Influence of nitrogen alloying on hydrogen embrittlement in AISI 304-type stainless steels
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I.
INTRODUCTION
IT is
now well established that when austenitic stainless steels are exposed to cathodic charging or gas environments of high or low hydrogen pressures, a remarkable degradation of mechanical properties can occur. ~-~ Also it is believed that there are links between hydrogen embrittlement (HE) and stress corrosion cracking (SCC). 6'7 Even though the detailed mechanism of hydrogen-induced cracking remains unknown, it is possible to connect cracking to such material variables as austenite stability, s'') stacking fault energy (SFE), ~~ and impurity content and segregation.N'12 The role of hydrogen-induced martensite in HE of austenitic stainless steels has frequently been discussed. ~2'~3'~4 On the other hand, HE has also been correlated with SFE, and a straightforward loss of ductility due to hydrogen charging is observed as SFE decreases. ~'~~One rationale for this correlation is that low SFE enhances planar slip of dislocations, thereby increasing slip distance and dislocation transport of hydrogen.~S In order to study these effects, nitrogen alloying can be used. This is because nitrogen stabilizes the austenite while having only a minor effect on SFE, 16'17even though it promotes co-planar glide probably by inducing a short range order. 16 The purpose of this study is to examine the effects of nitrogen alloying on hydrogen embrittlement in cathodically charged AISI 304-type austenitic stainless steels--both in solution annealed and sensitized conditions-- and to discuss the roles of austenite stability and SFE in embrittlement.
II.
EXPERIMENTAL METHODS
The compositions of the materials used in this study are shown in Table I. The materials were in the form of cold rolled foils of 0.3 mm in thickness. After annealing at SIMO-PEKKA HANNULA, formerly with Helsinki University of Technology, Laboratory of Physical Metallurgy, SF-02150 Espoo 15, Finland, is now a Research Fellow with the Department of Materials Sqience and Engineering, Cornell University, Ithaca NY 14853. HANNU HANNINEN and SEPPO TAHTINEN are Research Officers, Technical Research Centre of Finland, Metals Laboratory, Metallimiehenkuja 6, SF-02150 Espoo, Finland. Manuscript submitted November 1, 1983. METALLURGICALTRANSACTIONS A
1323 K for 30 minutes, the foils were quenched in water. Sensitization was introduced by annealing at 923 K for 24 hours. Flat tensile test specimens (gage length 20 ram) about 0.1 mm in thickness were prepared by grinding, the final thinning being performed with a 600 grit paper. Tensile test specimens were cathodically charged with hydrogen at room temperature in 1N H2SO4 solution containing 0.25 g of NaAsO2 per liter as a hydrogen recombination poison. A platinum anode was used. The current density was about 50 mA/cm 2 and charging time 48 hours. In order to measure the hydrogen contents of charged specimens, a series of materials was hydrogen charged for 60 hours by using conditions mentioned above. For determination of hydrogen content of steels a carrier gas technique (Leybold-Heraeus, Gmbh) was use
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