Effects of Microalloying and Heat-Treatment Temperature on the Toughness of 26Cr–3.5Mo Super Ferritic Stainless Steels

doi:10.1007/s40195-014-0070-2

Abstract

Abstract:

The effects of Ni content and heat-treatment process on the toughness of a super ferritic stainless steel with 26 wt% Cr and 3.5 wt% Mo were investigated. It was found that with the increase of Ni content, the Charpy impact toughness improved remarkably, and transformed from cleavage brittle fracture to the most ductile fracture. There were no obvious differences between the high- and low-Ni contents on the microstructure and mechanical properties since the addition of Ni did not influence crystal structure, phase composition, and precipitation of ultra pure ferritic stainless steels. Meanwhile, the heat-treatment process was a key point to maintain a high level of toughness by optimizing structure and removing detrimental precipitation, i.e., chi phase.

Key words: Super ferritic stainless steel, Nickel, Heat treatment, Toughness

Li Ma, Jian Han, Junqi Shen, Shengsun Hu. Effects of Microalloying and Heat-Treatment Temperature on the Toughness of 26Cr–3.5Mo Super Ferritic Stainless Steels[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(3): 407-415.

Figures/Tables 11

Table 1 Compositions of 26% Cr super ferritic stainless steels (wt%)

Steel	C	Si	Mn	Cr	Ni	Mo	Nb	Ti	N	Fe
No. 1	0.010	0.25	0.27	25.4	2.03	3.39	0.34	0.20	0.010	Bal.
No. 2	0.007	0.33	0.28	25.7	3.87	3.57	0.35	0.17	0.008	Bal.

Fig. 1 Calculated equilibrium molar fractions of precipitates of No. 1 steel a, b; No. 2 steel c, d

Table 2 Mechanical properties of hot-rolled plate of No. 1 and No. 2 steels

Steel	YS (MPa)	UTS (MPa)	EL (%)	Hardness (HV)
No. 1	695	791	18.0	292
No. 2	684	765	18.0	287

Fig. 2 Mechanical properties of hot-rolled No. 1 and No. 2 steels after annealed at various annealing temperature: a yield strength; b tensile strength; c elongation; d hardness

Fig. 3 Charpy impact values of No. 1 and No. 2 steels: a room temperature impact toughness of No. 1 and No. 2 variation with annealing temperatures; b impact toughness of No. 1 and No. 2 steels after annealed at 1,060 °C variation with test temperature

Fig. 4 Overall fracture appearances and SEM fractographs of No.1 steel after impacted at 40 °C and -40 °C: a overall fracture appearances at 40 °C; b SEM fractographs at 40 °C; c overall fracture appearances at -40 °C; d SEM fractographs at -40 °C

Fig. 5 Overall fracture appearances and SEM fractographs of No. 2 steel after impacted at 40 °C and -40 °C: a overall fracture appearances at 40 °C; b SEM fractographs at 40 °C; c overall fracture appearances at -40 °C; d SEM fractographs at -40 °C

Fig. 6 OM images of No. 1 steel a; No. 2 steel b annealed at 1,060 °C

Fig. 7 SEM image showing the typical precipitations in No. 1 steel a; No. 2 steel b annealed at 1,060 °C

Fig. 8 SEM image showing the typical precipitations in No. 2 steel after annealed at 980 °C a; annealed at 1,020 °C b

Fig. 9 SEM images and EDS results of the typical precipitations in No. 1 and No. 2 steels: a, c TiN; b, d chi phase

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