros ion-abras ive behaviours'>· 10. Also, no n-standard

al loys of minute scattered carbides and matrix alloys

(ex: ) present good response 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 ipation of carbides, and e rosion-abras io n res istant alloys req uire hi gh participation of carbides, making poss ible a greater harde ning of the st ructure; as a result of coo lin g and the sca tte ring of carbides the s tructure is capable of sustainin g the de te rioration mec hani sms as a conseq uence of deformation and cuttin g by abras ive

. I II . 12 partlc es .

However, addition of nicke l in hi gh-chrome a ll oys faci I itates fo rmation of pearl ite, increases hardness and tens ile strength of iron. Furthermore the presence of Ni reduces volume of carbides in the a ll oy. Therefore the action of Ni is of a complex nature, stabil is ing austenite which presents a greate r capac ity of carbon disso luti on than ferrite, and decreasing carbides precipitation. The presence of Ni in c rystal I i ne matrix , due to its smaller atomic diamete r, can counterweigh 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 which favours the ex it of carbiding e lements l'. The precipitation of carbides is also reduced in thi s situation.

There fore the purpose of the present work was the preparation and manufacture of high-chrome (23-34 per cent) a ll oys by determining the adequate combinations of chemical components through tests invol ving thermal treatments , in order to evaluate the influence on a ll oys so as to warrant the best mechanical behaviour in wear-out cond itions and corros ion res istance in sugar media as those found in industrial processes.

The preparation of hi gh-chrome iron a ll oys that co mbine res istance to wear-out and corros ion allows a greate r be havioural knowledge o f these materials ,

mainly for their use in applications in sugar industry where such deteriorations take place; and with an

inc reased knowledge of the practi a l and theoretical a lloying e le ment limitati ons.

Materials and Methods

Several high-chrome alloys wcre prepared in an induction furnace of 1.2 kg capac ity. high-frequency, with me lt in in ves tment casting, in flat coupons 70 x 70 x 5 mm representing the ave rage thi ck ness of cast Iron walls III sugar processing and handling equipments .

The criteria used to select a part icu la r c hem ical composition were based mainl y on hi gh carbon contents in order to obtain a good parti c ipati on of carbides. Chrome was used in amoun ts above 18 per cent (22-34 per cent), as it con tributes to the formation of structures with hi gh parti c ipati on of hard chrome carbides thu s ach iev i ng a good corrosion­eros ion res istance. Nicke l was used at 0.5-7.7 pe r cent, as it improves the formation of a passivation layer in corros ion processes; as up LO 8 per cent Ni is reco mmended in alloys used in sugar applica tions. On the other hand , ni ckel at about 2 pe r cent leve l provides be tte r mec hanical propert ies and causes auto-quenching in structures of hardened matri ces most adequate for erosion wear-out. Additionally, small proportions of other a lloyi ng e lements were added to cast iron alloys as g iven in Table I.

Furthermore, standard control samples of regul ar al loy were prepared , w hich conta ined different proportion of alloying e lements , in order to eva lu ate changes In the properti es of these experimental a ll oys. The resulting tes ted alloy samples were characterised by scanning elect ron microscopy (SEM) through EDAX. Hardness of a ll oy samples was mechani cally measured th rough standard procedures by means of a Rockwell C Tes ting Device so as to dete rmine res ilience, to know the capac ity o f the mate rial to absorb energy during e lasti c defor mation .

All wear-out trials were performed in situ at a Sugar Processing Plant ; in that connec tion, a bypass on the juice-mixing line was made . The medium conditions we re as foll o ws: pH 5.2, corresponding to organic acids at 30 °C; tangen tial acce le ration of 15-

ZUMELZU el al.: HIGH CHROMIUM CAST IRO N ALLOYS 585

Tabl e 1- Chemical composi tion of all oys

Alloy Chemical Composit ion, per cen t by Illass

2

3

4

5

C

0.50

0 .80

0.78

0.78

1.48

Nb < 0.3; and Co < 0. 1

Cr

35.80

23.50

24.15

33 .50

2 1.30

Ni Mo

0 .20 < 0.05

1.50 0.083

7.00 0.12

7.68 < 0.05

1.10 < 0.05

20 mis, in the presence of hard solids .(Si02) at 0.6-1.5 per cent by we ight. The tri a ls we re pe rfo rmed for

200 h in a 45-cm drum, in which all a ll oy samples were pl aced s imul ating blades spinning at 1,700 rpm .

The wear-out was dete rmined by weight loss when a ll oy samples were compared with a control

alloy (85 Cu-5 Sn-5 Zn-5 Pb).

To study the poss ible improvements in the wear­out res istance properties of cast alloy samples,

the rmal treatment was carri ed out which cons isted of

the austenisation at 975°C for 1 h, coo ling by moving a ir down to 300 °C, and keeping a lloy sampl es for

8 h. The results of cast alloys were compared with those of the rmal treatme nts to determine the ex iste nce

of a decrease in the wear-out prope rti es of hi gh­

chrome alloys.

In o rde r to evaluate the corrosion performance,

alloy lO x 10 mm samples were prepared . A

standardi sed e lectroche mical trial with pote nti osta ti c measurements was carri ed out to determine the

corros ion curre nt den sity (ieorr), measured at an

overpotential of 10 mY (ESC) afte r a time span of 30 min . In thi s the iron a ll oy samples were used as

e lectrodes; a platinum counter e lectrode, and a ca lome l re fe rence e lectrode were also e mployed . The

e lectro lyte used as a corrosive medium, which s imulates the indu stri a l sugar juice, is described in Table 2 .

The set of trials carri ed out with high-chrome iron alloys and the respective mic rostructura l

characte ri sati ons, allowed de termination of sampl es which performed bes t and correlated these results w ith applications in sugar process ing under wear-out and corros ion conditi ons.

Mn Cu Si Ti V

0.56 < 0.05 0.72 0 .1 6 0.02

0.29 0.04 1.73 0. 12 0.01

0.30 0.05 1.74 0 . 15 0.03

0.40 < 0.05 0.94 0 .1 8 0.03

0 .35 < 0.05 1.96 0.12 0.03

Table 2-Solution simulating the indu strial sugar j uice

Component

Phosphoric ac id

Sacharose

Sodium ch loride

Sili con

Water

Citric acid

6

N 5 b ,... ~4

N

E 1:. 3 -.. Cl 'i::'2 C\) CI)

::: a +-----,----1

14.4

Concen trati o n

6.0 g

140.0 g

400 ppm

I per cent

840 IllL

3.0 g

----fr- WITH TREATMEN T

CrlC

Figure I - Evaluation of wear resistance versus CrlC rati o of cast iron all oys

Results and Discussion

The the rmal treatme nt appl ied to the all oy samples obtained from casting reduced the average wear-out of the tested alloys , as shown in Figure I .

586 J SCI IND RES VOL 62 JUNE 2003

A ll oy sample I, used as cont ro l; was a low­ca rbon and high-chrome a ll oy which rep resents the standard compos iti on of chrome a ll oys de ve loped fo r use In ex treme wear-out cond iti ons. Wear-out res istance, under tes ted cond iti ons, decreased accord ing to the CrlC ratio. T he C proporti on influenced such dec rease th rough the formati on o f chrome carb ides and lead to the consequent chrome loss fro m the matrix whi ch caused the a ll oy structure to be susceptible to corros ion. Howeve r, hi gher chrome proporti ons a ll owed a high-chrome stainless matri x.

W hen a thermal treatme nt was appli ed to a lloy samp les 4 and 5, they showed be tte r wear-out res istance than the cont ro l sample I . Sampl es with a hi gher c hro me pro porti on increased the wear-out resistance, which was ev ident with a hi gher Ni conten t; therefore, therma l treatment of a ll oys and incorporati on o f Ni at 2 per cent leve l proved benefic ia l in increas ing resis tance.

Concerning hardness, a decreas ing trend was shown as the CrlC rati o increased (Figure 2) .

Greater hardness occurred in samples 2 and 5,

which were over 10 points in the a + (Cr, Fe)7C6 phase when compared to the contro l. T he increas ing carbon content favoured formation of a greater number of chro me carb ides at nicke l concentration of 1.5 per cent.

In the studied a ll oys, the mechanica l property, the res ilience first increased with CrlC ratio up to CrlC 30 .9, and then w ith furth er increase in CrIC, fro m 30. 9 to 71 .6, it decreased in a lmost a linear manner (Figure 3).

4J

:Ii

( 3) a: :r: 2)

~ ;:o Q)

~ 15 ell :r: 1O

5

0+------.-------.------,------,-------

144 ::9.4 3)9

OC

429 71.6

Figure 2 -- Evaluation of hardness as a function of the CrlC ra tio of cast iron alloys

~

'"

2.5

'E 2 u E en 1.5 ~ ---CIl U c: ,!!:! '~ 0.5 a:

o +------,------,------,-----,------,

14.4 29.4 30.9

eriC

42.9 71.6

Figure 3 -- Resi lience variat ion versus CrlC ratio of cast iron all oys

~

'" E u

<t .s. ... ... 0 0

1cxxx)

1CXXl

100

10

1

0.1

0.01 Alloy sarrple t-k>.

Figure 4 -- Evalu ati on of co rrosion suscept ibili ty versus cast iron alloys

All oy samples 3 and 4 exceeded the contro l sample I in res il ience. In these high-c hrome iron alloys, nicke l was significant in such an increase, s ince greater res ili ence va lues were obtained at a Ni content of 7 per cent.

The results of the corros ion suscept ibility tri a ls fo r high chrome iron a ll oys tested in suga r med ia are shown in Figure 4 .

Alloy sampl es 2 and 4 presented the best corrosion res istance. Sample 5 was hi gh in carbon and low in nicke l, and during its manu fac turing a passivati on layer was achieved whic h protected the al loy surface aga inst the sugar medium. O n the other hand , sample 3 wi th proporti ons 24 C r-7 Ni , a lso provided good corros ion res istance. A high chro me content, mainta ini ng the N i ra ti o, d id not lead to good results aga inst corros ion. There fo re, ca re mu st be taken in the rati os of these alloy ing e lemen ts.

ZUMELZU e/ al.: HIGH CHROMI UM CAST IRON ALLOYS 587

The samples showing the lowest susceptibility to corrosion were samples 2 and 4, with ioni c clisso lut ions of Fe, Cr, and Ni under 0.05 mg/L. The SEM of sample 4 showed no ev idence of surface damage (Figure 5) on the all oy caused by the electrolyte used, and this exh ibited a homogeneous morphology, without defects. The sample 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 contrary, the SEM of sample 5 (Figure 7) showed intergranular corrosion with ionic dissolutioll (b lack line) caused by chrome separati on from the alloy. The Figure 8 with the hi ghest magnificati on showed the damage morphology by the sugar medium between the ferrite grains and corros ion products (li ght areas) in sa mpless

Conclusions

Of the results obtained under the ex perimental conditions of thi s study the following conc lusions cou ld be drawn :

• The chemical compos ition has a strong influence on the properties of all oys, i. e., the characteri stics 01 carb ides regarding their proportion , size and shape; and the type of matrix formed.

• With regard to the structural characteri sti cs the amount of chrome ill the matri x of tested alloys proved to be significant and determinant of the proporti on, size, and di stance of carb ides which cluster themse lves formi ng small discontinuous co lonies on grain edges, with distances that vary from 3 to 6 ~l1n .

Figure 5-Surface morphology of sample 4 without attack

(SEM. X 6,400)

Figure 6 - Incipienl pits on the surfacc of sample 2 (SEM. X 500)

Figure 7 - Surface morphology of sample 5 with intcrgranular corros ion (SEM, X 500)

Figure 8 - Effcct of corrosion on the micrnst ruclUre of sample 5, damage morphology (SEM, 4, 26ll)

• Three attack mechanisms of corros ion on the alloys under study were observed; I' i~., ga lvanic action , pitting mi crocorros ion around impurities,

588 J SCIIND RES VOL 62 JUNE 2003

and intergranular corrosion due to chrome detachme nt from the matrix.

All the 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.

Acknowledgements

The authors gratefu ll y acknowledge the financial support provided by CONICYT Chile/CITMA proj ec t W 1998 02 159; and the Laboratory for Iron and Stee l marking, Universiteit Gent, Belgium.

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