Effect of vanadium micro-alloying on the microstructure evolution and mechanical properties of 718H pre-hardened mold steel

doi:10.1016/j.jmst.2019.04.033

Abstract

Abstract:

The effects of different contents of vanadium (V) (0.1, 0.2, and 0.3 wt%) on the microstructure evolution and mechanical properties of 718H steel were investigated. The precipitate was characterized by means of atom probe tomography (APT) and bright-field transmission electron microscopy (TEM). The increase in V content has great benefits for strength, but has an adverse effect on impact toughness. The strength increase can be attributed to the influence of V addition on dislocation density, misorientation gradient, and fine scale MC precipitates. Precipitation strengthening mainly contributes to the V-added steel by analyzing various strengthening mechanisms. However, fine scale MC precipitates can pin dislocation leading to a decrease in its mobility. A large number of immovable dislocations will increase the dislocation accumulation, internal stress and brittle cracking, resulting in a gradual decrease in impact toughness with the V addition. In addition, compared with V-free steel, the dissolved V content in austenite decreases the grain boundary energy and inhibits the diffusion of the C atoms, ultimately reducing the transformation range of pearlite (P).

Key words: 718H steel, Vanadium, Microstructure, Mechanical properties

Liu Hanghang, Fu Paixian, Liu Hongwei, Sun Chen, Du Ningyu, Li Dianzhong. Effect of vanadium micro-alloying on the microstructure evolution and mechanical properties of 718H pre-hardened mold steel[J]. J. Mater. Sci. Technol., 2019, 35(11): 2526-2536.

Figures/Tables 16

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

Test steel	C	Si	Mn	Cr	Ni	Mo	V	RE	O	N	Fe
0 V	0.34	0.31	1.53	2.02	1.03	0.19	0	0.01	0.0008	0.003	Balance
0.1 V	0.34	0.32	1.51	2.01	1.05	0.18	0.1	0.0095	0.0008	0.004	Balance
0.2 V	0.34	0.31	1.51	2.02	1.05	0.18	0.2	0.012	0.0008	0.005	Balance
0.3 V	0.35	0.33	1.53	2.02	1.03	0.19	0.3	0.01	0.0007	0.004	Balance

Fig. 1. (a) Sampling positions for mechanical properties tests, (b) heat treatment cycle.

Fig. 2. Mass fraction of constitute phases calculated by Thermo-Calc software of test steels: (a) 0.1 V, (b) 0.2 V, (c) 0.3 V, (d) 0.4 V.

Fig. 3. (a) XRD spectra, (b) dislocation densities of the tempered samples of test steels with different V contents.

Fig. 4. CCT diagrams of test steels with different V contents: (a) 0 V, (b) 0.1 V, (c) 0.3 V. (P represents pearlite, B represents bainite, and M represents martensite).

Fig. 5. Optical metallographs of original austenite grain structure of samples after quenching with different V contents: (a) 0 V, (b) 0.1 V, (c) 0.2 V, (d) 0.3 V.

Fig. 6. EBSD crystallographic analyses of the tempered samples of test steels with different V contents, step size is 0.15 μm, IPF orientation map: (a) 0 V, (b) 0.1 V, (c) 0.2 V, (d) 0.3 V; high angle grain boundaries (misorientation≥15°) map: (e) 0 V, (f) 0.1 V, (g) 0.2 V, (h) 0.3 V.

Table 2 Quantitative analysis of the test steels by EBSD crystallographic.

Test steel	Effective grain size (μm) at 620 °C
0 V	1.126
0.1 V	1.085
0.2 V	1.052
0.3 V	1.035

Fig. 7. XRD spectra of precipitates: (a) quenched precipitates in 0 V steel, (b) quenched precipitates in V-added steels, (c) tempered precipitates in V-added steels.

Fig. 8. Bright-field TEM micrographs and SAED pattern of tempered precipitates of test steels with different V contents: (a) 0 V, (b) 0.1 V, (c) 0.2 V, (d) 0.3 V.

Fig. 9. TEM micrographs of fine scale precipitates and dislocations of V-added steels: (a) 0.1 V, (b) 0.2 V, and (c) 0.3 V.

Fig. 10. (a) Observation of the precipitates morphology, (b, c) atom maps for different elements by APT in 0.2 V steel.

Fig. 11. 3D-reconstructed atom maps from selected subvolume of precipitate in zone B in 0.2 V steel.

Fig. 12. Observation of the precipitates morphology and atom maps for different elements by APT in 0 V steel.

Fig. 13. Evolution of mechanical properties in response to different additions of V elements in test steels: (a) hardness, (b) yield, tensile strength, elongation and section shrinkage, (c) impact properties.

Fig. 14. Fracture analysis of impact samples of test steels with different V contents: (a) 0 V, (b) 0.1 V, (c) 0.2 V, (d) 0.3 V, (e) EDS results of the inclusions at zone B. (B is the zone B morphology of the impact fracture).

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