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Accelerated creep-fatigue crack propagation in thermally aged type 316 stainless steel
ACCELERATED CREEP-FATIGUE CRACK PROPAGATION IN THERMALLY AGED TYPE 316 STAINLESS STEEL D. J. MKHEL and H. H. SMITH Naval Research Laboratory, Washington, DC 20375 U.S.A.
Abstract-Crack propagation experiments performed on thermally aged 316 stainless steel at 593°C have shown that an acceleration in the rate of crack propagation occurred when extended tension .hold periods were inciuded in the creep-fatigue cycle. The results demonstrate that the onset of accekrated crack propagation was coincident with the linkage of intergranular cavities associated with grain boundary precipitates and was accompanied by a change in failure mode. Analyses of the quantitative values obtained for average cavity spacing, x and size, p, show that the critical conditions necessary for cavity linkas were satisfied when the crack tip opening displaament equaled or exceeded a value given by ~(1 - n. The crack propagation rate was found to revert to the continuous cycling value when the duration of the hold period was reduced. The results suggest that both intergranular precipitates and creep-fatigue cycling must be present to produce the accelerated crack propagation. R&mn6-Des exp&riencea de propagation des fissures darts un a&r inoxydable 316 vieilli B 593°C ont mottttt qu’it se produisait une au&ration de la vitesse de propagation dea fissures torsqu’on introduisait des pMxles prolong&s de maintien en tension dans le cyck fluage-fatigue. Nos rtsultats montrent que le debut de I’aeoMration dc la propagation des fissures ccincide avec le contact des cavitts intergranulaires assoei&s aux precipitts intergranulaires, et qu’il est accompagntpar un changement du mode de rupture. L’analyse dea vakurs experimentales de l’espacement moyen 1 et de la taille moyennc j? des cavites montre que les conditions critiques n&aasaires B la mise en contact des ca_titts sont satisfaites lorsque k d&placement 9 l’extr2mitt de la fissure CgaIe ou dtpasse la valeur a(A - a. La vitesse de propagation des fissures reprend la valeur qu’elle avait lors des cycles continus, lorsqu’on diminue la durot de Ia pertode de maintien. Nos rtsuttats donnent B penser que la prtsena de prccipites intergranulaires et Ie cyefe fluage-fatigue sont tous deux ntcessaires & la propagation aoctltrtt des fissures.
Zusammcnfassung-Expcrimnte zum RiDfortschritt an thermisch gealtertem rostfreien Stahl 316 bei 593°C zeigten, daB der RiDfortschritt beschleunigt war, wenn der Kriech-Ermiidungszyklus verliingerte Haltezeiten bei der Zuglast enthielt. Die Ergebnisse zeigen, daB der Einsatz beschleunigten RiBfortschrittes mit dem Zusammenwachsen intergranularer Hohlraume an den Korngrenzausscheidungen zusammenfkl und mit einem Wechsel in der Bruchmode verbunden war. Die Auswertung der mittleren Abstinde 2 zwisehen den Hohlrilumen und der GrijBe der Hohlriiume p zeigt, daB die fib das Zusammenwachsen notwendige kritisehen Bedingungen erftillt waren, wenn die bffnung der R&p&e 2 c@ - a war. Es wurde gefunden, daB die Ri~~~hwindigkeit zu ihrem Wert bei der kontinuieriichen We~~l~lastung zurtickkchrte, wenn die Dauer der Haitezeiten verkleinert wurde. Diese Ergebnisse legen nahe, daB intergranulare Ausscheidungen und Kriech-Erm~dun~zyklen bei beschleunigtem RiBfortschritt vorhanden sein miissen.
1. INTRODUCI’ION The influence of either cyclic or static loading on the elevated temperature crack propagation behavior of structural materials has been the subject of nu~rous investigations. It is known, however, that many ekvated temperature applications will involve both cyc lit and static loading conditions where combined creep and fatigue conditions will be expected. Recent studies have shown that the combination of creep and fatigue may produce higher rates of crack propagation than when either continuous cycling or static loading conditions are experienced. For Type 316 stainless steel, Michel, Smith and Watson [I] have shown that the addition of a hold time period at the maximum cyclic load produced increased crack propagation rates in both annealed and cold worked 6.~ 2817-k.
material. The increased crack propagation rates were accompanied by a change from a transgranular to an inter~anu~r failure mode with hold time. Similar results have been reported by Shahini~ [a]. At elevated temperatures, it has been shown that austenitic stainless steels are metalhugically unstable. The intergranular precipitation of intermetallic phases is known to influence the fatigue crack propagation rates and failure mode of the austenitic stainless steels. Michel and Smith [33 and James[4] have shown that prior thermal aging reduced the rate of crack propagation during continuous cycling. With the addition of hold times, Michel and Smith [YJ found that thermal aging reduced the magnitude of the increase in crack propagation rate in annealed and cold worked Type 316 steel. The failure mode for the aged steel was found to remain transgranular for 999
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hold times up to I minute rather than to become inter~anular as observed for unaged steel. Therefore, it was hypothes~ed[~J that the effect of the intergranular precipitates was to suppress the transition to an intergranular failure mode by the reduction of grain boundary sliding and cavity formation at intergranular precipitates. The ir&oence of intergranular precipitates on the formation of cavities at elevated temperatures recently has been addressed by Raj 171. Related studies by Min and Raj [83 have suggested that specific stress and time conditions which promote cavity nucleation and that cavity linkage is critically related to the size
and spacing of the precipitates. A specific suggestion has been made by Lloyd [9], on the basis of semiquantitative evidence, that a critical relationship will exist between precipitate size and spacing and crack propagation rate observed in stainless steels under elevated temperature hold time conditions. The purpose of this paper is to present the results from an experimental study designed to investigate the relationship between intergranular precipitates, failure mode, cavity formation, and crack propagation performance of thermally aged Type 316 stainless steel at 593°Cduring creep-fatiguecycling. The results show that when the critical conditions necessary to achieve cavity linkage are achieved, accelerated crack propagation accompanied by a change in faihrre mode was observed. It is shown that both intergranular precipitate particles and combined cyclic and static loading conditions must be present to produce the accelerated crack propagation.
2 RXPRRIMRNTAL PROCRDURRS The chemical composition of the Type 316 stainless steel plate material, the thermal aging treatment, the specimen design, and the experimental procedures have been reported in &tail [3,5J. Material in both the annealed and 20% cold worked conditions was used in this study and, in order to provide a stabilized precipitate distribution during testing, specimen blanks from each material condition were thermally aged for 5000 hours in air at 593°C. Following final basin& the ~ngIe~~notc~ cantikver specimens were cycled under xer~to-tension loadiig using a sawtooth mode to a constant maximum load at 593°C in air. The nominal cyclic frequency was 0.17 Hz for continuous cycling (zero hold time) tests. To examine the relationship between hold time, crack propagation rate and failure mode, the m~imum cyo lit load was hefd constant for sekcted time periods of 1,2,4,8,16 and 32 min before returning to zero load. These hold times were applied sequentially to four specimens where the hold duration was increased or decreased at selected AK values. The loading and unloading rates were constant for all hoId times. Crack length was measured during testing using a traveling microscope. Specimen temperature was
INTERACTION
IN 316 STAINLESS
STEEL
achieved by induction heating, being continuously monitored and controlled to within f3”C. Crack propagation rates, da/dN, were determined from slopes of crack length versus number of cycIes plots and correlated with the sack-tip stress intensity factor range AK, computed according to the previously discussed expression r3.53:
6PL where Y = 1.99(u/W)“2 - 247 (ajW)3/2 + 12.97(a/W)5iz - 23.17 (a/W)‘@ + 24.80(dW)g’z, and P is the maximum load, I, is the distance from the crack plane to the point of load application, a is the total length ofcrack and notch, W is the specimen width, B is the specimen thickness, and BN is the net specimen thickness at the side groovea Corrections for crack tip plasticity were not apphed to the data. AR calculations were performed using a programmable digital csdcuiator according to previously described procedures [S]. The experimental data for da/&V and AK were plotted in Log-Log form for evaluation according to the power law ~~~n~p: dalcrN = CTarsy,
(2)
where C and m are constants which de&be the proportionality and the slope, respectively. It is well known that this relationship describes fatigue crack propagation behavior in metallic. materials over a wide range of temperatures. Conventional procedures were employed to prepare selected areas of the fatigue test specimens for transmission and scanning eke&on mierosoope (IRM and SEM) examination. Thin foils for TRM study were prepared by spark discharge machining from specimen areas adjacent to and outside of the plastic zone. The foils were thinned socording to previously reported techniques [lo] and were examined using a IEMGOOAmicroscope operated at 200 kV. Selected area diffraction and dark 6eld imaging procedures were used to identify the intergranular precipitates. The failure mode of all specimens was examined by sectioning each test specimen to remove the entire fracture surface and approximately 3 mm of adjoining material. In addition, 3 mm diameter discs of specimen material, both immediately adjacent to the hatture surface and from the bulk material, were prepared by e~~o~~~ing for examination of the grain boundary precipitates and separation due to sliding. The SEM examination was performed using a Coates and Welter Cwikscan 106Aoperated at an acceleration voltage of 900 volts. Quantitative measurements of the precipitate size and spacing and the cavity size and spacing were made using a particle size analyzer from SEM miaographs. Additional ~a~~~nts were made from TRM micrographs.
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results from the tests conducted with the zero and 1 min hold times within the factor of two to three predicted from Bow stress argmrxMs[9]. Third the introduction of a hold time of 16min produced an accelerated crack propagation rate which was not observed at the shorter hokl times. ~i~lly, for those tests where the 16 minute hold time was included in the cycle, the crack propagation rate returned to the ~ntinuous cycling value when the hold time was 002ERO A A I MIN reduced from 16 to 8 min. 0 2MIN Similar points to those for the aged, annealed steel X4MtN 0 6 UIN can be made from the results for the aged, 20% cold Q t6 YIN worked Type 3 16 stainkss steel as shown in Fig. 2 for the tests in Table 1. However, for the cold worked test initiated with a hold time of 4min, the initial crack propagation rate was found to be higher than for the test initiated with a 1min hold time. It is noted that the inclusion of a 32 min hold time, in one of the two sequential hold time tests (Table l), continued the accelerated crack propagation rate observed for the 16min hold time. The accelerated crack aviation rate for both the annealed and cold worked material seen in Figs 1 and 2 was accompanied by a change in failure mode from ~~s~~u~ to in~~~u~ for the Fig. 1. Aaekated txwk propagation in sofutioa pr~ly annealed, thermallyaged 3 16 stainless steel tested at 593’C 16min hold time. Figs 3 and 4 illustrate the fractoThe solid points represent results from, tests eomiucted graphic features for the axmeakd and cold worked with constant hold times. The open points mpresentresults specimens tested with sequcntiaxhold times. For both from sequentiat hold time test* material conditions, the fractographs clearly show that the transition to the intergranular failure mode
3.REStJL'IS The effects of sequential hold time cycks on the crack propagation performance of t~mally aged, annealed and WA cold worked Type 316 stainkss steel were studied at 593°C.Figure 1 shows the results for the aged, annealed material tested with hold times of 0, 1,2,4, 8 and 16 minutes from four separate tests (two with sequential hold times) as shown in Table 1, Four important points are indic&ed by the results in Fig. 1. First, as previously noted [3,5J, the crack propagation behavior of material tested with a one minute hold time remained nearly the same as that for material tested with continuous cycling (zero hold time) over the duration of the tests. Second, the crack propagation rates for tests conducted with sequential hold times of 8 min or less are in agreement with the
1 MIN
Table 1. Summary of ExperimentalDetails Microstructural Condition Solution Annealed; Thermally Aged So00 h at 593°C 20% Cold worked: Thermally aged XIOOh at 593°C
Test Specimen Temperature No. (“Cl SO 137 141 143 46
149 151
593 593 593 593 593 593 593
Hold Time (minf 0 1 1,x 1698 4.8, I6,8 1 4,8,16,32 1.24,16
STRESS
INTENSlTf
FhClOf4 RANGE. AK, MPn/Sii
Fig. 2. Accelerated crack propagation in 20% cotd worked, thermally aged 316 stainless steel tested at 593°C. The solid points represent results from a test conducted with a constant hold time. The open points represent results from sequential hold time tests.
Fig. 3. Scanning electron micrographs of the fracture surfaces dew&sped during fat&w crack propagation of solution anneakd, thermally aged 316 ststinless steef tested with sequent@ hotd times at 593°C. The direction of crack propagation is from left ta right in aU miexographa a. 1miu hoid time: b. 2 min hold time: c. 16min bold time: d. 8 min hold time.
also produced a pmmmced tendency for secondary in&granular cracking (Figs 3c and 4c). The reversion to the transgranular failure mode in the annealed specimen, produced by the reduction in hold time from 16 to 8 min is evident from she striations in Fig M. The results from SEM examination of 3 mm diameter discs adjacent to the fracture surface, following electropolishing, are shown in Figs 5 and 6. For the aged, annealed material, Figs Sa and b show the absence of grain boundary cracking and the present: of the inter~an~iar precipitates produced by the thermal aging for the 2 min h&d time region of the speoimen. However, Figs. SCand Sd clearly show the de+& opment of intergranular cavities associated with the grain boundary precipitates in the 16min hold time region of the specimen. In the aged, cold worked specimens, Figs. da and 6b illustrate the cavity formation and linkage in the region which had been tested with an 8 mitt hold time. For the 16 min hoM time region of this specimen. Figs. 6c and 6d further indicate the cavity development and linkage associated with the accelerated crack propagation rate of this
Wd time. The quantitative results for the precipitate size and spacing and cavity size and spacing are given in Table 2 for these specimens where these parameters were determined. It will be noted from Table 2 that in most cases the measured cavity sizes are less than the precipitate diameter This is a m&&on of the tendency for cavity mmlea~ at the grain boundaryprecipitata interface rather than by decohesion at the matrix-precipitate interface. Transmission electron microscopy (TEM) was employed to further study the relationship between the precipitates and cavity formation. Figs 7a and 7b illustrate the precipitate distrrbutions observed in the aged, anneakd and c&d worked Type 316 stainless steel, respectively, immediately adjacent to the fracture surfaoe in the 8 min hold time region. Figs 7c and 7d show the intergranular features for both materials conditions in the 16 min hold time region, Stereo micrographs revealed that many of the light areas adjacent to the grain boundary precipitate particles in Figs 7c and 7d were entirely within the TEM foils. This confirmed the presence of cavities in the 16min hold time regions or the specimen. Similar
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Fig 4. Scanning electron micrographs of the fracture surfaces developed during fatigue crack propagation of 20% cold worked, thermally aged 316 stainless steel tested with sequential hold times at 593°C. The direction of crack propagation is f!roin left to right in all micrographs. a 4 min hold time: b. 8 min hold time: c. 16 min hold time: d. 32 min hold time.
observations were made for the other specimens tested. The predominant precipitate composition was identified as Ml& in all specimens.
4. DISCUSSION The results developed in this study show that accelerated crack propagation in thermally aged Type 3 16 stainless steel at 593°C was achieved under creepfatigue conditions and was accompanied by a change in failure mode. The fractographic results also suggest that the accelerated crack propagation rate was directly associated with the linkage of intergranular cavities associated with the grain boundary precipitates. These points will be discussed in the light of the availabk evidence. As noted previously, Raj [7l has recently discussed the nucleation of grain boundary cavities at precipitates. His analysis indicates that the surface barrier to cavity nucleation will be lowest at the interface between grain boundaries and boundary precipitates with subsequent cavity growth dependent upon applied stress and temperature. This was found to be
the case in the present study (see Table 2). For cyclic loading conditions, grain boundary sliding can produce stress concentrations at the precipitate particles which reduces the time necessary for the onset of cavity nucleation. Subsequent work by Min and Raj [S] to demonstrate the relationship between precipitates, cavities, and crack propagation during fully plastic creep-fatigue cycling suggests that unsymmetrical hold cycles in tension are necessary for cavity nucleation. The interpretation of their results is dependent on the nearly one-to-one relationship between cavities and precipitate particles predicted from surface energy considerations. However, the qualitative or quantitative direct metallographic evidence for such cavity formation is absent from their paper. Based on the quantitative experimental evidence for cavity formation and coalesence in the present study, the critical size and spacing for the onset of accelerated crack propagation can be estimated. If it is taken that the onset of acceleration will occur coincident with linkage of the cavities then the crack propagation rate will be a function of the displacement across the grain boundaries at that point. The relationship
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Fig 5. Scanning electron mierographs of the intergranular fcnturcs in solution mm&d, thermdy aged 316 stainkss steel tested with scqucntirl hold timu at 593’C. a. a&b. Interqanu&r precipitates in the 2 min hold time region: c. and d Intergranular precipitates and avitia in
the 16 min hold time region associated with the aeeelcrated arek propagation.
between sack propagation rate, da/dN, and displacement can be written as:
where E is the Young’s modulus, u, is the yield stress, AK is the stress intensity factor and CTOD is the sack tip opening displacement. For both the annealed and cold worked material& Figs I and 2, the transition to aca!erated crack propagation is men to occur at da/dN values in the range from 1.5 to 2.5~m/qck. Cavity linkage, tkrefcre, should occur when one-half the sack tip displacement (l/2 CLOD) is about 1.5/rm and localized at the grain boundary region. Using the empirically obtained expression relating grain boundary displacement to cavity spacing [I I J.
v, = 0.23fexpG,
4
where )I is the slope ol the da/dN vs AJC plot and V, is the displacement. a value for the cavity spacing, x
can be estimated for the transition
to the accckrated
crack propagation case (16 mm hold time) For the cold worked material (tested with bold times of 4.8 16and32minkn==4andusingV.= l.Slun,avalue of I= 1.7~ is found. Inspection of Table 2 shows that the measured average cavity spacing was 1.25 p for the cold worked material in reasonable agreement with the calculated value from equation (4). A similar calculation for the annealed mat&al, using II = 9 and V, - 1.5cun, yields a value of 1= 3.9~ which is approximately a factor d three larw than the measured value of average cavity spacing in Table 2 for this material. Since equation (4) is strictly applicable only when all deformation is bcalized to the grainboundarits,tkd&fqanqar~thatsucb localization bad not entirely occurmd in the annealed material. Nemtbelers, tk results for both the cold worked and the annealed materials support tk idea that the transition to accelerated sack propagation was associated with cavity finkage when tk cavity spacing was approximately equal to tk gram boundary displacement. Recently, Lloyd [9] has suggested that, when both intergranular crack propagation and grain boundary cavities are present, a situation is reached where the
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Fig 6. Sanning dectnm microgr@s of the intergranuIar features in WA cold worked, thermally aged 316 staiakrr #teaI tatrd with aquaadrl hold timu at 593°C. a. and b. Intergranular cavity formation and linkage ijr the 8 min hold thm re#m: c and d. lntergranularcavity formation and Iinkage in the ldmia Wd time rq~lon mociattd with the aceekmted crack propagation.
onset of au&rated scribed by:
crack propagation tnay be de-
Using the estimated crack tip displacement
(l/2
CrOD) of 1.5pm at onset of tbc acceleration,a pro(9
portionality constant of unity, and the measured values of averagecavity spacingfromTable 2, unrea-
where~isthearaagecavitysim.LbydandWarcing [ 123 have agfpated th8t thi8 txpradoa cm bt
listic (negative) values of average cnvity size are obtained bromexpression(6) for both the cold worked and anne&d material&This suggestseither that the
f-CTOD~(fx-iS),
rewritten as:
#CI’ODsa@-m
(6)
aiticnl amditiom for the onset ofcavitylinkage may have oocmed at somewhat analkr values of l/2
CLOD or that the proportionality constant in exwherea is a positi= numbet to account for the reali- pression(6) is lqpx than unity. This formersuggeszation that strain IocaEmtion at the grain boundaries tion is not supportedby the evaluation of expression (6) using the experimental1 and p results for the 16 will lead to cavity linkages. Table 2. Microstructural Results Precipitates Specimen No.
Hold Time (mid
Spacing Size sue @ml @ml (cmr) max. min. ave. max. min. ave. max. min.
Cavities
ave.
max.
Spacing (ctm) min.
ave.
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minute hold time region, which yields 2 - p Y&CS Of 0.3 to 0.5 m assuming a pro~~ionality constant of unity. The fatter suggestion is realistic since the highly localized nature of the deformation in the presence cavities would lead to a proportionality constant unity or greater than unity. Since the magnitude the proportionality constant in expression (5) must larger than unity, its value may be determined equating the I- p value found from the data
of of of be by in
Table 2.0.3 to 0.5 /.q with the estimated value of l/2 CI’OD of 1.5 m. Doing this, values of from 3 to 5 are found for a, Therefore, the results for both cold worked and annealed materials support the idea that
a critical cavity size and spacing are coincident with the onset of accelerated crack propagation. Furthermore, the results show that both intergranular precipitates and creep-fatigue cycling must be present to produce accelerated crack propagation. It is recognized that a key element in the transition to acceIerated crack propagation is the growth of cavities to the critical size necessary to produce cavity linkage. In concept, both diffusional motion and plastic flow are necessary to produce cavity growth following nucleation at the precipitatograin boundary interface. The criteria for the cavity growth during creep-fatigue crack propagation conditions at elevated temperatures is developed in detail in a separate paper [13]. It is shown that cavity growth will occur during the tension hold portion of the creep-fatigue cycle by the accommodation of the crack tip displacement.
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I. Accelerated crack propa~tion was observed in both annealed and cold worked Type 316 stainless
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steel, thermally aged for 5000 h at 593°C and tested under sequential hold time conditions at 593°C. 2. For both annealed and cold worked conditions accelerated crack propagation was only observed when a hold time of 16 minutes was included in the fatigue cycle. At hold times of 0, 1, 2, 4 and 8 minutes, crack propagation rates consistent with previous studies at 593°C were observed. 3. The transition to accelerated crack propagation rate was accompanied by a change in failure mode from trans~anular to inter~anular in both annealed and cold worked materials. 4. It is concluded that the transition to the accelerated crack propagation rate was coincident with the development of the critical intergranular cavity size and spacing necessary to produce cavity linkage in accord with the relationship l/2 CLOD P a(;i - 3. Acknowledgem~ts-his research was supported by the Office of Naval Research. The experimental assistance of J. T. Atwell and E Woodali is gratefully acknowledged.
REFERENCES 1. D. J. Michel, H. H. Smith and H. E. Watson, Structural Materials for Service at Elevated Temperatures in Nuclear Power Generation, MPC-1, pp. 167-190, American Society of Mechanical Engineers (1975). 2. P. Shahinian, .I. Pres. Vessel Tech, Traris. ASME 98, 166 (1976).
3. D. J. Michel and H. H. Smith, ASME-MPC Symposium on Creep-Fatigue fnreruction, MPC-3, pp. 391-415, American Society of Mechanical Engineers (1976). 4. L. A. James, Metall. Trans S, 831 (1974). 5. D. J. Michel and H. H. Smith, E@ct ofHold Time and Thermal Aging on Elevated Temperature Fatigue Crack Propagation in Austenitic Stainless Steeis, NRL
5. CONCLUSIONS A direct relationship between the linkage of grain boundary cavities and the transition to accelerated crack propagation accompanied by an intergranular failure mode has been demonstrated. It is recognized that both combined cyclic-static loading and intergranular precipitates must be present to produce accelerated crack propagation at the elevated temperature in this study. Specific conclusions drawn from this study are:
IN 316 STAINLESS STEEL
6. 7. 8. 9. 10. 11. 12. 13.
Memorandum Report 3627, Naval Research Laboratory (1977). D. J. Michel and H. H. Smith, J. Eng. Mater. Tech. Trans. ASME, H 99,282 (1977). R. Raj, Acta Metafl. 26, 995 (1978). B. K. Min and R. Raj, Acta Metall. 26, 1007 (1978). G. J. Lloyd, Metal Sci. 13, 39 (1979). D. J. Michel, J. Moteff and A. J. Love& Acta Metall. 21, 1269 (1973). W. Pavinich and R. Raj, Metal!. Trans. (A) 8, 1917 (1977). G. J. Lloyd and J. Wareing, f. Eng, Mater. Tech. Tram ASME, 101, 275 (1979). G. J. Lloyd and D. J. Michel, (to he published).
Abstract-Crack propagation experiments performed on thermally aged 316 stainless steel at 593°C have shown that an acceleration in the rate of crack propagation occurred when extended tension .hold periods were inciuded in the creep-fatigue cycle. The results demonstrate that the onset of accekrated crack propagation was coincident with the linkage of intergranular cavities associated with grain boundary precipitates and was accompanied by a change in failure mode. Analyses of the quantitative values obtained for average cavity spacing, x and size, p, show that the critical conditions necessary for cavity linkas were satisfied when the crack tip opening displaament equaled or exceeded a value given by ~(1 - n. The crack propagation rate was found to revert to the continuous cycling value when the duration of the hold period was reduced. The results suggest that both intergranular precipitates and creep-fatigue cycling must be present to produce the accelerated crack propagation. R&mn6-Des exp&riencea de propagation des fissures darts un a&r inoxydable 316 vieilli B 593°C ont mottttt qu’it se produisait une au&ration de la vitesse de propagation dea fissures torsqu’on introduisait des pMxles prolong&s de maintien en tension dans le cyck fluage-fatigue. Nos rtsultats montrent que le debut de I’aeoMration dc la propagation des fissures ccincide avec le contact des cavitts intergranulaires assoei&s aux precipitts intergranulaires, et qu’il est accompagntpar un changement du mode de rupture. L’analyse dea vakurs experimentales de l’espacement moyen 1 et de la taille moyennc j? des cavites montre que les conditions critiques n&aasaires B la mise en contact des ca_titts sont satisfaites lorsque k d&placement 9 l’extr2mitt de la fissure CgaIe ou dtpasse la valeur a(A - a. La vitesse de propagation des fissures reprend la valeur qu’elle avait lors des cycles continus, lorsqu’on diminue la durot de Ia pertode de maintien. Nos rtsuttats donnent B penser que la prtsena de prccipites intergranulaires et Ie cyefe fluage-fatigue sont tous deux ntcessaires & la propagation aoctltrtt des fissures.
Zusammcnfassung-Expcrimnte zum RiDfortschritt an thermisch gealtertem rostfreien Stahl 316 bei 593°C zeigten, daB der RiDfortschritt beschleunigt war, wenn der Kriech-Ermiidungszyklus verliingerte Haltezeiten bei der Zuglast enthielt. Die Ergebnisse zeigen, daB der Einsatz beschleunigten RiBfortschrittes mit dem Zusammenwachsen intergranularer Hohlraume an den Korngrenzausscheidungen zusammenfkl und mit einem Wechsel in der Bruchmode verbunden war. Die Auswertung der mittleren Abstinde 2 zwisehen den Hohlrilumen und der GrijBe der Hohlriiume p zeigt, daB die fib das Zusammenwachsen notwendige kritisehen Bedingungen erftillt waren, wenn die bffnung der R&p&e 2 c@ - a war. Es wurde gefunden, daB die Ri~~~hwindigkeit zu ihrem Wert bei der kontinuieriichen We~~l~lastung zurtickkchrte, wenn die Dauer der Haitezeiten verkleinert wurde. Diese Ergebnisse legen nahe, daB intergranulare Ausscheidungen und Kriech-Erm~dun~zyklen bei beschleunigtem RiBfortschritt vorhanden sein miissen.
1. INTRODUCI’ION The influence of either cyclic or static loading on the elevated temperature crack propagation behavior of structural materials has been the subject of nu~rous investigations. It is known, however, that many ekvated temperature applications will involve both cyc lit and static loading conditions where combined creep and fatigue conditions will be expected. Recent studies have shown that the combination of creep and fatigue may produce higher rates of crack propagation than when either continuous cycling or static loading conditions are experienced. For Type 316 stainless steel, Michel, Smith and Watson [I] have shown that the addition of a hold time period at the maximum cyclic load produced increased crack propagation rates in both annealed and cold worked 6.~ 2817-k.
material. The increased crack propagation rates were accompanied by a change from a transgranular to an inter~anu~r failure mode with hold time. Similar results have been reported by Shahini~ [a]. At elevated temperatures, it has been shown that austenitic stainless steels are metalhugically unstable. The intergranular precipitation of intermetallic phases is known to influence the fatigue crack propagation rates and failure mode of the austenitic stainless steels. Michel and Smith [33 and James[4] have shown that prior thermal aging reduced the rate of crack propagation during continuous cycling. With the addition of hold times, Michel and Smith [YJ found that thermal aging reduced the magnitude of the increase in crack propagation rate in annealed and cold worked Type 316 steel. The failure mode for the aged steel was found to remain transgranular for 999
loo0
MICHEL
AVIYI SMITH.
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hold times up to I minute rather than to become inter~anular as observed for unaged steel. Therefore, it was hypothes~ed[~J that the effect of the intergranular precipitates was to suppress the transition to an intergranular failure mode by the reduction of grain boundary sliding and cavity formation at intergranular precipitates. The ir&oence of intergranular precipitates on the formation of cavities at elevated temperatures recently has been addressed by Raj 171. Related studies by Min and Raj [83 have suggested that specific stress and time conditions which promote cavity nucleation and that cavity linkage is critically related to the size
and spacing of the precipitates. A specific suggestion has been made by Lloyd [9], on the basis of semiquantitative evidence, that a critical relationship will exist between precipitate size and spacing and crack propagation rate observed in stainless steels under elevated temperature hold time conditions. The purpose of this paper is to present the results from an experimental study designed to investigate the relationship between intergranular precipitates, failure mode, cavity formation, and crack propagation performance of thermally aged Type 316 stainless steel at 593°Cduring creep-fatiguecycling. The results show that when the critical conditions necessary to achieve cavity linkage are achieved, accelerated crack propagation accompanied by a change in faihrre mode was observed. It is shown that both intergranular precipitate particles and combined cyclic and static loading conditions must be present to produce the accelerated crack propagation.
2 RXPRRIMRNTAL PROCRDURRS The chemical composition of the Type 316 stainless steel plate material, the thermal aging treatment, the specimen design, and the experimental procedures have been reported in &tail [3,5J. Material in both the annealed and 20% cold worked conditions was used in this study and, in order to provide a stabilized precipitate distribution during testing, specimen blanks from each material condition were thermally aged for 5000 hours in air at 593°C. Following final basin& the ~ngIe~~notc~ cantikver specimens were cycled under xer~to-tension loadiig using a sawtooth mode to a constant maximum load at 593°C in air. The nominal cyclic frequency was 0.17 Hz for continuous cycling (zero hold time) tests. To examine the relationship between hold time, crack propagation rate and failure mode, the m~imum cyo lit load was hefd constant for sekcted time periods of 1,2,4,8,16 and 32 min before returning to zero load. These hold times were applied sequentially to four specimens where the hold duration was increased or decreased at selected AK values. The loading and unloading rates were constant for all hoId times. Crack length was measured during testing using a traveling microscope. Specimen temperature was
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achieved by induction heating, being continuously monitored and controlled to within f3”C. Crack propagation rates, da/dN, were determined from slopes of crack length versus number of cycIes plots and correlated with the sack-tip stress intensity factor range AK, computed according to the previously discussed expression r3.53:
6PL where Y = 1.99(u/W)“2 - 247 (ajW)3/2 + 12.97(a/W)5iz - 23.17 (a/W)‘@ + 24.80(dW)g’z, and P is the maximum load, I, is the distance from the crack plane to the point of load application, a is the total length ofcrack and notch, W is the specimen width, B is the specimen thickness, and BN is the net specimen thickness at the side groovea Corrections for crack tip plasticity were not apphed to the data. AR calculations were performed using a programmable digital csdcuiator according to previously described procedures [S]. The experimental data for da/&V and AK were plotted in Log-Log form for evaluation according to the power law ~~~n~p: dalcrN = CTarsy,
(2)
where C and m are constants which de&be the proportionality and the slope, respectively. It is well known that this relationship describes fatigue crack propagation behavior in metallic. materials over a wide range of temperatures. Conventional procedures were employed to prepare selected areas of the fatigue test specimens for transmission and scanning eke&on mierosoope (IRM and SEM) examination. Thin foils for TRM study were prepared by spark discharge machining from specimen areas adjacent to and outside of the plastic zone. The foils were thinned socording to previously reported techniques [lo] and were examined using a IEMGOOAmicroscope operated at 200 kV. Selected area diffraction and dark 6eld imaging procedures were used to identify the intergranular precipitates. The failure mode of all specimens was examined by sectioning each test specimen to remove the entire fracture surface and approximately 3 mm of adjoining material. In addition, 3 mm diameter discs of specimen material, both immediately adjacent to the hatture surface and from the bulk material, were prepared by e~~o~~~ing for examination of the grain boundary precipitates and separation due to sliding. The SEM examination was performed using a Coates and Welter Cwikscan 106Aoperated at an acceleration voltage of 900 volts. Quantitative measurements of the precipitate size and spacing and the cavity size and spacing were made using a particle size analyzer from SEM miaographs. Additional ~a~~~nts were made from TRM micrographs.
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results from the tests conducted with the zero and 1 min hold times within the factor of two to three predicted from Bow stress argmrxMs[9]. Third the introduction of a hold time of 16min produced an accelerated crack propagation rate which was not observed at the shorter hokl times. ~i~lly, for those tests where the 16 minute hold time was included in the cycle, the crack propagation rate returned to the ~ntinuous cycling value when the hold time was 002ERO A A I MIN reduced from 16 to 8 min. 0 2MIN Similar points to those for the aged, annealed steel X4MtN 0 6 UIN can be made from the results for the aged, 20% cold Q t6 YIN worked Type 3 16 stainkss steel as shown in Fig. 2 for the tests in Table 1. However, for the cold worked test initiated with a hold time of 4min, the initial crack propagation rate was found to be higher than for the test initiated with a 1min hold time. It is noted that the inclusion of a 32 min hold time, in one of the two sequential hold time tests (Table l), continued the accelerated crack propagation rate observed for the 16min hold time. The accelerated crack aviation rate for both the annealed and cold worked material seen in Figs 1 and 2 was accompanied by a change in failure mode from ~~s~~u~ to in~~~u~ for the Fig. 1. Aaekated txwk propagation in sofutioa pr~ly annealed, thermallyaged 3 16 stainless steel tested at 593’C 16min hold time. Figs 3 and 4 illustrate the fractoThe solid points represent results from, tests eomiucted graphic features for the axmeakd and cold worked with constant hold times. The open points mpresentresults specimens tested with sequcntiaxhold times. For both from sequentiat hold time test* material conditions, the fractographs clearly show that the transition to the intergranular failure mode
3.REStJL'IS The effects of sequential hold time cycks on the crack propagation performance of t~mally aged, annealed and WA cold worked Type 316 stainkss steel were studied at 593°C.Figure 1 shows the results for the aged, annealed material tested with hold times of 0, 1,2,4, 8 and 16 minutes from four separate tests (two with sequential hold times) as shown in Table 1, Four important points are indic&ed by the results in Fig. 1. First, as previously noted [3,5J, the crack propagation behavior of material tested with a one minute hold time remained nearly the same as that for material tested with continuous cycling (zero hold time) over the duration of the tests. Second, the crack propagation rates for tests conducted with sequential hold times of 8 min or less are in agreement with the
1 MIN
Table 1. Summary of ExperimentalDetails Microstructural Condition Solution Annealed; Thermally Aged So00 h at 593°C 20% Cold worked: Thermally aged XIOOh at 593°C
Test Specimen Temperature No. (“Cl SO 137 141 143 46
149 151
593 593 593 593 593 593 593
Hold Time (minf 0 1 1,x 1698 4.8, I6,8 1 4,8,16,32 1.24,16
STRESS
INTENSlTf
FhClOf4 RANGE. AK, MPn/Sii
Fig. 2. Accelerated crack propagation in 20% cotd worked, thermally aged 316 stainless steel tested at 593°C. The solid points represent results from a test conducted with a constant hold time. The open points represent results from sequential hold time tests.
Fig. 3. Scanning electron micrographs of the fracture surfaces dew&sped during fat&w crack propagation of solution anneakd, thermally aged 316 ststinless steef tested with sequent@ hotd times at 593°C. The direction of crack propagation is from left ta right in aU miexographa a. 1miu hoid time: b. 2 min hold time: c. 16min bold time: d. 8 min hold time.
also produced a pmmmced tendency for secondary in&granular cracking (Figs 3c and 4c). The reversion to the transgranular failure mode in the annealed specimen, produced by the reduction in hold time from 16 to 8 min is evident from she striations in Fig M. The results from SEM examination of 3 mm diameter discs adjacent to the fracture surface, following electropolishing, are shown in Figs 5 and 6. For the aged, annealed material, Figs Sa and b show the absence of grain boundary cracking and the present: of the inter~an~iar precipitates produced by the thermal aging for the 2 min h&d time region of the speoimen. However, Figs. SCand Sd clearly show the de+& opment of intergranular cavities associated with the grain boundary precipitates in the 16min hold time region of the specimen. In the aged, cold worked specimens, Figs. da and 6b illustrate the cavity formation and linkage in the region which had been tested with an 8 mitt hold time. For the 16 min hoM time region of this specimen. Figs. 6c and 6d further indicate the cavity development and linkage associated with the accelerated crack propagation rate of this
Wd time. The quantitative results for the precipitate size and spacing and cavity size and spacing are given in Table 2 for these specimens where these parameters were determined. It will be noted from Table 2 that in most cases the measured cavity sizes are less than the precipitate diameter This is a m&&on of the tendency for cavity mmlea~ at the grain boundaryprecipitata interface rather than by decohesion at the matrix-precipitate interface. Transmission electron microscopy (TEM) was employed to further study the relationship between the precipitates and cavity formation. Figs 7a and 7b illustrate the precipitate distrrbutions observed in the aged, anneakd and c&d worked Type 316 stainless steel, respectively, immediately adjacent to the fracture surfaoe in the 8 min hold time region. Figs 7c and 7d show the intergranular features for both materials conditions in the 16 min hold time region, Stereo micrographs revealed that many of the light areas adjacent to the grain boundary precipitate particles in Figs 7c and 7d were entirely within the TEM foils. This confirmed the presence of cavities in the 16min hold time regions or the specimen. Similar
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Fig 4. Scanning electron micrographs of the fracture surfaces developed during fatigue crack propagation of 20% cold worked, thermally aged 316 stainless steel tested with sequential hold times at 593°C. The direction of crack propagation is f!roin left to right in all micrographs. a 4 min hold time: b. 8 min hold time: c. 16 min hold time: d. 32 min hold time.
observations were made for the other specimens tested. The predominant precipitate composition was identified as Ml& in all specimens.
4. DISCUSSION The results developed in this study show that accelerated crack propagation in thermally aged Type 3 16 stainless steel at 593°C was achieved under creepfatigue conditions and was accompanied by a change in failure mode. The fractographic results also suggest that the accelerated crack propagation rate was directly associated with the linkage of intergranular cavities associated with the grain boundary precipitates. These points will be discussed in the light of the availabk evidence. As noted previously, Raj [7l has recently discussed the nucleation of grain boundary cavities at precipitates. His analysis indicates that the surface barrier to cavity nucleation will be lowest at the interface between grain boundaries and boundary precipitates with subsequent cavity growth dependent upon applied stress and temperature. This was found to be
the case in the present study (see Table 2). For cyclic loading conditions, grain boundary sliding can produce stress concentrations at the precipitate particles which reduces the time necessary for the onset of cavity nucleation. Subsequent work by Min and Raj [S] to demonstrate the relationship between precipitates, cavities, and crack propagation during fully plastic creep-fatigue cycling suggests that unsymmetrical hold cycles in tension are necessary for cavity nucleation. The interpretation of their results is dependent on the nearly one-to-one relationship between cavities and precipitate particles predicted from surface energy considerations. However, the qualitative or quantitative direct metallographic evidence for such cavity formation is absent from their paper. Based on the quantitative experimental evidence for cavity formation and coalesence in the present study, the critical size and spacing for the onset of accelerated crack propagation can be estimated. If it is taken that the onset of acceleration will occur coincident with linkage of the cavities then the crack propagation rate will be a function of the displacement across the grain boundaries at that point. The relationship
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Fig 5. Scanning electron mierographs of the intergranular fcnturcs in solution mm&d, thermdy aged 316 stainkss steel tested with scqucntirl hold timu at 593’C. a. a&b. Interqanu&r precipitates in the 2 min hold time region: c. and d Intergranular precipitates and avitia in
the 16 min hold time region associated with the aeeelcrated arek propagation.
between sack propagation rate, da/dN, and displacement can be written as:
where E is the Young’s modulus, u, is the yield stress, AK is the stress intensity factor and CTOD is the sack tip opening displacement. For both the annealed and cold worked material& Figs I and 2, the transition to aca!erated crack propagation is men to occur at da/dN values in the range from 1.5 to 2.5~m/qck. Cavity linkage, tkrefcre, should occur when one-half the sack tip displacement (l/2 CLOD) is about 1.5/rm and localized at the grain boundary region. Using the empirically obtained expression relating grain boundary displacement to cavity spacing [I I J.
v, = 0.23fexpG,
4
where )I is the slope ol the da/dN vs AJC plot and V, is the displacement. a value for the cavity spacing, x
can be estimated for the transition
to the accckrated
crack propagation case (16 mm hold time) For the cold worked material (tested with bold times of 4.8 16and32minkn==4andusingV.= l.Slun,avalue of I= 1.7~ is found. Inspection of Table 2 shows that the measured average cavity spacing was 1.25 p for the cold worked material in reasonable agreement with the calculated value from equation (4). A similar calculation for the annealed mat&al, using II = 9 and V, - 1.5cun, yields a value of 1= 3.9~ which is approximately a factor d three larw than the measured value of average cavity spacing in Table 2 for this material. Since equation (4) is strictly applicable only when all deformation is bcalized to the grainboundarits,tkd&fqanqar~thatsucb localization bad not entirely occurmd in the annealed material. Nemtbelers, tk results for both the cold worked and the annealed materials support tk idea that the transition to accelerated sack propagation was associated with cavity finkage when tk cavity spacing was approximately equal to tk gram boundary displacement. Recently, Lloyd [9] has suggested that, when both intergranular crack propagation and grain boundary cavities are present, a situation is reached where the
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Fig 6. Sanning dectnm microgr@s of the intergranuIar features in WA cold worked, thermally aged 316 staiakrr #teaI tatrd with aquaadrl hold timu at 593°C. a. and b. Intergranular cavity formation and linkage ijr the 8 min hold thm re#m: c and d. lntergranularcavity formation and Iinkage in the ldmia Wd time rq~lon mociattd with the aceekmted crack propagation.
onset of au&rated scribed by:
crack propagation tnay be de-
Using the estimated crack tip displacement
(l/2
CrOD) of 1.5pm at onset of tbc acceleration,a pro(9
portionality constant of unity, and the measured values of averagecavity spacingfromTable 2, unrea-
where~isthearaagecavitysim.LbydandWarcing [ 123 have agfpated th8t thi8 txpradoa cm bt
listic (negative) values of average cnvity size are obtained bromexpression(6) for both the cold worked and anne&d material&This suggestseither that the
f-CTOD~(fx-iS),
rewritten as:
#CI’ODsa@-m
(6)
aiticnl amditiom for the onset ofcavitylinkage may have oocmed at somewhat analkr values of l/2
CLOD or that the proportionality constant in exwherea is a positi= numbet to account for the reali- pression(6) is lqpx than unity. This formersuggeszation that strain IocaEmtion at the grain boundaries tion is not supportedby the evaluation of expression (6) using the experimental1 and p results for the 16 will lead to cavity linkages. Table 2. Microstructural Results Precipitates Specimen No.
Hold Time (mid
Spacing Size sue @ml @ml (cmr) max. min. ave. max. min. ave. max. min.
Cavities
ave.
max.
Spacing (ctm) min.
ave.
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minute hold time region, which yields 2 - p Y&CS Of 0.3 to 0.5 m assuming a pro~~ionality constant of unity. The fatter suggestion is realistic since the highly localized nature of the deformation in the presence cavities would lead to a proportionality constant unity or greater than unity. Since the magnitude the proportionality constant in expression (5) must larger than unity, its value may be determined equating the I- p value found from the data
of of of be by in
Table 2.0.3 to 0.5 /.q with the estimated value of l/2 CI’OD of 1.5 m. Doing this, values of from 3 to 5 are found for a, Therefore, the results for both cold worked and annealed materials support the idea that
a critical cavity size and spacing are coincident with the onset of accelerated crack propagation. Furthermore, the results show that both intergranular precipitates and creep-fatigue cycling must be present to produce accelerated crack propagation. It is recognized that a key element in the transition to acceIerated crack propagation is the growth of cavities to the critical size necessary to produce cavity linkage. In concept, both diffusional motion and plastic flow are necessary to produce cavity growth following nucleation at the precipitatograin boundary interface. The criteria for the cavity growth during creep-fatigue crack propagation conditions at elevated temperatures is developed in detail in a separate paper [13]. It is shown that cavity growth will occur during the tension hold portion of the creep-fatigue cycle by the accommodation of the crack tip displacement.
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I. Accelerated crack propa~tion was observed in both annealed and cold worked Type 316 stainless
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steel, thermally aged for 5000 h at 593°C and tested under sequential hold time conditions at 593°C. 2. For both annealed and cold worked conditions accelerated crack propagation was only observed when a hold time of 16 minutes was included in the fatigue cycle. At hold times of 0, 1, 2, 4 and 8 minutes, crack propagation rates consistent with previous studies at 593°C were observed. 3. The transition to accelerated crack propagation rate was accompanied by a change in failure mode from trans~anular to inter~anular in both annealed and cold worked materials. 4. It is concluded that the transition to the accelerated crack propagation rate was coincident with the development of the critical intergranular cavity size and spacing necessary to produce cavity linkage in accord with the relationship l/2 CLOD P a(;i - 3. Acknowledgem~ts-his research was supported by the Office of Naval Research. The experimental assistance of J. T. Atwell and E Woodali is gratefully acknowledged.
REFERENCES 1. D. J. Michel, H. H. Smith and H. E. Watson, Structural Materials for Service at Elevated Temperatures in Nuclear Power Generation, MPC-1, pp. 167-190, American Society of Mechanical Engineers (1975). 2. P. Shahinian, .I. Pres. Vessel Tech, Traris. ASME 98, 166 (1976).
3. D. J. Michel and H. H. Smith, ASME-MPC Symposium on Creep-Fatigue fnreruction, MPC-3, pp. 391-415, American Society of Mechanical Engineers (1976). 4. L. A. James, Metall. Trans S, 831 (1974). 5. D. J. Michel and H. H. Smith, E@ct ofHold Time and Thermal Aging on Elevated Temperature Fatigue Crack Propagation in Austenitic Stainless Steeis, NRL
5. CONCLUSIONS A direct relationship between the linkage of grain boundary cavities and the transition to accelerated crack propagation accompanied by an intergranular failure mode has been demonstrated. It is recognized that both combined cyclic-static loading and intergranular precipitates must be present to produce accelerated crack propagation at the elevated temperature in this study. Specific conclusions drawn from this study are:
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6. 7. 8. 9. 10. 11. 12. 13.
Memorandum Report 3627, Naval Research Laboratory (1977). D. J. Michel and H. H. Smith, J. Eng. Mater. Tech. Trans. ASME, H 99,282 (1977). R. Raj, Acta Metafl. 26, 995 (1978). B. K. Min and R. Raj, Acta Metall. 26, 1007 (1978). G. J. Lloyd, Metal Sci. 13, 39 (1979). D. J. Michel, J. Moteff and A. J. Love& Acta Metall. 21, 1269 (1973). W. Pavinich and R. Raj, Metal!. Trans. (A) 8, 1917 (1977). G. J. Lloyd and J. Wareing, f. Eng, Mater. Tech. Tram ASME, 101, 275 (1979). G. J. Lloyd and D. J. Michel, (to he published).