Through-thickness distributions of residual stresses in two extreme heat-input thick welds: A neutron diffraction, contour method and deep hole drilling study

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Acta Materialia 61 (2013) 3564–3574 www.elsevier.com/locate/actamat

Through-thickness distributions of residual stresses in two extreme heat-input thick welds: A neutron diffraction, contour method and deep hole drilling study W. Woo a, G.B. An b,⇑, E.J. Kingston c, A.T. DeWald d, D.J. Smith e, M.R. Hill f a

Neutron Science Division, Korea Atomic Energy Research Institute, Daejeon 305-353, South Korea b Technical Research Laboratories, POSCO, Pohang 790-300, South Korea c Veqter Ltd., University Gate East, Bristol BS1 5UB, UK d Hill Engineering LLC, Rancho Cordova, CA 95670, USA e Department of Mechanical Engineering, University of Bristol, Bristol BS8 1TR, UK f Department of Mechanical and Aerospace Engineering, University of California, Davis, CA 95616, USA Received 10 December 2012; received in revised form 5 February 2013; accepted 12 February 2013 Available online 9 April 2013

Abstract Spatial variations of residual stresses were determined through the thickness of 70 mm thick ferritic steel welds created using low (1.7 kJ mm1) and high (56 kJ mm1) heat inputs. Two-dimensional maps of the longitudinal residual stress were obtained by using the contour method. The results were compared to neutron diffraction measurements through the thickness at different locations from the weld centerline. The deep hole drilling technique was utilized to confirm the maximum stress locations and magnitudes. The results show that significant tensile stresses (90% of yield strength) occur along the weld centerline near the top surface (within 10% of the depth) in the low heat-input specimen. Meanwhile, in the high heat-input weld, the peak stress moved towards the heat-affected zone at a depth of 40% of the thickness. Finally, the influence of residual stresses on potential fracture behavior of the welded joints is discussed. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Residual stress; Neutron diffraction; Contour method; Deep hole drilling; Welding

1. Introduction As the use of large-scale, high-strength metallic structures increases in civil engineering constructions and other industries, the need to safely operate the structures has led to an emphasis on fracture and fatigue-related failure assessments [1,2]. This is particularly when heavy-section or very thick steel plate and pipes are used. In particular, for welded structures the presence of tensile residual stresses is potentially detrimental to the integrity and performance of the structure because they can lead to abrupt crack initiation and fracture [3,4]. Furthermore, most

⇑ Corresponding author. Tel.: +82 54 220 8013; fax: +82 54 220 6825.

E-mail address: [email protected] (G.B. An).

manufacturing industries are aggressively pursuing new joining technologies, introducing high heat inputs into welded structures to improve manufacturing efficiency and productivity [5]. For example, instead of the conventional low heat-input (LHI) methods using less than 2 kJ mm1, new extremely high heat-input (HHI) joining techniques using over 50 kJ mm1 have emerged recently. The HHI processes inherently generate detrimental tensile residual stresses and potentially degrade the fatigue strength and life of the components. Through-thickness variations of the residual stresses are of particular importance in heavy-section and HHI welded structures [6]. However, due to the intrinsic limitations and difficulties in many of the residual stress measurement techniques very few methods have been applied to heavy-section welded structures [7–9].

1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.02.034

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This paper explains a detailed study undertaken to obtain through-thickness distributions of residual stresses in 70 mm thick ferritic steel plates welded using low and very high heat input processes. Three methods of residual stress measurement were used in this work: neutron diffraction (ND), the contour method (CM) and deep hole drilling (DHD). Neutron diffraction (ND) has become a well-established method for measuring residual stresses in the interior of engineering structures and components [10,11]. It has the unique advantage in providing volumeaveraged bulk measurements. However, most of the instruments for ND method are unable to increase their depth of penetration to much over 25 mm thickness in steel plate specimens due to insufficient neutron flux [12]. The contour method (CM) is a newly devised method for measuring residual stress over a cross-section [13,14]. Displacements of a cut surface occur due to the relaxation of the internal stress. The surface displacements are measured and the residual stresses are recreated using a finite element model. The forces required to ensure that the measured deformed surface is returned to its original position represent the residual stresses. The method provides a two-dimensional (2-D) map of the residual stresses normal to the cut surface. Like the contour method, deep hole drilling (DHD) is a mechanical strain relief technique for measuring residual stresses along a line through the component thickness [15,16]. The stresses are calculated via the distortions of a reference hole created through the thickness of interest. One can obtain a number of stress data along the depth profile; however, it is necessary to drill repeatedly at the other locations to obtain more detail across a section. Several studies have been reported that combine complementary measurement methods such as ND, CM and DHD [17–23]. Wimpory et al. compared the results of the residual stress obtained by the ND and the DHD methods in a 100 mm thick steel T-plate joint. However, to obtain the ND measurements, a 12.5 mm thick slice was removed from the joint and used to decrease the neutron beam path length [17]. Consequently, the out-of-plane stress field was modified. Prime et al. illustrated results for CM and ND measurements with both techniques having the ability to measure low-magnitude stresses (30 MPa, less than 0.05% of the elastic modulus) in 25.4 mm thick Al alloy friction stir welds [19]. Bouchard reported extensive results from round robin studies in the NeT European project in a 17 mm thick stainless steel bead-on-plate plate weld with the very low heat input (0.6 kJ mm1) [21]. Differences among the measured stresses obtained by ND, CM and DHD methods were attributed to the different sampling volumes (or spatial resolutions) among the techniques. Brown et al. used ND for the determination of the residual stresses in a uranium cylinder part welded by a localized electron beam [23]. Since uranium poses significant complexities for ND, caused by an orthorhombic structure, large grains and anisotropic mechanical/thermal properties, it was compared with the mechanical-based CM measurement. Results from both

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measurement processes were within 50 MPa of each other. In this paper, we present: (i) spatial variations of macroscopic residual stresses through the thickness of 70 mm thick ferritic steel weld specimens measured by three different methods (ND, CM and DHD); (ii) comparison of the through-thickness stress distributions between the LHI and HHI weld specimens; and (iii) most importantly the locations of the maximum tensile residual stress in each case. 2. Processing, microstructure and mechanical properties As-received commercial high-strength low-carbon steel (0.05 C, 0.1 Si, 1.2 Mn, 0.01 P and balance Fe, in wt.%) was made using a thermomechanically controlled processing (TMCP) method [2]. The steel was specially designed to permit the HHI (called EH40-TMCP) welding and processing. The base material had 20 lm sized equiaxed fine grains due to the typical hot rolling at 1150 °C and water quenching to 500 °C followed by air cooling to room temperature. Two plates (each 1000 mm long by 150 mm wide by 70 mm thick) were joined using LHI multi-pass flux cored arc welding and HHI one-pass electro-gas welding techniques. The macroscopic structures are shown in Fig. 2 with cross-sections extracted from the plates. The conventional LHI plates were welded using a heat input of 1.7 kJ mm1 using a welding current, voltage and electrode speed of 255 A, 32 V and 30 mm s1, respectively. The welding process provided a bead width of 50 mm on the top surface after 61 passes, with 21 layers welding in a groove of 30°, as shown in Fig. 2a. On the other hand, the HHI welding process experienced a very high heat input of 56 kJ mm1 generated using two electrodes traveling parallel to the rolling direction of the plate (moving up vertically) at a constant speed of 0.6 mm s1 using 400 A and 44 V. The weld was manufactured as one pass in a groove of 20°, producing a bead width of 40 mm on the top surface, as shown in Fig. 2b. The welding consumable for the LHI welding process was a typical commercial product (nominal composition: 0.04 C–0.37 Si–1.32 Mn– 0.016 P–0.006 S–1.53 Ni, wt.%). For the HHI welding process the consumable was specially designed for the HHI conditions (0.09 C–0.33 Si–1.62 Mn–0.01 P–0.006 C–0.62 Ni, wt.%). After welding, the two as-welded large plates were each cut slowly into three parts, with dimensions 230 mm long and 300 mm wide, by using a band saw, as shown in Fig. 1a. One smaller plate was provided for each residual stress measurement method, ND, CM and DHD. In the remainder of the paper, x, y and z directions denote longitudinal (welding), transverse and normal directions, respectively, as shown in Fig. 1. Microstructural characterization was performed on cross-sections of the two plates. Fig. 3 represents a composite picture of the micrographs of the sections. The locations for the optical microscopy were: centerline (0 mm), fusion line (15-18 mm from the centerline) and 1, 2, 5, 20 mm

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z

300

230

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230 ND

230

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Fig. 1. Schematic of the sample dimension. Measurement locations, contour in the cut surface and the reference core were shown for the neutron diffraction (ND), contour method (CM) and deep hole drilling (DHD), respectively.

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50 mm

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Fig. 2. Cross-sectional macrostructure of the 70 mm thick ferritic steel weld specimens: (a) the LHI multi-pass, (b) the HHI one-pass. Marked squares denote the location of optical micrographs.

from the fusion line at the mid-thickness of the specimen as marked by six squares, shown in Fig. 2. The continuous changes in the microstructure of the LHI and HHI welded specimens reveal typical variations of the grain size in each characteristic region associated with the weld, fusion line, coarse grain zone, fine grain zone and base material in the TMCP joint [24]. In particular, in Fig. 3a, the grain size of the coarse grain zone in the LHI welded specimen was 40 lm (obtained by the linear intercept method). For the HHI welded specimen, Fig. 3b shows that much larger grains of 80 lm are widespread up to 5 mm from the fusion line, which is consistent with sustained high temperature exposure resulting from the high heat input. Four tensile specimens were machined from each plate at the same locations of subsequent ND measurements, i.e., 0, 30, 60 and 100 mm from the weld centerline, Fig. 1, at the mid-thickness, with the gage length parallel

to the longitudinal direction (x). Following the ASTM E 8M-04 procedure the dimension of the tensile specimen was 6.25 mm diameter and 32 mm long in the gage section. All specimens were prepared using electrical-discharge machining (EDM) and tensile tests were performed at room temperature using a typical load frame with hydraulic wedge-grips and a constant cross-head velocity providing an initial strain rate of 6.7  104 s1. The yield and tensile strengths of welding consumables obtained from tensile specimens at the 0 mm location were 570 and 610 MPa (in the LHI welded specimen), 500 and 630 MPa (in the HHI welded specimen), respectively. At other locations (30, 60, 100 mm) the yield and tensile strengths were 430 and 540 MPa, respectively, both in the LHI and HHI welded specimens. Two more sets of tensile specimens were prepared from each plate to measure diffraction elastic constants (Ehkl) for the (1 1 0) and (2 1 1) dif-

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100 μm

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weld

Fusion line

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Fig. 3. Optical micrographs taken at the centerline (0 mm), fusion line and 1, 2, 5, 20 mm from the fusion line at the mid-thickness of: (a) LHI and (b) HHI.

fraction planes using neutron diffraction. Elastic lattice strains were measured as a function of the applied stress at intervals of 50 MPa. 3. Residual stress measurement methods 3.1. Neutron diffraction Spatially resolved neutron strain scanning was performed by using the residual stress instrument (RSI) at the Korea Atomic Energy Research Institute (KAERI) [25]. The residual stresses were measured through the thickness of the 70 mm thick welds, utilizing the two-peak combined methodology with the benefit of the wavelength dependence of neutron beam penetration [26,27]. First, ˚ and 1.55 A ˚ were selected for the difwavelengths of 2.39 A fraction planes (1 1 0) and (2 1 1) at scattering angles of 72.4° and 82.5°, respectively. Note that both Si (1 1 1) and Si (2 2 0) monochromators at take-off angles of 45° and 48° produced neutrons with the appropriate wavelengths. Such a configuration increased the penetration depth of neutrons significantly due to lower attenuation [26]. This was achieved in brief, through reducing the strain uncertainty (Dx), which is related to the peak height, H (=Hoel l) and the integral peak intensity, I (=Ioell) in the determination of the peak position [12]: pffiffiffi   2 ð1Þ ðDxÞ ¼ u2x =I ½1 þ 2 2B=H  where ux is the standard deviation of the peak, B is the background level, l is the attenuation coefficient, l is the path length and Ho and Io are the peak height and integral peak intensity at the zero path length, respectively. Since l is proportional to the total neutron cross-section (rt), the lower rt (attenuation) exponentially decreases the Dx as l increases. Secondly, the configuration of the diffractometer minimizes the actual beam path length and enhances the beam intensity [27]. For example, the (1 1 0) diffraction plane is advantageous for the transmission geometry and the (2 1 1) diffraction plane for the reflection geometry of

the welded specimens. The combined method is available because of the similar diffraction elastic constants (225.5 GPa) of the two diffractions [11]. The scattering gage volume of the neutron beam was defined by 4 mm wide and 8 mm (or 20 mm) high input slits and a 4 mm output slit. Then, the nominal scattering volumes of 4(x)  8(y)  4(z) mm3 for the diffraction patterns with their scattering vectors were configured to be parallel to the x direction and a volume of 20(x)  4(y)  4(z) mm3 for the y or z directions. Both volumes retained the 4 mm spatial resolution along the thickness direction (z). A total of 13 points were measured through the thickness of welded specimens starting from 5 mm from the top surfaces to 65 mm in 5 mm steps at locations from the weld centerline of 0, 30, 60 and 100 mm, as shown in Fig. 1. Note that all strain components were measured by using the (1 1 0) ˚ except that diffraction plane with a wavelength of 2.39 A the strain normal components (z) at the 30 to 40 depths (only three points) were measured by using the (2 1 1) dif˚ . Mostly, the fraction plane with a wavelength of 1.55 A measurement period was 1 h for each strain component, achieving a strain uncertainty of ±100 le. However, to obtain strains in the z-direction at mid-thickness locations, measurement times were up to 12 h. Diffraction peaks were analyzed using a least squares Gaussian fitting method in the RSI data analysis program. Once the peak position was determined, the elastic lattice strains (e) were calculated using e = coth(h  ho) = (d  do)/do, where the ho (do) and h (d) are the diffraction angles (d-spacings) for the stress-free and stressed materials at each position, respectively [11]. The generalized Hooke’s law was used to convert elastic strains (ex, ey, ez) to the residual stresses (rx, ry, rz) along the three orthogonal directions (x, y and z) in a given plate. Measured diffraction elastic constants were averaged using E110/211 = (2E110 + E211)/3. This resulted in 220 GPa (at the 0 mm location) and 226 GPa (at the 30, 60 and 100 mm locations) being used for the ND analysis. The difference between elastic constants E110 and E211 was less

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than ±10 GPa. The Poisson’s ratios m110 and m211 of 0.28 were used [11]. After strain scanning, the plate specimen, measured using the ND technique, was sectioned using electro-discharge machining to prepare a “stress free” reference sample. Comb-like reference samples were extracted along each line of strain scanning, as shown in Fig. 1. The combs were 10 mm long (x), 4 mm wide (y) and 5 mm deep (z). The stress-free lattice spacing (do) was carefully measured with the assumption that there was no steep variation of do along the weld direction (x) due to chemical composition changes [27,28]. Note that the gage volume was 8 mm3 (2  2  2 mm3) for all do measurements. 3.2. Contour method The contour method (CM) is a technique for the determination of the residual stress over a cross-section [13]. The displacements of the cut surface (the surface contour) are created as residual stresses are relaxed. The displacements are compared to an assumed flat surface contour. The residual stresses are computed using an elastic finite element model. The main experimental procedures include: (1) specimen cutting, (2) surface displacement measurement and (3) data reduction and analysis. A detailed description of the general methodology is given in Refs. [18,19]. It is assumed that only elastic relaxation occurs on cutting and there are no cutting-induced stresses. Each selected specimen was cut in half at the mid-length position as shown Fig. 1. This was done by using wire electro-discharge machining with a 100 lm diameter brass wire. Each welded specimen was submerged in temperature-controlled deionized water. A ‘‘skim cut’’ setting was used in the machining process to minimize cutting-induced stresses. A symmetrical clamping arrangement was used to minimize specimen movement as the stresses relaxed. After cutting, the normal direction displacements on the cut surfaces were measured using a scanning confocal laser probe. The entire cross-section contour was mapped using a regular grid 0.5  0.5 mm. The maximum peak-to-valley range of the contour was 200 lm for each plate. The measurement noise and surface roughness was filtered from the measured surface contour by fitting to a smooth analytical surface. The misfit (3.0 lm) was selected by minimizing the estimated uncertainty in the results. A three-dimensional elastic finite element model for stress calculation was constructed for one half of the welded specimen using ABAQUS 6.7. The sample geometry was meshed with hexahedral elements and it was assumed that each material was homogeneous, isotropic and linearly elastic. The smoothed surface contour was applied to the model as a displacement boundary condition, and linear elastic stress analysis then provides the stress field normal to the plane of sectioning (rx) in the welded specimen. Two values of elastic moduli were used assuming that E = 206 GPa (at the weld centerline) and 219 GPa at all other locations. Poisson’s ratio (m) was used to be 0.28 [11].

3.3. Deep hole drilling The DHD method is a mechanical strain relaxation technique used to determine through-thickness residual stresses by measuring strains during stress relief from the removal of a small amount of material [15,16]. The procedure is divided into the following stages: (1) reference bushes are attached to the front and back surfaces of the component to determine uncertainties, (2) a 1.5 mm diameter reference hole is gun drilled through the component and reference bushes, (3) the initial diameter (Uo) of the reference hole is measured at intervals of 0.2 mm through the depth and at 22.5° (angle, h) intervals around the circumference of the reference hole (i.e about the z-axis), (4) a cylindrical core of 5 mm diameter is trepanned along the z-axis using electro-discharge machining and finally (5) the diameter (U) of the reference hole is re-measured at the same locations. Since the diameter Uo represents the diameter with stresses and the diameter U represents the reference hole diameter when the stresses are relieved, the difference (DU = Uo  U) permits determination of the original residual stresses. For the plane stress condition (rz = 0) and an isotropic material, the normalized diameter distortion (Urr) is related to the applied remote stress field (rx, ry and rxy) by U rr ðhÞ ¼ DU=Uo ¼ 1=E½rx ð1 þ 2 cos 2hÞ þ ry ð1  2 cos 2hÞ þ 4rxy sin 2h

ð2Þ

where the x direction coincides with the h = 0 direction [29]. Inversion of Eq. (2) provides the in-plane (perpendicular to the axis of the reference hole) residual stresses. If a component contains high magnitude, tri-axial residual stresses, the material can undergo plastic relaxation during the trepanning process. To account for the plasticity a modification to the standard DHD technique, called the incremental DHD (iDHD), is adopted [16]. In brief, during the iDHD technique the core is extracted in incremental machining steps, and the diameter of the reference hole is measured between each increment. The DHD technique was used for a single stress vs. depth measurement on each of the LHI and HHI welded specimens. The measurement was taken 30 mm from the centerline (y) at the mid-length of the plate (x), as shown in Fig. 1. The conventional DHD method was applied to the LHI welded specimen. Due to the expected high stresses in the HHI welded specimen the iDHD method was applied in increments of 6 mm starting at a depth of 4 mm from the weld top surface. As with the contour method, E = 219 GPa m = 0.28. 4. Results 4.1. Residual stresses by neutron diffraction The measured distributions of residual stresses are shown in Figs. 4 and 5 for the LHI and HHI welded

W. Woo et al. / Acta Materialia 61 (2013) 3564–3574

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Stress (MPa) Fig. 4. Residual stresses through the thickness of the LHI specimen using neutron diffraction along (a) 0, (b) 30, (c) 60 and (d) 100 mm locations from the centerline. The longitudinal residual stress (rx) of an as-received base metal plate was profiled as a thick gray line [6].

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Stress (MPa) Fig. 5. Residual stresses through the thickness of the HHI specimen using neutron diffraction along (a) 0, (b) 30, (c) 60 and (d) 100 mm locations from the centerline.

specimens, respectively. Each figure shows the throughthickness variations of residual stresses through the four different measurement locations. The stress uncertainties were mostly less than ±50 MPa. For the LHI specimen the through-thickness stress profiles of the rx and ry exhibit a “U” shape distribution, as shown in Figs. 4a and b, with large tensile stresses near the top or the bottom surface of the plate. Smith et al. [7] reported a similar stress profile in the region adjacent to the heat-affected zone of 108 mm thick steel weld, with results obtained from the DHD method and a cutting method. Note that the maximum longitudinal residual stress (rx) of 530 MPa was measured at a depth of 5 mm below the top surface along the 0 mm profile, Fig. 4a. It corresponds to the stress that is 93% of the yield strength of the base material. Higher residual stresses are often found near to the top surface of multipass butt welds mainly due to the accumulated thermal

expansion/contraction and non-uniform plastic flow [30,31]. Meanwhile, the stress profiles obtained at the 60 and 100 mm locations, Fig. 4c and d, exhibit a “M” shape, with the rx and ry in compression up to 300 MPa near the surfaces (at depths of 5 and 65 mm) and balanced with tensile residual stresses (200 MPa) at depths of 20 and 50 mm. Note that the through-thickness rx profiles at 100 mm are close to that of an as-received base metal plate, which has experienced similar hot-rolling and waterquenching (shown as a thick gray line in Fig. 4d) [6]. Additionally, hydrostatic stresses (rx  ry  rz) of 100 MPa were found at positions near to the mid-thickness (20– 50 mm depth) and the plane stress condition (rz = 0) is invalid in the current thick plate specimen, Fig. 4d. The results obtained from the HHI welded specimen revealed a more uniform through-thickness distribution in the plate compared to the LHI plate, with Fig. 5a and

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Fig. 6. 2-D mapping of the longitudinal residual stress (rx): (a) LHI and (b) HHI by using the contour method (CM). (c) LHI and (d) HHI constructed by the neutron diffraction (ND) measurements.

b illustrating only mild fluctuations in magnitudes. It is obvious in the 2-D map of CM as shown in Fig. 6. Such uniformity through the thickness of the HHI specimen can be due to the large amount of heat all at once put into the welding groove, which has a low angle of 20°, Fig. 2b. The significant tensile rx developed in the profile at 30 mm from the weld centerline occurred at 20 mm below the top surface, Fig. 5b. The maximum rx was 490 MPa and corresponded to 114% of the yield strength of the base material. Such high values of the residual stresses (even exceeding the initial yield strength) have been routinely reported near the centerline of various low-carbon steel welds [1,6]. It has been attributed to the multi-axial nature of the residual stresses and the strain-hardening effect of material [3,23]. Similar to the LHI welded specimen, the HHI plate results, Fig. 5c and d, revealed an “M” shaped

distribution of residual stresses. It is interesting to note that the large amounts of heat associated with the HHI process reached up to the 60 mm location and destroyed the “M” shaped profile of the through-thickness stress profile, as shown in Fig. 5c. 4.2. Residual stresses by contour and deep hole drilling methods The results from the contour method provided 2-D maps of the rx. These are shown for the LHI welded specimen in Fig. 6a and for the HHI specimen in Fig. 6b. The stress uncertainty for the CM measurements was ±30 MPa. The residual stress maps show clear differences between the two heat inputs. For the LHI specimen, high tensile stresses exist within ±25 mm from the weld center-

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Stress (MPa) Fig. 7. Through-thickness distributions of the longitudinal residual stress (rx) using the contour method (CM); along (a) 0, 30 mm, (b) 60, 100 mm in the LHI and (c) 0, 30 mm, (b) 60, 100 mm in the HHI (extracted from Fig. 6). For comparison neutron diffraction (ND) results were marked (dots).

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Fig. 8. Through-thickness distributions of the longitudinal (rx) and transverse (ry) stresses using: (a) the deep hole drilling (DHD) for the LHI and (b) incremental DHD (iDHD) for the HHI along the 30 mm from the centerline. For comparison neutron diffraction (ND) results were marked (dots).

line near the top of the weld as shown in Fig. 6a. Compressive residual stresses exist at either side of the weld to achieve the necessary stress balance across the section of the LHI specimen. In contrast, the HHI welded specimen had a different distribution, revealing low tensile stress in the weld bead, but high tensile stresses in the heat affected zone, ±30 mm either side of the weld centerline and near mid-thickness, Fig. 6b. This trend is very similar to the ND results, illustrated in Fig. 6c and d, though it has much lower spatial resolution than CM due to the large spacing transverse to the weld. Using data extracted from the maps shown in Fig. 6, CM results were compared to those of ND. Fig. 7 shows profiles of rx on four through-thickness lines (at 0, 30, 60 and 100 mm from the weld centerlines). Overall, the trends for both measurement methods are similar, with the contour measurements slightly lower than the ND results. The CM profiles show a maximum stress of 380 MPa at a depth of 4 mm through the weld centerline for the LHI welded specimen. This is marked by an arrow in Fig. 7a. In the HHI welded specimen, the peak stress was 260 MPa at a depth of 32 mm along the 30 mm profile, as shown in Fig. 7c. A repeat CM measurement was performed on the same HHI welded specimen as that used for the ND measurements. Results of the repeat measurements, Fig. 7c, show similar profiles at the 0 and 30 mm locations. The residual stresses obtained from the DHD and iDHD measurements on the LHI and HHI welded specimens are shown in Fig. 8. Both plates had measurements made at a location of 30 mm from the weld centerline as shown in Fig. 1. For the LHI specimen, the rx and ry show similar profiles over the mid-thickness regions, but vary significantly from each other as they approach either

surface, Fig. 8a. The rx was tensile at a maximum value of 320 MPa at a depth of 1 mm, became compressive and reached to 175 MPa at a depth of 34 mm. The rx then increased again into a secondary tensile peak of 150 MPa at a depth of 50 mm and decreased to a compressive magnitude of 170 MPa at 63 mm. The ry exhibited a similar profile compared to the rx though the magnitudes were higher at most depths. For the HHI welded specimen, Fig. 8b shows that the rx and ry were similar in shape, but dissimilar in the stress magnitudes. The rx dominated and remained tensile throughout the thickness of the specimen, whereas the ry was slightly compressive near the surface of the plate. 5. Discussion 5.1. Through-thickness distribution of residual stresses in low and high heat-input welds The discussion starts by examining the difference in residual stress profiles between the LHI and HHI welds. First, Fig. 9 compares the distribution of the rx through the thickness of the LHI and HHI specimens measured by ND. It is clear that the profiles are different at the weld centerline (0 mm) and 30 mm from the weld centerline, Fig. 9a and b, respectively. Nevertheless, the profiles at distances of 60 and 100 mm from the weld centerline are similar, Fig. 9c and d. It is similar in the ry of LHI and HHI as shown in Figs. 4 and 5, respectively. The results show that the rx in the LHI was mostly higher than in the HHI at the weld centerline, Fig. 9a. This is reversed in the 30 mm profiles, Fig. 9b. More importantly, the maximum rx was located at a depth of 5 mm below the top surface at the weld centerline in the LHI and at a depth of 20 mm below

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LHI HHI

20 30 40 50 60 70

-400 -200

0

200

400

600 -400 -200

0

200

400

600 -400 -200

0

200

400

600 -400 -200

0

200

400

600

Stress (MPa) Fig. 9. Comparison of the longitudinal residual stress (rx) between the LHI and HHI using the neutron diffraction (ND) along (a) 0, (b) 30, (c) 60 and (d) 100 mm locations from the centerline.

the plate surface along the 30 mm profile in the HHI. These maximum stress locations were similarly found in the CM and DHD results (marked arrows) as shown in Figs. 7a–c and 8, respectively. A diagram based on the locations of the maximum residual stresses in the cross-section of the welded specimens is shown in Fig. 10. Additional data for the maximum values of the rx and ry, obtained from 50 and 80 mm thick ferritic steel welds are also shown. These measurements were also obtained using ND, CM and DHD measurement methods [6,32]. In the LHI specimens, the locations are shown along the centerline near the top surface within 10% of the plate depth ratio. In contrast, the maximum stresses occur at locations between 20 and 30 mm away and around mid-thickness (30 to 80% of the depth) in the HHI specimen. This dissimilarity is attributed to the different amounts of heat inputs and welding procedure (passes) followed by the constraints under cooling [31]. For instance, the large heat inputs put into the HHI specimen

Depth / Thickness

0.0

σ σ σ σ σ σ

0.2 0.4

LHI HHI

σ σ σ σ

σ

0.6

Table 1 The results of the maximum longitudinal residual stress (rmax x ), longituto the YS (rmax /YS) in dinal yield strength (YS) and the ratio of the rmax x x the 0 mm (centerline) and 30 mm location of the low heat-input (LHI) and high heat-input (HHI) specimens.

σ σ

0.8 1.0

0

10

20

30

reduce the yielding strength of the welding consumable and circumferential region, leading to a more extensive heat-affected zone, as shown in Fig. 3b. Thus, the softened area could be constrained by wide surrounding cold materials during cooling and it results in the movement of the peak stress location toward the base material region. Indeed, the higher tensile rx was developed, not along the weld centerline, but through the heat-affected zone at 30 mm from the weld centerline in the HHI welded specimen, Fig. 9. The peak value was 490 MPa, which exceeds the initial yield strength by 14%. This is summarized in Table 1. The implications of residual stresses on fracture behavior are important. Tensile residual stresses can reduce the load to fracture of a component and conversely local compression can confine fatigue crack growth to regions adjacent to the tearing of a crack edge [33–35]. For thick specimens, the through-thickness distribution of the residual stresses can be critical to capturing the closure of crack propagation [36,37]. Fig. 11 shows the 2-D maps of the transverse residual stresses (ry) in the LHI and HHI welded specimens. It should be noted that the contour plots were constructed from ND data and that they have large spacing transverse to the weld, which means that they should not be used for a detailed understanding of the spatial variation

40

50

60

70

Distance from the weld centerline (mm) Fig. 10. A diagram of the maximum residual stress locations in the LHI and HHI cases. It includes the stress components (rx and ry), measurement methods (ND, CM and DHD) and plate thickness (50, 70 and 80 mm). The y-axis indicates the depth ratio to the sample thickness.

Specimens

Location (mm)

rmax (MPa) x

YS (MPa)

rmax x =YS

LHI

0 30

530 300

570 430

0.93 0.70

HHI

0 30

250 490

500 430

0.50 1.14

W. Woo et al. / Acta Materialia 61 (2013) 3564–3574

(a) LHI

260 0

260

-200 260

depth (mm)

(b) HHI

5

(MPa) 400 300 200 100 0 -100 -200 -300 -400

15

260

25

260

35 45

260

55 65 0 10

20 30 40

50 60

70

80 90 100

Distance from the weld centerline (mm)

Fig. 11. 2-D mapping of the transverse residual stress (ry): (a) LHI and (b) HHI constructed by the neutron diffraction (ND) measurements.

of stress along the y direction. The spatial distributions of the two specimens are significantly different, similar to the differences in rx measured by CM (as shown in Fig. 6c and d). It is plausible that significant tensile stresses developed through the thickness; for example, at 30 mm location in the HHI specimen they create an easy path for crack propagation, if a crack were to initiate, Fig. 11b. In contrast, the compressive stresses near the mid-thickness of the LHI plate, Fig. 11a, can be an obstacle to crack growth. Consequently, the locations of the maximum residual stress and through-thickness spatial distributions provide an indication of the significance of crack initiation and the integrity of the final thick weld components. 5.2. Comparison of residual stresses from ND, CM and DHD measurements Three types of inherently different techniques were applied to measure residual stresses through the thickness of the different 70 mm thick welded specimens. 2-D mapping of rx was performed using the CM measurements. Areas of high stress were then extensively examined using the ND method, which provided through-thickness distributions of the three orthogonal stress components (rx, ry and rz). The ND results were then confirmed through the thickness at a selected location (at 30 mm) using the DHD method. When comparing the results from the ND and CM methods, there is general qualitative agreement in terms of the maximum stress locations and general character of the stress fields, as shown in Fig. 6. Nevertheless, there are differences in magnitude, as shown in Fig. 7, with

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results from CM generally providing lower magnitude than obtained from ND. In particular, the rx profile in the CM measurements was up to 200 MPa lower than results from the ND method, Fig. 7c. The literature shows some examples of ND giving higher stresses than CM in welded plates, mainly in the high stress region near the weld centerline [8,20–22], although this is not always the case [19,23]. Several reasons for the discrepancy have been suggested: (i) inappropriate “stress-free” reference specimens associated with phase and/or microstructure changes in the ND method [22,27,28]; (ii) different spatial averaging in the CM and ND methods [21,23]; and (iii) the effects of yielding of the cut surface as high stresses are released in the CM method [13]. Considering that there is relatively good agreement at most lower-stress locations, it may be that differences in peak magnitudes are due to local yielding during the contour cut. The results obtained from the application of ND and DHD methods reveal good similarities, Fig. 8. The stress profiles obtained from the DHD method determine the stress magnitudes in both the rx and ry directions. In Fig. 8b, the higher magnitudes in the rx compared to the ry is also manifested through the depth in the HHI welded specimen in both the ND and DHD methods. It is a benefit of using the iDHD process, especially when plastic unloading during the trepanning process because of the presence of high tri-axial stresses. Furthermore, the peaks at depths of 50 mm followed by the various stress fluctuations were successfully measured in both the ND and DHD measurements, Fig. 8a. Such fluctuation near the bottom of 30 mm line is a signature turning into the “M” shape found in the profiles of 60 or 100 mm locations, Fig. 4c and d. 6. Conclusions

1. Microstructure, longitudinal tensile properties and residual stresses were examined in 70 mm thick welded specimens created using low (1.7 kJ mm1) heat input (LHI) and high (56 kJ mm1) heat input (HHI). The average grain size of 40 lm in the coarse grain zone of the LHI welded specimen increased to 80 lm in the HHI welded specimen. The coarse grained zone extended to 5 mm away from the fusion line toward the base material in the HHI welded specimen. The diffraction and bulk elastic constants of the weld material were 3% and 6% lower than those of the base material in the LHI and HHI welded specimens, respectively. 2. In order to obtain full-field knowledge of the magnitudes and spatial distributions of the residual stresses, three methods were applied to measure residual stresses. The longitudinal stress component were two-dimensionally mapped over the cross-section using the contour method (CM) and compared to the neutron diffraction (ND) results measured along through-thickness lines at the weld centerline (0 mm) and 30, 60 and 100 mm loca-

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tions from the weld centerline of the specimens. Finally the maximum stress locations were confirmed by using the deep hole drilling (DHD) technique. 3. The LHI and HHI welded specimens exhibited significant differences in the spatial variations of the residual stresses through the thickness. The three measurement methods show that significant tensile stresses of 530 MPa (up to 93% of the initial yield strength) were developed along the weld centerline near the top surface (within 0.1 depth to thickness ratio) in the LHI welded specimen. However, in the HHI plate the maximum residual stress reached 490 MPa (114% of the base metal’s yield strength) with the peak location in the heat-affected zone 30 mm away from the weld centerline near the mid-thickness.

Acknowledgements This research activity was supported by the Nuclear Research and Development Program of the Korea Science and Engineering Foundation funded by the Korean government. It is also supported by POSCO project No. 20116118. The authors would like to thank C. Truman, J.U. Park, M.H. Kang, and J.W. Lee for their help. References [1] Masubuchi K. Analysis of welded structures. New York: Pergamon; 1980. [2] Ouchi C. ISIJ Int 2001;6:542. [3] Webster GA, Ezeilo AN. Int J Fatigue 2001;23:S357. [4] Withers PJ, Bhadeshia HKDH. Mater Sci Tech 2001;17:366. [5] Deng D, Kiyoshima S. Comp Mater Sci 2012;62:23. [6] Woo W, Em V, Mikula P, An GB, Seong BS. Mat Sci Eng A 2011;528:4120. [7] Smith DJ, Bouchard PJ, George D. J Strain Anal 2000;35:287. [8] Withers PJ, Turski M, Edwards L, Bouchard PJ, Buttle DJ. Int J Pres Ves Pip 2008;85:118. [9] Rossini NS, Dassisti M, Benyounis KY, Olabi AG. Mater Des 2012;35:572.

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