Surface and Grain Boundary Segregation on and in Iron and Steels — Effects on Steel properties
Segregacije na površini in na mejah zrn železa in jekla — vplivi na lastnosti jekel
H.J. Grabke, Max-Planck-lnstitut fur Eisenforschung GmbH, Dusseldorf, Deutschland
The surface and grain boundary segregation was studied for binary alloys Fe-A using Auger-Electron Spectroscopy, Low Energy Electron Diffraction and Photoelectron-Spectroscopy. As examples the sudace segregation of C, Si, Sn, O, and S on iron and the grain boundary segregation of P and Sn are deseribed. The segregation studies are correlated to metallurgical phenomena■ the effect ofsulfur on carburization and nitrogenation, the effects of phosphorus at grain boundaries on embrittlement of a turbine steel and stress corrosion cracking of carbon steels and the effect of tin on the creep of a turbine steel.
Key words: sudace segregation, grain boundary segregation, segregation thermodynamics, segregation struetures, sudace reactions, sudace diffusion, intergranular fracture, embrittlement of steels, intergranular corrosion, Langmiur-McLean creep of heat resistant steels
Segregacije na površini in na mejah zrn binarnih zlitin Fe-A so bile raziskane z metodami AES (spektroskopija Augerjevih elektronov), LEED (nizkoenergijski elektronski uklon) in XPS (rentgenska fotoeiektronska spektroskopija). Opisani so primeri segregacij C, Si, Sn, O in S na površini železa in P ter Sn na mejah zrn. Raziskave segregacij so povezane z metalurškimi procesi in pojavi: vpliv žvepla na naogljičenje in nitriranje, vpliv fosforja na mejah zrn na krhkost jekel za turbine in na napetostne korozijske razpoke ogljikovih jekel ter vpliv kositra na lezenje jekel za turbine.
Ključne besede: segregacije na površini, segregacije na mejah zrn, termodinamika segregacije, struktura segregacije, reakcije na površini, difuzija na površini, interkristalni prelom, krhkost jekel, lezenje pri visokotemperaturno odpornih jeklih
1 Introduction
In the metallurgy of iron and steels many phenomena and processes, such as carburization/decarburization, nitrogena-tion/denitrogenation, corrosion, surface diffusion, sintering, recrystallization. adhesion, frietion, wear, etc. are deter-mined decisively by the atomic composition of the surface. Also the atomic composition of the grain boundaries is very important in affecting the mechanical properties and the corrosion behavior of steels. These interfaces will normally be covered vvith impurity atoms from the gas phase or segregation of dissolved atoms from the bulk, and also the grain boundaries by segregation from the bulk.
The equilibrium segregation on iron surfaces and at grain boundaries
A(dissolved) = A(segregated)	(1)
vvhere A = C, Si, Sn, N, P, O, S... has been investigated for binary systems Fe-A by surface analytical methods. Con-centrations of the extraneous elements on the surface vvere determined by Auger-Electron-Spectroscopy (AES) in de-pendence on the bulk concentration rA and on the temperature of equilibration, and ordered struetures on surfaces vvere determined by LEED1-1 i.e. Lovv Energy Electron Diffraction. The binding modes vvere characterized by X-ray photoelectron spectroscopy (XPS)s~'4.
Impurities in steels can have strong effects on the mechanical properties and on the corrosion behavior, especially if they tend to enrich at the grain boundaries. Sn, P, As, Sb, S, Se, and Te segregate at grain boundaries and embrittle steels. The study of this segregation, vvhich is restrieted to a fevv monolayers in the interface, vvas possible since the development of AES. By fracture of spccimens inside an UHV chamber and by AES-analysis of the intergranular fracture faces, grain boundary concentrations of the embrit-tling elements could be determined. In our studies, grain boundary segregation of C, N, and B vvas also deteeted. These elements are not embrittling and, therefore, their de-teetion is possible only if an embrittling element such as S or P is present vvhich initiates intergranular fracture. In our AES-investigations on different steels mainly the impu-rity elements P and Sn vvere deteeted at grain boundaries. Obviously, other impurities such as As and Sb are present only in too small bulk concentrations, and sulfur is being scavenged by Mn so that the studies have been focused on the grain boundary segregation of P and Sn in iron, and on the effects of P and Sn on materials properties'5-30.
The equilibria vvere studied of grain boundary segregation of P and Sn in binary Fe P and Fe-Sn, temary, and quatcmary alloys, by equilibrating speeimens at different temperatures for sufficient time, aftervvards analyzing intergranular fracture faces by AES.
The thermodynantics of segregation as well for surface segregation and for grain boundary segregation are deseribed by the Langmuir-McLean equation:
6,/(l - ©i) = exp(—AG° / RT)
(2)
vvhere
the occupancy of the grain boundary sites with the segregating element i the mole fraetion in the bulk.
The free energy of segregation is
AG° = AH" - TASf
where
(3)
A H?:
A Sf:
the enthalpy of segregation the excess entropy of segregation.
According to thermodynamics the segregation inereases vvith decreasing temperature and vvith inereasing bulk con-centration.
Ali investigations of the surface segregation have been performed in the temperature and concentration range of the q- or 7-solid solution, in order to avoid the formation of three-dimensional compounds. Also the equilibria of grain boundary segregation have been established at elevated temperature vvithin this range of the phase diagram and vvere measured on rapidly quenched specimens.
In the follovving some examples are presented of studies on the surface segregation of C, Si, Sn, O and S vvhich demonstrate the principles of segregation thermodynamics and struetures. Studies on grain boundary segregation are deseribed of P and Sn. The results of the segregation studies are correlated to metallurgical phenomena, sueh as the effect of S on surface reactions, the effect of Sn on creep of heat resistant steels and the effects of P by grain boundary embrittlement and in corrosion.
2 Surface Segregation
2.1 Carbon
The surface segregation of carbon has been investigated in the temperature range 400-800°C on single crystals vvith carbon concentrations betvveen 10 to 100 ppm C1"3. On Fe(100) a c(2 x 2) strueture (Fig. 1) vvith 50 at% C is approached for high concentrations and lovv temperatures, this is the saturation coverage (0 = 1). The degree of coverage 6 decreases vvith inereasing temperature and decreasing bulk concentration, as expected from thermody-namics, see Fig. 2. These dependencies are deseribed by the Langmuir-McLean equation (2). This equation can be revvritten according to
ln
0i
1 - 0,
-Ali, RT
A Sf + —jjr- + In-r,-
(4)
For the čase of carbon on Fe(100) the segregation en-thalpy is - 85kJ/mol, as deri ved from the plot according to Eq. (4) shovvn in Fig. 3. Thus the segregation of carbon is strongly exothermic, vvhich is caused by the elastic en-ergy vvhich is sel free vvhen the carbon atom can leave the too narrovv interstitial sites in the lattice and pops up to the surface1.
a)
b)
Figure 1. Model of the c(2 X 2) adsorption strueture of carbon on the Fe(100) surface. derived from LEED studies a) top view b) cross seetion in (llOl direetion Slika 1. Model c(2 > 2) adsorpcijske strukture ogljika na površini Fe(l(X)). dobljen s pomočjo LEED raziskav a) pogled z vrha b) presek v smeri (110)
For carbon segregation on other orientations of iron, the results are not so simple and clear .
The binding mode of carbon on Fe( 100) has been char-acterized by photoelectron spectroscopy (XPS)l:. Spectra from single crystal surfaces vvith segregated C have been taken and have been compared vvith spectra of graphite and cementite (Fe, Cr)iC. (In order to obtain a thermodynami-cally stable cementite a Fe-2%Cr alloy had been carburized in CH4 - Hj). The photolines of the C ls photoelectrons of carbon are shovvn in Fig. 4.
The sample vvith 20 ppm C only shovvs the peak for segregated carbon, on the sample vvith 40 ppm C besides segregated carbon there is graphite deposited by oversatura-tion (at 600°C). The energies of the C ls electron levels are distinctly different from the energy level of C ls in graphite and in the carbide. The segregated carbon is a special state of carbon. The shift of energy, about —2.0 eV in compari-son to graphite, indicates that a certain electron transfer has taken plače from iron to carbon. The bond Fe-C is similar as in cementite, prevailing homopolar, but it is somevvhat stronger polarized than in the carbide.
2.2 Silicon
The surface segregation of Si has been studied on Fe-3%Si single crystals in the temperature range 450-900°C6. The dependence of surface concentration on temperature has been measured for the equilihrium of silicon segregation
0.9 10 1.1 12 13
103/I
Figure 3. Plot of the data in Fig. 2 on equilibrium segregation of earbon on Fe(100) according to the Langmuir-McLean equation Slika 3. Grafični prikaz podatkov s slike 2. ravnotežne koncentracije ogljika na površini Fe(lOO) v skladu z enačbo Langmuir-McLean
binding energy (eV)
Figure 4. Investigations of the binding of carbon by photoelectron spectroscopy (XPS) (1) segregated carbon and graphite on Fe(lOO) (2) segregated carbon on Fe(lOO) (3) graphite (4) carbon in cementite (Fe, Cr),C
Slika 4. Raziskave vezave ogljika z metodo XPS (1) segregirani ogljik in grafit na površini Fe(lOO) (2) segregirani ogljik na površini Fe(lOO) (3) grafit (4) ogljik in cement it (Fe, Cr)3C
on Fe(100). This dependence can be deseribed by the Langmuir-McLean equation (2). The result for the Gibbs' free energy of segregation is
A C" = -48000 + 157' (J/mol)	(5)
The value of the segregation enthalpy of silicon on iron A H!-- = —48 kJ/mol is much lower than for carbon. Thus carbon can displace silicon from the surface on Fe-3%Si samples vvhich contain small concentrations of carbon:
C(dissolved) + Si( segregated) =
= C(segregated) + Si(dissolved) (6)
This equilibrium of mutual displacement has been measured in dependence on temperature, see Fig. 5.
According to the higher value for Athe C-segregation prevails at lovver temperatures, for higher temperatures the carbon segregation decreases and silicon is able to segregate to the surface. This equilibrium vvas deseribed by equations vvhich consider the site competition of both segregating elements:
temperature , 'C
Figure 2. Equilibrium segregation of carbon on Fe(100), AES measurements of the surface concentration of carbon on samples vvith diflerent bulk concentration in dependence on temperature Slika 2. Ravnotežna segregacija ogljika na površini Fe(lOO), AES
meritve koncentracije ogljika na površini vzorcev z različno koncentracijo v masivnem materialu, v odvisnosti od temperature
cu CT>
a
CL> >
O (_)
CL>
cu C7)
cu -a
0.5
10
0Si(l - ©si - ©c) = ©c(l -©C-©Si) =
0.5-
»So
Afe
Sn
disordered
c (2-2) Sn
Figure 5. Surface segregation of silicon and carbon on Fe-3%Si(100) with 40 ppm carbon, mutual displacement of carbon and silicon in dependence on temperature Slika 5. Segregacija silicija in ogljika na površini Fe-3%Si< 100) z 40 ppm ogljika, vzajemna zamenjava ogljika in silicija v odvisnosti od temperature
15
ZO min
xSlexp(-AG°sjRT) (7) rcexp(-A G°C/RT) (8)
Here the thermodymic values for the Si-segregation stay unehanged, while the value for A//£ is inereased by the presence of Si.
2.3 Trn
The surface segregation of tin vvas investigated on (100) faces of Fe-4%Sn samples in the temperature range 600-800°C7. During segregation of tin at 600°C by diffusion there is a step in the increase of concentration vvith tinte, see Fig. 6a vvhich can be associated vvith the formation of the saturated c(2 x 2) strueture. Hovvever, the surface concentration continues to increase till a coverage of about 1.2 tin atoms per iron atoms is obtained. During this process the LEED-diffraction pattem vanishes, obviously a change of the surface strueture occurs from the ordered strueture to disorder. In this transition the binding mode of the segre-gated tin changes, as can be seen from the photolines. The binding energies of the electrons in the Sn 3d and Fe 2p levels shovv distinet changes in the transition. The d-levels of the segregated tin shift in the direetion to the values de-termined for pure tin. At the lovver surface concentration of tin the binding energies of the electrons are lovver, this may be interpreted similar as in the čase of carbon by an electron transfer from iron to the tin atoms in the ordered c(2 x 2) strueture. A tentative phase diagram is dravvn in Fig. 6b, vvith saturated surface struetures at © = 0.5 and 1.2 and heterogeneous regions in betvveen. A similar phase transition as on Fe(100) is observed also on Fe(lll), vvhile different complex ordered struetures are observed on Fe(110) vvith inereasing degree of coverage7. The tendency for surface segregation is very high for tin, trials vvith different small concentrations always led to high surface con-centrations © > 1, vvhich means that no thermodynamic data could be obtained but the segregation enthalpy of tin rnust be a rather high negative value.
Sn c (2-2)
Fe p(H) ♦ Sn disordered
Sn disordered ♦ c(2«2) Sn
-

Fe
3,5
b)
1,2
Satura t ion
'Sn
Figure 6. Surface segregation of tin on Fe-4%Sn(100) a) Kinetics of Sn segregation, measurement of the Auger peak ratio Asn/Apc after heating from room temperature to 600°C (in 6 min), after about 12 min c(2 x 2) strueture vvith 50 at% coverage as demonstrated by LEED diffraction pattem b) "Phase diagram" of the system Sn on Fe(100), stability ranges of the surface phases in dependence on temperature and degree of coverage Slika A. Segregacija kositra na površini Fe-4%Sn(100) a) kmetika segregacije kositra, meritve razmerja Augerjevih vrhov A sn/ Apo žarjenju od sobne temperature do 600°C (v 6 minutah), po približno 12 minutah c(2 x 2) strukture s 50% prekritjem, prikazano z LEED difrakcijskim modelom b) "Fazni diagram" sistema Sn na Fe(100), stabilna območja površinskih faz v odvisnosti od temperature in stopnje prekritja
2.4 Oxygen
The surface segregation of oxygen on iron cannot be investigated vvith the segregation method deseribed before, since the oxygen solubility in iron is very small. In spite of
that, segregation equilibria can be established, even at vvell-defined thermodynamic potential of the oxygen if a solid electrolyte celi is used: iron sample/oxygen-ion conducting solid electrolyte/reference electrode. Such celi, for exam-ple Fe/ThO: x Y2Oj/Cr - Cr203, can be inserted into a UHV system, and by controlling the celi voltage the chem-ical potential of oxygen can be fixed in and on the sample. Thus, the oxygcn segregation at a certain oxygen potential is established. With this set-up measurements are possible at oxygen potentials, which correspond to oxygen prcssures in the range 40—10—10 atm. Fig. 7 shovvs an exam-ple of a measurement of the oxygen segregation on an iron film, vapour deposited on the solid electrolyte. A tvvo-step isotherm is observed, obviously there are tvvo adsorption structures, at > 10"35 bar 02 vvith the ratio O/Fe = 0.25 and at > 10"25 bar vvith the ratio O/Fe = 1. The oxy-
Fp(100! - p (Z * Z) 0
Fe (100) - pd-UO
-35
bar
gen adsorption at very lovv oxygen prcssures < 10 02 can be related to oxygen adsorbed on steps, kinks, and other active sites of the surface. LEED and AES studies vvith iron single crystals5, vvhich had been sintered together vvith a mixture of Fe and the lovvest oxidc FeO, shovved a p(l x 1) structure at 800° C on Fe(100). This is in agree-ment vvith the degree of coverage O/Fe = 1 for oxygen pressures near the equilibrium Fe-FeO. Oxygen on Fe(100) is also embedded in central sites betvveen four iron atoms11, hovvever, in contrast to the other nonmetal atoms it reaches not only 50% coverage in a c(2 x 2) structure but 100% coverage in p(l x 1). The oxygen adsorption and oxidc nucleation can be depicted as shovvn in Fig. 8.
0 (od) on iron lilni, B00°L solid etectroly I o tet! Cr.Cr;0j I IhO,- V,0, / In
I /
log p0;(boi)
Figure 7. AES measurement of the surface segregation of oxygen 011
an iron film and of the oxidation to FeO. the oxygen potential is established by the solid electrolvte celi Cr. CrjOt/ThO; - Y:Of/Fe Slika 7. AES meritve segregaeije kisika na površini tanke plasti železa in oksidacije do FeO, kisikov potencial je bil določen s pomočjo trdne elektrolitske celice Cr, CnOi/ThOj - VjOj/Fe
2.5 Sulfur
Sulfur is extremely "surface active" on iron surfaces, even for very small bulk concentrations < 1 ppm in the stabil-ity range of the a-phase up to 900° C always saturation of the surface vvith sulfur vvas observed after equilibration2. The presence of sulfur on the iron surface strongly affects surface reaction kinetics in the čase of carburization and nitrogenation of iron2,4 and also the surface diffusion is influenced by adsorbed sulfur, the surface self-diffusivitv of iron is enhanced in the presence of adsorbed sulfur1'.
Figure 8. Model of oxygen adsorption and oxide nucleation on Fe(100) a) Adsorption structure p(2 x 2) b) Adsorption structure
p( 1 X 1) and FeO nucleation Slika 8. Model adsorpcije kisika in nukleacija oksida na površini Fe(100) a) Adsorpcijska struktura p(2 x 2) b) Adsorpcijska struktura p(l x 1) in nukleacija FeO
On Fe(100) the c(2 x 2) structure is obtained, up to high temperatures a very distinet LEED pattern of this structure can be observed. Fig. 9 shovvs this adsorption structure of segregated sulfur vvhich vvas proved by measurements and theoretical calculations of the LEED intensity energy curves. In this schematic the diameter of sulfur is assumed to be equal to the diameter of S2- ions. It is obvious that such dense coverage vvith sulfur ions vvill strongly retard any surface reactions, this is demonstrated in Fig. 10 for the carburization of iron in CH4 — H2 and the nitrogenation in N?. For these reactions the rate vvas determined in resistance relaxation measurements in flovving gas mixtures (at 1 bar), in many experintents, in vvhich the sulfur activity vvas given by the H2S — H2 ratio in the atmosphere4,32.
Determinations of the electron levels of segregated sulfur by photoelectron spectroscopy10 confirmed that these are very near to the electron levels of the doubly-ioniz.ed sulfur ion S2-. The binding energy of the S 2p and S 2s electrons is by some lOth of an eV higher than in the sulfides FeS and FeS2.
In steels the surface segregation of sulfur is somevvhat reduced by the presence of Mn, the sulfur is tied up in MnS, its solution concentration and therefore its surface concentration is desereased.
3 Grain Boundarv Segregation
3.1 Grain Boundary Segregation of Phosphorus in Ferrite and Austenite
3.1.1 Ferrite
The equilibrium grain boundary segregation of P in ferrite vvas studied on 7 Fe-P melts vvith P contents in the range 0.003 to 0.33 wt% P17. As expected, according to thermo-dynamics, the grain boundary concentration decreases vvith inereasing temperature and vvith decreasing bulk concentration, Fig. 11, and the data could be evaluated according to the Langmuir-McLean equation (4). The free enthalpy of grain boundary segregation of phosphorus in a-iron can be vvritten:
AGp =
-34 300 - 21.5/' (J/mol)
(9)
The presence of phosphorus in the grain boundaries in-duces grain boundary cmbrittlement, vvith inereasing grain
b)
Figure 9. Model of the c(2 X 2) adsorption structure of sulfur on the He(100) surface, derived fomi LEED studies a) top view and h) cross section in (110) direetion Slika 9. Model adsorpcijske sirukture c(2 x 2) žvepla na površini Fe(100), dobljen s pomočjo LEED raziskav a) pogled i vrha in b) presek v smeri (110)
boundary phosphorus concentration tlie fracture mode (at lovv temperatures) changes from transgranular to intergran-ular, see Fig. 12.
In a similar way the grain boundary segregation in Fe-C-P alloys vvas investigated17. Different concentrations in the range 10 to 100 ppm C vvere introduced to specimens vvith constant bulk concentrations of P by carburization in CH4 -H? mixtures. The specimens vvere annealed at different temperatures to establish the equilibrium grain boundary segregation of C" and P, quenched and analyzed by AES. The results indicatc displacement of phosphorus by carbon at the grain boundaries, according to
C(dissolved) + P(segregated) =
C(segregated) + P(dissolved) (10)
With inereasing carbon content the grain boundary concentration of phosphorus decreases and the grain boundary concentration of carbon increases, see Fig. 13. At 600°C the solubility limit of carbon is about 55 ppm, at higher concentrations cementite precipitates and no further changes of grain boundary concentrations are to be expected.
The inereasing carbon concentration in the bulk and in the grain boundaries causes a decrease in the percentage of intergranular fracture, thus carbon acts as a de-embrittling
Figure 10. Effect of sulfur on the rate of carburization and rate of nitrogenation, initial rates measured using the resistance-relaxation method in flouing gas mixtures CH4 - H; - H;S or V - H; - H2S at 1 bar, in dependence on the sulfur aetivilv given by the ratio
in the gas mixtures. The diagram also shows the adsorption isotherm of sulfur on iron al the reaction temperature 850°C derived
from the kinetic measurements Slika 10. Vpliv žvepla na stopnjo naogljičenja in stopnjo nitnranja. začetne stopnje so bile izmerjene ob uporabi uporovnorelaksacijske
metode v toku plinske mešanice CH4 - H; - H2S ali N: - H2 - HiS pri 1 bar, v odvisnosti od aktivnosti žvepla, ki je podana z razmerjem HiS/H: v plinski mešanici. Diagram prikazuje tudi adsorpcijsko izotermo žvepla na železu pri reakcijski temperaturi 850°C, dobljeno pri meritvah kinetike
element. The displacement of phosphorus and carbon according to Eq. (10) can be described by equations consid-ering the site competition of both elements.
0P/(1 - 6p - ©c) = xp exp( —A6'p/RT) (11) ©c/(1 " ©p - ©c) = J-c exp(-A6-Z/RT) (12)
From more extensive investigations29 in this system also the Gibbs' free energy could be determined for grain bound-ary segregation of carbon in ferrite vvhich is -72 kJ/mol at 500° C compared to -49 kJ/mol for phosphorus. This deseription of the displacement equilibrium (11) at grain boundaries corresponds to the result for the displacement equilibrium of carbon and silicon on the iron surface, see Eqs. (6) to (8).
In carbon steels vvith carbon contents higher than the maximum solubility in ferrite (0.02% at 735°C) the carbon concentration is given by equilibriuni vvith cementite and is relatively high in the critical temperature range concerning phosphorus segregation, thus grain boundary segregation in carbon steels generally vvill be lovv. Hovvever, the addition of carbide-forming elements, e.g. some percents of Cr or Mn, decreases the solubility of carbon, therefore the equi-librium (10) is shifted to higher grain boundary segregation of phosphorus, see Fig. 14. This effect is not caused by direct interaetion of Cr or Mn vvith P. by sonte "synergistic cosegregation" as assumed by other authors,33'34 but by the reduetion of carbon concentration in bulk and grain boundaries in the presence of (Fe. CrhC respectively (Fe. MnhC, vvhereby phosphorus can segregate to the grain boundaries. Fig. 14 demonstrates these results, grain boundary segregation in carbon-free alloys Fe-P and Fe-Cr-P is the same and unaffected by the presence of Cr, grain boundary seg-

regation in Fe-Cr-C-P, hovvever, is considerably increased compared to Fe-C-P17 35.
3.1.2 Austenite
Grain boundary segregation of P in austenite was inves-tigated after annealing of samples in the austenitic range and quenching in vvater25 30~32. The evaluation of data ob-tained with binary alloys containing 0.09, 0.145, and 0.20% P yielded a value of AG'j5 = -49 ± 4 kJ/mol at 1000°C, similar as for ferrite at 500° C. Thus, the grain boundary concentrations are considerable, between 15-30 at% of a monolayer for these alloys, see Fig. 15. As to be expected, the grain boundary concentration decreases with increasing temperature. Also in austenite the presence of carbon decreases grain boundary segregation of P and Eqs. (11) and (12) can be applied, the free energy of grain boundary segregation can be derived for carbon in austenite to be
AGc = -30 kJ/mol.
0	50	100
grain boundary concentration ot phosphorus [at%]
b)
Figure 12. Intergranular fracture caused by P grain boundary segregation a) Fracture face of sample with high grain boundary concentrations of phosphorus, fractured in the UHV system by impact at about -100°C b) the percentage of intergranular fracture is clearly related to the grain boundary concentration Slika 12. Interknstalni prelom povzročen zaradi segregacije P po mejah zm a) Prelomne ploskve vzorca z, visoko vsebnostjo fosforja, ki je segregiral po mejah zm; vzorec je bil prelomljen v UVV pri temperaturi okrog -100°C b) odstotek interknstalnega preloma je v vidni povezavi s koncentracijo na mejah zm
3.2 Grain Boundary Segregation ofTin in Ferrite
400 500 600 700 800 ageing tempercture l°C] Figure 11. Grain boundary concentrations of phosphorus detemiined by AES in Fe-P allovs of different P contents, plotted vs the equilibration temperature Slika 11. Koncentracija fosforja na mejah zm. določena z metodo AES v Fe-P zlitinah z različno vsebnostjo fosforja, grafično prikazana v odvisnosti od ravnotežne temperature
0.33 %p
0.046 % P
0.003 % P
0 018 %P 0009 % P
The effect of boron on phosphorus segregation in austenite is similar to the effect of carbon but much more pronounced. Even very small concentrations of boron in the range 5 to 30 ppm B strongly decrease the phosphorus segregation. Boron also was detected at the austenite grain boundaries30, its free energy of grain boundary segregation is relatively high, AGg % -100 kJ/mol.
For a study of the equilibrium grain boundary segregation of tin, 7 Fe-Sn melts were prepared vvith tin contents tn the range 0.02 to 0.2 wt% Sn2f\ The scatter of the measured values is very large, possibly due to a strong orienta-tion dependence of tin grain boundary segregation, see Fig. 16. Hovvever, the average values could be fitted vvith the Langmuir-McLean equation (2), the values vvere obtained

0.005
carbon eontent [wt%]
Figure 13. Grain boundary concentrations of phosphorus and of carbon in Fe-0.17%P at differenl carbon concentrations. Dependence of grain boundary concentrations on the bulk carbon concentration after equilibration at 600°C Slika 13. Koncentracija fosforja in ogljika na mejah /.m v Fe-0.17% P pri različnih vsebnostih ogljika. Odvisnost koncentracije na mejah zm od vsebnosti ogljika v masivnem materialu po doseženem ravnotežju pri 600° C
1,7.'/. Cr
0.013 7. P
0.048% P 0.0 W„ P 0.0177.P
(00
700
500	600
onneohng temperature I °C I Figure 14. Effects of carbon and chromium on the grain boundary segregation of phosphorus in Fe-P, Fe-Cr-P, Fe-C-P, and Fe-Cr-C-P alloys with about the same P concentration in dependence on the
equilibration temperature Slika 14. Vpliv ogljika in kroma na segregacijo fosforja po mejah zm v Fe-P, Fe-Cr-P, Fe-C-P in Fe-Cr-C-P zlitinah s približno enako vsebnostjo P v odvisnosti od ravnotežne temperature
for the free energy of segregation. AGŠn = -(22 500 ± 2 800) - (26.1 ± 0.9)T (J/mol) (13)
These data indicate a rather low tendency to grain boundary segregation of tin.
4GP" 32000 -17-1 I C" it,"" K 300 • 715-t»",
Y fP
F"igure 15. Grain boundary segregation in ferritic and austenitic Fe-P alloys calculated according to Ref. 17 and 30 Slika 15. Segregacija po mejah zrn v feritnih in avstenitnih Fe-P zlitinah, izračunanih v skladu z ref. 17 in 30
0 L
500
550
700
750
600	650
temperature (°C)
F"igure 16. Grain boundary segregation of tin in Fe-Sn alloys of different Sn eontent in dependence on temperature Slika 16. Segregacija kositra po mejah zm v Fe-Sn zlitinah z različno vsebnostjo Sn. v odvisnosti od temperature
Furthermore, tin can be displaced by carbon from grain boundaries. With inereasing carbon eontent in Fe-Sn-C al-loys the tin concentration at the grain boundaries decreases and the carbon concentration at the grain boundaries in-creases, simultaneously the tendency to intergranular fracture is reduced.
According to these results there is no great danger of tin segregation to grain boundaries for most steels. According to thermodynamics in equilibrium with usual bulk concentrations < 0.02 wt% Sn, the grain boundary concentration will be lovv, even small concentrations of dissolved and seg-regated carbon will keep the tin from the grain boundaries in carbon steels.
Furthermore, the diffusion of Sn in iron is slow, during usual processing and heat treatment of steels there will be no time for tin segregation to grain boundaries.
However, upon application of heat resistant steels at elevated temperatures > 500° C effects of tin are expected which result less from grain boundary segregation but more from surface segregation in cavities, since the tendency to surface segregation is very high7.
4 Effects on materials properties
4.1 Long-term embrittlement of a 3.5 NiCrMoV steel
For lovv pressure steam rotors NiCrMoV-steels are preferred since large rotors can be tempered and a good toughness is
achieved. Hovvever, these steels show a tendency to long-temi embrittlement, therefore their use had to be restricted to temperatures about 350°C. A sufficient ductility must be retained during long-term application up to 250,000 h. The ductility is affected by the grain boundary segregation of phosphorus.
For prediction and control of embrittlement, the grain boundary segregation of phosphorus in the 3.5 NiCrMoV-steels was studied24 and its effects on the ductility (transi-tion temperature) of that steel. Two melts were prepared with 0.048% and 0.10% P, samples vvere annealed at temperatures betvveen 400 to 500° C for different times and then the grain boundaries vvere analyzed by AES. The grain boundary concentration of phosphorus approaches equilib-rium, as shovvn in Fig. 17. The curves can be fitted apply-ing McLean's equation for diffusion-controlled segregation and the Langmuir-McLean equation for describing the equi-librium. The diffusivities of P are given by
D = 0.13-3 exp(—176 kJ/mol~x / RT) and the Gibbs' free energy of equilibrium segregation AG° = -46 350 + 0.5T (J/mol)
: i i i i; ;i| i i . i 3!	-i—,-------;—; r i; 11 j i l ii.im	
C > 75.0 I 0 ■ 1.30-10"V/s		
	- L^T I^-li 1	
- ---$ 1	-	
	1 I i ' ' -lili - —!—L.	M Mil I 1 1 1 1 1 11
11 m e in h
Figure 17. Kinetics of the grain boundary segregation of P in 3.5% NiCrMoV steel 0.048% P at 400°C. The curve is fitted using the values of equilibrium grain boundary concentration C and diffusion coefficient of phosphorus D. given in the diagram
Slika 17. Kinetika segregacije fosforja po mejah zm v 3.5% NiCrMoV jeklu z 0.048% P pri temperaturi 400°C. Krivulja je dobljena z uporabo vrednosti ravnotežne koncentracije ogljika na mejah zm in difuzijskega koeficienta fosforja D, danega v diagramu
With these data the grain boundary concentrations of P in the 3.5 NiCrMoC-steel vvere calculated for different bulk concentrations, in dependence on temperature and tirne, see Fig. 18. Values calculated for lovv temperatures and lovv bulk concentrations are vvell in agreement vvith AES-analyses of long-tinte annealed samples.
The transition temperature from the notch impact test is linearly correlated to the grain boundary concentration, for this steel the relation is ATT/Ac = 6.3 (K/at% P). Using these data the change in transition temperature can be calculated for a steel of knovvn phosphorus concentration vvhich occurs during its application tirne and it can be decided if the application temperature may be raised.
For a steel vvith the bulk concentration 50 ppm P after 105 h at 400° C an inerease of 19 at% in grain boundary concentration can be predieted vvhich corresponds to an inerease of transition temperature by about 120 K. At 350°C
i 1	. J... 11		i . 1111	
bi				
		1000 ;pm ? ..		.. -' '
			500	
				250,''
	------	~~~~~		mg.
				
				' 1 ...... 1 J 1 M M 1
is'	to2	io'	'0S
Figure 18. Curves calculated for different bulk concentrations of P, predieting the grain boundary segregation of P in 3.5% NiCrMoV steel during application al 400°C Slika 18. Krivulje izračunane za različne koncentracije P v masivnem materialu za napovedovanje segregacije P v 3.5% NiCrMoV jeklu med uporabo pri temperaturi 400° C
the equilibrium concentration vvould be higher, but the diffusion is much slovver, so the inerease of grain boundary concentration is 13.5 at% P and ATT ss 85°C. Accordingly, such steel cannot be applied at 400°C, for that purpose the bulk concentration of P must be even lovver (< 30 ppm).
Nowadays, "clean steels" are produced vvith < 25 ppm of P vvhich can be used at 400°C vvithout risk of embrittlement for up to 105 h.
4.2 Effects of tin on the creep of a 1% CrMoNiV steel
The creep properties of heat resistant steels to some extent depend on purity. Fraeture at lovv stress and after long creep life occurs by the formation and grovvth of cavities along grain boundaries, vvhen these cavities coalesce they form an intergranular fraeture path. The formation and grovvth of cavities are favored and accelerated by grain boundary and surface segregation. AES-analyses of creep samples after long-tirne creep tests at 550° C had shovvn P at grain boundaries and P and Sn in cavities36. The effect of P on creep vvas tested for a 1% CrMoNiV steel, additions of 0.06, 0.045 and 0.1% P caused an inerease of creep rate in the primary and secondary stage of creep37.
For elucidating the effect of tin, melts of the 1% CrMoNiV steel vvere prepared vvith 0.044, 0.022, 0.061 or 0.12 wt% Sn. The creep specimens vvere annealed at 550°C for 2000 to 5000 h to establish the grain boundary segregation equilibria vvhich vvere attained vvith relatively lovv grain boundary concentrations in the range 5 to 10%. of a mono-layer. The creep of these materials vvas measured at 550°C and loads of 200, 250, and 300 MPa (= Nitim"2). In the experiments at 200 and 250 MPa during 1200 h of creep range, hovvever, the tertiary creep starts earlier and leads to premature failure of the tin-doped steels, the earlier the higher the tin content, Fig. 19.
Investigation of the creep samples after the tests shovved cavity formation at the grain boundaries and the AES spec-tra shovved considerable Sn concentrations in the cavited areas.
hi many cavities also MnS particles vvere detected vvhich play an important role in the nueleation of voids (Fig. 20).
Tin has a high tendency (very negative A67° for surface segregation , but much less tendency for grain bound-ary segregation. This also leads to enhanced nueleation
0	~ 200 " ~ 700 "" 600 * 800 ~ ' 1000
TIME I h )
Figure 19. Hffeet of tin on the creep of 1% CrMoNiV steel at 550°C, creep curves at a load of 300 MPa for melts of different tin content Slika 19. Vpliv kositra na lezenje 1% CrMoNiV jekla pri 550°C, krivulje lezenja pri obremenitvi 300 MPa za taline z različno vsebnostjo kositra
and grovvth rate of cavities. Thus, small concentrations of impurities such as Sn and S can strongly affect the creep properties and creep life of heat resistant steels.
4.3 Effects of phosphorus in slress corrosion cracking and hydrogen induced cracking of carbon steels
Carbon steels show intergranular stress corrosion cracking (IGSCC) in certain potential ranges upon corrosion in hot nitrate and hot hydroxide solutions. Cathodic polarization in sulphuric acid vvith and vvithout arsenic causes hydrogen induced cracking (HIC). The resistance against IGSCC and HIC can be characteriz.ed by measuring the relative vvork of fracture in constant extension rate tests, i.e. the area belovv the stress-strain curves measured in the electrolyte related to the corresponding value measured in paraffin.
The effects of phosphorus on IGSCC and HIC vvere studied for steels vvith 0.15% and 0.4% or 2% Mn the phosphorus contents vvere 0.003, 0.03 and 0.05% P38'39. Constant strain rate tests vvere conducted at constant poten-tials in 55% Ca(NO,)2 at 75°C, in 5N NH4NO, at 75°C and 33% NaOH at 120°C. The strain rate vvas 10_6/s. The grain boundary concentrations of phosphorus varied in de-pendence on bulk concentrations and heat treatment of the steel as determined by AES.
Tests in 5N NH4NO3 at 75°C at constant potential shovved a decrease of resistance against IGSCC vvith in-creasing potential in the range -300 to 0 mVn, in this range of potentials the vvork of fracture decreases vvith increasing P-content of the steels. Fig. 21a. At potentials 0 mVn to 650 mVH the resistance against IGSCC is very lovv for ali steels, independent of P-content. In this range of potentials the grain boundaries are attacked already vvithout load and the failure is by stress-assisted intergranular corrosion. At even higher potentials > 650 mVH intergranular corrosion occurs vvithout any load, iron in nitrate disintegrates into grains16.
The behavior in 55% CafNO^h solution at 75°C is sim-ilar as in NH4NO3, hovvever, the steels are resistant against IGSCC up to -100 mVfj. The decrease of vvork of fracture is observed in the potential range from —100 to +100 mVH, the values being lovver for the high phosphorus steels, see Fig. 21b. At higher potentials the resistance against IGSCC is again very lovv and independent of P-content.
b)
Figure 20. Cavity formation in the fracture surface of steel vvith 0.061% Sn after a creep test at 300 MPa for 498 h a) SEM-photo of a fracture face vvith several creep cavities b) SEM-photo of a cavity vvith an MnS particle in the middle, points are indicated vvhere AES spectra have been taken Slika 20. Tvorba por na prelomni površini jekla z 0.061% Sn po testu lezenja pri 300 MPa in po 498 urah a) SEM posnetek prelomne
površine s številnimi porami, ki so nastale zaradi lezenja b) SEM posnetek pore z MnS vključkom v sredini, označena so mesta, kjer so bili posneti AES spektri
The effect of phosphorus on the IGSCC is caused by P-segregated at grain boundaries, vvith increasing grain bound-ary concentration of P the steels become less resistant. The grain boundary concentration is lovv for steels after nor-malizing and quenching, the grain boundary concentration increases upon holding the steels at 500° C.
300 MPa • 0 12 V. Sn o 0 0227. Sn ■ 0 00<.7. Sn
■	■ t* t-i k^tiAcVtM
F e.Mn
'1 I 1 I I T T T pr I 1 I j
■ ' ■ ■ I ■ ' ' ■ I ' ■ ' ■ I ■ ■ ■ ' I
500
electron vo!ts

fe
i . i . p-r-r-T j-r-r-r j-pr i , , | , , , i ] .Tr
100	500
electron vo its
-pm-T-j-
T I T I T T T T
500
electron volts c)
Figure 20. c) AES spectra from the points indicated in a), spectrum
1 MnS particle and cavity surface with Sn, 2 cavity surface with segregated Sn, 3 intergranuiar fracture face with less Sn Slika 20. c) AF.S spektri točk, ki so označene v a), spekter 1 MnS vključek in površina pore z Sn, 2 površina pore s segregiranim Sn. 3 interkristalno prelomljena ploskev z manj Sn
In the production of carbon steels the times usually are short during vvhich the steels are at teniperatures in the critical temperature range 400 to 600° C for the grain boundary segregation of phosphorus. The grain boundary concentra-tions may be somevvhat enhanced, for example, after coiling of hot rolled steel at relatively high temperature > 500°C.
On the other hand, carbon steels are less susceptible to effects of P than low alloy steels, since carbon in solution displaces phosphorus from grain boundaries. In steels al-
100-75-50
Fe - 0.15 % C - 0.4 %Mn
A 0.003%P x 0.03 7„P o 0.05 7. P
>
"5
25 -
1000
t o
-300
-200 -100 0 —► potential tmVEH]
a)
1000
0 -100 potential [mVtH
b)
1000
Figure 21. Intergranuiar stress corrosion cracking of carbon steels. relative vvork of fracture of carbon steels vvith different P contents,
tested after heat treatment (1 h, 940°C + 48 h, 500°C) in constant-extension-rate tests at lO^/sec a) in 5N NH4NO3, b) in
55% Ca(N03h, hot h at 75°C Slika 21. Intergranulama napetostna korozija ogljikovega jekla, relativno delo za prelom ogljikovega jekla z različno vsebnostjo P,
določeno po toplotni obdelavi (1 h. 940°C + 48 h, 500°C) pri konstantni stopnji raztezanja 10-6/s a) v 5N NH4NO3. b) in 55% CaiNOj); , v obeh primerih pri 75°C
loyed vvith carbide formi ng elements such as Mn and Cr the concentration of dissolved carbon is decreased and thus the grain boundary concentration of phosphorus can be higher (see Chapter 3.1). The effect of Mn on the grain boundary segregation of P vvas demonstrated, comparing a series of steels vvith 0.04% Mn and a series of steels vvith 2% Mn. The phosphorus segregation is enhanced for the high Mn-steels after comparable heat treatment, hovvever, the effect on the IGSCC in nitrates is less pronounced than expected.
In 33% NaOH at 120° C the corrosion potential is es-tablished at about -900 mVH, the IGSCC is observed at higher potentials, -900 to -500 mV^. The vvork of fracture
decreases in the range -900 to -700 mVH to a minimum value of about 25% for ali investigated steels, in this range the resistance against IGSCC is virtually independent of the phosphorus content of the steels. Above -700 mVH to -500 mVn the work of fracture increases vvith inereasing potential. This increase is clearly faster for the steels vvith lovver phosphorus content. Thus, in the range -700 mVH to -500 mVH the susceptibility against IGSCC in NaOH is enhanced by phosphorus38,39.
In 1 m H2SO4 at cathodic polarization the investigated steels shovv hydrogen induced stress corrosion cracking. The susceptibility tovvards HSCC is increased vvith the phosphorus content and reduced for the higher manganese content. The mode of cracking is transgranular. The suscep-tibility to HSCC is not related to the grain boundary concentration but to the bulk content of phosphorus. HSCC is correlated to the hydrogen uptake of the steels vvhich increases vvith the phosphorus content and decreases vvith the manganese content4^41.
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