Crack Propagation Behavior in Type 304 Stainless Steel Weldments at Elevated Temperature

Transcription

1 Crack Propagation Behavior in Type 304 Stainless Steel Weldments at Elevated Temperature Tests indicate that base metal can be used to predict weld metal behavior under the conditions of this study BY L. A. JAMES ABSTRACT. The fatiguecrack propagation behavior of weldments in stainless steel at 1000 F (538 C) was studied using the techniques of linearelastic fracture mechanics. Three different welding processes were evaluated: gastungstenarc welding (GTAW), submergedarc welding (SAW), and shieldedmetalarc welding (SMAW). In addition, a variety of crack orientations with respect to the weld were studied. For a given crack orientation, no difference was noted in the behavior between the three welding processes. It was also found that, in every welded specimen tested, the crack growth rate was within, or below the scatter band for base metal specimens tested under similar conditions. Therefore, it appears that test results obtained on base metal specimens may be conservatively used to evaluate the structural performance of welded components. Introduction Austenitic stainless steels are being utilized extensively for structural components in many new nuclear reactor designs and also in other high temperature applications. Welding is the joining process most often employed in such designs, and since welds are often the sites for defects from which a crack may propagate in service, knowledge of the fatiguecrack propagation behavior of weldments is essential. This paper uses linearelastic fracture mechanics to examine the fatiguecrack propagation behavior of weldments in AISI stainless steel at 1000 F (538 C). Fracture mechanics principles L. A. JAMES is Senior Research Engineer, Westinghouse Hanford Company, Richland, Washington. have already been extensively employed to characterize the fatiguecrack growth behavior of unwelded austenitic stainless steels, and many parameters are known to influence the crack growth behavior. For example temperature, 1 " 5 cyclic frequency, 4 * loading waveform, 7 cyclic stress ratio, 8 cold work crack orientation, 4 surrounding environment 9 10 alloy type, 3 6 and neutron irradiation" have all been shown to influence the cracking behavior of the austenitic stainless steels under certain conditions. In addition, fracture mechanics parameters have been shown to characterize the crack extension of an austenitic stainless steel under static loadings at elevated temperatures. 12 All of the above studies, however, have investigated the behavior of unwelded specimens, and only two recent studies have been devoted to the behavior of weldments. James 13 has investigated the effects of residual stresses upon the gastungstenarc (GTA) weldments in stainless steel at 75 F (24 C) and 1000 F (538 C) and Shahinian et al 14 have studied the behavior of submergedarc (SA) weldments in Types 304 and 316 stainless steels over the temperature range F (25593 C). The present study expands upon those of References 13 and 14 by examining the behavior of not only GTAW and SAW processes, but also the SMAW process, and a number of crack orientations relative to the weldment. Experimental Procedure The crack growth behavior of weldments produced by the three above mentioned welding processes was evaluated in the present study. In all three cases, the base metal was Type 304 stainless steel, and the filler metal was stainless steel. The chemical compositions of the three heats of base metal and the three heats of filler metal are given in Table 1, and the room temperature mechanical properties are listed in Table 2. The GTA welds were produced at the author's laboratory, while the SA and SMA welds were produced by Combustion Engineering, Inc. Plate 0.5 in. thick was used for the GTA welds, while 1.88 in. and 2.38 in. plates were used for the SA and SM/ 5 welds, respectively. All welds were of the doublevee groove type. The finished weldments were ground flat and the specimens were located such that a variety of orientations of the crack, relative to the weld, were obtained. A twoletter orientation system, similar in concept to that adopted by the ASTM, 15 was used. In this system the first letter indicates the direction of the applied loading, while the second letter denotes the direction of crack extension. In the present study, "A" is the direction of welding, "B" is normal to the direction of welding, and "C" is the thickness direction. The ASTM"Compact Specimen" 15 was utilized in this investigation. The specimen configuration is shown in Fig. 1, and the specimen dimensions and major test parameters are given in Table 3. It will be noted that three different specimen sizes (i.e. three different specimen widths "W") were used. Prior to testing, all specimens were precracked at room temperature at load levels below the test load. The specimens were fatigue cycled on an MTS feedbackcontrolled electrohydraulic testing machine using load as the control parameter. The temperature, cyclic frequency, cyclic stress ratio, and loading waveform were held constant for all tests: the temperature was 1000F (538 C), the frequency was 40 WELDING RESEARCH SUPPLEMENT! 173s

2 Table 1 Chemical Composition of Base and Filler Metals Used in This Study (wt. %) Material Producer/ heat No Arcos/ Y1340T Raco/ G.O.Carlson/ A Comb. Engr/ JAFA C Mn P Si Cr Ni Cu Mo Pb Ca Sn Ti V Ta Cb C < < < < / (a).01 (a) r (a) Cb + Ta cpm (0.667 Hz), the stress ratio R (R = K mm /K max) was 0.05,and a sawtooth loading waveform was used. The specimens were heated in an air circulating furnace, and the temperature was controlled to within ± 2 F (± 1 C). Testing was interrupted periodically to make readings of the crack length. The readings were made optically using a traveling microscope, and the number of loading cycles was noted for each crack length. The fatiguecrack growth rate, da/dn. was calculated by dividing each increment of crack extension, da. by the number of loading cycles producing that extension, AN. The stress intensity factor, K, was based on the average crack length for each increment, and the equation given in Reference 15 was used to perform this calculation. Fatiguecrack growth rates were then plotted as a function of the stress intensity factor. Results and Discussion The results for the tests conducted on base metal specimens are shown in Fig. 2. An approximately linear relationship is produced between log(da/dn) and log(ak), and this is in accordance with the simple power law relationship of Paris and Erdogan 16 _* = C(AK)" dn where AK = K m ax (1R) C,n = constants for a given material/environment combination Scatter bands are drawn through the data, and these scatter bands for the base metal behavior will be used as a basis of comparison for the results on welded specimens. It will be noted that the results for the three different heats of base metal fall within a common scatter band, indicating little or 174s APRIL 1973 no heattoheat variation. The test results for the GTA welded specimens are shown in Fig. 3, the SA specimens in Figs. 49, and the SMA welded specimens in Figs As previously mentioned, a number of orientations of the crack, relative to the weld, were examined. Therefore, in some specimens the crack would propagate from the base metal, through the heataffected zone, and into the weld metal, while in other tests the propagation would remain entirely within the weld metal. (One specimen, no. 14, was essentially a base metal specimen for, although it contained a weld, the crack propagation was entirely within the base metal.) A number of observations may be made concerning the behavior of the welded specimens shown in Figs First, in a number of the weldment tests, there is more apparent "scatter" of the data relative to base metal results. This is consistent with the observations of Shahinian et al 14 for s in stainless steel tested at 1100 F (593 C), and this would seem to indicate that the crack extension behavior in weldments is somewhat more discontinuous in nature. Also, some of the "scatter" in the results for the AB orientation (see Figs. 4 and 10) is probably due to the influence of the residual stresses produced during the welding operation James 13 showed that the residual stresses parallel to the direction of welding had a dramatic influence upon the crack propagation behavior of specimens with an AB orientation, and in the type of specimen employed in the present study, such differences in behavior as the crack extends would most likely manifest themselves as "scatter." The same residual stresses could also influence specimens with an AC orientation, and this may in part explain the dramatic slope transition exhibited by specimen no. 10 (see Fig. 5). A second observation is that, for a given crack orientation, there appears to be little or no difference in fatigue behavior between the three welding processes. For example, comparing the results for the orientation for the GTAW, SAW and SMAW processes (Figs. 3, 6, and 11, respectively) reveals essentially identical behavior. A third, and perhaps the most important, observation, is that in every welded specimen tested for each of the three processes, the fatiguecrack growth rate for the weldment was within or below the scatter band for base metal tested under similar conditions. This is consistent with the observations of Shahinian et al 14 who tested s in Types 304 and 316 stainless steels over the temperature range F (24593 C). Observations of crack growth rates in weldments being equal to, or lower than, those in base metal have been reported by other investigators for a wide variety of materials and temperatures: Popp and Coles 19 for GTA welds in Inconel 718 at 1000 F (538 C), Clark 20 for s in ASTM A533B steel over the range F (24288 C), and Bucci et al 21 for 5Ni steel tested at 75 F (24 C) and 260 F (162 C). The implication that may be drawn from the above discussion is that, at least for the case of the austenitic stainless steels at elevated temperatures, fatiguecrack propagation data gathered on base metal may be used to analyze either welded or unwelded components, and in the former case the result should be somewhat conservative. This statement is strictly true only for the materials and test conditions examined in this study and in Reference 14. Thermal aging, different frequencies, tensile holdtimes or other parameters might in

3 B 07 N Total rare earths <001 Ferrite Application Base metal for GTA Filler metal for GTA Base metal forsa Filler metal for SA Base metal for SMA Filler metal for SMA fluence the cracking behavior. This is presently being investigated in the author's laboratory, and will be the subject of a later publication. The microstructure of the weld is considerably different from that of the base metal, the former consisting of a fine twophase delta ferriteaustenite structure, while the latter is fully austenitic. It is quite possible that the finer weld microstructure with its many interfaces offers a greater resistance to the extension of fatigue cracks than does the coarser microstructure of the base metal. Figures 13 and 14 illustrate typical crack extension behavior on the microscopic scale. Figure 13 shows an while Fig. 14 shows an SMA weld, and the difference in delta ferrite content in the welds is evident (12.8% for SA and 7.1% for SMA). Although it is suspected that the presence of the secondphase delta ferrite influenced the crack growth behavior, the similarity in test results between the three welding processes suggests that the behavior was not overly sensitive to differences in ferrite content. Examination of Fig. 13 and 14 reveals a slight tendency for the crack to propagate preferentially through or near the secondphase particles. This is in agreement with the observations of Goodwin et al" who found that, in austenitic stainless steel stress rupture specimens, the fracture path almost exclusively followed the ferriteaustenite boundaries. Figure 14(a) illustrates a region of the crack where crack branching took place. Crack branching to a minor degree has been noted in previous studies of austenitic stainless steel base metal fatigue tested at elevated temperatures, 4 7 but there appears to be a stronger tendency for branching to occur in the weldment specimens. (Perhaps this could also help account for some o'f the greater "scatter" in the data noted in some cases.) It is interesting to note the cracked areas in Figure 14(a) that failed to etch during electroetching. Apparently current could not be conducted to these areas, and this suggests that some of the "branched" cracks were actually connected within the interior of the specimen. Excessive crack branching and/or deviation of the crack from the normal propagation direction was noted in specimens with a CB orientation in both SA and SMA weld specimens. Figure 15 illustrates this branching. Branching of this sort is at such a scale that the simple stress intensity factor solution of Reference 15 probably does not accurately represent the stress distribution in the cracktip vicinity. Data for crack growth with such gross branching is reported, but it is designated differently (see Figs. 9 and 12), and is not considered valid. This sort of branching was noted only in specimens with a CB orientation, and although the exact reason for the branching is unknown, it is possible that it was caused by strain constraints due to the weld geometry, or possibly due to residual stresses. Concluding Remarks The findings of this study may be summarized as follows: 1. For a given crack orientation, there appears to be little difference in the elevated temperature fatigue behavior between the GTAW, SAW and SMAW processes. 2. In every welded specimen tested for each of the three welding processes, the crack growth rate was within, or lower than, the scatter band for base metal specimens tested under similar conditions. This suggests that for the materials and test conditions described in this study, test results obtained on base metal specimens may be conservatively used to evaluate the structural performance of welded components. 3. The reason for the lower growth rates in welded specimens is unknown, but is possibly due to the fine duplex delta ferriteaustenite structure of the weld deposit. Apparently \ B t W 1.?ow j 0.39 W k 1,1 II i i II i l l II S+s, s 0.25 W SYM > 0 a j W 1 ~ w > Fig. 1 Fatigue Test specimen. See Table 3 for dimensions "B" and "W" Table 2 Room Temperature Mechanical Properties Material Producer/ heat no. 0.2% Y.S., T.S., psi psi (kg/mm (kg/mm 2 ) Elong., % R.A.. BHN Grain size ASTM Application Arcos/ Y1340T Raco/ G.O.Carlson/ A Comb. Engr/ JAFA (27.84) 56,900 (40.01) 39,350 (27.67) 37,000 (26.01) 57,000 (40.08) 77,050 (54.17) 88,200 (62.01) 77,000 (54.14) 82,000 (57.65) 82,800 (58.22) Base metal (GTAW) Filler metal (GTAW) 34 Base metal (SAW) Filler metal (SAW) Base metal (SMAW) Filler metal (SMAW) WELDING RESEARCH SUPPLEMENT! 175s

4 Table 3 Specimen Dimensions and Major Test Parameters Specimen number Material <a) ' B.M. (GTAW) B.M.(GTAW) B.M. (GTAW) B.M. (GTAW) B.M. (SAW) B.M. (SMAW) GTA weld GTA weld SAweld SAweld SMA weld SMA weld SMA weld Weld orient. AB AC BC CA CB AB CB "W," in (a) B.M. indicates base metal; other designations indicate weld metal. "B," in Load, lb STRESS INTENSITY FACTOR RANGE.AK. tg/!mm 3'2 0 NO. 1 D NO. 2 A NO. 3 V NO. d ipj SE METAL FOR GTA WELDS O NO. 5. SE METAL FOR SA WELDS O NO. 6. SE METAL FOR SMA WELDS STRESS INTENSITY ^ACTOR RANGE. AK. STRESS INTENSITY FACTOR RANGE, AK, LB/UN.I 3 '* MN/lmj3'2 Fig. 2 Fatiguecrack propagation behavior of stainless steel base metal specimens tested at 1000 F STRESS INTENSITY FACTOR RANGE, AK, kg/lthm! 3 ' 2 10? STRESS INTENSITY FACTOR RANGE, AK, kg/lmml 3 ' 2 SPECIMEN NO.7 ORIENTATION SPECIMEN NO. 8 ORIENTATION SPECIMEN NO. 9 AB ORIENTATION,3 5 O SPECIMEN 7 D SPECIMEN 8 TYPE 304 GAS TUNGSTEN ARC WELDMENTS 1000 F (538 CI, 40CPM, R0.05 J FILLER 1000 F (538 CI, 40CPM, R0.05 STRESS INTENSITY FACTOR RANGE, AK, LB/UN.I 3 ' 2 _L STRESS INTENSITY FACTOR RANGE, AK. LB/ON.P' 2 IO 1 STRESS INTENSITY FACTOR RANGE, AK, MN/lmt 3 ' 2 Fig. 3 Fatiguecrack propagation behavior of GTA weldments ( orientation) tested at 1000 F. Shaded area represents Fig. 4 Fatiguecrack propagation behavior of SA weldments (AB orientation) tested at 1000 F. Shaded area represents the many second phase particles retard crack extension. 176s I APRIL The welded specimens exhibited a stronger tendency for crack branching than did the base metal specimens. This was especially apparent in certain crack orientations. The formation of "extra" crack surfaces requires additional energy, and this could also help account for the generally lower rates of crack propagation observed in welded specimens. Acknowledgement This paper is based on work performed under United States Atomic Energy Commission contract AT(451) 1270 with the Westinghouse Hanford Company, a subsidiary of Westinghouse Electric Corporation. References 1. James, L. A. and Schwenk, E. B., "FatigueCrack Propagation Behavior of Stainless Steel at Elevated Temperatures." Metallurgical Transactions, Vol. 2, No. 2, pp , Shahinian, P., Smith, H. H. and Watson, H. E., "Fatigue Crack Growth in Type 316 Stainless Steel at High Temperature,"

5 STRESS INTENSITY FACTOR RANGE, AK, kg/lmml 3 ' 2 STRESS INTENSITY FACTOR RANGE. AK, kg/lmml 3 ' 2 "3 a ic< SPECIMEN NO. 10 AC ORIENTATION 10 r 1000 F 1538 CI, 40 CPM. R0.05 STRESS INTENSITY FACTOR RANGE, AK, LB/IIN.! 3 ' 2 10 "L TYPE 304 SUBMERGED ARC WELDMEN 10O0 F (538 C;. 40CPM R0.05 1? IO 5 STRESS INTENSITY FACTOR RANGE AK, LB/IIN.F 3 ' 2 IO 1 10' STRESS INTENSITY FACTOR SANGE, AK. MN/lml 3 ' 2 STRESS INTENSITY FACTOR RANGE. AK, MN/lmr jo 2 Fig. 5 Fatiguecrack propagation behavior of SA weldments (AC orientation) tested at 1000 F. Shaded area represents Fig. 6 Fatiguecrack propagation behavior of SA weldments ( orientation) tested at 1000 F. Shaded area represents "Journal of Engineering For Industry, Trans. ASME, Series B, Vol. 93, No. 4, pp , James. L. A.. "The Effect of Elevated Temperature Upon the FatigueCrack Propagation Behavior of Two Austenitic Stainless Steels," Mechanical Behavior of Materials, Vol. Ill, pp , The Society of Materials Science, Japan, 4. James. L.A.. "FatigueCrack Growth in 20% Cold Worked Type 316 Stainless Steel at Elevated Temperatures." Nuclear Technology, Vol. 16, No. 1, pp , 5. Shahinian, P., Smith, H. H. and Wat STRESS INTENSITY FACTOR RANGE, AK, kgmmmi 3 ' 2 STRESS INTENSITY FACTOR RANGE, AK, kg/lmml 3 ' 2 S SPECIMEN NO. 13 EC ORIENTATION,3 5 SPECIMEN NO. 14 CA ORIENTATION OL 10 5 YPE 308 FILLER 1000 F (538 CI, 40 CPM, R0.05 STRESS INTENSITY FACTOR RANGE, AK, LB/UN.' 3 ' 2 10> 1000 F (538 CI, 40 CPM, R0.05 ^ i I 10* ID 5 STRESS INTENSITY FACTOR RANGE, AK, LB/IIN.I 3 ' : STRESS INTENSITY FACTOR RANGE, AK. MN/lml 3 ' 2 Fig. 7 Fatiguecrack propagation behavior of SA weldments (BC orientation) tested at 1000 F. Shaded area represents Fig. 8 Fatiguecrack propagation behavior of SA weldments (CA orientation) tested at 1000 F. Shaded area represents WELDING RESEARCH SUPPLEMENT! 177s

6 STRESS INTENSITY FACTOR RANGE, AK, kg/lmml 3 ' 2 10 STRESS INTENSITY FACTOR RANGE, AK, kg/lmm, 3 ' 2 ~" 1 l,, SPECIMEN NO. 15 CB ORIENTATION,3 m.10 SPECIMEN NO. 16 AB ORIENTATION 3 5 o=10 CRACK DEVIATED FROM NORMAL DIRECTION V 3.4S 1000 F (538 C), 40 CPM. R0.05 L io 4 io 5 STRESS INTENSITY FACTOR RANGE, AK, LB/IIN.I 3 ' 2 TYPE 304 SHIELDED METAL ARC WELDMENT 1000 F (538 C), 40CPM, R0.05 I io 4 lip STRESS INTENSITY FACTOR RANGE, AK, LB/IIN.) 3 ' 2. I IO 1 Fig. 9 Fatiguecrack propagation behavior of SA weldments (CB orientation) tested at 1000 F. Shaded area represents IO 1 10' Fig. 10 Fatiguecrack propagation behavior of SMA weldments (AB orientation) tested at 1000 F. Shaded area represents STRESS INTENSITY FACTOR RANGE, AK, kg/lmm) 3 ' 2 ID 2 STRESS INTENSITY FACTOR RANGE' AK kg/lmml 3 ' 2, n " 2 "io" i 1 SPECIMEN NO. 17 ORIENTATION 3 g SPECIMEN NO. 18 CB ORIENTATION CRACK DEVIATED FROM NORMAL DIRECTION PS M : io " TYPE 304 SHIELDED METAL ARC WELDMENT 1000 F 638 Ci, 40 CPM, R0.05 iio' TYPE 304 SHIELDED METAL ARC WELDMENTS 1000 F (538 CI. 40 CPM, R0.05 STRESS INTENSITY FACTOR RANGE. AK. LB/IIN.F 3 ' 2 IO 4 STRESS INTENSIT> tactor RANGF. AK LB/IIN.' IO 1 STRESS INTENSITY FACTOR RANGE. AK, MN/(m 3'2 Fig. 11 Fatiguecrack propagation behavior of SMA weldments ( orientation) tested at 1000 F. Shaded area represents STRESS INTENSITY FACTOR RANG; AK MN/ImP' Fig. 12 Fatiguecrack propagation behavior of SMA weldments (CB orientation) tested at 1000 F. Shaded area represents son, H. E., "Fatigue Crack Growth Characteristics of Several Austenitic Stainless Steels at High Temperatures," Fatigue at Elevated Temperatures, ASTM STP520, in press. 178s I APRIL James, L. A., "The Effect of Frequency upon the FatigueCrack Growth of Stainless Steel at 1000 F," Stress Analysis and Growth of Cracks, Part I, pp , ASTM STP513, 7. James, L. A., "HoldTime Effects on the Elevated Temperature FatigueCrack Propagation of Stainless Steel," Nuclear Technology, Vol. 16, No. 3, pp ,

7 8. James, L. A., "The Effect of Stress Ratio on the Elevated Temperature FatigueCrack Propagation of Stainless Steel," Nuclear Technology, Vol. 14. No. 2, pp , 9. Shahinian, P., Watson, H. E. and Smith. H. H., "Fatigue Crack Growth in Selected Alloys for Reactor Applications," Journal of Materials, Vol. 7, No. 4, pp , 10. Wheeler, K. R. and James, L. A., "Fatigue Behavior of Type 316 Stainless Steel under Simulated Body Conditions," Journal of Biomedical Materials Research, Vol. 5, No. 3, pp , Shahinian, P., Watson, H. E. and Smith. H. H., "Effect of Neutron Irradiation on Fatigue Crack Propagation in Types 304 and 316 Stainless Steels at High Temperature," ASTM Sixth International Symposium on Effects of Radiation on Structural Materials, Los Angeles, June 12. James, L. A., "Some Preliminary Observations on the Extension of Cracks under Static Loadings at Elevated Temperature," International Journal Of Fracture Mechanics, Vol. 8, No. 3, pp , 13. James, L. A., "FatigueCrack Growth in Stainless Steel Weldments at Elevated Temperature," Jrnl. of Testing and Evaluation, Vol. 1, No. 1, pp. 5257, Shahinian, P., Smith, H. H. and Hawthorne, J. R., "FatigueCrack Propagation in Stainless Steel Weldments at High Temperatures," Welding Journal Res. Suppl., Vol. 51, No. 11, pp. 527s532s, 15. "PlaneStrain Fracture Toughness of Metallic Materials, E39972," 1972 Annual Book of ASTM Standards, Part 31, pp , American Society for Testing and Materials, 16. Paris, P. and Erdogan, F., "A Critical Analysis of Crack Propagation Laws," Journal of Basic Engineering, Trans. ASME, Series D, Vol. 85, No. 4, pp , Nagaraja Rao, N. R. and Tall, L., "Residual Stresses in Welded Plates," Welding Journal Res. Suppl., Vol. 40, No. 10. pp. 468s480s, Tall, L., "Residual Stresses in Welded Plates A Theoretical Study," Welding Journal Res. Suppl., Vol. 43, No. 1, pp. 10s23s, Popp, H. G. and Coles, A., "Subcritical Crack Growth Criteria for Inconel 718 at Elevated Temperatures," Proc. Air Force Conf. of Fatigue and Fracture of Aircraft Structures and Materials, H. A. Wood et al (Eds.), AFFDL TR 70144, pp. 7186, Clark, W. G., Jr., "Fatigue Crack Growth Characteristics of Heavy Section ASTM A533 Grade B, Class 1 Steel Weldments." ASME paper 70PVP24, Bucci, F. J., Greene, B. N. and Paris. P. C, "Fatigue Crack Propagation and Fracture Toughness of 5 Nickel and 9 Nickel Steels at Cryogenic Temperatures," Sixth National Symposium on Fracture Mechanics, Philadelphia, August 22. Goodwin, G. M., Cole, N. C. and Slaughter, G. M., "A Study of Ferrite Morphology in Austenitic Stainless Steel Weldments," Welding Journal Res. Suppl., Vol. 51, No. 9, pp. 425s429s, Fig. 13 Crack tip region of specimen 12. in the orientation 0.1 mm«[* 0.1 mmj U &m X^ Fig. 14 (a) A portion of the crack tn specimen 17. SMA weld in the orientation, (b) Crack tip region of specimen 17. SMA weld in the orientation. J 0 5 U_ i mm i Fig. 15 Crack tip region of specimen 18. SMA weld tn the CB orientation»»: / WELDING RESEARCH SUPPLEMENT! 179s