Metallurgical Behaviour and Carbon Diffusion in Buttering Deposits Prepared With and Without Buffer Layers

doi:10.1007/s40195-016-0487-x

Abstract

Abstract:

Use of a buttering deposit on ferritic steel in dissimilar metal weld (DMW) joint is a common practice in nuclear plants to connect pressure vessel components (ferritic steel) to pipelines (austenitic stainless steel). Carbon migration and metallurgical changes near fusion interface (ferritic steel-austenitic stainless steel) lead to a steeper gradient in material properties, and minimizing this gradient is the major challenge in the manufacturing of DMW joints. Inconel 82 is often deposited on ferritic steel material as buttering to reducing the gradient of physical and attaining material compatibility. Inconel 82/182 fillers are used to minimize the carbon migration, but the results are not truly adequate. In this paper, Ni-Fe alloy (chromium-free) has been used as the intermediate buffer layer in the weld buttering deposit to address the issue of carbon migration and subsequent metallurgical deterioration. The weld pads with and without buffer layers of Ni-Fe alloy have been investigated and compared in detail for metallurgical properties and carbon diffusivities. Ni-Fe buffer layer can significantly control the carbon migration which resists the metallurgical deterioration. It showed the better results in post-weld heat treatment and thermally aged conditions. The buttering deposit with Ni-Fe buffer layer could be the better choice for DMW joints requirements.

Key words: Dissimilar welds, Buttering, Carbon diffusion, Buffer layer, Metallurgical properties

Dinesh W. Rathod,Sunil Pandey,Sivanandam Aravindan,Pavan Kumar Singh. Metallurgical Behaviour and Carbon Diffusion in Buttering Deposits Prepared With and Without Buffer Layers[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(2): 120-132.

Figures/Tables 20

Table 1 Chemical composition of as-received base metal and filler metals

Base metal and filler metals	Elemental composition (wt%)
Base metal and filler metals	C	Cr	Ni	Fe	Mn	Nb	Mo
SA508Gr.3Cl.1-base metal	0.197	0.12	0.53	96.95	1.30	-	0.44
Ni-Fe alloy-ERNiFe-CI	0.025	0.01	53.01	43.24	0.73	-	-
Inconel 82-ERNiCr-3	0.022	21.88	71.25	1.52	2.86	2.45	-

Fig. 1 Schematic of weld pads of SA508Gr3Cl.1 substrate a without buffer layer and b with buffer layer

Table 2 Process parameters for buttering deposition with and without buffer layer

Weld pads	Filler metal	Layers	Current (A)	Voltage (V)	Layer thickness avg. (mm)	Heat input (KJ/mm)
Without buffer layer	Inconel 82	1st	95	9-11	2.0	1.64-1.74
		2nd			1.4	1.66-1.72
		3rd			1.3	1.62-1.72
		4th			1.3	1.66-1.70
With buffer layer	Ni-Fe alloy	1st	100	9-11	1.5	1.80-1.94
	Inconel 82	2nd	95	9-11	1.3	1.64-1.70
		3rd			1.6	1.62-1.70
		4th			2.0	1.66-1.72

Table 3 Chemical composition obtained from OES results confirmed by EPMA analysis

Specimen	EPMA (wt%)					Optical emission spectrometer (OES) (wt%)
Specimen	Element	Pt 1	Pt 2	Pt 3	Avg.	Optical emission spectrometer (OES) (wt%)
Without buffer layer (on surface of fourth layer)	C	-	-	-	-	0.011
	Cr	21.85	22.00	22.09	21.98	21.80
	Ni	70.04	69.02	70.07	69.71	71.85
	Fe	1.57	1.55	1.73	1.62	1.65
	Nb	1.55	2.33	1.67	1.85	1.84
With buffer layer (on surface of fourth layer)	C	-	-	-	-	0.020
	Cr	21.98	21.01	20.99	21.33	21.50
	Ni	69.97	69.46	69.26	69.56	70.79
	Fe	1.52	1.38	1.42	1.44	1.83
	Nb	2.11	2.31	2.25	2.22	2.00

Fig. 2 BSE images of EPMA traces across the interfaces of weld pads a without buffer layer and b with buffer layer

Table 4 Chemical composition in fourth buttering layers for as-buttered and thermally aged conditions of weld pads with and without buffer layer

Weld pads	Condition	Elemental composition in weight percentage
Weld pads	Condition	Layer	%C	%Cr	%Ni	%Fe	%Mn	%Nb
Without buffer layer and buttering (Inconel 82), thermal ageing for 240 h	As-buttered condition	4th	0.011	21.30	71.85	1.86	1.91	1.84
	330 °C thermally aged	4th	0.014	21.17	72.20	1.90	1.91	1.92
	450 °C thermally aged	4th	0.014	21.00	71.81	1.78	1.98	2.08
With buffer layer (Ni-Fe alloy) and buttering (Inconel 82), thermal ageing for 240 h	As-buttered condition	4th	0.020	21.50	71.79	1.83	1.83	2.00
	330 °C thermally aged	4th	0.022	21.21	71.95	1.91	1.77	1.81
	450 °C thermally aged	4th	0.022	21.31	72.17	1.89	2.01	2.06

Table 5 Diffusivity of carbon (D) in buttering layers for as-buttered and thermally aged conditions of weld pads with and without buffer layer

Weld Pads	Layer description	Diffusivity (D) in m²/s
Weld Pads	Layer description	As-buttered	330 °C aged	450 °C aged
Without buffer layer (Inconel 82)	Ferritic steel-first layer (SA508Gr3.Cl.1-Inconel 82)	2.13 × 10^-10	4.55 × 10^-13	4.55 × 10^-13
Without buffer layer (Inconel 82)	Third layer-fourth layer (Inconel 82-Inconel 82)	1.64 × 10^-11	1.63 × 10^-12	1.63 × 10^-12
With buffer layer (Ni-Fe alloy-Inconel 82)	Ferritic steel-first buffer layer (SA508Gr3.Cl.1-Ni-Fe alloy)	3.24 × 10^-11	6.24 × 10^-13	2.22 × 10^-12
With buffer layer (Ni-Fe alloy-Inconel 82)	Third layer-fourth layer (Inconel 82-Inconel 82)	↑ 4.46 × 10^-9	1.55 × 10^-13	1.55 × 10^-13

Fig. 3 Major alloying element variations by EPMA across interface and OES at subsequent layers for weld pad without buffer layer

Fig. 4 Major alloying element variations by EPMA across interfaces and OES at subsequent layers for weld pad with buffer layer

Fig. 5 Variations in dilution for major alloying elements by EPMA measurement in weld pads a without buffer layer and b with buffer layer

Fig. 6 Estimated martensite thickness and Ms temperature variations across the interfaces of weld pads a without buffer layer and b with buffer layer

Fig. 7 Ferritic steel and first layer interface of a Inconel 82 without buffer layer and b Ni-Fe alloy buffer layer

Fig. 8 HAZ of ferritic steel near weld interface for a Inconel 82 without buffer layer and b Ni-Fe alloy buffer layer

Fig. 9 HAZ of ferritic steel at 450 °C aged condition a without buffer layer and b with buffer layer

Fig. 10 Microstructure of first layer near the interface for a Inconel 82 buttering and b Ni-Fe alloy buffer layer

Fig. 11 Interior of buttering in fourth layer without buffer layer in a as-buttered condition and b 450 °C aged condition

Fig. 12 Interior of buttering in fourth layer with buffer layer in a as-buttered condition and b 450 °C aged condition

Fig. 13 Average micro-hardness in the HAZ, buttering and buffer layer of both weld pads with and without buffer layer

Fig. 14 Micro-hardness variations across the fusion interface of weld pad without buffer layer

Fig. 15 Micro-hardness variations across the fusion interfaces of weld pad with buffer layer

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