Generally, industrial components undergo manufacturing processes that may include thermo-mechanical treatments to enhance their structural properties and functionality. In most cases, these processes lead to the development of residual stresses. Residual stresses in components and structures include macroscopic residual stresses, which entirely depend on the material’s macroscopic physical properties (e.g., the elastic limit or the coefficient of thermal expansion), and microscopic residual stresses that depend on the microstructure of the material. To achieve good design and optimization of the industrial components, these residual stresses must be added to the external ones that the components may undergo.
Microscopic residual stresses do not only develop in multiphase materials but also in single phase material alloys. In the latter ones the stress varies among neighboring grains. Presently, the most common technique to calculate residual stresses is based on the diffraction methods. By these methods, and using neutrons and/or synchrotron radiation, the full tri-axial residual stress state can be determined. Even though these methods are efficient for macroscopic stresses, its use for calculation of the microscopic stresses in single phase alloys is still limited. This is due to several factors; principally, because it is not possible to associate diffraction peaks from different spatial directions to specific individual grains. As a result, rigorous investigations on the tri-axial residual stress state of individual grains have not yet been conducted.
A group of researchers at the National Center for Metallurgical Research, CENIM (of C.S.I.C.), in Madrid, Spain: Dr. Fernández, Dr. Ferreira-Barragáns, Dr. Ibáñez and Dr. González-Doncel have developed a multi-scale analysis method for calculating both the macroscopic and microscopic residual stresses in a single-phase alloy. Synchrotron radiation diffraction data in conjunction with suitable approximations were used in this case. Their work is currently published in the research journal, Materials and Design.
Several factors led to the success of this study. Specifically, the accomplishment is base on the use of a sample which had a microstructure with cylindrical symmetry (resulting from an extrusion process) and in that the residual stress state resulting from the quenching process imposed to the sample (cylinder) possessed the same (high) symmetry. In this way, and considering that only the cooling front advancing from the lateral surface of the cylinder is responsible of the residual stress developed in the quenching, it could be assumed that the residual stress of any given grain is only crystallographic orientation dependent. Under this hypothesis it can be considered that grains at a given distance to the central sample axes, and with a common hkl parallel to the extrusion axes direction (cylinder symmetry axes) develop the same stress. Under this framework, the separate contribution of both the macroscopic and microscopic residual stresses was analyzed using a composite material approach, where grains belonging to a given texture component represent a given composite material phase.
The authors confirmed the influence of the thermo mechanical treatments on the grain orientation dependence of the microscopic residual stress. The study also shows the interdependence between the microscopic and the macroscopic residual stresses. For example, it is proposed in the publication that it is possible to achieve a strong (and detrimental) microscopic residual stress together with a compressive (and beneficial) macroscopic stress at the near-surface region of a given component.
For the first time, a technique for calculating the two residual stresses in a single-phase alloy was investigated. Both of the results obtained in the experiment and that got from the finite element analysis affirmed the validity of this study. During the quenching process, high m-RS developed in the external regions as compared to the center region. The study will advance the design and manufacturing of the components as well as optimizing their performance.
Fernández, R., Ferreira-Barragáns, S., Ibáñez, J., & González-Doncel, G. (2018). A multi-scale analysis of the residual stresses developed in a single-phase alloy cylinder after quenching. Materials & Design, 137, 117-127.Go To Materials & Design