Coupling Effect Mechanism of the δ-Ferrite and M23C6 on the Mechanical Properties of 9Cr-Steel Deposited Metals

doi:10.1007/s40195-024-01760-7

Abstract

Abstract:

9Cr ferritic/martensitic (9Cr F/M) steels are considered ideal structural materials for various nuclear energy systems. However, δ-ferrite (δ), as a controlled phase, may occur in its welds. Three deposited metals with different carbon contents (0.04, 0.07, and 0.10 wt%) were investigated using experimental and finite element simulation methods. The results showed that the incomplete peritectic reaction, the incomplete δ to austenite phase transition, and the segregation of ferrite-stabilized elements led to the residual δ. The amount and morphology of δ significantly influence the mechanical properties. After increasing the carbon content, the increase in strength comes mainly from precipitation strengthening and dislocation strengthening, the presence of δ will reduce the strength. During the impact process, δ affects the absorbed energy for the stable crack growth through its morphology, and M₂₃C₆ affects the crack formation energy through its quantity. By decreasing the carbon content to a certain extent, the reduction of M₂₃C₆ content and the generation of large polygonal δ can effectively improve the toughness of 9Cr-steel deposited metals.

Key words: Deposited metals, 9Cr-steels, δ-ferrite, Mechanical properties

Qishan Sun, Shitong Wei, Shanping Lu. Coupling Effect Mechanism of the δ-Ferrite and M₂₃C₆ on the Mechanical Properties of 9Cr-Steel Deposited Metals[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(1): 121-138.

Figures/Tables 23

Fig. 1 a Butt weld groove after welding; b schematic diagram of welding joints and sampling positions; c dimensions of the impact sample; d dimensions of the tensile sample; e location of microstructural characterization; f cross-section sampling position of tensile sample; g cross-section sampling position of impact sample

Table 1 Welding parameters

Welding current (A)	Welding voltage (V)	Shielding gas	Gas flow rate (L/min)	Welding speed (mm/s)	Preheating temperature (℃)	Interpass temperature (℃)
180-182	13-16	99.9% Ar	15	1.7	250	150-200

Table 2 Chemical compositions of deposited metals (wt%)

Deposited metals	C	Cr	Mn	Mo	W	N	Ni	Si	V + Nb	Fe
04C	0.045	8.75	0.51	0.076	1.95	0.05	0.40	0.40	0.25	Bal.
07C	0.073	8.98	0.53	0.072	2.00	0.04	0.41	0.41	0.25	Bal.
10C	0.110	8.95	0.52	0.075	2.00	0.04	0.41	0.41	0.24	Bal.

Fig. 2 Microstructure images of deposited metals with different carbon contents: a-c the distribution of δ in 04C, 07C, and 10C samples, respectively; d-f SEM morphology of δ in 04C, 07C, and 10C samples, respectively; g-i the difference in the number of precipitates in 04C, 07C, and 10C samples, respectively

Table 3 FF in deposited metals according to Schneider's formula, with corresponding statistical results of δ and precipitates

Deposited metal	Cr_eq	Ni_eq	Ferrite factor (FF)	Area fraction of δ (%)	Mean radius of M₂₃C₆(nm)	Area fraction of M₂₃C₆ (%)
04C	12.21	2.98	9.23	22.97	35.45	5.75
07C	12.56	3.71	8.85	12.04	39.55	9.41
10C	12.53	4.70	7.83	2.33	58.95	18.6

Fig. 3 XRD results of the electrochemical extracted powder of deposited metals

Fig. 4 Representative TEM bright-field (BF) and STEM results of deposited metals with different carbon contents: a-c macro-view of precipitate distribution in 04C, 07C, and 10C samples; d zoom of the red rectangle in c and the diffraction pattern of M23C6; e the semiquantitative results of M23C6

Fig. 5 EBSD maps of deposited metals with different carbon contents: a-c inverse pole figures (IPFs) of 04C, 07C, and 10C samples, respectively; d-f distribution maps of grain boundaries in 04C, 07C and 10C samples, respectively

Table 4 Grain boundaries changes of deposited metals with different C contents

Deposited metal	LAGB length (μm)	HAGB length (μm)	Total length (μm)
04C	5581.88	3756.15	9338.03
07C	7093.45	6349.65	13,446.10
10C	11,917.53	14,461.92	26,379.45

Fig. 6 Tensile and impact properties of deposited metals with different carbon contents: a yield and tensile strength; b elongation; c stress-strain curve; d impact energy

Fig. 7 Thermo-calc calculation results of deposited metals: a variation of δ and γ fraction during high-temperature solidification; b variation of phase fraction of 10C sample; c Scheil non-equilibrium solidification calculation results

Fig. 8 SEM image of δ and the corresponding EDS mapping of Cr, W, and Si

Fig. 9 Evolution model of δ-ferrite with decreasing temperature: a incomplete peritectic reaction; b the δ-ferrite caused by segregation of dendrites; c solid-state phase transformation

Fig. 10 Results of nanoindentation experiments of deposited metals: a the load-depth curves of δ and martensite, respectively; b enlarged images of indentations of δ and martensite, respectively

Fig. 11 a Simulation of the microstructure of Fig. 2e; b the meshing diagram; c contour map of effective stress; d contour map of logarithmic strain

Table 5 Comparison of yield strength calculations for δ and martensite in the deposited metals

Deposited metal	${\sigma }_{\text{M}}$ (MPa)	${\sigma }_{\delta }$ (MPa)	${\sigma }_{\text{cal}}$ (MPa)	${\sigma }_{\text{exp}}$ (MPa)	Error between ${\sigma }_{\text{cal}}$ and ${\sigma }_{\text{exp}}$
04C	664.54	435.03	596.42	608.7	2.02%
07C	689.27	435.03	658.68	649.7	1.38%
10C	733.56	435.03	726.60	721.4	0.72%

Fig. 12 Cross-sectional characterization of the tensile specimens: a, b SEM image and inverse pole figure (IPF) of the 04C sample, respectively; c 07C sample; d 10C sample

Fig. 13 a XRD diffraction patterns of the three deposited metals; b zoom of the (200) diffraction peak

Table 6 Comparison of yield strength calculations for δ and martensite in the deposited metals

Deposited metal	${\sigma }_{\text{ppt},{M}_{23}{\text{C}}_{6}}$ (MPa)	${\sigma }_{\text{dis}}$ (MPa)	${\sigma }_{\delta }{V}_{\delta }$ (MPa)	Pct. of δ influence in ${\sigma }_{\text{exp}}$	${\sigma }_{\text{exp}}$ (MPa)
04C	258.54	446.66	99.93	16.4%	608.7
07C	338.40	494.93	52.38	8.1%	649.7
10C	440.00	569.63	10.14	1.4%	721.4

Fig. 14 Fracture morphologies of the impact specimens: a-c macro-morphology images and partially enlarged images of 04C, 07C and 10C samples, respectively; d-f the enlarged images of dimples in 04C, 07C and 10C samples, respectively

Fig. 15 Oscillographic impact curves and their eigenvalues results: a comparison of curves for 04C, 07C and 10C samples; b detailed division using curve 04C as an example; c absorbed impact energy in different stages

Table 7 Impact test results of the deposited metals

Deposited metal	W_in (J)	W_stable (J)	W_unstable (J)	W_r (J)	W_t (J)	F_m (kN)	F_iu (kN)	F_iu/F_m (%)
04C	55.22	75.46	0.86	4.53	136.06	20.57	13.39	65.1
07C	43.04	28.58	0.78	12.14	84.54	22.61	21.57	95.4
10C	40.05	60.58	1.42	12.45	114.05	22.96	16.48	71.8

Fig. 16 Cross-sectional characterization of the impact specimens: a KAM of 04C sample; b 04C sample; c 07C sample; d 10C sample

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Coupling Effect Mechanism of the δ-Ferrite and M₂₃C₆ on the Mechanical Properties of 9Cr-Steel Deposited Metals

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