Investigation on the Mechanisms Improving Fatigue Strength in

Author:  R. Weingartner, J. Hoffmeister and V. Schulze
Source:  Conf Proc 2014: ICSP-12 Goslar, Germany (pgs.31-36)
Doc ID:  2014046
Year of Publication:  2014
Abstract:  
Micropeening differs from shot peening by smaller blasting shots. From this, low penetration depths and high velocities result. Micropeening offers many prospects for mechanical surface treatment and optimization of surface properties. Besides high surface quality, high compressive residual stresses and strong work hardening near the surface, also grain refinement up to nanocrystalline sizes can be achieved. These properties affect the mechanical properties, particularly the fatigue limit of processed surfaces, in a complex manner. Despite the very low penetration depth in comparison to conventional shot peening, an unexpectedly high increase of fatigue limit is found. Therefore the correlations describing conventional shot peening cannot be transfered to micropeening in unrestrained manner. Especially the effect of grain sizes is of interest for estimating the reinforcing mechanisms achieved by surface treatment, for example by the concept of local fatigue strength. For an optimum improvement of fatigue properties of steel the achievable surface properties are explored. In particular, changes of the microstructure are of great interest. For this purpose, the depth distribution of compressive residual stresses, cold work and grain sizes have been studied, taking account of the peening conditions. The methods are mainly based on X-ray measurements and Focussed-Ion-Beam microscopy. In addition, the surface topography has been observed by scanning electron microscopy and the roughness has been measured by confocal microscopy. Finally in bending fatigue tests the fatigue strength has been determined in relation to the adjusted surface properties. The observed effects are described and discussed with the aim to elaborate the relations between the surface characteristics and the fatigue limit. Thus, it is shown, that micropeening is an effective tool for improving fatigue properties.