Numerical e experimental analysis of residual stresses generated during hardening of AISI 1045, 4140 and 4340 bars.

O objetivo deste trabalho é analisar a distribuição das tensões residuais que resultam da combinação das variações volumétricas resultantes dos gradientes térmicos e das transformações de fase que ocorrem durante a têmpera de cilindros de aço AISI/SAE 1045, 4140 e 4340. O modelo matemático usado para este objetivo utiliza o programa AC3 de modelagem de tratamentos térmicos (curvas de transformação, curvas de resfriamento, microestrutura e a dureza do material), para alimentar um modelo de elementos finitos, considerando acoplamento termo-mecânico e comportamento não linear elasto-plástico, para previsão de tensões residuais em cilindros de aço AISI/SAE 1045, 4140 e 4340 temperados em água. São apresentados, também, os resultados de observações metalográficas e perfis de dureza que confirmam qualitativamente as previsões do programa AC3. A verificação do modelo numérico por elementos finitos foi efetuada através da medição das tensões residuais nos cilindros de aço com o emprego da técnica de difração de raios X. A simulação numérica, através do método dos elementos finitos comprova, nos três casos estudados, a existência de tensões residuais de compressão na região superficial após o processo de têmpera e indica de maneira quantitativa e qualitativa que as tensões mais significativas são as tangenciais. Os resultados obtidos a partir do modelo numérico mostraram uma aderência significativa em comparação com os resultados experimentais.The aim of this work is to analyze the distribution of residual stresses resulting from combination of volumetric changes due to heat gradients and phase changes occurring during the quenching process of AISI/SAE 1045, 4140 and 4340 steel cylinders. The mathematical model used for this objective uses the AC3 program for modeling thermal treatments (transformation curves, cooling curves, microstructure and material hardness), whose results were fed into a finite element model, considering thermal-mechanical coupling and non-linear elastic-plastic behavior for forecasting of residual stresses in AISI/SAE 1045, 4140 and 4340 steel cylinders quenched in water. The observed microstructures and measured hardness confirmed qualitatively the previsions of the AC3 program. The results of finite element modeling were compared to experimental measurements of residual stresses measured at the surface, using X-Ray diffraction techniques. The finite element numerical simulation shows, for the three studied cases, the presence of compressive residual stresses in the surface region after a quenching process and indicates qualitatively and quantitatively that the most significant stresses are the tangential ones. The results obtained from the numerical model showed a significant adherence in comparison with the experimental results

Numerical and experimental analysis of a trip steel submitted to hot stamping and quenching and partitioning (Q&P) processes.

O desenvolvimento de métodos de simulação física e numérica tem criado novas possibilidades de otimização dos processos relacionados à estampagem com inclusão de processos industriais reais. Portanto, recorrendo à aplicação destes métodos de análise, é possível avaliar a transformação mecânica e as transformações de fase que ocorrem no material e prever as interações entre as propriedades dos materiais no processo de conformação, o comportamento constitutivo do material, as variáveis de otimização do processo, bem como a previsão das tensões e deformações a fim de estabelecer a melhor relação material-processo-desempenho. A introdução e crescente utilização de aços avançados de alta resistência (AHSS) em aplicações automotivas exige uma maior compreensão dos fenômenos físicos envolvidos no processamento termomecânico a fim de otimizar a performance da peça final fabricada. O presente trabalho teve como objetivo avaliar experimentalmente o processo de estampagem a quente, com posterior tratamento térmico de têmpera e partição e analisar as microestruturas formadas e suas propriedades mecânicas. A formação de microestruturas durante o processo de estampagem a quente e de têmpera e partição foi avaliada neste trabalho por simulação física em simulador termomecânico Gleeble, acoplado à uma linha de difração de raios X (XTMS) de feixe de luz síncrotron no Laboratório Nacional de Nanotecnologia (LNNano). Foram avaliadas a partição do carbono, a estabilidade térmica da austenita retida e a formação de microconstituintes resultantes da transformação da austenita durante resfriamento forçado (têmpera), seguido de partição de carbono em patamares isotérmicos. Foram utilizadas técnicas de caracterização com apoio de microscopia eletrônica (MEV-FEG e STEM), EBSD, tomografia de sonda atômica (APT) e avaliação de propriedades mecânicas por ensaios de tração e indentação instrumentada. A análise numérica foi realizada por meio do método dos elementos finitos (MEF) e por elementos finitos orientada a objetos (OOF, Object Oriented Finite Element Analysis) visando estabelecer correlações entre microestrutura e propriedades mecânicas, comparando com resultados experimentais. Os resultados e conclusões obtidos no projeto, além de possibilitarem a identificação dos mecanismos fundamentais de geração de microestruturas durante o processo, auxiliam no projeto de aços AHSS estampados a quente, usados principalmente na indústria automobilística, na busca pela redução do consumo de combustível, através da redução do peso, e pelo aumento da segurança dos passageiros.The development of numerical and physical simulation methods has created new possibilities regarding the optimization of metal forming processes, taking into account real industrial forming processes. Therefore, by applying such methods of analysis it is now possible to assess the material phase transformations and predict the interactions between material properties and the forming process, the constitutive behavior of the material, and optimize process variables as well as predicting the best material-process-performance relationship. The increasing usage of Advanced High Strength Steels (AHSS) in automotive applications demands a better insight of the physical phenomena involved in the thermomechanical processing in order to optimize the performance of the final manufactured part. Thermomechanical simulation of the hot stamping, quenching and partitioning process was carried out in a Gleeble machine coupled to the XTMS Synchrotron X-ray diffraction line at the National Nanotechnology Laboratory (LNNano). Carbon partitioning, carbon contents, and amount of retained austenite, martensite, bainite and ferrite was assessed online during the experiments. In addition, characterization techniques by optical, electron microscopy (FEG-SEM and STEM), EBSD, and Atom Probe Tomography (APT) were applied. Mechanical testing of subsize specimens of the processed steels was performed by means of tensile tests and macro and nanoindentation tests. The numerical analysis was performed using the finite element method (FEM) and object-oriented finite element technique (OOF). The results were compared with the experimental results of mechanical testing of specimens used in the thermomechanical simulations and with hot stamped sheets, where quenching and partitioning were carried out. The results and conclusions obtained in this project allow the identification of the fundamental mechanisms of the process, helping the design of the hot stamping process for AHSS steels used primarily in the automotive industry, seeking weight reduction to improve fuel economy and increased passenger safety

NUMERICAL AND EXPERIMENTAL ANALYSIS OF RESIDUAL STRESSES GENERATED DURING HARDENING OFAISI 4140 BAR

The aim of this work is to analyze the distribution of residual stresses resulting from the combination of volumetric changes due to heat gradients and phase changes occurring during the quenching process of an AISI/SAE 4140 steel cylinder. The mathematical model used for this objective is the AC3 modeling software of thermal treatments (transformation curves, cooling curves and microstructure), whose results were input in an finite element model, considering thermalmechanical coupling and non-linear elastic-plastic behavior, aiming the assessment of residual stresses in quenched 4140 steel cylinders. The observed microstructure confirms quantitatively and qualitatively the previsions of the AC3 Software. The results of the modeling are compared with the residual stresses measurements made using X-Ray diffraction techniques. The finite element numerical simulation shows the existence of 350 MPa compressive residual stresses in the surface region and indicates that the most significant stresses are tangential

NUMERICAL AND EXPERIMENTAL ANALYSIS OF RESIDUAL STRESSES GENERATED DURING HARDENING OFAISI 4140 BAR

The aim of this work is to analyze the distribution of residual stresses resulting from the combination of volumetric changes due to heat gradients and phase changes occurring during the quenching process of an AISI/SAE 4140 steel cylinder. The mathematical model used for this objective is the AC3 modeling software of thermal treatments (transformation curves, cooling curves and microstructure), whose results were input in an finite element model, considering thermalmechanical coupling and non-linear elastic-plastic behavior, aiming the assessment of residual stresses in quenched 4140 steel cylinders. The observed microstructure confirms quantitatively and qualitatively the previsions of the AC3 Software. The results of the modeling are compared with the residual stresses measurements made using X-Ray diffraction techniques. The finite element numerical simulation shows the existence of 350 MPa compressive residual stresses in the surface region and indicates that the most significant stresses are tangential

Characterization and methodology for calculating the mechanical properties of a TRIP-steel submitted to hot stamping and quenching and partitioning (Q&P)

Thermomechanical simulation of quenching, hot stamping, and quenching and partitioning processes of a high-strength TRIP-assisted steel were carried out in a Gleeble®3S50 thermo-mechanical simulator, coupled to the synchrotron X-ray diffraction line. The microstructures and mechanical properties were analyzed using Field Emission Gun Scanning Electron Microscopy (FEG-SEM), X-ray diffraction, and nanoindentation. The microstructures of thermomechanical treated specimens were modeled using the Object Oriented Finite Element (OOF) technique. The modeled microstructures were then fed into a finite element model to predict the mechanical behavior. By using a reverse algorithm method, the elasto-plastic mechanical properties of different microconstituents were determined. This was done through the analysis of instrumented nanoindentation loading-penetration curves. Tensile properties of the thermomechanical processed steels were measured by tensile testing of subsized specimens cut from samples processed on the Gleeble®3S50. The comparison between the experimental results and those of the reverse algorithm and the OOF modeled microstructure showed quite good agreement

Didactic Use of Virtual Reality in Colombian Universities: Professors’ Perspective

This paper presents quantitative research on the perception of the didactic use of virtual reality by university professors in Colombia, with special attention to the differences according to their area of knowledge, as the main variable, and gender and digital generation, as secondary variables. The study involved 204 professors from different Colombian universities. As an instrument, a survey designed for this purpose was used with four scales that were used to measure, on a Likert scale, different dimensions involving the participants’ perception of the use of virtual reality in the classroom. The answers were analyzed statistically and the differences in the perceptions have been identified by means of parametric statistical tests according to the following: (i) area of knowledge, (ii) gender, (iii) digital generation of the participants. The results showed that the participants expressed high valuations of virtual reality, despite having intermediate or low levels of digital competence. Gaps were identified in terms of area of knowledge, gender, and digital generation (digital natives or immigrants) with respect to opinions of virtual reality and digital competence. The highest valuations of virtual reality are given by professors of Humanities, and by digital natives. It is suggested that Colombian universities implement training plans on digital competence for professors and that these plans be aimed at strengthening knowledge of virtual reality

Didactic Use of Virtual Reality in Colombian Universities: Professors’ Perspective

This paper presents quantitative research on the perception of the didactic use of virtual reality by university professors in Colombia, with special attention to the differences according to their area of knowledge, as the main variable, and gender and digital generation, as secondary variables. The study involved 204 professors from different Colombian universities. As an instrument, a survey designed for this purpose was used with four scales that were used to measure, on a Likert scale, different dimensions involving the participants’ perception of the use of virtual reality in the classroom. The answers were analyzed statistically and the differences in the perceptions have been identified by means of parametric statistical tests according to the following: (i) area of knowledge, (ii) gender, (iii) digital generation of the participants. The results showed that the participants expressed high valuations of virtual reality, despite having intermediate or low levels of digital competence. Gaps were identified in terms of area of knowledge, gender, and digital generation (digital natives or immigrants) with respect to opinions of virtual reality and digital competence. The highest valuations of virtual reality are given by professors of Humanities, and by digital natives. It is suggested that Colombian universities implement training plans on digital competence for professors and that these plans be aimed at strengthening knowledge of virtual reality

Failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting. Part II: Thermal stress analysis by FEA

A martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting and used as a glass roller failed prematurely after six rolling campaigns. A previous fractographic investigation showed that the failure of the roll was caused by thermal stresses associated with microstructural embrittlement of the steel casting. Crack nucleation took place on the internal surface of the roll and the crack propagated by ductile intergranular mode. In the present work, the thermal stresses distribution was investigated by a thermal elastic–plastic model using finite element analysis in order to associate the crack growth with the thermal history of the roll during service. The proposed methodology based on the assessment of thermal stresses by finite element analysis together with coupled thermal–mechanical non-linear modelling and calculations using non-linear elastic–plastic fracture mechanics allowed a better understanding of the stable and unstable growth of a longitudinal crack until the premature failure of the roll during its cooling to room temperature after the sixth campaign. The stable and unstable crack growth and the thermal history of the roll (heating up, glass rolling and cooling operations during the six campaigns of the roll) could be related using the proposed calculation

Solving Spatial Vision Difficulties Related to the Instruction of Welded Joints by Using PDF-3D

In this work, exploratory quantitative research on the opinion of the students of a course on Geometry of Welded Joints, in an introductory welding course of secondary technical education curriculum, on the didactic use of PDF-3D in the development of the course is carried out. For this purpose, a survey designed by the authors was used, which the students answered once the study of the subject had been completed. As a result, it has been found that the valuations of the virtual tool PDF-3D are very high, mainly in relation to motivation and ease of use. The importance that students attach to the ease of solving the spatial comprehension difficulties inherent to welded joints -and the imperfections linked with them- positively influences the ratings of this educational technology. On the other hand, this study reveals that students prefer a mixed methodology in which the presence of virtual resources is greater than that of the master class. Finally, the results obtained suggest that, given the lack of knowledge on the part of teachers of the potential of PDF-3D in the educational field, it would be advisable to offer specific training on the didactic use of PDF-3D to technical education professors