CREEP RUPTURE LIFE OF PRE-STRAINED SUPERALLOY N07080

The creep of the pre-strained superalloy N07080 is described in this work. The pre-strain was achieved by warm rolling at 1050 o C.-The warm rolling was performed due to additional strengthening, i.e increasing of the superalloy hardness.-The pre-strain drastically reduces the creep rupture life of the superalloy compared to the creep rupture life of the standard heat treated superalloy.-The drastic reductionof the creep rupture life is result of rapid creep cavity nucleation on stress concentration sites along primary grain boundaries of the pre-strained superalloy.-Recrystallization eliminates potential sites for rapid cavity nucleation and prolongates the creep rupture life.


ISSN: 2320-5407
Int. J. Adv. Res. 9 (10), 1167-1176 1168 DRX occurs during hot deformation, SRX occurs during heat treatment in unloaded prestrained-workpiece, MDRX occurs by continued growth of the nuclei formed by dynamic recrystallization- [6]. Nickel superalloys have low values of the stacking fault energy (SFE), hence the dynamic recrystallization takes place discontinuouslydiscontinuous dynamic recrystallization (DDRX)- [5,7].-Nucleation of DDRX is usually 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].-Such initial structures with coarse grains exist in the superalloy-N07080-after solution annealing (1080°C/8 h).
The service life of an alloy operating in creep conditions covers the entire period of its exploitation.-This period is divided into the time required for the nucleation of the cavities and the remaining time until fracture (growth and coalescense of intergranular cavities).-In fully recrystallized structures, nucleation of the cavities occupies most of the life of the alloy- [9].-The service life of some alloys can be drastically reduced if they are pre-strained (work hardened) before use under creep conditions [10,11,12,13].
Cavity nucleation generally occurs at sites where stress concentration occurs. Such sites are mainly grain boundaries. In the case of cavitation along grain boundaries, it mainly occurs along grain boundaries that lie transversely to the direction of tensile stress- [12].-As the amount of deformation increases during the creep process, uneven elongation of individual grains in the polycrystalline material occurs.-Unevenly deformed grains must adapt to each other.-This can be achieved by sliding along the boundaries between adjacent grains [14].-Different inhomogeneities at grain boundaries such as solid particles or bulges are potential sites for stress concentration to occur. In the pre-strain alloy additional sities for stress concentration are sities of interaction of a slip band and a grain boundary. Internal stress coused by blocked slip bands in pre-strained alloy can lead to accelerated cavity nucleation at elevated temperature and a significant shortening of the creep rupture-lifetime- [11].-Grain size has a significant effect on the creep resistance of the superalloy. In general, a recrystallized coarse-grained microstructure has a longer creep life than a fine-grained microstructure [10,15].

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 electroslagremelting (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, φ115 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 (staring temperature 1160 o C) of the bars φ20 mm was carried out on light-section rolling mill SKET φ370 mm on four different dimensions: -round bars with diameter 15 mm (not intendend for additional warm rolling), -horizontal oval bars 13,0 x 21,4 mm (intendend for additional warm rollingone pass on bars φ15 mm with 10% of warm deformation), -vertikal oval bars 15,0 x 18,0 mm (intendend for additional warm rollingtwo passeson bars φ15 mm with 20% of warm deformation), -horizontal oval bars 14,0 x 25,2 mm (intendend for additional warm rolling -three 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 hardness, creep and metallographic 1169 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.
The creep test of each test bar was performed on test pieces with a diameter of 8 mm machined from rolled bars with a diameter of 15 mm.-Hardness tests were performed on a full cross section of rolled bars with a diameter of 15 mm. All tests 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 recrystalized bars. Also, all tested bars were heat treated by precipitation aging before testing.-Hardness testing results are shown in Figure 2, while creep test results are given in Table 2.    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 ( Figure 3).-The grains in the peripheral parts of the warm rolled bars are large, while in the central parts of the warm rolled bars the grain size depends on whether a recrystallization annealing has been performed and at what temperature.-Since the creep test specimens were obtained by machining from the rolled bar φ15 mm to the initial diameter of the creep test pieces of 8 mm, the microstructures of test pieces generally correspond to the microstructures of the bars core- (Figure 4).-The largest grains are in standard heattreated-bars (between G1 and G3, some of them larger then G1),-they are slightly smaller in warm rolled and fully recrystallized bars-(between G2 and G4), even smaller in warm rolled and partially recrystallized (1040 o C/1h) bars. In warm rolled and warm rolled and partially recrystallized bars (1000 o C), except coarse grains, there are very small grains in the necklaces (G8)- [16].