ESTUDO DO ENVELHECIMENTO DINÂMICO EM UM AÇO Nb Mn Cr Mo

Castro, Cynthia Serra Batista; Gonzalez, Berenice Mendonça; Barbosa, Ricardo Rodrigues

doi:10.4322/tmm.00502011

Cited by 3 publications

(1 citation statement)

References 7 publications

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“…Em aços estruturais microligados, o envelhecimento dinâmico (dynamic strain aging, DSA) já é conhecido desde o início dos anos como um mecanismo efetivo que atua na resistência à deformação em temperaturas elevadas e, por conseguinte, na resistência à fluência também. Esta manutenção das propriedades de resistência mecânica em altas temperaturas, juntamente com o envelhecimento dinâmico, deve-se à interação dinâmica entre discordâncias e clusters de substitucionais-intersticiais (Mn-C, Mn-N ou V-C) (CASTRO, 2008). Leslie (1982) (MORROW, 1974apud DOWLING, 2003.…”

Section: Envelhecimento Dinâmicounclassified

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

Castro¹

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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.

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Section: Envelhecimento Dinâmicounclassified

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

Castro¹

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Study of creep and creep crack growth fatigue of aging ASTM A297/A297M-19 HP steel modified with niobium

Gatti,

Bose Filho,

Carvalho

et al. 2024

J Braz. Soc. Mech. Sci. Eng.

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Study of creep and creep crack growth fatigue of aging ASTM A297 / A297M-19 HP steel modified with niobium

Gatti,

Filho,

de Carvalho

et al. 2023

Preprint

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The HP steel has been commonly used in structural components that work at high temperatures, up to approximately 1150 ºC, with 927 °C being a typical working temperature. In this work, the mechanical properties of ASTM A297 / A297M-19 HP steel modified with niobium were characterized at as-cast microstructural conditions and after aging at 927 ºC for 1000 hours through mechanical tests of hardness, tensile, creep, fatigue, and metallographic analyzes. It was observed that the heat treatment of aging with the time of 1000 hours at 927 ºC provided the precipitation of secondary carbides (Cr23C6) in the austenitic matrix of the alloy and the presence of G-phase. Thus, the heat treatment of aging caused intense precipitation in the interdendritic space, where the chromium carbides coalesced and became coarser. The hardness results showed an increase of hardness by approximately 17% in the aged. In the tensile tests, it was observed that the aging increased the value of the mechanical resistance parameters and decreased the total elongation to approximately 48% when compared to the values obtained in the as-cast condition. The results of the Charpy impact tests on specimens with V-notches, performed at a temperature of 25 ºC, presented values lower than the values obtained at a temperature of 927 ºC in all conditions tested. In the creep tests, for the as-cast material, small variations of the Norton Diagram's stress exponent (n) were observed in the temperature range of 871 to 1066 ºC. It was noted that at the highest temperature of 1093 ºC, the stress exponent showed a sharp decrease due to the transition into dislocation-climb controlled mechanism zone. The fatigue crack propagation tests showed that the crack propagation rate da/dN, for each condition of the material, is higher at 927 ºC than at 25 ºC since the material presented better ductility at high temperatures. In creep crack growth tests, it was observed that the lower initial stress intensity factor (Ki) applied provided a decrease in the crack growth rate in the second stage when compared to the higher Ki value. The experimental results raised in the development of this work provided values of the parameters of monotonic mechanical strengths, nucleation, and crack propagation by fatigue and creep, which are of fundamental importance for the characterization, from the perspective of time-dependent fracture mechanics, the ability of the ASTM A297 / A297M-19 HP modified with niobium to resist nucleation and crack propagation by fatigue and creep mechanisms at elevated temperatures.

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ESTUDO DO ENVELHECIMENTO DINÂMICO EM UM AÇO Nb Mn Cr Mo

Cited by 3 publications

References 7 publications

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

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

Study of creep and creep crack growth fatigue of aging ASTM A297/A297M-19 HP steel modified with niobium

Study of creep and creep crack growth fatigue of aging ASTM A297 / A297M-19 HP steel modified with niobium

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