QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY
Online ISSN : 2434-8252
Print ISSN : 0288-4771
Effect of Carbon content on Creep Rupture Strength and Microstructure in Heat Affected Zone of Heat Resistant Ferritic Steel
—Alleviation of Deterioration of Creep Rupture Strength in Heat Affected Zone of Heat Resistant Ferritic Steel—
Hiroyuki HirataKazuhiro Ogawa
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JOURNAL FREE ACCESS

2007 Volume 25 Issue 2 Pages 245-253

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Abstract

The effect of carbon content on creep rupture strength and microstructural change in heat affected zone (HAZ) in high temperature service was investigated in order to alleviate the deterioration of creep rupture strength in HAZ of heat resistant ferritic steel. Simulated weld thermal cycle at 1273K was applied to test specimens machined from 10%Cr-3%Co-3%W-V, Nb ferritic steel plates with carbon content ranging from 0.005% to 0.1%. After heat treatment at 1013K for 1.8ks, simulating post weld heat treatment (PWHT), the creep rupture strength was evaluated at 823K, and the various microstructural examinations were carried out.
By decrease of carbon content, the creep rupture time of simulated HAZ got longer and the deterioration of creep rupture strength in HAZ was alleviated. In HAZ with lower carbon content, the density of fine particles in diameter of less than 0.1μm, which were effective for dispersion strengthening, was higher after long term heating. Additionally, the growth rate of particles such as M23C6 type carbide and MX type carbo-nitride was slower in HAZ with low carbon content compared with in that with high carbon content. The growth of particle was roughly in accordance with the theoretical Ostwald ripening considering the proportion of growth of each particle.
The mechanism to explain the effect of carbon content was considered as the following. The proportion of M23C6 and MX dominated by chromium diffusion, which has a higher growth rate than that of MX dominated by vanadium diffusion, contributing to Ostwald ripening of particles becomes lower by decrease of carbon content because of the decrease of partially-dissolved M23C6 during weld thermal cycle. As a result, the growth of particle is delayed during heating and the dispersion strengthening by fine particle is maintained for long term in HAZ with lower carbon content.

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© 2007 by JAPAN WELDING SOCIETY
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