Abstract

An investigation was conducted to gain insight on the fracture resistance of carbon steels used in refinery service containing stress oriented hydrogen induced cracking (SOHIC) damage. The investigation included: (1) the development of SOHIC damage in the base metal and heat affected zone of both a conventional and “mc Resistant” grade of ASTM A516-70 steel, (2) fracture resistance testing of the produced defects after out gassing, and (3) comparison of the data and resulting fracture surface morphologies between the two steels. The depth and character of the some attack observed on the four systems as a result of the environmental exposure indicated the base metal region of “mc Resistant” steels to be the most susceptible, followed by the HAZ region of c.onventiorll11 steels. CTOD testing indicated similar toughness of the some damaged material to a conventional fatigue precracked sample in the absence of hydrogen charging. The fracture surface morphology of the some was different on the two steels and its appearance was related to differences in microstructure.

INTRODUCTION

Much of the equipment in petroleum operations is exposed to aqueous process environments containing H2S. A large percentage of this equipment is fabricated from carbon steel and can exhibit susceptibility to wet H2S cracking. In 1984, rupture of an amine absorber pressure vessel at a refinery near Chicago, Illinois led to increased industry concern regarding the wet H2S cracking of carbon steel equipment in refinery service [1]. Failure was attributed to a combination of sulfide stress cracking (SSC) at the toe of a circumferential weld and stress oriented hydrogen induced cracking (SOHIC) which initiated at the tip of the SSC crack and propagated along the fine grained heat affected zone (HAZ) region adjacent to the base metal in the through-thickness direction.

Resistance to SOHIC is of paramount importance. Shallow surface flaws produced by SSC or other defects which possess an inherent stress concentration are common. With adequate SOHIC resistance, these cracks are unlikely to grow to a size which will cause leakage or vessel rupture. As in the incident described above, SOHIC has been more commonly observed in the fine grained HAZ region adjacent to the base metal. This has been observed in the field and in the laboratory on conventional low strength carbon steels used in refinery service and in the laboratory on the advanced (“HIC Resistant”) steels. However, SOHIC susceptibility of “HIC Resistant” steels has not been restricted to the weldments. These steels have also demonstrated SOHIC susceptibility in the base metal [2, 3]. It has been shown that decreases in sulfur content, while decreasing susceptibility to HIC, may result in an increase in the susceptibility to SOHIC [4].

Little to no data is available in the literature governing the fracture resistance of low strength refinery plate steels containing SOHIC damage. Furthermore, toughness data which has been developed on refinery plate steels has typically been limited to LT or TL orientations.

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