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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