The mechanical behavior of a pearlitic steel was studied by means of tensile tests performed in the temperature interval from 298 to 773 K, at strain rates from 10 Ϫ4 to 10 Ϫ2 s Ϫ1, aiming to analyze the role of cementite decomposition in the dynamic strain aging (DSA) phenomena occurring in these steels. Typical features of DSA were observed: serrated flow (the Portevin-LeChatelier-PLC effect), the presence of a maximum and a minimum in ultimate tensile strength and reduction of area versus temperature curves, respectively, and a less evident maximum in the yield strength versus temperature curve. Apparent activation energies were calculated based on the onset of the PLC effect, the maximum in ultimate tensile strength and the minimum in reduction of area. Results suggest that changes in mechanical properties associated with DSA in pearlitic steels are related to cementite decomposition. Differences on the phenomenological aspects regarding DSA in pearlitic and in low carbon steels are discussed, based on the kinetics of the process.KEY WORDS: dynamic strain aging; Portevin-LeChatelier effect; pearlitic steels; cementite decomposition.sized that, for the values of e˙and T in between the end of the PLC effect and the maximum in s t , the work hardening rate was high and the ductility was very low, irrespectively of the content of soluble nitrogen in the steel. However, the mechanism responsible for these effects was not discussed in their work.The PLC effect and a maximum in the 10 % flow stress versus temperature was also observed by Tsuzaki et al.,18) in a pearlitic steel tested between 293 and 623 K, at strain rates of 3.3ϫ10 Ϫ4 and 3.3ϫ10 Ϫ3 s Ϫ1. The activation energies for these phenomena were not determined in their work and the observed DSA effects were associated with the interaction of carbon atoms in solid solution with dislocations in ferrite.The present work was undertaken to provide a more complete description of the phenomenological and kinetic aspects of dynamic strain aging in pearlitic steel. The aim was to verify if cementite decomposition plays an important role in DSA phenomena taking place in pearlitic steels, as it does in static strain aging, as well as to compare the behavior of this steel with that generally observed in low carbon steels. ExperimentalThe commercial grade pearlitic steel studied was received as a lead patented, 5.5 mm in diameter wire rod. Its chemical composition (in weight percent) was: 0.80 % C, 0.76% Mn, 0.0015% P, 0.005% S, 0.25% Si, 0.019% Al, 0.0038 % N. Tensile-test specimens 2.5 mm in diameter and with a gauge length of 25.0 mm were machined from wire rod samples spheroidized at 953 K for 24 h. After machining, the specimens were heated at 1 153 K for 20 min and lead patented in laboratory conditions at 793 K for 10 min. According to Fisher,21) the mean true interlamelar spacing, Sp, of pearlite formed at this temperature should be equal to 77 nm. Atomic force microscopy (AFM) images taken from the transverse section of patented test specimens confirmed that the...
A influência da adição de cromo no envelhecimento após deformação foi estudada em dois aços perlíticos stelmorizados e trefilados para uma redução de área de 86%, contendo um dos aços, 0,25% de cromo em sua composição química. Foram observados dois estágios de envelhecimento no intervalo de temperatura de tratamento considerado para cada aço. O mecanismo responsável pelo envelhecimento no primeiro estágio foi o ancoramento de deslocações por átomos de nitrogênio e carbono em solução sólida na ferrita. O segundo estágio foi associado ao ancoramento de deslocações em células nas interfaces ferrita-cementita. O efeito do cromo foi a diminuição no espaçamento interlamelar da perlita, o que intensificou os efeitos do envelhecimento nas propriedades mecânicas. Não foram observadas diferenças significativas nos valores da energia de ativação entre os aços no segundo estágio de envelhecimento, significando que pelo menos para o teor investigado, 0,25%, o cromo não afetou a estabilidade da cementita.
The influence of addition of chromium in the strain aging were studied in two stelmorized pearlitic steels, after drawing to an area reduction of 86%, contains one of the steels, 0,25% of chromium in the chemical composition. Two stages of strain aging were observed for both steels in the treatment temperature interval considered. The mechanism responsible for the strain aging in the first stage was the locking of dislocations by nitrogen and carbon atoms in solid solution in the ferrite. The second stage of aging was associated with dislocation locking in the ferrite/cementite interfaces. The effect of chromium was the reduction of the pearlitic interlamelar spacing, increasing the intensity of the strain aging effects in the mechanical properties. No significant differences were observed in the activation energy for the second of aging, meaning that at least for the investigated amount, 0,25%, chromium does not affect the stability of cementite
ResumoMelhorar a qualidade dos aços e dos processos tem sido uma rotina dos engenheiros metalurgistas e das siderúrgicas em um mercado que exige produtos com qualidade e preço competitivos. No processo de refino dos aços são feitos os ajustes de composição química e temperatura, além das modificações inclusionárias necessárias às características de qualidade dos produtos e as exigências para uma perfeita lingotabilidade. O processo de solidificação mais comum é o lingotamento contínuo, onde a continuidade de fluxo de aço líquido através das válvulas é assegurada através da temperatura adequada e presença de inclusões no estado líquido nas temperaturas de lingotamento. Neste trabalho foram utilizados os softwares Thermocalc e CEQCSI com o objetivo de avaliar o efeito dos elementos químicos carbono, silício e enxofre na janela de lingotabilidade de diagramas de fases alumínio vs cálcio. Os softwares se mostraram bastante apropriados e eficazes neste estudo, e os resultados indicam que os elementos estudados afetam diretamente a posição da janela de lingotabilidade dos aços carbono. A análise de amostra de tarugo de um aço 0,2%C via MEV mostrou que há grande heterogenidade de inclusões no aço acalmado ao alumínio e tratado com cálcio. Palavras-chave: Diagrama de fases; Thermocalc; Lingotamento contínuo; Cálcio.
ResumoOs aços carbono são materiais metálicos muito importantes na fabricação de peças para a indústria metal-mecânica. Parafusos, hastes, eixos, pistões, engrenagens são fabricados a partir desse tipo de aço, mais especificamente do aço SAE1040. O objetivo desse trabalho foi de avaliar a influência da presença de 0,2% em massa de cromo nas propriedades mecânicas e metalúrgicas de um aço com 0,4% em massa de carbono. A liga padrão SAE1040 e esta liga modificada com cromo foram submetidas a tratamentos térmicos de normalização, recozimento, têmpera e revenimento, e comparadas às durezas e microdurezas das diferentes amostras entre si. Os resultados obtidos evidenciaram que a adição de cromo é um método eficiente quando se deseja aumentar a resistência de um material. Observou-se que os valores de dureza e microdureza para o aço com a adição de cromo apresentaram-se maiores em relação ao aço padrão, como consequência das microestruturas obtidas, principalmente após têmpera e revenimento. A adição de 0,2% em massa de cromo no aço carbono influiu na melhora das propriedades mecânicas do aço tornando-o mais interessante na fabricação de peças para a indústria metal-mecânica. Palavras-chave: Cromo; Tratamento térmico; SAE1040; Ensaios mecânicos; Aço. INFLUENCE OF CHROMIUM MECHANICAL AND METALLURGICAL PROPERTIES OF STEEL SAE 1040 AbstractCarbon steels are very important materials in parts production to the metal-mechanical industry. Screws, rods, shafts, pistons, gears are manufactured to this kind of steel, more specifically SAE1040 steel. The objective of this study was to evaluate the influence of the presence of 0.2wt% of chromium in the mechanical and metallurgical properties of steel with 0.4wt% carbon. The standard alloy SAE1040 and this alloy modified by chromium were submitted to heat treatment of normalization, annealing, quenching and tempering, and was compared to the hardness and microhardness of different samples to each other. The results showed that the addition of chromium is an efficient method when you want to increase the resistance a material. It was observed that the values for hardness and microhardness for the steel with the addition of chromium were bigger in relation to standard steel, it because of the microstructures obtained after quenching and tempering. The addition of 0.2wt% of chromium in carbon steel influenced the improvement of mechanical properties of steel, making it more interesting for the manufacture of parts for the metalworking industry.
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