Hydrogen attack in an austenitic stainless steel

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I.

INTRODUCTION

H Y D R O G E N attack (HA) of steels is caused by the formation and growth of hydrocarbon (methane) bubbles upon their exposure to high pressure hydrogen (>1 to 2 MPa) at elevated temperatures (573 to 873 K). HA has been studied widely as reviewed by Shewmon et al.l Lowering the carbon activity of steels by additions of moderately strong carbide formers such as Cr and Mo is the principal means of controlling HA in low alloy pressure vessel steels such as 2.25 Cr-I MO, which is resistant to HA for temperatures up to 723 K. If this steel is tempered long enough to tie up all the carbon in the form of M23C6 carbides, with no more metastable carbides present, then the analysis of Geiger and Angeles 2 for pure Cr23C6 indicates that the equilibrium methane pressure generated with 20 MPa hydrogen is insufficient to cause bubble growth even at 873 K. However, preliminary transmission electron microscopy (TEM) studies revealed bubbles on alloy carbides in Tistabilized, type 321 austenitic stainless steel. The following study was undertaken to reveal the details of HA in these steels and to understand the factors leading to HA under conditions where low alloy steels are normally immune to such degradation.

II.

EXPERIMENTAL

The type 321 stainless steel had a composition, as determined by atomic absorption-analysis, of 17 Cr, 12 Ni, 2 Mn, 1 Si, 0.60 Ti, and 0.203 C in wt pct. The carbon level is high for this grade so that the Ti (0.7 at. pct) is not sufficient to completely stabilize C (0.8 at. pct) as TiC, although a substantial amount of this carbide should form on holding at elevated temperature. The material was hot rolled at 1523 K, annealed 15 minutes at the same temperature, water quenched, and deformed 2 pct at room temperature to straighten the sheet. Samples were exposed for 480 hours to hydrogen at 14 MPa pressure at 873 K. Specimens with and without hydrogen exposure were thinned and examined mainly in a JEOL 100-CX electron microscope fitted with a STEM attachment, although a few foils were viewed in a 200 kV instrument.

M.J. YACAMAN is with the Instituto de Ffsica, University of Mexico. T.A. PARTHASARATHY, Postdoctoral Researcher, and J. R HIRTH, Professor, are with the Engineering Department, The Ohio State University, Columbus, OH 43210. Manuscript submitted November 4, 1983. METALLURGICAL TRANSACTIONS A

Gas analysis of bubbles was achieved by fracturing samples in situ in an ultra-high vacuum chamber fitted with a secondary ion mass spectrograph (SIMS). The ion beam was scanned through the newly fractured surface and the spectrum was recorded.

III.

RESULTS

A. Unexposed Material The TEM observations showed that the steel prior to hydrogen exposure contained two main types of precipitates, MC and M23C6. The precipitates were identified by STEM microdiffraction with an electron beam size of 10 to 20 nm. A typical example of an MC carbide is presented in Figure 1. The MC carbides were found to occur within austenite grains or along dislocation lines in agreement with the results of Thor

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