Selective hardening and residual stress relaxation in shot peened Timetal 21S

Transcription

1 Selective hardening and residual stress relaxation in shot peened Timetal 21S M-C Berger, J.K. Gregory Technische Universitat Munchen, D Garching, Germany Abstract Through aging after a conventional mechanical surface treatment (Selective Surface Aging: SSA), the fatigue behavior of precipitation hardenable alloys can be further improved over the mechanically surface treated condition. Compressive residual stresses remaining in the age hardened surface layers may cause this attractive behavior. To better understand and control the SSA treatment, the precipitation hardening and the relaxation of residual stresses during aging were examined in surface layers of the shot peened metastable (3-Ti alloy Timetal 21s. Aging treatments were carried out at temperatures between 350 and 500 C, for two oxygen levels and two initial degrees of cold work. Although precipitation reactions occur early in the strongly cold worked surface, residual stresses relax rapidly, so that the resulting component can only utilize a compromise between high surface hardness and high compressive residual stresses. Introduction A selective surface aging treatment can be used to tailor the microstmcture of precipitation hardenable alloys from the surface to the core, thus giving rise to attractive improvements in fatigue performance (Gregory [1], Wagner [2]). This two step process can be applied to alloys with semi-coherent or incoherent precipitates. First a high degree of cold work is generated in the surface layer by a conventional mechanical surface treatment, shot peening for instance. An appropriate aging treatment is then carried out so that the precipitation reaction

2 342 Surface Treatment essentially takes place in the "skin". The treated part thus combines a high strength, crack-nucleation-resistant skin with a ductile, damage tolerant core. SSA treating the metastable P Ti-alloy Beta C leads to notched and bending endurance limits much higher than that of the fully aged condition. An improvement over conventionally surface treated specimens is sometimes obtained, depending strongly on aging conditions [1, 2]. This feature may be explained as follows. Conventional mechanical surface treatments generate compressive residual stresses in the skin, which are of course beneficial as they retard fatigue crack growth. The short time aging treatment of SSA relaxes these residual stresses to a certain extent, depending on aging conditions. Decisive for the endurance limit is however not the initial amount of compressive residual stresses (before loading) but what remains stable under cyclic loading. In this respect, residual stresses in a high strength material have been found to be more stable than in a low strength material (Vohringer [3]). Thus, by controlling the short aging treatment of SSA to preserve some residual stresses in the hardened surface layers, endurance limits exceeding those of conventionally surface treated parts may be obtained. A better understanding and control of SSA-treatments requires the characterization, during aging of mechanical surface treated material, of residual stress relaxation and aging reaction in the surface layers, as well as an understanding of interactions between these two mechanisms. Early precipitation reaction could indeed be expected to slow down thermal residual stress relaxation. For that purpose, the metastable (3 Ti-alloy Timetal 21s was considered. This alloy has been reported to have, in the aged condition, remarkable thermal stability and creep resistance between 290 C and 625 C, see O'Connel [4]. Both a low and high oxygen content were considered so as to determine the role of this strong solution solid strengthener and alpha stabilizer (O'Connel [4], Imam [5]). Additionally, the initial degree of cold work was varied by conducting solution treatments below and above the recrystallization temperature, obtaining partially (PR) and totally recrystallized (R) microstructures, respectively. Previous work (Berger [6]) addressed the kinetics of thermal residual stress decay in this alloy. The present work aims to give a view of SSA-mechanisms, dealing with the possibility of interactions between residual stress decay and precipitation hardening. Material and Experimental Procedures Material was supplied in plate form with 2.8 mm thickness by TIMET (USA) through Tisto (Germany). Chemical analyses are shown in Table 1, Table 2 presents the tensile properties of both alloys. Solution heat treatments (SHT) were performed in flowing argon above the (3-transus temperature, the metastable (3-matrix offering the greatest driving force for subsequent aging. After electropolishing 150 urn from the surface,

3 Surface Treatment 343 specimens were shot peened by injection to an Almen intensity of 0.29 mma. SSA-Treatment parameters are summarized in table 3. Table 1. Chemical composition of the two alloys (wt.-%). TEMETAL 21S low oxygen content (21s-) TIMETAL 21S high oxygen content (21s+) Mo Nb Al Si C Fe N O H (ppm) Table 2. Tensile properties of the two alloys. 1 21s- (R) L SHT YS 0.02% 850 MPa UTS 884 MPa El% 21s+(R) 18 1 SHT YS 0.02% 1010 MPa UTS 1015 MPa El % 14 I Table 3. SSA-treatment parameters. SHT + water quench Shot Peening 0.29 mma Aging partially recrystallized (PR) totally recrystallized (R) shots230 air pressure / coverage Temperatures 21s- 30min810 C 30 min 860 C 21s + 30min85S C 30 mm 900 C mean diameter: 0.71mm, 46-53HRC 7.5 bar/ lx 350 C, 400 C, 450 C, 500 C The aging reaction was characterized by means of microhardness measurements, carried out on (T,S)-sections of SSA-treated specimens. Macroscopic residual stresses were measured by x-ray diffraction using Cu- Kot radiation according to the sirr\ /-method, see Eigenmann [7]. Diffraction lines were acquired in the T-direction with a Position Sensitive Detector for 11 angles \j/ in the range and with in-plane specimen oscillation. On SSAtreated specimens, residual stresses could only be measured in the (3-phase, using the (310}p diffraction line. On the aged and shot peened specimen, measurements were also made in the a-phase, using the {123 3 ^-diffraction line. Relaxation kinetics were obtained on a single specimen after electropolishing 30-40jum from the shot peened surface. Residual stress profiles were not corrected. The x-ray elastic constant!/2 s^' *2 was estimated on the basis of monocrystalline stiffness moduli of a p-ti single crystal, see [8]. The effect of oxygen was taken into account by assuming that s^' V increases in the same way as the macroscopic E-modulus (Imam [5]) with increasing oxygen content. Averaging Voigt and Reuss-hypotheses (Bollenrath [9]) we obtained ~* MPa'i for Timetal 21s- and ^ rvqv for Timetal 21s+. Absolute values of residual stresses should be viewed with caution, since theoretical values of x-ray elastic constants were used.

4 344 Surface Treatment Results and discussion Selective hardening The short aging treatment after shot peening is intended to induce a precipitation reaction in the previously cold worked layers. To characterize the SSA-reaction, the microhardness difference between surface and core AHV0.05 was considered, plotted as a function of aging time for each aging temperature, see Fig. 1. The high initial degree of cold work present after shot peening has a weak effect on the surface hardness and on AHV0.05, see Fig. 1 at t=0. The slightly higher selective hardness as well as clearly broader {310}p diffraction lines of shot peened 21s- account for higher initial surface degrees of cold work that in 21s+. As highlighted in Fig. 1, a selective hardening response is possible even at temperatures as low as 350 C, its amount after a given time at a given temperature depending strongly on oxygen content. A high oxygen level apparently decreases the selective hardening reaction at a given temperature, which remains misunderstood, owing to insufficient information about oxygen influence on age hardening of highly cold worked materials. It is also difficult to definitively isolate the effect of oxygen, as the initial degree of cold work is lower in 21s+. Fig. 1 also illustrates the disadvantage of partially recrystallized material: carrying out SSA at an aging temperature above 450 C leads to rapid core age hardening and thus to a decrease of the selective hardening effect Figure 1. Selective hardening, AHV0.05, as a function of aging time.

5 Surface Treatment 345 Residual stress relaxation The shot peening treatment leads to high levels of compressive residual stresses with a maximum below the surface, Fig. 2. The higher absolute values calculated for 21s+ in comparison with 21s- appear reasonable in view of the solution strengthening effect of oxygen, see Table 2. Comparable results are obtained for the partially and fully recrystallized conditions. The amount of residual stress relaxation after aging is greater in the near surface region, probably because of the higher initial dislocation density. The maximum compressive residual stress seems thus to shift to greater depths as relaxation increases, Fig. 2. A complete assessment of residual stress relaxation was obtained by determining a^ urn below the surface and plotting G^/G^Q versus aging time t (G^: residual stress after t min at T C, G^%: initial residual stress). As partially and fully recrystallized conditions show no significant differences in behavior, the corresponding kinetics are not differentiated in Fig. 3. Relaxation takes place even at 350 C and is strongly dependent on oxygen content. The systematically lower residual stress decay observed for 21 s+ in comparison with 21s- at given aging conditions may be ascribed to the solution strengthening effect of oxygen as well as to the lower initial degree of cold work. Results are further analyzed by describing the time and temperature influence with a Zener- Wert-Avrami function, see Vohringer [3]: a^/(t%=exp{-[bxexp(-q/(rt)) x t]"}. (1) The relaxation behavior between 350 and 450 C of 21s- (R and PR) can be described by choosing m=0.5, log(b)=6.86 and Q=1.19 ev. For 21 s+ results can only be fit using a temperature-dependent m parameter, which denotes a possible temperature dependence of the relaxation mechanism which controls relaxation. Interactions between precipitation and dislocations movements controlling relaxation could be responsible for this unusual behavior Distance from the surface (^ Distance from the surface (um) Figure 2. Residual stress profiles after shot peening and two SSA treatments.

6 346 Surface Treatment 021s-350 C 021s-400 C 021s-450 C X21s-500 C 21s+350 C *21s+400 C 21s+450 C Figure 3. Thermal residual stress relaxation c 0.5 c ( 0.25 Interactions between precipitation and residual stress relaxation As highlighted in Fig. 1 and 3, surface age hardening and residual stress relaxation take place simultaneously during SSA: an increase in surface hardness must unfortunately be "paid for" with a corresponding residual stress decay, see Fig. 4. Another sign for the simultaneity of both mechanisms is the behavior of the full width at half maximum (FWHM) of {310}p diffraction lines during SSA, Fig. 5. As experimental conditions were kept constant, two phenomena could explain the observed systematic increase of FWHM during aging: a decrease in the size of coherent diffraction domains and an increase in microstresses. The precipitation in the p phase, decreasing the size of p diffraction domains, probably has an important influence on FWHM. The separation of the contributions of microstresses and domains by line profile analysis requires better quality data than for residual stress measurement, so that the behavior of microstresses during SSA remains unknown. 0 21s-350 C -*-21s+350 C 0-21s-400 C -+-21s+400 C -O- 21s-450 C -+-21s+450 C HV0.05 (at 30-40pim depth) Figure 4. G^/G*% as a function of surface inicrohardness Figure 5. Full width at half maximum of {310}p as a function of aging time.

7 Surface Treatment s+, A+SP, a-phase 21s+, A+SP, p-phase ~21s+, R SSA, p-phase T1-6A1-4V, a-phase Fit T1-6A1-4V, a-phase log(t) (min) Figure 6. Comparison of various relaxation kinetics at 450 C (T1-6A1-4V see Vohringer [10]). Rapid a-precipitation could be expected to retard residual stress relaxation. A direct comparison with relaxation behavior in a stable (3 Ti-alloy, i.e. without a- precipitation, is not possible, because such systems do not exist. To gain further information, a Timetal 21s+ specimen was shot peened after fully aging 12 h at 650 C (A+SP) and the residual stress relaxation measured in the a and (3-phases after aging at 450 C. It is reasonable to assume that the micro structure is stable in this case, i.e. a-precipitates neither form nor do they coarsen. In Fig. 6 corresponding results are plotted together with relaxation kinetics obtained at 450 C for SSA-treated 21s+ (p-residual stresses) and for shot peened Ti-6Al- 4V (a-residual stresses), see Vohringer [10]. Residual stresses are thermally more stable in previously fully aged Timetal 21s than in material which age hardens during the thermal treatment. The relaxation kinetics obtained are very similar to those of shot peened T1-6A1-4V, a feature which is confirmed by the comparable creep resistance of both alloys, see O'Connel [4]. Though accelerated aging reactions certainly result in a high a-fraction in the skin (see high hardness levels reached, Fig. 4), the typical relaxation behavior of a (a+p)-matrix is not observed. Conclusion After SSA treating the metastable (3-Ti alloy Timetal 21s, beneficial combinations of remaining compressive residual stresses and higher surface hardness can be obtained. The oxygen content influences not only the degree of cold work and the level of residual stresses obtained through shot peening, but also the degree of selective hardening and of relaxed residual stresses during subsequent aging. Though the precipitation reaction seems to begin very early in the cold worked surface layers, the thermal relaxation behavior of shot peened Timetal 21s is much faster than in a typical (a+p)-matrix. As precipitation and residual

8 348 Surface Treatment stress relaxation take place simultaneously, it seems reasonable to conclude that these processes are interdependent. Selectively surface aging to obtain a maximum selective hardening unfortunately leads to very low compressive residual stresses. A subsequent shot peening treatment, reintroducing residual stresses, could result in a novel and interesting improvement in fatigue behavior. Acknowledgements The authors would like to thank the Deutsche Forschungsgemeinschaft financial support through contract Gr. 974/2. for its References [1] Gregory, J.K., Miiller, C. & Wagner, L., Preferential Surface Aging: New Processes for the Improvement of the Fatigue Properties of Components Exposed to Mechanical Load (in German), Met all, 10, pp , [2] Wagner, C. & Gregory, J.K., Improve the Fatigue Life of Titanium Alloys- PartII,^^AMcg6/M^gn^ awfroc&m&s, 145, 50HH-50JJ, [3] Vohringer, O., Relaxation of Residual Stresses by Annealing or Mechanical Treatment, Advances in Surface Treatments, ed. A. Niku Lari, PergamonPr., Oxford, pp , [4] O'Connell, T., Timetal 21s, Section V, Materials Properties Handbook: Titanium, eds. R. Boyer, G. Welsch & E.W. Collings, ASM International, Materials Park, Ohio, pp , [5] Imam, M.A. & Feng, C.R., Effect of Oxygen on Transformation Kinetics in Timetal 21s Titanium Alloy, Advances in the Science and Technology oj Titanium Alloy Processing, eds. I. Weiss et al., The Minerals, Metals & Materials Society, pp , [6] Berger, M.C. & Gregory, J.K., Residual stress relaxation in shot peened Timetal 21s, M?f. &%. ^gg. /4, Vol. 263, pp , [7] Eigenmann, B. & Macherauch, E., X-Ray Investigation of Stress States in Materials (in German), Mat.-wiss. u. Werks toff tech., 26, pp , [8] Materials Properties Handbook: Titanium, eds. R. Boyer, G. Welsch & E.W. Collings, ASM International, Materials Park, Ohio, p. 99, [9] Bollenrath, F., Hauk, V.& Miiller, E.H., Calculation of the Poly crystalline Elastic Constants from the Elastic Constants of the Single Crystal (in German), Z MgW/Wg,58, pp , [10] Vohringer, O., Hirsch, T. & Macherauch, E., Relaxation of shot peening induced residual stresses of TiA16V4 by annealing or mechanical treatment, Proc. 5^. Int. Conf. Titanium, eds. G. Liitjering, U. Z wicker & W. Burk, Vol. 3, DGM, Oberursel, pp , 1985.