The Effect of the Pre-Existing VC on the Evolution of Precipitate and Mechanical Properties in the H13 Steel

Abstract

To further improve the mechanical properties of H13 steel at room and high temperatures, its precipitates were regulated based on the Thermo-Calc results. Scanning electron microscope (SEM), electron backscattering diffraction (EBSD), transmission electron microscope (TEM), and X-ray diffraction (XRD) Rietveld refinement were used to study the effect of the intercritical annealing on the microstructure and mechanical properties of H13 steel. The results show that the intercritical annealing at 850~95 °C increased the VC volume fraction from 2.23 to 3.03~3.48%. Increasing the VC volume fraction could inhibit the M7C3 precipitation from 10.01 to 6.63~5.72% during tempering. A large amount of VC also promoted the M23C6 precipitation during tempering at higher dislocation densities. The intercortical annealing simultaneously increased the elongation of H13 steel. An excellent combination (room temperature: ultimate tensile strength (UTS) of 898 MPa and total elongation (TEL) of 19.35%, 650 °C: UTS of 439 MPa, and TEL of 27.80%) could be obtained when intercritical annealing is performed at 900 °C. Meanwhile, after aging at 650 °C for 128 h, the room temperature UTS and TEL decreased by only 31 MPa and 0.52%, respectively.

Carbide evolution for a quenched and tempered Dievar steel

“…The enrichment and redistribution of Cr in pre-existing particles (i.e. larger spherical V-rich M 8 C 7 carbides) also gives the priority for Cr-rich carbides to nucleate at the interface of pre-existing carbides [23]. Additionally, larger elliptical Cr-rich M 23 C 6 carbides with an average length of 150–200 nm appear at relatively higher tempering temperatures or for a longer tempering time, Figure 6(g)–(i) and Table 2, which are also calculated as one of equilibrium carbides in Dievar steel.…”

“…As listed in Table 2, thermodynamically calculated equilibrium carbides in Dievar are V-rich MC, Cr-rich M 23 C 6 and Mo-rich M 6 C, which are slightly different from the identified carbides on quenching and tempering. The presence of V-rich primary carbides can retard the growth of prior austenite grains to further refine the martensitic structure on quenching [23]. Lower bainite forms during oil quenching in Dievar with carbon diffusion under the para-equilibrium condition, where bainitic carbides, which are supposed to be cementite based on the observation in a X37CrMoV5-1 steel [24], appear as parallel arrays at about 60° to the axis of the bainitic plate due to its precipitation in one variant of the orientation relationship [25].…”