Effect of High Temperature Water Environment on Cyclic Crack Growth through Typical Weld HAZ Microstructures of SA 533gr. B Steel

Abstract

Cyclic crack growth behaviour of SA 533gr. B steel specimens of a common heat with different microstructures was examined through tests in a simulated BWR water environment.
The variation in microstructure was provided by the following three different heat treatments, i. e.
(1) air-quenched and tempered, (2) oil-quenched and tempered and (3) as oil-quenched, each of which was to simulate the localized variability of typical alternated microstructures of the weld-heat affected zones appearing along fusion lines. Crack growth rate measurements were made using constant-ΔK type contoured double cantilever beam (CDCB) specimens of 30mm thickness. The test environment, flowing 288°C 0.1-0.2ppmO₂ water, was closely controlled by continuously refreshing the test section at a rate of 1000ml/min.
In the preliminary air tests, no essential difference was observed among all kinds of materials in their ΔK versus da/dN relationships for two stress ratio conditions, i. e. R=0.1 and 0.5. In the BWR water environment, the crack growth rates of those three types of materials were accelerated relative to the air environment results. The degrees of acceleration were not greatly different to each other under stress ratio, R=0.1, while considerable difference was noted in case of higher stress ratio, R=0.5. Under the high stress ratio test condition, the quenched material, which was with martensitic structure and hardening, showed the highest degree of environment sensitivity among others. The results obtained were consistent with authors' earlier observation on the specific microstructure-dependent variability of the cyclic crack growth rates when cracks were advancing along weld fusion lines through HAZ, where a considerable level of residual stresses remained.