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Through thickness measurement of residual stresses in a stainless steel cylinder containing shallow and deep weld repairs
International Journal of Pressure Vessels and Piping 82 (2005) 279–287 www.elsevier.com/locate/ijpvp
Through thickness measurement of residual stresses in a stainless steel cylinder containing shallow and deep weld repairs D. George, D.J. Smith* Department of Mechanical Engineering, University of Bristol, Bristol BS8 1TR, UK
Abstract A programme of residual stress measurements was undertaken using the deep hole method. The measurements obtained the throughthickness residual stress profiles before and after introducing deep and shallow part-circumferential weld repairs into a 37 mm thick stainless steel cylinder. Measured through-thickness distributions were obtained at mid-width and in the heat affected zone of both the original weld and repairs. The results show that the membrane and bending components of the in-plane residual stresses are generally increased when weld repairs are introduced. q 2004 Published by Elsevier Ltd. Keywords: Residual stress; Deep-hole method; Weld repair
1. Introduction Welded components are usually repaired during manufacture or in-service after removal of defects or degraded material. However, the magnitude and distribution of the residual stresses for repair welds are not well known. For the case of repair welds that are well embedded inside existing welds it is uncertain whether repair welds, not subjected to post weld heat treatment, contain higher residual stress than in the unrepaired weld. Numerous methods are available to measure stresses in steel. For example, there is a wide range of experimental techniques available to measure surface or through-thickness stresses, particularly residual stresses, in engineering components [1–3]. These are summarised in Fig. 1. These methods are generally classified into two categories, noninvasive and invasive. The degree of destruction depends on the extent of material removed from the component. Most non-invasive (or non-destructive) techniques are usually restricted to near-surface depths down to about 1 or 2 mm in steels. These methods are mainly developed from wave refraction measurements such as electro-magnetism [4] penetrating to a depth down to 10 mm, conventional X-ray [5] capable of measurement to a depth of about 0.02 mm * Corresponding author. 0308-0161/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.ijpvp.2004.08.006
and ultra-sonic [6] going down to a depth of about 2 mm. For steels greater penetration, down to 20 mm, can be achieved using neutron diffraction [7]. Invasive methods all rely on measurement of strain relaxation resulting from material removal. Semi-destructive techniques include centre-hole drilling [8], ring-coring [9] and deep-hole drilling [10]. Techniques using surface strain gauges (centre-hole and ring-coring) are usually able to measure residual stresses in depths from about 1 mm up to about 15 mm. In contrast the deep-hole drilling method is capable of measuring stress at depths up to 500 mm in steel by non-contact measurement of a reference hole drilled into the component. These semi-destructive methods allow several measurements to be made on the specimen but not at the same location. Finally, fully destructive techniques such as slotting [11], block removal, splitting and layering (BRSL) [12] and the inherent strain [13] methods enable through-thickness measurements of residual stress to be made. However, they destroy the specimen completely so that no further measurements can be obtained. The focus of this paper is the measurement of throughwall distribution of in-plane residual stresses in a girth weld and repair-welded stainless steel test component using the deep-hole method [14–17]. Residual stresses were measured on a stainless steel girth weld of about 37 mm wall thickness and 432 mm outside diameter with and without weld repairs,
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Fig. 1. Residual stress measurement methods in steel.
in the plane perpendicular to the normal of the specimen outer surface. The manufacture of the pipe is described, together with a summary of the deep hole method. First, the residual stresses were measured in the girth weld of the pipe, and then weld repairs introduced. Then measurements of residual stress associated with the repair welds were made.
process was employed using BS 2926 19.12.3LBR electrodes of diameter up to 5 mm. The final arrangement of the bored and welded headers, creating the Type 316H stainless steel test component, is shown in Fig. 2. Further geometric parameters,
2. Materials and geometry The results presented in this paper were obtained from a butt-welded stainless steel cylindrical test component manufactured from two forged AISI Type 316H stainless steel steam headers provided by British Energy. The parent material 1% proof stress was 272 MPa. The two forgings had an outside diameter of 432 mm and wall thickness of 63.5 mm. Each forging originally consisted of a pipe with a nozzle. The nozzle on each forging had been removed, leaving a pipe with the original hole for the nozzle intersection in place. The headers were then bore-machined to a target nominal thickness of 35 mm. However, the actual thickness varied from 32 to 39 mm, with a thickness of 37 mm being typical of that recorded at the residual stress measurement locations. After machining, the headers were solution heat treated (for 1 h at 1050 8C followed by air cooling) to remove residual stresses remaining from original fabrication of the headers and bore machining. One end of each bored header was further machined to create a J-groove girth weld preparation typical of that used for manufacturing steam raising pipe welds. The machined sections were then mounted on a mandrel and joined using a ‘down-hand’ welding technique by slowly rotating the sections. After a TIG root pass, the manual metal arc (MMA)
Fig. 2. General arrangement of butt-welded test component.
D. George, D.J. Smith / International Journal of Pressure Vessels and Piping 82 (2005) 279–287
281
Table 1 Summary of test component geometry and weld parameters Component Global geometry
Base material
Weld characteristics
Geometry of welds
Characteristics Outer radius, R0 (mm) Wall thickness, t (mm) Rm/t Stainless steel 0.2% Proof stress (MPa) 1% Proof stress (MPa) Designation Weld type No. of passes Average arc energy (kJ/mm) Weld material 1% Proof stress (MPa) Arc-length, b (degrees) Mean length of repair (mm) Width, W (mm) Depth, d (mm) Repair depth, d/t (%)
216 z37 5.3 AISI Type 316H 212 272 Girth MMA 26 2.14
WR1 MMA 24 0.85
WR2 MMA 17 0.79
WR3 MMA 12 0.88
316L 446 360
316L 446 19
316L 446 20
316L 446 20
–
66
73
76
32 37 –
25 28 76
24 9.5 26
24 7.5 20
3. Residual stress measurements
Measurement locations were chosen to provide insight to the distribution of residual stress that act as driving force for material failure for component operation at high temperature, particularly in HAZ regions. Location DH1 was in the HAZ at the very edge of the weld cap of the original girth weld and situated at 2258 from TDC. The second through thickness location, DH2, measured before weld repairs were introduced, was at 1358 from TDC and at the weld centre line. After weld repairs, WR1, 2 and 3, were introduced, deep hole measurements of residual stress were made at four locations. DH3 (908 from TDC) was circumferentially positioned at mid-length of WR1 and axially located in the heat affected zone of the original weld at the edge of the cap. DH4 (1808 from TDC) was in the centre (mid-length and mid-width) of repair weld WR2. DH5 (2708 from TDC), was adjacent to the mid-length position of WR3, and like DH3 axially located in the HAZ at the edge of the cap of the original butt-weld. A final measurement DH6 (2858 from TDC) was made at mid-width of the original weld and circumferentially located 25 mm beyond the end of weld repair WR3.
3.1. Locations
3.2. Measurement procedures and analysis
The deep hole method was used to measure the distributions of in-plane residual stresses through the thickness of the original girth weld and the repair welds. Two measurements, DH1 and 2, were made in the original girth weld before the repair welds were introduced. Then four other measurements, DH3, 4, 5 and 6, were made either in the repair weld or adjacent to the repair weld. The details of the locations are illustrated in Fig. 3, and further details are given in Table 2.
Since a number of publications provide detailed descriptions of the deep-hole method [10,14–17], only a brief summary of the method is provided here. The five steps in the method are also illustrated in Fig. 4. The first step consists of attaching reference blocks to the component. These act as a means of ensuring alignment and initial material for drilling. Then a 3.175 mm diameter hole is gun-drilled completely through the specimen.
material properties and weld characteristics are provided in Table 1. After completing deep hole residual stress measurement in the test component girth weld, three repair welds, (WR1, WR2 and WR3) were introduced into the weld at different circumferential locations. WR1 consisted of a short (198 arc length) deep (76% of the wall thickness) repair located at 908 from top dead center (TDC) of the pipe. Note that TDC corresponds to the line joining the centres of two nozzles in each of the forgings. WR2 and WR3 were also short weld repairs, but were shallower (26 and 20% of the wall thickness) compared to WR1. The circumferential positions of the repairs were chosen to minimize interaction with each other and with the header nozzles holes at TDC. The details of the geometry of each repair weld and their locations are illustrated in Fig. 3. The repair cavities, centred at mid-width and fully contained within the original weld, were excavated using grinding tools and rotary burs. Geometric dimensions of the repairs and the welding parameters are given in Table 1.
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Fig. 3. Location of measurements and geometry of weld repairs in the 37 mm thick butt-welded test component.
Using a gundrill provides a very straight hole and good surface finish. In the third step an air probe is introduced into the drilled hole to measure accurately the diameter of the reference hole. Measurements are made every 0.2 mm in depth and at every 108 angle around the hole. Trepanning
out a 20 mm diameter core containing the reference hole as its axis follows this. This releases the residual stresses present into the core. Finally, in step five, the deformed reference hole is re-measured using the air probe. The measured distortions are then transformed to residual
Table 2 Summary of deep hole measurement locations Deep hole number
Position relative to TDC (degrees)
Location
Notation for Fig. 8
DH1 DH2 DH3 DH4 DH5 DH6
225 135 90 180 270 285
Original as-welded girth weld, heat affected zone Original as-welded girth weld at weld centre line Short deep 76% weld repair, WR1, in original weld heat affected zone Short shallow 26% weld repair, WR2, in weld centre line of weld repair Short shallow 20% weld repair, WR3, in original weld heat affected zone Short shallow 20% weld repair, WR3, in original weld centre-line, 25 mm from WR3
Original HAZ Original WCL WR1-HAZ WR2-WCL WR3-HAZ WR3-25 mm-WCL
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Fig. 4. Steps in the deep hole residual stress measurement method.
stresses. The accuracy of the air probe system is of the order of 0.5 mm. The accuracy of measurement with a 0.5 mm error is of the order of G30 MPa for a Young’s modulus of 200 GPa. The residual stresses were calculated from the measured displacements using an elastic analysis. This analysis assumes that there is no plastic deformation when residual stresses relax during trepanning. The diametral distortion, 3, at the edge of the reference hole is a function of angle q around the hole and through the thickness z. The distortion is related to the applied remote stress field by:
3½q; z Z
Dd 1 Z fsxx f ½q; z C syy g½q; z C sxy h½q; zg d0 E
(1)
The functions f[q,z], g[q,z] and h[q,z] are given by: f ½q; z Z A½zf1 C B½z2 cosð2qÞg g½q; z Z A½zf1 K B½z2 cosð2qÞg h½q; z Z 4A½zB½zsinð2qÞ
(2)
where A[z] and B[z] are coefficients that represent the uniform expansion and eccentricity of the reference hole, respectively. A minimum of three measurements is necessary to evaluate the three in-plane stresses.
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If, however, measurements are obtained at n angles around the hole, a least squares fit to the measured distortions can be used to determine the stresses. At a given through thickness position z1 Eq. (1) is rewritten in matrix form as: 3 Z ½Ms
(3)
where the measured distortion and residual stress vectors are 3 Z ½3q ½q1 ; z1 ; 3q ½q2 ; z1 ; .; 3q ½qn ; z1 ; 3zz T ; Z ½sxx ; syy ; sxy T
(4)
The compliance matrix M is given by 2 3 f ½q1 ; z1 g½q1 ; z1 h½q1 ; z1 6 7 1 6 f ½q2 ; z1 g½q2 ; z1 h½q2 ; z1 7 7 ½M Z 6 7 E6 « « « 4 5 f ½qn ; z1
s
(5)
g½qn ; z1 h½qn ; z1
The optimum residual stress field s~ is obtained by using Pseudo-Inverse or Moore–Penrose inverse matrices, where s~ Z ½M 3~m
(6)
and where [M]*Z(MTM)K1MT is the Pseudo-Inverse of the matrix [M] and s~ is the optimum stress vector ½s~ xx; s~ yy; s~ xy T that best fits the measured distortions 3~m : In the next section the calculated residual stresses determined from measured distortions are shown. At all measurement locations, DH1 to 6, the results are shown in terms of the axial and hoop residual stresses. Measurements were taken along radial lines through the wall thickness, and the hoop and axial stresses were equated to s~ xx and s~ yy , respectively. It was found also that the shear stress s~ xy was smaller than the measurement error, estimated to be G30 MPa.
Fig. 5. Comparison of measured girth weld hoop and axial residual stress through wall profiles at the weld centre-line and in the heat affected zone (prior to repairs).
The measured tensile hoop stresses are generally greater than the axial at both weld centre-line and heat-affected zone locations. Comparing the magnitudes of the stresses at the two locations it is seen that the axial stresses are higher at the weld centre-line, whereas for hoop stresses the reverse is true.
4. Results and discussion
4.2. Repaired conditions
4.1. Girth weld (prior to repairs)
All experimental measurements for the girth weld and repair welds are illustrated in Figs. 6 and 7. Fig. 6 shows the distribution of the residual stress for measurements, DH2, 4 and 6, obtained on the weld centreline. Irrespective of the magnitude of the residual stresses the distributions on the weld centreline are very similar. However, the introduction of the shallow repair weld, WR2, raised the peak tensile hoop residual stress from about 300 MPa to just less than 500 MPa (as shown in Fig. 6a). The elevation of the hoop residual stress occurred completely through the depth of the repair weld (about 10 mm, see Table 2) and for depths up to 18 mm through the wall thickness. For greater depths the distribution of the hoop stress in the girth weld and repair welded cases were essentially identical. At a position
Two measurements, DH1 and 2 were made prior to the introduction of the three weld repairs. Fig. 5 compares the measured hoop and axial residual stress distributions at the weld centre-line with the heat-affected zone. In general, the measured stress distributions at the two locations are very similar to each other. The hoop stresses are highly tensile towards the outer surface and fall into compression at about 20 mm below the surface. The axial stress distributions illustrate a sinusoid shape, that is tensile towards the outer surface, dropping into compression in the central region and returning to tension towards the inner surface.
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Fig. 6. Comparison of measured residual stress profiles in the original girth weld with the repair welded at the weld centreline (i.e. mid-width of girth weld).
Fig. 7. Comparison of measured residual stress profiles in the original girth weld with the repair welded at the heat affected zone.
25 mm beyond the end of the repair weld, (in the original weld), the hoop residual stress was found to be essentially the same as if no repair had been introduced. In contrast, as shown in Fig. 6b, the axial residual stresses from each of the three measurements on the weld centre-line differ significantly. The introduction of the repair weld has raised the axial residual stress through the thickness by about 100 MPa at the weld centre-line. For example, the peak tensile axial stress was about 300 MPa for the original girth weld near to the outer surface of the component. In the repair weld the peak tensile residual stress was raised to about 400 MPa. Furthermore, through thickness regions that were compressive for the girth weld are now tensile after the introduction of weld repair WR2. Finally, unlike the hoop residual stress, at a position 25 mm beyond the end of the weld repair, the axial residual stress is reduced through the entire thickness by about 40 MPa. This evidence supports finite element simulations [18] for finite length repairs which show that tensile transverse membrane stress induced along the length of a repair is self-equilibrated by a compressive stress field beyond the repair ends.
The introduction of weld repairs also changes the through thickness residual stress distributions in regions adjacent to the heat affected zones of the original weld. Fig. 7 shows results obtained from deep hole measurements, DH1, 3 and 5, with DH1 corresponding to the as-welded state, DH3 for the deep weld repair WR1 and DH5 for the shallow weld repair WR3. The deep weld repair, WR1, hoop residual stresses, shown in Fig. 7a, near to the outer surface of the pipe remain the same as for the as-welded case. However, at greater depths there is no region of compressive hoop stress as observed for the as-welded case. Rather, the hoop residual stresses were found to be entirely tensile through the thickness. This is also the case for the shallow repair weld, WR3, although the tensile residual stresses were lower in magnitude than for the as welded case. The axial residual stress through the depth in the region of the heat affected zone was also increased with the introduction of repair weld, WR1. Fig. 7b shows that the peak tensile axial residual stress was increased from about 200 MPa for the girth weld to about 350 MPa for the deep weld repair. The near outer surface elevation of the axial residual stress by introducing a deep weld repair was also
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continued through the thickness of the pipe, with the exception of the residual stresses on the inner surface. For the shallow repair, WR3, there was again an increase in the axial residual stress, with the earlier compressive region observed for the as welded case increased to a tensile residual stress through the entire pipe wall. 4.3. Membrane and bending stresses It is instructive to examine the results from the measurements in terms of the membrane and bending stresses. These components were determined by integrating a piecewise linear fit to the measured data, shown in Figs. 5–7, to satisfy equilibrium. The resulting membrane and bending stresses, normalised with respect to the 1% proof stress of the weld metal, are shown in Fig. 8, where the bending stress has been plotted as a function of the membrane stress for both hoop and axial residual stresses. The through wall bending stress
corresponds to the stress at the outer pipe surface. Results are shown for the heat affected zone and weld centreline measurements in Fig. 8a and b, respectively. Also shown in Fig. 8, for each axis, is the ratio of the parent material 1% proof stress to the weld metal 1% proof stress (equal to 0.6). For the HAZ and weld centreline, in the girth weld and repair welds, the membrane stresses did not exceed about 50% of the 1% weld metal proof stress. Certainly at the weld centreline the bending residual stresses were generally higher than the membrane stresses. The results also show that the membrane residual stresses did not exceed the parent material 1% proof stress. At the weld centreline, all the hoop residual stresses contained high levels of bending stress compared to the membrane stress. The hoop bending stresses were substantially higher than the parent material 1% proof stress. The results summarised in Fig. 8 show that introducing repair welds increased the membrane residual stress. For example, for the deep weld repair, WR1, the axial and hoop membrane stresses in the heat affected zone were increased compared with the original girth weld. A similar trend is seen for the membrane residual stresses at the weld centreline for the shallow repair weld, WR2. Notably, in the region beyond the end of repair WR3 in the original weld metal, there was a reduction in the membrane and bending residual stresses in the axial direction compared with the original girth weld.
5. Concluding remarks The through-thickness distribution of in-plane residual stress at the weld centerline and in the heat affected zone of a 37 mm thick stainless steel pipe girth weld has been measured using the deep hole method. Residual stress measurements were made both before and after the introduction of part-circumference (208 arc length) repair welds of varying depths that were fully embedded in the original girth weld. The shapes of the measured distributions were broadly the same before and after the introduction of weld repairs However, weld repairs increased the axial membrane residual stress at the mid-length of the repair compared with the original girth weld.
Acknowledgements
Fig. 8. Membrane and bending stresses for girth weld and repair welded conditions (see Table 2 for notation).
The work at the University of Bristol was supported by funding from the Industrial Management Committee [HSE(NII), British Energy and BNFL]. Their support is gratefully acknowledged. The authors wish to thank staff at British Energy including Mr John Bouchard and Mr A Wilby for supervising manufacture of the test specimen. This paper is published with the permission of British Energy Generation Limited.
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References [1] Withers PJ, Bhadeshia HKDH. Residual stress, Part 1: Measurement techniques. Mater Sci Technol 2001;17:355–6. [2] Liu J. Handbook of measurement of residual stresses. Georgia, USA: Society for Experimental Mechanics, Inc., Fairmont Press Inc.; 1996. [3] Kandill FA, Lord JD, Fry AT, Grant PV. A review of residual stress measurement methods, a guide to technique selection, NPL Report, MATC(A) 04; 2001. [4] Keller HP, Kerkoff H Giffeler, Meinhardt RJ. Residual stresses and their influence on the integrity of pressure vessel, T&V Rheinland Institut for Material Testing, Report No. SB 203/89, 1789. [5] Noyan IC, Cohen JB. Residual stresses—measurement by diffraction and interpretation. Berlin: Springer; 1987. [6] Parlane AJA. The determination of residual stresses—a review of contemporary measurement techniques. Residual stresses in welded construction and their effects 1977;1(8):63–78. [7] Withers PJ, Webster PJ. Neutron and synchrotron X-ray strain scanning. Strain 2001;37(1):19–33. [8] Procter E. The basic centre hole technique, TWI Seminar, Newcastle: Residual Stresses in Welded Construction; 1985. [9] Keil S. Experimental determination of residual stresses with the ring core method and an on-line measurement system. Experimental techniques. The Soc for Exp Mech Inc 1993;17–24. [10] Bonner NW. Measurement of residual stresses in thick section steel welds. PhD Thesis, University of Bristol; 1996.
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[11] Prime MB. Residual stress measurement by successive extension of a slot: the crack compliance method. Appl Mech Rev 1999;52(2): 75–96. [12] Leggatt RH. Residual stresses and distortion in multi-pass butt welded joints in type 316 stainless steel. In: Proceedings of the international conference on residual stresses, 1, 1986. p. 997–1004. [13] Hill MR, Nelson DV. The inherent strain method for residual stress determination and its application to a long welded joint. In: PVP, 318. New York: ASME; 1995. p. 343–52. [14] Leggatt RH, Smith DJ, Smith SD, Faure F. Development and experimental validation of the deep hole method for residual stress measurements. J Strain Anal 1996;31:177–86. [15] Bonner NW, Smith DJ. Measurement of residual stresses using the deep hole method. PVP 1996;327:53–65. [16] Bonner NW, Smith DJ, Leggatt RH. Measurement of residual stresses in thick section steel welds Proceedings of the 10th international conference on experimental mechanics, A.A. Balkema 1994, p. 767–72. [17] Smith DJ, Bouchard PJ, George D. Measurement and prediction of through thickness residual stresses in thick section welds. J Strain Anal 2000;35(4):287–305. [18] Elcoate CD, Denis RJ, Bouchard PJ, Smith MC. Three dimensional multi-pass repair simulations. Int J Pressure Vessels Piping 2004 [Special Issue on Residual Stresses at Repair Welds].
Through thickness measurement of residual stresses in a stainless steel cylinder containing shallow and deep weld repairs D. George, D.J. Smith* Department of Mechanical Engineering, University of Bristol, Bristol BS8 1TR, UK
Abstract A programme of residual stress measurements was undertaken using the deep hole method. The measurements obtained the throughthickness residual stress profiles before and after introducing deep and shallow part-circumferential weld repairs into a 37 mm thick stainless steel cylinder. Measured through-thickness distributions were obtained at mid-width and in the heat affected zone of both the original weld and repairs. The results show that the membrane and bending components of the in-plane residual stresses are generally increased when weld repairs are introduced. q 2004 Published by Elsevier Ltd. Keywords: Residual stress; Deep-hole method; Weld repair
1. Introduction Welded components are usually repaired during manufacture or in-service after removal of defects or degraded material. However, the magnitude and distribution of the residual stresses for repair welds are not well known. For the case of repair welds that are well embedded inside existing welds it is uncertain whether repair welds, not subjected to post weld heat treatment, contain higher residual stress than in the unrepaired weld. Numerous methods are available to measure stresses in steel. For example, there is a wide range of experimental techniques available to measure surface or through-thickness stresses, particularly residual stresses, in engineering components [1–3]. These are summarised in Fig. 1. These methods are generally classified into two categories, noninvasive and invasive. The degree of destruction depends on the extent of material removed from the component. Most non-invasive (or non-destructive) techniques are usually restricted to near-surface depths down to about 1 or 2 mm in steels. These methods are mainly developed from wave refraction measurements such as electro-magnetism [4] penetrating to a depth down to 10 mm, conventional X-ray [5] capable of measurement to a depth of about 0.02 mm * Corresponding author. 0308-0161/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.ijpvp.2004.08.006
and ultra-sonic [6] going down to a depth of about 2 mm. For steels greater penetration, down to 20 mm, can be achieved using neutron diffraction [7]. Invasive methods all rely on measurement of strain relaxation resulting from material removal. Semi-destructive techniques include centre-hole drilling [8], ring-coring [9] and deep-hole drilling [10]. Techniques using surface strain gauges (centre-hole and ring-coring) are usually able to measure residual stresses in depths from about 1 mm up to about 15 mm. In contrast the deep-hole drilling method is capable of measuring stress at depths up to 500 mm in steel by non-contact measurement of a reference hole drilled into the component. These semi-destructive methods allow several measurements to be made on the specimen but not at the same location. Finally, fully destructive techniques such as slotting [11], block removal, splitting and layering (BRSL) [12] and the inherent strain [13] methods enable through-thickness measurements of residual stress to be made. However, they destroy the specimen completely so that no further measurements can be obtained. The focus of this paper is the measurement of throughwall distribution of in-plane residual stresses in a girth weld and repair-welded stainless steel test component using the deep-hole method [14–17]. Residual stresses were measured on a stainless steel girth weld of about 37 mm wall thickness and 432 mm outside diameter with and without weld repairs,
280
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Fig. 1. Residual stress measurement methods in steel.
in the plane perpendicular to the normal of the specimen outer surface. The manufacture of the pipe is described, together with a summary of the deep hole method. First, the residual stresses were measured in the girth weld of the pipe, and then weld repairs introduced. Then measurements of residual stress associated with the repair welds were made.
process was employed using BS 2926 19.12.3LBR electrodes of diameter up to 5 mm. The final arrangement of the bored and welded headers, creating the Type 316H stainless steel test component, is shown in Fig. 2. Further geometric parameters,
2. Materials and geometry The results presented in this paper were obtained from a butt-welded stainless steel cylindrical test component manufactured from two forged AISI Type 316H stainless steel steam headers provided by British Energy. The parent material 1% proof stress was 272 MPa. The two forgings had an outside diameter of 432 mm and wall thickness of 63.5 mm. Each forging originally consisted of a pipe with a nozzle. The nozzle on each forging had been removed, leaving a pipe with the original hole for the nozzle intersection in place. The headers were then bore-machined to a target nominal thickness of 35 mm. However, the actual thickness varied from 32 to 39 mm, with a thickness of 37 mm being typical of that recorded at the residual stress measurement locations. After machining, the headers were solution heat treated (for 1 h at 1050 8C followed by air cooling) to remove residual stresses remaining from original fabrication of the headers and bore machining. One end of each bored header was further machined to create a J-groove girth weld preparation typical of that used for manufacturing steam raising pipe welds. The machined sections were then mounted on a mandrel and joined using a ‘down-hand’ welding technique by slowly rotating the sections. After a TIG root pass, the manual metal arc (MMA)
Fig. 2. General arrangement of butt-welded test component.
D. George, D.J. Smith / International Journal of Pressure Vessels and Piping 82 (2005) 279–287
281
Table 1 Summary of test component geometry and weld parameters Component Global geometry
Base material
Weld characteristics
Geometry of welds
Characteristics Outer radius, R0 (mm) Wall thickness, t (mm) Rm/t Stainless steel 0.2% Proof stress (MPa) 1% Proof stress (MPa) Designation Weld type No. of passes Average arc energy (kJ/mm) Weld material 1% Proof stress (MPa) Arc-length, b (degrees) Mean length of repair (mm) Width, W (mm) Depth, d (mm) Repair depth, d/t (%)
216 z37 5.3 AISI Type 316H 212 272 Girth MMA 26 2.14
WR1 MMA 24 0.85
WR2 MMA 17 0.79
WR3 MMA 12 0.88
316L 446 360
316L 446 19
316L 446 20
316L 446 20
–
66
73
76
32 37 –
25 28 76
24 9.5 26
24 7.5 20
3. Residual stress measurements
Measurement locations were chosen to provide insight to the distribution of residual stress that act as driving force for material failure for component operation at high temperature, particularly in HAZ regions. Location DH1 was in the HAZ at the very edge of the weld cap of the original girth weld and situated at 2258 from TDC. The second through thickness location, DH2, measured before weld repairs were introduced, was at 1358 from TDC and at the weld centre line. After weld repairs, WR1, 2 and 3, were introduced, deep hole measurements of residual stress were made at four locations. DH3 (908 from TDC) was circumferentially positioned at mid-length of WR1 and axially located in the heat affected zone of the original weld at the edge of the cap. DH4 (1808 from TDC) was in the centre (mid-length and mid-width) of repair weld WR2. DH5 (2708 from TDC), was adjacent to the mid-length position of WR3, and like DH3 axially located in the HAZ at the edge of the cap of the original butt-weld. A final measurement DH6 (2858 from TDC) was made at mid-width of the original weld and circumferentially located 25 mm beyond the end of weld repair WR3.
3.1. Locations
3.2. Measurement procedures and analysis
The deep hole method was used to measure the distributions of in-plane residual stresses through the thickness of the original girth weld and the repair welds. Two measurements, DH1 and 2, were made in the original girth weld before the repair welds were introduced. Then four other measurements, DH3, 4, 5 and 6, were made either in the repair weld or adjacent to the repair weld. The details of the locations are illustrated in Fig. 3, and further details are given in Table 2.
Since a number of publications provide detailed descriptions of the deep-hole method [10,14–17], only a brief summary of the method is provided here. The five steps in the method are also illustrated in Fig. 4. The first step consists of attaching reference blocks to the component. These act as a means of ensuring alignment and initial material for drilling. Then a 3.175 mm diameter hole is gun-drilled completely through the specimen.
material properties and weld characteristics are provided in Table 1. After completing deep hole residual stress measurement in the test component girth weld, three repair welds, (WR1, WR2 and WR3) were introduced into the weld at different circumferential locations. WR1 consisted of a short (198 arc length) deep (76% of the wall thickness) repair located at 908 from top dead center (TDC) of the pipe. Note that TDC corresponds to the line joining the centres of two nozzles in each of the forgings. WR2 and WR3 were also short weld repairs, but were shallower (26 and 20% of the wall thickness) compared to WR1. The circumferential positions of the repairs were chosen to minimize interaction with each other and with the header nozzles holes at TDC. The details of the geometry of each repair weld and their locations are illustrated in Fig. 3. The repair cavities, centred at mid-width and fully contained within the original weld, were excavated using grinding tools and rotary burs. Geometric dimensions of the repairs and the welding parameters are given in Table 1.
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Fig. 3. Location of measurements and geometry of weld repairs in the 37 mm thick butt-welded test component.
Using a gundrill provides a very straight hole and good surface finish. In the third step an air probe is introduced into the drilled hole to measure accurately the diameter of the reference hole. Measurements are made every 0.2 mm in depth and at every 108 angle around the hole. Trepanning
out a 20 mm diameter core containing the reference hole as its axis follows this. This releases the residual stresses present into the core. Finally, in step five, the deformed reference hole is re-measured using the air probe. The measured distortions are then transformed to residual
Table 2 Summary of deep hole measurement locations Deep hole number
Position relative to TDC (degrees)
Location
Notation for Fig. 8
DH1 DH2 DH3 DH4 DH5 DH6
225 135 90 180 270 285
Original as-welded girth weld, heat affected zone Original as-welded girth weld at weld centre line Short deep 76% weld repair, WR1, in original weld heat affected zone Short shallow 26% weld repair, WR2, in weld centre line of weld repair Short shallow 20% weld repair, WR3, in original weld heat affected zone Short shallow 20% weld repair, WR3, in original weld centre-line, 25 mm from WR3
Original HAZ Original WCL WR1-HAZ WR2-WCL WR3-HAZ WR3-25 mm-WCL
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Fig. 4. Steps in the deep hole residual stress measurement method.
stresses. The accuracy of the air probe system is of the order of 0.5 mm. The accuracy of measurement with a 0.5 mm error is of the order of G30 MPa for a Young’s modulus of 200 GPa. The residual stresses were calculated from the measured displacements using an elastic analysis. This analysis assumes that there is no plastic deformation when residual stresses relax during trepanning. The diametral distortion, 3, at the edge of the reference hole is a function of angle q around the hole and through the thickness z. The distortion is related to the applied remote stress field by:
3½q; z Z
Dd 1 Z fsxx f ½q; z C syy g½q; z C sxy h½q; zg d0 E
(1)
The functions f[q,z], g[q,z] and h[q,z] are given by: f ½q; z Z A½zf1 C B½z2 cosð2qÞg g½q; z Z A½zf1 K B½z2 cosð2qÞg h½q; z Z 4A½zB½zsinð2qÞ
(2)
where A[z] and B[z] are coefficients that represent the uniform expansion and eccentricity of the reference hole, respectively. A minimum of three measurements is necessary to evaluate the three in-plane stresses.
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If, however, measurements are obtained at n angles around the hole, a least squares fit to the measured distortions can be used to determine the stresses. At a given through thickness position z1 Eq. (1) is rewritten in matrix form as: 3 Z ½Ms
(3)
where the measured distortion and residual stress vectors are 3 Z ½3q ½q1 ; z1 ; 3q ½q2 ; z1 ; .; 3q ½qn ; z1 ; 3zz T ; Z ½sxx ; syy ; sxy T
(4)
The compliance matrix M is given by 2 3 f ½q1 ; z1 g½q1 ; z1 h½q1 ; z1 6 7 1 6 f ½q2 ; z1 g½q2 ; z1 h½q2 ; z1 7 7 ½M Z 6 7 E6 « « « 4 5 f ½qn ; z1
s
(5)
g½qn ; z1 h½qn ; z1
The optimum residual stress field s~ is obtained by using Pseudo-Inverse or Moore–Penrose inverse matrices, where s~ Z ½M 3~m
(6)
and where [M]*Z(MTM)K1MT is the Pseudo-Inverse of the matrix [M] and s~ is the optimum stress vector ½s~ xx; s~ yy; s~ xy T that best fits the measured distortions 3~m : In the next section the calculated residual stresses determined from measured distortions are shown. At all measurement locations, DH1 to 6, the results are shown in terms of the axial and hoop residual stresses. Measurements were taken along radial lines through the wall thickness, and the hoop and axial stresses were equated to s~ xx and s~ yy , respectively. It was found also that the shear stress s~ xy was smaller than the measurement error, estimated to be G30 MPa.
Fig. 5. Comparison of measured girth weld hoop and axial residual stress through wall profiles at the weld centre-line and in the heat affected zone (prior to repairs).
The measured tensile hoop stresses are generally greater than the axial at both weld centre-line and heat-affected zone locations. Comparing the magnitudes of the stresses at the two locations it is seen that the axial stresses are higher at the weld centre-line, whereas for hoop stresses the reverse is true.
4. Results and discussion
4.2. Repaired conditions
4.1. Girth weld (prior to repairs)
All experimental measurements for the girth weld and repair welds are illustrated in Figs. 6 and 7. Fig. 6 shows the distribution of the residual stress for measurements, DH2, 4 and 6, obtained on the weld centreline. Irrespective of the magnitude of the residual stresses the distributions on the weld centreline are very similar. However, the introduction of the shallow repair weld, WR2, raised the peak tensile hoop residual stress from about 300 MPa to just less than 500 MPa (as shown in Fig. 6a). The elevation of the hoop residual stress occurred completely through the depth of the repair weld (about 10 mm, see Table 2) and for depths up to 18 mm through the wall thickness. For greater depths the distribution of the hoop stress in the girth weld and repair welded cases were essentially identical. At a position
Two measurements, DH1 and 2 were made prior to the introduction of the three weld repairs. Fig. 5 compares the measured hoop and axial residual stress distributions at the weld centre-line with the heat-affected zone. In general, the measured stress distributions at the two locations are very similar to each other. The hoop stresses are highly tensile towards the outer surface and fall into compression at about 20 mm below the surface. The axial stress distributions illustrate a sinusoid shape, that is tensile towards the outer surface, dropping into compression in the central region and returning to tension towards the inner surface.
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Fig. 6. Comparison of measured residual stress profiles in the original girth weld with the repair welded at the weld centreline (i.e. mid-width of girth weld).
Fig. 7. Comparison of measured residual stress profiles in the original girth weld with the repair welded at the heat affected zone.
25 mm beyond the end of the repair weld, (in the original weld), the hoop residual stress was found to be essentially the same as if no repair had been introduced. In contrast, as shown in Fig. 6b, the axial residual stresses from each of the three measurements on the weld centre-line differ significantly. The introduction of the repair weld has raised the axial residual stress through the thickness by about 100 MPa at the weld centre-line. For example, the peak tensile axial stress was about 300 MPa for the original girth weld near to the outer surface of the component. In the repair weld the peak tensile residual stress was raised to about 400 MPa. Furthermore, through thickness regions that were compressive for the girth weld are now tensile after the introduction of weld repair WR2. Finally, unlike the hoop residual stress, at a position 25 mm beyond the end of the weld repair, the axial residual stress is reduced through the entire thickness by about 40 MPa. This evidence supports finite element simulations [18] for finite length repairs which show that tensile transverse membrane stress induced along the length of a repair is self-equilibrated by a compressive stress field beyond the repair ends.
The introduction of weld repairs also changes the through thickness residual stress distributions in regions adjacent to the heat affected zones of the original weld. Fig. 7 shows results obtained from deep hole measurements, DH1, 3 and 5, with DH1 corresponding to the as-welded state, DH3 for the deep weld repair WR1 and DH5 for the shallow weld repair WR3. The deep weld repair, WR1, hoop residual stresses, shown in Fig. 7a, near to the outer surface of the pipe remain the same as for the as-welded case. However, at greater depths there is no region of compressive hoop stress as observed for the as-welded case. Rather, the hoop residual stresses were found to be entirely tensile through the thickness. This is also the case for the shallow repair weld, WR3, although the tensile residual stresses were lower in magnitude than for the as welded case. The axial residual stress through the depth in the region of the heat affected zone was also increased with the introduction of repair weld, WR1. Fig. 7b shows that the peak tensile axial residual stress was increased from about 200 MPa for the girth weld to about 350 MPa for the deep weld repair. The near outer surface elevation of the axial residual stress by introducing a deep weld repair was also
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continued through the thickness of the pipe, with the exception of the residual stresses on the inner surface. For the shallow repair, WR3, there was again an increase in the axial residual stress, with the earlier compressive region observed for the as welded case increased to a tensile residual stress through the entire pipe wall. 4.3. Membrane and bending stresses It is instructive to examine the results from the measurements in terms of the membrane and bending stresses. These components were determined by integrating a piecewise linear fit to the measured data, shown in Figs. 5–7, to satisfy equilibrium. The resulting membrane and bending stresses, normalised with respect to the 1% proof stress of the weld metal, are shown in Fig. 8, where the bending stress has been plotted as a function of the membrane stress for both hoop and axial residual stresses. The through wall bending stress
corresponds to the stress at the outer pipe surface. Results are shown for the heat affected zone and weld centreline measurements in Fig. 8a and b, respectively. Also shown in Fig. 8, for each axis, is the ratio of the parent material 1% proof stress to the weld metal 1% proof stress (equal to 0.6). For the HAZ and weld centreline, in the girth weld and repair welds, the membrane stresses did not exceed about 50% of the 1% weld metal proof stress. Certainly at the weld centreline the bending residual stresses were generally higher than the membrane stresses. The results also show that the membrane residual stresses did not exceed the parent material 1% proof stress. At the weld centreline, all the hoop residual stresses contained high levels of bending stress compared to the membrane stress. The hoop bending stresses were substantially higher than the parent material 1% proof stress. The results summarised in Fig. 8 show that introducing repair welds increased the membrane residual stress. For example, for the deep weld repair, WR1, the axial and hoop membrane stresses in the heat affected zone were increased compared with the original girth weld. A similar trend is seen for the membrane residual stresses at the weld centreline for the shallow repair weld, WR2. Notably, in the region beyond the end of repair WR3 in the original weld metal, there was a reduction in the membrane and bending residual stresses in the axial direction compared with the original girth weld.
5. Concluding remarks The through-thickness distribution of in-plane residual stress at the weld centerline and in the heat affected zone of a 37 mm thick stainless steel pipe girth weld has been measured using the deep hole method. Residual stress measurements were made both before and after the introduction of part-circumference (208 arc length) repair welds of varying depths that were fully embedded in the original girth weld. The shapes of the measured distributions were broadly the same before and after the introduction of weld repairs However, weld repairs increased the axial membrane residual stress at the mid-length of the repair compared with the original girth weld.
Acknowledgements
Fig. 8. Membrane and bending stresses for girth weld and repair welded conditions (see Table 2 for notation).
The work at the University of Bristol was supported by funding from the Industrial Management Committee [HSE(NII), British Energy and BNFL]. Their support is gratefully acknowledged. The authors wish to thank staff at British Energy including Mr John Bouchard and Mr A Wilby for supervising manufacture of the test specimen. This paper is published with the permission of British Energy Generation Limited.
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References [1] Withers PJ, Bhadeshia HKDH. Residual stress, Part 1: Measurement techniques. Mater Sci Technol 2001;17:355–6. [2] Liu J. Handbook of measurement of residual stresses. Georgia, USA: Society for Experimental Mechanics, Inc., Fairmont Press Inc.; 1996. [3] Kandill FA, Lord JD, Fry AT, Grant PV. A review of residual stress measurement methods, a guide to technique selection, NPL Report, MATC(A) 04; 2001. [4] Keller HP, Kerkoff H Giffeler, Meinhardt RJ. Residual stresses and their influence on the integrity of pressure vessel, T&V Rheinland Institut for Material Testing, Report No. SB 203/89, 1789. [5] Noyan IC, Cohen JB. Residual stresses—measurement by diffraction and interpretation. Berlin: Springer; 1987. [6] Parlane AJA. The determination of residual stresses—a review of contemporary measurement techniques. Residual stresses in welded construction and their effects 1977;1(8):63–78. [7] Withers PJ, Webster PJ. Neutron and synchrotron X-ray strain scanning. Strain 2001;37(1):19–33. [8] Procter E. The basic centre hole technique, TWI Seminar, Newcastle: Residual Stresses in Welded Construction; 1985. [9] Keil S. Experimental determination of residual stresses with the ring core method and an on-line measurement system. Experimental techniques. The Soc for Exp Mech Inc 1993;17–24. [10] Bonner NW. Measurement of residual stresses in thick section steel welds. PhD Thesis, University of Bristol; 1996.
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[11] Prime MB. Residual stress measurement by successive extension of a slot: the crack compliance method. Appl Mech Rev 1999;52(2): 75–96. [12] Leggatt RH. Residual stresses and distortion in multi-pass butt welded joints in type 316 stainless steel. In: Proceedings of the international conference on residual stresses, 1, 1986. p. 997–1004. [13] Hill MR, Nelson DV. The inherent strain method for residual stress determination and its application to a long welded joint. In: PVP, 318. New York: ASME; 1995. p. 343–52. [14] Leggatt RH, Smith DJ, Smith SD, Faure F. Development and experimental validation of the deep hole method for residual stress measurements. J Strain Anal 1996;31:177–86. [15] Bonner NW, Smith DJ. Measurement of residual stresses using the deep hole method. PVP 1996;327:53–65. [16] Bonner NW, Smith DJ, Leggatt RH. Measurement of residual stresses in thick section steel welds Proceedings of the 10th international conference on experimental mechanics, A.A. Balkema 1994, p. 767–72. [17] Smith DJ, Bouchard PJ, George D. Measurement and prediction of through thickness residual stresses in thick section welds. J Strain Anal 2000;35(4):287–305. [18] Elcoate CD, Denis RJ, Bouchard PJ, Smith MC. Three dimensional multi-pass repair simulations. Int J Pressure Vessels Piping 2004 [Special Issue on Residual Stresses at Repair Welds].