INFLUENCE OF WARM ROLLING AND RECRYSTALLIZATION ANNEALING ON MECHANICAL AND METALLOGRAPHIC PROPERTIES OF THE SUPERALLOY N07080

Additional strengthening of superalloy N07080 described in this work was achieved by warm rolling. Control of the ratio of strength and ductile properties of the superalloy is possible by appropriate selection of the amount of warm deformation and the appropriate selection of the partial recrystallization temperature. In addition, recrystallization annealing makes it possible to equalize the grain size across the cross section of the warm rolled bars, which before recrystallization differ significantly in size in the central and peripheral parts of the bars.


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Nickel superalloys have low values of the stacking fault energy (SFE), hence the dynamic recrystallization takes place discontinuously -discontinuous dynamic recrystallization (DDRX)? [5,7]. In metals with high values of SFE the continuous dynamic recrystallization (CDRX) occurs. DDRX takes place heterogeneously with clear nucleation and growth stage while CDRX takes place uniformly with no clear nucleation and growth stage. Unlike to metals with low SFE, in metals with high SFE the dissociation of the perfect dislocation into two partials dislocation is more difficult, so perfect dislocation may glide, climb and cross slip easily.? This enables rearrangement (forming of the subgrains boundary) and annihilation of dislocations decreasing their density through dynamic recovery (DRV) process. In this case recrystallisation is based on progressively increasing of the misorientation of subgrain boundaries which may lead to HAGBs at larger strains (CDRX). In the case of metals with low SFE perfect dislocations are dissociated into partials, so their cross-slip or climb are more difficult. Because of that forming of the subgrain structures during deformation by DRV is difficult, so dislocation density increases to a high level with possibility of their local accumulation. On locations where dislocation density becomes larger then critical dislocation density the nuclei of the new grains will form and then grow (DDRX). Nucleation of DDRX is usuali initiated on pre-existing grain boundaries. A necklace structure of equiaxed grains forms when there is a large difference between the initial grain size and the recrystallized grain size [8].? Initial structure? with coarse grains exist in the superalloy? N07080 (Nimonic 80A) after solution annealing (1080°C/8 h).

Experimental Research and Test Results
Superalloy N07080 according to standard ASTM B 637 for experimental research was produced by double melting. Primary melting was performed in a vacuum induction furnace (VIM). Remelting? was performed by electroslag? remelting (ESR) process. Achieved chemical composition of the superalloy after remelting (ESR ingot) is given in Table 1. The dimensions of the ESR ingot were φ126 mm at the bottom, φ 115 mm at the top, and the length was 305 mm. The weight of the ingot was 27.9 kg. Hot forging of ingot up to a diameter of 50 mm was performed on a hydraulic press 2 MN, and then on a pneumatic hammer 2,5 kN up to a diameter of 20 mm. The temperature interval of hot forging was between 950 o C and 1160 o C. Hot rolling (starting temperature 1160 o C) of the bars φ20 mm was carried out on light-section rolling mill SKET φ 370 mm on four different dimensions: 1. Round bars with diameter 15 mm (not intendend for additional warm rolling), 2. Horizontal oval bars 13,0 x 21,4 mm (intendend for additional warm rollingone pass on bars φ15 mm with 10% of warm deformation), 3. Vertical oval bars 15,0 x 18,0 mm (intendend for additional warm rollingtwo passeson bars φ15 mm with 20% of warm deformation), 4. Horizontal oval bars 14,0 x 25,2 mm (intendend for additional warm rollingthree passes on bars φ15 mm with 30% of warm deformation).
Additional warm rolling of the bars was carrid out after performing of solution annealing at 1080°C/8 h. The starting temperature for the warm rolling was 1050 o C.
After warm rolling all bars were cooled to room temperature on still air. All solution annealed and warm rolled bars were heat treated by final precipitation aging at 700°C/16 h.
Hot rolled bars φ15 mm that not intenden for additional warm rolling were used for mechanical and metallographic testing of the superalloy after standard heat treatment (1080°C/8h + 700°C/16h).? All thermal and thermomechanical treatments performed? on the bars are shown in Figure 1.

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The tensile test of each test bar was performed on test pieces with a diameter of 8 mm machined from rolled bars φ15 mm. ? Hardness and metallographic tests were performed on a full cross section of rolled bars φ15 mm. All test were performed on standard heat treated bars, solution annealed + warm rolled bars with diferent amount of deformation (10 %, 20% or 30% reduction of cross section), solution annealed + warm rolled + partially or fully recrystallized bars. Also, all tested bars were heat treated by precipitation aging before testing.
The tensile testing results are shown in Table 2, Figure 2 and Figure 3. The hardness testing results are shown in Figure 4. The microstructures of the different bars are shown in Figures 5 to? 8. The grain size in standard heat treated (1080°C/8 h + 700°C/16 h) bars is between G1 and G3. These bars also have larger grains than G1.? In bars that are warm rolled, there is no significant change in grain size as well as in those that are recrystallization annealed at 1000 o C/1h.? By performing recrystallization annealing at higher temperatures (1040 o C and 1080 o C), the grain size is reduced (grain size between G2 and G4). This grain size analysis does not include grains in necklaces, where the grains are corresponds to G8? size. In the bar 1080 o C/8h + 10% warm def. + 1040 o C/1h + 700 o C/16h as a result of partial recrystallization, grain size is between G2 and G6 [9].

Conclusions:-
By conducting warm deformation of the N07080 superalloy at 1050 o C after solution annealing it is possible to increase its strenght and hardness. These properties can be reduced by recystallization annealing. Complete recrystalization is possible at temperature of 1080 o C.? Annealing at lower temperatures does not allow reducing strength and hardness of the warm rolled bars on values that the superalloy has after standard heat treatment.? Therefore, at temperatures of 1000 o C and 1040 o C it is possible to achieve only partial recrystallization of the superalloy N07080.? By appropriate selection of the amount of warm deformation and the appropriate selection of the partial recrystallization temperature, it is possible to control? the ratio of strength and ductile properties of the superalloy N07080.? Due to the difference in stress state and cooling rate of the central and peripheral parts of the bars after warm rolling the microstructure of these parts of the bars differ.? Differences in microstructure between these parts of the warm rolled bars are reduced by conducting recrystallization annealing. Recrystallization of the outer parts of the warm rolled bars is generally carried out by a static recrystallization process, while recrystallization of the central parts begins as a dynamic and/or metadynamic recrystallization.