Papers by Author: Herve Bonnefoy

Abstract: Synchrotron X-ray diffraction is reliable to measure residual stresses and characterize existing phases close to substrate/coating interface. In the present study, we focus on the cobalt-alloys coating deposited on stainless steel forging tools via Plasma Transfer Arc (PTA) process. Forging tools always work in high temperature conditions. Since fatigue crack is often detected near to the interface, we used synchrotron radiation to characterize residual stress profiles both in the substrate and coating sides. Also, we defined phases inside diluted substrate layers and in the stellite coating using a Rietveld refinement.

Abstract: Hot forging tools are exposed to severe thermo-mechanical load which cause their failure. The main failure mechanisms are mechanical fatigue, thermal fatigue, plastic deformation and abrasive wear. The use of hardfacing is an effective solution to increase tool life and consequently reducing the cost of forged part. In this work the use of a cobalt-base hard-facing obtained by Plasma Transferred Arc process is studied. The hardfacing is used for tool failed by thermal cracking. The high rate of temperature variation and high temperature gradient at the tool surface induce material softening and surface plastic deformation. These plastic deformations are accumulated during each cycle and eventually cause cracks in the most exposed zones and then tool failure after a few thousand cycles. Microstructure characterization and mechanical behavior analyses are required to prove the quality of coating. Finite element method is used to calculate stress distribution in tool resulting from external loading in order to optimize the thickness and the choice of coating used. Evaluation of residual stresses by ultrasonic technique is used to validate the finite element results and analyze the stress state by non destructive method.

Abstract: The objective of this study is to investigate the effects of Heat Affected Zone produced by Plasma Transferred Arc process and residual stress in substrate steel 55NiCrMoV7. In this study, based on ABAQUS code, a sequentially coupled thermal, metallurgical, mechanical 2-D finite element model is developed. In the numerical simulations, the coating and Heat Affected Zone are used to predict the residual stresses. The simulation results revealed that the final residual stress in substrate steel do seem to be influenced by the HAZ.

Abstract: The Stellite 6 hardfacing alloys have been deposited on steel substrate using a Plasma Transferred Arc (PTA) with complex geometry. The residual strain of the PTA technology at the surface of coating layer and the interface were determined by neutron diffraction method. In the present work, a numerical model for the residual stresses formed during the PTA process with physical conditions and mechanical properties using the Abaqus code is analysed. The result reveals that the residual stresses obtained by the numerical simulation are in very good agreement with experimental results by the neutron diffraction.

Abstract: A full review for technological processes used today, in dental prostheses industry for titanium casting, both in Europe and in United States, reveal that, there is no equipment available on the market able to cast small titanium parts in secondary inert vacuum by induction melting and centrifugal casting. All these advanced processes together contribute to an original concept of the Denticast System. The computer controlled cast sequence is another important factor contributing to a robust system, able to reproduce the achieved technological quality in a reliable manner. Mechanical testing (tensile and hardness) shows that the different alloys used in Denticast project and cast using the prototype, are in most of the results superior to those obtained with two different commercial systems (pressure-vacuum and centrifugal systems). The samples are usable for dental prosthesis without any risk for the patient health.