Abstract This research shows two new microalloyed steels with niobium (Nb) for railroad wheels (Nb1 and Nb2) applied on heavy haul transportation (load over 30 tons/axle). The steels have mixed microstructure composed of pearlite and bainite (finished wheel rim section), and another one composed of martensite with bainite (black wheel section removed on machining) obtained by continuous cooling quenching heat treatment followed by tempering. The bainitic and martensitic microstructure are sensitive to the temper embrittlement phenomenon; therefore, knowing the embrittlement temperature range of these steels is mandatory in order to avoid such an event in the tempering process. Charpy specimens were extracted from the rim of railroad wheels, austenitized (860 ºC), quenched in water for four minutes, and tempered at different temperatures to obtain the embrittlement curve. Three materials were compared: AAR (Association of American Railroads) Class C steel with pearlitic microstructure (not sensitive to embrittlement), Nb1, and Nb2 microalloyed steels with martensitic, bainitic, and pearlitic microstructures. The microalloyed steels have niobium (Nb), molybdenum (Mo), and silicon (Si) addition. The effect of Si content and the microstructure (analyzed by Scanning Electronic Microscope (SEM)), regarding the embrittlement phenomenon, was also studied. The microstructure of Nb2 steel was analyzed by with Transmission Electronic Microscope (TEM) for improving its characterization.

During welding of the superduplex stainless steels are exposed to thermal cycles, allowing precipitation of deleterious phases as well as unbalance as the ferrite and austenite phases. This work analyzed the microstructural changes taking place on a plate ASTM A890/A890M - grade 6A steel submitted to arc welding one welding of pass conducted on a flat surface using coated electrode Zeron®100, in the as welded and post-welded heat treatment conditions. Microstructural characterization in the regions base metal, heat affected zone, and fusion zone were conducted by optical microscopy, scanning electron microscopy and X-ray diffraction. The post-weld heat treatment in the fusion zone changed the ferrite/austenite volumetric fraction from 59/41 to 52/48. Cr, Mo and Ni distributions in the fusion zone were altered after the post-weld heat treatment, with the concentration ratio ferrite/austenite being around 1.0; 1.1 and 0.9 to 1.1; 1.4 and 0.7, respectively. In addition, hardness reduced from 300 to 270 HV0.3 at the fusion zone.

Heavy haul transportation (load over 30 tons/axle), as well as the axle load, has been more and more used in Brazil and worldwide. The stress generated in the wheel-rail contact with loads up to 30 tons/axle is around 760 MPa, which causes premature wear and cracks of conventional wheels (AAR (Association of American Railroads) class C). Microalloyed wheels are fundamental on heavy haul transport, whose main function is to combine high hardness, ductility, and yield strength of the material in order to prevent shelling. The main purpose of this research is to develop a new microalloyed wheel steel with niobium addition that meets all the requirements of the AAR class D material with mixed microstructure composed of pearlite and bainite. The 0.71C/0.020Nb steel developed in this study (Nb material) for railroad achieved the standards required for AAR class D in all mechanical properties, with fracture toughness higher than the usual vanadium microalloyed steel used in comparison. The Continuous Cooling Transformation (CCT) diagram showed the presence of bainite even at very low cooling rate, in the range between 0.3 - 2 °C/s. These cooling rates to form bainite are much lower than those observed in other steels with similar composition.

Abstract: Fracture mechanics approach is important for all mechanical and civil projects that might involve cracks in metallic materials, and especially for those using welding as a structural joining process. This methodology can enhance not only the design but also the service life of the structures being constructed. This paper includes detailed consideration of several practical issues related to the experimental procedures to assess the fracture toughness in high strength low alloy steels (HSLA) using the crack tip opening displacement (CTOD) parameter, specifically pipeline steels for oil and gas transportation. These considerations are important for engineers who are new in the field, or for those looking for guidelines performing different procedures during the experimentation, which usually are difficult to understand from the conventional standards. We discuss on topics including geometry selection, number of replicate tests, fatigue precracking, test procedure selection and realization, reports of results and other aspects.

Os aços inoxidáveis austeníticos são materiais de alto valor agregado, representando uma parcela importante das ligas usadas, principalmente, nas indústrias aeronáutica, química, naval, alimentícia e biomecânica. Apresentam boas propriedades mecânicas aliadas à elevada resistência à corrosão. Quando submetido à deformação a frio, esses aços exibem uma fase metaestável denominada martensita induzida por deformação (ferromagnética), cuja formação aumenta a resistência mecânica e conformabilidade, permitindo sua ampla gama de aplicações. Aquecida acima da temperatura ambiente, a martensita induzida por deformação se transforma em austenita. Existem dados na literatura sobre a formação da martensita induzida por deformação em aços austeníticos 18Cr/8Ni, mas não há essa informação em ligas de alto teor de níquel como A286 (26Ni, 15Cr), Incoloy 800 (30-40 Ni, 21Cr) e Inconel (50Ni, 19Cr). Portanto esse trabalho foi realizado para verificar a formação de martensita induzida por deformação, após o trabalho a frio de ligas Fe-18Cr base com adição de até 60% Ni. A reversão dessa fase para austenita, após recozimento até 600ºC, também foi estudada. Microscopia óptica, ensaios de caracterização magnética e difração de raios X foram usados para analisar as transformações.

Austenitic stainless steels represent a significant portion of the alloys used in the aeronautical, chemical, shipbuilding, food processing and biomechanical industries. They combine good mechanical properties with high corrosion resistance. When subjected to cold deformation, these steels exhibit a metastable phase called: strain induced martensite (ferromagnetic), whose formation increases mechanical strength and formability, allowing for a wide range of applications. Heated from room temperature, the strain induced martensite transforms to austenite (non-magnetic). It is easy to find information in literature about the strain induced martensite for 18Cr/8Ni austenitic steels, but there is no data for high nickel alloys like A286 (26Ni, 15Cr), Incoloy 800 (30-40 Ni, 21Cr) and Inconel (50Ni, 19Cr). Therefore, this study aimed to verify the formation of strain induced martensite after cold working in Fe-18Cr base alloys with the addition of up to 60 %Ni. The reversion of this phase to austenite after annealing up to 600 ºC was also studied. Optical microscopy, magnetic characterization tests, and x-ray diffraction were used to analyze the transformations.

As propriedades de recuperação de forma da liga à base de Fe-Mn-Si-Cr-Ni-Co foram estudadas usando ensaio de compressão. Foram analisadas medidas de recuperação elástica (Re), recuperação de forma (Rf) e recuperação de forma total (R T = Re + Rf) em função do número de ciclos de treinamento. Os resultados indicaram que 3,3 foi a melhor razão entre altura (h o = 20 mm) e diâmetro (φo = 6 mm) para se conseguirem uma deformação homogênea e curvas de carregamento bem definidas. A maior contribuição para a R T foi atribuída à recuperação de forma. No último ciclo de treinamento, foi obtida uma R T = 90%, sendo 25% atribuída a Re.

The shape recovery properties of an Fe-Mn-Si-Cr-Ni-Co based alloy were studied using compression tests. Analyzed were the elastic recovery (Er), shape recovery (Sr) and total shape recovery (T SR = Er + Sr) measurements as a function of training cycles. The results indicated that 3.3 was the best ratio between height (h o = 20 mm) and diameter (φo = 6 mm) to obtain a homogeneous deformation and defined loading curves. A major contribution of T SR was attributed to shape recovery. In the last training cycle, a T SR = 90 % was obtained, where 25% was attributed to Er.

O material estudado consiste de uma série de ligas com composição base (% em peso) de 18Cr - 0,01C - 0,2Si - 0,4Mn e teores de níquel variando de zero a 60%. Analisou-se a microestrutura por microscopia ótica das ligas no estado recozido e após deformação à temperatura ambiente, 350 e 700ºC, de modo a simular as altas temperaturas alcançadas na usinagem dessas ligas. Foram também gerados diagramas de equilíbrio das ligas por termodinâmica computacional (Thermocalc) para se prever o comportamento dessas ligas em uma larga faixa de temperaturas. As condições teóricas de equilíbrio termodinâmico do sistema foram comparadas às microestruturas observadas, indicando que a fase CFC nas ligas com teor de níquel entre 10% e 30% em peso está, na realidade, em uma condição metaestável à temperatura ambiente. Foi observada transformação martensítica induzida por trabalho a frio na liga com 10%Ni, validando os cálculos computacionais.

The material studied consists of a series of alloys with a basic composition (weight %): 18Cr - 0.01 C - 0.2 Si - 0.4 Mn, and levels of nickel varying from zero to 60%. Alloys in the annealed condition and after deformation at room temperature, 350 and 700°C to simulate the high temperatures achieved during machining were observed by optical microscopy. Equilibrium diagrams were generated by computational thermodynamics (Thermocalc) to predict the behavior of these alloys in a wide range of temperatures. The theoretical thermodynamic equilibrium conditions of the system were compared to the observed microstructures, indicating that the CFC phase in alloys with nickel content between 10% and 30% is, in fact, in a metastable condition at room temperature. Martensitic transformation induced by cold work in the alloy with 10% Ni was observed, validating the computer calculations.

A resistência à corrosão por pite tem sido correlacionada com as condições morfológicas da superfície: um acabamento superficial liso contribui para diminuir o potencial de iniciação de pites. Esse estudo teve por objetivo investigar a relação entre a resistência à corrosão por pite e a rugosidade em superfícies usinadas do aço inoxidável superaustenítico ASTM A744 grau CN3MN. As amostras fundidas sofreram um torneamento cilíndrico com diferentes combinações das condições de corte, resultando, assim, em diferentes padrões de rugosidade superficial. As superfícies das amostras, então usinadas, foram caracterizadas quanto à rugosidade e ao endurecimento. Após submetidas a um teste por imersão, que acelerava a corrosão, essas superfícies foram examinadas num esteroscópio, tendo sido, também, determinada a perda de peso devido à corrosão. Observou-se que as amostras exibiram diferentes comportamentos com relação à resistência à corrosão, de acordo com as condições de usinagem aplicadas. Uma correlação entre a resistência à corrosão e a rugosidade superficial ficou evidente, assim como à perda de peso devido à formação de pites. Esse estudo identificou que a corrosão pode ser controlada através da seleção dos parâmetros de usinagem apropriados.

Pitting corrosion resistance has been correlated to the morphological conditions of the surface: a smooth surface finishing decreases the potential for pitting. This study aimed at investigating the relationship between pitting corrosion resistance and surface roughness in the machined surfaces of superaustenitic stainless steel ASTM A744 grade CN3MN. The samples of the casting steel were cylindrically turned sunder different combinations of cutting conditions, producing different surface roughness patterns. The surfaces of the samples, as machined, were characterized by roughness and hardness. After the application of an accelerated immersion corrosion test, these surfaces were examined in a stereoscope and the weight loss by corrosion was also determined. It was revealed that the samples exhibited different corrosion resistance behaviors, according to the machining conditions applied. A correlation between pitting resistance corrosion and machined surface roughness was evident, and also, the weight loss due to the formation of pits. This study has identified that corrosion can be controlled through the selection of appropriate machining parameters.

O aço inoxidável superaustenítico ASTM A 744 Gr. CN3MN é aplicado na fabricação de equipamentos que trabalham em ambientes sob corrosão severa com solicitação mecânica. Nesse trabalho, investigou-se a influência do tratamento térmico de envelhecimento na microestrutura e nas propriedades de impacto desse tipo de material. Foram realizados tratamentos térmicos de envelhecimento a 900°C por 1,5; 12; 24; 36 e 48 horas. Ensaios de impacto na temperatura ambiente e a -46°C foram realizados nas amostras tratadas termicamente. As análises microestruturais foram feitas por meio de microscopia eletrônica de varredura e difração de raios X. Concluiu-se que quanto maior a o tempo de exposição do material à temperatura de 900°C, menor é a energia absorvida no impacto. Com 1,5 horas o material apresentou redução na resistência ao impacto de 128 para 25 Joules. O tratamento térmico a 900°C por 48 horas causou a precipitação de algumas fases na matriz austenítica, sendo as mais prováveis: sigma (σ), chi (χ) e carboneto M23C6.

ASTM A 744 Gr. CN3MN superaustenitic stainless steel is employed in the manufacture of equipments designed to work in severely corrosive environments under mechanical loads. This research investigated the influence of aging heat treatments on the microstructure and impact properties of this type of material. These treatments were carried out at temperature of 900ºC for different periods of time: 1.5; 12; 24; 36 and 48 hours. Impact Charpy tests were conducted at room temperature and -46°C for all heat treated samples. The microstructural analyses were carried out by optical microscopy, scanning electron microscopy and X-ray diffraction. It was concluded that as long as the steel was exposed to 900ºC, the energy absorbed during impact was lower. After 1.5 hours at 900ºC the impact energy dropped from 128 to 25 Joules. The samples heat treated at 900ºC for 48 hours showed precipitation of some phases at the austenitic matrix: the most probable were sigma (σ), chi (χ) and M23C6 carbide.

Foram realizados ensaios de tração na temperatura ambiente em ligas Fe-18%Cr, com teor de níquel variando entre zero e 60% e correlacionaram-se as propriedades obtidas com os resultados de usinabilidade obtidos anteriormente pelo grupo. As ligas foram ensaiadas, também, a 350 e 700ºC, de modo a determinar suas propriedades, na faixa de temperatura alcançada, no local da usinagem. A liga ferrítica apresentou alongamento e limite de resistência menores que as ligas austeníticas, tanto à temperatura ambiente, como a 350°C, o que explica porque, na usinagem dessas ligas, a vida da ferramenta foi muito maior que nas ligas austeníticas (100 contra 20 a 50 minutos, respectivamente). No ensaio a 350ºC, temperatura atingida pela peça na região de usinagem, o aumento do teor de níquel elevou, tanto o limite de resistência das ligas austeníticas, o que dificulta a usinagem, como, também, elevou a ductilidade (alongamento), facilitando a adesão da liga sobre a ferramenta de corte e exigindo maior esforço na usinagem, com conseqüente redução da vida da ferramenta.

Tensile tests were carried out at room temperature in Fe-18% Cr alloys, with nickel content varying between zero and 60%, and the properties obtained were correlated with the results of machinability previously obtained by the group. The alloys were also tested at 350 and 700°C in order to obtain these properties in the range of temperature compatible with the machining process. The ferritic alloy presented lower elongation and UTS (ultimate tensile strength) than austenitic alloys at room temperature and 350°C, which explains why, in the machining process, the ferritic alloy shows much greater tool life if compared to austenitic alloys (100 against 20 to 50 minutes, respectively). In the test at 350°C, the temperature reached during machining, the increase in nickel content increased the UTS of austenitic alloys, hampering the machining, and also elevated the ductility (elongation) that produces a higher volume of material adhered on the tool. This higher adhesion capacity, allied to a higher mechanical strength, explains the increase of tool wear as a function of increasing nickel content in the alloy with a consequent reduction in the life of the tool.

O aço inoxidável superaustenítico ASTM A 744 Gr. CN3MN é aplicado na fabricação de equipamentos que trabalham em ambientes sob corrosão severa com solicitação mecânica. Nesse trabalho investigou-se a influência do tratamento térmico de solubilização na microestrutura e nas propriedades desse tipo de material. Foram realizados tratamentos térmicos de solubilização na faixa de temperaturas entre 1100 e 1250°C. Ensaios de impacto (Charpy) em temperatura ambiente e a -46°C foram realizados nas amostras tratadas termicamente. As análises microestruturais foram feitas por meio de microscopia eletrônica de varredura, eletrônica de transmissão e difração de raios X. Concluiu-se que, para maximizar a resistência ao impacto, a solubilização deve ser feita a 1200°C, pois tal medida produz a menor fração volumétrica de precipitados. As amostras solubilizadas a 1200 e 1240°C apresentaram fase sigma (s) e carboneto M6C.

This research investigated the influence of solution heat treatments on the microstructure and properties of this type of material. These treatments were carried out at temperatures ranging from 1100 to 1250ºC. Impact (Charpy) tests were conducted at room temperature and -46°C for all solution treated samples. The microstructural analyses were carried out by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. It was concluded that, to maximize the material's impact strength, the solution heat treatment should be done at 1200ºC, at which temperature the volumetric fraction of precipitates is lower than at other solution heat treatment temperatures. The samples that were solution heat treated at 1200 and 1240ºC presented sigma (s) and M6C carbide phases.

RESUMO Esse trabalho apresenta um estudo da dureza e do mee austenita-γ da liga inoxidável à base de ferro com efeito de memória de forma. Utilizando indentação instrumentada, foi possível determinar a dureza e o módulo de elasticidade das fases austenita-γ e martensita-ε separadamente. A fase martensítica apresentou uma dureza de 7,0 GPa, enquanto que a dureza da fase austenítica foi de 3,0 GPa. Os valores de módulo de elasticidade foram 202 e 137 GPa, para as fases martensita e austenita, respectivamente.

ABSTRACT This work presents a hardness study and elastic modulus for the ε-martensite and γ-austenite phases of an iron based shape memory alloy. Using instrumented indentation, it was possible to determine the hardness and elasticity modulus of these phases separately. The martensitic phase presented a hardness of 7.0 GPa and the austenitic phase presented a hardness of 3.0 GPa. The elastic modulus values were 202 and 137 GPa for the martensitic and austenitic phases, respectively.

The aim of this work was to study the effect of austenitic grain size (GS) reduction on the structural parameters of the epsilonhcp - martensite in stainless shape memory alloy (SMA). Rietveld refinement data showed an expansion in c-axis and a reduction in a and b-axis with thermo-mechanical cycles for all samples analyzed. Samples with 75 < GS (µm) < 129 were analyzed. It was also observed an increase of the unit cell volume in this phase with GS reduction. The smallest grain size sample (GS = 75 µm) presented a c/a ratio of 1.649, and approximately 90% of total shape memory recovery.

O objetivo desse trabalho é estudar a influência de adições de níquel sobre as propriedades mecânicas de ligas Fe-Cr. Para atender a esse propósito, foram preparadas diversas ligas com a composição básica 18%Cr - 0,01%C - 0,2%Si - 0,4%Mn, variando-se o teor de níquel (0, 10, 20, 40 e 60 % em peso). A caracterização das ligas foi feita por difração de raios X e análise térmica. Foi determinada a dureza das ligas no estado como recebido e após a imposição de deformação a frio. Foram realizados ensaios de tração à temperatura ambiente e a 350ºC, para verificar o efeito da temperatura sobre o comportamento mecânico das mesmas. O objetivo da caracterização mecânica das ligas é o de associar as propriedades mecânicas com as propriedades de usinabilidade, obtidas em estudo que está sendo desenvolvido em paralelo.

The aim of this work was to study the influence of Ni additions on the mechanical properties of Fe-Cr alloys. For this purpose, alloys were prepared with an 18%Cr-0.01%C-0.2%Si-0.4%Mn base composition and variable Ni content (0, 10, 20, 40 and 60 weight %). The alloys were characterized by X-Ray diffraction and thermal analyses. Their hardness was determined before and after cold deformation. Tension tests were carried out at room temperature and 350ºC to verify the effect of temperature on the mechanical behavior of the alloys. The mechanical characterization aim was to use these figures to correlate mechanical properties and machinability (results originating from a parallel study).

O Efeito de Memória de Forma em ligas inoxidáveis está associado à transformação martensítica não termoelástica gama (cfc)<FONT FACE=Symbol>«</FONT> épsilon (hc). Trabalhos recentes do grupo têm demonstrado que o tamanho de grão da fase austenítica é um dos parâmetros importantes no grau de recuperação de forma bem como em outras propriedades como a tensão de escoamento, dureza e teor de martensita induzida. Usando a técnica de EBSD, esse trabalho mostra que, além dos parâmetros anteriormente mencionados, o número de orientações das variantes da martensita decresce com o decréscimo do tamanho de grão.

The shape memory effect of stainless shape memory alloy is associated to non-thermolastic gamma (CFC) <FONT FACE=Symbol>«</FONT> epsilon (HCP) martensitic transformation. Recent results generated by our group have demonstrated that the parent austenite grain size is an important parameter for the degree of shape recovery and also in some others properties such as yield stress, hardness and volume fraction of thermal martensite. Using EBSD, this work shows that besides the above-mentioned parameters, the number of martensite orientation variants decreases when the grain size decreases.