ABSTRACT

Hydrogen Induced Cracking (HIC) is one of the major issues of line pipes exposed to sour environments. There are some guidelines on materials requirements for carbon and low alloy steels for H2S-containing environments in oil and gas production. Generally, HIC susceptibility is evaluated after the test duration, typically 96 hours, in accordance with NACE TM0284-2016. However, HIC propagation behavior during HIC test has not been fully understood.

In this study, a new in-situ HIC measurement method has been developed in order to make the connection between HIC propagation behavior and microstructure. This technique is based on the combination of an automatic ultrasonic wave inspection system and a scanning electron microscopy (SEM) observation. HIC propagation rate and HIC propagation behavior of carbon steels with different texture were investigated, using this in-situ technique. Texture components of tested steels were changed by controlled rolling process in the alpha-gamma dual phase region. {100} intensity parallel to the rolling plane was developed with increasing controlled rolling reduction in the alpha-gamma dual phase region. HIC propagation rate increased and crack length of HIC grew in a staircase pattern with time when the {100} texture was highly developed. In addition, HIC propagation behavior could be overlapped with fracture surface, just like a projection mapping. The overlapping could make the connection between HIC propagation behavior and HIC fracture surface. Microstructure and texture just under the HIC fracture surface was also characterized by SEM and an electron backscatter diffraction pattern (EBSP) method.

The results obtained in this study showed that HIC propagation behavior was affected by a texture. In addition, the new in-situ HIC observation technique, which can make the direct connection between HIC propagation behavior and microstructure, revealed a detailed HIC propagation behavior and an effect of microstructure.

INTRODUCTION

In oil and gas industry, pipe line for natural gas transportation is exposed to wet H2S environments (Sour environments) in some cases. The presence of H2S in the sour environment enhances hydrogen entry into the steel due to catalytic action of H2S. Absorbed hydrogen atoms in the steel diffuse and accumulate in a local area and finally cause several types of environmental cracking such as Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC) and Stress Oriented Hydrogen Induced Cracking (SOHIC). HIC is one of the major issues of line pipes exposed to sour environments. There are some guidelines on materials requirements for carbon and low alloy steels for H2S-containing environments in oil and gas production. In general, HIC susceptibility, such as crack sensitivity ratio (CSR), crack length ratio (CLR) and crack area ratio (CAR), is evaluated after the test duration, typically 96 hours, in accordance with NACE TM0284-20161.

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