New Insights into the Aging Embrittlement Mechanism of 30Cr2Ni4MoV Steel Containing Si and Mn

doi:10.1007/s40195-024-01759-0

Abstract

Abstract:

The aging embrittlement of 30Cr2Ni4MoV steel during service at high temperature has been attributed to the segregation of Si and Mn at grain boundary (GB). We report an alternative mechanism of aging embrittlement of 30Cr2Ni4MoV steel. Using atom probe tomography, it is found that the quenched and tempered (QT) 30Cr2Ni4MoV steel has already contained obvious Si and Mn segregations at GB, which means that the Si and Mn segregations at GB are not sufficient to induce aging embrittlement. It is discovered for the first time in aged 30Cr2Ni4MoV there newly precipitate many G-phases along GB, and Si and Mn segregations at GB of QT30Cr2Ni4MoV steel are the main reason for the precipitation of G-phase. The hard and brittle G-phase helps to promote crack initiation during impact deformation. Subsequently, the cracks can rapidly propagate along GB due to the distribution of G-phase and the segregation of Si and Mn along the GB, which leads to intergranular cracking and low impact energy as for aged 30Cr2Ni4MoV steel.

Key words: 30Cr2Ni4MoV steel, Aging embrittlement, Si and Mn, Grain boundary segregation, G-phase

Yongfeng Zheng, Xiaofeng Hu, Haichang Jiang, Lijian Rong. New Insights into the Aging Embrittlement Mechanism of 30Cr2Ni4MoV Steel Containing Si and Mn[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(12): 2163-2169.

Figures/Tables 7

Table 1 Chemical compositions and J factor of two 30Cr2Ni4MoV steels (mass fraction, %)

Steel	C	Si	Mn	V	Cr	Ni	Mo	S	P	Sn	Fe	J factor
0Si0Mn	0.25	0.02	0.04	0.12	1.91	3.66	0.41	0.002	0.005	0.001	Bal.	3.6
30Si70Mn	0.26	0.32	0.68	0.12	1.72	3.70	0.41	0.003	0.004	0.001	Bal.	50

Table 2 Tensile properties and FATT of QT and aged experimental steels

Steel	State	R_m (MPa)	R_p0.2 (MPa)	A (%)	FATT (°C)
0Si0Mn	QT	983 ± 7.1	858 ± 4.2	19.5 ± 0.0	− 92
0Si0Mn	Aged	866 ± 3.5	761 ± 1.4	21.7 ± 0.3	− 101
30Si70Mn	QT	965 ± 1.4	853 ± 1.4	19.5 ± 0.7	− 64
30Si70Mn	Aged	894 ± 0.0	782 ± 2.8	19.5 ± 0.7	> 200

Fig. 1 FATT curves of the 0Si0Mn and 30Si70Mn after QT or aging

Fig. 2 Macro a1-d1 and micro a2-d2 -SEM fracture morphologies of the 0Si0Mn and 30Si70Mn after QT or aging, the brittle fracture zone is the region marked with a black dashed line, the micro-SEM images are the magnified images of the rectangle areas with black solid line in macrofractographs. (CF—cleavage fracture, IGF—intergranular fracture)

Fig. 3 Atom mappings of alloy elements in the QT a and aged c 30Si70Mn. The 1-D composition profiles obtained with a diameter of 20 nm cylindrical area along the red arrow across the GB in the QT b and aged d 30Si70Mn. The peak concentration of Si, Mn and Ni at GB in QT and aged 30Si70Mn e; the peak concentration of P, Mo, Cr and V at GB in both 0Si0Mn and 30Si70Mn after QT or aging f

Fig. 4 SEM and TEM images of QT and aged 30Si70Mn; a, b QT 30Si70Mn; c, d aged 30Si70Mn; e-g the SAED patterns of the carbides outlined by the dashed circles; LB (lath boundary); PAGB (prior austenite grain boundary); green arrows represent block and round M23C6; orange arrows represent rod M2C; blue arrows represent granular MC; purple arrows represent G-phase

Fig. 5 a A bright-field TEM image of the GB region corresponding EDS mappings of the region marked with a purple rectangle; the SAED of the G-phase outlined by the red dashed circle; b line scan of Cr, Mn, Si, V, Ni, Mo concentration along the purple arrow in (a); c SEM images of IGF surfaces in aged 30Si70Mn impact tested at RT; a drawing of partial enlargement of (c) (white arrow) and corresponding EDS mappings of the region marked with a red rectangle; green arrows represent M23C6; purple arrows represent G-phase

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