Structure and properties of an austenitic stainless steel AISI 316L grade ASTM F138 after low temperature plasma nitriding

Os aços inoxidáveis austeníticos possuem restrições para a nitretação nas temperaturas convencionais, próximas de 550ºC, devido à precipitação intensa de nitretos de cromo na zona de difusão. Essa precipitação eleva a dureza, mas deteriora as propriedades de corrosão. O uso do processo de nitretação sob plasma permite introduzir nitrogênio em temperaturas inferiores a 450ºC, levando à formação de uma fina camada de austenita expandida pelo nitrogênio (gN). Essa fase possui uma estrutura cristalina mais bem representada pelo reticulado triclínico, com elevada concentração de nitrogênio em solução sólida supersaturada, a qual promove um estado de tensões residuais de compressão capaz de elevar a dureza do substrato de 4 GPa para valores próximos de 14 GPa. O Módulo de Elasticidade mantém-se próximo de 200 GPa após a nitretação.Austenitic stainless steels cannot be conventionally nitrided at temperatures near 550°C due to the intense precipitation of chromium nitrides in the diffusion zone. The precipitation of chromium nitrides increases the hardness but severely impairs corrosion resistance. Plasma nitriding allows introducing nitrogen in the steel at temperatures below 450°C, forming pre-dominantly expanded austenite (gN), with a crystalline structure best represented by a special triclinic lattice, with a very high nitrogen atomic concentration promoting high compressive residual stresses at the surface, increasing substrate hardness from 4 GPa up to 14 GPa on the nitrided case.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Numerical modeling of flow stress and grain evolution of an Mg AZ31B alloy based on hot compression tests

Magnesium alloys offer a wide range of applications in modern lightweight structures, although the correct forming parameters need to be found to reach a good combination of fine microstructure and the required mechanical properties. Several discrete and statistical methods have been proposed to simulate the dynamic recrystallization process and adopted to study microstructural evolution. However, the materials parameters necessary to develop these models are not widely available. Hence, industrial evaluation of these parameters is complex, unpractical for several types of material and time consuming for daily industrial applications. In that way, the thermomechanical behavior and grain size evolution modeling of the AZ31 alloy are proposed using isothermal compression data. Parameters to calculate coupled stress–strain–temperature parameters, dynamic recrystallization, volume fraction and grain size were obtained from the stress–strain curves. Then, the data were input in Deform-3D software to simulate the hot deformation process and verify with experimental data the consistency of the values obtained. Measured grains size agreed with the conducted modeling, showing the reliability of strain–stress and grain size data on predicting dynamic recrystallization phenomena.Postprint (author's final draft

Assessment of the cavitation erosion resistance in high nitrogen austenitic stainless steels: study of the wear mechanisms

Amostras do aço inoxidável UNS S31803, nitretadas em alta temperatura, com 0,9 % de nitrogênio em solução sólida foram submetidas a ensaios de cavitação vibratória em água destilada. As amostras foram, previamente, caracterizadas por meio de difração de elétrons retroespalhados, EBSD, num microscópio eletrônico de varredura (MEV). Posteriormente, durante os ensaios de cavitação, o dano superficial das amostras foi, periodicamente, avaliado por observação no MEV das superfícies desgastadas. O aço austenítico convencional UNS S30403 foi usado como material de comparação. Nas primeiras etapas dos ensaios de cavitação, ocorreu deformação plástica da superfície, que pôde ser caracterizada como altamente heterogênea na escala do tamanho de grão. Em etapas posteriores, ocorreu perda de massa por desprendimento de partículas de desgaste (debris), como conseqüência de fadiga de baixo ciclo. O início do dano ocorreu tanto no interior dos grãos como nos contornos de grão; os contornos de macla mostraram as regiões mais suscetíveis. Os grãos com planos (101) orientados aproximadamente paralelos à superfície das amostras apresentaram maior resistência ao desgaste que os grãos com outras orientações cristalográficas. A maior resistência ao desgaste dos grãos com textura (101) || superfície foi atribuída a uma menor tensão projetada para deformar, plasticamente, esses grãos. A diminuição da referida tensão se dá em função das tensões impostas na superfície cavitada pela implosão de bolhas de vapor.Specimens of a UNS S31803 steel were submitted to high temperature gas nitriding and then to vibratory pitting wear tests. Nitrided samples displayed fully austenitic microstructures and 0.9 wt. % nitrogen contents. Prior to pitting tests, sample texture was characterized by electron backscattering diffraction, EBSD. Later on, the samples were tested in a vibratory pit testing equipment using distilled water. Pitting tests were periodically interrupted to evaluate mass loss and to characterize the surface wear by SEM observations. At earlier pit erosion, stages intense and highly heterogeneous plastic deformation inside individual grains was observed. Later on, after the incubation period, mass loss by debris detachment was observed. Initial debris micro fracturing was addressed to low cycle fatigue. Damage started at both sites, inside the grains and grain boundaries. The twin boundaries were the most prone to mass-loss incubation. Grains with (101) planes oriented near parallel to the sample surface displayed higher wear resistance than grains with other textures. This was attributed to lower resolved stresses for plastic deformation inside the grains with (101) || surface texture.Colômbia - COLCIENCIASCoordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES

Investigation of material flow and thermomechanical behavior during friction stir welding of an AZ31B alloy for threaded and unthreaded pin geometries using computational solid mechanics simulation

In the friction stir welding process, the tool role in the material flow and its thermomechanical behavior is still not entirely understood. Several modeling approaches attempted to explain the material and tool relationship, but to this date, insufficient results were provided in this matter. Regarding this issue and the urgent need for trustful friction stir welding models, a computational solid mechanic's model capable of simulating material flow and defect formation is presented. This model uses an Arbitrary Lagrangian-Eulerian code comparing a threaded and unthread pin profile. The model was able to reproduce the tool's torque, temperatures, and material flow along the entire process, including the underreported downward flow effect promoted by threaded pin's. A point tracking analysis revealed that threads increase the material velocity and strain rate to almost 30% compared to unthreaded conditions, promoting a temperature increment during the process, which improved the material flow and avoided filling defects. The presented results showed the model's capability to reproduce the defects observed in real welded joints, material thermomechanical characteristics and high sensitivity to welding parameters and tool geometries. Nevertheless, the outcomes of this work contribute to essential guidelines for future friction stir welding modeling and development, tool design, and defect predictionPeer ReviewedPostprint (author's final draft

Dependence of wear and mechanical behavior of nitrocarburized/CrN/DLC layer on film thickness

Diamond-like carbon (DLC) films are amorphous metastable carbon form that provide interesting mechanical and tribological properties. The role of film thickness influence upon wear and mechanical properties is of interest and not yet fully reported. In this study, two samples of previously plasma nitrocarburized, quenched and tempered H13 steel were duplex treated. First, a physical vapor deposition (PVD) chromium nitride (CrN) layer was applied, followed by a top final diamond-like carbon layer applied by plasma-enhanced chemical vapor deposition (PECVD). To evaluate thicknesses influence on mechanical and wear properties of coatings, samples were treated using two different thicknesses of both layers. In this study, the thickest CrN and DLC case presented higher hardness and better tribological properties, however, its failure occurs in brittle fashion.Peer ReviewedPostprint (published version

Influence of hydrogen on the microstructure and fracture toughness of friction stir welded plates of API 5L X80 pipeline steel

In this work, the influence of hydrogen on the microstructure and fracture toughness of API 5L X80 high strength pipeline steel welded by friction stir welding was assessed. Samples were hydrogenated at room temperature for a duration of 10 h in a solution of 0.1 M H2SO4 + 10 mg L-1 As2O3, with an intensity current of 20 mA cm-2. Fracture toughness tests were performed at 0 °C in single-edged notched bending samples, using the Critical Crack Tip Opening Displacement (CTOD) parameter. Notches were positioned in different regions within the joint, such as the stir zone, hard zone, and base material. Hydrogen induces internal stress between bainite packets and ferrite plates within bainite packets. Besides, hydrogen acted as a reducer of the strain capacity of the three zones. The base metal had a moderate capacity to resist stable crack growth, displaying a ductile fracture mechanism. While the hard zone showed a brittle behavior with CTOD values below the acceptance limits for pipeline design (0.1–0.2 mm). The fracture toughness of the stir zone is higher than that of the base metal. Nevertheless, the stir zone displayed higher data dispersion due to its high inhomogeneity. Hence, it can also show a brittle behavior with critical CTOD values.Peer ReviewedPostprint (author's final draft

Spheroidized pearlite formation in hypereutectoid steel.

Estudou-se a cinética e os mecanismos de formação de perlita esferoidizada em aço hipereutetóide, por meio de técnicas de tratamentos isotérmicos interrompidos e dilatometria. O aumento da temperatura de austenitização promoveu deslocamento das curvas TTT da perlita para a direita. Nas amostras que, após austenitização, apresentaram carbonetos não dissolvidos na austenita, o deslocamento das curvas foi maior para superesfriamentos menores; nas amostras austenitizadas em temperaturas maiores e que apresentaram carbonetos não dissolvidos, o deslocamento das curvas TTT foi maior para superesfriamentos maiores. Observou-se que a formação de perlita esferoidizada ocorria pela movimentação de uma interface alfa-gama sobre austenita contendo carbonetos não dissolvidos. O aumento da fração volumétrica de carbonetos que acompanhava a reação, ocorria principalmente por crescimento dos carbonetos já existentes na austenita quando em contato com a interface. Foram observados, entretanto, nucleação de novos carbonetos na interface ferrita austenita e crescimento de carbonetos na ferrita após a passagem da interface. Foram medidas a fração volumétrica, o número de colônias por unidade de volume e a velocidade de crescimento de colônias de perlita formadas após austenitização em diferentes temperaturas. Observou-se que a velocidade de crescimento é constante e, assim como a taxa de nucleação, sensível à estrutura. A velocidade de crescimento foi maior nas colônias de perlita esferoidizada formadas a partir da austenita, contendo carbonetos não dissolvidos, do que nas de perlita lamelar resultantes da transformação da austenita de carbonetos. Estudou-se a cinética de crescimento das estruturas esferoidizadas utilizando-se modelo de difusão com gradiente de concentraçãoconstante. Os coeficientes de difusão aparentes calculados resultaram compatíveis com processos de difusão de carbono na austenita e difusão de elementos substitucionais ao longo da interface ferrita-austenita. Com base nestes resultados, discutiu-se a competição entre a formação de estruturas esferoidizadas e lamelares nos aços hipereutetóides.The mechanisms and kinetics of spheroidized pearlite formation on hypereutectoid steel has been studied. Isothermal heat treatments and dilatometric technics, were used. The increase of the austenitizing temperature caused the displacement of the IT curves to the right. On specimens which after austenitizing treatment showed undissolved carbides in austenite, the displacement of the curves was higher for smaller undercooling. On specimens austenitized under higher temperatures, which were free from undissolved carbides, the displacement of the IT curves was larger for higher undercooling. It was observed that the formation of spheroidized pearlite, took place through the movement of an alfa-gama interface upon austenite containing undissolved carbides. The increase in carbide volume fraction, that followed the reaction, took place mainly, by growth of undissolved carbides when in contact with the interface. However, nucleation of new carbides at the ferrite-austenite interface and growth of carbides in ferrite after the passage of the interface, were observed. The volume fraction, number of colonies per unit volume and the growth velocity of pearlite formed from different austenitizing temperatures, were measured. It was observed that the growth velocity is constant and as the nucleation rate structure sensitive. The kinetics of spheroidized pearlite growth was studied using a constant concentration gradient diffusion model. The apparent diffusion coefficients calculated were in accordance with diffusion of carbon in austenite and diffusion of substitutional elements in the ferrite-austenite interface. Based on the above results, the competition between the formation of spheroidized and lamellar pearlite in hypereutectoid steels is discussed

Plastic deformation of pearlitic microstructures in carbon steels.

Faz-se, inicialmente, uma revisão das teorias relativas ao endurecimento e dos mecanismos de deformação plástica de ligas de duas fases com precipitados grosseiros. Em particular, apresentam-se os resultados relativos às observações metalográficas de estruturas perlíticas deformadas em ensaio de tração e por trefilação. Desenvolve-se, na parte experimental, o estudo da deformação plástica de estruturas perlíticas de aço carbono eutetóide, através de ensaios de tração e observação metalográfica das estruturas de deformação em: perlita parcialmente esferoidizada, perlita grossa e perlita fina. Fez-se a caracterização dos modos de deformação de estruturas perlíticas para pequenas deformações. Obtiveram-se evidências de operação de sistemas de escorregamento na ferrita paralelos à interface /Fe3C. Observou-se que, nos primeiros estágios de deformação, as colônias se deformam como um empilhamento de cartas, em falhas de crescimento da perlita e em contornos de colônia. Registrou-se, paralelamente, a presença de escoamento nítido em estruturas perlíticas grosseiras. Observou-se que a curva tensão real x deformação plástica real do aço com estrutura perlítica, nessa faixa de deformações, segue uma função potencial com expoentes de encruamento em torno de 0,9. O aço com estrutura parcialmente esferoidizada apresenta uma função potencial com expoente de encruamento em torno de 0,65. Interpreta-se esses resultados com base nas observações metalográficas realizadas e nos micromecanismos propostos para a deformação do material. Discute-se esses resultados à luz dos modelos de encruamento de ligas de duas fases, propostos na literatura. Observou-se, além disso, que, para deformações maiores, ocorre a formação de bandas de cisalhamento que atravessam as colônias.Estas bandas são constituídas de escorregamento na ferrita e microtrincas nas lamelas de cementita. A cementita apresenta comportamento frágil no processo de deformação. Registra-se uma queda da taxa de encruamento do material quando se inicia o processo acima descrito. Discute-se, ainda, os mecanismos relativos à deformação plástica do agregado de colônias, aplicando-se critérios de compatibilidade entre microconstituintes no processo de deformação.Theories on hardening and mechanisms of plastic deformation of two-phase alloys with coarse precipitates, are reviewed. Emphasis is given on the results of the metallographic observations of pearlite deformed by tension tests and by drawing. In the experimental part of this dissertation, the study of the plastic deformation of pearlite in euctetoid carbon steel is developed by tension tests and metallographic observations of three deformed structures: partially spheroidized pearlite, coarse pearlite and fine pearlite. The deformation modes of these structures were pointed out, for small plastic straims. Slip in ferrite, on slip systems parallel to the /Fe3C interface, was observed. At the initial stages of the deformation, pearlite colonies deform like a deck of cards. Slip at pearlite growth faults and at colony boundaries was observed. Discontinuous yielding in coarse pearlite was also verified. It was noted that the true-stress x true-plastic-strain curve of the pearlitic steel, within this range of deformation may be described by a power function with a work hardening exponent around 0,9. For the partially spheroidized steel, the power function has a work hardening exponent around 0,65. These results are interpreted on the basis of the metallographic observations and of the micromechanisms proposed for the deformation of the material. Discussion of these results is presented by using the existing work hardening models for two-phase alloys. For greater deformations, shear bands formed across the colonies, were observed. These bands are composed by slip in ferrite and microcracks in cementite lamellas. Cementite presented a brittle behavior throughout the deformation. At the beginning of the bands formation, a decrease of the work hardening rate was verified. The plastic deformation mechanisms of the colonies aggregate are discussed, by applying compatibility criteria to the microstructure on the deformation process