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An analysis of creep damage in a welded low alloy steel rotor
MATERIALS SGlEMGEdk ENGlWEERlWG Materials Science and Engineering A234-236
A
(1997) 5555558
An analysis of creep damage in a welded low alloy steel rotor J.E. Indacochea *, R.A. Seshadri University
of Illinois
at Chicago,
Civil and Materials
Engineering
Department
(mc 246),
842 W. Taylor,
Chicago,
IL 60607-7023,
USA
Received 3 February 1997; received in revised form 4 April 1997; accepted 7 April 1997
Abstract An assessment of commonly used creep-rupture extrapolation techniques on weldments was undertaken. Gas tungsten arc and submerged arc welds were fabricated using l.OCr-lMo-0.25V rotor steel as base metal. Crossweld samples, which includes base metal, heat-affected zone and weld metal, were machined and submitted to isostress creep-rupture tests at 83.0 MPa between 690 and 620°C. Larson-Miller and Monkman-Grant plots were produced from these data; the parameters associated with the Larson-Miller method were estimated and extrapolations were done to the service temperatures. The actual lives of the submerged arc crosswelds were shorter than predicted. Monkman-Grant predictions were more conservative than LarsonMiller’s for the gas tungsten arc welds. The discrepancies are attributed in part to the heterogeneous microstructures in a crossweldment leading to different creep rates. Application of extrapolation methods to weldments should be done prudently; the heterogeneous microstructure of the heat-affected zone makes them less reliable. 0 1997 Elsevier Science S.A. Keywords:
CrMoV crosswelds; Isostress-rupture tests
1. Introduction CrMoV steelsare used in high temperature and high stress sections of power plant members [l-3]. Their good creep resistance is impaired by welding done during fabrication of assemblies and weld repair of service damaged rotors. Hence, evaluation of the creep service performance of weld repaired rotors becomes necessary. Due to time and resource constraints, creep tests are conducted at higher temperatures, so the service life is estimated by extrapolation. Creep failure is related to cracking at grain boundary triple points or to cavities nucleating on sulfides [4] and carbides [5,6]. Chen et al. [5] found that all detectable sulfides appear to nucleate cavities, and did not seeinterfacial cracking around carbides. Myers et al. [7] pointed that the idea of cavities solely nucleating at grain boundary sulfide particles is incompatible with cavity nucleation data; the MnS number is too low to justify alone the cavity nucleation rate. Several extrapolation methods have been proposed [8-l 11, but none give an accurate life prediction since they do not consider microstructure changes that occur *Corresponding author. Tel.: + 1 312 9965283; fax: + I 312 9962426; e-mail: [email protected] 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PZZ SO921-5093(97)00273-6
due to weld thermal cycle or exposure to test or operating temperatures. Original carbides change in composition and morphology due to thermal cycles. This work compares the life predictions for gas tungsten arc weld (GTAW) and submerged-arc crosswelds (SAW) of lCr-lMo-0.25V steel using Larson-Miller [8] and Monkman-Grant [ll] methods and explains the deviations between predicted and actual lives by relating it to the aging of carbides and microstructure of the heat-affected zone.
2. Experimental procedure Welds were fabricated using coupons machined from a retired rotor and a full scale rotor. The base metal was an ASTM A470, Class 8 HP/IP rotor (0.3l%C, l.O4%Cr, l.l4%Mo, 0.25%V, O.ll%Ni, 0.75%Mn, O.l8%Si, O.O16%P and O.O12%S). The filler metal: O.l2%C, 2.48%Cr, 0.98%Mo, 0.26%V, 0.56%Mn, 0. lO%Si, O.OlO%P, O.O03%S. The coupon welds were multipass GTAW. Two narrow-groove multipass welds were made at two locations in the full scale rotor with GTAW and SAW procedures. Crossweld test samples from the coupons and the full scale rotor were isostress rupture tested at 83 MPa between 580 and 690°C.
556
J.E. Indacochea,
R.A.
Seshadri
/ Materials
6.76
9
4.26
;
3.6
2.75
2
126 0.63
0.0
0.62
0.64
(l/T) Fig. 1. Larson-Miller and SAW welds.
extrapolation
0.36
x 1000,
0.68
0.7
0.72
v?-’
to service conditions
for GTAW
Creep strains were measured; samples were metallographically examined and the fracture surfaces inspected in the SEM. Selective transmission electron microscopy and energy dispersive X-ray analysis of carbides was performed.
3. Results and discussion 3.1. Application of extrapolation weldment life prediction
techniques for
Most studies [8-111 on extrapolation techniques use metals of uniform microstructure. Our study is on crosswelds of different microstructures between the weld metal and base metal. The rupture times of GTAW-crossweld samples from the full scale rotor weld were plotted versus the inverse of the temperature, following the Larson-Miller (L-M) equation (Fig. 1). Table Creep Heat 720 770 790 840 890
1 properties treatment
of the subzones temperature
(“C)
of HAZ
at 593”C/207 Time 934.6 137.5 102.0 154.7 407.2
MPa
to rupture
Science
and Engineering
A234-236
(1997)
555-558
The slope of the line is the L-M parameter and equals 33 100. The constant C was extrapolated to 18.0. This value falls within the range expected for these steels, 15-25 [12]. The extrapolation of the rupture life to temperatures of about 538 and 510°C yielded times of 4.6 and 29.3 years. A similar L-M plot for the SAW welds yielded a L-M parameter of 25 895 and 13.4 for C. The rupture life extrapolations yielded 2.5 and 10.6 years at 538 and 510°C respectively. GTAW and SAW crosswelds of the full scale rotor, were also tested at 580°C and 83.0 MPA. Based on the L-M plots a life of 7890 h was estimated for the SAW crossweld and 5208 h for the GTAW crossweld; actual lives of 3207 h and 5145 h were obtained for the SAW and GTAW. The Monkman-Grant technique was applied to the creep rupture data of the GTAW-crosswelds. A life prediction of 3088 h was made at 580°C which underestimated the actual life. Note that this plot is a log-log plot, so that any subtle variations in creep rates will not lead to significant changes in predicted rupture life; further, this method was not meant to be an extrapolation technique. The differences found between predicted and actual lives are not unexpected when applied to weldments, since all extrapolations techniques use metals of a single microstructure and uniform creep strength. In crosswelds, there is heterogeneity in creep strength introduced by the varying microstructures in the subzones of the HAZ. Oh [13] heat-treated base metal samples reproducing regions of the HAZ, these later were stressrupture tested at 593°C and 207 MPA. The results, summarized in Table 1, show different secondary creep rates which are expected for the distinct regions of the HAZ. The A, temperature is 780°C and the ICHAZ temperature 790°C. All GTAW-crosswelds failed in this region. Microhardness showed that this spot is the softest one of the weldments. The results in Table 1 correlate with these findings, and show that the ICHAZ has the highest steady-state creep rate. Metallographic analysis of the creep rupture specimen was done to establish if changes occurred that may explain discrepancies between predicted and actual lives and correlate with other studies [14].
[13] (h)
Secondary 0.00213 0.01740 0.02750 0.01230 0.00523
creep rate (‘% hk’)
Hardness 245 222 216 265 272
(HV,,,)
J.E. Indcmchea,
Fig. 2. Optical micrograph ICHAZ after creep rupture
3.2. A4etallurgical
of a GTAW testing.
R.A.
Seshadri
crossweld
that
i Materials
failed
Science
at the
analysis of’ creep rupture specimen
Microscopy of failed creep rupture specimens showed that crossweld specimens failed in the ICHAZ (Fig. 2). The HAZ on the other side of the crossweld specimen necked at a spot coinciding with the failure of the specimen. A high density of cavities was seen by the failure location as well as in the necked area. SEM analysis of the fracture surface showed cavities about the carbides. Microhardness of the failed specimen showed softening at the failure spot and in the necked area. Oh [15] also observed softening in the crossweld specimens tested at 593”C/131 MPa and cavities about the carbides; these facts suggest that the mechanism of failure is the same at 593”C/131 MPa and at 58O”C/83 MPa. The as-received rotor steel has a tempered bainitic matrix microstructure with carbides, type M2,C, (chromium rich), M,C (molybdenum rich) and M,C,
and Engineering
A234-236
(1997)
Fig. 4. Carbides found at the failure after creep rupture testing at 663°C
555-558
location of a GTAW and 83 MPa.
557
crossweld
(vanadium rich), as shown in Fig. 3 [15]. Carbides at the failed regions were found to have coarsened and spheroidized. Fig. 4 shows the carbides of the sample tested at 663°C no rod-type M,C or H-type were seen. The carbides of the sample tested at 607°C also spheroidized, but, a number of metastable M,C (rodtype) and H-type were still observed. In addition, an increase in the alloy content of the carbides occurred. The differences in carbide type and chemistry, imply microstructure changes which current extrapolation techniques do not consider. Thus, the difference between the predicted and actual lives of the samples studied at 580°C is most likely influenced by the type of carbides present.
4. Conclusions (1) Extrapolation methods must be applied cautiously to welds. The HAZ heterogeneous microstructure reduces their effectiveness. All stress-rupture samples fractured at the ICHAZ. This region has the highest secondary creep rate and lowest microhardness. (2) Carbides undergo significant coarsening and alloy enrichment, leading to loss of matrix strength and shorter rupture times.
References [I] [2] [3] [4]
Fig. 3. Carbides
in the as-retired
lCrrlMoo0.25V
rotor
steel.
Y. Kadoya, T. Goto. J. ISIJ 78 (10) (1992) 1601-1608. B.J. Cane, Acta Metall. 29 (1981) 1581ll591. N. Shin-Ya, J. Kyoono, S. Yokai, ISIJ Int., 1984, pp 5733579. K. Laha, K. Bhanusankararao, S.L. Mannan, Mater. Sci. Eng. Al29 (1990) 183. [5] I.W. Chen, A.S. Argon, Acta Metall. 15 (1981) 1321. [6] E.P. George, P.L. Li, D.P. Pope, Acta Metall. 35 (1987) 2471. [7] M.R. Myers, R. Pilkington, Mater. Sci. Eng. 95 (1987) 81.
558 [8] [9] [lo] [ll] [12]
J.E. Indacochea,
R.A.
Seshadri/Muterials
Science
F.R. Larson, .I. Miller, Trans. ASME 74 (1952) 7655771. S.S. Manson, A.M. Haferd, NACA, TN2890, 1953, p. 1890. R.L.Orr,O.D.Sherby,J.E.Dorn,Trans.ASM46(1954)113-118. F.C. Monkman, N.J. Grant, Proc. ASTM 56 (1956) 593-605. R. Viswanathan, Damage Mechanisms and Life Assessment of High Temperature Components, ASM Int., Metals Park, OH, 1989.
and Engineering
A2346236
(1997)
555-558
[13] Y.K. Oh, J. E Indacochea, Creep rupture behavior due to molybdenum-rich M,C carbide in l.OCr~l.OMoo0.25V bainitic steel weldment, 1997 in press. [14] K.R. Williams, J. Mater. Sci. Eng. 5 (28) (1977) 289-296. [15] Y.K. Oh, Heat affected zone stability of l.OCr-l.OMoo0.25V bainitic turbine rotor steels. PhD thesis, University of Illinois at Chicago, IL, 1994.
A
(1997) 5555558
An analysis of creep damage in a welded low alloy steel rotor J.E. Indacochea *, R.A. Seshadri University
of Illinois
at Chicago,
Civil and Materials
Engineering
Department
(mc 246),
842 W. Taylor,
Chicago,
IL 60607-7023,
USA
Received 3 February 1997; received in revised form 4 April 1997; accepted 7 April 1997
Abstract An assessment of commonly used creep-rupture extrapolation techniques on weldments was undertaken. Gas tungsten arc and submerged arc welds were fabricated using l.OCr-lMo-0.25V rotor steel as base metal. Crossweld samples, which includes base metal, heat-affected zone and weld metal, were machined and submitted to isostress creep-rupture tests at 83.0 MPa between 690 and 620°C. Larson-Miller and Monkman-Grant plots were produced from these data; the parameters associated with the Larson-Miller method were estimated and extrapolations were done to the service temperatures. The actual lives of the submerged arc crosswelds were shorter than predicted. Monkman-Grant predictions were more conservative than LarsonMiller’s for the gas tungsten arc welds. The discrepancies are attributed in part to the heterogeneous microstructures in a crossweldment leading to different creep rates. Application of extrapolation methods to weldments should be done prudently; the heterogeneous microstructure of the heat-affected zone makes them less reliable. 0 1997 Elsevier Science S.A. Keywords:
CrMoV crosswelds; Isostress-rupture tests
1. Introduction CrMoV steelsare used in high temperature and high stress sections of power plant members [l-3]. Their good creep resistance is impaired by welding done during fabrication of assemblies and weld repair of service damaged rotors. Hence, evaluation of the creep service performance of weld repaired rotors becomes necessary. Due to time and resource constraints, creep tests are conducted at higher temperatures, so the service life is estimated by extrapolation. Creep failure is related to cracking at grain boundary triple points or to cavities nucleating on sulfides [4] and carbides [5,6]. Chen et al. [5] found that all detectable sulfides appear to nucleate cavities, and did not seeinterfacial cracking around carbides. Myers et al. [7] pointed that the idea of cavities solely nucleating at grain boundary sulfide particles is incompatible with cavity nucleation data; the MnS number is too low to justify alone the cavity nucleation rate. Several extrapolation methods have been proposed [8-l 11, but none give an accurate life prediction since they do not consider microstructure changes that occur *Corresponding author. Tel.: + 1 312 9965283; fax: + I 312 9962426; e-mail: [email protected] 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PZZ SO921-5093(97)00273-6
due to weld thermal cycle or exposure to test or operating temperatures. Original carbides change in composition and morphology due to thermal cycles. This work compares the life predictions for gas tungsten arc weld (GTAW) and submerged-arc crosswelds (SAW) of lCr-lMo-0.25V steel using Larson-Miller [8] and Monkman-Grant [ll] methods and explains the deviations between predicted and actual lives by relating it to the aging of carbides and microstructure of the heat-affected zone.
2. Experimental procedure Welds were fabricated using coupons machined from a retired rotor and a full scale rotor. The base metal was an ASTM A470, Class 8 HP/IP rotor (0.3l%C, l.O4%Cr, l.l4%Mo, 0.25%V, O.ll%Ni, 0.75%Mn, O.l8%Si, O.O16%P and O.O12%S). The filler metal: O.l2%C, 2.48%Cr, 0.98%Mo, 0.26%V, 0.56%Mn, 0. lO%Si, O.OlO%P, O.O03%S. The coupon welds were multipass GTAW. Two narrow-groove multipass welds were made at two locations in the full scale rotor with GTAW and SAW procedures. Crossweld test samples from the coupons and the full scale rotor were isostress rupture tested at 83 MPa between 580 and 690°C.
556
J.E. Indacochea,
R.A.
Seshadri
/ Materials
6.76
9
4.26
;
3.6
2.75
2
126 0.63
0.0
0.62
0.64
(l/T) Fig. 1. Larson-Miller and SAW welds.
extrapolation
0.36
x 1000,
0.68
0.7
0.72
v?-’
to service conditions
for GTAW
Creep strains were measured; samples were metallographically examined and the fracture surfaces inspected in the SEM. Selective transmission electron microscopy and energy dispersive X-ray analysis of carbides was performed.
3. Results and discussion 3.1. Application of extrapolation weldment life prediction
techniques for
Most studies [8-111 on extrapolation techniques use metals of uniform microstructure. Our study is on crosswelds of different microstructures between the weld metal and base metal. The rupture times of GTAW-crossweld samples from the full scale rotor weld were plotted versus the inverse of the temperature, following the Larson-Miller (L-M) equation (Fig. 1). Table Creep Heat 720 770 790 840 890
1 properties treatment
of the subzones temperature
(“C)
of HAZ
at 593”C/207 Time 934.6 137.5 102.0 154.7 407.2
MPa
to rupture
Science
and Engineering
A234-236
(1997)
555-558
The slope of the line is the L-M parameter and equals 33 100. The constant C was extrapolated to 18.0. This value falls within the range expected for these steels, 15-25 [12]. The extrapolation of the rupture life to temperatures of about 538 and 510°C yielded times of 4.6 and 29.3 years. A similar L-M plot for the SAW welds yielded a L-M parameter of 25 895 and 13.4 for C. The rupture life extrapolations yielded 2.5 and 10.6 years at 538 and 510°C respectively. GTAW and SAW crosswelds of the full scale rotor, were also tested at 580°C and 83.0 MPA. Based on the L-M plots a life of 7890 h was estimated for the SAW crossweld and 5208 h for the GTAW crossweld; actual lives of 3207 h and 5145 h were obtained for the SAW and GTAW. The Monkman-Grant technique was applied to the creep rupture data of the GTAW-crosswelds. A life prediction of 3088 h was made at 580°C which underestimated the actual life. Note that this plot is a log-log plot, so that any subtle variations in creep rates will not lead to significant changes in predicted rupture life; further, this method was not meant to be an extrapolation technique. The differences found between predicted and actual lives are not unexpected when applied to weldments, since all extrapolations techniques use metals of a single microstructure and uniform creep strength. In crosswelds, there is heterogeneity in creep strength introduced by the varying microstructures in the subzones of the HAZ. Oh [13] heat-treated base metal samples reproducing regions of the HAZ, these later were stressrupture tested at 593°C and 207 MPA. The results, summarized in Table 1, show different secondary creep rates which are expected for the distinct regions of the HAZ. The A, temperature is 780°C and the ICHAZ temperature 790°C. All GTAW-crosswelds failed in this region. Microhardness showed that this spot is the softest one of the weldments. The results in Table 1 correlate with these findings, and show that the ICHAZ has the highest steady-state creep rate. Metallographic analysis of the creep rupture specimen was done to establish if changes occurred that may explain discrepancies between predicted and actual lives and correlate with other studies [14].
[13] (h)
Secondary 0.00213 0.01740 0.02750 0.01230 0.00523
creep rate (‘% hk’)
Hardness 245 222 216 265 272
(HV,,,)
J.E. Indcmchea,
Fig. 2. Optical micrograph ICHAZ after creep rupture
3.2. A4etallurgical
of a GTAW testing.
R.A.
Seshadri
crossweld
that
i Materials
failed
Science
at the
analysis of’ creep rupture specimen
Microscopy of failed creep rupture specimens showed that crossweld specimens failed in the ICHAZ (Fig. 2). The HAZ on the other side of the crossweld specimen necked at a spot coinciding with the failure of the specimen. A high density of cavities was seen by the failure location as well as in the necked area. SEM analysis of the fracture surface showed cavities about the carbides. Microhardness of the failed specimen showed softening at the failure spot and in the necked area. Oh [15] also observed softening in the crossweld specimens tested at 593”C/131 MPa and cavities about the carbides; these facts suggest that the mechanism of failure is the same at 593”C/131 MPa and at 58O”C/83 MPa. The as-received rotor steel has a tempered bainitic matrix microstructure with carbides, type M2,C, (chromium rich), M,C (molybdenum rich) and M,C,
and Engineering
A234-236
(1997)
Fig. 4. Carbides found at the failure after creep rupture testing at 663°C
555-558
location of a GTAW and 83 MPa.
557
crossweld
(vanadium rich), as shown in Fig. 3 [15]. Carbides at the failed regions were found to have coarsened and spheroidized. Fig. 4 shows the carbides of the sample tested at 663°C no rod-type M,C or H-type were seen. The carbides of the sample tested at 607°C also spheroidized, but, a number of metastable M,C (rodtype) and H-type were still observed. In addition, an increase in the alloy content of the carbides occurred. The differences in carbide type and chemistry, imply microstructure changes which current extrapolation techniques do not consider. Thus, the difference between the predicted and actual lives of the samples studied at 580°C is most likely influenced by the type of carbides present.
4. Conclusions (1) Extrapolation methods must be applied cautiously to welds. The HAZ heterogeneous microstructure reduces their effectiveness. All stress-rupture samples fractured at the ICHAZ. This region has the highest secondary creep rate and lowest microhardness. (2) Carbides undergo significant coarsening and alloy enrichment, leading to loss of matrix strength and shorter rupture times.
References [I] [2] [3] [4]
Fig. 3. Carbides
in the as-retired
lCrrlMoo0.25V
rotor
steel.
Y. Kadoya, T. Goto. J. ISIJ 78 (10) (1992) 1601-1608. B.J. Cane, Acta Metall. 29 (1981) 1581ll591. N. Shin-Ya, J. Kyoono, S. Yokai, ISIJ Int., 1984, pp 5733579. K. Laha, K. Bhanusankararao, S.L. Mannan, Mater. Sci. Eng. Al29 (1990) 183. [5] I.W. Chen, A.S. Argon, Acta Metall. 15 (1981) 1321. [6] E.P. George, P.L. Li, D.P. Pope, Acta Metall. 35 (1987) 2471. [7] M.R. Myers, R. Pilkington, Mater. Sci. Eng. 95 (1987) 81.
558 [8] [9] [lo] [ll] [12]
J.E. Indacochea,
R.A.
Seshadri/Muterials
Science
F.R. Larson, .I. Miller, Trans. ASME 74 (1952) 7655771. S.S. Manson, A.M. Haferd, NACA, TN2890, 1953, p. 1890. R.L.Orr,O.D.Sherby,J.E.Dorn,Trans.ASM46(1954)113-118. F.C. Monkman, N.J. Grant, Proc. ASTM 56 (1956) 593-605. R. Viswanathan, Damage Mechanisms and Life Assessment of High Temperature Components, ASM Int., Metals Park, OH, 1989.
and Engineering
A2346236
(1997)
555-558
[13] Y.K. Oh, J. E Indacochea, Creep rupture behavior due to molybdenum-rich M,C carbide in l.OCr~l.OMoo0.25V bainitic steel weldment, 1997 in press. [14] K.R. Williams, J. Mater. Sci. Eng. 5 (28) (1977) 289-296. [15] Y.K. Oh, Heat affected zone stability of l.OCr-l.OMoo0.25V bainitic turbine rotor steels. PhD thesis, University of Illinois at Chicago, IL, 1994.