Effects of Stabilization Heat Treatment on Microstructure and Mechanical Properties of Si-Bearing 15Cr-9Ni-Nb Austenitic Stainless Steel Weld Metal

doi:10.1007/s40195-022-01497-1

Abstract

Abstract:

Two 15Cr-9Ni-Nb austenitic stainless steel weld metals with 2.5% Si and 3.5% Si (namely 2.5Si and 3.5Si samples, respectively) were designed and prepared through tungsten inert gas (TIG) welding and then hold at 800 °C or 900 °C for 3 h for stabilization. The microstructure and mechanical properties were investigated both for the as-welded and after-stabilization heat treatment (SHT) weld metals. There are 3.0-4.0% martensite and 2.5-3.5% δ ferrite in the 2.5Si as-welded weld metal and 6.0-7.0% δ ferrite in the 3.5Si as-welded weld metal. After SHT, a large amount of martensite formed in the 2.5Si weld metal, and δ → γ transition occurred during the SHT process both for the 2.5Si and 3.5Si weld metals. There were a large amount of coarse NbC and few nanoscale NbC in the as-welded weld metal. During the SHT, a large amount of nanoscale NbC formed in the matrix, while a large number of G phases formed at the austenite grain boundaries and the δ/γ interfaces. The decrease in solid solution C and δ ferrite content led to the decline of the yield strength of the weld metal after SHT. The martensite formed in 2.5Si weld metal after SHT had less effect on strength because of its low carbon content. The G phases formed during the SHT reduced the impact energy of the weld metal because it promoted the intergranular fracture, while the δ → γ transition reduced the amount of the δ/γ interfaces and avoided the intergranular fracture, which was beneficial for the impact toughness of the weld metals.

Key words: Austenitic stainless steel, Weld metal, Stabilization heat treatment, Martensite, NbC, δ→γ transition

Yakui Chen, Dong Wu, Dianzhong Li, Yiyi Li, Shanping Lu. Effects of Stabilization Heat Treatment on Microstructure and Mechanical Properties of Si-Bearing 15Cr-9Ni-Nb Austenitic Stainless Steel Weld Metal[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(4): 637-649.

Figures/Tables 20

Table 1 Chemical composition of filler wire and weld metal (wt%)

Element	C	Si	Mn	Cr	Ni	Nb
2.5Si filler wire	0.057	2.45	1.64	14.03	8.28	0.73
2.5Si weld metal	0.059	2.49	1.44	14.07	8.47	0.71
3.5Si filler wire	0.064	3.37	1.63	13.92	8.31	0.72
3.5Si weld metal	0.065	3.43	1.45	13.96	8.38	0.70

Table 2 Welding parameters

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

Fig. 1 Schematic diagram of a weldment, location of b the impact sample and c the tensile sample in the weld metal, the dimensions of d the tensile sample and e the impact sample

Fig. 2 Calculated result by Thermo-Calc: a 2.5Si, b 3.5Si. (γ = austenite, δ = δ ferrite, σ = σ phase)

Fig. 3 OM images of the weld metals: a, d 2.5Si, b, e 2.5Si-800, c, f 2.5Si-900, g 3.5Si, h 3.5Si-800, i 3.5Si-900

Table 3 Phase contents of the as-welded weld metals

Weld metal	FN	Phase content
2.5Si	6.5	2.5%-3.5% δ ferrite + 3.0%-4.0% Martensite + Balanced Austenite
3.5Si	6.4	6.0%-7.0% δ ferrite + Balanced Austenite

Fig. 4 FNs of the as-welded and after-SHT weld metals

Fig. 5 a SEM and b TEM images, c corresponding SAED pattern of Martensite in 2.5Si-900 weld metal

Fig. 6 Characterization of δ ferrite: SEM image of a 3.5Si as-welded weld metal and b 3.5Si-800 weld metal, c 3.5Si-900 weld metal, TEM image of d 3.5Si-900 weld metal (γδ = austenite transitioned by δ ferrite during the SHT)

Fig. 7 XRD analysis results of precipitations in weld metal: a as-welded, b 800 °C/3 h, c 900 °C/3 h

Fig. 8 Microstructure characterization of the coarse-NbC in the matrix of the 2.5Si as-welded weld metal: a SEM image of NbC and δ ferrite, elements distribution of b Nb, c Cr, and d Fe

Fig. 9 Microstructure characterization of the coarse-NbC at the δ/γ interfaces of the 3.5Si as-welded weld metal: a SEM image, EDS semiquantitative result of b spectrum 1 (NbC), c spectrum 2 (δ ferrite), d spectrum3 (austinite)

Table 4 EDS semiquantitative result in Fig. 9

Fig. 10 Dark field image of the nanoscale-NbC in the matrix of the a 3.5Si as-welded, b 3.5Si-900 weld metal (Red arrows point to NbC of size 5-10 nm, yellow arrows point to NbC of size 10-20 nm)

Fig. 11 TEM characterization of G phase: a G phase at the grain boundary in 2.5Si-900 weld metal and b corresponding SAED pattern, e, f, g corresponding STEM-EDS mappings, c G phase at δ/γ interface in 3.5Si-900 and d corresponding SAED pattern

Fig. 12 Vickers microhardness test results: a 2.5Si as-welded weld metal, b 2.5Si-800 weld metal

Fig. 13 Surfaces of the tensile fracture of a, c 2.5Si as-welded and b, d 2.5Si -800 weld metal

Fig. 14 Changes in room temperature mechanical properties of weld metals: a yield strength, b tensile strength, c elongation, d impact energy

Fig. 15 Surfaces of the impact fracture of a, d 3.5Si as-welded, b, e 3.5Si-800, c, f 3.5-900 weld metal

Fig. 16 SEM micrographs of the 3.5Si-800 weld metal impact sample (longitudinal section)

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