Effects of Rare Earth elements on microstructure evolution and mechanical properties of 718H pre-hardened mold steel

doi:10.1016/j.jmst.2019.12.035

Abstract

Abstract:

718H Pre-hardened mold steels with different Rare Earth (RE) contents were prepared to investigate the influence of RE on microstructure evolution and mechanical properties through a series of experiments and theoretical analysis. The results indicated that the toal oxygen (T.O) content decreased from 15 ppm to 6 ppm with 0.022 wt% RE addition, which is attributed to the active chemical properties of RE elements. For test steels, RE additions of 0.012 wt% and 0.022 wt% were significantly effective for refining inclusions by eliminating 11.5% large-sized inclusions with diameter exceeding 10 μm compared with that of 0RE steel. RE addition contributed to modify irregular MnS and Al₂O₃ inclusions into ellipsoidal RE-inclusions (RE₂O₃, RES, RE₂O₂S and REAlO₃). The purification of molten steel and the modification of inclusions by RE treatment have significant effects on improvement of the fatigue crack growth tests (FCG) inhibition ability and impact energy as well as the isotropy. However, excessive addition of RE elements (0.07 wt%) seriously reduced the impact energy, ultimate tensile strength and FCG inhibition ability due to rapidly increase of the volume fraction of large-sized inclusions. In addition to the inclusions formed by RE treatment, trace solid solution RE atoms improve the stability of undercooled austenite, resulting in the transformation region of bainite and perlite of 0.07RE steel shifting to the bottom right and prolonging the incubation period compared with that of 0RE steel.

Key words: 718H steel, Rare Earth, Inclusion evolution, Phase, ransformation, Mechanical properties

Hanghang Liu, Paixian Fu, Hongwei Liu, Yanfei Cao, Chen Sun, Ningyu Du, Dianzhong Li. Effects of Rare Earth elements on microstructure evolution and mechanical properties of 718H pre-hardened mold steel[J]. J. Mater. Sci. Technol., 2020, 50: 245-256.

Figures/Tables 16

Table 1 Chemical compositions of test steels (wt%).

Steel	C	Si	Mn	Cr	Ni	Mo	P	S	Al	Ce	La
0RE	0.32	0.31	1.52	1.97	0.9	0.19	0.004	0.002	0.02	0	0
0.012RE	0.32	0.32	1.53	1.97	0.9	0.19	0.004	0.0008	0.02	0.0069	0.0044
0.022RE	0.32	0.33	1.52	1.97	0.9	0.19	0.004	0.0008	0.02	0.015	0.0068
0.07RE	0.32	0.32	1.51	1.97	0.9	0.19	0.004	0.0006	0.02	0.052	0.018

Fig. 1. SIMS analysis results in 0.07RE steel.

Fig. 2. The [O] contents of test steels with different residual contents of RE.

Fig. 3. Average diameter and size distribution of inclusions with different residual contents of RE: (a) average diameter; (b) size distribution.

Fig. 4. Morphologies and distributions of inclusions observed by 3D X-ray microtomography: (a) 0RE; (b) 0.012RE; (c) 0.022RE; (d) 0.07RE.

Fig. 5. SEM morphologies of inclusions of 0RE steel: (a) Al2O3+MnS; (b) MnS.

Fig. 6. SEM morphologies and EDS analysis of inclusions of test steels: (a) RE-O-S-Al inclusion of 0.012RE steel; (b) RE-O + MnS composite inclusion of 0.012RE steel; (c) RE-O + RE-S composite inclusion of 0.022RE steel; (d) RE-O-S inclusion of 0.022RE steel in A area; (e) RE-S inclusion of 0.07 RE steel in B area; (f) RE-O-S inclusions of 0.07RE steel in C and D areas.

Fig. 7. Thermodynamic calculation of equilibrium precipitation of inclusions during solidification by Thermo-Calc software: (a) 0 wt% Ce; (b) 0.012 wt% Ce; (c) 0.022 wt% Ce; (d) 0.07 wt% Ce; (e) mass fraction of constitute phases in test steel without Ce addition.

Table 2 Summary of the relationship between amount of RE and type of formed inclusions.

Sample	SEM results	Factsage results	Actual precipitation
0RE	MnS, MnS + Al₂O₃	MnS, Al₂O₃	MnS, MnS + Al₂O₃
0.012RE	RE-O-S-Al, RE-O-MnS	Ce₂O₂S, Ce₂O₃, CeAlO₃, MnS	RE₂O₂S, RE₂O₃, REAlO₃, MnS
0.022RE	RE-S, RE-O, RE-O-S	Ce₂O₂S, Ce₂O₃, SCe	RE₂O₂S, RE₂O₃, RES
0.07RE	RE-O-S, RE-S	Ce₂O₂S, SCe	RE₂O₂S, RES

Fig. 8. CCT diagrams and SEM microstructures corresponding to different cooling rates of 0RE and 0.07RE steels. 0RE: (a) 10 K/s; (b) 1 K/s; (c) 0.05 K/s; (d) 0.01 K/s. 0.07RE: (e) 10 K/s; (f) 1 K/s; (g) 0.05 K/s; (h) 0.01 K/s (M is martensite, B is bainite and P is pearlite; Ps is the start point of pearlite transformation and Pf is the finish point of pearlite transformation; Bs is the start point of bainite transformation and Bf is the finish point of bainite transformation).

Fig. 9. Variation of fatigue crack growth rates of specimens with different residual contents of RE.

Table 3 Paris law constants with different residual contents of RE.

718H steel	c	m
0RE	1.7965 × 10^-4	3.48682
0.012RE	1.68835 × 10^-4	2.86087
0.022RE	1.78651 × 10^-4	2.70431
0.07RE	0.92327 × 10^-4	3.65317

Fig. 10. Evolution of mechanical properties of test steels in response to different residual contents of RE: (a) yield strength, tensile strength, elongation and section shrinkage; (b) impact properties; (c) isotropic properties.

Fig. 11. SEM fracture morphologies and EDS analysis of transverse tensile specimens of 718H steels: (a) 0RE; (b) 0.012RE; (c) 0.022RE; (d) 0.07RE.

Fig. 12. Schematic diagrams of crack initiation and propagation of inclusions under in-situ tension (a), crack initiation (b), crack aggregation (c) and inclusion shedding (d).

Fig. 13. SEM fracture morphologies of longitudinal impact samples of 718H steels: (a) 0RE; (b) 0.012RE; (c) 0.022RE; (d) 0.07RE.

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