ISSN { I ' V0Il . . .

peclal Issue ~IZL,,

OUNDRY ENGINEERING Rr.. 7 '

< - f *

Puo~rsneo quarterly ss the organ d the Foundry Commiralon of the Pdkh Academy of Sciences

Influence of regenerative heat treatment on structure and properties of G20CrMo2 - 5

(L20HM) cast steel

"nstitute of Matcsials Engineering, Czestochowa University of Technology, Armii Ksajowcj 19, Czcstochowa, Folantl "Corresponding author. E-mail address: grisza @mim.pcz.czest.pl

Rcccivcd 26.02.2008; accepted in revised form 0 1 .Q4.2008

Abstract

Thc papcr prcscnts rcsuSts or rcscarch on thc influence of rcgcncrati\rc hcat treatment on thc structurc and propcrtics [hardncss. impact cncrgy) or L2O11M cnst stccl. Invcstipatcd material was taken from outer fmmc o f a turbinc which was scrviccd for t 67 424 hours a! thc tcmpcrauirc or 535 "C and prcssurc or 12.75 MPa. In psi-operating condition ~ h c invcstigntcd cast steel was cl~aracteri~cd by low impact cncrpy of I II ant1 hart3ncss of 139HV30. Analysis uF the irlflue~~ue ul ;lustcniriz;~t ion pariimctcrs (tcrnpcr;lturc and lime) has rcvealcd rhnt at thc tcinpcr;~turc rangc o f 895 + 955 "C (i-e. h3 +I- IO + 70 "C). for both 3 and 5 hours of holtl timc. rhc ohtaincd grain sizc amounls to 20 + 2511m. I has hccn pmved that tempcred bainitic-rcrri~ic and remit ic-hainiric-pearlit ic stn~cturc o f thc invcstigatcd cast stccl msurcs high impact cncrgy, i.c. K V > 1001. as well a5 hardncss. i.e. 11Y30. just at thc tcrnpcmturc of OXl) "C. 'I'crnpcmliirc or 7110 "C causcs i'~tnhcr incrclrsc of impact cncrgy along with a slight dccrcasc of hardncss. Morcovcr. it has hccn concludd that nppl ying oS under-annealing instcad or tcmpcring. nflcr standardizalion. guarantees scquircd impact cncrgy o f KY r 271.

Keywords: Hcat Treatment; Mechanical bmpcrties: Cast Stccl

I. Introduction

G2OCrMoZ - 5 (L20tlM) cast accl hclongs to a group of cast materials lridcly t~scd in powcr industry, siich as: nurcr frames of steam ttrrbincs. Sclf-study I ] of Cr - Mo cnst s~ccl structurc nftcr vsriuos scsvicc pcriods show that h c a ~ trcalmcnt hcforc scrvicc cnnsisrcd in normalizing annealing or full anncnlin~ ant! ncxt high-tcmpcraturc tcmpcring. Such tscatmcnt ;~llowctl lo ohrain fcrriric-pciirl iric or fcrrit tc-parlitic-hainit ic s~ructz rc in Cr - Mo cast stccl. Obtaincd strucrurcs cnsurcd achieving nrprnpcrtics rcyuircd for stccl c x t s by the norm.

h n y - ~ c r m opcri~tion of rbc slccl casts undcr crccp condirinns dqradcs t hc structure. Thcrc is slight decrease of ~ncch;rnia~l propcrt ics ant! largc dccrcasc o f impact cncr2y.

rrcqucntly bclow the lcvcl o f IOJ [ 2 4 ] . On rhc hisis of numerous scll-stodics [I. 51 i t has hccn noticccl that anscrurcs with prevailing bainite volumcrric fractinn o f arnund 9O + 95% arc subject to degradation mow slowly tluring long-tcrm operation.

i t is purposefi~l ro match siich hcnt rrcatmcnl pararnctcrs for steel casts, which would allow lo obtain thc structtlrc or high-temperature tcmpcrcd hainitc. Such a slnfcturc cnsurcr optimum corrclntion of mcchanicsl and plastic pmpmies IS. 61.

Thc papcr prcscnts thc influcncc o f rcgcncrarivc hcat treatment on stnlcutrc and p rop t i cs oh G2KrMo2 - 5 (L20HM) cast stccl.

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2. Investigated material amounted to 357 and 884 "C, rcspcctivcly. Flcat trcatmcnt of thc cast stcel consisted in austcnitizing of sninplcs For 3 hours at thc tcmpcratun? of 910 '% and subsctlucnr ri~l~r-hous Icrnpcring at

Table 1. Chcmical composition of the cast steel, Bwt. thc tcmp. range ah 680 + 700 "C. Morcovcr. For fulIy nnnmlcd samples a intcrcritical trcatmcnt (undcr nnncnling) was applicd at thc following tcmperaturcs: 770, 7'10 and X 1 i) "C. Ohscrvntion and rccord of thc rnicrosrmcturcs obrainccl ns a rcsrrlr or Srcal trcatmcnt, wcrc msdc by means ot" scanning JOEL ISM - 5400 microscopc. Thcrmodynarnic rcscarch was pcrformcd by mcans e l Thcrmo Cafe program for Windows (TCIV). Mcaszlrcmcnts

The inyestipred material was G20CrMo2 - 5 [UOHM) low o f thc rncchanical propcrtics (impact cncryy, hertlncss) wcrc

altoy cast stecl with chemical composition givcn in Table I. takcn according to current standard norms,

Tahlc 2 shows rcquircments regarding cheiical composition and propcrtics according to Polish Standard 171.

Tahlc 2. Rcquirements for chemical composition and rncchanical proprtics according to Potish Standard

Samples for testing wcrc takcn from outcr frame of steam turbinc scrviccd for 3 67 424 hours at Ihc tcmpcraturc of 535 "C and prcssurc of 12.75 MPa.

Aftcr service the investigated cast stcel rcvcalcd degradcd fcrritic - parl i t ic structure. Inside pcarlitc colonics spheroidizalion and coaguiation of carbides could he obscrvcd. On fcrrirc grain houndarics, numerous carbide prccipitatcs wcrc noticed. In somc arcas they formed ,,continuous grid" o f prcc~pltatbns. (Fig. I). IdcntiIicatian of carbides, whictr was carricd out by mcans of carbon exfraction replicas using transmission elccrron microscopc, rcvcalcd prcsencc of thc following carbides: M s . MliC i MSG inside ferrite and M3C inside pcarlitc.

After long-term operation the invcstigatcd cast steel was characterized by impact energy or 131 and hanlncss: I39HV30.

3. Methodology of research

Rcxarcb on thc influence of austenitization nararnctcrs on mean diarnclcr af formcr austenite grain was carried out nz thc temperature range or X95+9R5 '% for holding timc: 3 and 5 hours. ~rnages necessary For measurement o f Erain sizes wcn: recorded hy mcans of optical microscope - Axiovcrt 25. Computer aided anatysis a f imagc was done by m a n s of Imagepro Plus program, at thc assumed - and necessary For calculatians - number OF grains bctwccn 90&1000. Critical tcrnpcratures 4, and A,, for invcstigatd cast sreel wen: dctcrmincd by mcans of LS4 optical dilatomcler and thcy

Fig. I. Structure o f G20CrMe2 - 5 cast srccl :~rtcr npcriition

4. Rcscarch rcsuI ts

What has hccn dctcrmincd fnr Cr - Mo cast stccl. i s rhc influcncc o f austcnitiaation pnrarnclcrs (tcrnpcnturc and timc) on thc mean diamctcr o f formcr nustcni~c grain. Thc results arc prcscnted In Fig. 2.

Dilatamctric rcsts mndc i t possihlc to d n w a graph prcscnting the influcncc of cooling rntc on slnlctilrc and hardncss of G20 cast siccl. Fiy. 3.

110 ARCH1VES or FOUNDRY ENGINEERING Voturne 8 . S p e c i a l f s s u o $ 1 2 0 0 8 , 109-1 1 4

( Temperature of austenitization, OC I

Fig. 2, lnfluence of austenitization temperature and zime on the mcan diamctct or formcr austenize grain

10" 1 0'" 1 OD 10'

COOLING RATE, V,, Kfs

Fig. 3. Influence of cooling rare on structure and hardncss of thc cast stccE

AppIied hcat trcatment of the investigated cast stecl consisted in obtaining of the following structure types in the cast

steel: bainitic - rcrrilic. ferritic - bainizic - pearlitic and ferritic - pcarli~ic. Fig. 4 prescnts few demonstration structures, while Fig. 5 and 6 show thc influencc of temperature of tempering (annealing) on proprtics.

Thermodynamic bases of TCW program wcrc used in order to construct a graph for thc invcsrigatcd cast stcel, presenting stability of phases at the tcrnpcraturc range of 600 t 1200K (below and abovc of ql ternpcraturc), dcpcnding on carbon content - Fig. 7.

5. Discussion and conclusions

Performed rcscarch has proved that in Cr - Mo cast stccl the austenitization process at thc tempcraturc nngc of 895 + 955 "C Tor 3 as wcll as for 5 hours of anncaling, cnsurcs obtaining of the grain size of 20 + 25pm. lncrcasc of austcnitization tempcrature above 955 "C causes growth of formcr austcnitc grain. - Fig. 2.

Analysis of dilatometric curves has rcvcalod: that austenite, when subject to cooling at thc rate of v, , 2 0.027 Ws, gets transformed into ferrite and pcartitc. Cooling rntc at the range of 0.09 1 2 vx .5 5 0.042 K/s atlows to obtain fcrritic - pcarlitic - bainikic structure. Whilc at the cooling rate of v~ -5 2 0.208 Ws - ferritic - bainitic stmcturc is forrncd, with Gainitc fraction growing as the cooling rate increases. Applying cooling rate of vn .5 2 13.043 Ws aIlows to obtain bainitic structure with abouz 5% of fcrrite content (Fig. 3).

Obtaincd structures aftcr heat treatment, pscscnted in Fig. 4, reveal few carbide precipitates on boundaries of formcr austenitc grains and fcrrite grains, whilc on houndarics of bainite needles the precipilatcs arc numerous. F~irthcrmorc, in the case of pcarlite the proccsscs of rragrncnrazion nnd s~heroidization of carbides could be observed.

Investigations of mechanical properties aficr rcgcncrativc hcat trcatment have shown that for Cr - Mo cast stcel (with bainitic - ferritic and fcrritic - bainitic - ~carlitic structure) thc lowest tempering remperaturc - 680 "C cnsurcs high impact energy of KV > IOOJ and hardncss as wcIl.

Maximum tempering tcrnperaturc 700 "C (accortIing to Polish Standard) contributes to further incrcasc of impact cncrgy with hardncss dccrcasc of about 4 i 6% (Fig. 5. 6a. 6b).

Heat treatment (according to standard), which consists or normalizing anneating and high-vrnpcrrlturc tcrnpcring, docs not ensure required impact cnegy OF KV > 275 (Fig. 6c). According to self-studies [I, 81, thc rcason for low impact cncrgy after rcgcncrativc hcaz tmatmcnt was: privilcgcd carbides precipitation on grain boundaries and segregation of phosphon~s 10 ferrite grain boundarics. Applying of intcrcritical (a+$ annealing aftcr slandardization, through dissolution of carbidcs precipitated on grain boundaries (Fig. 7) and proccss of phosphorus desegregation, contributes to achieving requircd impact energy of KV > 27J (Fig. 6d). Hardncss of thc cast stccl aftcr such heat trcatmcnt is corn~arablc te thc condition aitcr service. At !he same tirnc intcrcritical trcalmcnt cailscs liirrhcr grain refinement, which is bcncficial for cast rnatcrinls.

A R C H I V E S of F O U N D R Y E N G I N E E R I N G V o l u m e 8 , Spec ia l I s s u e 1 1 2 0 0 8 , 109-1 1 4

Fig. 4. S'cnlctitrc of' 11ic cast stccl after: a) scrvicc; b) Ilci~t In'alrricr~l (ilflcr uridur-orir~u:~liriy) - li'rriiiu-pv;irlitic: h ) hci11 lrciltrncnt - krritic-hainitic-pcarli~ic; d) h c a ~ treatIncnt - hainitic-rcrrilic

Temperature, OC Temperature, O C i Fig. 5. Inllocncc of tclnpcriny ~crnpcri~ti~rc on thc propcnics: a) hardness: b) impact cncrsy or G20 CitSI accl wirh hilinitic-l'crri~ic

slructurc ( l ) and I'crri~~c-hn~nit ic-pcarliric stnlculrc (2)

1 12 A R C H I V E S of F O U N D R Y ENGINEE'RING V o l u m e 8 . Spec ia l I s s u e 7 1 2 0 0 8 , 1 0 9 - 1 1 4

190 190

rea 4 '

170 - t60 - 1150 - 140 - 130 -

TOO 685 690 695 700 680 685 690 695 700

Fig. 6. lnflucncc of thc tcmpcraturc of rcrnpcring (wncaling) on impact cncrgy and hardness of G20 cast stccl with thc foltowinp strwcturc: a) bainitic - fcrritk; b) Scrriric - bainiric - parlitic: c) rcrritic - pcarlitic a h tcmpcrinp;

d) fcrritic - pcarl i~ ic nrtcr ondcr-annealing

600

A O V f 5 0.20 0.25

MASS-PERCENT C

Fig. 7, lnflucncc of ~ h c tsrnperaturc on stability of phascs in Cr - Mn cast stccl. catculalcd by rncans of the TCW program

Acknowlcdgemcnts

Scientific work fitndcd by thc Ministry of Education and Scicncc in rhc ycnn 2006 + 2WJ as a rcsearch project No. DWM/46/CQST/2005.

Rcfcrcnces

[ I S. Stachura, G Golanski. Mctallogmphic and mechanical pmpcnics of slccl and cast srccl aftcr long ~ c r m scrvicc at clevatd leml.)eraturcs. Rcpn BZ - 202 - 1/61 rcscarchcs unpublished

121 A. Zklinski, J. Dobrzafiski, IF. Krttoli, Stnrlurnl changs In law atley cast stccl Cr- Mo - v aftcr long rim crccp scrvicc. JAM ME. 25. (2007). 33

131 S. Stachura. Thc changes aT stnlcturc and mcchnnical proprtics of stecls and cast stccls artcr tog icrm scrviccd at ctcvatcd tcmpratures, Encrgctic. 2, (1999). ID9 - 115 (in Polish)

[4] E. Molinic, R. Piques, A. Pineau, Bchaviour of a ICr - IMo - 0.25V stml aficr long - lcrm cxpsurc - I . Charpy impact toughness and crccp propx-ties, Fatique Fract. Engng Marcr. Stnlct., 14, 5 , (199E),531

A R C H l V E S of FOUNDRY ENGINEERIFZG Volume 8 , S p e c l a l Issue 1 1 2 0 0 8 , 109-114 113

[5] S. Stachun, J. Kupczyk. M. Gucwa, Optimization of [7] PN - 89flI- 83 157 ,,Steel castings crccp resistant nt clcvatcd szructurc and propcflics of Cr - Mo and Cr - Mo - V cast tcmpcraturcs, Gradcs." svcl intcndcd for usc at incscad tcmpcnturc, Przcglqd [8] G. Gotanski. S. Stachura, J. Kupcyk, 13. Kucharska - Odlcwnictwa 54,5,(2004), 402 - 408 Gajda, Ircat trcatmcnt o f cast stccl using normalization and

161 G. Golariski, S. Stachura. l3. Gajda, J. Kupcyk, lnflucncc of intcrcrit ical annealing, Arch. Foundry Eng., 7, (200?), t 23 thc cooling ratc on stmctun: and mcchanical propcrtics of L2 113MF cast stccl ancr scgcncntivc hcaz zrcatrncnt, Archivcs of Foundary, 6,2 I , 2006, 143 - 150 (in Polish)

A R C l i l V E S o f FOUNDRY E N G I N E E R I N G Volumo 8 , S p o c i a l t s s u o 1120D8. 109 -114