Macroscopic and microscopic evolutions of a shot-peened layer during isothermal recovery

  • PDF / 1,211,681 Bytes
  • 12 Pages / 612 x 792 pts (letter) Page_size
  • 33 Downloads / 147 Views

DOWNLOAD

REPORT


INTRODUCTION

RESIDUAL stresses are powerful means to improve the service life of many mechanical parts because they generally have some beneficial effects on the fatigue strength and on the corrosion resistance. Nowadays, more and more mechanical manufacturers consider the residual stress state as a quality parameter to be controlled. Whatever the type of treatment, mechanical (shot peening, rolling, or machining), thermochemical (nitriding or carburizing), or thermal (quenching), residual stresses are always produced by strain misfits or local material behavior differences. In each case, the exposure of treated parts in the recovery temperature range activates some microstructural mechanisms that lower the internal energy stored in the affected layers. This study aims at determining the macroscopic and microscopic changes of a work-hardened layer during its recovery that will constitute the basis of a future physical model. At the macroscopic scale, the consequent evolutions of the internal strains lower the residual stresses and their expected beneficial effects. All the existing models are based on a phenomenological approach of the thermal relaxation of residual stresses. Very often, this usual term of stress relaxation is not strictly rigorous because the total strain tensors vary in the normal direction to the treated surface. An accurate modeling must quantify the strain changes, which are a state variable of the material. The consequent evolution of the residual stress field is given by the mechanical equilibrium of the studied structure. Several experimental techniques will be used to study the residual stress evolutions and the possible dimensional changes. Most of the experiments will be carried out on a shot-peened layer. Their I. LILLAMAND and L. BARRALLIER, Doctors, and J.M. SPRAUEL, Professor, are with the MecaSurf Laboratory, ENSAM, 13617 Aix-enProvence, Cedex 1, France. R. CHIRON, Engineer, is with the PMTM Laboratory, CNRS, Paris 13 University, Villetaneuse, France. Manuscript submitted August 10, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A

analysis will allow them to determine a recovery strain tensor never discussed before. At the microscopic scale, no experimental results are available in the existing articles dealing with the recovery of the shot-peened layer. This phenomenon is often associated with the changes of crystal defect density, mainly dislocations, owing to the thermal energy contribution. Transmission electron microscopy (TEM) investigations do not allow the observation of mobile defects that are often recovered during plate preparation. The modified Warren–Averbach’s method that is summarized in this article is based on X-ray peak broadening analysis. Applied to a shot-peened layer, this method will provide the microstrural mechanisms that occur during the shot-peening process and a subsequent recovery of the studied steel. II. MATERIAL AND TREATMENTS All specimens were machined from the same rolled bar of 38Cr4Mo low alloy steel (french standard). Its chemical composition a