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The influence of prior ageing on creep damage development in two variants of Alloy 800
Author(s) -
ElMagd Essam,
Frères Patrick,
Nicolini Guido,
Schuster Hans,
Ennis Phil J.
Publication year - 1997
Publication title -
steel research
Language(s) - English
Resource type - Journals
eISSN - 1869-344X
pISSN - 0177-4832
DOI - 10.1002/srin.199701790
Subject(s) - creep , ostwald ripening , carbide , metallurgy , intergranular corrosion , alloy , materials science , precipitation , microstructure , ageing , growth rate , grain boundary , medicine , physics , geometry , mathematics , meteorology , nanotechnology
The influence of high temperature thermal ageing treatments on the development of intercrystalline creep damage in two variants of Alloy 800 has been investigated. Ageing up to 3000 h and creep testing were carried out at 800 and 900°C. The high temperature behaviour of the 800HT variant is discussed with reference to the effect of heat treatments on the microstructure. The metallographic methods by which the creep damage was quantitatively determined are described. The growth rate of intercrystalline microcracks was described using a statistical model and the dependence of crack growth rate on the thermal history for both 800HT and 800H was determined. The carbide precipitation and growth processes were determined as functions of the exposure temperature and duration. The results showed the three characteristic stages, precipitation, growth and coarsening (Ostwald ripening). The largest increase in the intergranular creep damage was found in Alloy 800HT within the first 1000 h, in which the precipitation of chromium carbides on the grain boundaries is substantially completed. The comparison between the two variants showed significant differences in the damage development. 800HT exhibited at 800 and 900°C higher creep crack growth rates with increasing exposure duration, but in 800H this effect was not clearly identified.