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Vol. 4
May Issue
Year 2003
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Science Update


in Vol. 4 - May Issue - Year 2003
Warm Peening - A Possibility to Induce Stabilized Compressive Residual Stresses



Author Dr.-Ing. Volker Schulze (Ph.D.)


Fig. 1: Distributions of residual stresses (left) and half widths (right) after different shot peening treatments.


Fig. 2: Influence of the number of cycles at a,s = 1000 N/mm2 on residual stresses (left) and half widths (right) vs. peening temperature


Fig. 3: W

Abstract: Warm peening is a new and not yet established modification of conventional shot peening treatments. It yields an enormous stabilisation of the compressive residual stresses induced and therefore allows a significant increase of the fatigue limit compared to conventional peening. Both effects are presented and discussed in the following paper. Keywords: warm peening, strain aging, stability of residual stresses, fatigue

Introduction

The positive effects of well established shot peening treatments on the fatigue limit are caused by precisely adjusting compressive residual stress and work-hardening states. Besides the size and the depth of the residual stresses their stability is of decisive importance. This can be impressively improved at steels by peening at temperatures between 170 and 350 °C instead of room temperature. This allows dynamic strain aging effects coming up during peening which lead to a more diffuse dislocation structure and a higher dislocation density. Additionally, static strain aging effects come up during cooling down to room temperature which pin the dislocations to their positions due to the formation of clouds of interstitially soluted carbon atoms or finest carbides. Due to these effects the dislocations are hindered in their motion and therefore the residual stress state is stabilized and fatigue effects are delayed [1]. In the following text a comparison of warm peening and conventional peening is given exemplarily at the quenched and tempered German steel 42CrMo4 (AISI 4140).

Material and Experimental Details

The investigations were performed on 2 mm thick flat specimens made of the German steel 42CrMo4 (AISI 4140), which was in a heat treatment state quenched and tempered at 450 °C showing a hardness of 430 HV. The shot peening treatments were all performed in an air blast device (type Baiker) with additional self-made devices for warm peening [2]. Cast steel shot S170 was used at a media flow rate of 1.0 kg/min and a peening pressure of 1.2 bar. At a 100 % coverage Almen intensities between 0.24 and 0.28 mmA were reached. While the residual stresses were measured using X-rays according to the sin2?-method [3], the fatigue properties were analysed at alternating bending tests at a frequency of 25 Hz using 25 - 30 tests per Wöhler curve. The fatigue limits were determined according to [4].

Experimental Results

The depth distributions of residual stresses and half widths (as a measure of the work-hardening state) given in Fig. 1 show that the compressive residual stresses at the surface, the depth of the compressively pre-stressed layer and the work-hardening at the surface are increased by warm peening at 310°C. Besides this, the roughness is increased from 8.1 µm after conventional peening to 11.3 µm after warm peening [5].

Fig. 2. shows the residual stresses and the half widths remaining after loading with a stress amplitude at the surface of 1000 N/mm2 for different numbers of cycles as a function of the peening temperature. This shows first that the values of compressive residual stresses and half widths directly after peening show maxima at peening temperatures of 290 or 310°C. During cyclic loading the half widths of the conventionally peened state (Tpeen = 20°C) are decreasing severely, whereas all warm peened states show no significant changes. The residual stress values after cyclic loading are also increased compared to the conventionally peened state. This effect is the most pronounced at 310°C, where not only the highest initial residual stresses but also the highest stabilization effects are achieved. This is caused by the diffuse and pinned dislocation structure originating form the dynamic and static strain aging effects mentioned above.
Due to the dislocation structure and the stability of the residual stresses warm peening at 310°C leads to severe improvements of the alternating bending strength Rab, which can be seen in Fig. 3. Compared to conventional peening an increase of 37 % could be observed which shows the high potential of warm peening for further increases of the fatigue properties [1].

Summary

The surface state, stability of residual stresses and the fatigue properties of quenched and tempered 42CrMo4 were analysed after conventional peening and warm peening at flat specimens. Dynamic and static strain aging effects lead to stabilized residual stresses and substantial increases of the alternating bending strength after warm peening compared to conventional peening.

The author expresses his thanks to Dr. R. Menig for performing the experiments and acknowledges the financial support by the Deutsche For­schungsgemeinschaft (DFG, German Research Agency) for the investigations presented.

Literature

[1]  R. Menig, V. Schulze, O. Vöhringer: Mat. Sci. Eng. A 335 (2002), 198-206.
[2] A. Wick, V. Schulze, O. Vöhringer: Materialwissenschaft und Werkstofftechnik, 30 (1999), 269-273.
[3] E. Macherauch, P. Müller: Z. angew. Physik, 13 (1961), 305-312.
[4] D. Dengel: Zeitschrift für Werkstofftechnik, 8 (1975), 253-261.
[5] A. Wick, V. Schulze, O. Vöhringer: Mat. Sci. Eng. A 293 (2000), 191-197.




Author: Dr.-Ing. Volker Schulze (Ph.D.)
For information:
Dr.-Ing. Volker Schulze (Ph.D)
Institut für Werkstoffkunde I
Universität Karlsruhe (TH), Kaiserstr. 12
76128 Karlsruhe, Germany
Tel: +49.721.608.2219
E-Mail: volker.schulze@mach.uni-karlsruhe.de
www.uni-karlsruhe.de/~iwk1