The present work is concerned with the effects of tungsten on the passivity and the kinetics of intermetallic phase formation and resultant embrittlement of Fe-29Cr superferritic stainless steels.
In chapter III, the influences of W on the passivity for Fe-29Cr stainless steels were quantitatively compared with those of Mo based on its atomic percent by examining the effects of these alloying elements on the passivation parameters such as pitting potential ($E_p$), primary passivation potential ($E_{pp}$) and critical anodic current density ($i_c$) that are determined from anodic polarization responses of the alloys in a chloride or an acidic solution. $E_p$ for the alloys, measured in 4 M $MgCl_2$ at 80℃, increased linearly with Mo or W contents in which the increasing rate of $E_p$ per addition of Mo was almost equal to that of W. For Fe-29Cr alloys containing both Mo and W, the $E_p$ also increased linearly with the sum of their atomic percent without any synergic effect on the $E_p$ by a special combination of Mo and W. In 20 % $H_2SO_4$ solution, $E_{pp}$ for Fe-29Cr alloys decreased linearly with the logarithmic value of atomic percent of Mo or W. $i_c$ for the alloys also decreased with Mo or W contents, showing a linear relationship between logarithmic values of $i_c$ and atomic percent of Mo or W. The effects of Mo on the passivation parameter of the alloys were found to be almost identical to those of W when compared on the scale of atomic percent. These similarities in the effects of Mo and W on the passivity of Fe-29Cr alloys appear to result from the fact that both elements are very similar in the chemical and electrical properties.
In chapter Ⅳ, the effects of W on the formation kinetics of intermetallic phases in Fe-29Cr ferritic stainless steels during aging at 850℃ were systematically investigated together with the resultant effects on mechanical and localized corrosion properties, and compared with those of Mo by examining the microstructure using scanning electron spectroscopy(SEM) with energy dispersive spectroscopy(EDS) and back scattered electron(BSE) imaging and by examining the degradation in mechanical and corrosion properties using Charpy V-notch impact, tensile, and anodic polarization tests.
For W-containing Fe-29Cr alloys precipitation of χ phase preceded that of σ phase with aging at 850℃, which was similar to the precipitation behavior of Mo-containing alloys.
For W-containing Fe-29Cr alloys, the Cr content of χ phase increased while the W content decreased as χ phase grew, which leaded to the χ phase composition of Fe-26Cr-22W in the case of 10 μm in size. On the contrary, in the case of alloy containing Mo, the compositonal change was so fast that the composition of the χ phase was Fe-26Cr-22Mo in the case of just 2 μm in size. This attributes to the fact that the diffusivity of W in Fe-29Cr alloy is far lower than that of Mo in the alloy. Furthermore, it was suggested that W plays a key role in stabilizing the χ phase in comparison with Mo, considering the result that the W content of 50% in the χ phase formed in the W-containing alloy was kept until it grew up to 2 μm.
In the case of Fe-29Cr-4Mo alloy, when Mo was completely substituted by W, both the growth of χ phase and the formation of σ phase were considerably delayed, resulting from the lower diffusivity of W than that of Mo. An addition of W more significantly affected the reduction of the formation rate of σ phase than that of the growth rate of χ phase.
Fe-29Cr-xMo-yW (x+y=4, x=0∼4 wt %) alloys were embrittled drastically by the formation of intermetallic phases. The fracture toughness of the alloys was abruptly lowered as the χ phase formed continuous or semi-continuous networks, whereas the ductility of the alloys was drastically decreased due to the formation of σ phase. The embrittlement of the alloys with aging was hindered by substituting W for Mo, resulting from the fact that the formation of both χ and σ phases was delayed as Mo was replaced by W.
The formation of χ phase hardly affected the decrease in $E_p$ of Fe-29Cr-4Mo, Fe-29Cr-4W, and Fe-29Cr-8W alloys. It was shown that $E_p$ of the alloys was closely related to the formation of σ phase: $E_p$ decreased slightly as the formation of σ phase commenced, and as the σ phase grew $E_p$ decreased drastically. This denotes that, in the vicinity of χ phase, Mo (or W) is depleted but Cr enriched, whereas, in the vicinity of σ phase, both Mo (or W) and Cr are depleted concurrently.
Though the growth rate of the σ phase in Fe-29Cr-8W alloy was higher than that in Fe-29Cr-4Mo alloy, $E_p$ decreased more slowly in Fe-29Cr-8W than in Fe-29Cr-4Mo. It is because that the W depletion by σ phase formation in Fe-29Cr-8W was less than the Mo depletion by σ phase formation in Fe-29Cr-4Mo, which was confirmed by the fact the W ratio of matrix/σ in Fe-29Cr-8W was higher than the Mo ratio of matrix/σ in Fe-29Cr-4Mo as the σ composition of each alloy was Fe-32Cr-10Mo and Fe-33Cr-11W.