Effects of hydrogen concentration on slow crack growth in stainless steels
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INTRODUCTION
THEdemand on materials to perform in more aggressive environments has increased in recent years and has resulted in many instances where system feasibility is determined by material limitations. Since hydrogen will be increasingly used in many oil refining, chemical, and energy-related processes under severe temperature and pressure conditions, for proper design and operation of these systems it is necessary to understand the deleterious effects of hydrogen on structural materials and to find materials that are resistant to hydrogen degradation. Austenitic stainless steels were considered to have good resistance to hydrogen embrittlement, and therefore they were frequently chosen for environments containing hydrogen. ~But a number of studies in this area 2-19has shown that austenitic stainless steels are also susceptible to brittle cracking and decreased measures of ductility for ductile failure, though not to the extent of embrittlement of ferritic or martensitic steels. Holzworth6 showed that the elongation to fracture of type 304L steel was decreased by 40 pet, to less than 20 pet elongation, after cathodically charging thin sheet specimens at room temperature. Because of the limited depth of hydrogen penetration, thin specimens are necessary to demonstrate embrittlement when charging is carried out at room temperature. 5,6 Metallographic studies indicated that embrittled specimens contained numerous secondary surface cracks, both transgranular and intergranular. 6'2~The previous studies of austenitic stainless steels have established that the degree of embrittlement in tensile tests increases as the time of hydrogen charging increases s,6 and is dependent on strain rate and temperature. Most severe embrittlement occurred at low strain rates and at temperatures at or below room temperature. Lagneborg 2~ pointed out that the absence of embrittlement at higher temperatures may be due to loss of hydrogen from the samples at testing temperatures.
S. SINGH, formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign, is now with Key Electricals Ltd., Calcutta, India. C. ALTSTETrER is Professor of Physical Metallurgy, Department of Metallurgy and the Materials Research Laboratory, University of Illinois at Urbana-Champaign, IL 61801. Manuscript submitted March 12, 1982.
METALLURGICALTRANSACTIONS A
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In addition to embrittlement during tensile testing, both transgranular and intergranular cracking have been seen in austenitic steels after cathodic charging at room temperature without any external application of stress. *'2j Additional studies in the present research program have led to a more complete description of cathodic charging e f f e c t s . 22'23 In the case of 304L, Holzworth and Louthan 4 demonstrated that charging also induced the transformation of austenite to a and e martensites, and this has led to the view that internal hydrogen embrittlement may be related to the martensitic transformation. However, as Holzworth pointed out, this view is not consistent w
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