On the phase transformation of type 304 stainless steel caused by cathodic hydrogen charging

Abstract

When FCC iron alloys are subjected to cathodic hydrogen charging, usually two types of phase transformation occur in the surface layer of specimens. The first type is the formation of an FCC [gamma][sub H] phase. The second type is the formation of an HCP [var epsilon][sub H] phase in Fe-Mn alloys. In ternary Fe-Ni-Mn alloys, the [gamma][sub H] phase appears in the nickel-rich composition and the [var epsilon][sub H] phase in the manganese-rich composition, but there is no indication of the hydrogen-induced transformation in the intermediate composition around Fe[sub 50]Ni[sub 20]Mn[sub 30]. The [gamma][sub H] and [var epsilon][sub H] phases are stable only in an aqueous solution under cathodic polarization but unstable in air around room temperature. After an outgassing of hydrogen, the [gamma][sub H] and [var epsilon][sub H] phases completely disappear and return to the FCC matrix. Austenite stability can be characterized empirically by the Ni equivalent value, which is determined by the alloy composition. The 304 stainless steel is designated in a somewhat broad range of composition, and different melts may, therefore, have different austenite stability. However, the influence of austenite stability on the hydrogen-induced transformation has never been examined in detail for the 304 stainless steel. Themore » purpose of this paper is to demonstrate different appearances of the hydrogen-induced transformation by using two kinds of the commercial 304 stainless steel with different Ni equivalent values.« less