Papers by Author: Sylvain Dancette

Abstract: C-Mn steel welds of the secondary circuit of pressurised water reactors might be subjected to static strain ageing (SSA) due to the segregation of free interstitial atoms, especially nitrogen, on dislocations at temperatures of 250°C-300°C in operating conditions. Thus, several parameters such as the initial free nitrogen content and the amount of prestrain are likely to affect the sensitivity of the material to SSA, both influences being investigated in this paper. To this end, a broad experimental campaign based on internal friction (IF) measurements was conducted to follow the evolutions of the solute content. Preliminary studies emphasized the presence of nitrogen solutes in the as-welded material, whose content tends to increase from 250°C to 300°C due to the dissolution of ε carbonitrides. Regarding the parametric study of SSA, it was shown that the amount of segregated nitrogen first increases with the amount of prestrain or the initial concentration of nitrogen solutes before saturating from a critical threshold. Its value depends on the other parameter, leading to either no effect, a continuous increase or an increase before saturation of segregation. Therefore, a cross influence of both parameters was highlighted.

Abstract: Subtransus multiaxial hot forging of α+β Ti-6Al-4V (TA6V) titanium alloy with a β-transformed microstructure aims at obtaining an equiaxed microstructure through α phase globularization. The activation of mechanisms involved in microstructural evolution, such as globularization, depends on parameters such as time, temperature, strain and strain rate. It is also sensitive to the crystallographic orientation of α-lamellae. As a result, multiaxial processing of titanium alloys leads to significant microstructural gradients depending on thermomechanical conditions and initial microstructure. In this study, we focused on the effect of complex thermomechanical paths on microstructural evolutions. Thanks to the MaxStrain Gleeble device, we were able to reproduce such thermomechanical treatments to β-transformed TA6V samples. Stress strain fields obtained with finite element modelling of the MaxStrain test were compared to experimental microstructure gradients. This experimental method offers the opportunity to get closer to industrial open die forging conditions.

Abstract: Advanced High Strength Steels (AHSS) are key materials in the conception of car body structures, permitting to reduce their weight while increasing their behavior in crash conditions. Nevertheless, the weldability of AHSS presents some particular aspects, in that complex failure types involving partial or full interfacial failure can be encountered more often than with conventional mild steels during destructive testing, despite high spot weld strength levels. This paper aims at characterizing the behavior of different AHSS spot welds under two quasi-static loading conditions, tensile shear and cross tension, often used in the automotive industry for the determination of their weldability. Interrupted cross tension and tensile shear tests were performed and spot welds failure was investigated with optical micrographs, SEM fractography and 3D-tomography in order to follow the three-dimensional crack paths due to the complex loading modes. A limited number of failure zones and damage mechanisms could be distinguished for all steel grades investigated. Moreover, numerical simulation of the tests was used to better understand the stress state in the weld and the influence of geometrical features such as weld size on the occurrence of the different failure types.