Estudo do comportamento em fadiga do aço ASTM A297 Gr HP modificado com nióbio em altas temperaturas

Abstract

This work aims to study the mechanical behavior at high temperatures of ASTM A-297 Gr HP steel changed with niobium. This type of steel is used for pipe supports that remain exposed long periods of time at operating temperatures near 900°C. In this study, was studied the microstructural composition, the mechanical strength, impact toughness and fatigue resistance, considering the effect of temperature test. To evaluate the effect of temperature and residence time in the microstructure and hence the mechanical properties were performed hardness, tensile and impact test for each of the conditions presented. All conditions of the aged material showed the presence of precipitates in the interdendritic boundaries and secondary precipitates within the austenitic matrix, causing the material to elevate its mechanical strength, compared to CR condition. After this step was studied the mechanical properties at elevated temperatures only for the conditions CR and T1000 (aged at 927 °C for 1000 hours). Tensile, impact and fatigue tests (ε-N and da/dN-ΔK), were performed at the following temperatures: 25°C, 871ºC, 899ºC, 927ºC, 954ºC and 982°C. To evaluate the impact tests, were used Charpy specimens of the conventional type (presence of the notch) and pre-cracked (pre crack more notch). In all test temperatures of traction, it was observed that the T1000 condition resulted in an increase of the value of mechanical strength and reduction of its ductility, compared to CR conditions. In tests at elevated temperatures, it was observed that the precipitation of carbides in the austenitic matrix of the secondary condition T1000 is able to avoid a sharp decline in mechanical strength compared with the proviso CR. The two conditions (CR and T1000) were sensitive to the presence of a stress raiser (notch further pre-crack) when subjected to impact tests. The results of tensile and impact fractographies corroborate the evidence found for each condition, indicating a greater share of ductile fracture in testing samples at elevated temperatures than at room temperature. Regarding the fatigue behavior of the material, it was found that the fatigue life of the material tested at 25°C was much greater than 927°C, especially in the region of high cycle fatigue. This fact can be attributed to two factors dependent on the time: premature nucleation due to a significant reduction in yield strength and due to the effect of surface oxidation of permanent slip bands. The tests of crack propagation by fatigue showed that the rate da/dN was greater at a temperature of 927°C than at 25°C, because of higher ductility that the material exhibits at high temperatures associated with the effects of oxidation and creep.

Propagação De Trinca Por Fadiga Do Açoastm a-297 Gr Hp Modificado Com Nióbio

“…Temperaturas CR T1000 Dureza (HV10) (5) 23ºC Com relação aos ensaios à temperatura ambiente, observa-se que à 927ºC, independentemente da condição microestrutural inicial, o material torna-se muito mais dúctil e, assim, a energia absorvida por impacto é muito maior, o que corresponde a um modo de fratura dúctil. Nota-se, também, que o valor de energia absorvida por impacto é maior na condição CR que na condição T1000.…”

“…Uma vez que a ponta da trinca ultrapassa esta região, a taxa de vai aumentando novamente. Nesta temperatura, a taxa de propagação de trinca por fadiga é levemente superior no material T1000, provavelmente devido à maior quantidade de precipitação secundária em sua matriz austenítica que torna o material menos dúctil (5) . No entanto, na temperatura de 927ºC não se nota uma diferença significativa na taxa de propagação em relação às diferentes condições microestruturais.…”

“…Notou-se também que, na temperatura de 23ºC, a taxa de propagação de trinca por fadiga é maior no material T1000 do que no CR, devido à grande quantidade de precipitados secundários existentes na matriz austenítica do primeiro. (5) Nos ensaios a 927ºC não foi notada diferença significativa de comportamento para as duas condições.…”

“…Encontram-se poucas informações na literatura sobre o efeito do envelhecimento nas propriedades mecânicas de fadiga-fluência desta liga. Segundo Castro et al, (5) ,a microestrutura dessa liga submetida a tratamento de envelhecimento fica praticamente estabilizada para tempos da ordem de 1000 horas.…”