Microstructure and Mechanical Properties of Nb- and Nb + Ti-Stabilised 18Cr-2Mo Ferritic Stainless Steels

doi:10.1007/s40195-019-00988-y

Abstract

Abstract:

To explore the optimum use of stabilised elements and study the influences of stabilisation in 18Cr-2Mo grades, the Nb and Nb + Ti microalloying investigation focused on the relationships of the microstructure and mechanical properties of the microalloyed 18Cr-2Mo ferritic stainless steel thick plates. Thermo-Calc calculation was performed to predict the equilibrium phase diagrams. Afterwards, the microstructure, i.e. grain size and precipitation, of as-annealed specimens was analysed by means of optical microscopy, scanning electron microscopy and transmission electron microscopy, X-ray diffraction and energy-dispersive spectroscopy. Also, electron backscatter diffraction mapping was constructed to characterise grain boundary. The mechanical properties, including tensile strength and impact toughness, were tested to correlate with the microstructure. The results show that the grain sizes of Nb-stabilised steel are comparatively smaller, which is related to the fine precipitation at the grain boundaries and beneficial to the impact toughness. The increase in its strength is not apparent due to the inhomogeneous grain sizes. The grain boundary characters are similar, which is not the main factor related to their mechanical properties. When Ti is added, TiN forms above the liquidus, and large TiN particles evidently impair impact toughness.

Key words: Ferritic stainless steels, Toughness, Precipitation, Microstructure

Jian Han, Zhixiong Zhu, Gang Wei, Xingxu Jiang, Qian Wang, Yangchuan Cai, Zhengyi Jiang. Microstructure and Mechanical Properties of Nb- and Nb + Ti-Stabilised 18Cr-2Mo Ferritic Stainless Steels[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(5): 716-730.

Figures/Tables 20

Table 1 Chemical composition (wt%) of studied steels

Fig. 1 Calculated equilibrium molar fractions of phases of Nb-stabilised steel: aY-axis from 0 to 1 mol; bY-axis from 0 to 0.20 mol. N#: 1 ferrite (BCC_A2); 2 Nb precipitate (FCC_A1#2); 3 laves phase (LAVES_PHASE_C14); 4 Z phase (Z_PHASE); 5 liquid (LIQUID); 6 sigma; 7 ferrite (BCC_A2#2)

Fig. 2 Calculated equilibrium molar fractions of phases of (Nb + Ti)-stabilised steel: aY-axis from 0 to 1 mol; bY-axis from 0 to 0.20 mol. NT#: 1 ferrite (BCC_A2); 2 Nb/Ti precipitate (FCC_A1#2); 3 laves phase (LAVES_PHASE_C14); 4 liquid (LIQUID); 5 sigma phase (SIGMA); 6 ferrite (BCC_A2#2)

Table 2 Composition of precipitates formed for studied steels

Fig. 3 OM images of studied steels: a Nb- stabilised; b (Nb + Ti)-stabilised

Fig. 4 Distributions of grain sizes for two studied steels

Fig. 5 SEM images of precipitation for studied steels: a Nb- stabilised steel; b (Nb + Ti)-stabilised steel

Fig. 6 SEM image and EDS analyses of precipitates for Nb-stabilised steel

Fig. 7 SEM image and EDS analyses of precipitates for (Nb + Ti)-stabilised steel

Fig. 8 TEM images and EDS analyses of precipitates for Nb-stabilised steel: a TEM images of precipitates 1-1a, 1-1, 1-2, 1-3 and b-e EDS analyses of precipitates 1-1a, 1-1, 1-2, 1-3; f TEM image of precipitate 2-1 and g EDS analysis of precipitate 2-1

Fig. 9 TEM images and EDS analyses of precipitates for (Nb + Ti)-stabilised steel: a, c TEM images of precipitates (Ti, Nb)N and b, d EDS analyses of precipitates (Ti, Nb)N; e TEM image of precipitates 3-1, 3-2 and f, g EDS analyses of precipitates 3-1, 3-2

Fig. 10 Orientation maps and misorientation-angle distributions of studied steels: a orientation map of Nb-stabilised steel; b grain boundary map of Nb-stabilised steel; c comparison of misorientation-angle distribution of two studied steels; d orientation map of (Nb + Ti)-stabilised steel; e grain boundary map of (Nb + Ti)-stabilised steel; f appendix

Fig. 11 GBSD results of Nb-stabilised and (Nb + Ti)-stabilised steels

Fig. 12 Low-Σ CSLB distributions of Nb-stabilised and (Nb + Ti)-stabilised steels

Table 3 Tensile properties of studied 18Cr-2Mo steels

Fig. 13 Charpy impact values of Nb-stabilised and (Nb + Ti)-stabilised steels from - 40 to 40 °C

Fig. 14 Overall fracture appearances of Nb-stabilised specimens impacted at: a 40 °C; d 0 °C; g - 40 °C and SEM fractographs of Nb-stabilised specimens impacted at: b, c 40 °C; e, f 0 °C; h - 40 °C

Fig. 15 Overall fracture appearances of (Nb + Ti)-stabilised specimens impacted at: a 40 °C; c 0 °C; e - 40 °C and SEM fractographs of NT# specimens impacted at: b 40 °C; d 0 °C; f - 40 °C

Fig. 16 (Ti, Nb)N/TiN particles appearing at initiation of cleavage fractures and corresponding EDS analyses for (Nb + Ti)-stabilised specimens impacted at: a, b 40 °C; c, d 0 °C; e, f - 40 °C

Fig. 17 a SEM and b EDS of (Ti, Nb)N particle

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