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The effects of applied stress on the intergranular phosphorus segregation in a chromium steel
A<,<, M~t,,llt,ryr<~r Vol. 29. QQ. 1631 10 1636. 19x1 Prmled in Great Brlta~n All r,ghts rccaned
~1-6160/Sl/091631-os1o2.oo/0 Copyright 0 1981 Pergamon Press Ltd
THE EFFECTS OF APPLIED STRESS ON THE INTERGRANULAR PHOSPHORUS SEGREGATION IN A CHROMIUM STEEL TETSUMORI Department
SHlNODAt
and TADAHISA NAKAMURAZ
Materials Science & Engineering. Tokyo Institute Nagatsuda, Midori, Yokohama 227. Japan
of Technology,
Abstract- -The mean Auger peak ratio (APR) of phosphorus, averaged over the intergranular fracture surface of each sample. has been measured on smooth and notched types of specimens, which were subjected to stress agings at 773 K for times up to 15 h under given stresses of tension or compression after a long term aging at 773 K under no stress. When the stress level for aging was smaller than a critical value, the mean APR of P of the smooth specimen under tension rapidly increased during the first 1 h aging and then decreased to around the initial value in 15 h of aging, whereas that of the compressed specimen decreased. nearly by the same magnitude as in the case of tension, during the first 1h and then similarly approached to the initial value in 15 h of aging. The mean P-APR in the vicinity of the notch root was significantly small compared with those at positions distant from the notch root in a stress aged notched specimen. RCsum&Nous avons mesure le rapport moyen du pit Auger (RPA) du phosphore, moyennt sur la surface de rupture intergranulaire de chaque Cchantillon, pour des kprouvettes avec ou sans entaille, soumises B des vieillissements sous contrainte detraction ou de compression a 773 K pour des durCes pouvant atteindre 15 heures, aprts un long vieillissement a 773 K en l’absence de contrainte. Lorsque la valeur de la contrainte &tait infkrieure a une valeur critique, le RPA moyen du phosphore des tchantilIons sans entaille, sollicit& en traction, augmentait rapidement au tours de la premitre heure, puis reprenait en 15 h de vieillissement sa valeur initiale tandis que pour les &hantillons en compression le RPA dtcroissait d’une quantiti analoque, puis se sapprochait aussi de la valeur initiale en 15 h de vieillissement. La valeur moyenne du RPA du P au voisinage de I’extrgmitC de l’entaille ttait nettement infirieur aux valeurs observkes en des points Cloignts de cette extrkmitt pour un tchantillon entail16 vieilli sous contrainte. Zusammenfassung-Das durchschnittliche Verhtiltnis des Augermaximums (APR) von Phosphor, gemittelt iiber die intergranulare Bruchfliiche einer jeden Probe, wurde an ebenen und eingekerbten Proben untersucht. Diese Proben waren unter definierter Spannungsbelastung im Druck oder Zug bei 773 K bis zu 15 Stunden lang ausgelagert worden, nachdem sie vorher bei 773 K ohne Spannung lange behandelt worden waren. Unterhalb einem kritischen Spannungswert stieg das APR von Phosphor bei zugbelasteten ebenen Proben wPhrend der ersten Stunde der Auslagerung rasch an und fiel dann bei 15 h Auslagerung ab auf etwa den urspriinglichen Wert. Dagegen verkleinerte sich das APR in druckbelasteten Proben wihrend der ersten Stunde urn etwa denselben Betrag wie bei Zugbelastung und nlherte sich dann wghrend 15 h Auslagerung in lhnlicher Weise dem urspriinglichen Wert. Das APR des Phosphors war bei gekerbten Proben nach Auslagerung in der NIhe der Kerbwurzel bedeutend kleiner als an weiter entfernten Stellen
1. INTRODUCTION In our previous paper [l], describing about the effect of applied stresses on the intergranular segregation of phosphorus in steel during aging at 773 K, it has been found that the applied stress has an effect not only on the kinetics of intergranular P-segregation, possibly through its effects on the diffusive rate of P in the interiors of grains, but also on the capabilities of grain boundaries to absorb P-atoms. The former may be t Permanent address: Hitachi Research Laboratory, Hitachi Ltd. Saiwaicho, Hitachi, Ibaraki 316. Japan. $ Present address: Mechanical Engineering. Technological University of Nagaoka, Kamitomioka. Aza-Nagamine 1603-1, Nagaoka. Niigata 949-54, Japan.
referred to as the ‘kinetic’ effect of applied stress and the latter as the ‘thermodynamic’ effect. We have made a proposal [2] for the latter effect. It is such that whenever P-atoms are added on or removed from a grain boundary, they will decrease the free energy of the system through their works against the normal traction which acts on the grain boundary under the applied stress, just as atomic particles have been considered to do so in diffusional creep [3,4]. In the previous study, however, the distinction of the thermodynamic effect from kinetic one was unsuccessfully made because we there adopted such specimens that had been unsaturated with P at grain boundaries for the stress aging tests. The objective of this investigation is, therefore, to 1631
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confirm the existence of the thermodynamic effect of applied stress distinctly, and to modify the proposal, in conjunction with the present results. In the present study, P-bearing chromium steel is used as a test material and Auger electron spectroscopy (AES) is used to measure the intergranular P-concentration. The specimens, unlike the ones in the previous report, have been subjected to a long period of aging at the test temperature under no stress prior to the stress aging tests, so that their grain boundaries have been saturated with P. Associated modifications of the model will be treated later in an accompanying paper.
2. EXPERIMENTAL
PROCEDURES
The test material had a chemical composition in mass percentage of 0.30 C, 0.28 Si, 0.49 Mn, 0.050 P, 0.07 S, 2.29 Cr, 0.01 MO, with trace elements less than 0.015, the balancing being Fe. The material was forged into rod-blanks, which were solution-treated at 1473 K for 4 h, followed by oil quenching and tempered at 973 K for 2 h, followed by water quenching. The material thus obtained was, then, isothermally aged at 773 K for 1000 h under no external stress, followed by rapid cooling. The material was subsequently stress aged at 773 K for time ranging from 1 to 1.5h. The stress aging tests were divided into the following three types; (a) tensile loading on smooth specimens, (b) compressive loading on smooth specimens, and (c) tensile loading on notched specimens. The tensile specimens for the test (a) were of either 4.5 or 9 mm in diameter, and the compressive specimen for (b) of 4 mm in diameter. Each of these tensile and compressive specimens for the tests (a) and (b) was subjected to a given constant load, equivalent to the initial applied stress ranging from 10 to 100 MPa, for either 1, 3 or 15 h. The load was applied to each specimen by means of a lever type of creep machine having the capacity of 1 ton (9.8 kN). Before a given stress was applied to a specimen, it was kept at temperature of 773 K, with an accuracy of 1°C. for a few hours in the absence of stress. Immediately after stress aging, the specimens were cooled rapidly. On the other hand, each of the notched specimens for the above cited test (c), whose geometry is schematically shown in Fig. 1, was aged for 3 h under a given constant load, equivalent to either 20, 40 or 100 MPa in the nominal stress on the notch section. The methods of heating, loading and cooling in this case were also the same as those described above. From each specimen thus prepared for the tests (a) and (b), a sample for Auger spectroscopy, of 3.7 mm in diameter with a notch section of 1.9mm in diameter at the center of the length, was made. The longitudinal direction of Auger samples was always remained parallel to the direction of loading during the stress aging. On the contrary, from each notched
SEGREGATION
IN STEEL Tension ?-
Fig. 1. Schematic illustrations for the geometry of the notched specimen and how to cut out the three Auger samples, A, B and C, from it (Unit in mm).
specimen for the test (c), three Auger samples were made at the three positions: A (the notch vicinity), B (the intermediate) and C (the opposite edge), as shown in Fig. 1. Auger samples thus prepared were fractured at a low temperature (_ 193 K) inside the test chamber of the Auger system, in a vacuum less than 7 x lo-” torr (70nPa). All samples, except some A-samples from the notched specimens, exhibited almost 100% intergranular fracture. The AES analyses are made similarly to that described previously [S]. The typical electron beam diameter was less than lOpm, which was much smaller than typical grain sizes of the order of 1OOpm. With each Auger samples, Auger measurements were made at more than 20 different grain boundary facets, selected over one quarter of the fractured surface of each Auger sample. Namely, in this one quarter of the fractured surface, there were only 20 or so number of facets which was appropriate to the AES analysis in view of the size and orientation of the facets to the electron beam. The grain boundary concentration of P was evaluated from the relative peak height ratio of its Auger peak-to-peak height at 120eV, I,, to that of iron at 703 eV, IFe. This ratio of IdIF, will be referred to as ‘Auger Peak Ratio’ of P (APR of P) or more simply ‘P-APR’ hereafter. The ‘mean’ APR of P, x,, for each Auger sample was defined as
where X,,i is the individual value of P-APR at i-th grain boundary facet, and N the total (maximum) number of measurements. The word ‘mean’ will be used in this paper, as defined above, to specifically indicate the average intergranular P-concentration for an individual Auger sample, unless otherwise noted.
SHINODA
NAKAMURA:
AND
PHOSPHORUS SEGREGATION IN STEEL
1633
.27 .26 .25 .2? .26 .25
3
.27
--
B
.26
2
.2f
Y 6
.27 .26 .25 0
IO
20
30
Number of Measurements
t K)
Fig. 2. The scatter of the cumuIative average of P-APR. C X&K, as a function of number of AES measurements, K, of four Auger samples from the same rod.
3. RESULTS
of these four values, 0.258, was used as a most probable initial value of the mean P-APR prior to the stress aging test. As can be seen from this figure, the scatter of the cumulative average of
3.1 The initial value of the meun P-APR and its scattering In order to know the mean P-APR of the specimen which had not yet been subjected to stress aging, that is, to know the initial value of the mean P-APR, the Auger measurements were made on four separate Auger samples machined from the same rod that had only received the preliminary aging for 1000 h under no stress. Figure 2 shows the scatter of a cumulative average of P-APR.
from the level of 0.258 was comparatively large when the number of K was smaller, but it always fell within the -&2.5”/; error band around 0.258, when the number of K was increased to more than 20, for any case of the four samples. Typical Auger electron spectra from one of the above samples prior to stress aging are presented in Fig. 3. Spectrum (1). in this figure. is for the 1st measurement, which was taken at a point on the fractured surface of the sample about a few minutes after it had been broken in a vacuum of about 7 x lo-“torr, and Spectrum (2) for the 21st measurement on the same sample which was done about 2 h later from the fracture, in the same level of
with number of Auger measurements, K, for the four Auger samples described above. The mean APR’s of P. x,, at the maximum number of measurements for these four samples were, as shown in this figure, respectively 0.261, 0.256, 0.257 and 0.256. The average
I
1
I
100
200
t
30
I
,
*
LOO
Sal
600
Electron cmrpy
’
i 700
1
(rV)
Fig. 3. Typical Auger electron spectra from a Auger sample prior to stress aging, under the vacuum level of 7 x lo-” torr. Spectrum (1) is for the 1st measurement, with the value of 1,/I,, being 0.279, and Spectrum (2) for the 21st measurement, with the In/IF, being 0.271.
AND
SHINODA
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NAKAMURA:
PHOSPHORUS
&0.257
1 s=o.o41
SEGREGATION
IN STEEL
P-APR of the specimens aged under compression, to the contrary, first decreased by about 8?/, relative to the initial mean of 0.258 in one hour of aging, but thereafter increased up to the same level as that of tension in 15 h of aging. These mean P-APR’s in 15 h of aging for the stress aged specimens were within an error band of *2.5% around 0.264. This value of 0.264, being another reference level adopted in the present study, was the average of the two mean P-APR’s; the initial mean of 0.258 and the mean of the specimen which was further aged for 15 h under no stress, 0.269. When the applied stress was greater than about 50MPa, the mean P-APR of the smooth specimen varied with aging time in a complicated fashion than when it was in the lower stress cases. Although the data for the higher stress cases were insufficient to draw a definite conclusion, at least the following points might be said; the mean P-APR for the higher compressive stresses varied with aging time as if it were the case of the lower tensile stresses. That for the higher tensile stresses, on the contrary, varied with aging time as if it were the case of the lower compressive stresses. However, in the case of the higher tensile stresses, there was a subsidiary peak at around 1 h of aging whereas in the case of the lower compressive stresses, no such peak was detected.
‘:i_m.itJm0.2
0.3
0401
APRof
a2
03
0.4
P
Fig. 4. Typical histograms of the frequency of occurrence of the APR of P for the specimens aged under various conditions: (a) and (b) for the specimens prior to stress aging, (c) for the one aged for 1 h at 30 MPa of tensile stress, and (d) for the one aged for 1 h at 30 MPa of compressive stress (‘frequency’ represents number of AES measurements).
vacuum. It can be seen from this figure that vacuum levels adopted in the present study are tolerable enough to measure the phosphorus concentrations at more than 20 different points on a specimen fractured .surface, although the surface contamination proceeds somewhat, through oxygen and carbon. even under these circumstances. 3.2 Tensile and compressive stress qficts on smooth specimens Figure 4 shows typical histograms of the frequency of occurrence of the APR of P for the prior to stress aging specimens [Figs 4(a) and (b)], for the specimen aged for 1 h at 30 MPa of tensile stress [Fig. 4(c)], and for the specimen aged for 1 h at 30 MPa of compressive stress [Fig. 4(d)]. In these figures, the value of S shows the standard deviation of x,, which was defined by
3.3 Tensile stress &ects on notched specimens Figure 6 shows the relationship between the mean P-APR of the Auger samples cut from the notched specimens and the positions from which they were cut out. It can be seen from this figure that the mean P-APR at A-position in the vicinity of the notch root was fairly small compared with those at the other two positions, B (the intermediate) and C (the opposite
Tension
0.30
As can be seen from these figures, the histogram for the two prior to stress aging samples resembled each other in its pattern. Compared with these, the histogram for the stress aged samples was quite different; the most populated class of P-APR in each histogram for the stress aged specimens apparently shifted to either a higher or a lower rank relative to that for the prior to stress aging specimens, depending upon the sign of the applied stress. Figure 5 summarizes the changes in the mean P-APR as a function of subsequent aging time for the smooth specimens aged under different magnitudes of tensile or compressive stresses. When the applied stress was than about 40 MPa, the mean P-APR of the specimens aged under tension first increased by about 10% from the initial value of 0.258 during the first 1 h of aging followed by decreasing down to a level close to the initial mean P-APR, in 15 h of aging. The mean
0
6 a 0 4
c
0.3or
1
Aging
Time (h)
Fig. 5. The changes in the mean P-APR as a function of subsequent aging time under various condition of applied stresses for the smooth specimens. The vertical bar shows the +2.5x error band around 0.264.
SHINODA
AND
0 0
4
NAKAMURA
:lOO 8
” 1 12
I
PHOSPHORUS
~ 16
Di stance from the Notch (mm) Fig. 6. The relationship tance from the notch
between the mean P-APR root for the stress aged specimens.
and disnotched
edge) positions. The mean P-APR’s at B and C-positions were comparable to or somewhat lower than the reference level of 0.264, above mentioned. Here we should have a word with the mean at A-position. This value represents the mean P-APR, averaged over more than 20 different grain boundary facets within a region very close to the notch root, say a few microns distant from the notch root, rather than the mean P-APR averaged over the whole fracture surface of a standard Auger sample as in the other cases. Figure 7 shows the histogram of the frequency of occurrence of P-APR in this narrow region very close to the notch root, together with those for B and C-positions in the notched specimen. As can be seen from this figure, the type of histogram at A-position was significantly dif-
A P R of P Fig. 7. Histograms of the frequency of occurrence of the APR of P for the samples from the stress aged notched specimens: (a) for A-sample from the specimen aged at 100 MPa. (b). (c) and (d) for A, B and C-samples, respectively. from the specimen aged at 20 MPa.
SEGREGATION
IN STEEL
1635
ferent from those at the other two positions, which resembled those shown in Fig. 4. Namely, in A-region, the values of P-APR on some grain boundary facets were excessively lowered. and then this resulted in the significant decrease in the mean P-APR for this region. It follows from this that the standard deviation of the P-APR distribution for this narrow region increased by a factor of about two than that for the other Auger samples. as shown in Fig. 7. Even with the fracture appearance, some difference was observed between A-sample and the other ones; some facets in A-fracture region looked like ‘dimple’ whereas no such facet was observed in the fracture surface of the other samples, which was of almost lOO”, intergranular fracture type. Another characteristic of the Auger spectra from this narrow region of A was that Auger peak ratio of oxygen became extremely high on some grain boundary facets. This increase in oxygen concentration in some grain boundary facets seemed to be irrespective of whether the concentration of P on the facets was high or not. However, this enhancement in oxygen concentration was observed only when the applied stress was 20 MPa but never when it was 40 or 100 MPa. 4. DISCUSSION In our previous paper [l]. a mention was made that an error in measuring the mean APR of P was estimated to be approximately k2.5”. when measurements were made repeatedly at least at more than 20 different points on each Auger sample under the same analytical condition. The present results also confirmed that the previous estimation of the percentage error of the mean P-APR being 1_2.5”, was reasonable. Namely, the cumulative average of P-APR for four different Auger samples. as shown in Fig. 2. fell surely into the scatter band of k 2.5”,, of a given mean when the number of measurements were increased to more than 20. Thus we can conclude that the relative error of the mean P-APR was of the order of f2.5”,, under the analytical condition of AES, adopted in the present study. The present data would have some other types of experimental errors, possibly due to the errors in aging temperature or applied stress. For example, as shown in Fig. 5, the subsequent aging for 15 h under no stress seems to have increased the mean P-APR by about 4”;, from its starting value of 0.258. This increase in the mean P-APR seems to have come from the slight difference in aging temperature between the preliminary aging for 1000 h under no stress and subsequent stress aging. Following McLean’s theory of the equilibrium segregation [6], the average temperature during the preliminary aging can be estimated to be higher by about 3 K than that for the subsequent stress aging. In this estimation we used the value of - 16 kcal/mole ( - 67 kJ/mole) as the binding energy of P-atoms to grain boundaries. being a possible one for steel [7]. In addition the accuracy of applied stress
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would be impared to some degree when the applied stress was decreased to the order of 10 MPa, because we used a lever type of creep machine with a comparatively large capacity of load in these experiments. This inaccuracy in applied stress might be responsible in some part for the fact that the mean P-APR for the tensile stress of 10 MPa, as shown in Fig. 5, was unexpectedly small at 3 h of aging. Despite these experimental errors, major parts of the mean P-APR change during stress aging are believed to have been caused by the inherent change in the intergranular P-concentration. This belief is supported firstly be the fact that the observed changes in the mean P-APR were so great that all themselves were unable to be accounted only in terms of the experimental errors, and secondly by the fact that, as shown in Fig. 4, the histograms of the P-APR distribution for the stress aged specimens were quite different from those for the prior to stress aging specimens; the whole family of P-APR values for the stress aged samples had a tendency to shift towards either higher or lower values, depending upon the sign of stress, relative to those for the prior to stress aging samples. These facts demonstrate that the intergranular P-concentration, once having been saturated or in equihbrium prior to the stress aging, certainly varied as a result of the application of stress. It would seem that these changes in the intergranular P-concentration were induced by the thermodynamic effect of stress, earlier mentioned. It is such that the capacity of a grain boundary to absorb P-atoms is altered as a result of the application of stress. Detailed discussion on this matter will be presented later in the next paper. The applied stress effect on the intergranular segregation of impurities such as P in steel seems to be also of practical importance in view of the deterioration of materials under loaded service. Some suggestions on this matter are possibly obtained from the present results. For example, if the enhancement in P-segregation at grain boundaries occurred in a material, loaded to a comparatively small magnitude of stress under the uniform stress regime as in the ordinary service conditions for materials, it would be neglected in practice for the long-term services, because such an applied stress effect would be vivid only in a short period of time after loading, as shown in Fig. 5. When the material contains a region with a sharp stress gradient such as around a crack, some applied stress effect seems to remain even at a long-term service. However, it seems to be such that P-atoms are depleted from the vicinity of a crack tip, as shown in Fig. 6, so that it may be also harmless in practice. The deterioration of materials, however, may be enhanced
SEGREGATION
IN STEEL
by stress application through quite different mechanisms other than intergranular P-segregation. For example, we saw the occurrence of oxygen enhancement in the notch vicinity grain boundaries (perhaps oxygen atoms would be added from the atmosphere), but its effect on the embrittlement of materials has remained unknown as yet. We must await further studies to clarify such matters.
5. SUMMARY The applid tensile or compressive stress effect on the intergranular P-segregation in a chromium steel aged at 773 K has been studied by means of Auger electron spectroscopy with use of the smooth and notched types of specimens, preliminarily aged for 1000 h in the absence of stress at the same temperature, 773 K. In the case of the smooth specimens subsequently aged at stresses less than about 40 MPa, the mean P-APR increases when the applied stress is tension, or decreases when it is compression, by g-10 percent relative to the initial value during the first 1 h of aging. This relative change is much greater than the percentage experimental error of f2.5% for the mean P-APR. The relative change in the mean P-APR, however, disappears almost completely after 15 h of aging under stress. In the smooth specimens aged at stresses above about 50 MPa, the mean P-APR vs aging time curve appears as if it were the curve for the lower stress but with the opposite sign. In the notched specimens aged under tension, the mean P-APR at the position very close to the notch root is fairly small compared with those at the other positions distant from the notch root along the notch section. This depletion of P-atoms from the grain boundaries very close to the notch root, unlike the case for the stress aged smooth specimens, occurs preferentially in some grain boundaries. Possible explanations for this experimental results will be presented in a next paper.
REFERENCES 1. T. Shinoda and T. Nakamura, Trans. Japan Inst. Metuls 21, 753 (1980). 2. T. Shinoda and T. Nakamura, Trans. Japan Inst. Metals 21, 781 (1980). 3. C. Herrine. Jr. Aonl. Phvs. 21. 437 (1950). 4. R. L. Cob%, Jr. &pl. &s. G, 1679 (1963). 5. T. Nakamura, T. Shinoda and H. Watanabe, Trans. Iron Steel Inst. Japan 19, 365 (1979). 6. D. McLean, Grain Boundaries in Metals. Clarendon Press, Oxford (1967). 7. M. P. Seah, Acta metall. 25, 345 (1977).
~1-6160/Sl/091631-os1o2.oo/0 Copyright 0 1981 Pergamon Press Ltd
THE EFFECTS OF APPLIED STRESS ON THE INTERGRANULAR PHOSPHORUS SEGREGATION IN A CHROMIUM STEEL TETSUMORI Department
SHlNODAt
and TADAHISA NAKAMURAZ
Materials Science & Engineering. Tokyo Institute Nagatsuda, Midori, Yokohama 227. Japan
of Technology,
Abstract- -The mean Auger peak ratio (APR) of phosphorus, averaged over the intergranular fracture surface of each sample. has been measured on smooth and notched types of specimens, which were subjected to stress agings at 773 K for times up to 15 h under given stresses of tension or compression after a long term aging at 773 K under no stress. When the stress level for aging was smaller than a critical value, the mean APR of P of the smooth specimen under tension rapidly increased during the first 1 h aging and then decreased to around the initial value in 15 h of aging, whereas that of the compressed specimen decreased. nearly by the same magnitude as in the case of tension, during the first 1h and then similarly approached to the initial value in 15 h of aging. The mean P-APR in the vicinity of the notch root was significantly small compared with those at positions distant from the notch root in a stress aged notched specimen. RCsum&Nous avons mesure le rapport moyen du pit Auger (RPA) du phosphore, moyennt sur la surface de rupture intergranulaire de chaque Cchantillon, pour des kprouvettes avec ou sans entaille, soumises B des vieillissements sous contrainte detraction ou de compression a 773 K pour des durCes pouvant atteindre 15 heures, aprts un long vieillissement a 773 K en l’absence de contrainte. Lorsque la valeur de la contrainte &tait infkrieure a une valeur critique, le RPA moyen du phosphore des tchantilIons sans entaille, sollicit& en traction, augmentait rapidement au tours de la premitre heure, puis reprenait en 15 h de vieillissement sa valeur initiale tandis que pour les &hantillons en compression le RPA dtcroissait d’une quantiti analoque, puis se sapprochait aussi de la valeur initiale en 15 h de vieillissement. La valeur moyenne du RPA du P au voisinage de I’extrgmitC de l’entaille ttait nettement infirieur aux valeurs observkes en des points Cloignts de cette extrkmitt pour un tchantillon entail16 vieilli sous contrainte. Zusammenfassung-Das durchschnittliche Verhtiltnis des Augermaximums (APR) von Phosphor, gemittelt iiber die intergranulare Bruchfliiche einer jeden Probe, wurde an ebenen und eingekerbten Proben untersucht. Diese Proben waren unter definierter Spannungsbelastung im Druck oder Zug bei 773 K bis zu 15 Stunden lang ausgelagert worden, nachdem sie vorher bei 773 K ohne Spannung lange behandelt worden waren. Unterhalb einem kritischen Spannungswert stieg das APR von Phosphor bei zugbelasteten ebenen Proben wPhrend der ersten Stunde der Auslagerung rasch an und fiel dann bei 15 h Auslagerung ab auf etwa den urspriinglichen Wert. Dagegen verkleinerte sich das APR in druckbelasteten Proben wihrend der ersten Stunde urn etwa denselben Betrag wie bei Zugbelastung und nlherte sich dann wghrend 15 h Auslagerung in lhnlicher Weise dem urspriinglichen Wert. Das APR des Phosphors war bei gekerbten Proben nach Auslagerung in der NIhe der Kerbwurzel bedeutend kleiner als an weiter entfernten Stellen
1. INTRODUCTION In our previous paper [l], describing about the effect of applied stresses on the intergranular segregation of phosphorus in steel during aging at 773 K, it has been found that the applied stress has an effect not only on the kinetics of intergranular P-segregation, possibly through its effects on the diffusive rate of P in the interiors of grains, but also on the capabilities of grain boundaries to absorb P-atoms. The former may be t Permanent address: Hitachi Research Laboratory, Hitachi Ltd. Saiwaicho, Hitachi, Ibaraki 316. Japan. $ Present address: Mechanical Engineering. Technological University of Nagaoka, Kamitomioka. Aza-Nagamine 1603-1, Nagaoka. Niigata 949-54, Japan.
referred to as the ‘kinetic’ effect of applied stress and the latter as the ‘thermodynamic’ effect. We have made a proposal [2] for the latter effect. It is such that whenever P-atoms are added on or removed from a grain boundary, they will decrease the free energy of the system through their works against the normal traction which acts on the grain boundary under the applied stress, just as atomic particles have been considered to do so in diffusional creep [3,4]. In the previous study, however, the distinction of the thermodynamic effect from kinetic one was unsuccessfully made because we there adopted such specimens that had been unsaturated with P at grain boundaries for the stress aging tests. The objective of this investigation is, therefore, to 1631
1632
SHINODA
AND
NAKAMURA:
PHOSPHORUS
confirm the existence of the thermodynamic effect of applied stress distinctly, and to modify the proposal, in conjunction with the present results. In the present study, P-bearing chromium steel is used as a test material and Auger electron spectroscopy (AES) is used to measure the intergranular P-concentration. The specimens, unlike the ones in the previous report, have been subjected to a long period of aging at the test temperature under no stress prior to the stress aging tests, so that their grain boundaries have been saturated with P. Associated modifications of the model will be treated later in an accompanying paper.
2. EXPERIMENTAL
PROCEDURES
The test material had a chemical composition in mass percentage of 0.30 C, 0.28 Si, 0.49 Mn, 0.050 P, 0.07 S, 2.29 Cr, 0.01 MO, with trace elements less than 0.015, the balancing being Fe. The material was forged into rod-blanks, which were solution-treated at 1473 K for 4 h, followed by oil quenching and tempered at 973 K for 2 h, followed by water quenching. The material thus obtained was, then, isothermally aged at 773 K for 1000 h under no external stress, followed by rapid cooling. The material was subsequently stress aged at 773 K for time ranging from 1 to 1.5h. The stress aging tests were divided into the following three types; (a) tensile loading on smooth specimens, (b) compressive loading on smooth specimens, and (c) tensile loading on notched specimens. The tensile specimens for the test (a) were of either 4.5 or 9 mm in diameter, and the compressive specimen for (b) of 4 mm in diameter. Each of these tensile and compressive specimens for the tests (a) and (b) was subjected to a given constant load, equivalent to the initial applied stress ranging from 10 to 100 MPa, for either 1, 3 or 15 h. The load was applied to each specimen by means of a lever type of creep machine having the capacity of 1 ton (9.8 kN). Before a given stress was applied to a specimen, it was kept at temperature of 773 K, with an accuracy of 1°C. for a few hours in the absence of stress. Immediately after stress aging, the specimens were cooled rapidly. On the other hand, each of the notched specimens for the above cited test (c), whose geometry is schematically shown in Fig. 1, was aged for 3 h under a given constant load, equivalent to either 20, 40 or 100 MPa in the nominal stress on the notch section. The methods of heating, loading and cooling in this case were also the same as those described above. From each specimen thus prepared for the tests (a) and (b), a sample for Auger spectroscopy, of 3.7 mm in diameter with a notch section of 1.9mm in diameter at the center of the length, was made. The longitudinal direction of Auger samples was always remained parallel to the direction of loading during the stress aging. On the contrary, from each notched
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Fig. 1. Schematic illustrations for the geometry of the notched specimen and how to cut out the three Auger samples, A, B and C, from it (Unit in mm).
specimen for the test (c), three Auger samples were made at the three positions: A (the notch vicinity), B (the intermediate) and C (the opposite edge), as shown in Fig. 1. Auger samples thus prepared were fractured at a low temperature (_ 193 K) inside the test chamber of the Auger system, in a vacuum less than 7 x lo-” torr (70nPa). All samples, except some A-samples from the notched specimens, exhibited almost 100% intergranular fracture. The AES analyses are made similarly to that described previously [S]. The typical electron beam diameter was less than lOpm, which was much smaller than typical grain sizes of the order of 1OOpm. With each Auger samples, Auger measurements were made at more than 20 different grain boundary facets, selected over one quarter of the fractured surface of each Auger sample. Namely, in this one quarter of the fractured surface, there were only 20 or so number of facets which was appropriate to the AES analysis in view of the size and orientation of the facets to the electron beam. The grain boundary concentration of P was evaluated from the relative peak height ratio of its Auger peak-to-peak height at 120eV, I,, to that of iron at 703 eV, IFe. This ratio of IdIF, will be referred to as ‘Auger Peak Ratio’ of P (APR of P) or more simply ‘P-APR’ hereafter. The ‘mean’ APR of P, x,, for each Auger sample was defined as
where X,,i is the individual value of P-APR at i-th grain boundary facet, and N the total (maximum) number of measurements. The word ‘mean’ will be used in this paper, as defined above, to specifically indicate the average intergranular P-concentration for an individual Auger sample, unless otherwise noted.
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.27 .26 .25 .2? .26 .25
3
.27
--
B
.26
2
.2f
Y 6
.27 .26 .25 0
IO
20
30
Number of Measurements
t K)
Fig. 2. The scatter of the cumuIative average of P-APR. C X&K, as a function of number of AES measurements, K, of four Auger samples from the same rod.
3. RESULTS
of these four values, 0.258, was used as a most probable initial value of the mean P-APR prior to the stress aging test. As can be seen from this figure, the scatter of the cumulative average of
3.1 The initial value of the meun P-APR and its scattering In order to know the mean P-APR of the specimen which had not yet been subjected to stress aging, that is, to know the initial value of the mean P-APR, the Auger measurements were made on four separate Auger samples machined from the same rod that had only received the preliminary aging for 1000 h under no stress. Figure 2 shows the scatter of a cumulative average of P-APR.
from the level of 0.258 was comparatively large when the number of K was smaller, but it always fell within the -&2.5”/; error band around 0.258, when the number of K was increased to more than 20, for any case of the four samples. Typical Auger electron spectra from one of the above samples prior to stress aging are presented in Fig. 3. Spectrum (1). in this figure. is for the 1st measurement, which was taken at a point on the fractured surface of the sample about a few minutes after it had been broken in a vacuum of about 7 x lo-“torr, and Spectrum (2) for the 21st measurement on the same sample which was done about 2 h later from the fracture, in the same level of
with number of Auger measurements, K, for the four Auger samples described above. The mean APR’s of P. x,, at the maximum number of measurements for these four samples were, as shown in this figure, respectively 0.261, 0.256, 0.257 and 0.256. The average
I
1
I
100
200
t
30
I
,
*
LOO
Sal
600
Electron cmrpy
’
i 700
1
(rV)
Fig. 3. Typical Auger electron spectra from a Auger sample prior to stress aging, under the vacuum level of 7 x lo-” torr. Spectrum (1) is for the 1st measurement, with the value of 1,/I,, being 0.279, and Spectrum (2) for the 21st measurement, with the In/IF, being 0.271.
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&0.257
1 s=o.o41
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P-APR of the specimens aged under compression, to the contrary, first decreased by about 8?/, relative to the initial mean of 0.258 in one hour of aging, but thereafter increased up to the same level as that of tension in 15 h of aging. These mean P-APR’s in 15 h of aging for the stress aged specimens were within an error band of *2.5% around 0.264. This value of 0.264, being another reference level adopted in the present study, was the average of the two mean P-APR’s; the initial mean of 0.258 and the mean of the specimen which was further aged for 15 h under no stress, 0.269. When the applied stress was greater than about 50MPa, the mean P-APR of the smooth specimen varied with aging time in a complicated fashion than when it was in the lower stress cases. Although the data for the higher stress cases were insufficient to draw a definite conclusion, at least the following points might be said; the mean P-APR for the higher compressive stresses varied with aging time as if it were the case of the lower tensile stresses. That for the higher tensile stresses, on the contrary, varied with aging time as if it were the case of the lower compressive stresses. However, in the case of the higher tensile stresses, there was a subsidiary peak at around 1 h of aging whereas in the case of the lower compressive stresses, no such peak was detected.
‘:i_m.itJm0.2
0.3
0401
APRof
a2
03
0.4
P
Fig. 4. Typical histograms of the frequency of occurrence of the APR of P for the specimens aged under various conditions: (a) and (b) for the specimens prior to stress aging, (c) for the one aged for 1 h at 30 MPa of tensile stress, and (d) for the one aged for 1 h at 30 MPa of compressive stress (‘frequency’ represents number of AES measurements).
vacuum. It can be seen from this figure that vacuum levels adopted in the present study are tolerable enough to measure the phosphorus concentrations at more than 20 different points on a specimen fractured .surface, although the surface contamination proceeds somewhat, through oxygen and carbon. even under these circumstances. 3.2 Tensile and compressive stress qficts on smooth specimens Figure 4 shows typical histograms of the frequency of occurrence of the APR of P for the prior to stress aging specimens [Figs 4(a) and (b)], for the specimen aged for 1 h at 30 MPa of tensile stress [Fig. 4(c)], and for the specimen aged for 1 h at 30 MPa of compressive stress [Fig. 4(d)]. In these figures, the value of S shows the standard deviation of x,, which was defined by
3.3 Tensile stress &ects on notched specimens Figure 6 shows the relationship between the mean P-APR of the Auger samples cut from the notched specimens and the positions from which they were cut out. It can be seen from this figure that the mean P-APR at A-position in the vicinity of the notch root was fairly small compared with those at the other two positions, B (the intermediate) and C (the opposite
Tension
0.30
As can be seen from these figures, the histogram for the two prior to stress aging samples resembled each other in its pattern. Compared with these, the histogram for the stress aged samples was quite different; the most populated class of P-APR in each histogram for the stress aged specimens apparently shifted to either a higher or a lower rank relative to that for the prior to stress aging specimens, depending upon the sign of the applied stress. Figure 5 summarizes the changes in the mean P-APR as a function of subsequent aging time for the smooth specimens aged under different magnitudes of tensile or compressive stresses. When the applied stress was than about 40 MPa, the mean P-APR of the specimens aged under tension first increased by about 10% from the initial value of 0.258 during the first 1 h of aging followed by decreasing down to a level close to the initial mean P-APR, in 15 h of aging. The mean
0
6 a 0 4
c
0.3or
1
Aging
Time (h)
Fig. 5. The changes in the mean P-APR as a function of subsequent aging time under various condition of applied stresses for the smooth specimens. The vertical bar shows the +2.5x error band around 0.264.
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:lOO 8
” 1 12
I
PHOSPHORUS
~ 16
Di stance from the Notch (mm) Fig. 6. The relationship tance from the notch
between the mean P-APR root for the stress aged specimens.
and disnotched
edge) positions. The mean P-APR’s at B and C-positions were comparable to or somewhat lower than the reference level of 0.264, above mentioned. Here we should have a word with the mean at A-position. This value represents the mean P-APR, averaged over more than 20 different grain boundary facets within a region very close to the notch root, say a few microns distant from the notch root, rather than the mean P-APR averaged over the whole fracture surface of a standard Auger sample as in the other cases. Figure 7 shows the histogram of the frequency of occurrence of P-APR in this narrow region very close to the notch root, together with those for B and C-positions in the notched specimen. As can be seen from this figure, the type of histogram at A-position was significantly dif-
A P R of P Fig. 7. Histograms of the frequency of occurrence of the APR of P for the samples from the stress aged notched specimens: (a) for A-sample from the specimen aged at 100 MPa. (b). (c) and (d) for A, B and C-samples, respectively. from the specimen aged at 20 MPa.
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ferent from those at the other two positions, which resembled those shown in Fig. 4. Namely, in A-region, the values of P-APR on some grain boundary facets were excessively lowered. and then this resulted in the significant decrease in the mean P-APR for this region. It follows from this that the standard deviation of the P-APR distribution for this narrow region increased by a factor of about two than that for the other Auger samples. as shown in Fig. 7. Even with the fracture appearance, some difference was observed between A-sample and the other ones; some facets in A-fracture region looked like ‘dimple’ whereas no such facet was observed in the fracture surface of the other samples, which was of almost lOO”, intergranular fracture type. Another characteristic of the Auger spectra from this narrow region of A was that Auger peak ratio of oxygen became extremely high on some grain boundary facets. This increase in oxygen concentration in some grain boundary facets seemed to be irrespective of whether the concentration of P on the facets was high or not. However, this enhancement in oxygen concentration was observed only when the applied stress was 20 MPa but never when it was 40 or 100 MPa. 4. DISCUSSION In our previous paper [l]. a mention was made that an error in measuring the mean APR of P was estimated to be approximately k2.5”. when measurements were made repeatedly at least at more than 20 different points on each Auger sample under the same analytical condition. The present results also confirmed that the previous estimation of the percentage error of the mean P-APR being 1_2.5”, was reasonable. Namely, the cumulative average of P-APR for four different Auger samples. as shown in Fig. 2. fell surely into the scatter band of k 2.5”,, of a given mean when the number of measurements were increased to more than 20. Thus we can conclude that the relative error of the mean P-APR was of the order of f2.5”,, under the analytical condition of AES, adopted in the present study. The present data would have some other types of experimental errors, possibly due to the errors in aging temperature or applied stress. For example, as shown in Fig. 5, the subsequent aging for 15 h under no stress seems to have increased the mean P-APR by about 4”;, from its starting value of 0.258. This increase in the mean P-APR seems to have come from the slight difference in aging temperature between the preliminary aging for 1000 h under no stress and subsequent stress aging. Following McLean’s theory of the equilibrium segregation [6], the average temperature during the preliminary aging can be estimated to be higher by about 3 K than that for the subsequent stress aging. In this estimation we used the value of - 16 kcal/mole ( - 67 kJ/mole) as the binding energy of P-atoms to grain boundaries. being a possible one for steel [7]. In addition the accuracy of applied stress
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would be impared to some degree when the applied stress was decreased to the order of 10 MPa, because we used a lever type of creep machine with a comparatively large capacity of load in these experiments. This inaccuracy in applied stress might be responsible in some part for the fact that the mean P-APR for the tensile stress of 10 MPa, as shown in Fig. 5, was unexpectedly small at 3 h of aging. Despite these experimental errors, major parts of the mean P-APR change during stress aging are believed to have been caused by the inherent change in the intergranular P-concentration. This belief is supported firstly be the fact that the observed changes in the mean P-APR were so great that all themselves were unable to be accounted only in terms of the experimental errors, and secondly by the fact that, as shown in Fig. 4, the histograms of the P-APR distribution for the stress aged specimens were quite different from those for the prior to stress aging specimens; the whole family of P-APR values for the stress aged samples had a tendency to shift towards either higher or lower values, depending upon the sign of stress, relative to those for the prior to stress aging samples. These facts demonstrate that the intergranular P-concentration, once having been saturated or in equihbrium prior to the stress aging, certainly varied as a result of the application of stress. It would seem that these changes in the intergranular P-concentration were induced by the thermodynamic effect of stress, earlier mentioned. It is such that the capacity of a grain boundary to absorb P-atoms is altered as a result of the application of stress. Detailed discussion on this matter will be presented later in the next paper. The applied stress effect on the intergranular segregation of impurities such as P in steel seems to be also of practical importance in view of the deterioration of materials under loaded service. Some suggestions on this matter are possibly obtained from the present results. For example, if the enhancement in P-segregation at grain boundaries occurred in a material, loaded to a comparatively small magnitude of stress under the uniform stress regime as in the ordinary service conditions for materials, it would be neglected in practice for the long-term services, because such an applied stress effect would be vivid only in a short period of time after loading, as shown in Fig. 5. When the material contains a region with a sharp stress gradient such as around a crack, some applied stress effect seems to remain even at a long-term service. However, it seems to be such that P-atoms are depleted from the vicinity of a crack tip, as shown in Fig. 6, so that it may be also harmless in practice. The deterioration of materials, however, may be enhanced
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by stress application through quite different mechanisms other than intergranular P-segregation. For example, we saw the occurrence of oxygen enhancement in the notch vicinity grain boundaries (perhaps oxygen atoms would be added from the atmosphere), but its effect on the embrittlement of materials has remained unknown as yet. We must await further studies to clarify such matters.
5. SUMMARY The applid tensile or compressive stress effect on the intergranular P-segregation in a chromium steel aged at 773 K has been studied by means of Auger electron spectroscopy with use of the smooth and notched types of specimens, preliminarily aged for 1000 h in the absence of stress at the same temperature, 773 K. In the case of the smooth specimens subsequently aged at stresses less than about 40 MPa, the mean P-APR increases when the applied stress is tension, or decreases when it is compression, by g-10 percent relative to the initial value during the first 1 h of aging. This relative change is much greater than the percentage experimental error of f2.5% for the mean P-APR. The relative change in the mean P-APR, however, disappears almost completely after 15 h of aging under stress. In the smooth specimens aged at stresses above about 50 MPa, the mean P-APR vs aging time curve appears as if it were the curve for the lower stress but with the opposite sign. In the notched specimens aged under tension, the mean P-APR at the position very close to the notch root is fairly small compared with those at the other positions distant from the notch root along the notch section. This depletion of P-atoms from the grain boundaries very close to the notch root, unlike the case for the stress aged smooth specimens, occurs preferentially in some grain boundaries. Possible explanations for this experimental results will be presented in a next paper.
REFERENCES 1. T. Shinoda and T. Nakamura, Trans. Japan Inst. Metuls 21, 753 (1980). 2. T. Shinoda and T. Nakamura, Trans. Japan Inst. Metals 21, 781 (1980). 3. C. Herrine. Jr. Aonl. Phvs. 21. 437 (1950). 4. R. L. Cob%, Jr. &pl. &s. G, 1679 (1963). 5. T. Nakamura, T. Shinoda and H. Watanabe, Trans. Iron Steel Inst. Japan 19, 365 (1979). 6. D. McLean, Grain Boundaries in Metals. Clarendon Press, Oxford (1967). 7. M. P. Seah, Acta metall. 25, 345 (1977).