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The high temperature deformation and fracture of mild steel weldments in thick section 12Cr12Mo14V pipe
Int. J, Pres. Ves. & Piping 50 (1992) 243-254
The High Temperature Deformation and Fracture of Mild Steel Weldments in Thick Section ~Cr ~Mo ~V 1 Pipe M. C. C o l e m a n Nuclear Electric, Bedminster Down, Bristol, UK
& J. D. P a r k e r Department of Materials Engineering, University College, Singleton Park, Swansea, UK
ABSTRACT Safe, reliable operation of high energy pipework is critically dependent on accurate knowledge regarding the performance of weldments. A pressure vessel experiment, involving long-term testing of thick-section weldments under controlled conditions, is described. Details of strain accumulation and damage development are explained in terms of differences in local weldment microstructure and properties. These variations lead to redistribution of creep stresses so that behaviour cannot simply be predicted using a single stress value. Implications for the operating performance of in-service components are discussed.
INTRODUCTION Steam pipework in power generating plant operates under conditions of high temperature and pressure such that long term damage due to creep can occur. Normal design considerations, e.g. technical performance, cost, availability, etc., are such that in most cases main steam and hot reheat piping systems are manufactured from low alloy steels. Consideration of specific design life calculations then indicates that this pipework should be serviceable for at least 100000h. In general, service experience supports these calculations with the majority of 243
244
M. C. Coleman, J. D. Parker
piping system components exhibiting little evidence of creep damage after operation for periods well in excess of 100 000 h. However, whilst most components can achieve and exceed design lifetimes, in-service damage has been found at weldments. 1'2 Several general categories of damage have been identified. If problems due to gross manufacturing defects are ignored, service experience suggests that specific types of creep cracking may develop in particular ranges of operating time. A schematic illustration of this behaviour is given in Fig. 1. 3,4 The distributions in Fig. 1 are identified as: (1)
(2)
(3)
Curve B--Circumferential Cracking in the weld heat affected zone (HAZ). This occurs early in service, or in extreme cases during post weld heat treatment, due to relaxation of residual welding stresses in brittle microstructures. Curve C--Transverse Weld Metal Cracking. Damage has been observed to form on columnar grain boundaries in the weld which are predominantly parallel to the axis of the pipe, i.e. transverse to the weld. The susceptibility to cavitation and microcracking is promoted by relatively low temperatures of post weld heat treatment. Curve D--Type IV Cracking. In this case, damage develops in the partially transformed region of the H A Z and results in circumferential cracking.
The final curve in Fig. 1, identified as A, is shown as that representative of long term life in the absence of any of the other damage types. However, the details of this true life distribution have not yet been clearly defined either in terms of the timing, orientation or location of cracking. Research projects and programmes of plant Curve A
CurveC
/ %1 i °
I/' ,, "../li / 1-10000 4 0 - 6 0 0 0 0
60-80000
True life
Operating t i m e (h)
Fig. 1.
Schematic representation of weld failure modes in ferritic pipe welds. 3
Deformation and fracture of mild steel weldments
245
monitoring are in progress to develop the necessary algorithms to establish the rate of damage development for different weldment microstructures and operating regimes. From a detailed understanding of the factors affecting performance, it will then be possible to accurately define lifetime distributions for long term damage modes. A major contribution to the understanding of weldment performance has been achieved through a range of experiments on full-sized, thick-section weldments. A significant programme concerned with the creep behaviour of weldments in ½CrlMo 1V steel piping has been carried out at Marchwood Engineering Laboratories (MEL). Details of the overall programme have been described previously. 5"6 One aspect of this work has examined the effect on component creep behaviour of variations in weld metal-parent metal strength. Thus, weldments were manufactured in normalised and tempered ½Cr 1Mo 1V steel using mild steel, 1Cr ½Mo steel, 2~Cr 1Mo steel and ½Cr Mo V steel electrodes and tested under controlled conditions. The results of mild steel weldments are discussed in the present paper.
EXPERIMENTAL PROCEDURE The weldments were manufactured from the same and tempered main steam pipe. Mild steel manual electrodes were used in standard J preparations, as in Fig. 2. The electrode diameters used ranged from
cast of normalised metal arc (MMA) shown in the inset 2.5 mm in the root
7] Pressure connection
Mild steel ~ " welds \ /
112 CrMoV
+
~A
+ + '~/:==g*
~C ~E
+= g *
~F
+= o
~G
= 0 Hoop and a x i a l s t r a i n g a u g e s + Creep pips
I
~
p a r e n t pipe ~
Weld p r e p a r a t i o n detail
/o l-Store .
Fig. 2.
/
/
~
/
¢
/
/
Pressure vessel geometry and detail of the weld preparation.
246
M. C. Coleman, J. D. Parker
area, through 3-25 m m and 4 m m to 5 m m for the majority of each weld. Welding was carried out at a preheat of 200 °C within the current and voltage range r e c o m m e n d e d by the electrode manufacturers. On completion, each weld was post weld heat treated for 3 h at 700 °C-110 °C within the range specified by C E G B Standard 23584, 1974. The pressure vessel geometry is shown in Fig. 2 and consisted of two ICr IMo IV forged end caps and three hot drawn seamless pipe sections joined by four weldments. The pipe dimensions were 6 0 m m wall thickness, t, and 350 m m outside diameter, giving an outside to inside diameter ratio of 1.52. The weld centreline spacing was 6t so that elastic interactions between welds were negligible. 7 One end cap and one pipe-pipe weld were made from mild steel weld metal while the other two welds were made using 1Cr Mo V weld metal. However, only data from the mild steel weldments are reported here. The pressure vessel was installed in the testing facility at MEL. 8 Testing was carried out at 565°C and 455bar, representing an acceleration factor compared with plant of 10 based on ISO stress rupture data. 9 Heating to the test temperature was achieved using an air circulation bell furnace. Once at temperature, the vessel was pressurised with steam incrementally up to the test pressure which in all cases was achieved within a period of about 1 h. A short time was allowed to elapse after each pressure increment to ensure that temperature gradients in the vessel were kept to a minimum. At the end of each test period the vessel was returned to ambient conditions for various inspections by reducing first pressure and then temperature. Deformation in the vessel was monitored continuously at the test temperature, both during pressurisation and at testing pressure, using Planar capacitance gauges. ~° Strain data were also obtained intermittently at ambient temperature from direct measurements across reference creep pips. Details of these techniques have been given previously. 11 However, it is important to note that in order to avoid any localised cracking or weld metal interaction effects, the creep pips were attached to the pipe adjacent to the weldments and not directly to the surface of the weld metals. As a result, the weld metal hoop strain is inferred from the deformation recorded in the parent material immediately adjacent to the weld H A Z , while the axial data arise from a combination of weld metal, H A Z and pipe deformation. Following manufacture and after each test period, non-destructive examination (NDE) of the weldments was carried out using magnetic particle and ultrasonic inspection techniques. These methods generally detect defects with linear dimensions greater than --3 mm. Information on weldment microstructure and microdamage was obtained by performing metallurgical replication at selected inspections.
Deformation and fracture of mild steel weldments
247
P R E S S U R E VESSEL RESULTS The vessel was subjected to 19 separate test periods, with the periods varying between 500 and 2500 h. After 23 671 h cracking had become extensive and the test was terminated. Elastic deformation
In all cases, a linear relationship between hoop and axial strain and pressure was observed on loading, indicating that deformation was elastic, Fig. 3. Hoop and axial strains in the pipe to pipe weldment and the parent material were similar, giving a hoop to axial strain ratio of about 4. In the end cap weldment, however, the hoop strain was reduced and the axial strain enhanced, so that the hoop strain was approximately equal to the axial strain. 400
300 ¢/
C u0v . X C
200-
q
o
~oo r
/o
0
1
7
I
I
100
200
300
400
500
Pressure (bar)
Fig. 3.
The hoop and axialstrains measured during pressurisation a t 5 6 5 ° C .
248
M. C. Coleman, J. D. Parker
The hoop to axial strain ratio on the outer surface of an elastically loaded cylinder, where the radial stress, or~, is zero, is given by: hoop strain axial strain
oH - v o A -
oA -
(1)
voH
where OH and oA are the hoop and axial stresses respectively, and v is the Poisson's ratio, 0.3, for elastic conditions. The hoop to axial strain ratio of 4 is consistent with a hoop to axial stress ratio of 2 on the surface of the pipe. However, at the centre of the weld adjacent to the end cap, the hoop to axial stress ratio of about unity shows that the end cap geometry modifies the elastic stresses. Creep
deformation
Deformation was monitored using creep pips and capacitance strain gauges. In general, good agreement was obtained with the two methods. Thus, the results are presented as average strains measured from the creep pips since these data accurately represent the trends observed from the respective pressure vessel locations. Parent material deformation
The hoop and axial strain accumulation observed in the parent material is given in Fig. 4(a). In the hoop direction positive strain developed 0-3 0.2 0.1
J
0 -0-1 3 c2
/
/
11 i
I
i
& 0 3
/ ///
2
Fig. 4. Average creep strain observed in (a) the parent pipe, (b) the mild steel pipe/pipe weld, and (c) the mild steel pipe/end cap weld. The solid lines and dashed lines represent hoop and axial data respectively.
/ /
1 0
10 20 Time, hx1000
Deformation and fracture of mild steel weldments
249
throughout the test. Initially, the strain rate decreased for approximately 7000 h, appeared to pass through a m i n i m u m and then remained approximately constant with a rate of - 1 x 10 -7 h -1. The total average hoop strain observed in the test was - 0 . 2 5 % . For the axial strain, after a very small initial transient a zero creep rate was established. The total average axial strain observed was - 0 - 0 1 % .
Pipe~pipe weld The hoop and axial strain accumulation observed in the pipe/pipe weld are given in Fig. 4(b). A primary stage of decreasing creep rate was observed for approximately 7000 h after which the deformation rate gradually increased. Positive strain was monitored in both the measurement directions, with total strains of 0.85% and 2.8% for h o o p and axial orientations respectively. Pipe~end cap weld The hoop and axial strain accumulation observed in the p i p e / e n d cap weld are given in Fig. 4(c). The general trend in behaviour was similar to that of the pipe/pipe weld, i.e. following the initial transient of decreasing creep rate the rate increased to the end of the test. However, in the h o o p direction lower strains were observed at the side of the weld nearest the end cap compared to that detected on the pipe side of the weld. Thus, total hoop strains observed were approximately 0.35% and 0.6% for the end cap and pipe sides of the weld respectively. In the axial direction a total strain accumulation of - 2 . 7 % was detected.
Damage development Circumferential cracking was detected in the pipe/pipe weld by magnetic particle inspection after 1500 hours. This cracking occurred in the weld metal close to the fusion boundary on one side of this weld. Similar cracks were initiated at the other pipe side interfaces after 3500 h. These cracks grew in the circumferential direction and other cracks nucleated so that all of these defects were fully circumferential after about 9000 h. Growth of these defects through the wall thickness was monitored by periodic inspections using shear wave ultrasonics. These techniques estimated that the radial depth of these circumferential cracks ranged from approximately 40 m m for the deepest crack to 10 m m for the shallowest defect. Cracking parallel to the vessel axis was also found in the heat affected zone of these welds. Early stages of this damage development were monitored using replication; however, after
250
M. C. Coleman, J. D. Parker
approximately 21 000 h, axial macrocracks were identified in the H A Z on the pipe side of each weld. These cracks were initiated due to creep voiding in the coarse grained regions of the H A Z . The pipe/pipe weld was sectioned after the 23 671 h testing. Metallographic preparation and examination of sections from the weld confirmed the trend of damage indicated by the NDE. However, the degree of cracking, in terms of radial extent, was less than suggested by the ultrasonic test data. Post-test examination revealed that the weldment microstructure consisted of regions of coarse columnar structure separated by areas of refined microstructure. In general it appeared that the circumferential cracking had been confined to the coarse grained regions with crack growth in the refined microstructure observed only for the surface connected defect. Thus, although defects were present at a depth of - 4 0 mm from the outside surface, the cracking was not continuous. The pattern of damage observed at the end cap interface was generally similar. However, the timing of cracking was different. Thus, the circumferential weld metal cracking was delayed and the axial H A Z damage enhanced compared to the results at the pipe side of the welds.
DISCUSSION Approaches for the design of high temperature pipework typically require that welds are manufactured with filler material that exhibits properties matching those of the parent. Clearly the welds in the present test vessel have been deliberately selected to be of significantly lower strength than the ½Cr ½Mo IV steel base material. Nevertheless, evaluation of the vessel behaviour from a design approach and then consideration of the detailed vessel results provide useful information regarding the service performance of weldments. Design calculations are normally based on the component mean diameter hoop stress applied to relevant materials data. In the present programme, uniaxial creep data were generated 12 from samples of weld metal, base metal and coarse grained bainite, typical of H A Z material. The rupture behaviour of these samples at 565 °C is shown in Fig. 5. These data should then facilitate accurate predictions of vessel behaviour. The mean diameter hoop stress for the present vessel was 110 MPa. Thus, application of this stress to Fig. 5 indicates lifetimes of - 5 0 h, 31 000 h and 55 000 h for the weld metal, base metal and H A Z respectively. Furthermore, in each case, since damage normally occurs perpendicular to the principal stress direction, cracking would be expected to be predominantly axial. Clearly, the results from the
Deformation and fracture of mild steel weldments
251
400 200
"
a. 100
;
~
~
~
~
.
A. Z.
--"~'-,..~I Pa rent
~o ~o
(/I
5O ~ . ~ -
M i l d steel
o
20
L 102
, 103
J 104
, 105
i 106
Time (h)
Fig. 5. Stress rupture data for the mild steel weld metal, the ½Cr ½Mo IV steel parent ~Mo ~V steel simulated HAZ (adapted from Ref. 12). metal and the ~Cr ' ~
present test vessel do not agree with this simplified approach. Despite the fact that the creep strength of the weld metal was significantly weaker than that of the parent, a weld metal to parent metal ratio of 1690: 1 has previously been determined based on the results of uniaxial testing, ]2 nearly 24000h of testing were completed without catastrophic failure. Furthermore, whilst secondary damage was detected in the H A Z no indications of significant creep damage were detected in the base metal where the observed creep strain was - 0 . 2 5 % . The pattern of experimental observations can be explained on the basis of stress redistribution during creep. A study of the stresses developed in thick-section weldments has been reported previously. ]1 This study, performed using finite element techniques, modelled the weld, H A Z and parent regions of piping welds assuming elastic-steady state creep behaviour. This work showed that the hoop stresses in piping weldments were redistributed so that regions of low creep strength shed stress to regions of higher creep resistance. The hoop and axial steady state creep stress distributions predicted for the outside surface of a mild steel piping weld are shown in Fig. 6. From these data the stresses for the weld centre, H A Z and parent are 20 MPa (axial), 135 MPa (hoop) and 96 MPa (hoop) respectively. The stress directions, given in parentheses, indicate that the maximum principal stress in the weld is in fact in the axial direction. Thus, damage in the weld would be predicted to form circumferentially. Detailed consideration of the finite element
252
M. C. Coleman, J. D. Parker
~E If'Of
°
~100~-
HAZ
• : •
LRemote parent eta
I
(a)
~E~. ~"z100= ~ ~ i/1
60
--
20
-"
Fig. 6.
metal
WeLd metat centre L
I
Remote parent
~ ' ~
:.
.~
10 100 1000 Weld metat/parent metal creep rate ratio (b)
Variation of the surface stationary state (a) hoop stress and (b) axial stress for selected weldment locations for different combinations of weld metal/parent metal creep strength."
analysis reveals that the peak weld metal stress of 35 MPa (axial) occurs close to the fusion boundary. Consideration of Fig. 5 indicates lifetimes of - 2 3 000 h for the weld metal and H A Z and 58 000 h for the parent. Clearly, these predictions are in reasonable agreement with the damage trend observed in the pressure vessel test. Detailed analysis of the results for the end cap weldment has not been completed. However, it is clear that the geometry of the vessel end cap has modified both the elastic and creep stresses developed at this location. In particular the damage, at least up to 23 671 h, shows that the circumferential cracking in the weld is reduced and the axial cracking in the H A Z is enhanced compared to pipe-pipe geometries. Thus, it appears that, for specific damage types, the distribution of service lifetimes will be critically dependent upon local microstructureproperties and geometry. The present work indicates that techniques taking account of these factors are available to provide a realistic methodology for assessment.
Deformation and fracture of mild steel weldments
253
CONCLUDING REMARKS The present p r o g r a m m e illustrates many of the difficulties associated with prediction of weldment performance. Firstly, even in a situation where a weld metal of low creep strength has been deliberately used to manufacture a thick section weld, damage can form in different locations with different orientations. Whilst this behaviour can be reasonably explained on the basis of stress redistribution from material zones of different creep strength, accurate prediction of performance must take ductility into account. Thus, a weldment produced with good strength and ductility should provide optimal performance. Furthermore, for such welds, the true service life should be determined by the piping hoop stress. However, the current work suggests that design approaches based on the mean diameter hoop stress are in themselves conservative and therefore piping systems should be capable of operating lives greatly in excess of design values.
ACKNOWLEDGEMENT This work was performed while the authors were at Marchwood Engineering Laboratories.
REFERENCES 1. Toft, L. H. & Yeldham, D. E., Weld performance in high pressure steam generating plant in the Midlands Region CEGB. Int. Conf. on Welding Research Related to Power Plant, 1972, CEGB, London. 2. Plastow, B., Tort, L. H. & Yeldham, D. E., Service experience and evaluation of steam-pipe performance in the Midlands Region CEGB. Conf. on Creep Behaviour of Piping, 1974, I Mech E, London, C49/74. 3. Williams, J. A., Methodology for high temperatues failure analysis. Symposium on the Behaviour of Joints in High Temperature Materals, 1981, Commission of European Communities, Petten, The Netherlands, North Holland Publishers. 4. Price, A. T. & Williams, J. A., The influence of welding on the creep properties of steels. In Recent Advances in Creep and Fracture of Engineering Materials and Structures, ed. B. Wilshire & D. R. J. Owen, Pineridge Press, 1982, p. 265. 5. Rowley, T. & Coleman, M. C., A collaborative programme on the correlation of test data for the design of welded steam pipes, 1972, CEGB Note R/M/N710.
254
M. C. Coleman, J. D. Parker
6. Price, A. T., Creep in large scale experimental pressure vessels. Conf. on Failure of Components Operating in the Creep Range, 1976, I. Mech. E., London, 29. 7. Waiters, D. J., The stress analysis of cylindrical butt-welds under creep conditions, 1976, CEGB Note RD/B/N3716. 8. Coleman, M. C., Figler, R. & Williams, J. A., Crack growth monitoring in pressure vessels under creep conditions. In Detection and Measurement of Cracks, 1976, Welding Institute, Cambridge, pp. 40-4. 9. Hickin, W. D., Plastow, B. & Davison, J. K., A graphical presentation of the ISO stress rupture data for tube and pipe steels, 1972, CEGB Memorandum, SSD.MID/M.3/72. 10. Noltingk, B. E., McLachlan, D. F. A., Owen, C. K. V. & O'Neill, P. C., High stability capacitance strain gauges for use at elevated temperatures. Proc. Inst. Elec. Eng., 119 (1972)897. 11. Coleman, M. C., Parker, J. D. & Waiters, D. J., The behaviour of ferritic weldments in thick section ½Cr ½Mo XV pipe at elevated temperature. Int. J. Pres. Ves. & Piping, 18 (1985) 277-310. 12. Browne, R. J., Cane, B. J., Parker, J. D. & Waiters, D. J., Creep failure analysis of butt welded tubes. Int. Conf. on Creep and Fracture of Engineering Materials and Components, Swansea, 1981, 645-59.
The High Temperature Deformation and Fracture of Mild Steel Weldments in Thick Section ~Cr ~Mo ~V 1 Pipe M. C. C o l e m a n Nuclear Electric, Bedminster Down, Bristol, UK
& J. D. P a r k e r Department of Materials Engineering, University College, Singleton Park, Swansea, UK
ABSTRACT Safe, reliable operation of high energy pipework is critically dependent on accurate knowledge regarding the performance of weldments. A pressure vessel experiment, involving long-term testing of thick-section weldments under controlled conditions, is described. Details of strain accumulation and damage development are explained in terms of differences in local weldment microstructure and properties. These variations lead to redistribution of creep stresses so that behaviour cannot simply be predicted using a single stress value. Implications for the operating performance of in-service components are discussed.
INTRODUCTION Steam pipework in power generating plant operates under conditions of high temperature and pressure such that long term damage due to creep can occur. Normal design considerations, e.g. technical performance, cost, availability, etc., are such that in most cases main steam and hot reheat piping systems are manufactured from low alloy steels. Consideration of specific design life calculations then indicates that this pipework should be serviceable for at least 100000h. In general, service experience supports these calculations with the majority of 243
244
M. C. Coleman, J. D. Parker
piping system components exhibiting little evidence of creep damage after operation for periods well in excess of 100 000 h. However, whilst most components can achieve and exceed design lifetimes, in-service damage has been found at weldments. 1'2 Several general categories of damage have been identified. If problems due to gross manufacturing defects are ignored, service experience suggests that specific types of creep cracking may develop in particular ranges of operating time. A schematic illustration of this behaviour is given in Fig. 1. 3,4 The distributions in Fig. 1 are identified as: (1)
(2)
(3)
Curve B--Circumferential Cracking in the weld heat affected zone (HAZ). This occurs early in service, or in extreme cases during post weld heat treatment, due to relaxation of residual welding stresses in brittle microstructures. Curve C--Transverse Weld Metal Cracking. Damage has been observed to form on columnar grain boundaries in the weld which are predominantly parallel to the axis of the pipe, i.e. transverse to the weld. The susceptibility to cavitation and microcracking is promoted by relatively low temperatures of post weld heat treatment. Curve D--Type IV Cracking. In this case, damage develops in the partially transformed region of the H A Z and results in circumferential cracking.
The final curve in Fig. 1, identified as A, is shown as that representative of long term life in the absence of any of the other damage types. However, the details of this true life distribution have not yet been clearly defined either in terms of the timing, orientation or location of cracking. Research projects and programmes of plant Curve A
CurveC
/ %1 i °
I/' ,, "../li / 1-10000 4 0 - 6 0 0 0 0
60-80000
True life
Operating t i m e (h)
Fig. 1.
Schematic representation of weld failure modes in ferritic pipe welds. 3
Deformation and fracture of mild steel weldments
245
monitoring are in progress to develop the necessary algorithms to establish the rate of damage development for different weldment microstructures and operating regimes. From a detailed understanding of the factors affecting performance, it will then be possible to accurately define lifetime distributions for long term damage modes. A major contribution to the understanding of weldment performance has been achieved through a range of experiments on full-sized, thick-section weldments. A significant programme concerned with the creep behaviour of weldments in ½CrlMo 1V steel piping has been carried out at Marchwood Engineering Laboratories (MEL). Details of the overall programme have been described previously. 5"6 One aspect of this work has examined the effect on component creep behaviour of variations in weld metal-parent metal strength. Thus, weldments were manufactured in normalised and tempered ½Cr 1Mo 1V steel using mild steel, 1Cr ½Mo steel, 2~Cr 1Mo steel and ½Cr Mo V steel electrodes and tested under controlled conditions. The results of mild steel weldments are discussed in the present paper.
EXPERIMENTAL PROCEDURE The weldments were manufactured from the same and tempered main steam pipe. Mild steel manual electrodes were used in standard J preparations, as in Fig. 2. The electrode diameters used ranged from
cast of normalised metal arc (MMA) shown in the inset 2.5 mm in the root
7] Pressure connection
Mild steel ~ " welds \ /
112 CrMoV
+
~A
+ + '~/:==g*
~C ~E
+= g *
~F
+= o
~G
= 0 Hoop and a x i a l s t r a i n g a u g e s + Creep pips
I
~
p a r e n t pipe ~
Weld p r e p a r a t i o n detail
/o l-Store .
Fig. 2.
/
/
~
/
¢
/
/
Pressure vessel geometry and detail of the weld preparation.
246
M. C. Coleman, J. D. Parker
area, through 3-25 m m and 4 m m to 5 m m for the majority of each weld. Welding was carried out at a preheat of 200 °C within the current and voltage range r e c o m m e n d e d by the electrode manufacturers. On completion, each weld was post weld heat treated for 3 h at 700 °C-110 °C within the range specified by C E G B Standard 23584, 1974. The pressure vessel geometry is shown in Fig. 2 and consisted of two ICr IMo IV forged end caps and three hot drawn seamless pipe sections joined by four weldments. The pipe dimensions were 6 0 m m wall thickness, t, and 350 m m outside diameter, giving an outside to inside diameter ratio of 1.52. The weld centreline spacing was 6t so that elastic interactions between welds were negligible. 7 One end cap and one pipe-pipe weld were made from mild steel weld metal while the other two welds were made using 1Cr Mo V weld metal. However, only data from the mild steel weldments are reported here. The pressure vessel was installed in the testing facility at MEL. 8 Testing was carried out at 565°C and 455bar, representing an acceleration factor compared with plant of 10 based on ISO stress rupture data. 9 Heating to the test temperature was achieved using an air circulation bell furnace. Once at temperature, the vessel was pressurised with steam incrementally up to the test pressure which in all cases was achieved within a period of about 1 h. A short time was allowed to elapse after each pressure increment to ensure that temperature gradients in the vessel were kept to a minimum. At the end of each test period the vessel was returned to ambient conditions for various inspections by reducing first pressure and then temperature. Deformation in the vessel was monitored continuously at the test temperature, both during pressurisation and at testing pressure, using Planar capacitance gauges. ~° Strain data were also obtained intermittently at ambient temperature from direct measurements across reference creep pips. Details of these techniques have been given previously. 11 However, it is important to note that in order to avoid any localised cracking or weld metal interaction effects, the creep pips were attached to the pipe adjacent to the weldments and not directly to the surface of the weld metals. As a result, the weld metal hoop strain is inferred from the deformation recorded in the parent material immediately adjacent to the weld H A Z , while the axial data arise from a combination of weld metal, H A Z and pipe deformation. Following manufacture and after each test period, non-destructive examination (NDE) of the weldments was carried out using magnetic particle and ultrasonic inspection techniques. These methods generally detect defects with linear dimensions greater than --3 mm. Information on weldment microstructure and microdamage was obtained by performing metallurgical replication at selected inspections.
Deformation and fracture of mild steel weldments
247
P R E S S U R E VESSEL RESULTS The vessel was subjected to 19 separate test periods, with the periods varying between 500 and 2500 h. After 23 671 h cracking had become extensive and the test was terminated. Elastic deformation
In all cases, a linear relationship between hoop and axial strain and pressure was observed on loading, indicating that deformation was elastic, Fig. 3. Hoop and axial strains in the pipe to pipe weldment and the parent material were similar, giving a hoop to axial strain ratio of about 4. In the end cap weldment, however, the hoop strain was reduced and the axial strain enhanced, so that the hoop strain was approximately equal to the axial strain. 400
300 ¢/
C u0v . X C
200-
q
o
~oo r
/o
0
1
7
I
I
100
200
300
400
500
Pressure (bar)
Fig. 3.
The hoop and axialstrains measured during pressurisation a t 5 6 5 ° C .
248
M. C. Coleman, J. D. Parker
The hoop to axial strain ratio on the outer surface of an elastically loaded cylinder, where the radial stress, or~, is zero, is given by: hoop strain axial strain
oH - v o A -
oA -
(1)
voH
where OH and oA are the hoop and axial stresses respectively, and v is the Poisson's ratio, 0.3, for elastic conditions. The hoop to axial strain ratio of 4 is consistent with a hoop to axial stress ratio of 2 on the surface of the pipe. However, at the centre of the weld adjacent to the end cap, the hoop to axial stress ratio of about unity shows that the end cap geometry modifies the elastic stresses. Creep
deformation
Deformation was monitored using creep pips and capacitance strain gauges. In general, good agreement was obtained with the two methods. Thus, the results are presented as average strains measured from the creep pips since these data accurately represent the trends observed from the respective pressure vessel locations. Parent material deformation
The hoop and axial strain accumulation observed in the parent material is given in Fig. 4(a). In the hoop direction positive strain developed 0-3 0.2 0.1
J
0 -0-1 3 c2
/
/
11 i
I
i
& 0 3
/ ///
2
Fig. 4. Average creep strain observed in (a) the parent pipe, (b) the mild steel pipe/pipe weld, and (c) the mild steel pipe/end cap weld. The solid lines and dashed lines represent hoop and axial data respectively.
/ /
1 0
10 20 Time, hx1000
Deformation and fracture of mild steel weldments
249
throughout the test. Initially, the strain rate decreased for approximately 7000 h, appeared to pass through a m i n i m u m and then remained approximately constant with a rate of - 1 x 10 -7 h -1. The total average hoop strain observed in the test was - 0 . 2 5 % . For the axial strain, after a very small initial transient a zero creep rate was established. The total average axial strain observed was - 0 - 0 1 % .
Pipe~pipe weld The hoop and axial strain accumulation observed in the pipe/pipe weld are given in Fig. 4(b). A primary stage of decreasing creep rate was observed for approximately 7000 h after which the deformation rate gradually increased. Positive strain was monitored in both the measurement directions, with total strains of 0.85% and 2.8% for h o o p and axial orientations respectively. Pipe~end cap weld The hoop and axial strain accumulation observed in the p i p e / e n d cap weld are given in Fig. 4(c). The general trend in behaviour was similar to that of the pipe/pipe weld, i.e. following the initial transient of decreasing creep rate the rate increased to the end of the test. However, in the h o o p direction lower strains were observed at the side of the weld nearest the end cap compared to that detected on the pipe side of the weld. Thus, total hoop strains observed were approximately 0.35% and 0.6% for the end cap and pipe sides of the weld respectively. In the axial direction a total strain accumulation of - 2 . 7 % was detected.
Damage development Circumferential cracking was detected in the pipe/pipe weld by magnetic particle inspection after 1500 hours. This cracking occurred in the weld metal close to the fusion boundary on one side of this weld. Similar cracks were initiated at the other pipe side interfaces after 3500 h. These cracks grew in the circumferential direction and other cracks nucleated so that all of these defects were fully circumferential after about 9000 h. Growth of these defects through the wall thickness was monitored by periodic inspections using shear wave ultrasonics. These techniques estimated that the radial depth of these circumferential cracks ranged from approximately 40 m m for the deepest crack to 10 m m for the shallowest defect. Cracking parallel to the vessel axis was also found in the heat affected zone of these welds. Early stages of this damage development were monitored using replication; however, after
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M. C. Coleman, J. D. Parker
approximately 21 000 h, axial macrocracks were identified in the H A Z on the pipe side of each weld. These cracks were initiated due to creep voiding in the coarse grained regions of the H A Z . The pipe/pipe weld was sectioned after the 23 671 h testing. Metallographic preparation and examination of sections from the weld confirmed the trend of damage indicated by the NDE. However, the degree of cracking, in terms of radial extent, was less than suggested by the ultrasonic test data. Post-test examination revealed that the weldment microstructure consisted of regions of coarse columnar structure separated by areas of refined microstructure. In general it appeared that the circumferential cracking had been confined to the coarse grained regions with crack growth in the refined microstructure observed only for the surface connected defect. Thus, although defects were present at a depth of - 4 0 mm from the outside surface, the cracking was not continuous. The pattern of damage observed at the end cap interface was generally similar. However, the timing of cracking was different. Thus, the circumferential weld metal cracking was delayed and the axial H A Z damage enhanced compared to the results at the pipe side of the welds.
DISCUSSION Approaches for the design of high temperature pipework typically require that welds are manufactured with filler material that exhibits properties matching those of the parent. Clearly the welds in the present test vessel have been deliberately selected to be of significantly lower strength than the ½Cr ½Mo IV steel base material. Nevertheless, evaluation of the vessel behaviour from a design approach and then consideration of the detailed vessel results provide useful information regarding the service performance of weldments. Design calculations are normally based on the component mean diameter hoop stress applied to relevant materials data. In the present programme, uniaxial creep data were generated 12 from samples of weld metal, base metal and coarse grained bainite, typical of H A Z material. The rupture behaviour of these samples at 565 °C is shown in Fig. 5. These data should then facilitate accurate predictions of vessel behaviour. The mean diameter hoop stress for the present vessel was 110 MPa. Thus, application of this stress to Fig. 5 indicates lifetimes of - 5 0 h, 31 000 h and 55 000 h for the weld metal, base metal and H A Z respectively. Furthermore, in each case, since damage normally occurs perpendicular to the principal stress direction, cracking would be expected to be predominantly axial. Clearly, the results from the
Deformation and fracture of mild steel weldments
251
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present test vessel do not agree with this simplified approach. Despite the fact that the creep strength of the weld metal was significantly weaker than that of the parent, a weld metal to parent metal ratio of 1690: 1 has previously been determined based on the results of uniaxial testing, ]2 nearly 24000h of testing were completed without catastrophic failure. Furthermore, whilst secondary damage was detected in the H A Z no indications of significant creep damage were detected in the base metal where the observed creep strain was - 0 . 2 5 % . The pattern of experimental observations can be explained on the basis of stress redistribution during creep. A study of the stresses developed in thick-section weldments has been reported previously. ]1 This study, performed using finite element techniques, modelled the weld, H A Z and parent regions of piping welds assuming elastic-steady state creep behaviour. This work showed that the hoop stresses in piping weldments were redistributed so that regions of low creep strength shed stress to regions of higher creep resistance. The hoop and axial steady state creep stress distributions predicted for the outside surface of a mild steel piping weld are shown in Fig. 6. From these data the stresses for the weld centre, H A Z and parent are 20 MPa (axial), 135 MPa (hoop) and 96 MPa (hoop) respectively. The stress directions, given in parentheses, indicate that the maximum principal stress in the weld is in fact in the axial direction. Thus, damage in the weld would be predicted to form circumferentially. Detailed consideration of the finite element
252
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analysis reveals that the peak weld metal stress of 35 MPa (axial) occurs close to the fusion boundary. Consideration of Fig. 5 indicates lifetimes of - 2 3 000 h for the weld metal and H A Z and 58 000 h for the parent. Clearly, these predictions are in reasonable agreement with the damage trend observed in the pressure vessel test. Detailed analysis of the results for the end cap weldment has not been completed. However, it is clear that the geometry of the vessel end cap has modified both the elastic and creep stresses developed at this location. In particular the damage, at least up to 23 671 h, shows that the circumferential cracking in the weld is reduced and the axial cracking in the H A Z is enhanced compared to pipe-pipe geometries. Thus, it appears that, for specific damage types, the distribution of service lifetimes will be critically dependent upon local microstructureproperties and geometry. The present work indicates that techniques taking account of these factors are available to provide a realistic methodology for assessment.
Deformation and fracture of mild steel weldments
253
CONCLUDING REMARKS The present p r o g r a m m e illustrates many of the difficulties associated with prediction of weldment performance. Firstly, even in a situation where a weld metal of low creep strength has been deliberately used to manufacture a thick section weld, damage can form in different locations with different orientations. Whilst this behaviour can be reasonably explained on the basis of stress redistribution from material zones of different creep strength, accurate prediction of performance must take ductility into account. Thus, a weldment produced with good strength and ductility should provide optimal performance. Furthermore, for such welds, the true service life should be determined by the piping hoop stress. However, the current work suggests that design approaches based on the mean diameter hoop stress are in themselves conservative and therefore piping systems should be capable of operating lives greatly in excess of design values.
ACKNOWLEDGEMENT This work was performed while the authors were at Marchwood Engineering Laboratories.
REFERENCES 1. Toft, L. H. & Yeldham, D. E., Weld performance in high pressure steam generating plant in the Midlands Region CEGB. Int. Conf. on Welding Research Related to Power Plant, 1972, CEGB, London. 2. Plastow, B., Tort, L. H. & Yeldham, D. E., Service experience and evaluation of steam-pipe performance in the Midlands Region CEGB. Conf. on Creep Behaviour of Piping, 1974, I Mech E, London, C49/74. 3. Williams, J. A., Methodology for high temperatues failure analysis. Symposium on the Behaviour of Joints in High Temperature Materals, 1981, Commission of European Communities, Petten, The Netherlands, North Holland Publishers. 4. Price, A. T. & Williams, J. A., The influence of welding on the creep properties of steels. In Recent Advances in Creep and Fracture of Engineering Materials and Structures, ed. B. Wilshire & D. R. J. Owen, Pineridge Press, 1982, p. 265. 5. Rowley, T. & Coleman, M. C., A collaborative programme on the correlation of test data for the design of welded steam pipes, 1972, CEGB Note R/M/N710.
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6. Price, A. T., Creep in large scale experimental pressure vessels. Conf. on Failure of Components Operating in the Creep Range, 1976, I. Mech. E., London, 29. 7. Waiters, D. J., The stress analysis of cylindrical butt-welds under creep conditions, 1976, CEGB Note RD/B/N3716. 8. Coleman, M. C., Figler, R. & Williams, J. A., Crack growth monitoring in pressure vessels under creep conditions. In Detection and Measurement of Cracks, 1976, Welding Institute, Cambridge, pp. 40-4. 9. Hickin, W. D., Plastow, B. & Davison, J. K., A graphical presentation of the ISO stress rupture data for tube and pipe steels, 1972, CEGB Memorandum, SSD.MID/M.3/72. 10. Noltingk, B. E., McLachlan, D. F. A., Owen, C. K. V. & O'Neill, P. C., High stability capacitance strain gauges for use at elevated temperatures. Proc. Inst. Elec. Eng., 119 (1972)897. 11. Coleman, M. C., Parker, J. D. & Waiters, D. J., The behaviour of ferritic weldments in thick section ½Cr ½Mo XV pipe at elevated temperature. Int. J. Pres. Ves. & Piping, 18 (1985) 277-310. 12. Browne, R. J., Cane, B. J., Parker, J. D. & Waiters, D. J., Creep failure analysis of butt welded tubes. Int. Conf. on Creep and Fracture of Engineering Materials and Components, Swansea, 1981, 645-59.