Understanding the effect of weld parameters on the microstructures and mechanical properties in dissimilar steel welds

Understanding the effect of weld parameters on the microstructures and mechanical properties in dissimilar steel welds

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Available online at www.sciencedirect.com

ScienceDirect

Procedia Manufacturing 00 (2019) 000–000 www.elsevier.com/locate/procedia

Available online at www.sciencedirect.com Procedia Manufacturing 00 (2019) 000–000

www.elsevier.com/locate/procedia

ScienceDirect Procedia Manufacturing 35 (2019) 986–991

2nd International Conference on Sustainable Materials Processing and Manufacturing (SMPM 2019) 2nd International Conference on Sustainable Materials Processing and Manufacturing (SMPM 2019) Understanding the effect of weld parameters on the microstructures and mechanical properties in dissimilar steel welds Understanding the effect of weld parameters on the microstructures Dilipand Kumar Singh a, Vikram Sharma b , Ritwik Basu c* and Mostafa Eskandari d mechanical properties in dissimilar steel welds Department of Mechanical and Manufacturing Engineering, MIT, Manipal Academy of Higher Education (MAHE), Manipal-576104, India c b Researcha,and Development Division,b Tata Steel Ltd, Jamshedpur 831001, India Eskandari d Dilip cKumar Singh Vikram Sharma , Ritwik Basu and Mostafa School of Metal Construction Skills, Bhartiya Skill Development University, Jaipur- 302042, Rajasthan d a Department of Materials Science &Engineering, Faculty Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran India Department of Mechanical and Manufacturing Engineering, MIT,ofManipal Academy of Higher Education (MAHE), Manipal-576104, b Research and Development Division, Tata Steel Ltd, Jamshedpur 831001, India c School of Metal Construction Skills, Bhartiya Skill Development University, Jaipur- 302042, Rajasthan d Department of Materials Science &Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran Abstract a

*

Welding of dissimilar steel grades, such as stainless and medium carbon steel, is becoming increasingly challenging in Abstract petrochemical refineries, as well as in many mining and mineral processing operations. Problems encountered in dissimilar weld joints during in-service applications are associated with formation of brittle phases and undesired residual stresses that develop, Welding of cracks dissimilar steel grades, such as stainless medium carbon steel, is becoming in resulting in or failures in the component. Many ofand these problems can be largely tackled by increasingly tailoring the challenging microstructure petrochemical as steps. well asIt in many and mineral processing Problems encountered in dissimilar weld during the weldrefineries, processing has beenmining demonstrated in this work thatoperations. welding process variables can have a direct impact joints in-service applications are associated with of formation of brittle phases and undesired that develop, on theduring resulting microstructure and that the properties the material can be fine-tuned through residual a better stresses understanding of the resulting in cracks or failures in the component. Many of these problems can be largely tackled by tailoring the microstructure different microstructural mechanisms. Few different weld specimens of a stainless (SS 304) and a medium carbon (EN 8) steels duringprepared the weldbyprocessing It has been demonstrated in this work thatbywelding process variables have technique. a direct impact were changing steps. the weld parameters (current, voltage, speed) tungsten inert gas (TIG) can welding The on the resulting microstructure development and that the properties of the using material be fine-tuned through a better understanding of the studies on the microstructural were performed thecan electron backscattered diffraction (EBSD) techniques. different microstructural mechanisms. different weld specimens of a stainless (SS in 304) andofa the medium carbon of (EN 8) steels Large microstructural differences wereFew brought out between these samples, especially terms distribution phases and were prepared by changing the weld parameters (current, voltage, speed) by tungsten inert gas (TIG) welding technique. fractions of grain boundary. The tensile strength measured for these welded joints had a strong bearing on the presence of The low studiesgrain on the microstructural werepresents performed using theapproach electrontobackscattered diffraction (EBSD) techniques. angle boundaries (LAGB).development This brief study a systematic establish microstructure-mechanical property Large microstructural differences werejoints. brought out between these samples, especially in terms of the distribution of phases and relationship in dissimilar steel welded fractions of grain boundary. The tensile strength measured for these welded joints had a strong bearing on the presence of low angle grain (LAGB).by This brief study © 2019 Theboundaries Authors. Published Elsevier B.V. presents a systematic approach to establish microstructure-mechanical property relationship in dissimilar steel welded joints. Peer-review under responsibility of the organizing committee of SMPM 2019. © 2019 2019 The The Authors. © Authors. Published Published by by Elsevier Elsevier B.V. B.V. Peer-review under responsibility of the organizing committee of SMPM 2019. Peer-review under responsibility of the organizing committee of SMPM 2019.

* Corresponding author. Tel.: +91 7840022252. E-mail address: [email protected] * Corresponding Tel.: +91 7840022252. 2351-9789 © 2019 author. The Authors. Published by Elsevier B.V. E-mail address: [email protected] Peer-review under responsibility of the organizing committee of SMPM 2019. 2351-9789 © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of SMPM 2019.

2351-9789 © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of SMPM 2019. 10.1016/j.promfg.2019.06.046

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Keywords: Dissimilar welding; TIG welding; electron backscattered diffraction (EBSD); stored energy; tensile strength

1. Introduction The topic of dissimilar metal welding covers a wide range of materials and production techniques. A better knowledge of the dissimilar welding process enables industries to create more sustainable products that consume less energy and are easier to recycle. Welding of dissimilar steels poses challenges in the fabrication of pressure vessels, heat exchangers in power generation industries and many equipment used in petrochemical and oil-gas fields [1, 2]. These manufacturing difficulties are attributed to metallurgical shortcomings such as formation of brittle phases, solidification and hydrogen-induced cracking, which can lead to component failure during in-service applications [14]. It is also seen that the weld properties can be largely controlled by tailoring the microstructures. A comprehensive understanding on grain boundaries, local in-grain misorientation, stored energy of dislocations and its effect on the strength of the weld joint is presented here. One commonly adopted process for joining dissimilar metal is the tungsten inert gas (TIG) welding. TIG welding promotes a sustainable environment by consuming less energy. A high level of weld quality and precision is also achieved through this technique [5, 6]. All welded specimens were prepared through TIG welding technique in the present investigation. 2. Experimental setup and procedure In this study, stainless steel (SS 304) and medium carbon steel (EN 8) were used in cold-rolled and annealed states. These different steels were laterally welded end to end using the standard TIG welding technique. 309L, a stainless steel grade filler was used during welding. Five different samples, generically named as H1, H2, H3, H4 and H5, were generated at different heat inputs by altering the welding current, voltage and speed. The sample processing conditions are summarized in table 1. Table 1. Weld parameters used in the study. Sample

Voltage (V)

Current (A)

Speed

Heat Input (kJ mm-1)

H1

13.6

100

2.67

0.5093

H2

13.2

95

2.27

0.5524

H3

17.5

90

3.55

0.4436

H4

12.6

100

3.55

0.3549

H5

12.6

100

4.57

0.2757

(mm s-1)

Fig. 1. (a) Schematic representation of the welded specimen together with the orientation of the tensile specimen. The reference directions (RD/ rolling direction, TD/ transverse direction and ND/ normal direction) are shown. Courtesy of The Welding Institute (TWI); (b) Geometry of the

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tensile specimen used in the present study. Samples were prepared as per the ASTM E8M standards. Hatched area shows the approximate location of the weld zone. Courtesy of S. Sattari.

For all the weld specimens, the sample surface used for optical and EBSD investigation was the RD- TD plane shown in figure 1a. Tensile specimens as shown in figure 1b were sectioned from the weld plates as per the ASTM E8M standards. The sample sections were made in a way that the weld area was around the center of the sample gauge. The tensile tests were carried out in a 50 kN capacity ITW BISS Universal Testing Machine. The deformation of the specimens was carried out at 20 kN / min until the specimens completely failed. EBSD measurements were carried out for all samples in a Zeiss Supra 25 field emission scanning electron microscope (FE–SEM) equipped with an Oxford Instruments Nordlys 2 EBSD detector operated at 20 keV. For the acquisition of the raw diffraction data the Oxford Instruments’ AZTEC 2.0 data acquisition software was used. The scans of all samples were made with an identical step size of 0.5 μm. The captured raw data was processed for further analysis with the post-processing software from Oxford Instruments’ Channel 5. For analysis on the grain boundary fractions, misorientation and stored energy, the EBSD data was treated with a post-processing software- Channel 5 from Oxford Instruments. Grain boundaries (GBs) in this study were identified as a region continuously delimited by misorientation higher than 5 deg. GBs above 15 deg were defined as highangle grain boundaries (HAGBs), while boundary misorientations between 5 and 15 deg were categorized as medium-angle boundaries (MAGBs). Low-angle boundaries typically formed substructures within a grain and ranged between 2 and 5 deg. Grain misorientations for calculating the stored energies were measured taking into account the average point-to-point misalignment within an identified grain [7]. The stored energy per unit volume due to a grain boundary dislocation (Eb) was obtained from the Read Shockley equation [8] by multiplying the average energy per unit boundary area (γ ̅) to the area per unit volume:

Eb = SV   = 3

 d ECD

(1)

where, dECD refers to the average grain size calculated from equivalent circle diameter for all misorientations ranging between 5 deg< θ <62.8 deg. 3. Results and discussions 3.1. Microstructural evoloution in the weld metal (WM) region It is understood that the mechanical properties of weld samples are highly dependent on the microstructure of the weld area. The present study thus discusses the characteristic weld microstructure. The microstructural development of the weld metal region is summarized in figure 2. The characteristic microstructures exhibits elongated lamellar grain feature, serration in grain boundaries and two- phase distribution of iron bcc (ferrite) and fcc (austenite) phases. The weld samples H3, H4 and H5 show a mixed distribution of austenite and ferrite with different phase fractions and grain size. Samples H1 and H2 practically had no ferrite.

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Fig. 2. Microstructures in the weld metal (WM) region represented through IPF and phase maps under different heat input condition for all the weld specimens. The characteristic microstructures include elongated grains, serration in grain boundaries and two- phase distribution of bcc (δ) and fcc (γ). Weld microstructures of Samples H3, H4 and H5 exhibits a mixed phase distribution.

The scientific findings of many previous studies have found that the retained ferrite with a complex morphology that forms during the solidification of an austenitic stainless steel weld, is usually δ-ferrite [9]. The formation of δferrite is controlled by various factors such as alloying elements present in the stainless steel filler, composition variations of the filler and non-equilibrium cooling of the weld [10]. It appears in figure 2, that in sample H4, the austenite was the primary solidifying or leading phase and delta ferrite solidified from the rest melt. During the course of solidification of primary γ austenite, rejection of Cr into the melt was a possibility between the growing γ dendrites which initiated δ ferrite to form in these regions [11]. The solidification process in sample H5 weld was probably the reverse of this phenomena, the δ ferrite being the leading phase and austenite solidified from rest melt. In this situation, δ ferrite during its formation, caused nickel to be rejected before the advancing interface during solidification until the concentration in the melt was sufficient to form γ austenite and this phase continued to grow. The microstructure for the sample H3 suggests that the ferrite solidified from the melt directly and the austenite was likely to be precipitated from the solid ferrite under slow rate of cooling. The microstructure observation for sample H5 showed fine arrangement of δ ferrite grains around γ grains. As such no inter-networking between the γ/γ boundaries were observed. It appears that the growth of δ ferrite inhibited boundary migration and growth of γ grains by exerting a pressure on the boundaries that counteracted the driving potential for migration of boundaries. Though heat input did not relate to misorientation developments in the microstructures as shown in figure 3a, but it did show a linear relationship with the grain boundary fractions. This is represented through lines drawn across the data points shown in figure 3b. The results indicate that an increase in heat input resulted in a decrease in cooling rate, which ultimately led the sample to stay at higher temperatures for a long time, resulting in a migration of boundaries and grain growth caused by the reduction of the interfacial energy of boundaries. It is also expected that under the clamping force to hold the two pieces of base metals together, the joining interface of the work pieces could have caused restricted contraction of the weld metal (WM) resulting in plastic deformation simultaneously with partial recrystallization during cooling. This could result in an increased fraction of high angle grain boundaries.

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Fig. 3. (a) Average misorientation as a function of heat input; (b) grain boundary fractions as a function of heat input.

3.2. Mechanical properties and its correlation to microstructure Tensile tests were conducted to evaluate the strength of the joints. All samples failed in the weld area without substantial necking. The results of the tensile tests show that the strength and the elongation to failure vary according to a linear trend. The results of tensile tests are summarized in table 2. Table 2. Ultimate tensile strength (UTS), yield strength (YS) and % elongation values to failure for each sample under different heat inputs. Sample

Heat Input (kJ mm-1)

H1

0.5093

H2

0.5524

H3

0.4436

H4

0.3549

H5

0.2757

UTS (MPa)

YS (MPa)

Elongation (%)

593

271

24.5

475

168

12

536.5

256.

23.4

573

224

32

455

244

10.5

For the various microstructural parameters, the variation in strength is closely related to the low angle grain boundary distribution, see figure 4a. However, in the present study, the yield strength shows no noticeable change. It can be seen that the LAGBs within the grain form a discrete dislocation array. With a further increase of the LAGB density, these dislocation walls are refined, thus obstructing the movement of mobile dislocations, resulting in increase in the strength of the welded joints under application of stresses. The increase in ductility with increase in strength is mainly due to the movement of LAGB walls under applied stress. The mobility of LAGB units depends on the speed at which dislocations at the boundaries can climb and the driving force behind the mobility of these subgrain matrices is the stored energy associated with these boundaries. As shown in figure 4b, the LAGB fraction relates to the stored energy calculated from the Read-Shockley equation. The decrease in the strength and ductility of sample H5 with the increase of the LAGB may be associated with a lower stored energy, which reduces the mobility of these dislocations to undergo a deformation, thus leading to fracture at lower strength.

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Fig. 4. With UTS and YS, LAGB fraction represented for all conditions of welded samples. The low angle faction correlates well with UTS. Sample E, however, shows deviations from this behavior; (b) Fraction LAGB and stored energy estimated from the Reade-Shockley equation for all samples.

4. Conclusions Five different weld samples under varying heat inputs were generated. Significant differences in the weld microstructures were identified. The characteristic microstructures included large elongated grains constituting a single phase γ austenite, serrations in grain boundaries and two-phase distribution of γ austenite and δ ferrite grains of refined sizes. Differences in local in grain misorientation and grain boundary fractions were clear for all sample microstructures. Tensile strength (UTS) was closely related to the LAGB fractions. A higher UTS corresponds to higher LAGB fractions. This is due to higher presence of finely spaced dislocation walls which inhibits the movement of mobile dislocations. It has also been suggested that higher ductility with increased LAGBs result due to climb of dislocation walls. The movement of these dislocation walls is influenced by the stored energy associated with these LAGBs References [1] Joseph, A., Rai, S., Jayakumar, T. and Murugan, N. (2005). Evaluation of residual stresses in dissimilar weld joints. International Journal of Pressure Vessels and Piping, 82(9), pp.700-705. [2] Arivazhagan, N., Singh, S., Prakash, S. and Reddy, G. (2007). An assessment of hardness, impact strength, and hot corrosion behaviour of friction-welded dissimilar weldments between AISI 4140 and AISI 304. The International Journal of Advanced Manufacturing Technology, 39(7-8), pp.679-689. [3] Lippold, J. and Kotecki, D. (2005). Welding metallurgy and weldability of stainless steels. Norwood Mass. [4] Jafarzadegan, M., Feng, A., Abdollah-zadeh, A., Saeid, T., Shen, J. and Assadi, H. (2012). Microstructural characterization in dissimilar friction stir welding between 304 stainless steel and st37 steel. Materials Characterization, 74, pp.28-41. [5] Sadeghian, M., Shamanian, M. and Shafyei, A. (2014). Effect of heat input on microstructure and mechanical properties of dissimilar joints between super duplex stainless steel and high strength low alloy steel. Materials & Design, 60, pp.678-684. [6] I. Hajiannia, M. Shamanian, M. Kasiri, Mater. Des. 50 (2013); 566–573. [7] Badheka, V., Basu, R., Omale, J. and Szpunar, J. (2016). Microstructural Aspects of TIG and A-TIG Welding Process of Dissimilar Steel Grades and Correlation to Mechanical Behavior. Transactions of the Indian Institute of Metals, 69(9), pp.1765-1773. [8] Cao, W., Gu, C., Pereloma, E. and Davies, C. (2008). Stored energy, vacancies and thermal stability of ultra-fine grained copper. Materials Science and Engineering: A, 492(1-2), pp.74-79. [9] López, B. and Rodriguez-Ibabe, J. (2016). Metallurgical Aspects Affecting Thermomechanical Processing of Ti Based Microalloyed Steels. Materials Science Forum, 879, pp.84-89. [10] Suutala, N., Takalo, T. and Moisio, T. (1979). The relationship between solidification and microstructure in austenitic and austenitic-ferritic stainless steel welds. Metallurgical Transactions A, 10(4), pp.512-514.. [11] Hunter, A. and Ferry, M. (2002). Phase formation during solidification of AISI 304 austenitic stainless steel. Scripta Materialia, 46(4), pp.253-258.