Effects of Aging at 550 °C on α′-Martensitic Transformation of High Si-Bearing Metastable Austenitic Stainless Steel Weld Metal

doi:10.1007/s40195-024-01697-x

Abstract

Abstract:

The high Si-bearing 15Cr-9Ni-Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment (SHT) at 850 °C for 3 h. The effects of 550 °C aging on the α′-martensitic transformation of the as-welded and the SHT weld metals were investigated. The results showed that the weld metal had poor thermal stability of austenite. The precipitation of NbC during the 850 °C SHT made the thermal stability of the local matrix decrease and led to the formation of a large amount of C-depleted α′-martensite. The precipitation of coarse σ-phase at the δ-ferrite led to the Cr-depleted zone and the formation of Cr-depleted α′-martensite at the early stage of 550 °C aging. The homogenized diffusion of C and Cr in the matrix during 550 °C aging led to the restoration of austenitic thermal stability and the decrease of α′-martensite content. The C-depleted α′-martensite content in the SHT weld metal decreased rapidly at the early stage of aging due to the fast diffusion rate of the C atom in the matrix, while the Cr-depleted α′-martensite decreased at the later stage of aging due to the decreased diffusion rate of the Cr.

Key words: Metastable austenitic stainless steel weld metal, α'-martensitic transformation, δ-ferrite, σ-phase

Yakui Chen, Shitong Wei, Dong Wu, Shanping Lu. Effects of Aging at 550 °C on α′-Martensitic Transformation of High Si-Bearing Metastable Austenitic Stainless Steel Weld Metal[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(9): 1467-1479.

Figures/Tables 20

Table 1 Welding parameters

Welding current (A)	Welding voltage (V)	Welding speed (m/min)	Wire feed speed (m/min)	Shield gas	Gas flow rate (L/min)	Interpass temperature (°C)
180-182	13-13.5	0.1	1	99.999% Argon	15	<100

Fig. 1 a Schematic diagram of the weldment, b location of microstructural characterization, c location of the tensile samples, d dimensions of the tensile samples

Table 2 Chemical composition of the filler wire and weld metal (wt%)

Elements	Ni	C	Cr	Mn	Si	Nb	N	S	P
Filler wire	8.28	0.057	14.03	1.64	2.45	0.73	0.0052	<0.001	<0.005
Weld metal	8.47	0.059	14.07	1.44	2.49	0.71	0.0016	0.0008	<0.005

Fig. 2 a, b OM images of AW weld metal, c, d SEM images of AW weld metal

Table 3 FN value and phase content of the AW and SHT weld metals

Weld metals	FN	Phase content (mass fraction)
AW	~6.5	~3 δ-ferrite+~3.5% martensite+balanced austenite
SHT	~40.2	~1.8 δ-ferrite+~65.3% martensite+balanced austenite

Fig. 3 a, b OM images of SHT weld metal, c, d SEM images of SHT weld metal (γδ indicated the austenite that has been transformed from δ-ferrite during the SHT)

Fig. 4 TEM dark field images of the nanoscale NbC in a AW weld metal, b SHT weld metal

Fig. 5 Characterization of the δ-ferrite decomposition of the AW-500 h weld metal: a SEM image, b TEM image, c combined STEM map of Cr, Ni, and Nb, corresponding SAED pattern of d σ-phase, e residual δ-ferrite, f G-phase

Table 4 STEM semiquantitative results of the precipitates at δ-ferrite in the AW-500 h weld metal (at.%)

Precipitate types	Cr	Ni	Si	Mn	Nb	Fe	Total
σ-phase (Fe-Cr)	38.66	2.98	10.55	5.81	0.09	41.91	100.00
G-phase (A₁₆Mn₆Si₇)	3.36	49.37	13.00	22.41	0.45	11.41	100.00

Fig. 6 SEM images of δ-ferrite decomposition in a AW-1000 h weld metal, b AW-3000 h weld metal, c AW-5000 h weld metal

Table 5 Contents of the δ-ferrite and σ-phase in the aged weld metals

Weld metals	Phase contents	Weld metals	Phase contents
AW-500 h	~2.4% δ+~0.5% σ	SHT-500 h	~1.8% δ
AW-1000 h	~1.8% δ+~1.1% σ	SHT-1000 h	~1.1% δ+~0.7% σ
AW-3000 h	~1.2% δ+~1.7% σ	SHT-3000 h	~0.9% δ+~0.9% σ
AW-5000 h	~1.2% δ+~1.7% σ	SHT-5000 h	~0.9% δ+~0.9% σ

Fig. 7 SEM images of δ-ferrite decomposition in a SHT-500 h, b SHT-1000 h, c SHT-3000 h, d SHT-5000 h

Fig. 8 Characterization of the martensite in the AW-500 h weld metal: a, b OM images, c TEM images, d, e SEM images, f SAED pattern of the martensite

Fig. 9 EDS line scanning result of Cr in the AW-500 h weld metal

Fig. 10 OM images of a AW-1000 h, b AW-3000 h, c AW-5000 h

Fig. 11 Variations of FN values and contents of martensite in the a AW, b SHT weld metals after aging for different time

Fig. 12 OM images of a SHT-500 h, b SHT-1000 h, c SHT-3000 h, d SHT-5000 h

Fig. 13 Yield strengths of the aged AW and SHT weld metals

Fig. 14 Microhardness test result of a AW-1000 h, b SHT-1000 h

Fig. 15 DSC test result of the phase transition temperature of the SHT weld metal: a cooling process, b heating process

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