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GRAIN BOUNDARY NON-EQUILIBRIUM SEGREGATION IN STEELS
R. Faulkner
To cite this version:
R. Faulkner. GRAIN BOUNDARY NON-EQUILIBRIUM SEGREGATION IN STEELS. Journal de
Physique Colloques, 1990, 51 (C1), pp.C1-133-C1-138. �10.1051/jphyscol:1990119�. �jpa-00230277�
COLLOQUE DE PHYSIQUE
Colloque Cl, supplement au n o l , Tome 51, janvier 1990
GRAIN BOUNDARY NON-EQUILIBRIUM SEGREGATION IN STEELS
R . G . FAULKNER
Institute of Polymer Technology and Materials Engineering.
Loughborough, University of Technology, Loughborough, Leics LE11 3TU, Great-Britain
A model quantifying the kinetics and magnitude of non-equilibrium and equilibrium grain houndary segregation in n~etallic solids is described. The major input parameters for the model are: vacancylimpurity binding energies; diffusion data for impurities and complexed impurity- vacancy pairs; vacancy formation energies; and grain size. The principal thermal treatment parameters controlling non-equilibrium segregation are shown to be solution treatment femoerature and the coolina rate emnloved b reduce the material tr, room temaerature. The values of these parameters'ieading to km-mum segregation differ greatly depending upon which impurity element in the matrix is being considered. Examples of the extent of silicon and phosphorus qrain houndary segregation expected in steels as a function of austenitising iernpkrature,-cooling rate and subsequent tempering will b e shown. Experimental measurements of qrain boundary secireaation of phosphorus in a microalloyed
steel will be described. These hea;ur&nents a& n?ide with the high spatial resolution microana.hrticat method known as iield emission aun scannins transmission electron microscopy
(FEGSTEM).
Supplementary data. concerning the mechanic4 properties of lerritic!martensitic - steels wilt be aresentpd. These data. ~ r o v i d e indirect evidence of arain boundary seoreaation influencing mechanical properties such as impact fractsre energy. ~ecomrnend&ion~ are. made about how to alleviste non-equilibrium and equilibrium grain boundary segregation in steelaSegreqatinn to interphase and grain boundaries in metallic solids has been acknowledged for some time. McLean {l) was the first to quantify equilibrium segrega?ion. In remnt years non-equilibrium segregation has been recognised as important afler the early work of Aust et al (2) afld Anthony (3).
Attempts have been made to unify the con-equiiihrjum and equi!ibrium approaches ((4).
Considerable work has been aimed at qrrsntifying impurity segregation in steels. e.g., B (5) and P (6). Specific measurement o! the finely segregated layers on boundaries has rzcently been made possible with the FEGSTEM anaiysis technique (7). f$;s paper demonstrates the prediction of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990119
COLLOQUE DE PHYSIQUE
segregation of Si and P in several steels a.nd confirms the modeling directly with FEGSTEM and indirec!ly wilh mechanical property measurensents.
2.
- MODEL
DETAlLSa) Equilibrium Segregation (€S]
McLean's model is used ( l ). This assumes that for an impuri?y with binding energy to the l a k e . Eh, at any temperature T. there wiil be an increased concentration of that impuriiy in a monolayer on boundaries and interfaces. cb. The driving force for this me&.anism is the reduction of energy.
Eb.of the impurity a!om on placing i! in the shain-free environmen! e!!be disordered boundary.
McLean refined these ideas to e.ccount for time. correctly realising that a finite time is required to accumulate the mofiolayer and thst this time is mntrolied by the diifiisiviv of the impurity in the matrix, Di. In the unified treatment of ref. (4). ES in the quench a.nd during subsequent ageing are treated separately and summed to give the final ES segregafion parameter.
h) Non-equilibrium Segregation (NES]
The methods described in ( 4 ) are used to calculate segregation occurring by the NES mechanism.
The mechanism is based on the creation of vacancy concen?ration gradients in tho neighbrrurhood of vacancy sinks during cooiing from a high iemperajure, Ti. ( 2.3 j. Vacancies dragged down ihis gradient W-ll also pull impurity atoms with them if there is a net attractive interaction. Thus impurities with a strong binding with vacancies will accumulate in considerable quantities in the sink regions of the microstrudure. i.e., interfaces.
The magnitude of the process is time-dependent. An effective %me,
t
is calculated iron1 the quench rate,6 .
and the starting tempereture, Ti. To allow for ageing a.fter cooling. an additional time. tA, can be added to t. Equations defining the varis.bles so far mentioned can be found in ref ( 4 ).HES produces an accumulation of impurity over distances of iip to several microns. The impuritf atoms in this enriched zone are not in their relsxod state as are :te &toms segregated hy the ES mechanism and so they will tend to diffuse back down itreir own concentration gradients. This desegregation process is imagined to begin &er a critics! time. t,, which deprnds on the grain size, d. and the relative diffusion rates of the impurity 8!oms: Di= and the vaca.ncy-impurily complexes. D,.
c] Combined ES 2nd NES
The actual concentrations resulting from ES and NES are shown in Figs. l and 2. For convenience, We profiles are integrated between O and 103 nm to give an accurate p i d ~ r e o: the overall segregation. This amount is represented in the niodel as a level concentration in a zone 1 nm on one side of the inteeace. Sin=
c,
(ES) is betiewed to be a m~nofayzr segrega:ion esending 0.25 nm into the grain edjssent to B e grain boundary. !!ie cnnren:ra!ion lstrel cnr~eopnndIng to Wls !ram f i e equivaient segreiptior. in a 1 nm wide zone adjacent i o fhe bounday is c,iESlil (see ideaiised cunre in Fig. 1 ). In tbe case ofc,
(NES), the model producer values crf 5 (~IESI at 10 nm intewais. i tIS assumed tha: segregated zones wil: commorrly ex?~nd ou! to ?E8 rim. Therefore !0 values of
c,
HE^^ are summed in the modei to give an e!fec?ivt: integration of the NES concentration profile- The average concentration at these ten points is multipired hy 10 ?a yiefd the effective concentrafion of Ihe segtsgant should i! alf be located in the I nm wide zcrie on either side D! !he interface. (see idealised curve in Fig. 2)
The total seqregatian concefiBation, less the cnncefitration coti:ribiGian from the matrix* cg, is onten the r y z b o l F in graphs stiw#n in Figs. 3.45.7 and 9.
3. - APPLICATIONS OF THE MODEL
a) Segregation of B. P, Si. t:b, Mo arid V in a FerriticiNariensiiic See1
Fig. 3 shows an example o l the isometric plots produced by the model which show how F varies with staHing tomperdure. Ti. and moling r ~ t e . The case chosen is for F in a fe~fitls!ma.rier,sitic steel. 1.4214. whose con~position is @sen in TaMe 1. S i m i l ~ r plots czn he produced far E, Si, Hb, MO a.nd V. li is cleariy seen that segregation increases as Ti increases and wtien c~0:ing rate is eq~ivafont to anegedive t h e !,. No ageing is assumed in this Figwe.
The ageing ~ f i e c t is to increase the ES wntribiition to a maxirritiiii afier increasingly shofier tixes 82 higher ageing temperatures. This is shown in Fig. 4. The material has been heal treated at a solution treatmerit tempeiature,Cr,] of 108@"C fnr l i 2 hour, fnliowed by air coo:irig at 6.5O S-! and then ten~pering at 700 or 750°C. The NES contr'ibution during ageing is also shown in F i g 5. For this application, 1, has been exceeded dirring the slow air cooling and desegrega.!ion is in prngrzss throughout the 8-geing treatment. For standardisation purposes, the n~s?rix level of P in the alloy is induded in both Figures. Table 2 indicates the critical cooling rates necessary :or maximum segregation of all the elements mentioned, together with the effect of cooiing rate and ageing (tempering) on the amount of segrega:ion.
b) Si Segregation and Fracture Resistance of Ferritic;Martensitic Steel , 1.4914
Inrpacifracture studies of 1.4914 in various heat trested condi::lcns indicate a dear change in energy absorbed a! fracture as a function of cooling rate in material tempered a? EOOOG. (Fig. 6
1.
This effect is less obvious 8.Rer longer tempering times and at higher tempering temperatures.
From agoing cuwes siml!nr to those of Figs. 4 and 5. the element w3ich appPais mas! significantly affected Dy changes in the cooling rate in the 6.5 to 220" S-l ra-nge during tempering d fiOO°C is Si.
(see Fig. 7 ). This shows that Si segregation pea.ks after about 1.6 hours at EOOnC in material cooled at 500 S-1- After the ssme ageing time in the faster or slower cooled materials. the predicted Si segregation is lower-
c) P Segregation in a CMn f.ficroal!oyed Steel
P o d weld beat treatment of the stee: whose camposition is listed in Table 1 rzsii::~ in s trough in ducti!ity with time as shown in Fig. 8. The exact position of %is trough depends on the cooling rate emplnyed from the welding temperatore. Using the model together with dais described in ref. j 6 ), fke predictetf P grain bouadzry segregstion in the steel as z?unc!ion of ageing t h e SO8 OG is shown in Fig. 9 . Tine predidions of the model awe confirmed experimentafly by analytical election microscopy (H3501 FEGSTEMJ of lath boundaries in tough and brittle samples. Fig. 10 . Samp!e 81 clea.rly shows an accunwlaiion of P within 103 nm o i the bounuary. Other elements tried with the nlodel do not segregsi.e a! the appropriate rate to predict segregation peaks S? the titnes
corresponding to the trouphs in ductility.
The behaviour of the alloying eiernents in 1.4914 indicated by Figs. 3 and 7 and in Table 2 fof Si and P suggests that atom migration by nos-equilibrium segreqatiori rilechanis~,~ could occur for most of the efemen!s considered during t;*pical hardening and tempering operations for this steel. Typicat quertch rales for water, oit and air are 220, 50 and 6.5 @ S-l respec!ive!y. So all of the elen?en!s except 8. bat especially P and Si. wil: segrega-:E durikcj cooling from Ette anilealing temperature by any of these cooling procedures. 8uSseqtient tempering at 70rt"G is shown in Table 2 and Fig. 5 io
,
remove niost non-equiiibriux seg~egation effects aiter one hour. However, if tempering is ~1 a iower
Cl-136 COLLOQUE DE PHYSIQUE
temperature. then several days may be required to desegregate the atoms which ha.ire migrated to the boundaries. Tbis i s illustrated for Si i n Fig. 7
.
where the tempering temperature is 6000C.The outcome of the fracture analysis is that for fast. impact fracture. segregation can be of high importance- Fig. 6 shows t h ~ t Si is beneficial in ferriticlmartensiiic steels. On the other hand.
Figs. 8 and 9 show that P is hamiful to impact fracture resistance in plain cs.rbon steels. This would suggest from the McMahon (9) ana!ysis that the surface energy of steel increases in the presence of Si but is reduced when P segregation occurs.
From the a b w e discussion. several guidelines can be established for heat treatment of sfeels to minimise lath and grain boundary segregation effects.
1. Normalising. austenitising and annealing treatments should be I..ept to as fow a tentperature as possible.
2. Tempering treatments should be at as high a temperature and for as long a t i n e as possible.
3. The fine grain lath structure resulting from fast quenches and low temperature len~pering treatments is advantageous for reducing segregation.
4. Si and P are particularly sensitive to cooling rate variations in steels cooled 81 conventional heat treatment rates.
5
-
CONCLUSIONSA new unified equilibri~m and non-equilibrium segregation model for interfacial segregation i n metals as a function olthermal treatment is described. Hs application to pradically obsewed problems associated with lath and grain boundary segregation i n ferriticlmartensiiic and pla.in carbon steels is discussed. During normal commercial heat treatment of steels. Si and P are the main eiements likely to be undergoing maximum segregation. The advent of more sophisticated analytical eledron microsmpy (FEGSTEM) has enabled confirmation of the nlodel predictions and the case of P i n plain carbon steel is described. It is shown how the segregation model is capable of predicting how steel heat treatnient schedules can be modified to alleiliate harmful segregation effects.
6
-
REFERENCES1. D. Mdean. 'Grain Boundaries in Metals'. W o r d University Press. London, 1957. p.131.
2. KT. Autt S.J. Armijo. E.F. Roch. and J.A. Westbrooke. Trans. ASM..& (1967). 360 3. T.R. Antkony. A d a Met. 11, (1969). 603
4- RG. Faulkner, A d a Met.
B
(1987). 2905 5. L. Karlsson and H. Worden, Ads.. Met..= (1998).13 6. M.P. Seah. Actrr. Met., (l 977). 3457. D.B. Williams and A.D. Rornig. Ultra.microscopy. LJ3 (1989).38
8. R.G. Faulkner-To be published in Materials Science and Techfiology. 1989.
9. C.J. McMabon and V-Vitek. Acla. Met.. 22, (197g). 507
Table 1. Allov
-
com~ositibn.
( ~ 1 % ) . ,Steel Si Mn Cr Ni
M
MO S V P Nb B N C Fe1.4914 0.45 0.35 11.3 0.7
-
0.5-
0.3-
0.25 70 ppm 0.029 0.1 1 Bal.Tabtel. Segregation intensitits, EF, as a function of cooling rate, 0. and ageing time in 1.4914 cooled from 1080°C
ST. 1080'C Pcak
J3erncni 9 , ( S ) EF 0 (S-') J @) EF
Cl-138 COLLOQUE DE PHYSIQUE
Fig.
4
EfTecI o r ageing on P equilibrium segregation in 1.4914.l l
;;::l
lD Hotrlx 1.1.1
0.01
0 0.005 0010 0015 0.020 0.025 0.0SO ''0 50 1CD 150 200 2X
Time l h1 Coollng rote (*C C')
F14 5
E R ~ C I o r ageing on FFig
6
Franurc energy in 1.4914 as a function of coolifnoncqu.ilibrium vgrcgation in 1.4914. rate.
Fig.
7
Effect of ageing a1 600°C on Si segregation with various cooling rates. 1080'C wlution treatment.Effecl of Ageing Time a t 60@C o n P Segregation t o Lath Boundaries for different Cooling R a t e s
I CD S - 4-9
-
4.7
-
0 2 4 6
Xme (h)
---
uinh0; .
D lDDD .-
S l G (A) ''WO
