Correlations of Ni Contents, Formation of Reversed Austenite and Toughness for Ni-Containing Cryogenic Steels

doi:10.1007/s40195-016-0496-9

Abstract

Abstract:

It has been widely demonstrated that addition of Ni in low-carbon steels can effectively improve the cryogenic toughness, but the mechanism behind it has yet to be clarified. In the present work, the evolutions of microstructure and mechanical properties after quenching and tempering for Ni-containing cryogenic steels with different Ni contents (3.5-9 wt%) were investigated. The results showed that after quenching and tempering, the Ni-containing cryogenic steels were composed of tempered martensite and reversed austenite. The volume fraction of reversed austenite has increased from 0 up to 6.3% when the Ni content increases from 3.5% to 9%. The Charpy impact tests indicated that the low-temperature toughness was markedly improved with the increase in Ni content, which can be correlated with the increase in reversed austenite amount. The main contribution of reversed austenite to the toughness lies in: (1) the elimination of cementite precipitates improved the plastic deformation capacity of matrix, and (2) the crack propagation is hindered through plastic deformation.

Key words: Ni-containing cryogenic steel, Microstructure, Reversed austenite, Impact toughness, Crack propagation

Meng Wang, Zhen-Yu Liu, Cheng-Gang Li. Correlations of Ni Contents, Formation of Reversed Austenite and Toughness for Ni-Containing Cryogenic Steels[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(3): 238-237.

Figures/Tables 14

Table 1 Chemical composition of Ni-containing cryogenic steels (wt%)

Alloy	C	Mn	Si	Ni
3.5Ni	0.058	0.73	0.23	3.42
5Ni	0.057	0.71	0.21	5.07
7Ni	0.062	0.72	0.17	7.11
9Ni	0.055	0.71	0.20	8.93

Table 2 Heat treatment process parameters

Alloy	Austenitizing temperature (°C)	Holding time (min)	Tempering temperature (°C)	Holding time (min)
3.5Ni	810	40	610	60
5Ni	810	40	610	60
7Ni	830	40	600	60
9Ni	830	40	600	60

Fig. 1 SEM micrographs of a 3.5Ni, b 5Ni, c 7Ni, d 9Ni steels after quenching and tempering treatments

Fig. 2 EBSD analysis results of a 3.5Ni, b 5Ni, c 7Ni, d 9Ni steels (red color corresponds to fcc austenite phase), e the size distribution of the reversed austenite

Fig. 3 Volume fraction of the reversed austenite as a function of Ni content

Fig. 4 TEM micrographs of a 3.5Ni, b, c 5Ni, d 7Ni, e 9Ni steels after quenching and tempering treatments

Fig. 5 EPMA line scan analyses of C, Ni and Mn: a 3.5Ni, b 7Ni steel

Fig. 6 Variation of strengths and elongation with Ni content

Fig. 7 Variations of Charpy impact energy of test steels with the function of a test temperature and b Ni content

Table 3 Percentage of total absorbed fracture energy, initiation energy and propagation energy for four test steels fractured at -196 °C

Alloy	Total absorb energy (J)	Initiation energy (J)	Propagation energy (J)	Propagation to total absorb energy (%)
3.5Ni	12	7	5	41.7
5Ni	35	17.9	17.1	48.9
7Ni	133	39	94	70.7
9Ni	196	53.6	142.4	72.7

Fig. 8 Variations of Charpy impact energy with the function of the volume fraction of reversed austenite

Fig. 9 SEM fractographs of Charpy impact specimens fractured at -196 °C: a 3.5Ni, b 5Ni, c7Ni, d 9Ni steels

Fig. 10 SEM micrographs of the cross-sectioned area beneath the fracture surface of CVN impact specimens tested at -196 °C: a 3.5Ni, b, c 5Ni, d 9Ni steels

Fig. 11 EPMA map analysis results of the cross-sectioned area beneath the fracture surface of CVN impact specimens tested at -196 °C for 5Ni steel

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