Hydrogen Induced Slow Crack Growth in Stable Austenitic Stainless Steels
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I.
INTRODUCTION
I T is well established that austenitic stainless steels, whether stable or unstable, undergo both internal and gaseous hydrogen embrittlement (i.e., hydrogen induced ductility reduction),la although the existence of hydrogen induced slow crack growth is still controversial.3-6 It has been shown 3 that in type 304L and 18-6-9 stainless steel, no hydrogen induced slow crack growth is observed in a 52 MPa hydrogen gas atmosphere over periods as long as six months. In contrast with this, Ohnishi et alr reported that sustained load cracking occurred in a notched tensile specimen of type 304L in 9.8 MPa hydrogen. Eliezer et al.5 investigated the effect of austenite stability on sustained load cracking with a notched tensile specimen. Their results showed that hydrogen induced slow crack growth could occur at room temperature when type 304 or type 304L stainless steel was stressed while undergoing cathodic charging or exposed to 0.1 MPa external hydrogen gas pressure and ~' martensite was detected on the fracture surface. Type 310 or 316L stainless steels exhibited no slow crack growth, and no c~' martensite was found. If type 304 specimens were tested in gaseous hydrogen at 160 ~ that is, well above the Mo temperature, no slow crack growth or c~' martensite was observed. Therefore, they concluded that the formation of ~' martensite was necessary for slow crack growth and that no slow crack growth could occur for a stable austenitic stainless steel. 5 The same conclusion was obtained by Briant6 and Singh. 7 So far, studies of slow crack growth in the stable austenitic stainless steels are scarce.8 In addition, the exact role of the ~' martensite in slow crack growth has not been clarified. The present work is intended to investigate whether slow crack growth can occur in a stable austenitic stainless steel and whether the formation of a' martensite is a necessary condition for slow crack growth in the austenitic stainless steels. WU-YANG CHU, Associate Professor, and CHI-MEI HSIAO, Professor, are at Beijing University of Iron and Steel Technology, Beijing, China. JING YAO is an Engineer of Institute of Aeronautical Materials. Manuscript submitted March 11, 1983.
METALLURGICALTRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
The chemical compositions of the materials are listed in Table I. Thin single-edge notched tensile specimens (Figure l(a)) were prepared from cold rolled foils (about 0.23 mm thick). The specimens were annealed in an argon atmosphere at 1050 ~ and quenched into oil. The final thinning was performed by grinding with a No. 400 grit paper. For the type 321 steel, modified WOL type constant deflection specimens (Figure l(b)) were also used. All specimens were cathodically charged at room temperature in a 1 N H2SO4 solution containing 0.25 g/1 of AS203. The current density was 5 • 103 A / m 2 for the constant load specimens and (1 - 20) x 103 A/m 2 for the constant deflection specimens. The Kt values were calculated using the following expression for single-edge notched specimen: 9
K,=
Bw
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