Weld repair of grade 91 steel without post-weld heat

Materials Research Innovations

ISSN: 1432-8917 (Print) 1433-075X (Online) Journal homepage: http://www.tandfonline.com/loi/ymri20

Weld repair of grade 91 steel without post-weld heat treatment S. J. Brett & K. C. Mitchell To cite this article: S. J. Brett & K. C. Mitchell (2013) Weld repair of grade 91 steel without postweld heat treatment, Materials Research Innovations, 17:5, 312-317 To link to this article: http://dx.doi.org/10.1179/1432891713Z.000000000253

Published online: 22 Oct 2013.

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Date: 15 March 2016, At: 14:05

Weld repair of grade 91 steel without postweld heat treatment

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S. J. Brett*1 and K. C. Mitchell2 A weld repair technique to be used without post-weld heat treatment has been developed for use on grade 91 steel. The approach makes use of standard (non- modified) 9CrMo weld metal. The work has extended an approach used successfully on the low alloy steel KCrMoV, which utilised a low strength 2CrMoL weld metal, to the more advanced steel grade 91, using an equivalently lower strength weld metal, standard (non-modified) 9CrMo. This has considerably lower creep strength than matching modified 9CrMo weld metal. Two variants of standard 9CrMo weld metal were chosen for investigation: a specially commissioned low carbon 9CrMoL version, with carbon below the normal minimum for this grade of weld metal (0?05 wt-%), and a conventional batch of 9CrMo weld metal, but selected to have carbon in the bottom half of the normal range. Comparison between the 9CrMoL weld metal and the standard 9CrMo weld metal, on the basis of residual stress level and creep and fracture toughness properties, has shown the latter to be the better option. The most likely repair scenario envisaged was to a retrofit grade 91 header on a UK coal fired power station. The goal was to achieve a lifetime for such a repair greater than the 4 year period between major overhauls for a typical power station of this type, corresponding to .20 kh operating hours. Keywords: Grade 91 steel, cold weld repair, repair without post weld heat treatment

This paper is part of a special issue on Energy Materials

Introduction Grade 91 steel has been in use in the UK power industry since the late 1980s. It was first used on retrofit applications, in which earlier 2Cr1Mo headers suffering from thermal fatigue generated ligament cracking were replaced by grade 91 components with thinner wall and improved stub arrangements. In a number of cases, however, the replacement components, typically operating at 580uC, have been found to suffer from type IV cracking in the parent heat affected zone (HAZ) at branch and attachment welds.1,2 The problem has been severe enough for four retrofit headers, out of an estimated total UK population of y100, to require replacement for a second time. In addition, a growing number of other retrofit headers remaining in service have required regular inspection and extensive grinding out of cracking. In the light of this experience, generating companies faced the prospect of having at some stage to carry out extensive weld repairs on a grade 91 header, with the possible accompanying complications of multiple postweld heat treatments along the header body, and the associated risks of unplanned outage time and delays in return to service. Historically cold welding on low alloy steels had been successfully carried out to avoid these complexities and minimise repair time, and the aim of 1

Department of Mechanical, Materials & Manufacturing Engineering, University of Nottingham, UK RWE npower, Windmill Hill Business Park, Swindon, UK

2

*Corresponding author, email [email protected]

ß W. S. Maney & Son Ltd. 2013 Received 12 February 2013; accepted 25 April 2013 DOI 10.1179/1432891713Z.000000000253

this project was to extend the same approach to grade 91 steel. In difficult repair situations such as those presented by a cracked header, the risks of avoiding post-weld heat treatment may be no greater than the risks of carrying out conventional repair with heat treatment. Against this background, RWE npower decided to investigate the use of a cold weld repair method that could be used without post-weld heat. The philosophy adopted was to use a relatively weak standard (i.e. non-modified) 9CrMo weld metal. This built on an earlier development of a cold weld repair method for KCrMoV pipework welds, which had utilised a weak low carbon 2CrMoL weld metal.3 This had successfully led to the repair of 65 steam pipework welds without post-weld heat treatment by the end of 2011. In this case, in comparison with conventional repair, the use of a relatively weak weld metal had shown the following advantages: (i) a lower peak residual weld stress in the as welded (As-W) condition arising from the lower tensile strength of the weld metal (ii) a faster decay in service of the residual stress level, which is present arising from the lower creep strength of the weld metal (iii) a higher proportion of the fixed strain accumulation occurring across the weld during the decay of the residual stress being absorbed by the weld metal, reducing the strain to be accommodated by the parent HAZ. The use of a relatively weak weld metal without postweld heat treatment is not a new concept. Before its


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Weld repair of grade 91 steel without post-weld heat treatment

adoption for the welding of KCrMoV in the UK, a similar approach had been used in the former Soviet Union.4,5 RWE npower subsequently took part in a joint TWI/EWI project looking at the use of 2CrMoL for welding 2JCr1Mo/CrMoV steels.6 More recently, in the case of the weld repair of grade 91, the use of grade 24 weld metal has been proposed.7 Whichever type of weaker weld metal is chosen, clearly it must be strong enough to survive the subsequent period of service required for the repaired component. For the purpose of this exercise, this was defined, as a minimum, to be the 4 year period between major overhauls, typically y20 kh operation.


Test welds Two variants of manual metal arc 9CrMo weld metal were considered: a conventional batch of 9CrMo weld metal, but selected to have carbon in the bottom half of the normal range, and a specially commissioned low carbon 9CrMoL version. The two weld metals were found to have carbon levels of 0?06 and 0?04 wt-% respectively either side of the minimum normally specified for this grade of weld metal (0?05 wt-%). In line with the earlier KCrMoV/2CrMoL programme, this project aimed to subject large section test welds, large enough to represent real repair welds, to a heat treatment designed to simulate, as far as possible, stress relief in service. The goal was to demonstrate that the welds could both avoid reheat cracking and retain mechanical properties sufficient to provide an acceptable subsequent life in service. Residual stress measurements were carried out in the As-W and simulated in service stress relieved (SISR) conditions to demonstrate that the welds had absorbed the strains associated with residual weld stress relaxation before they were cut up to provide specimens for the test programme. It was considered that this would provide the best means of evaluating the fracture and creep properties of a cold weld in service. Whereas the earlier programme had been targeted on pipework butt weld repairs and had utilised a full section butt weld geometry, the present exercise was targeted on branch welds. Because of the potential difficulties of obtaining appropriate creep rupture and fracture toughness test specimens from a branch weld, it was decided instead to weld a large rectangular trough geometry. Welding was carried out by Doosan Babcock at Tipton. Four large section excavation welds, 2506150635 mm deep (see Fig. 1) designated A–D, were made in lengths of new as received thick section P91 pipe, two of them to provide the As-W properties and two to provide the SISR properties. The batches of 9CrMoL and 9CrMo were each used for an As-W and SISR weld, as summarised Table 1 below. One pipe section contained the As-W welds, the other the SISR welds. Each pair of welds were positioned diametrically opposite each other. Details of the P91 pipe used are given in Table 2, and the compositions of the weld metal batches are given in Tables 3 and 4. Metrode Chromet 9 Specification: BS EN 1599 E CrMo9 B 32 H5, AWS A5?5-06/ASME SFA 5?5: E8015B8 Batches were chosen to have carbon at low end of normal range for this type of weld metal (0?05–0?10 wt%). Metrode batch no.: W025733 – 3?2 mm electrodes All weld metal analysis/wt-% (BS EN 10204:2004: 3?1)

1 Excavation in pipe supplied

Metrode batch no.: W025382, 4?0 mm electrodes All weld metal analysis/wt-% (BS EN 10204:2004: 3?1) Metrode Chromet 9L Specification: BS EN 1599 E CrMo9 B 32 H5, AWS A5?5-06/ASME SFA 5?5: E8015-B8L Batches were specially commissioned to have carbon below normal minimum for this type of weld metal (0?05 wt-%). Metrode batch no.: WB17933R32, 3?2 mm electrodes All weld metal analysis/wt-% (BS EN 10204:2004: 3?1) Metrode batch no.: W026960, 4?0 mm electrodes All weld metal analysis/wt-% (BS EN 10204:2004: 3?1) All four welds were made using a procedure employing 3?2 mm electrodes for the first layer of weld metal in contact with the parent, and a subsequent fill with 4?0 mm electrodes. The weld metal was deposited as stringer beads with 50% overlap on the preceding bead. Apart from this, no further measures were adopted to reduce HAZ grain size. A preheat of 200uC and a maximum interpass temperature of 300uC were applied, but no subsequent post-weld heat treatment was carried out.

Residual stress measurement The intended purpose of this project was to demonstrate that large scale cold welds could be made in grade 91 material without producing reheat cracking during the period in which the As-W residual stress would decay away in service. Residual stress measurements were carried out immediately on the As-W welds A and B, while SISR welds C and D were given a heat treatment at 600uC for 336 h (2 weeks). Residual stress measurements were then carried out on these welds to demonstrate that stress relief had occurred. The SISR temperature of 600uC was considered to be close enough to the operating temperature of a typical retrofit header (580uC) to be a realistic representation of service conditions while providing a degree of acceleration in Table 1 Summary of test welds Weld metal

As-W

SISR

Chromet 9 (9CrMo) Chromet 9L (9CrMoL)

A B

C D


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Table 2 Pipe details*; chemical composition/wt-% C

Mn

Si

S

P

Al

Cr

Ni

Mo

V

Nb

N

0.10

0.47

0.31

0.002

0.012

0.014

8.66

0.28

0.95

0.212

0.071

0.066


*Producer: Vallourec & Mannesmann Tubes; dimensions: 310 mm o.d.650 mm t; cast identity: 2328–75 776; heat treatment: normalised 45 min 1060uCzair cool, tempered 120 min 760uCzair cool; room temperature mechanical properties: yield 494 MPa, UTS 666 MPa, el 23%; hardness (Brinell):199–207; room temperature Charpy: 204–226 J.

the residual stress decay. The welds were inspected after the SISR and found to be free of significant defects. The two lengths of P91 pipe were sent to Veqter at Bristol University for through wall residual stress measurement using the deep hole drilling technique. The As-W results, measured at the centre of each weld, are shown for the 9CrMo and 9CrMoL weld metals in Figs. 2 and 3 respectively. The distributions are broadly similar. In both cases, the stresses are compressive at the outer surface, although this might be a result of the grinding carried out on the welds to prepare them for non-destructive testing (NDT). In both cases, the peak stresses (y450 MPa) are located 5–20 mm below the outer surface, are higher in the axial direction and decrease towards the bore. The 9CrMoL distribution shows a distinct minimum near the bottom of the weld excavation, although this is not apparent in the 9CrMo distribution. The SISR results are shown for the 9CrMo and 9CrMoL weld metals in Figs. 4 and 5 respectively. Again the distributions are broadly similar. The peak stresses are now much lower, particularly in the area of the weld excavations (25–100 MPa). The artificial stress relief has therefore achieved its purpose, providing evidence that such repairs would relieve themselves very rapidly in service.

Test programme The SISR welds C and D were sectioned to provide creep rupture specimens. The As-W welds A and B and the remaining part of weld C were sectioned to provide Charpy and fracture toughness tests.

Creep rupture Creep rupture testing was carried out by SERCO. Since the object of this part of the project was to evaluate long term creep rupture properties, only the SISR welds C and D were tested. The tests were carried out on crossweld specimens orientated in the hoop direction and containing half weld metal and half parent material, as illustrated in Fig. 6. This geometry was chosen to allow for failure either in the weak weld metal or in the type IV zone. Testing was carried out at 600uC. Table 3 Details of 9CrMo weld metal

The rupture results are shown in Fig. 7, where the cross-weld results obtained for the two weld metals are compared with type IV data from a number of grade 91 casts generated in other test programmes. All these data are for conventional welds with post-weld heat treatment. They are a mixture of published and unpublished results, the published being shown in Ref. 2. In the current exercise, all the cross-weld specimens failed in the weld metal. It can be seen that, while the 9CrMoL results fall significantly short of the type IV data, the 9CrMo cross-weld results fall just within the type IV scatter band. This weld metal has therefore been shown to have a creep rupture strength equal to the lower end of the type IV range for conventionally welded grade 91 material. Since in practice this is the material most likely to require repair for type IV cracking in service, it can be argued that the weld metal creep strength is optimum for this type of repair.

Fracture Fracture toughness testing was carried out at SERCO using unloading compliance to provide J–R curves. The primary requirement for fracture toughness is that the critical defect in the cold weld should be of a size that can be comfortably detected by NDT. In practice, this means a through wall dimension .3 mm. A simple estimate of critical defect sizes for the two weld metals in the As-W condition can be obtained from the basic fracture mechanics relationship K~sðpaÞ1=2 where K is the fracture toughness in MPa m1/2, s is the stress in MPa and a is the defect half length in m. The half length corresponds to the total defect size for a surface breaking defect and half the defect size for an embedded defect. The cold weld can be considered most vulnerable to fast fracture on startup as plant returns to service. Because an initial level of warming will be required to produce through wall thermal gradients sufficient to generate failure stress, this will be most likely to occur at a temperature above ambient, and a test temperature of 100uC was adopted as an estimated worst case. It was Table 4 Details of 9CrMoL weld metal

C

Mn

Si

S

P

Cr

Ni

Mo

Cu

C

Mn

Si

S

P

Cr

Ni

Mo

Cu

0.06

0.70

0.32

0.006

0.014

9.3

0.33

1.13

0.02

0.044

0.78

0.36

0.008

0.011

9.76

0.31

1.07

…

C

Mn

Si

S

P

Cr

Ni

Mo

Cu

C

Mn

Si

S

P

Cr

Ni

Mo

Cu

0.06

0.78

0.42

0.005

0.012

8.8

0.29

1.11

0.02

0.041

0.66

0.30

0.009

0.010

8.7

0.29

0.98

0.03


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2 Deep hole drilling measured residual stress distribution for 9CrMo weld in As-W condition: shear direction is through wall; figure is taken from Veqter report for RWE npower

3 Deep hole drilling measured residual stress distribution for 9CrMoL weld in As-W condition: shear direction is through wall; figure is taken from Veqter report for RWE npower

4 Deep hole drilling measured residual stress distribution for 9CrMo weld in SISR condition: shear direction is through wall; figure is taken from Veqter report for RWE npower


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5 Deep hole drilling measured residual stress distribution for 9CrMoL weld in SISR condition: shear direction is through wall; figure is taken from Veqter report for RWE npower

6 Schematic of cross-weld rupture specimen geometry containing half weld and half parent

further assumed that the maximum stress that will be imposed is the 0?2% proof stress at 100uC, and a summary of critical defect sizes calculated on this basis is shown in Table 5. It appears that either weld metal would have adequate toughness in the As-W condition. The estimated critical defect sizes should be readily detectable by NDT in both cases, although the 9CrMo weld metal has a slightly better margin against fast fracture. Toughness of the latter should then increase significantly in service.

Discussion The cold welds must be evaluated against the twin demands of acceptable creep strength and acceptable

fracture toughness. In the case of creep strength, the requirement is that the weld metal must have sufficient strength to absorb the creep strain associated with the decay of residual weld stress in service and then provide a useful working life. In the case of fracture toughness, the requirement is that the weld has sufficient toughness to survive the initial start-up after repair and early subsequent service. The creep rupture tests have shown that the 9CrMo weld metal, with carbon at the lower end of the normal range, has a strength equal to that of the type IV zone in material at the bottom end of the parent scatter band. If used for repairing the weakest material therefore, this weld metal can be expected to have a service life comparable to that of a conventional repair with postweld heat treatment in the same material. In contrast, the 9CrMoL weld metal, with carbon below the normal range, has a rupture life significantly shorter than that of the 9CrMo weld metal or the type IV zone in material at the bottom end of the parent scatter band. Either weld metal would have adequate toughness in the As-W condition. The estimated critical defect sizes should be readily detectable by NDT in both cases, although the 9CrMo weld metal has a slightly better margin against fast fracture. A relatively limited period of operation will reduce the residual stress and improve the toughness, removing toughness as a structural integrity issue. The 9CrMo weld metal was found to have a peak residual stress only marginally higher than that of the 9CrMoL weld metal, and the level of stress relaxation during the SISR was only slightly lower. There is therefore little benefit in terms of residual weld stress in choosing the 9CrMoL weld metal in preference to the 9CrMo weld metal to weigh against the better creep strength of the latter.

Conclusions

7 Creep rupture results at 600uC for SISR 9CrMo and 9CrMoL welds compared to grade 91 type IV data

The conclusion from this project is that standard (nonmodified) 9CrMo weld metal with carbon at the lower end of the normal range is suitable for the temporary repair without post-weld heat treatment of grade 91 steel, possessing both adequate toughness in the As-W condition and acceptable creep rupture strength.


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Table 5 Minimum critical defect size derived from fracture toughness testing Weld

0.2% proof at 100uC/MPa

Fracture toughness at 100uC/MPa m1/2

Min critical defect size–surface breaking/mm

Min critical defect size –embedded/mm

9CrMo As-W 9CrMoL As-W 9CrMo SISR

702 702 558

103 80 155

6.9 4.1 24.6

13.7 8.3 49.1


Acknowledgements This paper is published with the permission of RWE npower. The authors would also like to thank P. Gilbride, Doosan Babcock Tipton, for manufacturing the test welds; M. Blackburn, RWE’s Workshops Ferrybridge, for organising the heat treatment of the SISR welds; E. Kingston and L. Chidwick, Veqter Bristol, for the residual stress measurements; and C. Austin, S. May and P. Hutchinson at SERCO for the creep rupture and fracture toughness testing.

3.

References

6.

1. S. J. Brett: ‘In-service failures of modified 9Cr (grade 91) components’, Proc. IMechE Seminar on ‘Forensic investigation of power plant failures’, London, UK, March 2005, IMechE. 2. S. J. Brett: ‘Early type IV cracking on two retrofit grade 91 steel headers’, Proc. IIW Int. Conf. on ‘Safety and reliability of welded

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components in energy and processing industry’, Graz, Austria, July 2008, IIW, Verlag der Technischen Universita¨t Graz, 225. S. J. Brett and K. C. Mitchell: ‘The weld repair of butt welds in aged KCrMoV steam pipework using lower strength flux cored arc weld metal without post weld heat treatment’, Proc. 3rd Int. Conf. on ‘Integrity of high temperature welds’, London, UK, April 2007, Institute of Materials, Minerals and Mining, IOM Communications Ltd., 177. A. E. Anokhov et al: ‘Performance of cast casings of 20KhMFL steel repaired with pearlitic electrodes without heat treatment’, Welding production no. 3, 17–19, 1985. A. E. Anokhov, F. A. Khromchenko and I. V. Fedina: ‘A new method of repair welding cast components of chromium–molybdenum–vanadium steel without heat treatment’, Welding production no. 10, 15–17, 1986. L. M. Friedman: ‘EWI/TWI controlled deposition repair welding procedure for 1?25%Cr–0?5%Mo and 2?25%Cr–1Mo Steels’, Weld. Res. Council Bull, 1996, 412, 27–34. J. Vekeman and S. Huysmans: ‘Cold weld repair of T91’, Proc. 2nd Int. ECCC Conf. on ‘Creep and fracture in high temperature components – design and life assessment’, Zurich, Switzerland, April 2009, EMPA, 272.


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