Microstructure and Mechanical Properties of High-Nitrogen Austenitic Stainless Steels Subjected to Equal-Channel Angular Pressing

doi:10.1007/s40195-016-0370-9

摘要/Abstract

Abstract:

Three high-nitrogen stainless steels with different N contents were successfully processed by equal-channel angular pressing for one pass, and their microstructures and mechanical properties were investigated. It was found that the microstructure of the billet was heterogeneous across the billet thickness, resulting in the difference in the mechanical properties due to the different deformation conditions. A relatively low strength, high uniform elongation, and high work-hardening rate for the samples at the bottom of the billet was achieved in comparison with those processed at the top. Meanwhile, it was observed that the density of deformation twins increased with the content of N; accordingly, the strength and elongation of the alloys increase with the content of N, resulting in a good strength-ductility combination.

Key words: High-nitrogen, stainless, (HNS), steels, Equal-channel, angular, pressing, (ECAP), N, content, Twinning, Strength, Ductility

. [J]. 金属学报英文版, 2016, 29(2): 140-149.

Fu-Yuan Dong, Peng Zhang, Jian-Chao Pang, Qi-Qiang Duan, Yi-Bin Ren, Ke Yang, Zhe-Feng Zhang. Microstructure and Mechanical Properties of High-Nitrogen Austenitic Stainless Steels Subjected to Equal-Channel Angular Pressing[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(2): 140-149.

图/表 13

Table 1 Chemical compositions of austenitic stainless steels (wt%)

	C	Mn	Si	Cr	N	Mo	P	S
HNSS-65	0.017	15.7	0.28	18.24	0.83	2.26	0.005	0.003
HNSS-83	0.044	15.8	0.18	18.62	0.65	2.78	0.013	0.004
HNSS-99	0.020	16.1	0.31	17.98	0.99	2.27	0.005	0.003

Fig. 1 a Schematic illustration of a billet before and after ECAP for a single pressing through the die; b illustration of billet for ECAP and the coordinate systems; c illustration of tensile specimen; d selected positions for tensile specimens on the transverse section of the ECAP billet

Fig. 2 Microstructure of the as-received HNS steel was characterized by EBSD

Fig. 3 Microstructure of the HNSS-83 steel characterized by EBSD: a, b at the top of the billet after ECAP; c, d at the bottom of the billet after ECAP

Fig. 4 a, b Activation of the limited slip systems with the planar gliding of dislocations and the Taylor lattice formation; c, d dissociated partial dislocations with the wide stacking fault; e, f formation of the HDDWs and microbands

Fig. 5 TEM micrographs of the deformation microstructure of HNS-83 steels after one ECAP pass: a a higher magnification observation of a nanotwin bundle; b twin intersections; c-e the nanotwined regions with shear bands; f twin-matrix lamellae are vanished within the shear band

Fig. 6 a-c Typical tensile engineering stress-strain curves of the samples at different locations of the billets; d-emechanical properties, including YS, UTS, and UE, as a function of the N content

Table 2 Tensile properties of three HNS samples at different locations of the ECAP billets

	Distance from center (mm)	-2.25	-0.75	0.75	2.25
HNSS-65	YS (0.2%) (MPa)	950	1030	1132	1201
	UTS (MPa)	1151	1121	1214	1303
	UE (%)	13.7	7.87	1.91	1.59
HNSS-83	YS (0.2%) (MPa)	1005	1087	1211	1264
	UTS (MPa)	1233	1207	1302	1385
	UE (%)	15.6	8.91	3.08	1.76
HNSS-99	YS (0.2%) (MPa)	1046	1105	1277	1345
	UTS (MPa)	1281	1257	1343	1412
	UE (%)	17.6	13.4	3.78	1.83

Fig. 7 a Values of the Vickers microhardness for HNS steels recorded along the Z-direction on the cross-sectional plane after ECAP; b, c mechanical properties including YS, UTS, and UE, as a function of the distance from center of the billets

Fig. 8 Work-hardening rate Θ versus true strain a-c and versus true stress d-f for the HNSS steel samples at different positions of the ECAP billets

Fig. 9 Relationship between UTS and UE of HNS steels after ECAP, showing the effects of increasing N content and SPD processing on the mechanical properties

Fig. 10 a-c Typical bright-field TEM images and corresponding SAED patterns (insets) for the deformation twins in the ECAP-processed samples: a HNSS-65; b HNSS-83; c HNSS-99; d-f statistical distributions of T/M lamellar thickness corresponding to a-c

Fig. 11 a Influence of adding N on the twinning stress (σ t) and the dislocation-solute interaction (σ y), leading to higher twin density; b enhanced strength-ductility synergy obtained by increasing twin density

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