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Investigation of local tensile strength and ductility properties of an X100 submerged arc seam weld
Materials Science & Engineering A 768 (2019) 138475
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Investigation of local tensile strength and ductility properties of an X100 submerged arc seam weld Nazmul Huda a, *, James Gianetto b, Yuquan Ding a, Robert Lazor c, Adrian P. Gerlich a a
Centre for Advanced Materials Joining (CAMJ), Department of Mechanical and Mechatronic Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada b Canmet MATERIALS, Natural Resources, Hamilton, Ontario, L8P 0A5, Canada c TC Energy Corp, Calgary, Alberta, T2P 5H1, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: MA Fracture Strength Ductility Void
Tensile testing is the most common method to determine the mechanical properties of a weld joint, yet the local tensile properties can be different from the global properties when stress localization occurs. In the current study, Digital Image Correlation was used to explain the difference in local and global tensile behavior in different sections of a submerged arc specimen. Tensile tests are used to compare the base metal and weldment properties of a two-pass submerged-arc welded X100 pipe steel in terms of strength and ductility. The base metal exhibits superior tensile strength compared to specimens machined from the second pass (Pass 2) or the overlap weld region spanning between Passes 1 and 2, corresponding to the reheated heat affected zone. Both cross-weld tensile specimens fractured in the fine grained heat affected zone (FGHAZ). Although the overlapped tensile specimen achieved higher global strain compared to the Pass 2 weld specimen, it sustained lower local strain, which promoted fracture in the HAZ. In addition, the overlapped tensile specimen exhibited a higher degree of strain hardening. The martensite-austenite (MA) microconstituents in the FGHAZ of Pass 2 contained higher carbon than the MA in the overlapped FGHAZ, which led to increased MA hardness as measured by nano indentation. A higher fraction of MA with a finer spacing in the FGHAZ of the overlapped HAZ zone provides higher strength and strain hardening but lower ductility. The tensile fracture was dominated by void formation from the hard MA/ferrite interfaces in all tensile specimens, and so an increased fraction of MA led to accelerated microvoid coalescence, and reduced ductility in the overlapped HAZ specimens. In-situ strain monitoring technology via Digital Image Correlation was used to identify the potential fracture location during tensile test. X-ray tomography clearly revealed microvoid formation and coalescences of voids.
1. Introduction Demand for oil and gas is increasing across the world, and the most economical and reliable mode for their transmission is by pipelines. However, construction and installation of large diameter pipelines re mains challenging due to stringent integrity and reliability requirements that must be met for demanding applications, particularly where high longitudinal strains may occur. Utilization of higher strength pipe, with adequate strain capacity, and proven toughness are important properties needed for construction of such transmission pipelines. Various high strength low alloy steel (HSLA) grades such as X70, X80, X100 and X120 (which are designated based on their specified
minimum yield strength in terms of kilopound per square inch) have been incorporated into the American Petroleum Institute (API) standard. These steels offer good pipe body mechanical properties, including strength and toughness; however, during both pipe manufacturing and pipeline construction application of fusion welding may make it chal lenging to achieve these properties. In the case of pipe manufacturing, the submerged arc welding pro cess is often used as the joining method, particularly for larger diameters and thicknesses (>12 mm). To improve productivity, the seam welds are often produced by multiple-wire automatic submerged-arc welding using a two-pass procedure where one pass is deposited from the inside and a second pass is made on the outside of the pipe.
* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Huda), [email protected] (J. Gianetto), [email protected] (Y. Ding), robert_ [email protected] (R. Lazor), [email protected] (A.P. Gerlich). https://doi.org/10.1016/j.msea.2019.138475 Received 17 August 2019; Received in revised form 27 September 2019; Accepted 29 September 2019 Available online 30 September 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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It is widely known that fusion welding can deteriorate steel pipe properties depending on the thermal cycles experienced in the heataffected zone (HAZ). Frequently, the HAZ is considered a critical re gion of the welded joint, since embrittlement (loss of ductility and toughness) and/or softening can occur in this region. The HAZ of single pass welds can be divided into the coarse-grained HAZ (CGHAZ), finegrained HAZ (FGHAZ), inter-critical HAZ (ICHAZ) and sub-critical HAZ (SCHAZ) based on the peak temperature reached during welding as a function of distance from the fusion line [1]. In addition, pipe materials are often too thick to join using a single weld pass, and this demands multiple pass procedures to complete the joint. Multiple pass welding causes reheating of previous HAZ regions by the successive pass(es), such that previous HAZ structures are further modified by a second thermal cycle [1]. As a result, considerable attention has been paid to developing a better understanding of the influence of different HAZ microstructures on their resultant mechanical properties. Several researchers have investigated the influence of MA on HAZ properties, especially toughness. For example, Fairchild et al. reported that low toughness in the HAZ of an HSLA steel can be mainly attributed to the presence of MA islands [2]. Kim et al. showed that single thermal cycle specimens exhibit higher toughness at 10 � C compared to spec imens produced with multiple thermal cycles It was also shown that among the different reheated zones, the intercritically reheated (ICR) CGHAZ exhibited the lowest toughness [3]. The reason for deteriorated properties following a reheating thermal cycle was related to the for mation of MA islands. Liao et al. [4] investigated the effect of cooling times on toughness for the CGHAZ (following a single thermal cycle) and the effect of peak temperature on the ICRCGHAZ (following a second thermal cycle). They indicated that the toughness of material after a single thermal cycle decreased with increasing cooling time. It was also reported that the toughness deteriorated after a reheating thermal cycle with peak temperatures between 973-1023 K was subjected to a constant “t8/5” cooling time of 6s (indicating the time between 800� to 500 � C or Δt800-500 ¼ 6s) [5]. Interestingly, there is ongoing research that is aimed at developing high-performance pipe, which includes the presence of MA. Pipe steels utilized in strain-based design are currently under investigation to pre vent fracture of pipelines by providing high deformation capacity [6]. It has been shown that effective use of MA can be considered a useful approach for developing high-performance materials [6]. For this pur pose, pipeline steels consisting of mainly ferrite-bainite or bainite-MA with highly refined structures that have been developed to attain high ductility and toughness [7]. Although the role of MA has been widely investigated in terms of the effect on toughness, its role in tensile strength, ductility, and strain hardening, has been seldom studied. To-date, work has revealed that increases in ultimate strength promoted by higher fractions of MA was observed by Han et al. [8]. Lanzillotto et al. also reported increased strength from the presence of MA in dual phase steel [9]. Reduced yield strength in X80 linepipe steel was reported by Pedrosa et al. and attributed to the disappearance of MA when material was austenitized and quenched in the first thermal cycle, then later aged in the second thermal cycle at different temperature [10]. In contrast, the presence of MA had been pinpointed as the reason for increased strength by other researchers [11], indicating it can be a desirable component of steel. Yong et al. and Chen et al. showed crack deviation around the MA which resulted in increased strength in tensile tests [12,13]. In addition to strength, studies on the influence of MA on ductility are also rather sparse. In the current authors recent work, it was observed that MA deteriorates HAZ ductility in Gas Metal Arc Welded (GMAW) joints of X80 pipe material [14]. In addition, the application of a potential tempering cycle to a welded specimen shifted the tensile fracture loca tion from the HAZ to the base metal by decomposing detrimental MA, which led to a drastic increase in strength and ductility [15]. The initial loss of ductility was attributed to void formation at the MA/ferrite
interfaces. Further research by Lambert et al. [16] and King et al. pin pointed MA as the crack initiator in low-temperature tensile tests [1,16]. In the previously mentioned work by the authors, it was revealed that a higher fraction of MA provided higher strength but reduced ductility in X80 pipe material. In addition, a higher strain hardening capacity was also associated with higher weld strength when greater fractions of MA were present [14]. The above mentioned investigation mostly considered global ductility (whole gauge length ductility) during tensile testing. However, during tensile deformation, the deformation becomes localized and significant localization of strain (rather than global elongation) occurs in the fracture location. Hence, the local strain distribution differs considerably from the global strain. In the current investigation, the goal was to compare the difference in local and global ductility in specimens extracted from the second pass (Pass 2), versus the overlapping weld region spanning Passes 1 and 2 of submerged arc welded X100 spec imen. In addition, the fracture behaviour and influence of MA on local tensile properties by comparing cross-weld specimens, which contained different HAZ regions from an X100 submerged arc seam weld has been analyzed. The effect of MA on strength, ductility and work hardening was a key aspect of this investigation, and a combination of micro structure characterization, hardness testing, strain mapping in tensile tests are performed to delineate the behaviour of the as-welded versus reheated HAZ regions. 2. Materials and experimental procedure Commercially produced large diameter (914 mm) thick wall (19 mm) X100 pipe manufactured with a two-pass submerged arc seam weld was used in this study. The typical heat input for submerge arc welding is 1.8–2.8 kJ/mm. The base metal chemical composition of the X100 pipe was analyzed using inductively coupled plasma emission spectroscopy and the results are reported in Table 1. The polished specimens were prepared based on the ASTM:E3 stan dard. A 2% Nital etchant was used to reveal the microstructures of the base metal and HAZ regions, while a LePera etchant (25 ml distilled water þ 25 ml Picric acid with 0.25 g Na2S2O5) was used to reveal MA regions (white) by optical microscopy according to ASTM E407-07. An Olympus microscope BX51 was used to observe the microstructureand further details of the MA internal structure were characterized using a Zeiss LEO Scanning Electron Microscope (SEM) and images for subse quent quantification of MA area fraction using Photoshop 6.The image processing method is shown in following Fig. 1. The image produced in Fig. 1b is used to calculated area fraction of MA and average size of MA (length and width average as shown in Fig. 1c). The line intercept method was used for calculating the spacing between the MA features. It was not possible to perform XRD exclusively in the FGHAZ since the zones were too small. XRD was performed on the base metal, Pass 2 HAZ and overlapped HAZ to investigate the fraction of phases in each location. These two regions were isolated by cutting small sections using a precision cutter and confirmed using area fraction measurements indicating that 95% of the Pass 2 HAZ and 83% of overlapped HAZ areas were isolated for XRD analysis. The retained austenite was determined using a Bruker D8 instrument for XRD analysis (using Cu Kα radiation) [17]. SEM analysis including Auger Electron Spectroscopy (AES) was performed using a JEOL 9500F system to map the carbon distribution in MA and particles on the fractured surface. The surface was ion milled to remove surface carbon and oxygen before performing AES analysis. Wavelength Dispersive Spectroscopy (WDS) was used to quantify the carbon content of MA using an accelerating voltage of 20 kV with 10.78 nA (3% variation) beam current. To standardize the analysis, WDS was performed on a similar steel with known carbon content and a martensitic microstructure (to provide a uniform carbon distribution). Vickers microhardness testing was performed using a 300 g load and 15s dwell time to produce mapping across the part of weld. The average hardness value was reported based on five tests. A Hystron Tribo 2
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Table 1 Chemical composition of X100 base metal, wt%. C
Mn
Si
Cr
Ni
Cu
Al
Mo
Nb
N
S
Ca
O
0.06
1.81
0.10
0.03
0.52
0.3
0.04
0.26
0.03
0.0023
0.005
0.005
0.001
Fig. 1. Martensite-austenite grain size measurement and spacing measurement method (a) SEM image used to produce binary image using Photoshop (b) Binary image from Photoshop (c) Average grain size measurement method.
tomography was performed to determine void location and observe void coalescence, with a minimum resolution of 2.3 μm.
indenter was also used with a 2000 μN load to perform nanoindentation on individual MA regions. SEM images were taken after each indenta tion to confirm and identify the MA at each specific location. A total of six cross-weld tensile specimens (three for each condition) were produced by precision electrical discharge machining from Pass 2 as well as spanning the overlapped region near the mid-thickness of the submerged arc weld. The Pass 2 specimen was considered to represent single cycled HAZ regions, whereas the overlapped specimen contained the reheated regions as well as single cycled HAZ regions from both Pass 1 and 2. The tensile specimens were produced following ASTM E8M standard dimensions, except due to curvature of pipe the overall length was reduced. Small-scale flat tensile test specimen were oriented in the hoop direction with dimensions of 87 mm overall length, 25 mm gauge length, 6 mm gauge width and 1 mm thickness. The macrograph of the submerged arc weld is shown in Fig. 2a. A schematic of the single and overlapped thermal cycle area and HAZ regions is shown in Fig. 2b. The location of tensile specimens for Pass 2 and the overlapped condi tion is also shown in Fig. 2b. A Tinius Olsen HK10T servo-mechanical tensile frame with a maximum 10 kN load capacity was used to perform tensile tests at room temperature. Strain monitoring was per formed using Digital Image Correlation (DIC) system from Correlated Solutions with a subset size of 29 and step size 7 (1.1 mm X 1.1 mm square) used throughout testing. Global yield strengths of both Pass 2 and the reheated (overlapping) thermal cycled HAZ were determined based on a 0.2% offset method from stress-strain curves. To investigate straining behaviour of fractured and unfractured sides of tensile spec imen, interrupted tensile tests were performed on separate Pass 2 and overlapped specimens. The HAZ zones were carefully marked on the specimens and the strain was monitored in each of the zones. X-ray
3. Experimental results 3.1. Macro and microstructure The microstructure of the base metal contains ferrite and secondary MA phases, as shown in Fig. 3a, and in more detail in the SEM image in Fig. 3b. The fraction of MA is 7.6% in the base metal while the remaining matrix is ferrite. Variation in the internal structure of MA was observed in base metal using high magnification SEM images. The weld metal region, fusion line, and extent of the HAZ (including CGHAZ, FGHAZ and ICHAZ) as well as the overlapped reheated regions are evident in Fig. 2a and b. The maximum HAZ width for Pass 1 (single thermal cycle region) varies between 3.83-4.00 mm, while the HAZ width of Pass 2 (single thermal cycle region) varies between 3.02-4.03 mm, indicating that the heat inputs of the two passes were comparable. However, in the overlapped reheated region, the HAZ width was 3.60 mm due to the bead shapes of the respective passes. Meanwhile, the weld metal microstructure was comprised of acicular ferrite. 3.2. Hardness distribution The base metal hardness was measured to be 247 � 12.5 HV as an average through the thickness of the plate. The hardness distribution for Pass 1, Pass 2 and overlapped HAZ and weld region are shown in Fig. 4. The Pass 2 weld metal hardness is lower than Pass 1. A comparison between Pass 1 Pass 2, and the overlapped HAZ hardness indicates that
Fig. 2. (a) Macrograph of submerged arc welding in X100 material (b) Schematic of submerged arc welding with the location of tensile specimens. 3
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Fig. 3. (a) Lower magnification image of base metal (b) Higher magnification image of base metal.
3.3. Nano-indentation of MA The MA hardness values in the base metal were 4.90 � 1.04 GPa. The FGHAZ of overlapped HAZ MA hardness were 5.83 � 1.16 GPa at a distance of 1.30 mm from fusion line while for the FGHAZ of Pass 2 HAZ MA hardness was 7.17 � 1.07 GPa. Force-displacement curves obtained from both the MA and ferrite matrix are shown in Fig. 8a. It can be concluded that MA in the FGHAZ of Pass 2 are harder than those in the FGHAZ of overlapped HAZ at a distance of 1.30 mm from fusion line as shown in Fig. 8b. The lower hardness of the MA regions within the FGHAZ of the overlapped HAZ suggests that those contain lower carbon. This can be explained as follows: during reheating, carbon can diffuse out of the structure, which reduces the hardness of MA in the reheated HAZ. In addition, it has been observed that some of the nanoindentation curves from the FGHAZ of overlapped HAZ showed discontinuous deformation which is called “pop-in” behaviour, and can occur in tempered MA (see Fig. 8c). Pop-in behaviour is often attributed to a reduction in dislocation density [18]. The FGHAZ of Pass 2 and FGHAZ of the overlapped HAZ contained MA with a hardness higher than the MA in X100 material investigated by Choi et al. [19]. To investigate the difference in MA hardness between the overlapped FGHAZ and Pass 2 FGHAZ, WDS was used to determine the carbon percentage in the MA at the 1.30 mm distance. WDS analysis has a sensitivity of 0.01%, and spectral resolution as slow as 2eV, and showed that the MA in the FGHAZ of Pass 2 contained higher carbon (0.099–0.181 wt%) than in the FGHAZ (0.073–0.083 wt%) of over lapped HAZ. Ferrite in the FGHAZ of Pass 2 contains 0.07 wt% carbon, while FGHAZ of overlapped HAZ ferrite also contained 0.07 wt%. The presence of austenite was detected in all of the zones as shown in Fig. 9, although the intensity of austenite peak is low.
Fig. 4. (a) Hardness mapping across the submerged welding specimen.
the hardness for Pass 2 HAZ is lower than in Pass 1 or the overlapped HAZ. The microstructures for the different distances from the fusion line for Pass 2 and overlapped HAZ are shown in Fig. 5. It can be observed that the overlapped HAZ microstructure is finer for all the distances from fusion line compared to the HAZ of Pass 2 as shown in Fig. 5. The SEM images were taken at 1.25, 1.50 and 1.75 mm distances from the fusion are shown in Fig. 6. The microstructure of the FGHAZ for Pass 2 and FGHAZ for overlapped HAZ in those locations consisted of combinations of ferrite and MA. The average area fraction of MA at all three distances for Pass 2 FGHAZ locations was 4.93 � 0.73%. The FGHAZ microstructure for the overlapped HAZ contained ferrite and MA, in which the area fraction of MA across the 1.25–1.75 mm distance from the fusion line was 5.57 � 0.41%. SEM observations of the FGHAZ structures at distances 1.25–1.50 mm from the fusion line after LePera etching are shown in Fig. 7a and b. A significant number of white structures were observed at this distance for FGHAZ of Pass 2, which represent untempered MA [14]. However, the number of white structures at the same distance within the overlapped zone FGHAZ was less than in the FGHAZ of Pass 2. SEM observation in the FGHAZ of the overlapped HAZ shows that some of the MA are homogenous and micron-sized (see Fig. 7c), while some MA is slightly larger and contains evidence of carbides within the structure, indicating that those MA regions are auto-tempered, as shown in Fig. 7d. Line intercept measurements reveal that the spacing between MA in the FGHAZ region of Pass 2 (5.11 � 0.7 μm) was higher than in the FGHAZ of the overlapped HAZ (4.27 � 0.57 μm). In addition, the maximum size of MA in the FGHAZ of Pass 2 (3.97 � 0.56 μm) was higher in those positions in comparison to the FGHAZ (3.27 � 0.46 μm) of the overlapped HAZ. The average size of MA in the FGHAZ of Pass 2 was also higher than the FGHAZ in the overlapped HAZ.
3.4. Tensile tests The base metal yield strength was 776 � 2.5 MPa, while the tensile and yield strengths for the Pass 2 specimen were 665 � 6.5 and 653 � 4 MPa respectively. However, tensile and yield strength for overlapped HAZ specimens were 712 � 5 and 683 � 8.5 MPa. The average ductility of X100 base metal was 10.0 � 1.1%, however, the Pass 2 and overlapped specimens exhibited ductility values of 4.8 � 0.5% and 5.7 � 0.2%, respectively. Stress-strain curves for all tensile tests are shown in Fig. 11a. It can be observed that the base metal exhibits the highest tensile and yield strength (Fig. 10a), while the Pass 2 and overlapped specimens exhibited lower yield and tensile strength. Moreover, it can be observed that the overlapped specimen reaches higher strength than the Pass 2 specimen. In addition, the overlapped specimen offered better ductility than the Pass 2 specimen. This a result of the overlapped specimen having more uniform strain hardening behaviour while the Pass 2 had negligible strain hardening between the yield and ultimate tensile stress. A strain-time graph was plotted for all tensile tests (Fig. 10b), which indicates that the overlapped specimen strained later than the single thermal cycle (Pass 2) specimen. It should be noted that the global 4
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Fig. 5. Microstructures of HAZ regions at various distances from fusion line for the Pass 2 and overlapped reheated region.
strains will also depend on both the properties and width of the softened HAZ region versus the gauge length of the tensile specimen, however the HAZ widths are comparable in both specimens in Fig. 10 when compared to the much longer overall gauge length. The macrograph of the fractured specimen is shown in Fig. 10c and d. All the fractures occurred in the HAZ locations with shearing for all specimens. The fracture occurred at a distance of 1.31 mm from the fusion line for both specimens, which was found to be the softest loca tion based on hardness results. It is interesting to observe that although both specimens fractured at the same distance from the fusion line, the overlapped specimen achieved better strength, ductility and work hardening, which is worth further investigation. Local strain distributions across the gauge length (25 mm) were
investigated in the fractured specimen. Though the global strain for base metal was 10.0% (average), localized strain in the fracture location for base metal reached 45%. A similar phenomenon is observed for the Pass 2 and overlapped specimens, in which local strains for the Pass 2 spec imen reach 35% engineering strain locally at the fracture location, while the overlapped specimen reached 33%. It was observed that this value (at the fractured side of the weld) was consistently 2.3% greater for all three tests. This indicates that at fracture location of the FGHAZ of overlapped HAZ actually sustained less strain than the FGHAZ of Pass 2, despite the higher global strain. However, the reason for higher global strain in the FGHAZ of the overlapped HAZ is related to the increased strain on the other (unfractured) side of the weld. It was observed that the strain values for the FGHAZ of overlapped HAZ unfractured side 5
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Fig 6. (a,b,c) Microstructure in FGHAZ of Pass 2 at distances of 1.25, 1.50 and 1.75 mm from fusion line and (d,e,f) Microstructure in FGHAZ of overlapped HAZ at the same distances from fusion line.
were consistently higher by 1.7% (max. 4.0%) than FGHAZ of Pass 2 for all three tests. Localized strain distribution in different zones of interrupted tensile tests are shown in Fig. 11. It can be observed that the FGHAZ of Pass 2 was subjected to a strain of 18.5% where the fracture would potentially occur (Fig. 11a), while the global strain was 3.2%. However, the FGHAZ on the unfractured side only sustained 2.5% strain (Fig. 11a). In contrast, the potential overlapped FGHAZ fracture location sustains 16.7% strain when subjected to 4.6% global strain, while the other overlapped FGHAZ experienced 8.5% local strain. The difference in local strain distribution in potential fracture location between Pass 2 and overlapped FGHAZ may result from ferrite grain size, MA micro constituents size and distribution which will be discussed later.
FGHAZ were 0.3 � 0.3, 1.7 � 0.2, and 2.1 � 1.0%, respectively. The density of voids was 0.001, 0.004 and 0.005/μm2, respectively for the base metal, Pass 2 FGHAZ and overlapped FGHAZ, with sizes in all samples being comparable and varying from less than 1–10 μm. The sizes of voids in three specific size ranges: below 1 μm (smaller void), between 1 to 3 μm (intermediate size void) and more than 3 μm (large void) were quantified, based on three images per specimen and sum marized in Fig. 12. It can be observed that the base metal, FGHAZ of Pass 2, and overlapped FGHAZ, all contained notable fractions of small, in termediate and large voids. However, the fraction of all sizes of voids was lowest in the base metal. The void fraction in the FGHAZ of Pass 2 is low for both intermediate and large voids in comparison to the over lapped FGHAZ. It is also observed that 91% of the voids initiated from the MA/ferrite interfaces, as shown in the inset image of Fig. 12. Fractography indicates that all of the specimens exhibited dimples, which is consistent with ductile fracture in all cases. However, the FGHAZ of Pass 2 and overlapped FGHAZ fracture surface contains a greater number of large voids as shown in Fig. 13. It is also observed that some of the voids contain particles, presumably inclusions. AES analysis from the surfaces of several of these particles indicates that in the base metal they are rich in carbon, sulfur, and manganese. Their composition is shown in Table 2, where the atomic fraction of carbon is 25.2 at% for the particles versus 8 at% for the matrix. The carbon content on the particles observed on the fracture surface is much higher than the
3.5. Fracture mechanism during tensile testing 3.5.1. Void nucleation Significant numbers of voids were observed close to the tensile fracture surface in the base metal, Pass 2 and overlapped specimens using optical microscopy. SEM analysis also confirmed the presence of voids close to the fracture surfaces after tensile testing. The area fraction of voids was calculated based on three SEM images. Quantification of these voids was performed in which the fraction of voids close to the fracture location for the base metal, Pass 2 FGHAZ and overlapped 6
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Fig. 7. (a) LePera etched microstructure in FGHAZ of Pass 2 at distance 1.30 mm (b) LePera etched microstructure in overlapped FGHAZ at distance 1.30 mm (c) untempered MA in FGHAZ of Pass 2 (d) Tempered MA in FGHAZ of overlapped HAZ.
Fig. 8. (a) Force-displacement curves from base metal, FGHAZ and FGHAZ MA and ferrite (F) (b) Average MA hardness (c) Pop in behaviour from tempered MA in FGHAZ of overlapped HAZ. 7
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work [8]. Interrupted tensile tests were performed on the base metal with global strain of 8%, 5%, 3% and 1%. X-ray tomography was per formed in the area identified as the fracture location for 8% and 5% strained specimen. Results from the 5% interrupted tensile test of X100 base metal are shown in Fig. 14. It can be seen that there are clustering of voids in potential fracture locations, although there are some other scattered voids away from the localized clustering. It can also be observed that the voids are coalescing with each other at the locations of clustering. A total of 535 voids were identified by x-ray tomography. The total volume fraction of voids was 0.015%, while their individual vol umes varied between 11.93 to 52,189.3 μm3. SEM observation from a specimen strained to 1% suggests that void initiation can occur imme diately after the yield point. 4. Discussion 4.1. Differences in microstructure and hardness The hardness difference between Pass 2 HAZ and overlapped HAZ (Fig. 4) can be explained by the difference in grain size, MA fraction and distribution. It was observed that the HAZ ferrite grains in Pass 2 were clearly coarser than those in the overlapped HAZ (see Fig. 6). The coarse ferrite lath (potentially due to slower cooling rate) is associated with lower hardness. In addition, MA fractions were lower in the FGHAZ of Pass 2, and the spacing between MA was larger than within the FGHAZ of the overlapped specimen. The combination of coarser ferrite laths, lower MA fraction and higher spacing between MA could lead to lower hardness in the FGHAZ of Pass 2 than the FGHAZ of overlapped HAZ. Although the FGHAZ of Pass 2 exhibited lower hardness than the FGHAZ of the overlapped HAZ, the nano-indentation results suggest that
Fig. 9. XRD spectra (Cu K alpha) near austenite peak in base metal, Pass 2, and the overlapped HAZ, with ferrite peak of 327506 counts (truncated).
carbon percentage of MA regions measured by WDS. 3.5.2. Void formation and void coalescence To investigate the role of void formation and coalescence of X100 base metal, tensile tests were monitored using the DIC strain measure ment system. Several tensile tests were intentionally interrupted at a critical strain value, and the fracture location was subsequently identi fied. The details of this method of has been explained in author’s other
Fig. 10. (a) Stress-strain curves (for 25 mm gauge length in each condition) (b) Strain-time plot (c) Macrograph of fractured overlapped specimen (d) Macrograph of fractured Pass 2 specimen. 8
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Fig. 11. Localized strain distribution across gauge length for (a) Pass 2 specimen subjected to 3.2% global strain and (b) Overlapped specimen subjected to 4.6% global strain (targeted gauge lengths are indicated by the gauge marks (white dots) on the horizontal lines in the images.
4.2. Strength and work hardening The specimen obtained from the overlapped HAZ regions possessed higher global yield and tensile strength than the Pass 2 specimen. It can be observed from Fig. 4 that the overlapped HAZ hardness is higher than the Pass 2 HAZ, which accounts for the higher tensile and yield strength in the former. The difference in hardness is due to the difference in microstructure as discussed above. In addition to hardness, increased dislocation interactions may contribute towards higher strength. Fig. 6 shows that the number of MA regions is less in the Pass 2 FGHAZ compared to the overlapped FGHAZ. Okada et al. [20] showed that high concentration plastic strain occurred in the soft phase around elongated and massive MA which could be due to a higher dislocation density which is also reported in author’s previous work [15]. Magula et al. [21] reported that MA could as act as an obstacle to dislocation movement. More benign and uniform distributions of MA in the FGHAZ of the overlapped zone may give more resistance to dislocation movement, which in turn provides higher strength in this zone. However, disloca tion movement is easier in the FGHAZ of Pass 2 since the spacing be tween MA is larger. In addition to hardness, increased strain hardening could contribute to the high strength of overlapped HAZ. Yan et al. [22] also observed “round house” stress-strain curve behaviour, with strain hardening phenomena in HSLA steel when tempering was performed at 700 � C. High straining ability for this temperature was related to a strong interaction of dislocations and interstitials with grain boundaries and finely dispersed precipitates of carbides and carbo-nitrides. Zuo et al. [23] also observed a similar effect for the stress-strain behaviour for X70 steel. MA in the FGHAZ of overlapped HAZ were found to be tempered, and contained carbides, as shown in Fig. 7d. Carbides in the overlapping FGHAZ might interact with dislocation and enhance strain hardening which results in higher strength. It was shown by electron-backscattered diffraction (EBSD) that an X80 weld HAZ microstructure contained a high fraction of MA with more tempered MA structure. This led to more
Fig. 12. Void fraction adjacent to the fracture surface vs void size distribution in base metal, Pass 2 FGHAZ and overlapped FGHAZ.
the MA hardness in Pass 2 FGHAZ is higher than the MA in the over lapped FGHAZ. The difference in MA microconstituents hardness be tween Pass 2 and overlapped FGHAZ can be attributed to carbon content of MA. WDS analysis showed that the MA regions in the FGHAZ of Pass 2 contain a higher percentage of carbon than those in the FGHAZ of overlapped HAZ. The difference in carbon percentage led to a higher hardness in the MA of the FGHAZ of Pass 2. Meanwhile, the MA in the FGHAZ of overlapped HAZ had lower carbon content. The above dis cussion suggests that grain size and dispersed distribution of MA had a greater contribution to hardness in the FGHAZ of overlapped HAZ. 9
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Fig. 13. Tensile fracture surface observation (a) Base metal (b) Pass 2 FGHAZ (c) Overlapped FGHAZ.
difference in carbon content between MA and ferrite in FGHAZ of overlapped HAZ. However, the difference in carbon content between MA and ferrite is higher in FGHAZ of Pass 2, which is also evident in the nano-indentation results. The difference in mechanical properties be tween MA and matrix is very important in tensile tests. The hardness difference between untempered MA and ferrite in the FGHAZ of Pass 2 is higher than the FGHAZ of overlapped HAZ. Although, the difference between tempered MA and ferrite was smaller in the FGHAZ of over lapped HAZ. From the WDS and nano-indentation results, it can be concluded that FGHAZ of overlapped HAZ properties are more similar to the base metal properties, which is also clear from Fig. 11. Both over lapped HAZs strained in a similar way to the base metal until 8.5% elongation. In contrast, the strain is more localized in one Pass 2 HAZ. The MA properties in the FGHAZ of overlapped HAZ are close to its surrounding matrix, which might lead to deformation uniformly across the microstructure during the tensile test. However, there is a more significant difference between the MA and ferrite matrix in the FGHAZ of Pass 2, which might lead to deformation of the softer ferrite matrix compared to the hard MA which caused strain to localize easily in one of Pass 2 HAZ. Chen et al. [27] reported that at room temperature ferrite yields more easily and has great capability to sustain deformation compared to harder MA, although tempering of hard MA will enhance ductility. Although, the overall ductility is higher for the overlapped specimen, the localized ductility is lower for the overlapped specimen compared to the Pass 2 HAZ. This can be explained by the fact that a non-uniform material was evaluated during tensile testing, and this would enhance the differences in strain localization observed using DIC. In Fig. 11, the Pass 2 specimen was subjected to 3.2% global strain, while the localized strain in potential fracture location was 18.5%. In comparison, the overlapped specimen was subjected to 4.6% global strain, in which case the localized strain in potential fracture location was 16.7%. However, the unfractured side of the overlapped specimen sustained 8.5% strain, while Pass 2 experienced only 2.5% strain. A similar phenomenon has
Table 2 Elemental composition (%at) using Auger for the particle on the fracture surface of the base metal. Particles Matrix
C
O
Fe
S
Ca
Mn
Total
25.2 8
28.2 39.9
29.5 50.4
12.1 1.1
0 0.5
5 0
100 99.9
strain hardening than for the microstructure with low fraction MA and a correspondingly lower fraction of tempered MA structure, which pro duced higher ultimate tensile strength [14]. In the current study, it can be observed that the overlapped specimen exhibited strain hardening while the Pass 2 specimen does not show such behavior. That may result from a high fraction of MA and tempered MA in the FGHAZ of the overlapped specimen. The size of MA can also affect strength due to the initiation of voids. It has been reported that coarse particles initiate voids before smaller ones [24,25], and it is recognized that premature formation of voids can significantly affect tensile strength [26]. An average MA grain size of 1 μm has been reported to be critical for tensile properties [15]. It was observed that the average size of MA in the FGHAZ of Pass 2 was larger than for the FGHAZ of overlapped HAZ. In addition, the maximum size of MA in the FGHAZ of Pass 2 is higher in fracture location than FGHAZ of overlapped HAZ. This implies that it is easier to form voids in the FGHAZ of Pass 2, which did not lead to as much strain hardening as in the FGHAZ of overlapped HAZ which results in lower strength. 4.3. Global Vs. local ductility in relation to void formation It can be observed from Fig. 10 that overlapped specimens have higher global ductility than the Pass 2 specimens and both fractured in the FGHAZ. The MA hardness of FGHAZ of overlapped is found lower than the FGHAZ of Pass 2. WDS analysis showed that there is little
Fig. 14. (a) Void mapping area (600 μm � 1700 μm) using X-Ray tomography in X100 base metal for 5% interrupted tensile test (b) Magnified area of centre region in Fig. 14a showed microvoid and void coalescence. 10
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be shared at this time due to technical or time limitations.
been observed for full fracture tensile specimen which may relate to void formation. It is observed that the overlapped FGHAZ contains more voids than the FGHAZ of Pass 2. In the current investigation, it was observed that the FGHAZ of the overlapped specimen contains more MA and a higher number of MA produces more voids. In addition, the size of voids in the FGHAZ of the overlapped HAZ was larger than in the FGHAZ of Pass 2. This suggests that void formation and void coalescence is easier in FGHAZ of the overlapped specimen than the FGHAZ of Pass 2. A higher number of MA regions led more void formation during tensile testing, which will lead to a loss in localized ductility in the overlapped specimen, although the overall extension was higher than that of the Pass 2 specimen. In the authors previous work, it was observed that a loss of ductility occurs relative to the number of voids [14]. It was observed that higher number of MA promoted formation of more voids and that would lead to reduced ductility [14]. In addition, it has been reported that random void arrays with a larger spacing increased ductility; while reducing the spacing or increasing void size decreases ductility [26].
Acknowledgments The authors wish to acknowledge the Natural Sciences and Engi neering Research Council of Canada (NSERC) for financial support. Further financial support was provided by TC Energy Corp. References [1] C. Davis, J. King, Metall. Mater. Trans. A 25 (1994) 563–573. [2] D. Fairchild, N. Bangaru, J. Koo, P. Harrison, A. Ozekcin, Weld. J. 70 (1991), 321. s-329. s. [3] B. Kim, S. Lee, N. Kim, D. Lee, Metall. Trans. A 22 (1991) 139–149. [4] J. Liao, K. Ikeuchi, F. Matsuda, Trans. JWRI 23 (1994) 223–230. [5] J. Liao, K. Ikeuchi, F. Matsuda, Nucl. Eng. Des. 183 (1998) 9–20. [6] S. Tsuyama, H. Nakamichi, K. Yamada, S. Endo, ISIJ Int. 53 (2013) 317–322. [7] H. Kimura, T. Yokota, N. Ishikawa, S. Kakihara, J. Kondo, in: 2016 11th International Pipeline Conference, American Society of Mechanical Engineers, 2016. V003T005A024-V003T005A024. [8] N. Huda, A.P. Gerlich, Canweld Conference, 2019. [9] C. Lanzillotto, F. Pickering, Met. Sci. 16 (1982) 371–382. [10] I.R.V. Pedrosa, R.S.d. Castro, Y.P. Yadava, R.A.S. Ferreira, Mater. Res. 16 (2013) 489–496. [11] T. Tagawa, T. Miyata, S. Aihara, K. Okamoto, Tetsu-To-Hagane 79 (1993) 1176–1182. [12] Y. Zhong, F. Xiao, J. Zhang, Y. Shan, W. Wang, K. Yang, Acta Mater. 54 (2006) 435–443. [13] L.-h. Chen, Y.-l. Kang, X.-h. Li, D. Wen, G. Liu, J. Univ. Sci. Technol. Beijing 31 (2009) 983. [14] N. Huda, R. Lazor, A.P. Gerlich, Metall. Mater. Trans. A 48 (2017) 4166–4179. [15] N. Huda, Y. Ding, A.P. Gerlich, Mater. Sci. Technol. 33 (2017) 1978–1992. [16] A. Lambert, X. Garat, T. Sturel, A. Gourgues, A. Gingell, Scr. Mater. 43 (2000) 161–166. [17] N. Huda, A.R. Midawi, J. Gianetto, R. Lazor, A.P. Gerlich, Materials Science and Engineering: A, 2016. [18] A. Gouldstone, H.-J. Koh, K.-Y. Zeng, A. Giannakopoulos, S. Suresh, Acta Mater. 48 (2000) 2277–2295. [19] B.-W. Choi, D.-H. Seo, J.-i. Jang, Met. Mater. Int. 15 (2009) 373–378. [20] H. Okada, K. Ikeuchi, F. Matsuda, I. Hrivnak, (1995). [21] V. Magula, Z. Li, H. Okada, F. Matsuda, Trans. JWRI 20 (1991) 69–75. [22] W. Yan, L. Zhu, W. Sha, Y.-y. Shan, K. Yang, Mater. Sci. Eng. A 517 (2009) 369–374. [23] X. Zuo, Z. Zhou, Mater. Res. 18 (2015) 36–41. [24] W. Roberts, B. Lehtinen, K.E. Easterling, Acta Metall. 24 (1976) 745–758. [25] N. Huda, Y. Wang, L. Li, A.P. Gerlich, Mater. Sci. Eng. A (2019) 138301. [26] P. Magnusen, E. Dubensky, D. Koss, Acta Metall. 36 (1988) 1503–1509. [27] J. Chen, Y. Kikuta, T. Araki, M. Yoneda, Y. Matsuda, Acta Metall. 32 (1984) 1779–1788.
5. Conclusions A comparison of between the local tensile properties of single and double thermal cycled HAZ regions of the submerged arc seam weld in an X100 pipe was carried out and the following conclusions were drawn: 1 The region of overlapping and reheated HAZ between two weld passes exhibited higher yield and tensile strengths as well as hard ness compared to the second pass (Pass 2) of the submerged arc weld. In addition, the overlapped HAZ exhibits higher strain hardening and global ductility. 2. The fracture occurred in FGHAZ for both the Pass 2 and overlapped HAZ. A high fraction of MA and smaller spacing between particles might provide enhanced dispersion strengthening in FGHAZ of overlapped HAZ, which provides higher strength in this zone. 3. Although, the FGHAZ of overlapped specimen exhibits higher global strain than Pass 2 FGHAZ, localized strain distributions revealed that the FGHAZ of overlapped specimen exhibited lower strain than for the FGHAZ of Pass 2. A higher fraction of MA led to the larger and denser void formation in FGHAZ of overlapped HAZ, which causes a decrease in local ductility. Data availability The raw/processed data required to reproduce these findings cannot
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Investigation of local tensile strength and ductility properties of an X100 submerged arc seam weld Nazmul Huda a, *, James Gianetto b, Yuquan Ding a, Robert Lazor c, Adrian P. Gerlich a a
Centre for Advanced Materials Joining (CAMJ), Department of Mechanical and Mechatronic Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada b Canmet MATERIALS, Natural Resources, Hamilton, Ontario, L8P 0A5, Canada c TC Energy Corp, Calgary, Alberta, T2P 5H1, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: MA Fracture Strength Ductility Void
Tensile testing is the most common method to determine the mechanical properties of a weld joint, yet the local tensile properties can be different from the global properties when stress localization occurs. In the current study, Digital Image Correlation was used to explain the difference in local and global tensile behavior in different sections of a submerged arc specimen. Tensile tests are used to compare the base metal and weldment properties of a two-pass submerged-arc welded X100 pipe steel in terms of strength and ductility. The base metal exhibits superior tensile strength compared to specimens machined from the second pass (Pass 2) or the overlap weld region spanning between Passes 1 and 2, corresponding to the reheated heat affected zone. Both cross-weld tensile specimens fractured in the fine grained heat affected zone (FGHAZ). Although the overlapped tensile specimen achieved higher global strain compared to the Pass 2 weld specimen, it sustained lower local strain, which promoted fracture in the HAZ. In addition, the overlapped tensile specimen exhibited a higher degree of strain hardening. The martensite-austenite (MA) microconstituents in the FGHAZ of Pass 2 contained higher carbon than the MA in the overlapped FGHAZ, which led to increased MA hardness as measured by nano indentation. A higher fraction of MA with a finer spacing in the FGHAZ of the overlapped HAZ zone provides higher strength and strain hardening but lower ductility. The tensile fracture was dominated by void formation from the hard MA/ferrite interfaces in all tensile specimens, and so an increased fraction of MA led to accelerated microvoid coalescence, and reduced ductility in the overlapped HAZ specimens. In-situ strain monitoring technology via Digital Image Correlation was used to identify the potential fracture location during tensile test. X-ray tomography clearly revealed microvoid formation and coalescences of voids.
1. Introduction Demand for oil and gas is increasing across the world, and the most economical and reliable mode for their transmission is by pipelines. However, construction and installation of large diameter pipelines re mains challenging due to stringent integrity and reliability requirements that must be met for demanding applications, particularly where high longitudinal strains may occur. Utilization of higher strength pipe, with adequate strain capacity, and proven toughness are important properties needed for construction of such transmission pipelines. Various high strength low alloy steel (HSLA) grades such as X70, X80, X100 and X120 (which are designated based on their specified
minimum yield strength in terms of kilopound per square inch) have been incorporated into the American Petroleum Institute (API) standard. These steels offer good pipe body mechanical properties, including strength and toughness; however, during both pipe manufacturing and pipeline construction application of fusion welding may make it chal lenging to achieve these properties. In the case of pipe manufacturing, the submerged arc welding pro cess is often used as the joining method, particularly for larger diameters and thicknesses (>12 mm). To improve productivity, the seam welds are often produced by multiple-wire automatic submerged-arc welding using a two-pass procedure where one pass is deposited from the inside and a second pass is made on the outside of the pipe.
* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Huda), [email protected] (J. Gianetto), [email protected] (Y. Ding), robert_ [email protected] (R. Lazor), [email protected] (A.P. Gerlich). https://doi.org/10.1016/j.msea.2019.138475 Received 17 August 2019; Received in revised form 27 September 2019; Accepted 29 September 2019 Available online 30 September 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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It is widely known that fusion welding can deteriorate steel pipe properties depending on the thermal cycles experienced in the heataffected zone (HAZ). Frequently, the HAZ is considered a critical re gion of the welded joint, since embrittlement (loss of ductility and toughness) and/or softening can occur in this region. The HAZ of single pass welds can be divided into the coarse-grained HAZ (CGHAZ), finegrained HAZ (FGHAZ), inter-critical HAZ (ICHAZ) and sub-critical HAZ (SCHAZ) based on the peak temperature reached during welding as a function of distance from the fusion line [1]. In addition, pipe materials are often too thick to join using a single weld pass, and this demands multiple pass procedures to complete the joint. Multiple pass welding causes reheating of previous HAZ regions by the successive pass(es), such that previous HAZ structures are further modified by a second thermal cycle [1]. As a result, considerable attention has been paid to developing a better understanding of the influence of different HAZ microstructures on their resultant mechanical properties. Several researchers have investigated the influence of MA on HAZ properties, especially toughness. For example, Fairchild et al. reported that low toughness in the HAZ of an HSLA steel can be mainly attributed to the presence of MA islands [2]. Kim et al. showed that single thermal cycle specimens exhibit higher toughness at 10 � C compared to spec imens produced with multiple thermal cycles It was also shown that among the different reheated zones, the intercritically reheated (ICR) CGHAZ exhibited the lowest toughness [3]. The reason for deteriorated properties following a reheating thermal cycle was related to the for mation of MA islands. Liao et al. [4] investigated the effect of cooling times on toughness for the CGHAZ (following a single thermal cycle) and the effect of peak temperature on the ICRCGHAZ (following a second thermal cycle). They indicated that the toughness of material after a single thermal cycle decreased with increasing cooling time. It was also reported that the toughness deteriorated after a reheating thermal cycle with peak temperatures between 973-1023 K was subjected to a constant “t8/5” cooling time of 6s (indicating the time between 800� to 500 � C or Δt800-500 ¼ 6s) [5]. Interestingly, there is ongoing research that is aimed at developing high-performance pipe, which includes the presence of MA. Pipe steels utilized in strain-based design are currently under investigation to pre vent fracture of pipelines by providing high deformation capacity [6]. It has been shown that effective use of MA can be considered a useful approach for developing high-performance materials [6]. For this pur pose, pipeline steels consisting of mainly ferrite-bainite or bainite-MA with highly refined structures that have been developed to attain high ductility and toughness [7]. Although the role of MA has been widely investigated in terms of the effect on toughness, its role in tensile strength, ductility, and strain hardening, has been seldom studied. To-date, work has revealed that increases in ultimate strength promoted by higher fractions of MA was observed by Han et al. [8]. Lanzillotto et al. also reported increased strength from the presence of MA in dual phase steel [9]. Reduced yield strength in X80 linepipe steel was reported by Pedrosa et al. and attributed to the disappearance of MA when material was austenitized and quenched in the first thermal cycle, then later aged in the second thermal cycle at different temperature [10]. In contrast, the presence of MA had been pinpointed as the reason for increased strength by other researchers [11], indicating it can be a desirable component of steel. Yong et al. and Chen et al. showed crack deviation around the MA which resulted in increased strength in tensile tests [12,13]. In addition to strength, studies on the influence of MA on ductility are also rather sparse. In the current authors recent work, it was observed that MA deteriorates HAZ ductility in Gas Metal Arc Welded (GMAW) joints of X80 pipe material [14]. In addition, the application of a potential tempering cycle to a welded specimen shifted the tensile fracture loca tion from the HAZ to the base metal by decomposing detrimental MA, which led to a drastic increase in strength and ductility [15]. The initial loss of ductility was attributed to void formation at the MA/ferrite
interfaces. Further research by Lambert et al. [16] and King et al. pin pointed MA as the crack initiator in low-temperature tensile tests [1,16]. In the previously mentioned work by the authors, it was revealed that a higher fraction of MA provided higher strength but reduced ductility in X80 pipe material. In addition, a higher strain hardening capacity was also associated with higher weld strength when greater fractions of MA were present [14]. The above mentioned investigation mostly considered global ductility (whole gauge length ductility) during tensile testing. However, during tensile deformation, the deformation becomes localized and significant localization of strain (rather than global elongation) occurs in the fracture location. Hence, the local strain distribution differs considerably from the global strain. In the current investigation, the goal was to compare the difference in local and global ductility in specimens extracted from the second pass (Pass 2), versus the overlapping weld region spanning Passes 1 and 2 of submerged arc welded X100 spec imen. In addition, the fracture behaviour and influence of MA on local tensile properties by comparing cross-weld specimens, which contained different HAZ regions from an X100 submerged arc seam weld has been analyzed. The effect of MA on strength, ductility and work hardening was a key aspect of this investigation, and a combination of micro structure characterization, hardness testing, strain mapping in tensile tests are performed to delineate the behaviour of the as-welded versus reheated HAZ regions. 2. Materials and experimental procedure Commercially produced large diameter (914 mm) thick wall (19 mm) X100 pipe manufactured with a two-pass submerged arc seam weld was used in this study. The typical heat input for submerge arc welding is 1.8–2.8 kJ/mm. The base metal chemical composition of the X100 pipe was analyzed using inductively coupled plasma emission spectroscopy and the results are reported in Table 1. The polished specimens were prepared based on the ASTM:E3 stan dard. A 2% Nital etchant was used to reveal the microstructures of the base metal and HAZ regions, while a LePera etchant (25 ml distilled water þ 25 ml Picric acid with 0.25 g Na2S2O5) was used to reveal MA regions (white) by optical microscopy according to ASTM E407-07. An Olympus microscope BX51 was used to observe the microstructureand further details of the MA internal structure were characterized using a Zeiss LEO Scanning Electron Microscope (SEM) and images for subse quent quantification of MA area fraction using Photoshop 6.The image processing method is shown in following Fig. 1. The image produced in Fig. 1b is used to calculated area fraction of MA and average size of MA (length and width average as shown in Fig. 1c). The line intercept method was used for calculating the spacing between the MA features. It was not possible to perform XRD exclusively in the FGHAZ since the zones were too small. XRD was performed on the base metal, Pass 2 HAZ and overlapped HAZ to investigate the fraction of phases in each location. These two regions were isolated by cutting small sections using a precision cutter and confirmed using area fraction measurements indicating that 95% of the Pass 2 HAZ and 83% of overlapped HAZ areas were isolated for XRD analysis. The retained austenite was determined using a Bruker D8 instrument for XRD analysis (using Cu Kα radiation) [17]. SEM analysis including Auger Electron Spectroscopy (AES) was performed using a JEOL 9500F system to map the carbon distribution in MA and particles on the fractured surface. The surface was ion milled to remove surface carbon and oxygen before performing AES analysis. Wavelength Dispersive Spectroscopy (WDS) was used to quantify the carbon content of MA using an accelerating voltage of 20 kV with 10.78 nA (3% variation) beam current. To standardize the analysis, WDS was performed on a similar steel with known carbon content and a martensitic microstructure (to provide a uniform carbon distribution). Vickers microhardness testing was performed using a 300 g load and 15s dwell time to produce mapping across the part of weld. The average hardness value was reported based on five tests. A Hystron Tribo 2
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Table 1 Chemical composition of X100 base metal, wt%. C
Mn
Si
Cr
Ni
Cu
Al
Mo
Nb
N
S
Ca
O
0.06
1.81
0.10
0.03
0.52
0.3
0.04
0.26
0.03
0.0023
0.005
0.005
0.001
Fig. 1. Martensite-austenite grain size measurement and spacing measurement method (a) SEM image used to produce binary image using Photoshop (b) Binary image from Photoshop (c) Average grain size measurement method.
tomography was performed to determine void location and observe void coalescence, with a minimum resolution of 2.3 μm.
indenter was also used with a 2000 μN load to perform nanoindentation on individual MA regions. SEM images were taken after each indenta tion to confirm and identify the MA at each specific location. A total of six cross-weld tensile specimens (three for each condition) were produced by precision electrical discharge machining from Pass 2 as well as spanning the overlapped region near the mid-thickness of the submerged arc weld. The Pass 2 specimen was considered to represent single cycled HAZ regions, whereas the overlapped specimen contained the reheated regions as well as single cycled HAZ regions from both Pass 1 and 2. The tensile specimens were produced following ASTM E8M standard dimensions, except due to curvature of pipe the overall length was reduced. Small-scale flat tensile test specimen were oriented in the hoop direction with dimensions of 87 mm overall length, 25 mm gauge length, 6 mm gauge width and 1 mm thickness. The macrograph of the submerged arc weld is shown in Fig. 2a. A schematic of the single and overlapped thermal cycle area and HAZ regions is shown in Fig. 2b. The location of tensile specimens for Pass 2 and the overlapped condi tion is also shown in Fig. 2b. A Tinius Olsen HK10T servo-mechanical tensile frame with a maximum 10 kN load capacity was used to perform tensile tests at room temperature. Strain monitoring was per formed using Digital Image Correlation (DIC) system from Correlated Solutions with a subset size of 29 and step size 7 (1.1 mm X 1.1 mm square) used throughout testing. Global yield strengths of both Pass 2 and the reheated (overlapping) thermal cycled HAZ were determined based on a 0.2% offset method from stress-strain curves. To investigate straining behaviour of fractured and unfractured sides of tensile spec imen, interrupted tensile tests were performed on separate Pass 2 and overlapped specimens. The HAZ zones were carefully marked on the specimens and the strain was monitored in each of the zones. X-ray
3. Experimental results 3.1. Macro and microstructure The microstructure of the base metal contains ferrite and secondary MA phases, as shown in Fig. 3a, and in more detail in the SEM image in Fig. 3b. The fraction of MA is 7.6% in the base metal while the remaining matrix is ferrite. Variation in the internal structure of MA was observed in base metal using high magnification SEM images. The weld metal region, fusion line, and extent of the HAZ (including CGHAZ, FGHAZ and ICHAZ) as well as the overlapped reheated regions are evident in Fig. 2a and b. The maximum HAZ width for Pass 1 (single thermal cycle region) varies between 3.83-4.00 mm, while the HAZ width of Pass 2 (single thermal cycle region) varies between 3.02-4.03 mm, indicating that the heat inputs of the two passes were comparable. However, in the overlapped reheated region, the HAZ width was 3.60 mm due to the bead shapes of the respective passes. Meanwhile, the weld metal microstructure was comprised of acicular ferrite. 3.2. Hardness distribution The base metal hardness was measured to be 247 � 12.5 HV as an average through the thickness of the plate. The hardness distribution for Pass 1, Pass 2 and overlapped HAZ and weld region are shown in Fig. 4. The Pass 2 weld metal hardness is lower than Pass 1. A comparison between Pass 1 Pass 2, and the overlapped HAZ hardness indicates that
Fig. 2. (a) Macrograph of submerged arc welding in X100 material (b) Schematic of submerged arc welding with the location of tensile specimens. 3
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Fig. 3. (a) Lower magnification image of base metal (b) Higher magnification image of base metal.
3.3. Nano-indentation of MA The MA hardness values in the base metal were 4.90 � 1.04 GPa. The FGHAZ of overlapped HAZ MA hardness were 5.83 � 1.16 GPa at a distance of 1.30 mm from fusion line while for the FGHAZ of Pass 2 HAZ MA hardness was 7.17 � 1.07 GPa. Force-displacement curves obtained from both the MA and ferrite matrix are shown in Fig. 8a. It can be concluded that MA in the FGHAZ of Pass 2 are harder than those in the FGHAZ of overlapped HAZ at a distance of 1.30 mm from fusion line as shown in Fig. 8b. The lower hardness of the MA regions within the FGHAZ of the overlapped HAZ suggests that those contain lower carbon. This can be explained as follows: during reheating, carbon can diffuse out of the structure, which reduces the hardness of MA in the reheated HAZ. In addition, it has been observed that some of the nanoindentation curves from the FGHAZ of overlapped HAZ showed discontinuous deformation which is called “pop-in” behaviour, and can occur in tempered MA (see Fig. 8c). Pop-in behaviour is often attributed to a reduction in dislocation density [18]. The FGHAZ of Pass 2 and FGHAZ of the overlapped HAZ contained MA with a hardness higher than the MA in X100 material investigated by Choi et al. [19]. To investigate the difference in MA hardness between the overlapped FGHAZ and Pass 2 FGHAZ, WDS was used to determine the carbon percentage in the MA at the 1.30 mm distance. WDS analysis has a sensitivity of 0.01%, and spectral resolution as slow as 2eV, and showed that the MA in the FGHAZ of Pass 2 contained higher carbon (0.099–0.181 wt%) than in the FGHAZ (0.073–0.083 wt%) of over lapped HAZ. Ferrite in the FGHAZ of Pass 2 contains 0.07 wt% carbon, while FGHAZ of overlapped HAZ ferrite also contained 0.07 wt%. The presence of austenite was detected in all of the zones as shown in Fig. 9, although the intensity of austenite peak is low.
Fig. 4. (a) Hardness mapping across the submerged welding specimen.
the hardness for Pass 2 HAZ is lower than in Pass 1 or the overlapped HAZ. The microstructures for the different distances from the fusion line for Pass 2 and overlapped HAZ are shown in Fig. 5. It can be observed that the overlapped HAZ microstructure is finer for all the distances from fusion line compared to the HAZ of Pass 2 as shown in Fig. 5. The SEM images were taken at 1.25, 1.50 and 1.75 mm distances from the fusion are shown in Fig. 6. The microstructure of the FGHAZ for Pass 2 and FGHAZ for overlapped HAZ in those locations consisted of combinations of ferrite and MA. The average area fraction of MA at all three distances for Pass 2 FGHAZ locations was 4.93 � 0.73%. The FGHAZ microstructure for the overlapped HAZ contained ferrite and MA, in which the area fraction of MA across the 1.25–1.75 mm distance from the fusion line was 5.57 � 0.41%. SEM observations of the FGHAZ structures at distances 1.25–1.50 mm from the fusion line after LePera etching are shown in Fig. 7a and b. A significant number of white structures were observed at this distance for FGHAZ of Pass 2, which represent untempered MA [14]. However, the number of white structures at the same distance within the overlapped zone FGHAZ was less than in the FGHAZ of Pass 2. SEM observation in the FGHAZ of the overlapped HAZ shows that some of the MA are homogenous and micron-sized (see Fig. 7c), while some MA is slightly larger and contains evidence of carbides within the structure, indicating that those MA regions are auto-tempered, as shown in Fig. 7d. Line intercept measurements reveal that the spacing between MA in the FGHAZ region of Pass 2 (5.11 � 0.7 μm) was higher than in the FGHAZ of the overlapped HAZ (4.27 � 0.57 μm). In addition, the maximum size of MA in the FGHAZ of Pass 2 (3.97 � 0.56 μm) was higher in those positions in comparison to the FGHAZ (3.27 � 0.46 μm) of the overlapped HAZ. The average size of MA in the FGHAZ of Pass 2 was also higher than the FGHAZ in the overlapped HAZ.
3.4. Tensile tests The base metal yield strength was 776 � 2.5 MPa, while the tensile and yield strengths for the Pass 2 specimen were 665 � 6.5 and 653 � 4 MPa respectively. However, tensile and yield strength for overlapped HAZ specimens were 712 � 5 and 683 � 8.5 MPa. The average ductility of X100 base metal was 10.0 � 1.1%, however, the Pass 2 and overlapped specimens exhibited ductility values of 4.8 � 0.5% and 5.7 � 0.2%, respectively. Stress-strain curves for all tensile tests are shown in Fig. 11a. It can be observed that the base metal exhibits the highest tensile and yield strength (Fig. 10a), while the Pass 2 and overlapped specimens exhibited lower yield and tensile strength. Moreover, it can be observed that the overlapped specimen reaches higher strength than the Pass 2 specimen. In addition, the overlapped specimen offered better ductility than the Pass 2 specimen. This a result of the overlapped specimen having more uniform strain hardening behaviour while the Pass 2 had negligible strain hardening between the yield and ultimate tensile stress. A strain-time graph was plotted for all tensile tests (Fig. 10b), which indicates that the overlapped specimen strained later than the single thermal cycle (Pass 2) specimen. It should be noted that the global 4
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Fig. 5. Microstructures of HAZ regions at various distances from fusion line for the Pass 2 and overlapped reheated region.
strains will also depend on both the properties and width of the softened HAZ region versus the gauge length of the tensile specimen, however the HAZ widths are comparable in both specimens in Fig. 10 when compared to the much longer overall gauge length. The macrograph of the fractured specimen is shown in Fig. 10c and d. All the fractures occurred in the HAZ locations with shearing for all specimens. The fracture occurred at a distance of 1.31 mm from the fusion line for both specimens, which was found to be the softest loca tion based on hardness results. It is interesting to observe that although both specimens fractured at the same distance from the fusion line, the overlapped specimen achieved better strength, ductility and work hardening, which is worth further investigation. Local strain distributions across the gauge length (25 mm) were
investigated in the fractured specimen. Though the global strain for base metal was 10.0% (average), localized strain in the fracture location for base metal reached 45%. A similar phenomenon is observed for the Pass 2 and overlapped specimens, in which local strains for the Pass 2 spec imen reach 35% engineering strain locally at the fracture location, while the overlapped specimen reached 33%. It was observed that this value (at the fractured side of the weld) was consistently 2.3% greater for all three tests. This indicates that at fracture location of the FGHAZ of overlapped HAZ actually sustained less strain than the FGHAZ of Pass 2, despite the higher global strain. However, the reason for higher global strain in the FGHAZ of the overlapped HAZ is related to the increased strain on the other (unfractured) side of the weld. It was observed that the strain values for the FGHAZ of overlapped HAZ unfractured side 5
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Fig 6. (a,b,c) Microstructure in FGHAZ of Pass 2 at distances of 1.25, 1.50 and 1.75 mm from fusion line and (d,e,f) Microstructure in FGHAZ of overlapped HAZ at the same distances from fusion line.
were consistently higher by 1.7% (max. 4.0%) than FGHAZ of Pass 2 for all three tests. Localized strain distribution in different zones of interrupted tensile tests are shown in Fig. 11. It can be observed that the FGHAZ of Pass 2 was subjected to a strain of 18.5% where the fracture would potentially occur (Fig. 11a), while the global strain was 3.2%. However, the FGHAZ on the unfractured side only sustained 2.5% strain (Fig. 11a). In contrast, the potential overlapped FGHAZ fracture location sustains 16.7% strain when subjected to 4.6% global strain, while the other overlapped FGHAZ experienced 8.5% local strain. The difference in local strain distribution in potential fracture location between Pass 2 and overlapped FGHAZ may result from ferrite grain size, MA micro constituents size and distribution which will be discussed later.
FGHAZ were 0.3 � 0.3, 1.7 � 0.2, and 2.1 � 1.0%, respectively. The density of voids was 0.001, 0.004 and 0.005/μm2, respectively for the base metal, Pass 2 FGHAZ and overlapped FGHAZ, with sizes in all samples being comparable and varying from less than 1–10 μm. The sizes of voids in three specific size ranges: below 1 μm (smaller void), between 1 to 3 μm (intermediate size void) and more than 3 μm (large void) were quantified, based on three images per specimen and sum marized in Fig. 12. It can be observed that the base metal, FGHAZ of Pass 2, and overlapped FGHAZ, all contained notable fractions of small, in termediate and large voids. However, the fraction of all sizes of voids was lowest in the base metal. The void fraction in the FGHAZ of Pass 2 is low for both intermediate and large voids in comparison to the over lapped FGHAZ. It is also observed that 91% of the voids initiated from the MA/ferrite interfaces, as shown in the inset image of Fig. 12. Fractography indicates that all of the specimens exhibited dimples, which is consistent with ductile fracture in all cases. However, the FGHAZ of Pass 2 and overlapped FGHAZ fracture surface contains a greater number of large voids as shown in Fig. 13. It is also observed that some of the voids contain particles, presumably inclusions. AES analysis from the surfaces of several of these particles indicates that in the base metal they are rich in carbon, sulfur, and manganese. Their composition is shown in Table 2, where the atomic fraction of carbon is 25.2 at% for the particles versus 8 at% for the matrix. The carbon content on the particles observed on the fracture surface is much higher than the
3.5. Fracture mechanism during tensile testing 3.5.1. Void nucleation Significant numbers of voids were observed close to the tensile fracture surface in the base metal, Pass 2 and overlapped specimens using optical microscopy. SEM analysis also confirmed the presence of voids close to the fracture surfaces after tensile testing. The area fraction of voids was calculated based on three SEM images. Quantification of these voids was performed in which the fraction of voids close to the fracture location for the base metal, Pass 2 FGHAZ and overlapped 6
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Fig. 7. (a) LePera etched microstructure in FGHAZ of Pass 2 at distance 1.30 mm (b) LePera etched microstructure in overlapped FGHAZ at distance 1.30 mm (c) untempered MA in FGHAZ of Pass 2 (d) Tempered MA in FGHAZ of overlapped HAZ.
Fig. 8. (a) Force-displacement curves from base metal, FGHAZ and FGHAZ MA and ferrite (F) (b) Average MA hardness (c) Pop in behaviour from tempered MA in FGHAZ of overlapped HAZ. 7
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work [8]. Interrupted tensile tests were performed on the base metal with global strain of 8%, 5%, 3% and 1%. X-ray tomography was per formed in the area identified as the fracture location for 8% and 5% strained specimen. Results from the 5% interrupted tensile test of X100 base metal are shown in Fig. 14. It can be seen that there are clustering of voids in potential fracture locations, although there are some other scattered voids away from the localized clustering. It can also be observed that the voids are coalescing with each other at the locations of clustering. A total of 535 voids were identified by x-ray tomography. The total volume fraction of voids was 0.015%, while their individual vol umes varied between 11.93 to 52,189.3 μm3. SEM observation from a specimen strained to 1% suggests that void initiation can occur imme diately after the yield point. 4. Discussion 4.1. Differences in microstructure and hardness The hardness difference between Pass 2 HAZ and overlapped HAZ (Fig. 4) can be explained by the difference in grain size, MA fraction and distribution. It was observed that the HAZ ferrite grains in Pass 2 were clearly coarser than those in the overlapped HAZ (see Fig. 6). The coarse ferrite lath (potentially due to slower cooling rate) is associated with lower hardness. In addition, MA fractions were lower in the FGHAZ of Pass 2, and the spacing between MA was larger than within the FGHAZ of the overlapped specimen. The combination of coarser ferrite laths, lower MA fraction and higher spacing between MA could lead to lower hardness in the FGHAZ of Pass 2 than the FGHAZ of overlapped HAZ. Although the FGHAZ of Pass 2 exhibited lower hardness than the FGHAZ of the overlapped HAZ, the nano-indentation results suggest that
Fig. 9. XRD spectra (Cu K alpha) near austenite peak in base metal, Pass 2, and the overlapped HAZ, with ferrite peak of 327506 counts (truncated).
carbon percentage of MA regions measured by WDS. 3.5.2. Void formation and void coalescence To investigate the role of void formation and coalescence of X100 base metal, tensile tests were monitored using the DIC strain measure ment system. Several tensile tests were intentionally interrupted at a critical strain value, and the fracture location was subsequently identi fied. The details of this method of has been explained in author’s other
Fig. 10. (a) Stress-strain curves (for 25 mm gauge length in each condition) (b) Strain-time plot (c) Macrograph of fractured overlapped specimen (d) Macrograph of fractured Pass 2 specimen. 8
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Fig. 11. Localized strain distribution across gauge length for (a) Pass 2 specimen subjected to 3.2% global strain and (b) Overlapped specimen subjected to 4.6% global strain (targeted gauge lengths are indicated by the gauge marks (white dots) on the horizontal lines in the images.
4.2. Strength and work hardening The specimen obtained from the overlapped HAZ regions possessed higher global yield and tensile strength than the Pass 2 specimen. It can be observed from Fig. 4 that the overlapped HAZ hardness is higher than the Pass 2 HAZ, which accounts for the higher tensile and yield strength in the former. The difference in hardness is due to the difference in microstructure as discussed above. In addition to hardness, increased dislocation interactions may contribute towards higher strength. Fig. 6 shows that the number of MA regions is less in the Pass 2 FGHAZ compared to the overlapped FGHAZ. Okada et al. [20] showed that high concentration plastic strain occurred in the soft phase around elongated and massive MA which could be due to a higher dislocation density which is also reported in author’s previous work [15]. Magula et al. [21] reported that MA could as act as an obstacle to dislocation movement. More benign and uniform distributions of MA in the FGHAZ of the overlapped zone may give more resistance to dislocation movement, which in turn provides higher strength in this zone. However, disloca tion movement is easier in the FGHAZ of Pass 2 since the spacing be tween MA is larger. In addition to hardness, increased strain hardening could contribute to the high strength of overlapped HAZ. Yan et al. [22] also observed “round house” stress-strain curve behaviour, with strain hardening phenomena in HSLA steel when tempering was performed at 700 � C. High straining ability for this temperature was related to a strong interaction of dislocations and interstitials with grain boundaries and finely dispersed precipitates of carbides and carbo-nitrides. Zuo et al. [23] also observed a similar effect for the stress-strain behaviour for X70 steel. MA in the FGHAZ of overlapped HAZ were found to be tempered, and contained carbides, as shown in Fig. 7d. Carbides in the overlapping FGHAZ might interact with dislocation and enhance strain hardening which results in higher strength. It was shown by electron-backscattered diffraction (EBSD) that an X80 weld HAZ microstructure contained a high fraction of MA with more tempered MA structure. This led to more
Fig. 12. Void fraction adjacent to the fracture surface vs void size distribution in base metal, Pass 2 FGHAZ and overlapped FGHAZ.
the MA hardness in Pass 2 FGHAZ is higher than the MA in the over lapped FGHAZ. The difference in MA microconstituents hardness be tween Pass 2 and overlapped FGHAZ can be attributed to carbon content of MA. WDS analysis showed that the MA regions in the FGHAZ of Pass 2 contain a higher percentage of carbon than those in the FGHAZ of overlapped HAZ. The difference in carbon percentage led to a higher hardness in the MA of the FGHAZ of Pass 2. Meanwhile, the MA in the FGHAZ of overlapped HAZ had lower carbon content. The above dis cussion suggests that grain size and dispersed distribution of MA had a greater contribution to hardness in the FGHAZ of overlapped HAZ. 9
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Fig. 13. Tensile fracture surface observation (a) Base metal (b) Pass 2 FGHAZ (c) Overlapped FGHAZ.
difference in carbon content between MA and ferrite in FGHAZ of overlapped HAZ. However, the difference in carbon content between MA and ferrite is higher in FGHAZ of Pass 2, which is also evident in the nano-indentation results. The difference in mechanical properties be tween MA and matrix is very important in tensile tests. The hardness difference between untempered MA and ferrite in the FGHAZ of Pass 2 is higher than the FGHAZ of overlapped HAZ. Although, the difference between tempered MA and ferrite was smaller in the FGHAZ of over lapped HAZ. From the WDS and nano-indentation results, it can be concluded that FGHAZ of overlapped HAZ properties are more similar to the base metal properties, which is also clear from Fig. 11. Both over lapped HAZs strained in a similar way to the base metal until 8.5% elongation. In contrast, the strain is more localized in one Pass 2 HAZ. The MA properties in the FGHAZ of overlapped HAZ are close to its surrounding matrix, which might lead to deformation uniformly across the microstructure during the tensile test. However, there is a more significant difference between the MA and ferrite matrix in the FGHAZ of Pass 2, which might lead to deformation of the softer ferrite matrix compared to the hard MA which caused strain to localize easily in one of Pass 2 HAZ. Chen et al. [27] reported that at room temperature ferrite yields more easily and has great capability to sustain deformation compared to harder MA, although tempering of hard MA will enhance ductility. Although, the overall ductility is higher for the overlapped specimen, the localized ductility is lower for the overlapped specimen compared to the Pass 2 HAZ. This can be explained by the fact that a non-uniform material was evaluated during tensile testing, and this would enhance the differences in strain localization observed using DIC. In Fig. 11, the Pass 2 specimen was subjected to 3.2% global strain, while the localized strain in potential fracture location was 18.5%. In comparison, the overlapped specimen was subjected to 4.6% global strain, in which case the localized strain in potential fracture location was 16.7%. However, the unfractured side of the overlapped specimen sustained 8.5% strain, while Pass 2 experienced only 2.5% strain. A similar phenomenon has
Table 2 Elemental composition (%at) using Auger for the particle on the fracture surface of the base metal. Particles Matrix
C
O
Fe
S
Ca
Mn
Total
25.2 8
28.2 39.9
29.5 50.4
12.1 1.1
0 0.5
5 0
100 99.9
strain hardening than for the microstructure with low fraction MA and a correspondingly lower fraction of tempered MA structure, which pro duced higher ultimate tensile strength [14]. In the current study, it can be observed that the overlapped specimen exhibited strain hardening while the Pass 2 specimen does not show such behavior. That may result from a high fraction of MA and tempered MA in the FGHAZ of the overlapped specimen. The size of MA can also affect strength due to the initiation of voids. It has been reported that coarse particles initiate voids before smaller ones [24,25], and it is recognized that premature formation of voids can significantly affect tensile strength [26]. An average MA grain size of 1 μm has been reported to be critical for tensile properties [15]. It was observed that the average size of MA in the FGHAZ of Pass 2 was larger than for the FGHAZ of overlapped HAZ. In addition, the maximum size of MA in the FGHAZ of Pass 2 is higher in fracture location than FGHAZ of overlapped HAZ. This implies that it is easier to form voids in the FGHAZ of Pass 2, which did not lead to as much strain hardening as in the FGHAZ of overlapped HAZ which results in lower strength. 4.3. Global Vs. local ductility in relation to void formation It can be observed from Fig. 10 that overlapped specimens have higher global ductility than the Pass 2 specimens and both fractured in the FGHAZ. The MA hardness of FGHAZ of overlapped is found lower than the FGHAZ of Pass 2. WDS analysis showed that there is little
Fig. 14. (a) Void mapping area (600 μm � 1700 μm) using X-Ray tomography in X100 base metal for 5% interrupted tensile test (b) Magnified area of centre region in Fig. 14a showed microvoid and void coalescence. 10
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be shared at this time due to technical or time limitations.
been observed for full fracture tensile specimen which may relate to void formation. It is observed that the overlapped FGHAZ contains more voids than the FGHAZ of Pass 2. In the current investigation, it was observed that the FGHAZ of the overlapped specimen contains more MA and a higher number of MA produces more voids. In addition, the size of voids in the FGHAZ of the overlapped HAZ was larger than in the FGHAZ of Pass 2. This suggests that void formation and void coalescence is easier in FGHAZ of the overlapped specimen than the FGHAZ of Pass 2. A higher number of MA regions led more void formation during tensile testing, which will lead to a loss in localized ductility in the overlapped specimen, although the overall extension was higher than that of the Pass 2 specimen. In the authors previous work, it was observed that a loss of ductility occurs relative to the number of voids [14]. It was observed that higher number of MA promoted formation of more voids and that would lead to reduced ductility [14]. In addition, it has been reported that random void arrays with a larger spacing increased ductility; while reducing the spacing or increasing void size decreases ductility [26].
Acknowledgments The authors wish to acknowledge the Natural Sciences and Engi neering Research Council of Canada (NSERC) for financial support. Further financial support was provided by TC Energy Corp. References [1] C. Davis, J. King, Metall. Mater. Trans. A 25 (1994) 563–573. [2] D. Fairchild, N. Bangaru, J. Koo, P. Harrison, A. Ozekcin, Weld. J. 70 (1991), 321. s-329. s. [3] B. Kim, S. Lee, N. Kim, D. Lee, Metall. Trans. A 22 (1991) 139–149. [4] J. Liao, K. Ikeuchi, F. Matsuda, Trans. JWRI 23 (1994) 223–230. [5] J. Liao, K. Ikeuchi, F. Matsuda, Nucl. Eng. Des. 183 (1998) 9–20. [6] S. Tsuyama, H. Nakamichi, K. Yamada, S. Endo, ISIJ Int. 53 (2013) 317–322. [7] H. Kimura, T. Yokota, N. Ishikawa, S. Kakihara, J. Kondo, in: 2016 11th International Pipeline Conference, American Society of Mechanical Engineers, 2016. V003T005A024-V003T005A024. [8] N. Huda, A.P. Gerlich, Canweld Conference, 2019. [9] C. Lanzillotto, F. Pickering, Met. Sci. 16 (1982) 371–382. [10] I.R.V. Pedrosa, R.S.d. Castro, Y.P. Yadava, R.A.S. Ferreira, Mater. Res. 16 (2013) 489–496. [11] T. Tagawa, T. Miyata, S. Aihara, K. Okamoto, Tetsu-To-Hagane 79 (1993) 1176–1182. [12] Y. Zhong, F. Xiao, J. Zhang, Y. Shan, W. Wang, K. Yang, Acta Mater. 54 (2006) 435–443. [13] L.-h. Chen, Y.-l. Kang, X.-h. Li, D. Wen, G. Liu, J. Univ. Sci. Technol. Beijing 31 (2009) 983. [14] N. Huda, R. Lazor, A.P. Gerlich, Metall. Mater. Trans. A 48 (2017) 4166–4179. [15] N. Huda, Y. Ding, A.P. Gerlich, Mater. Sci. Technol. 33 (2017) 1978–1992. [16] A. Lambert, X. Garat, T. Sturel, A. Gourgues, A. Gingell, Scr. Mater. 43 (2000) 161–166. [17] N. Huda, A.R. Midawi, J. Gianetto, R. Lazor, A.P. Gerlich, Materials Science and Engineering: A, 2016. [18] A. Gouldstone, H.-J. Koh, K.-Y. Zeng, A. Giannakopoulos, S. Suresh, Acta Mater. 48 (2000) 2277–2295. [19] B.-W. Choi, D.-H. Seo, J.-i. Jang, Met. Mater. Int. 15 (2009) 373–378. [20] H. Okada, K. Ikeuchi, F. Matsuda, I. Hrivnak, (1995). [21] V. Magula, Z. Li, H. Okada, F. Matsuda, Trans. JWRI 20 (1991) 69–75. [22] W. Yan, L. Zhu, W. Sha, Y.-y. Shan, K. Yang, Mater. Sci. Eng. A 517 (2009) 369–374. [23] X. Zuo, Z. Zhou, Mater. Res. 18 (2015) 36–41. [24] W. Roberts, B. Lehtinen, K.E. Easterling, Acta Metall. 24 (1976) 745–758. [25] N. Huda, Y. Wang, L. Li, A.P. Gerlich, Mater. Sci. Eng. A (2019) 138301. [26] P. Magnusen, E. Dubensky, D. Koss, Acta Metall. 36 (1988) 1503–1509. [27] J. Chen, Y. Kikuta, T. Araki, M. Yoneda, Y. Matsuda, Acta Metall. 32 (1984) 1779–1788.
5. Conclusions A comparison of between the local tensile properties of single and double thermal cycled HAZ regions of the submerged arc seam weld in an X100 pipe was carried out and the following conclusions were drawn: 1 The region of overlapping and reheated HAZ between two weld passes exhibited higher yield and tensile strengths as well as hard ness compared to the second pass (Pass 2) of the submerged arc weld. In addition, the overlapped HAZ exhibits higher strain hardening and global ductility. 2. The fracture occurred in FGHAZ for both the Pass 2 and overlapped HAZ. A high fraction of MA and smaller spacing between particles might provide enhanced dispersion strengthening in FGHAZ of overlapped HAZ, which provides higher strength in this zone. 3. Although, the FGHAZ of overlapped specimen exhibits higher global strain than Pass 2 FGHAZ, localized strain distributions revealed that the FGHAZ of overlapped specimen exhibited lower strain than for the FGHAZ of Pass 2. A higher fraction of MA led to the larger and denser void formation in FGHAZ of overlapped HAZ, which causes a decrease in local ductility. Data availability The raw/processed data required to reproduce these findings cannot
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