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Effects of warm peening on fatigue life and relaxation behaviour of residual stresses in AISI 4140 steel
Materials Science and Engineering A293 (2000) 191 – 197 www.elsevier.com/locate/msea
Effects of warm peening on fatigue life and relaxation behaviour of residual stresses in AISI 4140 steel A. Wick, V. Schulze *, O. Vo¨hringer Institut fu¨r Werkstoffkunde I, Uni6ersita¨t Karlsruhe (TH), Kaiserstr. 12, 76131 Karlsruhe, Germany Received 18 February 2000; received in revised form 19 May 2000
Abstract A new device has been built which allows shot peening in an air blast machine at elevated temperatures. The effects of conventional shot peening and peening at elevated temperatures on the characteristics of regions close to the surface, on the stability of residual stresses and half widths of X-ray interference lines and on the fatigue strength are presented for a quenched and tempered AISI 4140 steel (German grade 42CrMo4). The alternating bending strength is increased by warm peening compared with conventional shot peening. Additional investigations of samples conventionally peened and then annealed confirm that these effects are due to the stability of the dislocation structure, which is highly affected by strain ageing effects. This causes an additional benefit owing to higher stability of the residual stresses induced. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Warm peening; Fatigue strength; Residual stresses; Strain ageing; Steel
1. Introduction Shot peening is an often used surface treatment process applied to improve the fatigue strength and fatigue life of cyclically loaded components. This improvement is achieved by inducing compressive residual stresses and work hardening effects in areas close to the surface. Beside the magnitude of the residual stresses, their stability during quasistatic and cyclic loading is of great importance [1 – 4]. As the possible increase in the fatigue resistance by using conventional shot peening treatments is limited, it is necessary to modify the shot peening process. One possibility is to preload the component in tensile direction while shot peening, which causes higher compressive residual stresses in the longitudinal direction close to the surface [5 – 7]. An alternative way in the case of steels is to carry out the process at elevated temperatures, which has been studied in only a few investigations so far [8], none of them systematic. * Corresponding author. Tel.: +49-721-6082219; fax: + 49-7216088044. E-mail address: [email protected] (V. Schulze).
For that, an air blast system was supplemented with an air flow heater in order to perform warm peening (peening at elevated temperatures) [9–11]. The investigations were performed with flat samples made from AISI 4140 steel (German grade 42CrMo4) in a quenched and tempered condition. The characteristics of the regions close to the surface as well as their stability and the fatigue strength at alternating bending are compared for differently shot peened variants.
2. Material and specimen geometry The investigations were carried out at AISI 4140 steel (German grade 42CrMo4) with the chemical composition 0.40 C, 0.98 Cr, 0.17 Mo, 0.18 Si, 0.63 Mn, 0.01 P, 0.03 Al and the rest Fe (all in wt.%). The samples were machined from flat material and ground to a thickness of 2.2 mm. Thereafter the samples were austenitized for 20 min at 850°C, oil quenched down to 25°C, tempered at 450°C for 2 h and cooled in a vacuum furnace. After the heat treatment, the samples were ground to a thickness of 2.0 mm in order to eliminate distortions in the flatness of the specimens. After grinding, small compressive residual stresses with s rs B 100 N mm − 2
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 0 3 5 - 2
A. Wick et al. / Materials Science and Engineering A293 (2000) 191–197
192
Fig. 1. Specimen geometry.
were found close to the surface. The final geometry of the samples is shown in Fig. 1.
3. Experimental procedures The shot peening treatments were performed using an air blast machine, which is shown schematically in Fig. 2. For shot peening at elevated temperatures, a second air flow was heated up to a maximum of 500°C in an electric flow heater (FH). This air flow was mixed with the cold air and the peening media in a special nozzle [9–11]. This system allows a sample temperature while shot peening of Tpeen 5290°C. The samples were peened from both sides simultaneously in order to avoid distortions. Cast iron shot S 170 (mean diameter
0.43 mm) with hardness 56 HRC was used at a peening pressure of 1.2 bar and a media flow rate of 1.0 kg min − 1. The resulting coverage was about 98%. After conventional shot peening, several samples were annealed at 300°C for 20 min in air. In order to research the differences in the residual stress states and their stability, the residual stresses in longitudinal direction of the specimens were determined using the X-ray technique. The {211}-interference lines of the ferritic phase were determined at nine c-angles between −60° and +60° using CrKa-radiation and analyzed according to the sin2 c-method [12]. Neglecting the elastic anisotropy, a Young’s modulus E= 210 000 N mm − 2 and a Poisson’s ratio n=0.28 were used. The depth distribution of the residual stresses was determined by iterative electrolytic removal of thin
Fig. 2. Air blast system with supplemented stress peening device (SP) and air flow heater (FH).
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using 25–30 specimens for each S–N-curve. For all variants only the curves representing a failure probability of 50% according to the arcsin p method are given [14]. The sample geometry was changed by cutting off the dovetails of the samples in a way that 110 mm long specimens remained. Tests in order to determine the stability of the surface residual stresses by alternating bending at a fixed initial stress amplitude were all performed using a single specimen. The test was interrupted at predefined numbers of cycles, the residual stress values of the upper and the lower side were measured and averaged and then the test was restarted.
4. Experimental results
Fig. 3. Depth distribution of the residual stresses (a) and of the half widths (b) of differently shot peened states.
Fig. 4. S – N curves for ground and differently shot peened states.
surface layers and subsequent X-ray measurements. These residual stress values were corrected for changes in the residual stress state during the removal process according to the method by Moore and Evans [13]. The half width values indicated were determined as an average of those measured at c = −15, 0 and + 15°. They are a measure of micro residual stresses, work hardening and dislocation density, respectively. The alternating bending tests were carried out on alternating bending machines at a frequency of 25 Hz
The different shot peening methods — conventional shot peening, warm peening at Tpeen = 290°C and conventional shot peening plus annealing — cause clear differences in the surface roughness, the residual stress state and the micro deformation state of the samples, which is characterized by the half widths of the X-ray interference lines. The surface roughness Rz is increased from 8.1 to 11.3 mm by increasing the peening temperature from room temperature to 290°C. The residual stress distribution of the different variants are shown in Fig. 3(a), and the distribution of the half widths in these samples is given in Fig. 3(b). The conventionally peened sample shows compressive residual stresses of −610 N mm − 2 directly at the surface and the depth x0 where the residual stresses change their sign is also relatively low (x0 = 0.165 mm). These values are only slightly higher for the samples peened at Tpeen = 290°C. Annealing at 300°C for 20 min reduces the residual stress values in the region close to the surface. After annealing, a surface value of −473 N mm − 2 is measured, compared to − 610 N mm − 2 for the conventionally peened sample. The depth x0, however, is not significantly affected by the annealing treatment. The depth distribution of the half widths of the conventionally peened sample shows values increasing from 2.7° in 2u at the core region to 3.3° in 2u at the surface. For the variants peened at Tpeen = 290°C, the half width close to the surface region is much higher (HWs : 3.55° in 2u). By annealing, a small reduction of the half width close to the surface was determined. The S–N-curves of the conventionally peened and warm peened and the shot peened plus annealed state at alternating bending tests are compared to the S–Ncurve of the ground state in Fig. 4. Shot peening at room temperature increases the fatigue strength Rab for about 90 N mm − 2 from 440 to 530 N mm − 2 compared to the ground condition. Compared to that, peening at Tpeen = 290°C increases the fatigue strength for additional 20% to Rab = 640 N mm − 2. After conventionally shot peening and annealing, the fatigue strength de-
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creased compared to the variant conventionally shot peened to Rab = 500 N mm − 2 ( −6%). In order to get an explanation for the effect of the different peening treatments on the fatigue behaviour, the stability of the surface residual stresses was investigated in detail. For that purpose, specimens were loaded at different fictitious surface stress amplitudes 3005s*a,s 51000 N mm − 2 for different numbers of cycles up to failure or 107 cycles. The results of these investigations can be seen in Fig. 5 for conventionally peened samples (top), samples warm peened at 290°C (middle) and samples annealed after conventionally shot peening (bottom). For all three peening variants the residual stresses at the surface (left hand side) relax more rapidly with increasing loading amplitude during the first cycle as well as during further cycling. A linear correlation between residual stresses and the logarithm of the number of cycles N can be recognized for wide
intervals of N] 1. In the conventionally peened samples, however, the residual stresses relax more than in the warm peened and the annealed samples. Pronounced differences can be seen especially for N 51 where the residual stresses in the warm peened and the annealed samples show a higher stability than those conventionally peened. For N] 1, the residual stresses in the variant peened at room temperature relax also more than in the variant peened at Tpeen = 290°C and that conventionally peened and annealed afterwards. The resulting surface values of the half widths HWs related to their initial values HWs,0 are also shown in Fig. 5 (right hand side). The half widths of the conventionally peened samples relax for s*a,s ] 700 N mm − 2. With increasing loading amplitude, an increasing relaxation is observed. The relaxation of the micro residual stresses is much more pronounced for the samples peened at room temperature compared to the other two
Fig. 5. Residual stresses (a, c, e) and half widths (b, d, f) versus number of cycles for different stress amplitudes for the conventionally shot peened, the warm peened (290°C) and the shot peened and annealed state.
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5. Discussion A comparison of all peening states needs a discussion of the stability of residual stresses for the states exposed to temperature during or after shot peening compared to the conventionally shot peened variant. The relaxation of the residual stresses can be subdivided into a quasistatic (N5 1) and a cyclic (N] 1) phase [1]. During the first phase, the residual stresses are not changed while the applied load at the surface s*s is smaller than a critical load s*s,crit. The critical load can be found by plotting the residual stress values after the first cycle versus the applied load s*s . This is given for the samples conventionally peened in Fig. 6(a), for those warm peened in Fig. 6(b) and for those annealed in Fig. 6(c). The data points at N=1 show that the critical load at which quasistatic residual stress relaxation begins, is much smaller for the conventionally peened variant (s*s,crit = 310 N mm − 2) than for the variant peened at 290°C (s*s,crit = 500 N mm − 2) and for the variant annealed after shot peening (s*s,crit = 690 N mm − 2). Using the critical load and the initial residual stress value at the surface, the compressive yield strength of the peened surface region Re(c),s can be estimated. Since shot peening generates a biaxial residual stress state with nearly identical components in longitudinal and transverse direction, Re(c),s can be calculated using the v. Mises hypothesis [15]: 2 rs 2 rs rs Re(c),s = (s rs s + s* s,crit) + (s s ) − (s s + s* s,crit)s s
Fig. 6. Residual stresses versus loading stresses or stress amplitudes for the conventionally shot peened (a), the warm peened (290°C) (b) and the shot peened and annealed state (c).
peening variants. For the warm peened and the annealed variant, the measured values after cycling are often slightly higher than the initial values. Only for s*a,s ]900 N mm − 2 and N ] 104 a significant decrease can be seen.
(1)
Applying Eq. (1) for the three variants investigated, clear differences of Re(c),s can be recognized (Table 1). For all variants, however, the resulting compressive yield strength at the surface is smaller than that of the core region, which was found to be 1300 N mm − 2 in compression tests [16]. This is due to the Bauschingereffect, which is quite distinct in quenched and tempered states of steels [17]. However, the work softening is much smaller for the warm peened samples than for the samples peened at room temperature. This is due to dynamic and static strain ageing effects occurring during and after warm peening. During the annealing of the conventionally peened samples, static strain ageing also occurs. Dynamic strain ageing results in a more diffuse and stable dislocation structure caused by pinning of dislocations by soluted carbon atoms and formation of fine carbides. The residual stress relaxation during cyclic loading due to cyclic creep is characterized by a linear reduction of residual stresses with increasing logarithm of the number of cycles for 15N5 104. Therefore, the residual stress values at the surface after 104 cycles can be taken as a measure of the degree of cyclic residual stress relaxation. These values are shown in Fig. 6 for the three different variants versus the magnitude of the applied loading amplitude at the surface s*a,s. The linear
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fit of these points intersects the line fitted through the residual stresses measured at N =1 at the critical loading amplitude s*a,crit, which is a measure of the onset of cyclic residual stress relaxation. By using the residual stresses at the surface after N = 1 and the assumption that the residual stress state at the surface is still almost axisymmetric, the cyclic yield strength at the surface R cycl e,s can be calculated similar to Eq. (1). These values are listed in Table 2. It can be seen that R cycl e,s for the conventionally peened samples is much smaller than for the warm peened state and for the samples annealed after shot peening. Moreover, the ratio of the calculated cyclic yield strength at the surface and that for the core region R cycl found in push – pull tests [18] indicates that the e cyclic work-softening typical for quenched and tempered material states does not appear in the warm peened cycl variant (R cycl =1.07) and is significantly reduced e,s /R e by the annealing treatment after shot peening (R cycl e,s / =0.92). The increase of R cycl R cycl e e,s in the warm peened samples and the samples annealed after conventional shot peening is assumed to be the result of a very strong pinning of dislocations by clouds of carbon atoms and fine carbides which exist due to strain ageing effects. This pinning is so strong that even at highest loading amplitudes s*a,s, the pinned dislocations cannot move. Therefore, dislocations that were newly generated during cyclic loading cause the residual stress relaxation. The increase of the half widths for these variants during alternating bending tests (Fig. 5) is a further hint for this assumption. With this discussion the effects of the different peening treatments on the fatigue strength can be interpreted. Compared to the conventionally peened condition, the significantly higher fatigue strength of the warm peened variant cannot only be explained by the slightly higher compressive residual stresses. Moreover, their higher stability shown before seems to be the main reason for this behaviour. The variant annealed after shot peening has a smaller fatigue strength than the conventionally
peened variant. This is due to the fact that annealing causes a marked residual stress reduction (Fig. 3(a)). The remaining compressive residual stresses and the dislocation structure are very stable due to static strain ageing effects occurring during annealing. But they cannot improve the fatigue strength as the residual stresses are lower compared to the conventionally peened state. This is in contrast to literature, where it was shown that annealing after shot peening can improve not only the residual stress stability but also the fatigue behaviour, when the annealing parameters are optimized [19]. This may be due to the fact that in Ref. [19] materials and material states like recrystallized steel SAE 1045, austenitic stainless steel AISI 304 and magnesium wrought alloy AZ 31 are used, which show severe relaxation of residual stresses during cyclic loading after conventional peening processes. Therefore in these states the fatigue life improvement is almost fully due to work hardening of the surface regions. After annealing, when the macro residual stresses may be reduced but the micro residual stresses due to work hardening may remain at high values, the strain ageing effects may cause improved fatigue lives in these materials. At high strength steels however, it is necessary to get high and stabile residual stresses which seems to be achievable by warm peening treatments as shown before.
6. Conclusions Shot peening at elevated temperatures increases the fatigue strength of samples made of quenched and tempered AISI 4140 compared to conventional shot peening. As the warm peened samples have only slightly higher compressive residual stresses than the conventionally peened ones, this is caused by the high stability of the residual stresses and half widths after peening at elevated temperatures. They are due to strain ageing processes during and after warm peening,
Table 1 Quasistatic surface yield strength in compression s*s,crit (N mm−2) Tpeen =20°C Tpeen =290°C Tpeen =20°C+300°C, 20 min
310 500 690
−2 s rs ) s (N mm
−600 −660 −480
Re(c),s (N mm−2) 801 1008 1019
Re(c),s/Re(c) 0.6 0.78 0.78
Table 2 Cyclic surface yield strength for differently shot peened variants s*s,crit (N mm−2) Tpeen =20°C Tpeen =290°C Tpeen =20°C+300°C, 20 min
514 714 690
−2 s rs ) s (N mm
−520 −620 −455
−2 R cycl ) e,s (N mm
895 1156 1000
cycl R cycl e,s /R e
0.82 1.07 0.92
A. Wick et al. / Materials Science and Engineering A293 (2000) 191–197
which lead to a more diffuse dislocation structure and a pinning of dislocations by solute carbon atoms and/or fine carbides. Annealing after conventional shot peening reduces the fatigue strength because the residual stresses are decreased and their increased stability during cyclic loading is then of minor influence. Shot peening at elevated temperatures seems to be the most appropriate treatment to increase the fatigue strength of high strength steels.
References [1] H. Holzapfel, V. Schulze, O. Vo¨hringer, E. Macherauch, Mater. Sci. Eng. A248 (1998) 9–18. [2] O. Vo¨hringer, in: E. Macherauch, V. Hauk (Eds.), Eigenspannungen, Entstehung-Messung-Bewertung, DGM, Oberursel, Germany, 1983, pp. 49–83. [3] H. Hanagarth, O. Vo¨hringer, E. Macherauch, in: K. Iida (Ed.), Proc. ICSP 4, Tokyo, 1990, pp. 337–345.
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[4] V. Schulze, K.-H. Lang, O. Vo¨hringer, E. Macherauch, in: J. Champaigne (Ed.), Proc. ICSP 6, San Francisco, CA, 1996, pp. 403 – 412. [5] J.C. Straub, D. May, The Iron Age, 1949, pp. 66 –70. [6] R. Zeller, Materialpru¨f. 35 (1993) 218 – 221. [7] F. Engelmohr, B. Fiedler, Mod. Fert. Technol., 1991, pp. 77–91. [8] M. Schilling-Praetzel, F. Hegemann, G. Gottstein, in: D. Kirk (Ed.), Proc. ICSP 5, Oxford, 1993, pp. 227 – 238. [9] A. Wick, V. Schulze, O. Vo¨hringer, Mat.-wiss. Werkstofftech. 30 (1999) 269 – 273. [10] A. Wick, V. Schulze, O. Vo¨hringer, in: Nakonieczny (Ed.), Proc. ICSP 7, Warsaw, 1999, pp. 110 – 116. [11] A. Wick, V. Schulze, O. Vo¨hringer, in: Nakonieczny (Ed.), Proc. ICSP 7, Warsaw, 1999, pp. 102 – 109. [12] E. Macherauch, P. Mu¨ller, Z. Angew. Phys. 13 (1961) 305–312. [13] M.G. Moore, W.P. Evans, SAE Trans. 66 (1958) 340–345. [14] D. Dengel, Z. Werkstofftechn. 8 (1975) 253 – 261. [15] R. v. Mises, Z. Angew. Math. Mech. 8 (1928) 161 –185. [16] H. Holzapfel, Dr.-Ing. Thesis, Universita¨t Karlsruhe (TH), 1994. [17] B. Scholtes, O. Vo¨hringer, Ha¨rterei-Techn.-Mitt. 41 (1986) 347– 354. [18] D. Eifler, Dr.-Ing. Thesis, Universita¨t Karlsruhe (TH), 1981. [19] I. Altenberger, B. Scholtes, Scripta Mater. 41 (1999) 873–881.
Effects of warm peening on fatigue life and relaxation behaviour of residual stresses in AISI 4140 steel A. Wick, V. Schulze *, O. Vo¨hringer Institut fu¨r Werkstoffkunde I, Uni6ersita¨t Karlsruhe (TH), Kaiserstr. 12, 76131 Karlsruhe, Germany Received 18 February 2000; received in revised form 19 May 2000
Abstract A new device has been built which allows shot peening in an air blast machine at elevated temperatures. The effects of conventional shot peening and peening at elevated temperatures on the characteristics of regions close to the surface, on the stability of residual stresses and half widths of X-ray interference lines and on the fatigue strength are presented for a quenched and tempered AISI 4140 steel (German grade 42CrMo4). The alternating bending strength is increased by warm peening compared with conventional shot peening. Additional investigations of samples conventionally peened and then annealed confirm that these effects are due to the stability of the dislocation structure, which is highly affected by strain ageing effects. This causes an additional benefit owing to higher stability of the residual stresses induced. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Warm peening; Fatigue strength; Residual stresses; Strain ageing; Steel
1. Introduction Shot peening is an often used surface treatment process applied to improve the fatigue strength and fatigue life of cyclically loaded components. This improvement is achieved by inducing compressive residual stresses and work hardening effects in areas close to the surface. Beside the magnitude of the residual stresses, their stability during quasistatic and cyclic loading is of great importance [1 – 4]. As the possible increase in the fatigue resistance by using conventional shot peening treatments is limited, it is necessary to modify the shot peening process. One possibility is to preload the component in tensile direction while shot peening, which causes higher compressive residual stresses in the longitudinal direction close to the surface [5 – 7]. An alternative way in the case of steels is to carry out the process at elevated temperatures, which has been studied in only a few investigations so far [8], none of them systematic. * Corresponding author. Tel.: +49-721-6082219; fax: + 49-7216088044. E-mail address: [email protected] (V. Schulze).
For that, an air blast system was supplemented with an air flow heater in order to perform warm peening (peening at elevated temperatures) [9–11]. The investigations were performed with flat samples made from AISI 4140 steel (German grade 42CrMo4) in a quenched and tempered condition. The characteristics of the regions close to the surface as well as their stability and the fatigue strength at alternating bending are compared for differently shot peened variants.
2. Material and specimen geometry The investigations were carried out at AISI 4140 steel (German grade 42CrMo4) with the chemical composition 0.40 C, 0.98 Cr, 0.17 Mo, 0.18 Si, 0.63 Mn, 0.01 P, 0.03 Al and the rest Fe (all in wt.%). The samples were machined from flat material and ground to a thickness of 2.2 mm. Thereafter the samples were austenitized for 20 min at 850°C, oil quenched down to 25°C, tempered at 450°C for 2 h and cooled in a vacuum furnace. After the heat treatment, the samples were ground to a thickness of 2.0 mm in order to eliminate distortions in the flatness of the specimens. After grinding, small compressive residual stresses with s rs B 100 N mm − 2
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 0 3 5 - 2
A. Wick et al. / Materials Science and Engineering A293 (2000) 191–197
192
Fig. 1. Specimen geometry.
were found close to the surface. The final geometry of the samples is shown in Fig. 1.
3. Experimental procedures The shot peening treatments were performed using an air blast machine, which is shown schematically in Fig. 2. For shot peening at elevated temperatures, a second air flow was heated up to a maximum of 500°C in an electric flow heater (FH). This air flow was mixed with the cold air and the peening media in a special nozzle [9–11]. This system allows a sample temperature while shot peening of Tpeen 5290°C. The samples were peened from both sides simultaneously in order to avoid distortions. Cast iron shot S 170 (mean diameter
0.43 mm) with hardness 56 HRC was used at a peening pressure of 1.2 bar and a media flow rate of 1.0 kg min − 1. The resulting coverage was about 98%. After conventional shot peening, several samples were annealed at 300°C for 20 min in air. In order to research the differences in the residual stress states and their stability, the residual stresses in longitudinal direction of the specimens were determined using the X-ray technique. The {211}-interference lines of the ferritic phase were determined at nine c-angles between −60° and +60° using CrKa-radiation and analyzed according to the sin2 c-method [12]. Neglecting the elastic anisotropy, a Young’s modulus E= 210 000 N mm − 2 and a Poisson’s ratio n=0.28 were used. The depth distribution of the residual stresses was determined by iterative electrolytic removal of thin
Fig. 2. Air blast system with supplemented stress peening device (SP) and air flow heater (FH).
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193
using 25–30 specimens for each S–N-curve. For all variants only the curves representing a failure probability of 50% according to the arcsin p method are given [14]. The sample geometry was changed by cutting off the dovetails of the samples in a way that 110 mm long specimens remained. Tests in order to determine the stability of the surface residual stresses by alternating bending at a fixed initial stress amplitude were all performed using a single specimen. The test was interrupted at predefined numbers of cycles, the residual stress values of the upper and the lower side were measured and averaged and then the test was restarted.
4. Experimental results
Fig. 3. Depth distribution of the residual stresses (a) and of the half widths (b) of differently shot peened states.
Fig. 4. S – N curves for ground and differently shot peened states.
surface layers and subsequent X-ray measurements. These residual stress values were corrected for changes in the residual stress state during the removal process according to the method by Moore and Evans [13]. The half width values indicated were determined as an average of those measured at c = −15, 0 and + 15°. They are a measure of micro residual stresses, work hardening and dislocation density, respectively. The alternating bending tests were carried out on alternating bending machines at a frequency of 25 Hz
The different shot peening methods — conventional shot peening, warm peening at Tpeen = 290°C and conventional shot peening plus annealing — cause clear differences in the surface roughness, the residual stress state and the micro deformation state of the samples, which is characterized by the half widths of the X-ray interference lines. The surface roughness Rz is increased from 8.1 to 11.3 mm by increasing the peening temperature from room temperature to 290°C. The residual stress distribution of the different variants are shown in Fig. 3(a), and the distribution of the half widths in these samples is given in Fig. 3(b). The conventionally peened sample shows compressive residual stresses of −610 N mm − 2 directly at the surface and the depth x0 where the residual stresses change their sign is also relatively low (x0 = 0.165 mm). These values are only slightly higher for the samples peened at Tpeen = 290°C. Annealing at 300°C for 20 min reduces the residual stress values in the region close to the surface. After annealing, a surface value of −473 N mm − 2 is measured, compared to − 610 N mm − 2 for the conventionally peened sample. The depth x0, however, is not significantly affected by the annealing treatment. The depth distribution of the half widths of the conventionally peened sample shows values increasing from 2.7° in 2u at the core region to 3.3° in 2u at the surface. For the variants peened at Tpeen = 290°C, the half width close to the surface region is much higher (HWs : 3.55° in 2u). By annealing, a small reduction of the half width close to the surface was determined. The S–N-curves of the conventionally peened and warm peened and the shot peened plus annealed state at alternating bending tests are compared to the S–Ncurve of the ground state in Fig. 4. Shot peening at room temperature increases the fatigue strength Rab for about 90 N mm − 2 from 440 to 530 N mm − 2 compared to the ground condition. Compared to that, peening at Tpeen = 290°C increases the fatigue strength for additional 20% to Rab = 640 N mm − 2. After conventionally shot peening and annealing, the fatigue strength de-
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creased compared to the variant conventionally shot peened to Rab = 500 N mm − 2 ( −6%). In order to get an explanation for the effect of the different peening treatments on the fatigue behaviour, the stability of the surface residual stresses was investigated in detail. For that purpose, specimens were loaded at different fictitious surface stress amplitudes 3005s*a,s 51000 N mm − 2 for different numbers of cycles up to failure or 107 cycles. The results of these investigations can be seen in Fig. 5 for conventionally peened samples (top), samples warm peened at 290°C (middle) and samples annealed after conventionally shot peening (bottom). For all three peening variants the residual stresses at the surface (left hand side) relax more rapidly with increasing loading amplitude during the first cycle as well as during further cycling. A linear correlation between residual stresses and the logarithm of the number of cycles N can be recognized for wide
intervals of N] 1. In the conventionally peened samples, however, the residual stresses relax more than in the warm peened and the annealed samples. Pronounced differences can be seen especially for N 51 where the residual stresses in the warm peened and the annealed samples show a higher stability than those conventionally peened. For N] 1, the residual stresses in the variant peened at room temperature relax also more than in the variant peened at Tpeen = 290°C and that conventionally peened and annealed afterwards. The resulting surface values of the half widths HWs related to their initial values HWs,0 are also shown in Fig. 5 (right hand side). The half widths of the conventionally peened samples relax for s*a,s ] 700 N mm − 2. With increasing loading amplitude, an increasing relaxation is observed. The relaxation of the micro residual stresses is much more pronounced for the samples peened at room temperature compared to the other two
Fig. 5. Residual stresses (a, c, e) and half widths (b, d, f) versus number of cycles for different stress amplitudes for the conventionally shot peened, the warm peened (290°C) and the shot peened and annealed state.
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5. Discussion A comparison of all peening states needs a discussion of the stability of residual stresses for the states exposed to temperature during or after shot peening compared to the conventionally shot peened variant. The relaxation of the residual stresses can be subdivided into a quasistatic (N5 1) and a cyclic (N] 1) phase [1]. During the first phase, the residual stresses are not changed while the applied load at the surface s*s is smaller than a critical load s*s,crit. The critical load can be found by plotting the residual stress values after the first cycle versus the applied load s*s . This is given for the samples conventionally peened in Fig. 6(a), for those warm peened in Fig. 6(b) and for those annealed in Fig. 6(c). The data points at N=1 show that the critical load at which quasistatic residual stress relaxation begins, is much smaller for the conventionally peened variant (s*s,crit = 310 N mm − 2) than for the variant peened at 290°C (s*s,crit = 500 N mm − 2) and for the variant annealed after shot peening (s*s,crit = 690 N mm − 2). Using the critical load and the initial residual stress value at the surface, the compressive yield strength of the peened surface region Re(c),s can be estimated. Since shot peening generates a biaxial residual stress state with nearly identical components in longitudinal and transverse direction, Re(c),s can be calculated using the v. Mises hypothesis [15]: 2 rs 2 rs rs Re(c),s = (s rs s + s* s,crit) + (s s ) − (s s + s* s,crit)s s
Fig. 6. Residual stresses versus loading stresses or stress amplitudes for the conventionally shot peened (a), the warm peened (290°C) (b) and the shot peened and annealed state (c).
peening variants. For the warm peened and the annealed variant, the measured values after cycling are often slightly higher than the initial values. Only for s*a,s ]900 N mm − 2 and N ] 104 a significant decrease can be seen.
(1)
Applying Eq. (1) for the three variants investigated, clear differences of Re(c),s can be recognized (Table 1). For all variants, however, the resulting compressive yield strength at the surface is smaller than that of the core region, which was found to be 1300 N mm − 2 in compression tests [16]. This is due to the Bauschingereffect, which is quite distinct in quenched and tempered states of steels [17]. However, the work softening is much smaller for the warm peened samples than for the samples peened at room temperature. This is due to dynamic and static strain ageing effects occurring during and after warm peening. During the annealing of the conventionally peened samples, static strain ageing also occurs. Dynamic strain ageing results in a more diffuse and stable dislocation structure caused by pinning of dislocations by soluted carbon atoms and formation of fine carbides. The residual stress relaxation during cyclic loading due to cyclic creep is characterized by a linear reduction of residual stresses with increasing logarithm of the number of cycles for 15N5 104. Therefore, the residual stress values at the surface after 104 cycles can be taken as a measure of the degree of cyclic residual stress relaxation. These values are shown in Fig. 6 for the three different variants versus the magnitude of the applied loading amplitude at the surface s*a,s. The linear
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fit of these points intersects the line fitted through the residual stresses measured at N =1 at the critical loading amplitude s*a,crit, which is a measure of the onset of cyclic residual stress relaxation. By using the residual stresses at the surface after N = 1 and the assumption that the residual stress state at the surface is still almost axisymmetric, the cyclic yield strength at the surface R cycl e,s can be calculated similar to Eq. (1). These values are listed in Table 2. It can be seen that R cycl e,s for the conventionally peened samples is much smaller than for the warm peened state and for the samples annealed after shot peening. Moreover, the ratio of the calculated cyclic yield strength at the surface and that for the core region R cycl found in push – pull tests [18] indicates that the e cyclic work-softening typical for quenched and tempered material states does not appear in the warm peened cycl variant (R cycl =1.07) and is significantly reduced e,s /R e by the annealing treatment after shot peening (R cycl e,s / =0.92). The increase of R cycl R cycl e e,s in the warm peened samples and the samples annealed after conventional shot peening is assumed to be the result of a very strong pinning of dislocations by clouds of carbon atoms and fine carbides which exist due to strain ageing effects. This pinning is so strong that even at highest loading amplitudes s*a,s, the pinned dislocations cannot move. Therefore, dislocations that were newly generated during cyclic loading cause the residual stress relaxation. The increase of the half widths for these variants during alternating bending tests (Fig. 5) is a further hint for this assumption. With this discussion the effects of the different peening treatments on the fatigue strength can be interpreted. Compared to the conventionally peened condition, the significantly higher fatigue strength of the warm peened variant cannot only be explained by the slightly higher compressive residual stresses. Moreover, their higher stability shown before seems to be the main reason for this behaviour. The variant annealed after shot peening has a smaller fatigue strength than the conventionally
peened variant. This is due to the fact that annealing causes a marked residual stress reduction (Fig. 3(a)). The remaining compressive residual stresses and the dislocation structure are very stable due to static strain ageing effects occurring during annealing. But they cannot improve the fatigue strength as the residual stresses are lower compared to the conventionally peened state. This is in contrast to literature, where it was shown that annealing after shot peening can improve not only the residual stress stability but also the fatigue behaviour, when the annealing parameters are optimized [19]. This may be due to the fact that in Ref. [19] materials and material states like recrystallized steel SAE 1045, austenitic stainless steel AISI 304 and magnesium wrought alloy AZ 31 are used, which show severe relaxation of residual stresses during cyclic loading after conventional peening processes. Therefore in these states the fatigue life improvement is almost fully due to work hardening of the surface regions. After annealing, when the macro residual stresses may be reduced but the micro residual stresses due to work hardening may remain at high values, the strain ageing effects may cause improved fatigue lives in these materials. At high strength steels however, it is necessary to get high and stabile residual stresses which seems to be achievable by warm peening treatments as shown before.
6. Conclusions Shot peening at elevated temperatures increases the fatigue strength of samples made of quenched and tempered AISI 4140 compared to conventional shot peening. As the warm peened samples have only slightly higher compressive residual stresses than the conventionally peened ones, this is caused by the high stability of the residual stresses and half widths after peening at elevated temperatures. They are due to strain ageing processes during and after warm peening,
Table 1 Quasistatic surface yield strength in compression s*s,crit (N mm−2) Tpeen =20°C Tpeen =290°C Tpeen =20°C+300°C, 20 min
310 500 690
−2 s rs ) s (N mm
−600 −660 −480
Re(c),s (N mm−2) 801 1008 1019
Re(c),s/Re(c) 0.6 0.78 0.78
Table 2 Cyclic surface yield strength for differently shot peened variants s*s,crit (N mm−2) Tpeen =20°C Tpeen =290°C Tpeen =20°C+300°C, 20 min
514 714 690
−2 s rs ) s (N mm
−520 −620 −455
−2 R cycl ) e,s (N mm
895 1156 1000
cycl R cycl e,s /R e
0.82 1.07 0.92
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which lead to a more diffuse dislocation structure and a pinning of dislocations by solute carbon atoms and/or fine carbides. Annealing after conventional shot peening reduces the fatigue strength because the residual stresses are decreased and their increased stability during cyclic loading is then of minor influence. Shot peening at elevated temperatures seems to be the most appropriate treatment to increase the fatigue strength of high strength steels.
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