Formation and evolution of layered structure in dissimilar welded joints between ferritic-martensitic steel and 316L stainless steel with fillers

doi:10.1016/j.jmst.2019.05.047

Abstract

Abstract:

Dissimilar high-energy beam (HEB) welding is necessary in many industrial applications. Different composition of heat-affected zone (HAZ) and weld metal (WM) lead to variation in mechanical properties within the dissimilar joint, which determines the performance of the welded structure. In the present study, appropriate filler material was used during electron beam welding (EBW) to obtain a reliable dissimilar joint between reduced-activation ferritic-martensitic (RAFM) steel and 316 L austenitic stainless steel. It was observed that the layered structure occurred in the weld metal with 310S filler (310S-WM), which had the inferior resistance to thermal disturbance, leading to severe hardening of 310S-WM after one-step tempering treatment. To further ameliorate the joint inhomogeneity, two-step heat treatment processes were imposed to the joints and optimized. δ-ferrite in the layered structure transformed into γ-phase in the first-step normalizing and remained stable during cooling. In the second-step of tempering, tempered martensite was obtained in the HAZ of the RAFM steel, while the microstructure of 310S-WM was not affected. Thus, the optimized properties for HAZ and 310S-WM in dissimilar welded joint was both obtained by a two-step heat treatment. The creep failure position of two dissimilar joints both occurred in CLAM-BM.

Key words: Dissimilar welding with fillers, Electron beam welding, Layered structure, Post weld heat treatment, Microstructural evolution

Liu Guoliang, Yang Shanwu, Ding Jianwen, Han Wentuo, Zhou Lujun, Zhang Mengqi, Zhou Shanshan, Misra R.D.K., Wan Farong, Shang Chengjia. Formation and evolution of layered structure in dissimilar welded joints between ferritic-martensitic steel and 316L stainless steel with fillers[J]. J. Mater. Sci. Technol., 2019, 35(11): 2665-2681.

Figures/Tables 24

Table 1 Composition of base metals and filler metals (wt%).

Sample	C	N	Si	Mn	S	P	Ni	Cr	W	V	Ta	Mo	Fe
CLAM	0.098	0.0085	0.29	0.52	0.0084	0.0072	/	8.83	1.60	0.21	0.13	/	Bal.
316 L	0.025	0.023	0.69	0.99	0.024	0.0025	10.2	17	/	/	/	2.18	Bal.
310S	0.05	0.0466	0.51	0.94	0.027	0.0010	19.29	25.42	/	/	/	1.1	Bal.
Nickel	0.06	/	0.08	0.04	0.004	0.002	>99.5	/	/	/	/	/	0.04

Fig. 1. Experimental set-up used for electron beam welding CLAM steel and 316 L stainless steel with filler foils.

Table 2 Electron beam welding parameters.

Welding speed	10 mm/s	Scanner function	Circular oscillation
Working distance	400 mm	Amplitude	1 mm
Acceleration voltage	90 KV	Frequency	100 HZ
Focus current	1750 mA	Heat input	1.89 kJ/cm
Welding current	21 mA	Width of top surface	4 mm

Fig. 2. Cross-sectional optical microscopic images in as-welded state for (a) 310S-filler welded joint, and (b) Ni-filler welded joint.

Fig. 3. Composition distribution in (a) 310S-WM, (b) Ni-WM, and (c) Schaeffler diagram predicting the microstructure of dissimilar welded joints. The position in Fig. 3(a) and (b) corresponded to the boxes 1-16 in Fig. 2(a) and (b), respectively.

Table 3 Calculated composition of resultant WMs (wt%).

Sample	C	N	Si	Mn	Ni	Cr	W	V	Ta	Mo
Ni-WM	0.059	0.014	0.45	0.68	16.80	11.63	0.65	0.085	0.053	1.03
310S-WM	0.053	0.023	0.53	0.83	8.57	15.91	0.52	0.068	0.042	1.27

Fig. 4. Microstructure of resultant WMs in as-welded state: (a) optical micrograph of 310S-WM etched by Beraha’s II reagent (austenite is white, and ferrite is black), characteristic areas marked with boxes are analyzed by EBSD; (b, c) magnified Euler maps corresponding to boxes b and c, respectively, in Fig. 4(a), showing dendritic austenite of 310S-WM near base metals. Euler map represents for FCC phase, band contrast (BC) map represents for BCC phase; (d) enlarged SEM micrograph of the solidified dendritic structure of 310S-WM etched by aqua-regia; (e) optical micrograph of Ni-WM etched by Beraha’s II reagent; (f, g) Euler maps corresponding to boxes f and g in Fig. 4(e), respectively, exhibiting the coarse austenitic columnar grain of Ni-WM; (h) enlarged SEM micrograph of cellular structure in Ni-WM electron-etched by oxalic acid solution. The insets in Fig. 4(d) and (h) were the result of EDS-line corresponding to yellow line in Fig. 4(d) and (h), respectively.

Fig. 5. (a) SEM micrograph of layered structure in 310S-WM in as-welded state etched by aqua-regia, the inset was the BC map of layered structure, blue color represents BCC phase, (b) results of EDS point test corresponding to the points in Fig. 5(a) and (c) SEM micrograph of CLAM-HAZ in as-welded state etched by Villella’s reagent, revealing coarse lath martensite with some δCLAM-ferrite near fusion line.

Fig. 6. Mechanical properties of dissimilar joints in as-welded state, cross-sectional hardness distribution of (a) 310S-filler welded joint, (b) Ni-filler welded joint, (c) the engineering stress-strain behaviors of longitudinal tensile for WMs and (d) optical macroscopic of cross-section of transverse tensile specimens after tests as well as the test results. The points in Fig. 6(a) and (b) corresponding to the indentations in Fig. 2(a) and (b), respectively.

Fig. 7. Mechanical properties of dissimilar joints after PWDT, cross-sectional hardness distribution of (a) 310S-filler welded joint, (b) Ni-filler welded joint, (c) the engineering stress-strain behaviors of longitudinal tensile for WMs and (d) optical macroscopic of cross-section of transverse tensile specimens after tests as well as the test results.

Fig. 8. SEM micrographs of (a) CLAM-HAZ after PWDT etched by Villella’s reagent, showing tempered martensite, the δCLAM-ferrite still existing near fusion boundary and (b) Ni-WM after 740 °C/2 h electron-etched by oxalic acid solution. The insets in Fig. 8(a) was the enlarged SEM micrographs.

Fig. 9. Micrographs of layered structure in 310S-WM after PWDT: (a) optical micrograph (OM) etched by Beraha II’s reagent; (b) SEM micrograph of layered structure zone etched by Kalling’s reagent; (c) enlarged SEM micrograph of the layered structure, exhibiting a large amount of σ-phase existed in this zone. The inset was EDS-line corresponding to yellow line in Fig. 9(c); (d) SEM image showing the δpe-ferrite in 310S-WM matrix, the inset was the corresponding EDS of the σ-phase of Fig. 9(d).

Fig. 10. Schematic of two types of evolution for layered structure in 310S-WM (I) direct tempering and (II) normalizing plus tempering.

Fig. 11. Micrographs of layered structure in 310S-WM after PWNT: (a) optical micrograph (OM) etched by Beraha’s II reagent; (b) SEM image of layered structure zone etched by Kalling’s reagent; (c) results of average composition of areas in Fig. 11(b); (d) enlarged SEM micrograph of the 310S-WM matrix with δpe-ferrite after PWNT.

Fig. 12. Cross-sectional hardness distribution of (a) 310S-filler welded joint after PWNT, (b) the average hardness in four zones of 310S-filler welded joints in various conditions, (c) Ni-filler welded joint after PWNT, (d) the average hardness in four zones of Ni-filler welded joints in various conditions and the mechanical properties of dissimilar joints after PWNT, (e) the engineering stress-strain behaviors of longitudinal tensile for WMs and (f) optical macroscopic of cross-section of transverse tensile specimens after tests as well as the test results.

Fig. 13. SEM micrographs of (a) CLAM-HAZ after PWNT etched by Villella’s reagent, exhibiting fully tempered martensite, fine PAGs and no evidence of δCLAM-ferrite and (b) Ni-WM after PWNT with electron-etched by oxalic acid. The inset in Fig. 13(a) was the enlarged SEM micrograph.

Fig. 14. Formation mechanism of layered structure in dissimilar high-energy beam welding: (a) sketch of stirred weld pool; (b) formation of peninsula, layered structure as well as unmixed zone (UMZ); (c) liquidus surface projection of ternary Fe-Cr-Ni phase diagram [41]. Raw materials and resultant weld metals were marked in diagram based on the Schaeffler contents to obtain the corresponding liquidus points; (d) vertical section at 70%-Fe for the Fe-Ni-Cr phase diagram [42], and solidus lines relevant for solidification sequence of Ni-WM and 310S-WM. The vertical orders (I-IV) were the solidification sequence of these two dissimilar weld metals, corresponding to the data in Table 4.

Fig. 15. SEM micrograph of the peninsula and its surrounding microstructure etched by Villella’s reagent in as-welded state in cross-section of (a) 310S-filler welded joint, (b) Ni-filler welded joint. The insets are the EDS distribution in Fig. 15(a) and (b), respectively. The SEM micrographs showing the peninsula and layered structure in horizontal-section in as-welded state for (c) 310S-filler welded joint and (d) Ni-filler welded joint.

Table 4 Transformation sequence of the microstructure in the dissimilar WMs.

Materials	Position	Solidification mode	Phase transformation path	Final solidified microstructure
Ni-WM	(Ⅰ) Ni-WM matrix	A	L→L + A→A	Coarse columnar (width 100-200 μm) consisting of cellular grains with same orientation
Ni-WM	(Ⅱ) alloy-depleted zone in Ni-WM, less ? 4% Ni	A	L→L + A→A	Coarse columnar (width 100-200 μm) consisting of cellular grains with same orientation
310S-WM	(Ⅲ) 310S-WM matrix	FA	L→L+δ→L+δ+A→δ+A	Austenite dendritic structure with δ_pe-ferrite distributed along the solidified dendritic boundary
310S-WM	(Ⅳ) Layered structure zone in 310S-WM, less ? 2% Ni	F	L→L+δ→δ→δ+A	δ_L-ferrite dispersed in γ-phase, existed a large amount of phase-interface (δ_L/γ)

Table 5 Evolution of three types of δ-ferrite in the dissimilar joints during the PWHTs.

Item	δ_CLAM-ferrite	δ_L-ferrite	δ_pe-ferrite
Composition	9Cr	14Cr-7Ni	Chromium ? 16Cr, Nickel ? 9Ni
As-welded	Incompletely transformation of δ_CLAM → γ at fast cooling, δ_CLAM formed in the CLAM-CGHAZ near fusion boundary	Incompletely transformation of δ_L → γ due to a relatively lower transformation temperature, formed a mass of lath δ_L-ferrite	Formed through peritectic-eutectic reaction, δ_pe-ferrite was distributed along the solidified dendritic boundary
PWDT	δ_CLAM-ferrite still existed, some carbide formed in the HAZ	a large amount of σ-phase formed through eutectoid decomposition of δ_L-ferrite (δ_L→σ+γ₂)	σ-phase formed through eutectoid decomposition of δ_pe-ferrite (δ_pe→σ+γ₂)
PWNT	δ_CLAM-ferrite completely transformed into γ-phase at 980 °C, sheared into martensite after cooling, finally formed uniform tempered martensite after tempering (δ_CLAM→γ→M→tempered M)	Most of δ_L-ferrite transformed to γ-phase at 980 °C, and remained stable at room temperature (δ_L → γ)	Due to the higher Cr content, δ_pe-ferrite was not eliminated at 980 °C, formed σ-phase through δ_pe→σ+γ₂ during subsequent tempering

Fig. 16. SEM fractographs of the longitudinal tensile samples after test (a) L-310S-as-welded, (b) L-310S-PWDT, (c) L-310S-PWNT, (d) L-Ni-as-welded, (e) L-Ni-PWDT and (f) L-Ni-PWNT.

Table 6 The Md30 value of γ-phase in the resultant WMs.

Samples	310S-WM	Ni-WM	316 L-BM
Md₃₀ (°C)	14.3 °C	-157.9 °C	-54.6 °C
Deformation at 20 °C	TRIP	No TRIP	No TRIP

Fig. 17. Transverse creep test of dissimilar welded joints in PWNT condition: (a) creep strain versus time curves; (b) hardness profiles after creep test; (c) cross-section macrographs of transverse creep fracture specimens. The points in Fig. 17(b) corresponding to the indentations in Fig. 17(c).

Table 7 Dilution rates of WMs.

Raw materials	Dilution rates
	310S-WM	Ni-WM
CLAM	32.85%	40.68%
316 L	48.22%	47.28%
310 s filler	18.93%	/
Ni filler	/	12.04%

References

[1]	R. Mittal, B.S. Sidhu, J. Mater. Process.Technol., 220(2015), 76-78.
[2]	V.D. Vijayananda, J. Vanaja, C.R. Das, K. Mariappan, A. Thakur, S. Hussain, G.V.Prasad Reddy, G. Sasikala, S.K. Albert, Mater. Sci. Eng. A, 742(2019), 432-441.
[3]	Y. Li, J. Wang, E. Han, W. Wu, H. Hanninen, J. Mater. Sci.Technol., 35(2019), 545-559.
[4]	Z. Shen, Y. Ding, J. Chen, B.S. Amirkhiz, J. Wen, L. Fu, A.P. Gerlich, J. Mater. Sci.Technol., 35(2019), 1027-1038.
[5]	C. Liu, Q. Shi, W. Yan, C. Shen, K. Yang, Y. Shan, M. Zhao, J. Mater. Sci.Technol., 35(2019), 266-274.
[6]	B. Kim, F. Sabourin, M. Merola, L. Giancarli, R. Villari, P. Maio, F. Lucca, M. Marconi, B. Levesy, Fusion Eng. Des., 89(2014), 1969-1974.
[7]	A. Cardella, E. Rigal, L. Bedel, P. Bucci, J. Fiek, L. Forest, L.V. Boccaccini, E. Diegele, L. Giancarli, S. Hermsmeyer, G. Janeschitz, R. Lasser, A.L. Puma, J.D. Lulewicz, A. Moslang, Y. Poitevin, E. Rabaglino, J. Nucl. Mater., 329-333(2004), 133-140.
[8]	S. Kano, A. Oba, H.L. Yang, Y. Matsukawa, Y. Satoh, H. Serizawa, H. Sakasegawa, H. Tanigawa, H. Abe, Nucl. Mater. Energy, 9(2016), 300-305.
[9]	N. Hara, S. Nogami, T. Nagasaka, A. Hasegawa, H. Tanigawa, T. Muroga, Fusion Sci. Technol., 56(2009), 318-322.
[10]	H. Fu, T. Nagasaka, N. Kometani, T. Muroga, W. Guan, S. Nogami, K. Yabuuchi, T. Iwata, A. Hasegawa, M. Yamazaki, S. Kano, Y. Satoh, H. Abe, H. Tanigawa, Fusion Eng. Des., 98-99(2015), 1968-1972.
[11]	G. Liu, S. Yang, W. Han, L. Zhou, M. Zhang, J. Ding, Y. Dong, F. Wan, C. Shang, R.D.K. Misra, Mater. Sci. Eng. A, 722(2018), 182-196.
[12]	V. Thomas Paul, T. Karthikeyan, Arup Dasgupta, C. Sudha, R.N. Hajra, S.K. Albert, S. Saroja, T. Jayakumar, Metall. Mater. Trans. A, 47(2016), 1153-1168.
[13]	H. Serizawa, D. Mori, Y. Shirai, H. Ogiwara, H. Mori, Fusion Eng. Des., 88(2013), 2466-2470.
[14]	H. Serizawa, D. Mori, H. Ogiwara, H. Mori, Fusion Eng. Des., 89(2014), 1764-1768.
[15]	S. Kano, A. Oba, H.L. Yang, Y. Matsukawa, Y. Satoh, H. Serizawa, H. Sakasegawa, H. Tanigawa, H. Abe, Nucl. Mater. Energy, 9(2016), 300-305.
[16]	S.K. Albert, C.R. Das, S. Sam, P. Mastanaish, M. Patel, A.K. Bhaduri, T. Jayakumar, C.V.S. Murthy, R. Kumar, Fusion Eng. Des., 89 (7-8) (2014), 1605-1610.
[17]	Z. Sun, Int. J. Press. Vessel. Pip., 68(1996), 153-160.
[18]	A.K. Bhaduri, S. Venkadesan, P. Rodriguez, P.G. Mukunda, Int. J. Press. Vessel. Pip., 58(1994), 251-265.
[19]	J. Akram, P. Rao Kalvala, M. Misra, I. Charit, Mater. Sci. Eng. A, 688(2017), 396-406.
[20]	K. Furuya, M. Ida, M. Miyashita, H. Nakamura, J. Nucl. Mater., 386-388(2009), 963-966.
[21]	T. Soysal, S. Kou, D. Tat, T. Pasang, Acta Mater., 110(2016), 149-160.
[22]	Y.K. Yang, S. Kou, Sci. Technol. Weld. Join., 13(2008), 318-326.
[23]	Y. Zhang, H. Jing, L. Xu, Y. Han, L. Zhao, B. Xiao, Mater. Charact., 139(2018), 279-292.
[24]	C. Du, X. Wang, L. Hu, J. Mater. Process.Technol., 256(2018), 78-86.
[25]	K. Hao, G. Li, M. Gao, X. Zeng, J. Mater. Process.Technol., 225(2015), 77-83.
[26]	J. Onoro, J. Mater. Process.Technol., 180(2006), 137-142.
[27]	A.L. Schaeffler, Metal. Prog., 56(1949), p. 680B.
[28]	H. Inoue, T.T. Koseki, S. Okita, M. Fuji, Weld. Int., 11(1997), 937-949.
[29]	H. Inoue, T.T. Koseki, S. Okita, M. Fuji, Weld. Int., 11(1997), 876-887.
[30]	S. Kou, Welding Metallurgy,(second ed.), John Wiley, New York (2003), 243-247.
[31]	S. Sam, C.R. Das, V. Ramasubbu, S.K. Albert, A.K. Bhaduri, J. Nucl. Mater., 455(2014), 343-348.
[32]	A. Hishinuma, A. Kohyama, R.L. Klueh, D.S. Gelles, W. Dietz, K. Ehrlich, J. Nucl. Mater., 258-263(1998), 193-204.
[33]	C.C. Hsieh, W. Wu, J. Alloys. Compd., 506(2010), 820-825.
[34]	S. Kou, Transport Phenomena and Materials Processing, John Wiley and Sons, New York (1996), 57-60.
[35]	C. Herrera, D. Ponge, D. Raabe, Acta Mater., 59(2011), 4653-4664.
[36]	C. Shek, D. Li, K. Wong, J. Lai, Mater. Sci. Eng. A, 266(1999), 30-36.
[37]	D. Li, Y. Gao, J. Tan, F. Wang, J. Zhang, Scr. Metall., 23(1989), 1319-1321.
[38]	K.H. Lo, C.H. Shek, J.K.L. Lai, Mater. Sci. Eng. R, 65(2009), 39-104.
[39]	O.D. Sherby, Acta Metall., 10(1962), 135-147.
[40]	Y. Wang, L. Li, R. Kannan, Mater. Sci. Eng. A, 714(2018), 1-13.
[41]	H. Lukas, P. Agraval, M.S.I. Stuttgart, 2007.
[42]	J.C. Lippold, W.F. Savage, Weld. J., 12(1979), 362s-374s.