In-Depth Understanding the Retained Austenite Stability on the Susceptibility of Multi-Alloying Ultra-Strength Steel to Hydrogen-Induced Cracking

doi:10.1007/s40195-024-01809-7

Abstract

Abstract:

Hydrogen-induced cracking (HIC) is one of the most complex material problems that hydrogen can diffuse into and interact with microstructure, degrading their mechanical properties. Microstructural modification is an effective way to enhance the resistance to HIC. The present study focused on the relationship between the retained austenite (RA) and HIC behavior in NiCrMoV/Nb multi-alloying ultra-strength steel. Results demonstrated that the maximum volume fraction of RA of 9.31% was obtained for QL₃₀T specimen. After the deep cryogenic pretreatment, the volume fraction of RA reduced to 8.8%. RA could reduce the effective diffusion coefficient, while deep cryogenic pretreatment increased the susceptibility of the steel to HIC by a maxim of 14.8%. This was mainly due to the transformation of retained austenite into martensite, degrading the mechanical properties under hydrogen-charged condition. In addition, the deep cryogenic pretreatment had a significant effect on the crack initiation and propagation, with the intergranular (IG) fracture becoming the dominant fracture mode where an increase in the number of secondary cracks in the section. The interfaces of RA and matrix, as well as the grain boundaries, were the preferred sites for cracks initiation.

Key words: Hydrogen-induced cracking, Multi-alloying ultra-strength steel, Retained austenite, Deep cryogenic pretreatment

Chao Hai, Kang Huang, Cuiwei Du, Xiaogang Li. In-Depth Understanding the Retained Austenite Stability on the Susceptibility of Multi-Alloying Ultra-Strength Steel to Hydrogen-Induced Cracking[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(4): 691-704.

Figures/Tables 15

Fig. 1 Schematic diagram of heat treatment procedures: a QT; b QLT

Fig. 2 Microstructure images of a QT, b QL15T, c QL30T and d QL60T specimens

Fig. 3 TEM images illustrating the microstructures of a QT, b QL15T, c QL30T, d QL60T specimens, e the corresponding dark-field micrograph and f selected area electron diffraction patterns of QL60T specimens

Fig. 4 X-ray diffraction patterns of the four test specimens: a no pretreatment; b deep cryogenic pretreatment

Table 1 Volume fraction of retained austenite obtained from XRD results

Specimens	No pretreatment volume fraction of RA (%) [27]	Deep cryogenic pretreatment volume fraction of RA (%)	Transformation (%)
QT	4.8 ± 0.3	2.3* ± 0.2 (low)	-
QL₁₅T	3.2 ± 0.2* (low)	2.8 ± 0.3* (low)	-
QL₃₀T	9.3 ± 0.3	8.8 ± 0.4	5 ± 0.3
QL₆₀T	6.7 ± 0.1	6.1 ± 0.3	10 ± 0.2

Fig. 5 Hydrogen permeation curves of the four specimens with and without deep cryogenic pretreatment: a QT; b QL15T; c QL30T; d QL60T

Table 2 Hydrogen permeation parameters of four different specimens

Specimens	I_∞/A (A/m²)	t_b (s)	D_eff (10^-11 m²/s)	C_ap (mol/m³)	N_t (10²⁸ m⁻³)	Refs.
QT	0.52	3098	2.10	257.83	3.14	[27]
QT-DCT	0.72	3032	2.11	347.69	4.23	-
QL₁₅T	0.53	3243	2.02	271.87	3.46	[27]
QL₁₅T-DCT	0.49	3174	2.06	244.92	3.06	-
QL₃₀T	0.42	3400	1.92	226.65	3.03	[27]
QL₃₀T-DCT	0.46	3240	1.95	233.93	2.92	-
QL₆₀T	0.54	3243	1.98	278.58	3.62	[27]
QL₆₀T-DCT	0.57	3250	2.01	291.49	3.72	-

Fig. 6 SSRT curves of the a QT, b QL15T, c QL30T, and d QL60T specimens test in air and dynamic hydrogen charging with a stain rate of 5 × 10−6 s−1

Table 3 Mechanical properties of uncharged and H-charged specimens with and without deep cryogenic pretreatment

Steel	Yield stress (MPa)	Tensile stress (MPa)	Elongation (%)	HE index (%)
QT-air	953 ± 17	988 ± 14	11.7 ± 0.2	-
QT-DCT-air	950 ± 11	974 ± 10	11.1 ± 0.3	-
QT-H	950 ± 14	978 ± 10	5.0 ± 0.2	57.3
QT-DCT-H	956 ± 15	971 ± 18	3.1 ± 0.1	72.1
QL₁₅T-air	913 ± 10	972 ± 11	14 ± 0.2	-
QL₁₅T-DCT-air	953 ± 20	1004 ± 11	13.6 ± 0.2	-
QL₁₅T-H	911 ± 13	944 ± 12	5.7 ± 0.3	58.01
QL₁₅T-DCT-H	965 ± 20	980 ± 21	5.0 ± 0.4	64.3
QL₃₀T-air	896 ± 11	947 ± 8	14.5 ± 0.3	-
QL₃₀T-DCT-air	864 ± 13	941 ± 7	14.1 ± 0.2	-
QL₃₀T -H	907 ± 18	991 ± 13	6.9 ± 0.5	52.4
QL₃₀T-DCT-H	883 ± 812	941 ± 10	6.6 ± 0.4	53.2
QL₆₀T-air	904 ± 10	957 ± 10	14.9 ± 0.4	-
QL₆₀T-DCT-air	872 ± 13	921 ± 20	14.3 ± 0.2	-
QL₆₀T-H	899 ± 12	948 ± 9	8.3 ± 0.4	44.3
QL₆₀T-DCT-H	903 ± 11	942 ± 13	5.9 ± 0.3	58.8

Fig. 7 Overview and high-magnification images of fracture surfaces morphologies for the uncharged specimens: a QT; b QL15T; c QL30T; d QL60T; e QT-DCT; f QL15T-DCT; g QL30T-DCT; h QL60T-DCT

Fig. 8 Overview and high-magnification images of fracture surfaces morphologies for the H-charged specimens: a QT; b QL15T; c QL30T; d QL60T; e QT-DCT; f QL15T-DCT; g QL30T-DCT; h QL60T-DCT

Fig. 9 SEM micrographs of fractured steels under dynamic hydrogen charging of the specimens after SSRT: a1 and a2 QT, b1 and b2 QL15T, c1 and c2 QL30T, d1 and d2 QL60T

Fig. 10 SEM micrographs of fractured steels with deep cryogenic pretreatment under dynamic hydrogen charging: a1-a4 QT, b1-b4 QL15T, c1-c4 QL30T, and d1-d4 QL60T steels

Fig. 11 Band contrast (BC), inverse pole figure (IPF) and (Geometrically necessary dislocation) GDN maps of secondary cracks on the side surface of different specimens under dynamic hydrogen charging conditions: a QT; b QL30T; c QT-DCT; d QL30T-DCT

Fig. 12 Evolution of retained austenite after tensile test as determined by XRD: a QT steel; b QL15T; c QL30T; d QL60T

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