Comparative study of austenite-ferrita transformation in two if steels with different chemical compositions.

Os a?os IF s?o livres de elementos intersticiais, como o C e o N, na matriz ferr?tica. Estes elementos s?o controlados no processo de refino na aciaria, a partir da aplica??o de t?cnicas de desgaseifica??o a v?cuo, juntamente com a adi??o de elementos estabilizantes, como Ti e Nb. Durante a fabrica??o e em algumas aplica??es dos a?os IF, como por exemplo soldagem, a transforma??o de fase austenita?ferrita e o efeito dos elementos de liga na sua cin?tica t?m forte influ?ncia na microestrutura final.Neste contexto, este trabalho caracterizou microestruturalmente e comparou as cin?ticas de transforma??o de fase de dois tipos de a?os IF com composi??es qu?micas diferentes, sendo eles o IF-Ti e o IF-TiNb. Por meio dos resultados obtidos, foi poss?vel concluir que o a?o IF-TiNb, como fabricado, possui uma microestrutura mais refinada devido ? presen?a do Nb e seu papel na lamina??o controlada do a?o. Al?m disso, observou-se que as temperaturas cr?ticas Ar3 e Ar1 do a?o IF-TiNb s?o sistematicamente menores do que as temperaturas medidas para o a?o IF-Ti. Como a fra??o dos elementos microligantes em solu??o s?lida ? muito pequena, atribui-se este efeito ? diferen?a no teor de Mn dos a?os estudados, que parece potencializar o refino de gr?o na etapa final de resfriamento da chapa de a?o laminada.The IF steels are free of interstitial elements, such as C and N, in the ferritic matrix, which are controlled during the steel refining process using vacuum degassing techniques, andstabilizing element additions (Ti and Nb). During the steel manufacturing and in some applications, welding for example, the austenite?ferrite phase transformation and the effect of the alloying elements on the phase transformation kinetics have strong influence on steel final microstructure. In this context, this work characterized the steel microstructures and compared the phase transformation kinetics of two types of IF steels with different chemical compositions: the IF-Ti and the IF-TiNb. The obtained results showed that the IF-TiNb steel has the finer microstructure due to the presence of Nb and its effect on controlled rolling. Besides that, it was observed that the critical temperatures Ar3 and Ar1 for the IF-TiNb steel are systematically smaller than the obtained for the IF-Ti steel. As the fraction of microalloying elements in solid solution is too low, the difference on studied steel Mn contents, possibly is the main cause of this. The Mn content seems to potentiate the ferritic grain refinement during the final steel cooling

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

Study of the kinetics of martensitic transformation in the USISAR 80T microalloyed steel.

No presente trabalho, estudou-se a aplica??o de modelos cin?ticosde transforma??o martens?ticadispon?veis na literatura t?cnica a fim de modelar curvas de fra??o da fase austenita transformada em martensita para o a?o USISAR 80T, estrutural, microligado, de boa temperabilidade e considerado de boa soldabilidade. Ensaios dilatom?tricos foram realizados com taxa de aquecimento de 3?C/s at? a temperatura de austenitiza??o de 920?C, tempo de encharque de 180 segundos e taxas de resfriamento de 25?C/s, 50?C/s, 75?C/s e 100?C/s. Um dos modelos estudados apresentou boa correla??o com os dados experimentais para todas as taxas de resfriamento estudadas, sendo poss?vel ent?o a obten??o de constantes caracter?sticas da transforma??o estudada e a proposi??o de um modelo de previsibilidade para o a?o USISAR 80T.In this work, martensitic kinetic models, available in the technical literature, were studied aiming to model and to predict the austenite to martensite transformation for the USISAR 80T steel, which is a structural, microalloyed, with high hardenability and considered a great weldable steel. Dilatometrictests were performed with a heating rate of 3?C/s until 920?C, with a soaking time of 180 seconds and cooling rates of 25?C/s, 50?C/s, 75?C/s and 100?C/s.One of the studied models showed good agreement with the experimental data for all the employed cooling rates, and it was possible to determinetransformation characteristic constants and suggest a previsibility model for the USISAR 80T steel

Proposition of an empirical model for determination of critical temperatures during continuous cooling in heat affected zones of IF steels welded by the TIG process.

A?os IF s?o amplamente empregados na ind?stria automobil?stica. No decorrer das aplica??es de soldagem, eles passam pela transforma??o de fase austenita (?) ? ferrita (?) que ocorre por nuclea??o e crescimento de gr?o e, em condi??es isot?rmicas, ? descrita pela equa??o JMAK. Todavia, as transforma??es de fases p?s-soldagem acontecem a partir de resfriamento cont?nuo. Assim, o presente trabalho caracterizou a cin?tica de transforma??o ? ? ? e prop?s um modelo emp?rico para prever as temperaturas cr?ticas durante o resfriamento cont?nuo em zonas termicamente afetadas de tr?s tipos de a?os IF. Amostras dos a?os foram submetidas a ensaios dilatom?tricos com diferentes taxas de resfriamento cont?nuo. Determinaram-se as temperaturas cr?ticas de transforma??o de fase (???) e obteve-se uma equa??o emp?rica que correlacionou a temperatura cr?tica com a taxa de resfriamento e que viabilizou a previsibilidade da cin?tica de transforma??o de fases. Processo de soldagem TIG mecanizado foi aplicado nos a?os IF com monitoramento de ciclos t?rmicos e, em regi?es da ZTA com condi??es de austenitiza??o e taxa de resfriamento similares ?s utilizadas nos ensaios dilatom?tricos, verificou-se que a equa??o emp?rica obtida ? uma alternativa vi?vel para prever as temperaturas cr?ticas e a microestrutura durante o resfriamento cont?nuo na ZTA destes materiais.IF steels are widely used in the automotive industry. In the course of the welding applications, they undergo the transformation of austenite (?) ? ferrite (?) phase that occurs by nucleation and grain growth and, under isothermal conditions, is described by the JMAK equation. However, post-weld phase transformations occur under continuous cooling conditions. Thus, the present work characterized the transformation kinetics ? ? ? and proposed an empirical model to predict critical temperatures during continuous cooling in heat affected zones of three types of IF steels. Samples of the steels were submitted to dilatometric tests with different rates of continuous cooling. The critical phase transformation temperatures (? ? ?) were determined and an empirical equation was obtained which correlated the critical temperature with the cooling rate, which enabled the predictability of phase transformation kinetics. Mechanized TIG process was applied to IF steels with thermal cycling monitoring and, in regions of the HAZ with austenitizing conditions and cooling rates similar to those used in the dilatometric tests, it was verified that the obtained empirical equation is a viable alternative to predict the critical temperatures and the microstructure during the continuous cooling in the HAZ of these steels

Characterization of the isothermal phases transformations and the respectives products in three eutectoid steels for railroad application.

Artigos recentemente publicados v?m destacando a import?ncia do conhecimento sobre a cin?tica de transforma??o de fases em a?os eutet?ides aplicados em ferrovias, visando o aprimoramento de processos de fabrica??o e de soldagem. Nesse sentido, o presente trabalho apresenta um estudo de caracteriza??o da cin?tica de decomposi??o isot?rmica da austenita em tr?s a?os atualmente utilizados no mundo para a fabrica??o de trilhos ferrovi?rios, sendo dois de classifica??o premium e um de classifica??o standard. Os a?os estudados foram caracterizados em seu estado de entrega e, por meio de ensaios de dilatometria, as temperaturas cr?ticas de austenitiza??o e os intervalos de tempo de decomposi??o da austenita foram medidos. Diagramas tempo-temperatura-transforma??o (TTT) foram determinados. A influ?ncia de alguns par?metros como o espa?amento interlamelar perl?tico original e o tamanho de gr?o austen?tico pr?vio sobre as transforma??es de fases e sobre as microestruturas resultantes, foram discutidos. Mostrou-se ainda que, a partir de tratamentos isot?rmicos bem planejados, ? poss?vel a obten??o de uma microestrutura majoritariamente bain?tica em a?os eutetoides com composi??es qu?micas relativamente simples.Recently published papers have been enhancing the importance of understanding about the kinetics of phase transformation in eutectoid steels applied to railways, aiming the improvement of the manufacturing and the welding processes. In this sense, this work presents a characterization study on the austenite?s isothermal decomposition kinetics in three steels currently used worldwide in the manufacturing of railway tracks, being two of then premium and the other standard. The steels studied were characterized at initial state and, through dilatometry, the austenitization critical temperatures and the austenite decomposition?s time intervals were measured. Time-temperature-transformation (TTT) diagrams were determined. The influence of some parameters such as the original pearlite interlamellar spacing and the prior austenitic grain?s size on the phase transformations and the resulting microstructures, was discussed. The study also showed that stemming from well-planned isothermal treatments, it is possible to obtain a mostly-bainitic microstructure in eutectoid steels of fairly simple chemical compositions

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

Evaluation of hydrogen-Induced cracking resistance of the In625 laser coating system on a C-Mn steel substrate.

The corrosion of C-Mn steels in the presence of hydrogen sulfide (H2S) represents a significant challenge to oil production and natural gas treatment facilities. The failure mechanism induced by hydrogen-induced cracking (HIC) in a Inconel 625 coating / C-Mn steel has not been extensively investigated in the past. In the present work, an API 5CT steel was coated with In625 alloy using laser cladding and the HIC resistance of different regions, such as the coating surface, the substrate and HAZ, were evaluated. SEM observations illustrated that all HIC cracks were formed at the hard HAZ after 96h of exposure. No HIC cracks were observed in the substrate and the In625 coating after the same exposure duration. Pitting was recorded in the substrate caused by non-metallic inclusion dissolving

Efeito de diferentes tratamentos t?rmicos sobre microestrutura e microdureza de um sistema a?o C-Mn/revestimento de inconel 625.

A utiliza??o de tubos de a?o C-Mn na ind?stria de ?leo e g?s ? muito comum desde o nascimento desta ind?stria. Mesmo com a grande evolu??o da metalurgia destes a?os ao longo das ?ltimas d?cadas, o desafio de desempenho em ambientes severos de corros?o devido ?s caracter?sticas reativas (H2S, CO2) do petr?leo e seus derivados, imp?em uma nova era para a fabrica??o de tubos. A solu??o tecnol?gica mais direta seria a utiliza??o de a?os com alta liga (Ni, Cr) e/ou superligas de n?quel. No entanto, os custos de produ??o tornariam invi?veis pelo alto custo destes materiais. Neste contexto, ? crescente iniciativas de pesquisas para o desenvolvimento de overlay e/ou cladding, ou seja, revestimentos de a?os C-Mn usando superligas de n?quel e a?os inoxid?veis. Neste trabalho, foi utilizada a deposi??o de uma camada da superliga de n?quel Inconel 625 atrav?s do processo laser. Uma das desvantagens da deposi??o a laser ou qualquer outro processo de fus?o ? a forma??o de uma regi?o de descontinuidade microestrutural no a?o, chamada de ZTA (zona termicamente afetada pelo calor). Neste estudo, diferentes rotas de tratamentos t?rmicos foram investigadas com o objetivo de eliminar a ZTA e preservar as caracter?sticas originais do substrato e do revestimento.The application of C-Mn steel pipe in the oil & gas industry is quite common since starting of this industry. Even with the great evolution of the metallurgy of this type of steel over the last decades, the challenge is to overcome the corrosion severity caused by sour (H2S, CO2) species present in the petroleum and its derivates. The direct technological solution would be to replace the C-Mn steel by Ni-based superalloys and/or stainless steels. However, the high costs of these materials would make several projects impracticable. In this context, there are several initiatives in order to develop overlay and/or cladding, that means, coatings of C-Mn steels using superalloys and stainless steels. In this work, it was used an overlay deposition of Inconel 625 on the surface of a C-Mn steel using the laser process. A disadvantage of laser deposition, as it is for any deposition melting process, is the formation of a microstructural discontinuity in the steel substrate caused by the thermal cycle (HAZ = heat affected zone). In this study, different heat treatment routes were investigated aiming to eliminate the HAZ preserving the original characteristics of the substrate and of the coating