Simulación numérica del temple por inducción en aceros de baja aleación y análisis de la influencia de las tensiones residuales en el rolling contact fatigue

Los capítulos 3 y 4 están sujetos a confidencialidad por la autora. 244 p.El proceso de temple por inducción es una técnica de endurecimiento superficial que utiliza cada vez más en la industria debido a las ventajas que ofrece sobre otros tratamientos térmicos más convencionales. Habitualmente se utiliza en componentes críticos que están sometidos a cargas elevadas y a contactos de alta presión, lo que requiere una elevada dureza superficial. A pesar de que el interés de la industria en este tratamiento térmico es relevante, la definición de los parámetros más determinantes del proceso está generalmente limitado de forma empírica al know-how de los técnicos y a la experiencia previa de los agentes del sector, aumentando los costes asociados y el time-to-market, puesto que el diseño del proceso normalmente se realiza mediante procedimientos de prueba-error. Al ser un proceso multifísico con numerosas interacciones entre campos físicos, la simulación del temple por inducción es altamente compleja y computacionalmente muy costosa. En la revisión de la literatura se ha observado que la aplicación del temple por inducción en componentes complejos industriales se ve obstaculizada por la falta de modelos numéricos capaces de predecir sus consecuencias. La simulación numérica es, por lo tanto, clave en la definición optimizada del proceso y su implementación eficaz en la industria moderna. Además, el estudio de las implicaciones que el temple por inducción tiene en el comportamiento en servicio de los componentes endurecidos, como es el caso de la fatiga, es una tarea a la que ingenieros y científicos han dedicado su atención en los últimos años, aunque aún no está del todo resuelto.En esta tesis doctoral se aborda la problemática de la simulación del proceso de temple por inducción para tratar de solventar las limitaciones encontradas en la literatura. Se ha trabajado en el desarrollo de un modelo numérico para simular la fase de calentamiento por inducción de materiales ferromagnéticos de forma eficiente, acoplando los campos electromagnético y térmico a través de un modelo semi-analítico. Este modelo se ha validado experimentalmente en cilindros del acero de baja aleación 42CrMo4 obteniendo resultados más precisos y un 80 \% más rápidos que utilizando softwares comerciales ya existentes. Adicionalmente, se ha desarrollado un modelo multifísico acoplado para simular la fase de enfriamiento del proceso de temple por inducción. Este modelo acopla las físicas térmica, mecánica y microestructural y se ha validado experimentalmente en términos de predicción de microestructura, dureza y generación de tensiones residuales. El modelo desarrollado, a diferencia de otros softwares comerciales, permite evaluar el impacto que tienen en los resultados del proceso las diferentes aproximaciones de cálculo de los fenómenos que ocurren durante el temple. En esta tesis, se ha investigado el impacto del Transformation Induced Plasticity (TRIP) en el acero 42CrMo4, concluyendo que los modelos de cálculo deberían incluir este efecto para mejorar las predicciones de tensiones residuales. Finalmente, se han combinado los modelos de simulación desarrollados con técnicas experimentales para investigar la influencia de las tensiones residuales en el comportamiento a rolling contact fatigue (RCF) en cilindros templados por inducción. En este estudio, se ha desarrollado una metodología de cálculo para incorporar las tensiones residuales en los análisis de vida a RCF y se ha estudiado la influencia de las tensiones residuales numérica y experimentalmente mediante un ensayo de three-ball-on-rod modificado. Se ha observado que las tensiones residuales de compresión en la superficie extienden la vida del componente y modifican la profundidad a la que se produce el daño más crítico. Para el análisis numérico se ha utilizado el criterio multiaxial de Dang Van y se han comparado tres magnitudes de cortadura críticas (Tresca, cortadura ortogonal y cortadura octaédrica) en términos de predicción de vida útil y localización del daño crítico. Los resultados numéricos y experimentales indican que la magnitud de cortadura ortogonal predice resultados más precisos. Se espera que las contribuciones realizadas en esta tesis doctoral reduzcan la actual brecha entre los modelos simplificados generalmente desarrollados en la literatura y los casos industriales de mayor complejidad, permitiendo que los procedimientos empíricos de prueba-error se reduzcan considerablemente y aumentando el control sobre el resultado obtenido con el proceso. Con este cambio de paradigma, se espera obtener mayor eficiencia en términos económicos, temporales y energéticos a nivel industrial, reduciendo también el número de defectivos

Mixed material bonding : Effects of the properties of the adhesive on the thermal shape distortion

Adhesive bond-line read-through is the terminology used in the automotive industry that describes a visible distortion of an adherent over the adhesive bond-line. Usually, this is a result of the difference in the coefficients of thermal expansion of the substrates and the adhesive. First of all, previous work is presented, as well as some information about the terms that are used throughout the project. A linear thermal simulation is carried out in order to analyse the bond-line read-through curvature defects produced by the adhesive properties in a typical bonding from the automotive industry. The analysed models belong to a carbon fibre and epoxy composite material plate bonded with a metal plate that can be made of steel, aluminium and magnesium. The use of carbon fibre composites is one of the main focuses in the automotive industry due to the decrease of environmental and therefore economic impact. The adhesives used for the analysis are liquid or paste adhesive and adhesive tapes from the 3M™ VHB™ series. Several equations are obtained from the results, which can be used to calculate the curvature produced by a specific adhesive for these combinations. The use of these equations is limited to certain material properties and thicknesses that are presented through the work. The outcome of this project provides an opportunity to make the adhesive selection easier, based on the maximum required substrate curvature

Numerical and experimental investigation of residual stresses during the induction hardening of 42CrMo4 steel

The usage of induction hardening in the industry has increased in the last years due to its efficiency and repeatability. Induction hardening produces a hard martensitic layer on the specimen surface, which is accompanied by the generation of compressive residual stresses in the hardened case and tensile stresses in the untreated core. Residual stresses generated by induction hardening greatly impact on fatigue performance, as they act as crack growth retardants. In this work, a multiphysical coupled finite element model is developed to simulate induction hardening and compute the final residual stress state of the specimens along the microstructural transformations and hardness evolution. The impact of the transformation induced plasticity strain in the stress-state of the specimen during the process is also studied. The experimental validation shows that considering the transformation induced plasticity in induction hardening simulations improves the residual stress predictions, concluding that this effect should be included to achieve good residual stress predictions, especially in the subsurface region

Numerical and experimental investigation on the residual stresses generated by scanning induction hardening

Induction hardening is widely used in the industry as a surface heat treatment that improves the surface and the subsurface hardness of components greatly. The hardened case, which usually is a few mm, highly impacts the surface and structural integrity of the component. In this work, we simulate the scanning induction hardening process by means of finite element modeling. The computed hardness, microstructure, and residual stress profile are compared with experimentally measured data using several surface and subsurface characterization techniques. A very good agreement is found between the simulated and experimentally measured residual stresses, which were characterized by the incremental hole drilling technique

Predicting the induction hardened case in 42CrMo4 cylinders

Induction hardening has the potential to produce favorable surface integrity that can improve fatigue performance and extend the lifetime of a component. The localized superficial heating provided by induction is the main advantage of this process, as it allows the core to remain intact and, therefore, ductile, while the surface is hardened. Achieving favorable characteristics in the hardened case is of great importance, as this process is usually applied to load bearing and wear-susceptible metallic components. The simulation of the hardening process by induction heating is a complex and challenging task at which many efforts have been directed in the last years. Due to the numerous interactions of the many physics that take part in the process (electromagnetic, thermal, microstructural and mechanical), a highly coupled finite element model is required for its numerical simulation. In this work, a semi-analytical induction heating model is used to compute the induction hardening process, predicting the size and shape of the hardened layer and the distribution of the hardness. Using the semi-analytical model allows the computational time to be much faster compared to a fully coupled model using a commercial software, where the time consumption for the presented 2D case is reduced by 20 %. Experimental validation is presented for cylindrical 42CrMo4 billets heated by a short solenoidal inductor, which shows good agreement with the predicted results, reaching an average error of 3.2 % in temperature estimations

Influence of induction hardening residual stresses on rolling contact fatigue lifetime

Rolling contact fatigue is a unique mode of fatigue that components under cyclic contact loading experience. In this work, the impact of induction hardening residual stresses in rolling contact fatigue lifetime is investigated experimentally and numerically using the Dang Van multiaxial criterion. Various residual stress fields from induction hardening are simulated using the finite element method and are mapped into a classical monocontact finite element model. The impact of induction hardened residual stresses on the lifetime of a component has been investigated, and the importance of incorporating the residual stress profile into fatigue life assessments is affirmed

A semi-analytical coupled simulation approach for induction heating

The numerical simulation of the induction heating process can be computationally expensive, especially if ferromagnetic materials are studied. There are several analytical models that describe the electromagnetic phenomena. However, these are very limited by the geometry of the coil and the workpiece. Thus, the usual method for computing more complex systems is to use the finite element method to solve the set of equations in the multiphysical system, but this easily becomes very time consuming. This paper deals with the problem of solving a coupled electromagnetic - thermal problem with higher computational efficiency. For this purpose, a semi-analytical modeling strategy is proposed, that is based on an initial finite element computation, followed by the use of analytical electromagnetic equations to solve the coupled electromagnetic-thermal problem. The usage of the simplified model is restricted to simple geometrical features such as flat or curved surfaces with great curvature to skin depth ratio. Numerical and experimental validation of the model show an average error between 0.9% and 4.1% in the prediction of the temperature evolution, reaching a greater accuracy than other analyzed commercial softwares. A 3D case of a double-row large size ball bearing is also presented, fully validating the proposed approach in terms of computational time and accuracy for complex industrial cases