Carbides in Vanadis PM steel

Figure 1: Optical micrographs of V4 steel: (a) as-cast,(b) as-sprayed, Scale bars: 20, 10 µm.

Figure 2: X-ray diffraction spectrum of the as-sprayed V4 steel.

Figure 3: TEM micrographs: (a) twined martensite, (b) lath martensite, (c) retained austenite (bright field) and its diffraction patterns, (d) retained austenite (dark field) and standardization of diffraction patterns. Scale bars: 0.5 µm.

Figure 4: SEM micrographs of the as-cast V4 steel. Scale bars: 30, 5 µm.

Figure 5: (a) TEM micrograph of VC in the as-sprayed V4 steel, (b) selected area diffraction patterns in [011] zone axis of VC. Scale bar: 0.5 µm.

Figure 6: TEM micrograph of the M7C3 carbide in the as-sprayed V4 steel and its selected area diffraction patterns. Scale bar: 0.5 µm.

Figure 7: Comparison of V4 annealed microstructures: (a) spray forming, (b) powder metallurgy. Scale bars: 10 µm.

Carbide name: VC, M7C3
Record No.: 738
Carbide formula: VC, M7C3
Carbide type: MC, M7C3
Carbide composition in weight %: No data
Image type: LM, SEM, TEM
Steel name: Vanadis 4
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: PM high alloyed steels
Steel composition in weight %: 1.5% C, 1.0% Si, 0.4% Mn, 8.0% Cr, 1.5% Mo, 4.0% V.
Heat treatment/condition: The steels were melted in a vacuum induction furnace and then cast into rods as feedstock for spray forming. The feedstock was heated in an induction furnace and soaked at above melting point for 20 min. The molten metal flow rate was set at approximately 0.1 kg/s, using N2 with a pressure of 2.2 MPa as atomizing gas. The atomized droplets were cooled and driven towards a revolving substrate to form a condensed product. The distance from the nozzle to the substrate was set as 360 mm, and the copper substrate rotated at a speed of 10 rpm during atomization and deposition. The whole spray forming process was completed in about 40 s, and a gaussshaped billet with about 130 mm in diameter and 30 mm in height was obtained.
The as-sprayed billet was machined into specimens with a thickness of 16 mm. The specimens were then heated at a rate of 10C/min to the rolling temperatures of 850C, 900C, 950C, 1050C, 1150C, respectively. After holding for 15 min, the specimens were rolled in a single 60% reduction pass, and then cooled to room temperature in the sand. Three groups of as-rolled specimens were annealed at 850C, 900C, 950C respectively, with each group containing five specimens rolled at different temperatures. After being isothermal held for 2 h, the specimens were cooled to 500C at a rate of 30C/h, and then cooled in air. The most ideal microstructure, which was obtained by the as-sprayed steel rolled at 1050C and annealed at 900C, was chosen from the annealed specimens and compared with that of the commercial V4 steel made by powder metallurgy (By UDDEHOLM, Sweden).
Note: The as-sprayed microstructure of the Vanadis 4 steel was studied. It was observed that the as-sprayed microstructure consists of martensite, retained austenite and uniformly distributed spheroidal carbides. The KurdiumovSachs crystallographic relation between martensite and retained austenite is confirmed by transmission electron microscope. No macro segregation or network carbides are found. This can be explained in terms of the rapid solidification of spray forming for microstructure refining. The steel obtained by the new processing has a uniform carbide distribution, and the average carbide size is even smaller than the equivalent in the powder metallurgical Vanadis 4 steel. It was proved that spray forming can be considered as a new and high-efficient way to replace powder metallurgy for the production of Vanadis 4 steel.

As shown in Fig. 1a, the microstructure of the as-cast V4 steel contains coarse eutectic carbides rich in Mo, Cr and V, which segregate at the prior austenite grain boundaries during solidification. These carbide arrangements will lead to very low toughness and ductility of the materials. Fig. 1b shows the microstructure of the as-sprayed material consisting of fine, homogeneous and fully spheroidal grains ranging from 8 to 10 µm, which was substantially finer than the conventionally cast equivalent. No carbide networks or large irregular carbides were found, and the fine spheroidal carbides were uniformly distributed throughout the microstructure. The carbides located at the grain boundary were relatively larger compared with those inside the grains.
The X-ray diffraction analysis revealed the presence of martensite, and a considerable quantity of metastable retained austenite in the matrix (Fig. 2). The cause of the retained austenite, as reported for spray formed D2 and rapid solidification D2 steel, is that much of the alloying elements were dissolved in the austenite during the spray forming and the rapid solidification stabilized the austenite and therefore decreased the martensite start temperature Ms.

The TEM observation showed that the microstructure was predominantly twined martensite. Some acicular martensite and lath martensite were occasionally found. The representative morphology of the martensite was shown in Fig. 3a and b. It could be seen from Fig. 3c and d that the retained austenite was arranged in form of layers between martensite strips. The selected area diffraction pattern showed the Kurdiumov.Sachs orientation relation of (011)alpha'//(111)gamma between the austenite and the martensite.

Carbides are important phases providing the steel with high hardness and high wear resistance. The TEM analysis of the cold work steel showed two types of carbides in the matrix: vanadium riched MC (VC) and chromium riched M7C3 ((Cr, Fe)7C3). It is interesting to compare the morphology of the carbides in the as-sprayed microstructure with that in the ascast one. As shown in Fig. 4a, many strip or short rod coarse eutectic VC carbides (shown by arrows) are located along the grain boundaries. But in the as-sprayed microstructure, fine spheroidal VC carbides are uniformly distributed on the grain boundaries, no coarse VC carbides could be found. The morphology of VC is shown in Fig. 5a. This difference could be explained by comparing the characteristics of VC formation during solidification process. The vanadium is a strong carbide-forming element, and the vanadium carbides obtained by conventional cast could be divided into primary carbides and eutectic carbides. The primary vanadium carbides precipitate first from the molten steel during the solidification process, and they grow without restriction in various directions. Thus the primary carbides have a trend of forming large conglomerations. Other eutectic carbides are in shape of strip or shot rod. This is because the VC is a typical faceted crystal, but the austenite is non-faceted crystal, therefore they tend to form divorced eutectic and the VC carbides grow along the austenite grain boundaries during solidification.

The uniform distribution of the spheroidal MC carbides in the as-sprayed materials indicated that the eutectic reaction was greatly restrained as the result of the rapid solidification. The microstructural characteristics of the as-sprayed materials critically depend upon the solidification history of the droplets. During the spray forming process, the melt stream is broken into finer droplets in diameters ranging from 5 to 200 µm. The small droplets might have solidified completely, the medium sized partially solidified and the larger still in their liquidity when they arrived on the deposition surface. The droplets will ideally form a mushy zone consisted of liquid and solidified metal and the thickness is only a few millimetres on the surface of the billet. The mushy zone is expected to exist during the whole spray forming process. When the droplets with high velocities hit the surface of the mushy zone, dendrites form during flight experience and semi solidified droplets may break up and remelt. Those dendritic fragments having not been remelt will be the new nuclei for the rapid solidification which is promoted by the comparably cold atomizing gas. The formation of spheroidal grains is proposed to evolve from the homogenization of dendrites without being deformed extensively, or from the growth and coalescence of the deformed or fractured dendrites.
The atomized droplets undergo a rapid solidification by the effective convection of surrounding gas, and therefore, solute segregation can be greatly depressed. In addition, the impact of the droplets causing turbulences in the mushy zone leads to further homogeneous thermal balance and chemical composition. The amount of the residual liquid that could be at the eutectic chemical composition is reduced due to the nonequilibrium solidification process. Therefore the formation of the elongated vanadium carbide is greatly suppressed and results in fine spheroidal carbides.
The typical morphology of the VC carbide and its diffraction patterns with zone axis of [011] are shown in Fig. 5. It is of interest to note that the superlattice spots existed in the diffraction pattern, which revealed the ultrafine structure of the VC. The VC is in face-centered cubic (FCC) crystal structure, and the atom diameter ratio of the carbon and the vanadium is rC/rM=0.57(<0.59). The smaller the ratio the higher the trend to form simple close-packed structure. The V atoms are packed as face-centered cubic and the C atoms filled in the octahedron interstice. But some C atoms may be absent in the octahedron interstice of the crystal, which makes the atomic ratio of C to V in the lattice less than 1. For example, the chemical formula of vanadium carbide may vary form VC0.5 to VC.

Some eutectic M7C3 carbides existed along the grain boundaries in the as-cast materials, and others precipitated from the matrix and their morphology are shown in Fig. 4b, which is region A in Fig. 4a. They were spherical and uniformly distributed since they precipitated from the matrix. Most of the M7C3 carbides in the as-sprayed microstructure are spheroidal with diameter of about 180 nm, and most of them are distributed inside the grains. Few dog-bone-like M7C3 carbides, as shown in Fig. 6, are observed. However, they are only about 34 m long and are far smaller than those in the as-cast microstructure.

After spray forming, the billets were hot rolled and then annealed at different conditions in order to optimize the processing parameters. The best microstructure obtained, Fig. 7a, consists of fine carbides dispersed homogeneously in the ferrite matrix. It was obtained by the as-sprayed steel rolled at 1050C and then annealed at 900C. The average carbide size is even smaller than the equivalent in the powder metallurgical V4 steel (Fig. 7b). The crucial reason for obtaining the excellent microstructure is attributed to the spray forming used for microstructure refinement. It is important to note that, unlike conventional cast high alloy steels, the as-sprayed steels showed good workability at temperatures ranging from 850 to 1150C as no crack have been found in the specimens, which have experienced severe deformation. The enhanced workability of the as-sprayed steel is also attributed to the refined microstructure with small equiaxed grains and evenly distributed fine spheroidal carbides. The large deformation is usually used on conventional cast high alloy steels in order to break up the coarse carbides, and the steels are sensitive to crack during hot working because of the coarse grains and coarse carbide networks.
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