** Fachgebiet Werkstoffwissenschaften, Universitat-Gesamthochschule Paderborn, FRG *** lnstitut fur Werkstoffkunde I, Universitat Karlsruhe (TH), FRG

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1 INFLUENCE OF SHOT PEENING RESIDUAL STRESSES ON THE TENSILE DEFORMATION BEHAVIOUR OF NiCr22Co12Mo9 D. VIERECK*, D. LOHE**, 0. VC)HRINGER*** and E. MACHERAUCH"' *ABB Kraftwerke AG Mannheim, FRG ** Fachgebiet Werkstoffwissenschaften, Universitat-Gesamthochschule Paderborn, FRG *** lnstitut fur Werkstoffkunde I, Universitat Karlsruhe (TH), FRG ABSTRACT Tensile specimens manufactured from 2 mm thin sheets of the nickel-base superalloy NiCr22Co12Mo9 were shot peened with cast steel shot S 170 and S 330, respectively. The resulting depth distributions of macro and micro residual stresses are hardly influenced by the shot size. Uniaxial tensile tests at temperatures between 293 and 1473 K were performed with unpeened and shot peened specimens. Below 1123 K, the onset of plastic deformation is shifted to very low stresses, the workhardening rate is increased and the elongation at fracture is reduced by shot peening. Above 1123 K, the stress-strain curves of unpeened and shot peened specimens are similiar. At 1323 K, however, the elongation at fracture of specimens peened with shot S 330 is markedly reduced. This results from a strong enhancement of carbide precipitation in the shot peened surface area during heating the specimens to test temperature. INTRODUCTION Nickel-base superalloys strengthened by solute atoms and carbides are often used as structural materials for combustion chambers of landbase or aircraft gas turbines. Due to fabrication processes and surface cleaning treatments by blasting, local deformations occur near the surface of these complex structures, resulting in the formation of macro and micro residual stresses varying across the combustion chamber wall. In contrast to many investigations concerning the influence of residual stresses on the deformation behaviour of steelsl aluminium and titanium alloys (see e.g. [I]), only few investigations were performed on high temperature alloys [2,3]. The present paper deals with the influence of macro and micro residual stresses on the tensile deformation behaviour of the nickel-base superalloy NiCr22Co12Mo9, which was studied in the course of an extensive research programme on the behaviour of combustion chamber materials under static, quasistatic and cyclic loadings 1451.

2 EXPERIMENTAL PROCEDURE The material under investigation was supplied as sheet of 2 rnm thickness under the trade mark of Nicrofer 5520Co by the Vereinigte Deutsche Metallwerke (VDM). The chemical composition of this alloy (wt.- %: 11.9 Co, 22.0 Cr, 8.88 Mo, 0.12 Fe, 0.01 Mn, 0.07 Si, C, 0.92 Al, 0.44 Ti, bal. Ni) is similar to lnconel 617. The alloy, which was solution heat treated and quenched by the manufacturer, had an average grain size of 0.12 mm. Tensile specimens with a gauge length of 11 mm and a cross section of mrn2 were machined and then shot-peened in an air blast shot-peening equipment [4]. Cast steel shots S 170 and S 330 with a hardness of HRC and HRC, respectively, were used at a pressure of 1.6 bar until a coverage of 98 % was achieved. Macro residual stresses were determined with X-rays by evaluating the 1220)-interference lines according to the sin2$-method using CrKa-radiation. Tensile tests with a strain rate of = 3x s-l were performed with unpeened and shot peened specimens at different temperatures using a computer controlled screw driven testing machine and an induction heating system [6]. More details about the experimental procedure are given in [4]. RESULTS The upper part of Fig. 1 shows the depth distribution of macro residual stresses produced by shot peening. Independently of the shot size used, the compressive residual stresses show an absolute maximum of -700 ~ /mm2 at the surface and a relative maximum of about -620 ~ /mm2 at a distance of 0.02 rnm below the surface. In the lower part of the figure, the corresponding half width of the {220)-interference lines are given. The values decrease from 2.5 " (shot S 170) and 2.9" (shot S 330), respectively, at the surface to about 1.2" in the core of the specimens. distance from surface [mm] Figure 1. Depth distributions of macro residual stress (upper graph) and half width (lower graph) after shot peening. In Fig. 2, nominal stress - plastic strain curves of unpeened specimens (lefthand side) as well as of specimens peened with shot S 170 (middle) and shot S 330 (right-hand side) are compiled. In each case, four temperature ranges with different kinds of nominal stress - plastic strain curves exist. Below 400 K (range I), the curves are smooth, reveal a strong workhardening effect and a distinct influence of temperature on the flow stress. Between 400 and 973 K (range ll), the nominal stress - plastic strain curves are serrated and the influence of temperature on the flow stresses is rather small. Between 973 K 4 T K (range Ill) the curves become smooth again and reveal a strong influence of temperature on the ultimate tensile strength. R, In this case, Rm does not correspond to the

3 onset of geometrical softening due to necking. At temperatures between 1100 and 1473 K (range IV) almost no workhardening occurs. Plots of nominal stress versus plastic strain show continuously decreasing flow stresses. However, plots of true stresses versus plastic strain are characterized by constant flow stresses [4]. Figure 2. Influence of temperature on the nominal stress - plastic strain curves of unpeened specimens (a), specimens peened with cast steel shot S 170 (b) and S 330 (c). Below 1100 K, the slopes do/d q of the stress-total strain curves of unpeened specimens in the quasi-elastic defomation regime correspond well to known values of Young's modulus evaluated by the resonance technique or other methodes [7,8]. Above 1100 K, however, a strong drop of the measured values occurs, as can be seen from Fig. 3, which can not be attributed to the temperature-dependence of Young's modulus alone. Below 1100 K the corresponding do/det-values of shot peened specimens fall far below those of unpeened specimens and are obviously influenced by microplastic deformations. Above K, however, the differences between shot peened and unpeened specimens are rather small. = as rece~ved state o = \ \ \ \% A --- / -/ SO temperature [ K I Figure 3. Influence of temperature on the slope of the stress -total strain curves in the quasi-elastic deformation regime (Young's modulus and modulus, respectively). From Fig. 4 a) it can be seen that at temperatures below 1100 K, specimens peened with shot S 170 reveal lower and specimens peened with S 330 higher 0.1 % proof stresses Rpo.1 than unpeened ones. in the case of 0.5 % proof stress RpOS5 (Fig. 4 b), both shot peening treat-

4 1200 = as rece~ved state SO temperature [ K I Figure 4. Influence of temperature on 0.1 % proof stress (a), 0.5 % proof stress (b), ultimate tensile strength (c) and elongation at fracture (d). ments result in higher values compared with the unpeened state. Above 1100 K, however, the measured RpOSl and RpOa5 values are almost identical. Furthermore, at all temperatures no differences appear with respect to ultimate tensile strength Rm (Fig. 4 c). Fig. 4 d shows that a complex influence of shot peening on elongation at fracture exists. At about 1100 K and at 1473 K, the measured values are almost identical. Between 293 and 1000 K, the data of shot peened specimens fall distinctly below those of unpeened ones. Furthermore, the elongation at fracture of specimens peened with shot S 330 steeply decreases if the temperature is raised from 1100 to 1323 K. DISCUSSION The uniaxial tensile deformation behaviour of unpeened and shot peened states of the nickel-base superalloy NiCr22Co12Mo9, which is characterized by different kinds of stress-strain curves in four temperature ranges (Fig. 2), is controlled by specific deformation mechanisms [4,9]. In range I, the flow stress is determined - besides dislocation and solid solution hardening - by thermally activated dislocation movement across short range obstacles. In range II, the serrated stress-strain curves are caused by dynamic strain ageing processes due to elastic interactions between slip dislocations and diffusing alloying atoms. The strong temperature dependency of the ultimate tensile strength Rm in range Ill is a result of a microstructural softening effect caused by rearrangements of the dislocation structure with tangles and cells to a typical subgrain structure. Finally, in range IV the deformation behaviour is controlled by steady state creep processes. Independently of these deformation mechanisms, in the quasi-elastic deformation regime at temperatures below OC the slopes d u/d c are significantly reduced by shot peening. This behavlour can be interpreted by a surface-core model which consists of an element

5 distance from surface [mml Figure 5. Depth distributions of shot peening induced macro residual stresses as received lml representing the workhardened surface area with compressive macro residual * stresses and an element representing the core of the specimen bearing tensile macro residual stresses [I,4]. In Fig. 5, the depth distribution of macro residual stresses is extrapolated into the core of the specimen assuming a triangular residual stress-distance profile and allowing for the balance of forces. The superposition of these tensile macro residual stresses in the core area with the applied tensile loadings initiates at room temperature localized microplastic deformations at relatively small stresses. Therefore, compared with Young's modulus, reduced d 0 /d c t-vaiues are determined (Fig. 3). Although the macro residual stresses show an increasing relaxation with increasing temperature and time caused by thermal activation [I 1, similar reduced d 0 /d c t-values can be observed 3s long as macro residual stress relaxation is not complete. Only at temperatues above 1100 K, the slopes du/d rt in the quasi-elastic regime of unpeened and shot peened specimens are the same due to the completely relaxed residual stress state. The marked decrease of the values measured above K with increasing temperature is combined with creep deformations appearing already at very low stresses 141. plastic stmin [%I Faure 6. Nominal stress - plastic strain curves of the as received state and the shot peened states at different temperatures a) 293 K, b) 873 K, Tensile tests with relatively small plastic strains in the range up to ep = 1 % yield strong relaxation effects on the macro residual stress state [1,4]. However, the workhardened surface area, which is characterized by the depth distribution of half widths of X-ray interference lines (Fig. I), does not change significantly during tensile deformation 141. Therefore, as shown in Fig. 6a and b, at plastic strains ep 3 1 %, the flow stress measured at 293 and 873 K, respectively, increases with increasing shot eening induced workhardening of the

6 surface areas. At temperatures above 1100 K, the surface workhardening is subjected to a complete thermal relaxation during heating to test temperature and its influence on the flow stresses vanishes as demonstrated by the stress-strain curves in Fig. 6c and d for T = 1323 and 1473 K, respectively. The smaller values of elongation at fracture of shot peened specimens below 1123 K are caused by the workhardened surface area, which yields to matrix-carbide interface decohesions and hence failure of the specimen at smaller tensile strains compared to unpeened specimens. However, TEM-investigations of shot peened specimens tested at 1323 K show that large molybdenum-rich M6C-carbides originate especially in the surface area during the heating to this test temperature [4]. In this case the significant reduction of the elongation at fracture is caused by deformation induced void formation in the matrix-carbide interface. ACKNOWLEDGMENT The financial support for these investigations by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. They were performed within the 'Sonderforschungsbereich 167'. REFERENCES Vbhringer, O., in Advances in Surface Treatments Vol. 4, ed. Niku-Lari, A., Pergamon Press, Oxford, 1987, Fujioka, J., Murase, H. and Matsuda, S., in Proc. of the 4th lnt. Conf. on Pressure Vessel Techn., Vol. 1, London 1980, Cook, P.,H., in Nucl. Techn., 1984, 66, Viereck, D., Dr.-ha. thesis, Universitiit Karlsruhe, 1989, Fortschritt-Berichte VDI, Reihe 5, Nr. 202, VDI Verlag Dusseldorf (1 990) Merckling, G., Dr.-lna. thesis, Universitiit Karlsruhe, 1989 Viereck, D., Merckling, G., Lohe, D., Vbhringer, 0. and Macherauch, E., in Werkstoff~rufunq 1987, Deutscher Verband fur Materialprijfung, Berlin 1987, Schepp, P., Dr.-lna. thesis, Universitat Erlangen-Nurnberg, 1982 Rothmann, M. F., in Technical Review 1984, Cabot Corp., Kokomo, USA Viereck, D., Merckling, G., Lang, K.-H., Eifler, D. and Ldhe, D. in Festiakeit und Verformuna bei hoher Tem~eratur, ed. Schneider, K., DGM-Inforrnationsgesellschaft, 0 berursel (FRG), 1989,