Fatigue crack growth rates on the weld metal of high heat input submerged arc welding.

Submerged arc welding (SAW) with high heat input and more than one wire has been used to weld thick plates in order to reduce the number of passes and consequent increase of productivity in the shipbuilding and oil industries. Additional characteristics of SAW such as high penetration and relative ease to produce welds with good finishing and without discontinuities contribute to this process has being widely applied. However, the use of very high heat inputs leads to the formation of a large melting pool and large amount of liquid metal, and it submits the weld region to long-term thermal cycles with low cooling rates. This scenario contributes to the formation of thick solidification structures and, at the end of the cooling, results in a microstructure consisting mainly of grain boundary ferrite with low mechanical strength and large grain size. This research evaluated fatigue cracks growth rates in welding metals with heat input higher than 10 kJ/mm made of the new SAW technique, called Integrated Cold Electrode? with addition of non-energized (cold) wire. Single pass welds were made on 25mm thick EH36 steel varying the chemical composition of the wire (filler metal). Microstructure, mechanical properties and da/dN??K graphics were made with the propagation of the crack in the longitudinal direction of the weld. For all tested conditions, propagation rates were similar to Paris regime base metals. However, the welds presented a crack propagation threshold higher than base metal, and fatigue threshold of higher heat input welds was increased using a molybdenum-containing filler metal. It was attributed to the presence of a higher amount of acicular ferrite in this particular welding microstructure

Damage evolution in a tensile specimen of a ductile stainless steel.

This paper presents a study about the fracture mechanism of a ferritic stainless steel (UNS S44400 type) during a tensile test. The applied materials for the experimental procedures were 25 specimens of the steel, machined in the rolling direction. Each specimen was submitted to standard polishing procedures. One of the samples, in the original state, was structurally characterized by reflected light optical microscopy. The other samples were submitted to tensile tests with a constant displacement rate. Three samples were tested until failure (complete tests) and the others just until specific strain values, when the tests were interrupted and the samples were characterized by using optical and scanning electron microscopy. The main objective of these characterizations was to evaluate the structural damage evolution and to identify the fracture mechanism for the tested conditions. A methodology to quantify the damage evolution by surface roughness, identified by optical microscopy, was proposed. A new index - Damage by Diffuse Reflection Index (DRI) - was proposed to quantify the damage evolution in function of the specimen deformation. It was possible to confirm the ductile behavior of the studied steel and that the main fracture mechanism was the traditional dimpled rupture

Damage evolution in a tensile specimen of a ductile stainless steel.

This paper presents a study about the fracture mechanism of a ferritic stainless steel (UNS S44400 type) during a tensile test. The applied materials for the experimental procedures were 25 specimens of the steel, machined in the rolling direction. Each specimen was submitted to standard polishing procedures. One of the samples, in the original state, was structurally characterized by reflected light optical microscopy. The other samples were submitted to tensile tests with a constant displacement rate. Three samples were tested until failure (complete tests) and the others just until specific strain values, when the tests were interrupted and the samples were characterized by using optical and scanning electron microscopy. The main objective of these characterizations was to evaluate the structural damage evolution and to identify the fracture mechanism for the tested conditions. A methodology to quantify the damage evolution by surface roughness, identified by optical microscopy, was proposed. A new index - Damage by Diffuse Reflection Index (DRI) - was proposed to quantify the damage evolution in function of the specimen deformation. It was possible to confirm the ductile behavior of the studied steel and that the main fracture mechanism was the traditional dimpled rupture

Tenacidade ? fratura e propaga??o de trinca de fadiga de uma superliga INCONEL 600.

No presente trabalho procurou-se verificar a tenacidade ? fratura e a resist?ncia ? propaga??o de trinca de fadiga de uma superliga de n?quel do tipo INCONEL 600. Trata-se de uma liga Ni-Cr-Fe, utilizada em componentes internos de geradores de energia termonuclear. Corpos-de-prova do tipo C(T) foram confeccionados no sentido longitudinal de conforma??o mec?nica, com espessura de 7 mm e rela??o B/W (espessura por largura) igual a 1/4. Os ensaios foram realizados na temperatura ambiente. Os ensaios de tenacidade ? fratura tiveram como objetivo a determina??o da curva de resist?ncia integral J versus incremento de trinca ?a e o valor cr?tico Ji de inicia??o de crescimento de trinca. Foi determinado tamb?m o valor de CTOD de carga m?xima. Estes ensaios foram realizados de acordo com a Norma ASTM E 1820. Os ensaios de fadiga foram realizados para a determina??o da taxa de propaga??o de trinca da/dN em fun??o da for?a motriz ?K de propaga??o de trinca de fadiga, desde o limiar de propaga??o at? a fratura. Os ensaios foram realizados para R (raz?o entre tens?es) igual a 0,1 , de acordo com a Norma ASTM E 647. Os resultados mostraram valores elevados para as propriedades estudadas, indicando que o material atende as especifica??es para utiliza??o na ind?stria termonuclear. Observou-se que a t?cnica da normaliza??o para modelamento da curva de resist?ncia J-?a ? conservadora, necessitando de corre??o. Observou-se tamb?m que o modelo matem?tico de Collipriest representa bem a forma sigmoidal da curva da/dN x ?K

Caracteriza??o do fechamento de trinca de fadiga numa liga Al 7475 pr?-deformada e com shot peening.

No presente trabalho procurou-se verificar o efeito de estiramento em tra??o e de tratamento mec?nico superficial de shot peening na resist?ncia ? propaga??o de trinca de fadiga de uma liga de alum?nio do tipo Al 7475-T7351. Trata-se de uma liga Al-Zn-Mg-Cu, com tratamento t?rmico de superenvelhecimento, para emprego aeron?utico. Corpos-de-prova do tipo C(T) foram confeccionados no sentido longitudinal de conforma??o mec?nica, e posteriormente ensaiados de acordo com a Norma ASTM E 647, para a determina??o da taxa de propaga??o de trinca da/dN em fun??o da for?a motriz ?K de propaga??o de trinca de fadiga, desde o limiar de propaga??o at? a fratura. Os ensaios foram realizados para R (raz?o entre tens?es) igual a 0,1 e 0,5. A espessura dos corpos-de-prova foi de 5 mm. O efeito do estiramento foi estudado em corpos-de-prova com 0%, 3% e 5% de deforma??o pl?stica. O efeito de shot peening foi estudado em corpos-de-prova com e sem este tratamento superficial. A resist?ncia ? propaga??o de trinca de fadiga diminui com o aumento do estiramento, mas pode ser melhorada atrav?s do shot peening. Quanto maior o valor de R, maior a taxa de propaga??o de trinca. Todos estes efeitos foram estudados e explicados a partir da determina??o da varia??o da carga de fechamento de trinca em fun??o da for?a motriz ?K, assim como atrav?s de an?lise fratogr?fica.Aluminum alloys are widely used in aeronautical design due to their good mechanical properties and low densities. Among these alloys, 7475 (Al-Zn-Mg-Cu), modified from 7075, is successfully used due to strength similar to that of 7075 and superior fracture toughness. In this study, strips of 7475-T7351 (overaged) were subjected to two tensile pre-strain levels of 3% and 5%, and surface treatment of shot peening. Using compact tension C(T) specimens with 5 mm thickness, fatigue crack growth (FCG) tests were conducted in air under constant amplitude loading at a stress ratio of 0.1 and 0.5, with specific emphasis at the fatigue threshold. The results showed that FCG resistance decreased with an increase in the pre-strain level, but increased with shot peening. Fatigue crack closure and fractographic analysis were used to explain the different behavior

Evaluation of standard API casing connections and parametric API buttress improvement by finite element analysis.

Threaded and coupled connections generally present a non-uniform stress distribution, which is related to the higher stiffness of the box when compared to the pin. The non-uniform stress distribution can contribute to fatigue cracks and other failure modes in high pressure oil wells. An API 5CT P110 steel obtained from a seamless pipe was mechanically characterized. FEA models were carried out to investigate and compare two API casing connections under make-up torque and tensile efforts. A parametric study was performed using numerical models to determine the influence of some geometric features on the behavior of the API Buttress threaded connection. The API Buttress connection supported higher tensile loads than the API Short Round, however both standard connections showed high stress concentration in the last engaged thread and a non-uniform stress distribution. The manufacturing of grooves at the first and the last engaged threads proved to be an efficient way to reduce the stress concentration of Buttress casing connection and could be an alternative to the development of new products

Simulation of flash-butt welding process of a railway steel. Part 1 : residual stress analysis via FEM.

Trilhos longos, soldados pelo processo Flash Butt Welding (FBW), s?o a realidade das ferrovias brasileiras de m?dia e alta carga por eixo. Embora apresentem caracter?sticas desej?veis do ponto de vista do comportamento din?mico da via, as soldas s?o regi?es de descontinuidade estrutural e mec?nica onde se originam tens?es residuais, e que est?o associadas a falhas prematuras por fadiga. Simula??es num?ricas termomec?nicas, fisicamente n?o-lineares, no dom?nio do tempo, pelo M?todo dos Elementos Finitos (MEF), foram empregadas para avaliar o desenvolvimento de tens?es residuais originadas durante o processo de soldagem. Uma nova abordagem para a inclus?o do aporte de calor envolvido no processo ? proposta. Os resultados num?ricos s?o comparados a medidas experimentais de tens?es residuais superficiais e aspectos macrogr?ficos das juntas, incluindo largura da Zona Termicamente Afetada (ZTA) e dados de taxas de resfriamento. Os resultados mostram boa correla??o entre as an?lises num?ricas e medidas experimentais de tens?es residuais. Aspectos fundamentais relacionados ao desenvolvimento de tens?es residuais s?o esclarecidos a partir da correla??o entre an?lises num?rica e experimental. Ademais, verifica-se que os modelos computacionais podem ser utilizados na previs?o de pontos cr?ticos para nuclea??o de trincas por fadiga e/ou avaliar efeitos de vari?veis de processo sobre o campo de tens?es residuais.Long rails, welded by the Flash-Butt Welding (FBW) process, are the reality of the Brazilian railroads for medium and high axle loads. Although they present desirable characteristics concerning the dynamic behavior of the track, welded joints are regions of structural and mechanical discontinuity where high residual stresses originate, and, consequently, premature fatigue failures may take place. This paper employs the Finite Element Method (FEM) to carry out transient, physically non-linear thermo-mechanical analyses to evaluate residual stresses evolved in the welding process. A new approach is proposed to take into account the heat input involved in the process. The numerical results are compared to experimentally measured surface residual stresses, and to the macrographic joint aspects, including the HAZ width and cooling rate data. The results show a good correlation between the numerical and experimental measurements of residual stresses. Fundamental aspects related to the development of residual stresses are clarified, correlating numerical and experimental analyses. In addition, it is verified that the computational models can be used to predict critical crack nucleation points by fatigue, and/or to evaluate effects of process parameters on the residual stress field