This study was carried out to investigate the effect of precipitation behavior of R-phase and its effect on the mechanical properties, corrosion resistance and impact toughness. This study was also investigated the effect of W substitution on the precipitation behavior of R-phase and impact toughness in super duplex stainless steels.
The result obtained from this study are as follow:
When tungsten is substituted for molybdenum, volume fraction of ferrite phase was decrease, while austenite phase was increased.
As the R-phase was mainly precipitated at the interface of ferrite/austenite phases and the inside of the ferrite phase in the initial stage of aging, volume fraction of R-phase was increased, and then decreased by transformation of R-phase was transformed into σ-phase with an increase of aging time.
The ferrite phase was decomposed into new austenite phase() and σ-phase by aging treatment.
The σ-phase was precipitated at late aging time then R-phase
When tungsten is partial substituted for molybdenum, precipitation of R-phase was facilitated, while the precipitation of σ-phase and the decomposition of ferrite phase ware delayed and inhibited
The R-phase was an intermetallic compound, which represented the higher molybdenum and tungsten concentration than the matrix, and also showed the higher molybdenum and tungsten concentration than the σ-phase.
In the initial stage of aging, the precipitation of R-phase did not change the hardness and strength, but elongation was slowly decreased. Furthermore, the hardness and the strength was grown by the increase of aging time, but the elongation was rapidly decreased.
These results show that R-phase didn''t affect significantly the hardness and the strength, but it affected the elongation.
The pitting potential (Ep) has slowly increased as the R-phase precipitated with an increase of aging time. Then it was rapidly decreased as the σ-phase precipitated.
The R-phase deduced the pitting potential, but it had smaller effect than σ-phase.
With an increase of aging time, the impact toughness decreased rapidly and then, slowly decreased.
The impact toughness was decreased by precipitation of R and σ-phases, decreasing in initial stage of aging was affected to precipitation of R-phase then σ-phase.
The impact toughness was decreased by solution strengthening of W with W substitution for some amounts of Mo.