Surface and grain boundary segregation related to the temper embrittlement of a 214 Cr-1Mo steel

108

Applications of Surface Science 15 (1983)108—119 North-Holland Publishing Company

SURFACE AND GRAIN BOUNDARY SEGREGATION RELATED TO THE TEMPER EMBRITFLEMENT OF A 23Cr-iMo STEEL P. HO

“,

D.F. MITCHELL and M.J. GRAHAM

Division of Chemistry, National Research Council of Canada, Ottawa, Canada KJA 0R9 Received 9 June 1982; accepted for publication 20 September 1982

Temper embnttlement of 2~Cr—lMo steels is considered to be promoted by the segregation of impurities, such as P, to grain boundaries. Analysis by scanning Auger microscopy and XPS of intergranular fracture surfaces indicates the presence of — 0.2 monolayer P, most of which is in a chemisorbed state. Fe 3C (with some Cr and Mo) is also precipitated at grain boundaries. Surface segregation studies at 500—625°Cusing X-ray emission analysis combined with RHEED shows that only P (no C, Cr or Mo) segregates to a free surface, and that the P is chemisorbed, rather than3exp( present — E/RT) in a cm2/s crystalline with E state. = 45The ±4 kcal/mole. P diffusion coefficient is determined to be — 2X 10

1. Introduction 23Cr—i Mo steels are widely used for thick-walled structural applications. Previous work [1—141has indicated that this steel is susceptible to temper embrittlement caused by the segregation of Mn and Si, together with impurities such as P and Sn, to prior austenite grain boundaries [1,21.Evidence for the impurity segregation in 23Cr—IMo steel and various other alloys has been provided by Auger electron spectroscopy [1—9,15—26],the field ion microprobe [27], as well as related techniques [1—26]. The addition of Mo to alloy steels inhibits temper embrittlement, and this is often attributed to a positive interaction between Mo and P [1,9—11,15].The presence of C in the base steel complicates the segregation phenomenon, as Mo can be precipitated as a carbide [1—4,7,15,21,24,28—30]during tempering or step-cooling, and the matrix is depleted of Mo [1,9—11,151.The P released then segregates at a rate consistent with the rate of carbide precipitation [1]. Cr is also reported to cosegregate with P and to enhance embrittlement of 23Cr— 1Mo steels [1,2]. For Fe—0.04%C alloys doped with Sb, Sn, As or P, Rellick and

*

NRC Guest Worker/Centre for the Study of Materials, University of Toronto, Toronto, Canada M5S 1A4.

0378-5963/83/0000—0000/$03.00 © 1983 North-Holland

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Temper embrittlement of 2~Cr— iMo by segregation

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McMahon [31] have proposed that embrittlement occurs when these impurity elements are rejected from precipitating carbides during cooling. Despite the extensive work related to segregation in 23Cr— lMo steels, questions remain as to the quantity of P (and other impurities) at grain boundaries, whether or not a Mo—P compound is present, and whether segregation of impurities to free surfaces is similar to segregation to grain boundaries [17,32—40].Both scanning Auger microscopy (SAM) and X-ray photoelectron spectroscopy (XPS) have been employed to examine grain boundary segregation, and surface segregation of P has been studied using reflection high energy electron diffraction (RHEED) combined with X-ray emission analysis (XRE) [41]. Microstructural examination of grain boundary carbides will be included in a future report [42].

2. Experimental The chemical composition of the 23Cr—lMo steel which contains 0.038 wt% P is listed in table 1. Samples were austenitized for 1 h at 925°C then water-quenched, tempered for 1 h at 675°C then air-cooled, and then stepcooled from 595°C as shown in fig. 1. This embrittling heat treatment produces about one-third the increase in impact transition temperature that results from 60,000 h service in the temperature range 325—575°C[3]. The alloy grain size is 30—50 ~sm. Specimens for grain boundary segregation studies were fractured in-situ at 80°Cin a vacuum of 2 x 10_b Torr in a Physical Electronics Industries Inc. (PHI) 590 scanning Auger microprobe equipped with XPS. Typical operating conditions for Auger analysis were: a beam energy of 5 keV, a beam size of 2 ~smand a beam current of 0.2 isA. Auger depth profiling was performed using 1 keV xenon ions rastered over an area 6 mm2 ion current density 6 ~A/cm2. The elemental sensitivity factors for Auger analysis were determined —

—

—

—

Table I Chemical composition of 2~Cr—lMosteel *) Element

Conc. (wt%)

Element

Conc. (wt%)

Element

Conc. (wt%)

C Cr Mo Mn

0.11 2.25 1.01 0.46

Si P Sn S

0.37 0.038 0.001 0.005

Al Ni V Fe

0.035 <0.01 0.003 Ba].

~ Steel supplied by W.R. Tyson, CANMET, Energy, Mines and Resources C~tnada,Ottawa, Canada KIA 0E4.

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Temper embrittlement of 2~Cr— IMo by segregation

AUSTENITIZE 925°C lb TEMPER

I

STEP—COOL

615’ li’~

595°

i1540’

I

F—

15h

525° 24h

411° 12h

L_~

46h

w

~FC

I

w

\315°

H WA

AC

AC

TIME

—~

Fig. I. Heat treatment to promote temper embrittlement in 2 ~Cr—I Mo steels (WQ AC air-cool and FC furnace-cool).

water-quench,

using an Fe—Ni—Cr—P—B amorphous alloy, a Ni—l3Mo alloy and Fe3C (cementite) as standards. (The composition of the amorphous alloy was found to be uniform on a large scale (— cm’s) by atomic absorption and on a fine scale (— ~um’s) by Auger analysis.) The peak positions, modulation voltages used and the sensitivity factors obtained are summarized in table 2. XPS spectra were recorded using a Mg X-ray source and a PHI 15-255G double pass cylindrical mirror analyzer attachment, which was operated with a 50 eV pass energy. XPS spectral peaks are referenced only to the Au 4f7/2 peak (83.8 eV). Table 2 Auger elemental sensitivity factors and modulation voltages used at primary beam voltage of 5 keV Element

Auger peak (eV)

Modulation voltage (eV)

Elemental sensitivity factor

Fe a) Ni a) Cr a) pa) Ba>

703 848 529 120 179 272 187

6 6 3 3 3 2 3

0.220 0.302 0.432 0.720 0.120 0.123 0.265

Mo *) b) ‘~

C)

Fe—Ni—Cr—P—B amorphous alloy. Cementite, Fe3C. Ni—Mo alloy.

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Temper embrittlement of 2~Cr— iMo by segregation

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For surface segregation studies, specimens were analyzed in a RHEED/XRE system, the details of which have been described previously [41]. One specimen was austenitized and tempered as shown in fig. 1, electropolished and then transferred to the RHEED/XRE machine. After bakeout the specimen was

P

2

Mo

Fig. 2. (a) Scanningelectron micrograph of a fracture surface of embrittled 2~Cr—lMosteel. Auger scans for P and Mo (along line indicated) are shown and the Auger analysis of points 1—4 is given in table 3. Marker represents 20 tam. (b) Auger map of P.

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Temper embrittlement of 2~Cr— iMo by segregation

sputter-cleaned and then annealed at temperatures from 500—625°C. The quantity of P segregating to the surface was determined from the emitted X-ray signal. After reaching surface saturation at a particular temperature the specimen was held at 675°Cfor 1 h to restore a uniform concentration of P in the near surface volume. Repeat experiments at 575 and 600°Con another sample reproduced the surface P segregation data reasonably well. During these segregation experiments the specimen surface was examined by RHEED for indications of any crystalline compound formation.

3. Results and discussion 3.1. Grain boundary segregation 3.1.1. Impurity analysis Fig. 2a shows a scanning electron micrograph of a typical fracture surface. Auger analysis of both intergranular (grain boundary) and transgranular fracture areas shown in fig. 2a are given in table 3 where the atomic percent (at%) concentrations are based on the equation of Davis et al. [43] which assumes a homogeneous composition within the analysis volume. As seen in the table, the concentrations of C, P, Mo and Cr are high at grain boundaries compared with those on transgranuiar regions. Auger line scans of P and Mo are displayed in fig. 2a, and the increased P segregation is highlighted by the Auger map shown in fig. 2b. A typical Auger spectrum of a grain boundary area is shown in fig. 3, in which the C peak has the characteristic carbide shape [24]. S was not observed to segregate to grain boundaries, and this is probably due to MnS formation [2,5,44].

Table 3 Chemical analysis of fracture surface (see fig. 2a) Element

Fe Cr C Mo P *) b)

Conc. (at%) Point 1 a)

Point 2

91.4 2.0 4.9 1.0 0.7

76.9 4.7 8.2 3.8 6.3

Transgranular. Intergranular (grain boundary).

b)

Point 3 a)

Point 4

90.0 2.9 4.7 1.2 1.2

79.3 4.0 8.2 3.6 4.9

b)

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Temper embrittlement of 2~Cr— iMo by segregation

Fe~

113

-

Fe I 0

200

I

I

I

400 600 800 ELECTRON ENERGY, cv

1000

Fig. 3. Auger spectrum of an intergranular (grain boundary) fracture area of embrittled 2~Cr—1Mo steel. Modulation voltage, 4 eV.

3.1.2. Auger sputter profiles Sputter profiles for C, P and Mo as well as Cr and Fe from an intergranular fracture surface are shown in fig. 4. As seen in the figure, the concentration of C is 20% during early sputtering. This C predominates most probably as —

SPUTTER TIME, mm Fig. 4. Auger sputter profiles of an intergranular (gran boundary) fracture area of embrittled 2~Cr—1Mosteel. Sputter tate with I keY xenon: — 4 A/mm based on Fe 203 standards; 2 A atomic layer.

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cementite, Fe3C, which is removed after 65 A of sputtering. (An Fe3C standard preferentially lost C during sputtering to give an apparent homogeneous at% C concentration 2 1%.) The small amounts of Cr and Mo are also probably present as carbides [1]. It should be noted that the concentration of elements on intergranular fracture surfaces does vary from area to area and from sample to sample. For example, the C was found to vary from 8 to 23 at% and the P from 4 to 8 at% (cf. fig. 4 and table 3). Different C values are due to variations in the amount of Fe3C present at grain boundaries. The grain boundary P concentration (fig. 4) falls rapidly during the first 2 mm (8 A) of sputtering while a smaller change is observed for the Mo concentration. Area measurements of the profiles indicate the ratio of P: Mo removed during this period to be 5: 1. The reduction of Mo (and P) during sputtering is better seen in fig. 5. The P peak is a doublet which may indicate the presence of both chemisorbed P and the chemical interaction, perhaps with Mo, of some segregated P [17]. The P peak shape is similar to that for P in the amorphous alloy which gives some confidence in using a sensitivity factor obtained from the latter in the analysis of fracture surfaces. —

—

—

—

—

3.1.3. Auger thin film analysis The thin film analysis model developed at NRC [45] has been used to determine that the P coverage on an intergranular fracture area is 0.10 monolayer based on the sputter profile data fig. 4, and definingfor1 2. shown Grain in boundary coverages monolayer to contain 1.6 X 1015 atoms/cm

P

-

10

115

Mo(x4)

Sputter Time, mm

120

125

0.8

75

180 185

190

ELECTRON ENERGY, eV Fig. 5. Auger spectra of P and Mo during initial sputtering in I keV xenon of an intergranular (grain boundary) fracture area of embrittled 2~Cr—1Mosteel. Modulation voltage, 3 eV.

P. Ho et a!. / Temper embrittlement of 2~Cr— iMo by segregation

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segregated elements can also be estimated from the Auger peak-to-peak (P—P) heights using an approximation developed by Hondros and Seah [46]. The coverage C~of element x on an Fe-based substrate is given by

cxo.18I~s--,

(1)

where ‘Fe is the Fe P—P height at 703 eV, I~is the P—P height for element x, E~ is the Auger electron energy in eV for element x, and SFe and S~are the relative sensitivity factors for Fe and element x, respectively. Using the parameters listed in table 2 and the P—P heights from fig. 3, the P coverage is calculated to be approximately 0.11 monolayer (1 monolayer m 1.4 x iO~ atoms/cm2). This P coverage is thus in good agreement with the results from the thin film analysis model. If it is assumed that P is distributed roughly equally on each of the two fracture surfaces, then the total concentration of P segregated at grain boundaries would be — 0.2 monolayer.

3.1.4. XPS studies An attempt has been made using XPS to determine the chemical state of elements present on grain boundary fracture surfaces. This is difficult, firstly because of the large analysis area of the technique (— 50 mm2) which is an order of magnitude larger than the total fracture surface area, and secondly because of the lack of XPS data from appropriate standards such as Fe and Mo carbides and phosphides. The spectra obtained from fracture surfaces are shown in fig. 6 and the binding energies of the various elements are given in table 4. It may be speculated that the 2 eV shift in the C peak from graphite [47] is due to the formation primarily of Fe 3C, which is consistent with the Auger data. The P and Mo binding energies (P 2p, 129.3 eV; Mo 3d5/2, 228.0 eV) may be compared with values obtained from the Fe—Ni—Cr—P—B amorphous alloy (P 2p, 129.3 eV) and the Ni—Mo alloy (Mo 3d5/2~,227.7 eV). —

Table 4 XPS binding energies of elements present on fracture surfaces of embnttled 2~Cr—lMosteel, referenced to the Au 4f712 peak at 83.8 eY Element

Binding energy (eV, ±0.2)

P 2p Mo 3d)/2 C 2P3/2 ls Cr 2P Fe

129.3 228.0 286.2 574.2

3/2

706.8

116

l~ 36 128

P. Ho et a!. / Temper embrittlement of 2~Cr— IMo by segregation

I (~ 232 22~ 288

280584 576

720

712

704

BINDING ENERGY, eV Fig. 6. XPS spectra from a fracture surface of embrittled 2 ~Cr—IMo steel.

It should be noted that both the XPS and Auger P peaks from the amorphous alloy have the same line shape as the P peak from a fracture surface. The XPS data are thus consistent with the statement that most of the P on grain boundaries is present in a chemisorbed state. As with the Auger data, the possibility that a minor amount may be combined with Mo to form Mo phosphides, as has been suggested by previous workers [1,48,49], cannot be ruled Out.

3.2. Surface segregation On annealing the 23Cr—lMo steel at temperatures from 500 to 625°Conly P was observed by XRE to segregate to the free surface. No segregation of C, Cr, Mo or S was observed; the C concentration, for example, remained at 0.3 monolayer throughout the heating period. The rate of P segregation is shown in fig. 7, where it is seen that the amount of P increases with increasing temperature up to 600°C; between 600 and 625°C it decreases a little. At 600°C,if it is assumed that all the P is in the first atomic layer, then the ratio of the amount segregated to the bulk concentration approaches 400. The rate of P surface segregation was also followed during the step-cooling procedure (fig. 1). The P concentration increased to 0.35 monolayer in 1 h of annealing at 595°C, and then remained unchanged during the rest of the procedure. A simple model [50] which describes the kinetics of surface segregation assumes that the concentration of the segregant is initially uniformly distributed throughout the matrix and diffuses to the surface by volume diffusion until the surface is saturated. The time r to reach saturation is then given by —

—

P. Ho et at

~0.4

4

0.3 0.2

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Temper embrittlement of 2~Cr— iMo by segregation

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1..600°C

~.~—55O°

-(

500*

______

5

TIME, h Fig. 7. Segregation of P to a 2~Cr—lMosteel surface at different temperatures. The error bars at 500 and 600°Cindicate the deviation in 3 repeat experiments. (The quantity of P is calculated assuming that P and S have the same sensitivity, the latter having been determined from radiotracer techniques.)

(2)

T~T/D~(F/2C5),

where F is the saturation coverage at the surface, C, ~S the initial cdncentration of the segregant, and D~is the volume coefficient. Usingatthedifferent data of 3, diffusion the diffusion coefficients fig. 7 and c~ = 5.7 x l0~ atoms/cm temperatures can be calculated and a plot of ln D~versus 1 / T is shown in fig. 8, which yields the diffusion coefficient, D~= 2 x 10 ~ exp( E/RT) cm2/s with E = 45 ±4 kcal/mole. These approximate diffusion coefficients —

T, *C 625 I

600 I

575 I

550 I

500

I

35

34

0

moIe~ lIT K,x IO~ Fig. 8. Arrhenius plot of P diffusion coefficient from 500 to 625°C.

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Temper embritt!ement of 2 ~Cr — iMo by segregation

and activation energy are in reasonable agreement with reported values in the literature for P diffusion in iron and various alloys [3,18,37,38,50—54]. Specimen surfaces were examined by RHEED during the surface segregation experiments, and no crystalline compound formation was ever observed. The detection limit of the technique to crystalline phases is extremely high, iO-4 monolayer). Thus, it may be concluded lOll g/cm2 (equivalent to that P segregated at the surface is present in a non-crystalline state. —

—

4. Conclusions Auger analysis of intergranular fracture surfaces of embrittled 2~Cr—lMo steels indicates the precipitation at grain boundaries of Fe 3C (with some Cr and Mo) and the presence of — 0.2 monolayer P, the majority of which is in a chemisorbed state. In contrast, surface segregation studies at 500—625°Cshow that no C, Cr or Mo is segregated to the free surface. Only P is detected and, as for grain boundary segregation, it is present in a non-crystalline state.

Acknowledgements The authors would like to thank Mr. G.I. Sproule for his assistance with the Auger and XPS analysis, and Drs. W.R. Tyson, J. Bowker, M.R. Piggott and G.C. Weatherly for helpful discussions.

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