A FIM/ATOM-PROBE STUDY OF PHASE TRANSFORMATIONS IN MOLYBDENUM STEELS

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A FIM/ATOM-PROBE STUDY OF PHASE

TRANSFORMATIONS IN MOLYBDENUM STEELS

I. Stark, G. Smith

To cite this version:

I. Stark, G. Smith. A FIM/ATOM-PROBE STUDY OF PHASE TRANSFORMATIONS IN MOLYBDENUM STEELS. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-447-C6-452.

�10.1051/jphyscol:1987673�. �jpa-00226881�

JOURNAL DE PHYSIQUE

Colloque C6, supplCment a u n O l l , Tome 48, novembre 1987

A FIM/ATOM-PROBE STUDY OF PHASE TRANSFORMATIONS IN MOLYBDENUM STEELS

I. Stark and G.D.W. Smith

Department of Metallurgy, Oxford University, Parks Road, Oxford OX1 3PH, U.K.

AlSXRMPFIM/AP,T!%,~ X-ray analysis ard dilatonetry irdicate that both 3 C precipitation and a s o l u t e d r e g l i k e effect are associated with reacticn retaz&tim i n F d H b allay. %ere is no e v i h for molybdennn or carbcn/wlybdenm clustering in awtenite.

INIIICDUCPIZH

The i n h i b i t i n g e f f e c t s of mlybdmm rn the t r a m f o m i c n of Fe++b alloys are d i a m m [1-6], but t h e i r o r i g i n d m mlear. What is clear is that l dof mlyb&mu of only a.few tenths of an atomic per cent significantly retard the rate of deccmposition of a t s t e d t e t o f e r r i t e 4% carbids at a l l temperatures. This e f f e c t i s p a r t i c u l a r l y marked a t i n t e n d l a t e teapetahlres ( 50 C

-

^{703 C )}

resulting in the deep lsys in the lllkwws of mny mlyWenrm steels. Similar effects are observed with other solute additions tut n b t o the sare degree a s with molylxbum.

The molybdenum effect has been mriously attributed t o (1) Mo/C clustering in the azsrenite pinning t h e i n t e r f a c e [4], ( 2 ) s o l u t e d r a g at the transPonnaticn ineerfaoe [3,6,7] a d (3)Merphsse interface p r e c i p i t a t i o n [8,9]. There i s no strong p r e f e r e n t i a l evidence f o r any one of these hypotheses, principally because the m , l y ~ c a r b a d i n t e r f a c e interactiau appear t o be on an a t i c d eand are

~ c e s s i b t t o the experhmtal t d i n t q ~ ~ ~ applied previcusly. In the present iinrestigathn the uee of a field-ion microscope/atom-probe has pruvided the f i r s t analysis on a suEficiadly fine scale of the atomic processes associated with mtardathn both above and helm the b y region of the 'ITCarrves. A f i e l d a n i s s i m FlCFM has also k e n used t o investigate grain-hdary -tion.

-

Fe+.20wt%C-0.52wt;ZMo and Fe43.32wt%C-2.05wt% alloys ere EhOSen on the h i s of their mnrkemy different TlT amw [ref.l p.119.,ref.2,p.106]. Transfonmtion t hndmtes in

9

fomer,ohtt can t&e hcurs or days in the latter. I n the 2& all9 it is p a r t i q l y slow betwem 650 C ad 550 C and a d e p b a y f o r m s i n t h e ~ c u r v e . S p e c i m e u s ~ h a P D g e n i s e d a t 1 2 M C f o r 3 d a y s a d ~ q r a r h e d ~ o b r i n e . Following r e a u s t e n l t i s a t i o n a t 1050 C they were quen&ed h t o a salt bath or transferred to -her

furnace. Specimens were transformed as bars or discs of thfckwss 0.5-fmn i n order t o &eve w i d cooling. No significant dacattmisaticn was observed.

OFIM/AP a n a l y s i s was carried out using a VG Scientific FIMlUl [lo] with the tfq hpxature set at 90 K. Molybdenum atoms image very brightly in necn and the disMbutim of mlybdenm c d d be d e d u o e d f r o m t b F I M ~ .

X-ray analyses *re obtained using the VG HD501 fiel-ssim SIB4 at the ZKAFA HaweU laboratories by Dr. J.M.TLtchm~9h.

The TIT auve for a F e 0 . 3 ~ ~ - 1 . 9 ~ alloy was determind using dilatanetry. lhts is expected t o d i f f e r s l i g h t l y from t h a t f o r t h e 2.0- alloy as the tmperature rerge of each reacticn regime is a satsitive f m c t i m of mlylxbum content [2]

.

To t e s t whether clustering is assodated with the deep b y in the 2.05wtXEb allcy, the distribution of molybdenum i n a u s t e n i t e under these conditions was determined. I n a l l instances neither FIM observatim nor atomprole analysis sIowed any -sth that ~ l y b & m n e l u s t e e had tdtaz p k . Ihe results obtained fmn a 20,0a)-icn sanple a f t e r 23 hours at 570 C are s b m in table 1 below. Ihe laJ

X'

value supports the lull hypothesis that the distribution of mlyWanm is rwdan. Analysis of the spct*

between successive molybdenum ataos also -tsd a rzadaP rllstributim. S q l e axxi spa* amlysis also s l m d no widence of carbon/m,lybdenrm asmdation.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987673

C6-448 JOURNAL DE _PHYSIQUE

Table 1 : % c c n p o s i t h of b a t a n sarples (20-atan step).

Mo per 40-atan w l e Observed W t e d i f randan

0 633 622

1 282 297 %-squared = 5.5

2 67 69

h

18 11 prolnblllty

>

11%

(2) pol/AP ad 2PN anal+ af f e r r i t e 0

Optical microscopy revealed four t m f o n n a t i c n regimes between the u p p r nose a t 730 C ad M

,

cad1 of which was slwwn by FIM/AP analysis t o be associated with a particular type of k / C interactfa,. llle aralyses are smmarised in table 2 and the mLcrmtnrctures i n figs. 1-3 d 6.

Dilatometry slwwed t h a t the lower transformation region, usually represented by a sirgle set of TIT-curves, i n f a c t comprises two closely-overlapping regimes ( f i g . 4) with distinctly rLCEfere~u mlcrc6 tructures.

Ihe four g n e r a l reactial reginw wre:

(a)Above 590 C: The 0 i n i t i a l decomposition product ws f e r r i t e which f i r s t F o n d alorg t l e prior a u s t e n i t e grain-boundaries ( f i g . l b ) . Throughout t h i s range ahnost a l l of tleilulylxhrun kcwas incorporated i n t o carbides. I n i t i a l l y , these a r e approxhwtely s t o i c h i a e t r l c YC du?re sole of tlle molybdenum s i t e s a r e occupied by ircn uterus. In the l a t t e r s ~ a g r s , wlcre ~ r l l t e fonlr;, boLl~ Elf tuxl molybdeny-enriched E C are 0@3erved i n the

-

^{FlM sp~ji~m.}

^{9 )5C}

arecLpltdLe wra ~ n i ~ d y railers ( 1 ( ~ 2 0 0 & X 300-Aa t 730 C and spheraida ( l ~ 2 . 5 d ) a t 670 C- 90 C (fig.la). Mos~ of UE! r a l l e t s were s i m i l a r l y oriented alt- tlere

vre

^{a F w} occepticns The size iud s l q ~ of t k railers was rile sse a f t e r both ooe how: a d ane day at 730 C.

A n example of the mlcroetructure f o d a t 595 C and associared with slow r e t i a t kLncLLcs is s l m i n the e l e c t r o n micrograph ( f i g . 6). 'Iho density of dislocations i n tlle f e r r i t e w a particularly ldgh a dcarbides.

( b ) ~ t 570'~: grain-boundary dotri-rp118, needles am3 blocky f e r r i t e =re all olselwd (fig.2b).

The range of f e r r i t e morp1~Logies m rdlected i n an e q d l y diverse r a g e of p r d p i t a t e s . Figure 2a shows a case h e r e rcda and elagated plates precipitated in two 100 directlcns. I n otirer instawes elle rods were aligned a l q a single dfrecticn o r were present as an array of rcugt~ splrralds of raditts

MR.

Because of the s i z e and of the carbides, the analyses w r e certain! to cuztah s a w o x i t r i h ~ t i m from the f e r r i t e , but t h e observed Mo:C r a t i o of just less than 2:1 ws identical t o that a t higher temperatures.

(c)510-555 C: blocky 0 needles -re the doninant rroq>lmlogy (fig.3b). 'lltese c o n t a W s r a l L (1-10fi3), b r i g l 1 t l p i m g l ~ preclpitatea a c h appeared i n clusters t-acller t l m a t rardan i n tJw fcrrlte. nSe =re present i n very high concentratiaw a t the l m a r g l e b a u d a r l s kt- ferrLte d t s (Eig.3a). 111eLr dlstLnct e l e c t r o n d i f f r a c t i o n pattern indlooted tl& t l r y w r e preclpitatcs rat1:lrcr tlun cluskrx. rile average r a t i o of mlybdennn t o carbon i n these precipitates ws al>proxhmtely 2:l. Ihe dawity of single b r i g h t s p o t s i n the f e r r i t e irdicated that i t retained a substantial amunt of oolybchm arl Llds ws c o d i d by the analyses.

TP1 slmwed that d y

-

^of the f e r r i t e units containerl cementite wlrreas F C m s a o d p r s e n t . n~

cementite shape and distrihltim d t e r 8 mlmtes m t e d tW r e p l m a s k of C by FSC I d IX*

y e t become s i g n i f i c a n t . I h e i m d i c a t i m was that m s t of t l e KC observed 1 4 fon2d1u-LNI austenite decomposition: The s i n g l e cementite particle analysed o x i t a i d t l e bulk mlybbmm level

T%.76~0.24

~ p c ~ . 9 8 ~ 0 . 0 ? . 3 ) 1 ~

(d)Below 500 C: packets of f i n e , characteristically b a l d t i c wdles fonnrl. 0 llle a d y precipitate present i n these d e s vas cementite.

I n t h i s f l l l o y , t h g transformation was complete witllin tllc f i r s t ftw dmtm a t a l l teqeratures between 510 C and 725 C. t$C p r e c i p i t a t i o n was observed thraghoat the rar\ge lut f e r d t e 1 ~ 1 y L d 3 m u 1-1s bere generally close t o the W c level (fig.8b). The AP analysts are s l m ~ i n table 3.

0 0 0

(a)*620 C: Sphcmldal t$.C f o m d a t 620 C a d 725 C. (he of the specit- transfonued for 103 s x o d s a t 620 C cartalned a highlydemrated I ~ Q h - ~ l c f e r d t e t f e r r i t e bouodary. llle docorarim \mi restricted t o the intedece i n the form of a sub-mmolayer covering rather t l m an array of Interface precipitates.

Evaporation resulted i n t h e i n t e r f a c e mving a s a smooth curve towards t l r tip apex with the me e z e p t i m s l n m in figure Sa, whem i t was kinked taranls a Y C precipitate. Analysis w a w r i e d out

when t h e boundary was c l o s e t o the apex and the zone of h t e r s e c t i m of the interface with the t i p surface was wider than the probe hole. I n t h i s way the boundary contribution was mdmised. 'lhe mlybdenrn and carbon 1 4=re 15-20at% and 10-15arX respectively. Zhe ccnpoaitim profile (figme 5b) shlws that 5-lOaa of 8ulpiur and sane phosphorus were a@ present. Identical decoration WE observed m two occasicns in t h i s alloy aFter tramformation a t 525 C. Ccnsideraticn of the awtedte grain s i z e and t h e mmber of specimens dried w s t e d that i t was d k e l y that a U of tkse were prlor austenite grain Lumdari83.

(b)595'~: TIN s t w d

-

M2C precipitation i n dl f e r r i t e grains f n m

the

e m l i e s t steges.

(c)52?j0c: The low-angle f e r r i t e / f e r r i t e boundaries =re uuch less hmvily decorated thm thase formed i n t h e 2.05wt%Mo a l l o y under tlle sam caditions. Zhe diffe- ^WE M yw r t h w the relative d i f f e r a r e Cn bulk mlyWeram content.

m

x-ray d p s i s of tran9formtial i n t e r f ~ ~ e s

Analyses obtained from transformagion i n t e r f a c e s in the 0.52wtXMD end 2.0- at2ays aEter 15 seconds and 23 hrxlrs respectively a t 595 C are s l u m with the associated SIPI higea of precipitatefree i n t e r f a c a (ffg. 7). W y s i s of th sl-t g r a d q interface seetiara in the 0.5- d a y , which are t h e l i k e l i e s t s i t e s of segregation, is extreoely difficult due t o their highly c-luted nature. 'Ihe smooth interface s l m i n the ,SIlPl ndcrcgraph is s d t a b l e for analysis, but m y nct be represmtative of a l l interEaces. Molybdenm a d phospllorus enrichnent is observed in the 2.- allqr but nct in the 0.52wtDb alloy.

The a n a l y s i s shown i n t a b l e 1 was obtained w h e r e ~ 2 0 atoms were detected per 1x of material evaporated. Additional analyses wre oMraIned over a range of &table q l e and step sizes and in no instance vas tllere any evidare of clustering. 'Ihe analysis of the betwsm irrllvidual w1yMenm atoms shows no clustering m on atunic scale. 'Ihe s w i t i v f t y of the spadrrg analysis is reduoed ly the 5CB: inst-t detection efficiency and the 2-3nn diameter area f n m where auressively-Qtected icns my o r i g i n a t e . Consequently t h e AP a n a l y s i s does nct totally disc& the possibility that sam 2-3 atom clusters fom, altkmgh i t makes i t very fnprobable.

Because carbon d i f f u s e s when mrterrsite forms on q,- the observed distributicn will nct be the same a s i n t h e a u s t e n i t e . Therefore the observed absence of a carbm/mQ%&nm aasodatfm is of q u e s t i d l e s i g n i f i c a m .

Precipitation end Segregation

The m s t obvious result of the imrestigoticn i s t h a t there i s saue C predpitaticn i n both alloys throughout t h e intermediate transformation temperature range (i.e%st a hand k d c u the by).

Dilatometry has a l s o sllown t h a t MZC p r e c i p i t a t i o n j u s t below t h e deepest p a r t of thelTl!bey is associated with a decomposition process that differs distinctly f r a a the w e dU t e umphology observed a t lmr tenperaturn.

FIM/M analyses shows t h a t tlle rate of reactim a d the mcmt of t$C predpitaticn are inversely r e l a t e d . That t h e cxtent of p r e c i p i t a t i o n i s not s h p l y related t o the d i f f e r a n e s in c e t i m is conEinned by tables 2&3 a c h glve the mlyb&mmI c m e n t of f e r r i t e end s b m that precipitate &mity depends on more than t h e bulk molyLxhm level. It i s the effects which came precipitation t o teccme extensive t h a t a r e t h e centre of debate, t h e p r i d p a l . questicn bekg "Is prectpitatim a cause o r effect of the d q e s i n i n t e r f e velodty a d transformation rate?".

Solute-drag tlieory predicts t h a t mlybdenm is segregated a t the interface, which then

-

^{a t a}

r a t e a t which t h i s enhanced molybdenum l e v e l can be raintained. Prectpitatim a t these slcu mwlrrg boundaries i s tllils a secondary process. It i s almost certain that interf- that are so intemely decorated nuve rrore slowly than tlwse tlmt are solute-free. Zhe relatimship tetwsm p d p i t a t e h i t y and reaction r a t e is m r e ccnsistent, althougti the precipitate density observed a t a f d t e / a u s t e n i t e i n t e r f a c e seems t o be too low t o N l y a m m t for i t s haw velocity and

m

X-ray analysis slum the p r e c i p i t a t e - f r e e regions i n the 2.05wt.% Pb alloy a t 595 C t o be highly auiched in mlybcknm. At the some t e m p e r a t u r e i n t h e 0.52wtX Mo a l l o y , the f e r r i t e / a u s t e n i t e i n t e r f a c e i s not obvicxlsly mlybdemwemiched.

It seems l i k e l y t h a t a c m b i n a t i o n of t h e tw processes aocamts for the o b d behavlau. Zhe r e l a t i v e importance of these & q e 6 with canposition, precipitation irrreaaf&y daninant at higher molylxknnu and c a r h levels. Because of the a t t r a c t i m of w1yWenm f o r carbcn a d because of a

C6-45 0 JOURNAL DE PHYSIQUE

s i z e e f f e c t , a higher wlybdenum l e v e l is expected at the i n t e r f e i n all of the alloys, h r t tk e v i d e n c e s u g g e s t s t h a t transformation i s strongly inhibited c d y &ere there is s i g n i f i c a n t precipitation. The pinn&tg e f f e c t s of C a r e suggested by the high density of dislocatians in the f e r r i t e a t 1 9a d ~525 ~C in ~tkZ.OS*rdl: dlq

9

the interface kink d a t e d wffh c e of the fat p r e c i p i t a t e s formed i n the 0.52wtXMo a l l o y a t 620 C. A t higher taperatures, *re the energetically expensive s t e r a t i o n of l a t t i c e Qfects is not possible, different p r m s e s may M b i t tnursfomticn.

F e r r i t e folmafipl my require ledges that are high emugh to overgrow interphase 5 C or, &re 5 C fonus a t the growth f r m t i n a eutectoid -r, tnursformtian proceed at a rate gwemed bj intedaoe wlyMenm diffusial.

axmmms

(1) No evidence f o r Mo o r Mo/C c l u s t e r i n g hm been ohenred in amtenfte in FeC* alloys u d e r caditione t h m t r a m f o w a t h is stralgly irhibited.

(2) Some 3 C formation a c c a q a d e s trarrrfolmaticn both above a d below the bay in both alloys. The extent of this precipitation is not sinply related t o the the hulk of velslybderun a d carcarbon.

(3) The

'

'molybdenum effect" is probably a combination of the e f f e c t s of segregation and precipitation. The r e l a t i v e import- of these can charge ctring the aurse of transformtian as the a u s t e n l t e becomes progressively carbon-enriched. Significant effects are associated with extemive irmrporaticm af mlybdenm into 3 C .

-

^{(1) AW,Atlas d} Iaothermnl wd

Cmlim

Trwsformation M w m s . (2) 'Ihelning,K-E,Steel & its Heat l h s t n e n t s . h t t e n w t h s , 2nd ed.

(3) 8-xwLl e t al, M. Preps. and Ph&e Trws. i n

a.

Mat., &t.Soc. 3Lpe,(1986) ,p.445.

(4) Sharma,R, z drndy,G.R., Met-Trws., 4, (1974), p.939.

(5) Ehkshia,H.K.D.H., J.Wt.Sci..

3

(1983), p.1473.

(6) Aarmsc*1 e t al, ihid,

3

(1985), p.4232.

(7) Bumto,M., and Aammm,H.I. @t.Trans. (1986), p.1385.

(8) Purdy,G.R., Acta Met.

,a

(1978), p.477 (9) Purdy,G.R., ibid, p.487.

(lO)Ceteeo,A. et al, J.de PIysw6,artppl. to 3(1984),329.

(11)CTabkqH.J. e t al, Surf.& Int.Analysts,~p.2(12,(1987)

~abl. 2' 8 Atom-probe analyees of re-0.33wtXC-2.OSutXHo

o~ Time at T Ferrite Carblde

( C) (minutes) atXC

*

^{o atXMotC atXC f a - atXM0tG}

730 6 0 0 0.05 0.03 35 1.0 56 1.1

725 1200 0 0.20 0.08 33 1.6 63 2.2

670 420 0 0.07 0.05 40 2.5 6 0 3.1

620' 1680 0.01 0.01 0.13 0.04 33 1.7 65 2.4 25 1.4 5.7 0.7

590 6600 0 0.13 0.04 43 8 57 9

;;::

0.13 0.06 0.70 0.13 44 2.7 56 3.0 0.08 0.03 0.33 0.05 381.5 6 2 2 . 0 550 1140 0 0.8 0.16 3 4 3 . 4 66 4.6 540 34 0.06 0.04 1.22 0.15 32 10 68 15 533 16 0.06 0.02 0.91 0.05 34 6 66 8

526' 8 0 1.02 0.06 25 6 75 9

24 2.0 1.0 0.4 470 4 0.09 0.03 1.18 0.09 31 4.3 0.6 0.6 430 0.5 0.10 0.02 1.30 0.06 25 1.2 1.2 0.3 Table 3 I r Atorprobe analyses of ~e-0.2OwtXC-0.52wtXMo

8

^{Time at T Ferrite Carbide}

( C) (minutes) atXC f G atXMofG etXC t G atXMotG 725 16 0.09. 0.02 0.20 0.03 33 15 44 11

625 1.7 0.13 0;02 0.31 0.03

-- --

6001 3 0 0.07 0.02 0.38 0.05 29 7.5 71 12

526 8 0 0.19 0.04 22 1.2 1.1 0.3

43 11 57 14 513 1.1 0.04 0.03 0.36 0.07 23 2.3 1.2 0.5

I

-

In the mjority of caaes tlie analyses given summarlse those obtalned tr- aeveral apecimena.

* -

The carbides ere too m a l l for the Fe content to be eccuretely determined.

1

-

lko dlstinct forms of ti C have been enalysed.In one I4 is principally Mo.

whilst i n thn other i t fsbmainly Fe.

Pigs. 1-3 ELM observation of the cllanges in &C precipitation and overall ferrite 526 C in a Fe-0.32C-2.05Ho 0 w t X alloy.

B _i

I *

* .

^r

5-

~ i g . 2a 5 7 0 " ~ , 95 hours Fig. 2b 570'~. 23 hours

Fig. 3a 526 0 C, 8 minutes Fig. 3b 540 0 C, 16 minutes

70@l

^{10% *}

I

_./ Figure 4

... ----.--'

~7

^z: Dilatometrically determined TTT-

\ curve for Fe-0.33wtXC-1.96wtXMo.

The 10% transformation curve is shown.

I

C6-452 JOURNAL DE PHYSIQUE

F i g u r e 5a F i g u r e 5b

Fig. 5 I n t e r f e r e n c e of a 3 C p r e c i p i t a t e w i t h a h e a v i l y d e c o r a t e d f e r r t e / f e r r i t e

d

boundary and a n a l y s i s a c r o s s t h i s boundary. Fe-0.20wtXC-0.52wtXM0, 620 C, 100s.

F i g u r e 6

Micrograph of Fe-0.32wt;bC-2.05wtMo showing a transformatdon i n t e r f a c e a f t e r 23 hours a t 595 C. Note t h e h i g h d i s l o c a t i o n d e n s i t y i n f e r r i t e , p a r t i c u l a r l y around M2C.

0.4pm

Fig. 7 STEM X-ray a n a l y s e s of f e r r i t e / a u s t e n i t e i n t e r f a c e s . Fig. 7a Fe-0.32wtXC-2.OawtXMo

2 3 hours 595 C.

f e r r i t e

P o s i t i o n wt XMo

i n t e r f a c e (1) 4.0

"

( 2 ) 3.9

" ( 3 ) 1.8

lOnm i n t o f e r r i t e ( 4 ) 1 .O, 50nm i n t o a u s t e n i t e ( 5 ) 1.3

wtXP P o s i t i o n

1.1 f e r r i t e

0.7 ^{t I}

0.6 t t

0.6, a u s t e n i t e

0.5 ^{I t}

I t 1 1

i n t e r £ a c e

I I I I I t

1

.o

0.4 N o P i n

0.3

0.2 any of t h e 0.2

0.6, a n a l y s e s . 0.3,

0.5, 0.5

*

⁼ Less t h a n 2 s t a n d a r d d e v i a t i o n s .