Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Influence of inhomogeneity on several length scales on the local mechanical properties in V-alloyed all-weld metal

Influence of inhomogeneity on several length scales on the local mechanical properties in... Recently, a new, vanadium alloyed welding consumable with a minimum yield strength of 1100 MPa was developed. The mechanical properties of welding consumables for gas metal arc welding are usually classified by producing and testing all- weld metal samples, which are typically a multipass weld. Chemical and microstructural fluctuations of a vanadium alloyed all- weld metal sample on a macro- and microscale and their influence on the local mechanical properties were investigated. On a macroscale, hardness mappings show a pattern of hard and soft zones which can differ up to 60 HV. Despite the existence of these fluctuations, undersized Charpy V-notch tests revealed no significant difference between the last weld bead and the underlying ones. It is explained how vanadium and its tendency to form precipitates affect both the hardness inhomogeneity and the toughness homogeneity. On a microscale, segregations of several alloying elements and significant grain size fluctuations were found. Their influence on fluctuations of the mechanical properties is discussed as well. . . . . . Keywords Filler materials Weld metal Toughness Hardness Homogeneity Multipass welding 1 Introduction chemical composition of the welding consumable is crucial. In order to classify the mechanical properties of the welding High-strength steels are frequently welded by gas metal arc consumable itself, the production of all-weld metal samples welding with matching welding consumables to produce com- according to DIN EN ISO 15792-1 is required. In this proce- ponents with a high load bearing capacity and a comparatively dure, a large gap is buffered and then filled layer by layer with low weight. These components can be found in e.g. vehicles or the welding consumable. The result is a multilayer structure cranes, where a low component weight can save a significant similar to Fig. 1 with a reproducible number of beads depend- amount of energy. The mechanical properties of the produced ing on the chosen welding parameters. This sample design (and welds depend on their geometry, number of layers and the used especially the buffering) eliminates influences from the chem- welding parameters. Of course, also the choice of a suitable ical composition of the base material and should lead to a chemical composition of the weld metal which is only influ- enced by the chemical composition of the filler wire. Recommended for publication by Commission II - Arc Welding and Filler Metals Such a multipass weld contains several sources of inhomo- geneity on different scales. At first, the bead will solidify in a * Phillip Haslberger cellular or dendritic manner, which generates interdendritic phillip.haslberger@unileoben.ac.at segregations of alloying elements [1, 2]. These segregations can affect the formation of different ferritic constituents in the Department of Physical Metallurgy and Materials Testing, weld metal during further cooling. This was described by Montanuniversitaet Leoben, Leoben, Austria Keehan et al. [2] for high-strength weld metals produced by voestalpine Stahl GmbH, Linz, Austria shielded metal arc welding and by Powell and Herfurth [3]for gas metal arc welds. In another study, Haslberger et al. pointed Institute of Materials Science, Joining and Forming, Graz University of Technology, Graz, Austria out that these segregations could also influence the formation of clusters and precipitates in V-alloyed weld metal [4]. voestalpine Böhler Welding Austria GmbH, Kapfenberg, Austria After delta ferrite formation, austenite columns will form Present address: voestalpine Wire Technology GmbH, upon cooling. Their size and shape may change throughout Leoben, Austria 1154 Weld World (2018) 62:1153–1158 Table 1 Used welding parameters for the production of all-weld metal samples Current (A) Voltage (V) Welding speed Heat input Interpass (cm/min) per unit length temperature (kJ/cm) (°C) 250 30 50 9 150 Fig. 1 Schematic of an all-weld metal sample for the investigated type of welding consumables. The area shaded light gray is the buffer zone the weld bead. This phenomenon was investigated by Zhang using the welding parameters shown in Table 1.Intotal, and Farrar [5], who showed that the width of austenite columns the weld metal consisted of 7 layers with 21 beads (Fig. 1). can change throughout one weld bead depending on the nickel The chemical composition of the weld metal is shown in content. These factors (segregation and column growth) may Table 2. The amount of alloying elements guarantees a influence the microstructure from a microscopic point of view. fully martensitic weld metal for the expected cooling time From a macroscopic point of view, every deposited bead is between 800 and 500 °C of about 5 s. For an evaluation of reheated by subsequent welding passes. Depending on the peak the local mechanical behaviour of the weld metal, hardness temperature, this can effect remelting, reaustenitization, or tem- mappings and local Charpy V-notch tests were carried out. pering of the microstructure. As a result, zones with the original The Vickers hardness mappings were conducted with a or altered microstructures exist in the weld, which was described load of 2 kg and a step size of 0.5 mm. Subsized Charpy in several studies [6–10]. The size of zones with different types of V-notch samples with a cross section of 5 × 10 mm were microstructure (i.e., columnar grains and refined equiaxed or re- prepared (Fig. 2), dividing the all-weld metal sample in an crystallized grains) is dependent on the used welding parameters upper, middle and lower area. In the upper samples, a large [6]. The influence of the area fraction of columnar grains and part of the notch length was located in the last deposited recrystallized grains on the impact toughness is reported diversely bead, while the middle and lower samples contained a mix- [10]. While some indicated that a high amount of reheated mate- ture of differently reheated material. Three samples were rial is beneficial for impact toughness [11], others suggested that a tested at each temperature according to DIN EN ISO 148- low amount of reheated material is advantageous because of a 1, and the measured values were converted to the values higher homogeneity of the weld [12]. Another study claimed that for a 10 × 10 mm sample by applying a geometric factor the influence of the amount of reheated material is negligible [3]. accounting for the size of the fractured area compared to a Therefore, it can be summarized that the influence of the amount standard size sample [14]. of reheated material on the impact toughness strongly depends on For the investigations of interdendritic segregations, elec- the investigated material and its microstructural condition. tron probe microanalysis measurements were performed at an Currently, a V-alloyed welding consumable with a yield acceleration voltage of 15 keVand a probe current of 600 nA. strength of 1100 MPa and a minimum impact energy of 47 J A step size of 500 nm was chosen to ensure the ability to at − 20 °C was developed [13]. The produced all-weld metal resolve the segregations. Light optical microscopy of the nital samples were martensitic and contained V(C,N) precipitates etched sample was used to identify zones with different mi- [4, 13]. The purpose of this study is to demonstrate the exis- crostructural appearance on a macro scale. Electron backscat- tence of chemical and microstructural inhomogeneities in this ter diffraction (EBSD) measurements at 30 keV and 10 nA type of martensitic multipass weld metal and to clarify their with a step size of 80 nm were used to determine local effec- influence on the mechanical properties of the weld. Special tive grain sizes inside the last deposited bead. The effective emphasis is put on the comparison between the last deposited grains were defined by their misorientation tolerance angle of bead and the middle of the all-weld metal due to their differ- 15° [15], and their size was averaged by area. More informa- ence in thermal history. Hardness mappings and subsized tion regarding sample preparation and EBSD parameters can Charpy V-notch tests are used to determine fluctuations of be found in [16], which specifically addresses the microstruc- the local mechanical properties of the multipass weld. tural characterization of all-weld metal samples with light op- Microprobe investigations and microstructure comparisons tical microscopy and EBSD. are used to interpret the observed mechanical behaviour. Table 2 Chemical composition of the investigated material (m%) 2 Materials and methods CSi Mn Cr Mo Ni Cu V Fe The investigated all-weld metal samples were produced by 0.08 0.57 1.27 0.78 0.73 3.03 0.11 0.2–0.5 Rest gas metal arc welding according to DIN EN ISO 15792-1 Weld World (2018) 62:1153–1158 1155 Fig. 2 Schematic of the prepared subsized Charpy V-notch samples. The approximate shape of the last weld bead is outlined 3 Results 3.1 Local impact toughness The converted Charpy impact values from − 60 to + 20 °C for all three locations in the multipass weld are depicted in Fig. 3. Generally, the impact toughness increases gradually with in- creasing temperature. There is no significant difference be- tween upper, middle and lower part of the all-weld metal, and therefore no significant difference between top bead and tempered beads despite their diverse thermal history. 3.2 Hardness mappings For an assessment of hardness fluctuations in the multipass welds, several hardness mappings were conducted over the Fig. 4 a Nital etched all-weld metal sample after hardness mapping. The melting line of the last deposited bead is indicated by a black line. b whole weld metal, and the results are exemplarily shown on Colour-coded hardness map of the V-alloyed sample superimposed on one obtained map. The hardness indents covered a large part the image of the nital etched sample. The arrow points at the soft zone in of the sample, which is shown in Fig. 4a, where also the the last weld bead position of the last, untempered bead is marked by a black line. The position of the last bead is different compared to corresponding weld beads and their heat-affected zones, lead- the sample used for the Charpy V-notch tests. However, this ing to an average hardness of ca. 380 HV. should not affect the interpretation of the results. The resulting hardness map is shown in Fig. 4b. Varying hardness values 3.3 Interdendritic segregations between 350 and 410 HV were observed. The yellow and orange colours (arrow in Fig. 4b) in the last bead show that Microprobe mappings of the elements Mn, Cr and V were it is softer than the surrounding material. In the heat-affected conducted to assess the existence and the amount of zone below the last bead, a harder area with ca. 410 HVand a interdendritic segregation in the material. All measured ele- soft zone with ca. 370 HV follow. Throughout the rest of the ments clearly segregated, which is visible in Fig. 5. The red all-weld metal, hard and soft zones appear in the shape of the points in the Mn maps (Fig. 5a, b) originate from Mn-rich inclusions, which are homogeneously distributed over the whole weld metal. Consequently, they are not included in the further evaluation and interpretation of the interdendritic segregations visible in the map. Visually Mn segregates more than Cr (Fig. 5c, d) and V (Fig. 5e, f). The amount of segre- gation was evaluated by calculating the ratio of the maximum local element concentration and the average concentration over the whole map. Both Mn and Cr showed maximum con- tents of about 120% of the average value in the segregated regions. For V, this value was even higher with 190%. No difference between the top bead and the middle beads was observed, implying that reheating did not cause a redistribu- Fig. 3 Results of the subsized Charpy V-notch tests from − 60 to + 20 °C tion of alloying elements on this length scale. 1156 Weld World (2018) 62:1153–1158 Fig. 5 Microprobe mappings of sample with 0.2 m.% V. The distributions of Mn (a, b), Cr (c, d)and V(e, f) are shown for the top bead and the middle beads, respectively 3.4 Grain size fluctuations 4 Discussion The nature of the multipass welding process implies the exis- Several methods were used to investigate the influence of tence of grain size fluctuations between the passes due to inhomogeneity on different scales on the local mechanical reheating. This phenomenon of a mixture of zones with co- lumnar grains and zones with equiaxed fine grains is well described in literature [6, 7, 10, 12, 17, 18] and exemplarily shown in Fig. 6 for the current material. On a different scale, grain size fluctuations inside a single weld bead may occur. Therefore, EBSD measurements were conducted at several positions in the last bead for a determination of the local ef- fective grain size, which was defined by a tolerance angle of 15° [15, 19]. This tolerance angle ensures that martensitic blocks are separated and that the grain size correlates to the fracture behaviour of the material [15]. The results in Table 3 clearly indicate the existence of fluctuations of the effective grain size within the bead. Additionally, due to the irregular nature of the martensitic microstructure, the standard devia- tion at each position is high. However, there was no evidence for changes in microstructure caused by the interdendritic seg- regations. The influence of the irregular microstructures on Fig. 6 Last deposited bead and its surroundings etched with nital. Below the bead, a zone consisting of equiaxed fine grains is visible different scales will be discussed in the next section. Weld World (2018) 62:1153–1158 1157 Table 3 Local effective segregations and grain size. Nevertheless, it can be concluded Distance to the fusion Effective grain grain size in the last that enrichments of elements like Mn, Cr and especially V will line (mm) size (μm) deposited bead change the potential for precipitate formation. The resulting dependingonthe 0.52 2.6 ± 1.6 distance to the fusion line heterogeneity of the precipitate population will induce fluctu- 2.4 2.7 ± 1.4 ations in the local impact energy. 2.5 3.1 ± 2.1 Furthermore, as each welding pass will create a heat- 3.8 2.8 ± 1.6 affected zone inside the weld metal with areas of coarse- 3.9 2.8 ± 1.7 grained microstructures and fine-grained microstructures, a multilayer weld is always inhomogeneous in terms of grain size. The intercritical reheated zone may also contain areas properties of a martensitic all-weld metal sample produced with fresh martensite which leads to a local deterioration of from a new type of welding consumables. both hardness and toughness [22, 23]. This soft zone is clearly In multipass welds, a weld bead will be heat treated by visible in Fig. 4b. Additionally, the formation of precipitates is subsequent welding passes [7, 12]. In the case of a martensitic dependent on the reheating temperature. Hence, a Charpy V- weld metal, this implies the assumption that the reheating will notch sample will contain a mixture of zones with different cause a tempering of the martensite which should generally grain sizes (Fig. 6), microstructural constituents and precipi- increase the toughness [1]. Consequently, the last bead should tate populations, which may also lead to fluctuations in the have a higher strength and a lower toughness than the rest of impact energy values. Contrarily, Fig. 3 shows as high stan- the weld. However, the type of welding consumables investi- dard deviations for the top bead as for the rest of the weld. gated in this study is alloyed with V. This element is known for Therefore, it can be concluded that these heterogeneities in the its potent strengthening effect in steels because of its tendency middle of the multipass weld are balanced over a whole to form precipitates during reheating [4, 20, 21]. Therefore, Charpy V-notch sample and the impact energy of the mixed hardness mappings and local Charpy V-notch tests were ap- microstructure will always be similar despite the existing plied to study the local mechanical properties and the differ- hardness fluctuations. ence between the last weld bead and the underlying beads in this type of material. The local Charpy V-notch tests delivered unexpected re- sults. From Fig. 3, it can be concluded that there is no signif- 5 Conclusions icant difference between the impact toughness of the last bead (upper part) and the rest of the weld. Contrarily, the hardness An all-weld metal sample was produced by gas metal arc mappings showed that the last bead is softer than the under- welding with a V-alloyed welding consumable, resulting in a lying structure (Fig. 4). This behaviour can be explained by multipass weld with 21 weld beads. This type of material was looking at the precipitate population in the material. In an investigated regarding its local mechanical properties and mi- atom probe study on the same type of material [4], it was crostructural appearance with local Charpy V-notch tests, proven that precipitates do not exist in the last bead. hardness mappings, microprobe, light optical microscopy Compared to the rest of the weld, the toughness is hence not and electron backscatter diffraction. The findings can be sum- decreased by precipitate formation, but also there is no tem- marized as follows: pering of the martensitic matrix. These counteracting mecha- nisms result in a parity of the Charpy V-notch energies for all & There was no significant difference between the local im- parts of the weld. The absence of V-rich precipitates in the last pact energies of the last bead and the underlying weld. The bead is also responsible for its comparatively low hardness. last bead does not contain precipitates, but is also not Both the impact energy and the hardness show significant tempered. In the underlying weld, the tempering of the fluctuations. The standard deviation of the impact energy was martensitic microstructure counteracts the toughness loss up to 6 J (Fig. 3). The hardness map depicts values between by precipitate formation. 350 and 410 HV for the weld metal (Fig. 4). These fluctua- & The hardness of the last bead was lower compared to the tions may stem from interdendritic segregations of alloying rest of the weld because of the absence of V-rich elements as well as grain size heterogeneity in the weld metal. precipitates. As shown in Fig. 5, several alloying elements tend to segre- & Multipass welding produces heat-affected zones inside the gate during solidification in the investigated system. The ef- weld with fluctuations of grain size and V-rich precipitate fect of these segregations on the martensitic matrix is hard to populations. quantify. According to Table 3, the grain size varies depending & Interdendritic segregations and effective grain size fluctu- on the location of the EBSD scan. However, there is no obvi- ations are responsible for high standard deviations of the ous evidence for a connection between interdendritic local impact toughness values. 1158 Weld World (2018) 62:1153–1158 Acknowledgements Open access funding provided by deposits. Mater Des 30:1902–1912. https://doi.org/10.1016/j. Montanuniversität Leoben. The K-Project Network of Excellence for matdes.2008.09.023 Metal JOINing is fostered in the frame of COMET—Competence 10. Amrei MM, Monajati H, Thibault D, Verreman Y, Germain L, Centers for Excellent Technologies by BMWFW, BMVIT, FFG, Land Bocher P (2016) Microstructure characterization and hardness dis- Oberösterreich, Land Steiermark, Land Tirol and SFG. The programme tribution of 13Cr4Ni multipass weld metal. Mater Charact 111: COMET is handled by FFG. 128–136. https://doi.org/10.1016/j.matchar.2015.11.022 11. Jorge JC, Souza LF, Rebello JM (2001) The effect of chromium on the microstructure/toughness relationship of C–Mn weld metal de- Open Access This article is distributed under the terms of the Creative posits. Mater Charact 47:195–205. https://doi.org/10.1016/S1044- Commons Attribution 4.0 International License (http:// 5803(01)00168-1 creativecommons.org/licenses/by/4.0/), which permits unrestricted use, 12. Bhadeshia HKDH, Svensson LE (1989) The microstructure of sub- distribution, and reproduction in any medium, provided you give appro- merged arc-weld deposits for high-strength steels. J Mater Sci 24: priate credit to the original author(s) and the source, provide a link to the 3180–3188. https://doi.org/10.1007/BF01139039 Creative Commons license, and indicate if changes were made. 13. Haslberger P, Holly S, Ernst W, Schnitzer R (2018) Microstructure and mechanical properties of high-strength steel welding consum- ables with a minimum yield strength of 1100 MPa. J Mater Sci 53: 6968–6979. https://doi.org/10.1007/s10853-018-2042-9 References 14. Chao YJ, Ward JD, Sands RG (2007) Charpy impact energy, frac- ture toughness and ductile-brittle transition temperature of dual- 1. Grong O, Matlock DK (1986) Microstructural development in mild phase 590 steel. Mater Des 28:551–557. https://doi.org/10.1016/j. and low-alloy steel weld metals. Int Met Rev 31:27–48. https://doi. matdes.2005.08.009 org/10.1179/imtr.1986.31.1.27 15. Kim M-C, Jun Oh Y, Hwa Hong J (2000) Characterization of 2. Keehan E, Karlsson L, Andrén H-O, Bhadeshia H (2006) New boundaries and determination of effective grain size in Mn-Mo-Ni developments with C-Mn-Ni high-strength steel weld metals, part low alloy steel from the view of misorientation. Scr Mater 43:205– a—microstructure. Weld J 85:200s–210s 211. https://doi.org/10.1016/S1359-6462(00)00392-4 3. Powell GLF, Herfurth G (1998) Charpy V-notch properties and 16. Haslberger P, Holly S, Ernst W, Schnitzer R (2017) Microstructural microstructures of narrow gap ferritic welds of a quenched and characterization of martensitic all-weld metal samples. Pract tempered steel plate. Metall Mater Trans A Phys Metall Mater Sci Metallogr 54:513–532. https://doi.org/10.3139/147.110464 29:2775–2784. https://doi.org/10.1007/s11661-998-0318-4 4. Haslberger P, Holly S, Ernst W, Schnitzer R (2018) Precipitates in 17. Kikuta Y, Araki T, Yoneda M, Yoshida H, Suga T (1982) The microalloyed ultra-high strength weld metal studied by atom probe reheated zone toughness of multipass weld metal (report 1). J tomography. Weld World 62:713–719. https://doi.org/10.1007/ Japan Weld Soc 51:359–365. https://doi.org/10.2207/qjjws1943. s40194-018-0581-y 51.359 5. Zhang Z, Farrar RA (1995) Columnar grain development in C-Mn- 18. Surian E, Ramini de Rissone M, De Vedia L (2005) Influence of Ni low-alloy weld metals and the influence of nickel. J Mater Sci molybdenum on ferritic high- strength SMAWall-weld-metal prop- 30:5581–5588. https://doi.org/10.1007/BF00356690 erties. Weld J 84:53–62 6. Yongyuth P, Ghosh PK, Gupta PC, Patwardhan AK, Prakash S 19. Mine Y, Hirashita K, Takashima H, Matsuda M, Takashima K (1992) Influence of macro/microstructure on the toughness of Ball (2013) Micro-tension behaviour of lath martensite structures of weld^ multipass submerged arc welded C-Mn steel deposits. ISIJ carbon steel. Mater Sci Eng A 560:535–544. https://doi.org/10. Int 32:771–778 1016/j.msea.2012.09.099 7. Reed RC, Bhadeshia HKDH (1994) A simple model for multipass 20. Gladman T (1997) The physical metallurgy of microalloyed steels. steel welds. Acta Metall Mater 42:3663–3678. https://doi.org/10. The Institute of Materials, London 1016/0956-7151(94)90432-4 21. Lagneborg R, Siwecki T, Zajac S, Hutchinson B (1999) The role of 8. MythiliR,ThomasPaulV,SarojaS,VijayalakshmiM, vanadium in microalloyed steels. Scand J Metall Raghunathan VS (2003) Microstructural modification due to 22. Akselsen OM, Solberg JK, Grong O (1988) Effects of martensite- reheating in multipass manual metal arc welds of 9Cr-1Mo steel. austenite (M-A) islands on intercritical heat-affected zone tough- J Nucl Mater 312:199–206. https://doi.org/10.1016/S0022- ness of low carbon microalloyed steels. Scand J Metall 17:194–200 3115(02)01680-X 23. Matsuda F, Ikeuchi K, Fukada Yet al (1995) Review of mechanical 9. Avazkonandeh-Gharavol MH, Haddad-Sabzevar M, Haerian A and metallurgical investigations of MA constituent in welded joint (2009) Effect of copper content on the microstructure and mechan- in Japan. Trans JWRI 24:1–24 ical properties of multipass MMA, low alloy steel weld metal http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Welding in the World Springer Journals

Influence of inhomogeneity on several length scales on the local mechanical properties in V-alloyed all-weld metal

Download PDF
6 pages

Loading next page...
 
/lp/springer-journals/influence-of-inhomogeneity-on-several-length-scales-on-the-local-5ZqpGBFjj6

References (26)

Publisher
Springer Journals
Copyright
Copyright © 2018 by The Author(s)
Subject
Materials Science; Metallic Materials; Continuum Mechanics and Mechanics of Materials; Theoretical and Applied Mechanics
ISSN
0043-2288
eISSN
1878-6669
DOI
10.1007/s40194-018-0636-0
Publisher site
See Article on Publisher Site

Abstract

Recently, a new, vanadium alloyed welding consumable with a minimum yield strength of 1100 MPa was developed. The mechanical properties of welding consumables for gas metal arc welding are usually classified by producing and testing all- weld metal samples, which are typically a multipass weld. Chemical and microstructural fluctuations of a vanadium alloyed all- weld metal sample on a macro- and microscale and their influence on the local mechanical properties were investigated. On a macroscale, hardness mappings show a pattern of hard and soft zones which can differ up to 60 HV. Despite the existence of these fluctuations, undersized Charpy V-notch tests revealed no significant difference between the last weld bead and the underlying ones. It is explained how vanadium and its tendency to form precipitates affect both the hardness inhomogeneity and the toughness homogeneity. On a microscale, segregations of several alloying elements and significant grain size fluctuations were found. Their influence on fluctuations of the mechanical properties is discussed as well. . . . . . Keywords Filler materials Weld metal Toughness Hardness Homogeneity Multipass welding 1 Introduction chemical composition of the welding consumable is crucial. In order to classify the mechanical properties of the welding High-strength steels are frequently welded by gas metal arc consumable itself, the production of all-weld metal samples welding with matching welding consumables to produce com- according to DIN EN ISO 15792-1 is required. In this proce- ponents with a high load bearing capacity and a comparatively dure, a large gap is buffered and then filled layer by layer with low weight. These components can be found in e.g. vehicles or the welding consumable. The result is a multilayer structure cranes, where a low component weight can save a significant similar to Fig. 1 with a reproducible number of beads depend- amount of energy. The mechanical properties of the produced ing on the chosen welding parameters. This sample design (and welds depend on their geometry, number of layers and the used especially the buffering) eliminates influences from the chem- welding parameters. Of course, also the choice of a suitable ical composition of the base material and should lead to a chemical composition of the weld metal which is only influ- enced by the chemical composition of the filler wire. Recommended for publication by Commission II - Arc Welding and Filler Metals Such a multipass weld contains several sources of inhomo- geneity on different scales. At first, the bead will solidify in a * Phillip Haslberger cellular or dendritic manner, which generates interdendritic phillip.haslberger@unileoben.ac.at segregations of alloying elements [1, 2]. These segregations can affect the formation of different ferritic constituents in the Department of Physical Metallurgy and Materials Testing, weld metal during further cooling. This was described by Montanuniversitaet Leoben, Leoben, Austria Keehan et al. [2] for high-strength weld metals produced by voestalpine Stahl GmbH, Linz, Austria shielded metal arc welding and by Powell and Herfurth [3]for gas metal arc welds. In another study, Haslberger et al. pointed Institute of Materials Science, Joining and Forming, Graz University of Technology, Graz, Austria out that these segregations could also influence the formation of clusters and precipitates in V-alloyed weld metal [4]. voestalpine Böhler Welding Austria GmbH, Kapfenberg, Austria After delta ferrite formation, austenite columns will form Present address: voestalpine Wire Technology GmbH, upon cooling. Their size and shape may change throughout Leoben, Austria 1154 Weld World (2018) 62:1153–1158 Table 1 Used welding parameters for the production of all-weld metal samples Current (A) Voltage (V) Welding speed Heat input Interpass (cm/min) per unit length temperature (kJ/cm) (°C) 250 30 50 9 150 Fig. 1 Schematic of an all-weld metal sample for the investigated type of welding consumables. The area shaded light gray is the buffer zone the weld bead. This phenomenon was investigated by Zhang using the welding parameters shown in Table 1.Intotal, and Farrar [5], who showed that the width of austenite columns the weld metal consisted of 7 layers with 21 beads (Fig. 1). can change throughout one weld bead depending on the nickel The chemical composition of the weld metal is shown in content. These factors (segregation and column growth) may Table 2. The amount of alloying elements guarantees a influence the microstructure from a microscopic point of view. fully martensitic weld metal for the expected cooling time From a macroscopic point of view, every deposited bead is between 800 and 500 °C of about 5 s. For an evaluation of reheated by subsequent welding passes. Depending on the peak the local mechanical behaviour of the weld metal, hardness temperature, this can effect remelting, reaustenitization, or tem- mappings and local Charpy V-notch tests were carried out. pering of the microstructure. As a result, zones with the original The Vickers hardness mappings were conducted with a or altered microstructures exist in the weld, which was described load of 2 kg and a step size of 0.5 mm. Subsized Charpy in several studies [6–10]. The size of zones with different types of V-notch samples with a cross section of 5 × 10 mm were microstructure (i.e., columnar grains and refined equiaxed or re- prepared (Fig. 2), dividing the all-weld metal sample in an crystallized grains) is dependent on the used welding parameters upper, middle and lower area. In the upper samples, a large [6]. The influence of the area fraction of columnar grains and part of the notch length was located in the last deposited recrystallized grains on the impact toughness is reported diversely bead, while the middle and lower samples contained a mix- [10]. While some indicated that a high amount of reheated mate- ture of differently reheated material. Three samples were rial is beneficial for impact toughness [11], others suggested that a tested at each temperature according to DIN EN ISO 148- low amount of reheated material is advantageous because of a 1, and the measured values were converted to the values higher homogeneity of the weld [12]. Another study claimed that for a 10 × 10 mm sample by applying a geometric factor the influence of the amount of reheated material is negligible [3]. accounting for the size of the fractured area compared to a Therefore, it can be summarized that the influence of the amount standard size sample [14]. of reheated material on the impact toughness strongly depends on For the investigations of interdendritic segregations, elec- the investigated material and its microstructural condition. tron probe microanalysis measurements were performed at an Currently, a V-alloyed welding consumable with a yield acceleration voltage of 15 keVand a probe current of 600 nA. strength of 1100 MPa and a minimum impact energy of 47 J A step size of 500 nm was chosen to ensure the ability to at − 20 °C was developed [13]. The produced all-weld metal resolve the segregations. Light optical microscopy of the nital samples were martensitic and contained V(C,N) precipitates etched sample was used to identify zones with different mi- [4, 13]. The purpose of this study is to demonstrate the exis- crostructural appearance on a macro scale. Electron backscat- tence of chemical and microstructural inhomogeneities in this ter diffraction (EBSD) measurements at 30 keV and 10 nA type of martensitic multipass weld metal and to clarify their with a step size of 80 nm were used to determine local effec- influence on the mechanical properties of the weld. Special tive grain sizes inside the last deposited bead. The effective emphasis is put on the comparison between the last deposited grains were defined by their misorientation tolerance angle of bead and the middle of the all-weld metal due to their differ- 15° [15], and their size was averaged by area. More informa- ence in thermal history. Hardness mappings and subsized tion regarding sample preparation and EBSD parameters can Charpy V-notch tests are used to determine fluctuations of be found in [16], which specifically addresses the microstruc- the local mechanical properties of the multipass weld. tural characterization of all-weld metal samples with light op- Microprobe investigations and microstructure comparisons tical microscopy and EBSD. are used to interpret the observed mechanical behaviour. Table 2 Chemical composition of the investigated material (m%) 2 Materials and methods CSi Mn Cr Mo Ni Cu V Fe The investigated all-weld metal samples were produced by 0.08 0.57 1.27 0.78 0.73 3.03 0.11 0.2–0.5 Rest gas metal arc welding according to DIN EN ISO 15792-1 Weld World (2018) 62:1153–1158 1155 Fig. 2 Schematic of the prepared subsized Charpy V-notch samples. The approximate shape of the last weld bead is outlined 3 Results 3.1 Local impact toughness The converted Charpy impact values from − 60 to + 20 °C for all three locations in the multipass weld are depicted in Fig. 3. Generally, the impact toughness increases gradually with in- creasing temperature. There is no significant difference be- tween upper, middle and lower part of the all-weld metal, and therefore no significant difference between top bead and tempered beads despite their diverse thermal history. 3.2 Hardness mappings For an assessment of hardness fluctuations in the multipass welds, several hardness mappings were conducted over the Fig. 4 a Nital etched all-weld metal sample after hardness mapping. The melting line of the last deposited bead is indicated by a black line. b whole weld metal, and the results are exemplarily shown on Colour-coded hardness map of the V-alloyed sample superimposed on one obtained map. The hardness indents covered a large part the image of the nital etched sample. The arrow points at the soft zone in of the sample, which is shown in Fig. 4a, where also the the last weld bead position of the last, untempered bead is marked by a black line. The position of the last bead is different compared to corresponding weld beads and their heat-affected zones, lead- the sample used for the Charpy V-notch tests. However, this ing to an average hardness of ca. 380 HV. should not affect the interpretation of the results. The resulting hardness map is shown in Fig. 4b. Varying hardness values 3.3 Interdendritic segregations between 350 and 410 HV were observed. The yellow and orange colours (arrow in Fig. 4b) in the last bead show that Microprobe mappings of the elements Mn, Cr and V were it is softer than the surrounding material. In the heat-affected conducted to assess the existence and the amount of zone below the last bead, a harder area with ca. 410 HVand a interdendritic segregation in the material. All measured ele- soft zone with ca. 370 HV follow. Throughout the rest of the ments clearly segregated, which is visible in Fig. 5. The red all-weld metal, hard and soft zones appear in the shape of the points in the Mn maps (Fig. 5a, b) originate from Mn-rich inclusions, which are homogeneously distributed over the whole weld metal. Consequently, they are not included in the further evaluation and interpretation of the interdendritic segregations visible in the map. Visually Mn segregates more than Cr (Fig. 5c, d) and V (Fig. 5e, f). The amount of segre- gation was evaluated by calculating the ratio of the maximum local element concentration and the average concentration over the whole map. Both Mn and Cr showed maximum con- tents of about 120% of the average value in the segregated regions. For V, this value was even higher with 190%. No difference between the top bead and the middle beads was observed, implying that reheating did not cause a redistribu- Fig. 3 Results of the subsized Charpy V-notch tests from − 60 to + 20 °C tion of alloying elements on this length scale. 1156 Weld World (2018) 62:1153–1158 Fig. 5 Microprobe mappings of sample with 0.2 m.% V. The distributions of Mn (a, b), Cr (c, d)and V(e, f) are shown for the top bead and the middle beads, respectively 3.4 Grain size fluctuations 4 Discussion The nature of the multipass welding process implies the exis- Several methods were used to investigate the influence of tence of grain size fluctuations between the passes due to inhomogeneity on different scales on the local mechanical reheating. This phenomenon of a mixture of zones with co- lumnar grains and zones with equiaxed fine grains is well described in literature [6, 7, 10, 12, 17, 18] and exemplarily shown in Fig. 6 for the current material. On a different scale, grain size fluctuations inside a single weld bead may occur. Therefore, EBSD measurements were conducted at several positions in the last bead for a determination of the local ef- fective grain size, which was defined by a tolerance angle of 15° [15, 19]. This tolerance angle ensures that martensitic blocks are separated and that the grain size correlates to the fracture behaviour of the material [15]. The results in Table 3 clearly indicate the existence of fluctuations of the effective grain size within the bead. Additionally, due to the irregular nature of the martensitic microstructure, the standard devia- tion at each position is high. However, there was no evidence for changes in microstructure caused by the interdendritic seg- regations. The influence of the irregular microstructures on Fig. 6 Last deposited bead and its surroundings etched with nital. Below the bead, a zone consisting of equiaxed fine grains is visible different scales will be discussed in the next section. Weld World (2018) 62:1153–1158 1157 Table 3 Local effective segregations and grain size. Nevertheless, it can be concluded Distance to the fusion Effective grain grain size in the last that enrichments of elements like Mn, Cr and especially V will line (mm) size (μm) deposited bead change the potential for precipitate formation. The resulting dependingonthe 0.52 2.6 ± 1.6 distance to the fusion line heterogeneity of the precipitate population will induce fluctu- 2.4 2.7 ± 1.4 ations in the local impact energy. 2.5 3.1 ± 2.1 Furthermore, as each welding pass will create a heat- 3.8 2.8 ± 1.6 affected zone inside the weld metal with areas of coarse- 3.9 2.8 ± 1.7 grained microstructures and fine-grained microstructures, a multilayer weld is always inhomogeneous in terms of grain size. The intercritical reheated zone may also contain areas properties of a martensitic all-weld metal sample produced with fresh martensite which leads to a local deterioration of from a new type of welding consumables. both hardness and toughness [22, 23]. This soft zone is clearly In multipass welds, a weld bead will be heat treated by visible in Fig. 4b. Additionally, the formation of precipitates is subsequent welding passes [7, 12]. In the case of a martensitic dependent on the reheating temperature. Hence, a Charpy V- weld metal, this implies the assumption that the reheating will notch sample will contain a mixture of zones with different cause a tempering of the martensite which should generally grain sizes (Fig. 6), microstructural constituents and precipi- increase the toughness [1]. Consequently, the last bead should tate populations, which may also lead to fluctuations in the have a higher strength and a lower toughness than the rest of impact energy values. Contrarily, Fig. 3 shows as high stan- the weld. However, the type of welding consumables investi- dard deviations for the top bead as for the rest of the weld. gated in this study is alloyed with V. This element is known for Therefore, it can be concluded that these heterogeneities in the its potent strengthening effect in steels because of its tendency middle of the multipass weld are balanced over a whole to form precipitates during reheating [4, 20, 21]. Therefore, Charpy V-notch sample and the impact energy of the mixed hardness mappings and local Charpy V-notch tests were ap- microstructure will always be similar despite the existing plied to study the local mechanical properties and the differ- hardness fluctuations. ence between the last weld bead and the underlying beads in this type of material. The local Charpy V-notch tests delivered unexpected re- sults. From Fig. 3, it can be concluded that there is no signif- 5 Conclusions icant difference between the impact toughness of the last bead (upper part) and the rest of the weld. Contrarily, the hardness An all-weld metal sample was produced by gas metal arc mappings showed that the last bead is softer than the under- welding with a V-alloyed welding consumable, resulting in a lying structure (Fig. 4). This behaviour can be explained by multipass weld with 21 weld beads. This type of material was looking at the precipitate population in the material. In an investigated regarding its local mechanical properties and mi- atom probe study on the same type of material [4], it was crostructural appearance with local Charpy V-notch tests, proven that precipitates do not exist in the last bead. hardness mappings, microprobe, light optical microscopy Compared to the rest of the weld, the toughness is hence not and electron backscatter diffraction. The findings can be sum- decreased by precipitate formation, but also there is no tem- marized as follows: pering of the martensitic matrix. These counteracting mecha- nisms result in a parity of the Charpy V-notch energies for all & There was no significant difference between the local im- parts of the weld. The absence of V-rich precipitates in the last pact energies of the last bead and the underlying weld. The bead is also responsible for its comparatively low hardness. last bead does not contain precipitates, but is also not Both the impact energy and the hardness show significant tempered. In the underlying weld, the tempering of the fluctuations. The standard deviation of the impact energy was martensitic microstructure counteracts the toughness loss up to 6 J (Fig. 3). The hardness map depicts values between by precipitate formation. 350 and 410 HV for the weld metal (Fig. 4). These fluctua- & The hardness of the last bead was lower compared to the tions may stem from interdendritic segregations of alloying rest of the weld because of the absence of V-rich elements as well as grain size heterogeneity in the weld metal. precipitates. As shown in Fig. 5, several alloying elements tend to segre- & Multipass welding produces heat-affected zones inside the gate during solidification in the investigated system. The ef- weld with fluctuations of grain size and V-rich precipitate fect of these segregations on the martensitic matrix is hard to populations. quantify. According to Table 3, the grain size varies depending & Interdendritic segregations and effective grain size fluctu- on the location of the EBSD scan. However, there is no obvi- ations are responsible for high standard deviations of the ous evidence for a connection between interdendritic local impact toughness values. 1158 Weld World (2018) 62:1153–1158 Acknowledgements Open access funding provided by deposits. Mater Des 30:1902–1912. https://doi.org/10.1016/j. Montanuniversität Leoben. The K-Project Network of Excellence for matdes.2008.09.023 Metal JOINing is fostered in the frame of COMET—Competence 10. Amrei MM, Monajati H, Thibault D, Verreman Y, Germain L, Centers for Excellent Technologies by BMWFW, BMVIT, FFG, Land Bocher P (2016) Microstructure characterization and hardness dis- Oberösterreich, Land Steiermark, Land Tirol and SFG. The programme tribution of 13Cr4Ni multipass weld metal. Mater Charact 111: COMET is handled by FFG. 128–136. https://doi.org/10.1016/j.matchar.2015.11.022 11. Jorge JC, Souza LF, Rebello JM (2001) The effect of chromium on the microstructure/toughness relationship of C–Mn weld metal de- Open Access This article is distributed under the terms of the Creative posits. Mater Charact 47:195–205. https://doi.org/10.1016/S1044- Commons Attribution 4.0 International License (http:// 5803(01)00168-1 creativecommons.org/licenses/by/4.0/), which permits unrestricted use, 12. Bhadeshia HKDH, Svensson LE (1989) The microstructure of sub- distribution, and reproduction in any medium, provided you give appro- merged arc-weld deposits for high-strength steels. J Mater Sci 24: priate credit to the original author(s) and the source, provide a link to the 3180–3188. https://doi.org/10.1007/BF01139039 Creative Commons license, and indicate if changes were made. 13. Haslberger P, Holly S, Ernst W, Schnitzer R (2018) Microstructure and mechanical properties of high-strength steel welding consum- ables with a minimum yield strength of 1100 MPa. J Mater Sci 53: 6968–6979. https://doi.org/10.1007/s10853-018-2042-9 References 14. Chao YJ, Ward JD, Sands RG (2007) Charpy impact energy, frac- ture toughness and ductile-brittle transition temperature of dual- 1. Grong O, Matlock DK (1986) Microstructural development in mild phase 590 steel. Mater Des 28:551–557. https://doi.org/10.1016/j. and low-alloy steel weld metals. Int Met Rev 31:27–48. https://doi. matdes.2005.08.009 org/10.1179/imtr.1986.31.1.27 15. Kim M-C, Jun Oh Y, Hwa Hong J (2000) Characterization of 2. Keehan E, Karlsson L, Andrén H-O, Bhadeshia H (2006) New boundaries and determination of effective grain size in Mn-Mo-Ni developments with C-Mn-Ni high-strength steel weld metals, part low alloy steel from the view of misorientation. Scr Mater 43:205– a—microstructure. Weld J 85:200s–210s 211. https://doi.org/10.1016/S1359-6462(00)00392-4 3. Powell GLF, Herfurth G (1998) Charpy V-notch properties and 16. Haslberger P, Holly S, Ernst W, Schnitzer R (2017) Microstructural microstructures of narrow gap ferritic welds of a quenched and characterization of martensitic all-weld metal samples. Pract tempered steel plate. Metall Mater Trans A Phys Metall Mater Sci Metallogr 54:513–532. https://doi.org/10.3139/147.110464 29:2775–2784. https://doi.org/10.1007/s11661-998-0318-4 4. Haslberger P, Holly S, Ernst W, Schnitzer R (2018) Precipitates in 17. Kikuta Y, Araki T, Yoneda M, Yoshida H, Suga T (1982) The microalloyed ultra-high strength weld metal studied by atom probe reheated zone toughness of multipass weld metal (report 1). J tomography. Weld World 62:713–719. https://doi.org/10.1007/ Japan Weld Soc 51:359–365. https://doi.org/10.2207/qjjws1943. s40194-018-0581-y 51.359 5. Zhang Z, Farrar RA (1995) Columnar grain development in C-Mn- 18. Surian E, Ramini de Rissone M, De Vedia L (2005) Influence of Ni low-alloy weld metals and the influence of nickel. J Mater Sci molybdenum on ferritic high- strength SMAWall-weld-metal prop- 30:5581–5588. https://doi.org/10.1007/BF00356690 erties. Weld J 84:53–62 6. Yongyuth P, Ghosh PK, Gupta PC, Patwardhan AK, Prakash S 19. Mine Y, Hirashita K, Takashima H, Matsuda M, Takashima K (1992) Influence of macro/microstructure on the toughness of Ball (2013) Micro-tension behaviour of lath martensite structures of weld^ multipass submerged arc welded C-Mn steel deposits. ISIJ carbon steel. Mater Sci Eng A 560:535–544. https://doi.org/10. Int 32:771–778 1016/j.msea.2012.09.099 7. Reed RC, Bhadeshia HKDH (1994) A simple model for multipass 20. Gladman T (1997) The physical metallurgy of microalloyed steels. steel welds. Acta Metall Mater 42:3663–3678. https://doi.org/10. The Institute of Materials, London 1016/0956-7151(94)90432-4 21. Lagneborg R, Siwecki T, Zajac S, Hutchinson B (1999) The role of 8. MythiliR,ThomasPaulV,SarojaS,VijayalakshmiM, vanadium in microalloyed steels. Scand J Metall Raghunathan VS (2003) Microstructural modification due to 22. Akselsen OM, Solberg JK, Grong O (1988) Effects of martensite- reheating in multipass manual metal arc welds of 9Cr-1Mo steel. austenite (M-A) islands on intercritical heat-affected zone tough- J Nucl Mater 312:199–206. https://doi.org/10.1016/S0022- ness of low carbon microalloyed steels. Scand J Metall 17:194–200 3115(02)01680-X 23. Matsuda F, Ikeuchi K, Fukada Yet al (1995) Review of mechanical 9. Avazkonandeh-Gharavol MH, Haddad-Sabzevar M, Haerian A and metallurgical investigations of MA constituent in welded joint (2009) Effect of copper content on the microstructure and mechan- in Japan. Trans JWRI 24:1–24 ical properties of multipass MMA, low alloy steel weld metal

Journal

Welding in the World – Springer Journals

Published: Aug 13, 2018

There are no references for this article.