Influence of Austenitizing Temperature on the Microstructure and Corrosion Resistance of 55Cr18Mo1VN High-Nitrogen Plastic Mould Steel

doi:10.1007/s40195-016-0506-y

Abstract

Abstract:

The influence of austenitizing temperature on the microstructure and corrosion resistance of 55Cr18Mo1VN high-nitrogen plastic mould steel was investigated. The microstructure, elemental distribution and Cr-depleted zone of different heat-treated samples were investigated by X-ray diffraction, electron probe microanalyzer analysis, and transmission electron microscopy. The corrosion resistance was evaluated using electrochemical measurements, and the analysis of passive film was carried out by X-ray photoelectron spectroscopy. The results indicated that the volume fraction of precipitates decreased, and the homogeneity of elements was improved with increasing austenitizing temperature. The degree of Cr-depleted zone around coarse M₂₃C₆ was severer than that around M₂N, and pitting corrosion initiated preferentially around M₂₃C₆. The corrosion resistance of the samples increased with the austenitizing temperature. With the increase in austenitizing temperature, the passive film was thickened and Cr(III)Cr2O3 in the inner layer of passive film was enriched, which enhanced the corrosion resistance of the steel. The higher content of nitrogen in solid solution at higher austenitizing temperature contributed to the increased intensity of CrN and NH3, leading to the increase in pH value in the pit, and promoting the repassivation of 55Cr18Mo1VN steel.

Key words: High-nitrogen plastic mould steel, Austenitizing temperature, Microstructure, Corrosion resistance, Cr-depleted zone

Hua-Bing Li1,Wei-Chao Jiao,Hao Feng,Zhou-Hua Jiang,Cui-Dong Ren. Influence of Austenitizing Temperature on the Microstructure and Corrosion Resistance of 55Cr18Mo1VN High-Nitrogen Plastic Mould Steel[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(12): 1148-1160.

Figures/Tables 16

Table 1 Chemical composition of 55Cr18Mo1VN high-nitrogen plastic mould steel (wt%)

C	Si	Mn	Cr	Mo	N	V	P	S
0.54	0.45	0.40	17.30	0.98	0.26	0.10	0.023	0.0021

Fig. 1 Heat treatment process curves of the specimens

Fig. 2 XRD patterns of the 55Cr18Mo1VN samples after different heat treatments: a diffraction angles from 40° to 100°; b detail information between 47° and 54°

Fig. 3 Microstructure of the heat-treated 55Cr18Mo1VN: a annealed and austenitized at b 940 °C; c 1020 °C; d 1100 °C

Fig. 4 Elemental distribution maps (Cr, N and C) via EPMA of samples austenitized at: a 940 °C; b 1020 °C; c 1100 °C

Fig. 5 TEM results of precipitates in the sample austenitized at 940 °C: a Cr23C6; b Cr2N

Fig. 6 Evolution of OCP of 55Cr18Mo1VN steels in 3.5 wt% NaCl after cathodic polarization

Fig. 7 Potentiodynamic polarization curves of 55Cr18Mo1VN steels in 3.5 wt% NaCl

Fig. 8 Nyquist plots of heat-treated 55Cr18Mo1VN steels in 3.5 wt% NaCl

Table 2 Fitting parameters of 55Cr18Mo1VN steels obtained from EIS spectra

Specimens (°C)	R_s (Ω cm²)	CPE		R_p (Ω cm²)
Specimens (°C)	R_s (Ω cm²)	n(0-1)	Q₀/(Ω^-1 cm^-2 sⁿ)
Annealed	2.986	0.7536	2.126 × 10^-4	2.382 × 10⁴
940	4.399	0.7764	1.710 × 10^-4	5.613 × 10⁴
980	1.382	0.8726	1.000 × 10^-4	7.248 × 10⁴
1020	3.702	0.8730	7.832 × 10^-5	9.721 × 10⁴
1060	3.021	0.8744	7.620 × 10^-5	1.209 × 10⁵
1100	2.158	0.8865	6.330 × 10^-5	1.670 × 10⁵

Fig. 9 Detailed XPS spectra of a, b Cr 2p3/2, c, d Fe 2p3/2 recorded from sputtering surface of passive film for 10 and 80 s

Fig. 10 Variation of Cr(III) to (Fe(II)+Fe(III)) ratio in passive film with etching time for the samples austenitized at different temperatures

Fig. 11 Detailed XPS spectra of N 1 s recorded from the outmost a and sputtering surface b of the passive film for 10 s on samples austenitized at different temperatures

Fig. 12 SEM images of the pit initiation sites on the sample austenitized at 940 °C: a metastable pits; b the local magnification of the metastable pits; c EDS analysis of precipitates exhibited in b

Fig. 13 a, c TEM images of precipitates in the sample austenitized at 940 °C: a Cr23C6; c Cr2N; b corresponding Cr-depleted zone of a; d corresponding Cr-depleted zone of c

Fig. 14 Schematic illustration of pitting corrosion mechanism for 55Cr18Mo1VN steel austenitized at a low, b high temperatures

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