Simultaneously Enhancing Strength, Ductility and Corrosion Resistance of a Martensitic Stainless Steel via Substituting Carbon by Nitrogen

doi:10.1007/s40195-023-01528-5

Abstract

Abstract:

Two martensitic stainless steels of 2Cr12Ni6 type hardened and tempered at 773 K have been studied: the first with 0.2% carbon content and the second with partial replacement of carbon by nitrogen (C0.1N0.1) in the first steel. It is found that the partial substitution of carbon with nitrogen contributed to an increase in ductility and strength of the steel, presumably due to the formation of more dispersive carbonitrides. Meanwhile, the addition of nitrogen suppressed the precipitation of carbonitrides, so that the solid solution strengthening effect of C0.1N0.1 did not decrease significantly after tempering treatment. In addition, the partial replacement of carbon by nitrogen contributed to improved ability against pitting corrosion (PC) in chloride-containing medium (3.5%NaCl at 303 K). The higher resistance to PC of tempered nitrogen-containing steel is apparently due to the lower content of massive carbonitrides, especially the reduced aggregation at grain boundaries. This leads to a lower acidity and aggressiveness of the test solution near the sample surface due to the accumulation of NH4⁺ ammonium ions in it. As a result of nitrogen addition, exception for Cr₂₃C₆ and VC, Cr₂N and (Cr, V) N type precipitates have also been found in C0.1N0.1 steel and this is consistent with the thermodynamic calculation results. In conclusion, substituting carbon by nitrogen in traditional martensitic stainless steel could realize the simultaneous improvement of multiple properties of martensitic stainless steels. This result provides a promising composition optimization route to develop novel martensitic stainless steels.

Key words: Martensitic stainless steel, Strength and ductility, Pitting, Carbonitrides

Fuyang Li, Jialong Tian, Huabing Li, L.M. Deineko, Zhouhua Jiang. Simultaneously Enhancing Strength, Ductility and Corrosion Resistance of a Martensitic Stainless Steel via Substituting Carbon by Nitrogen[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(5): 705-716.

Figures/Tables 14

Table 1 Chemical compositions of the experimental steels (wt%)

Steel	C	N	Ni	Cr	V	Fe
C0.1N0.1	0.094	0.1075	6.45	12.2	0.219	Bal.
C0.2	0.204	0.0050	6.58	12.4	0.213	Bal.

Fig. 1 Equilibrium phase fraction variations with temperature predicted by Thermal-Calc for a C0.1N0.1 steel and b detail enlarged picture of C0.1N0.1 steel; c C0.2steel and d detail enlarged picture of C0.2 steel

Fig. 2 a Evolution of hardness as a function of tempering time in two experimental steels. Error bars correspond to one standard deviation from the mean value. b, c Optical images taken for counting the size of prior austenite grain (PAG) in C0.1N0.1 and C0.2 specimens, as well as the statistical results

Table 2 Mechanical property of the experimental steels

Steel	YS (MPa)	UTS (MPa)	EL (%)	AR (%)
C0.1N0.1	866 ± 7	1604 ± 9	15 ± 1	63.7 ± 1
C0.2	859 ± 5	1416 ± 8	12.8 ± 0.2	62.5 ± 1

Fig. 3 a Engineering stress-strain curves of C0.1N0.1/C0.2 specimens after tempering treatments; b, c corresponding fracture morphologies observed by SEM

Fig. 4 SEM images indicating carbonitrides in C0.1N0.1 specimen a and c, and carbonitrides in C0.2 specimen b, d

Fig. 5 a and c Macroscopic morphologies after immersion in 3.5% NaCl solution at 303 K. b, d Depth of maximum pitting pit of C0.1N0.1 and C0.2

Fig. 6 SEM images of a C0.1N0.1, b C0.2 samples after tempering treatment

Fig. 7 TEM images taken from C0.1N0.1 sample after tempering treatment showing N, Cr, C, V and Fe element mappings

Fig. 8 TEM micrographs for C0.2 and the corresponding STEM EDS of C, Cr and Fe element mappings

Table 3 Percentage of various phases at 773 K (%)

Steel	Ferrite	Austenite	Cr₂₃C₆	Cr₂N	(Cr,V)N
C0.1N0.1	94.898	2.4692	1.7629	0.42324	0.44659
C0.2	93.505	2.9496	3.5457	0	0

Fig. 9 Thermal-Calc simulation on Gibbs free energy of the precipitated phase with the (C + N) = 0.2 composition at 500 °C as a function of the nitrogen content. a Gibbs free energy of (Cr,V)N, Cr2N and Cr23C6, b Gibbs free energy of Cr23C6 with nitrogen content when carbon is fixed

Fig. 10 Thermal-Calc simulation on diffusion coefficient of a the BCC with the (C + N) = 0.2 composition as a function of the nitrogen content at 773 K, b the BCC as a function of the nitrogen content at 773 K when carbon is fixed

Fig. 11 Microstructure evolution of experimental steels during the heat treatment

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