ABSTRACT

Resistance testing of steel to Sulfide Stress Cracking (SSC) can be performed according to the four methods given in NACE standard TM0177. However, for SSC testing of low alloyed line pipe steel for longitudinally submerged-arc welded (SAWL) large-diameter pipes of a grade up to X65 it is well established to conduct four-point bend tests according to the new NACE TM0316 standard on machined test specimens with reduced thickness. SSC testing was performed using transverse full-size four-point bend specimens with a thickness up to 35 mm, machined from base material and longitudinal weld, at loads up to 90 % of the actual yield strength of the pipe material. Finite element simulations have been made in order to analyze the stress distribution in the full-size four-point bend specimen under the high load present during the test.

The NACE TM0177 Method D “Double Cantilever Beam” (DCB) corrosion test is frequently used for the determination of the fracture toughness of higher strength steels such as Oil Country Tubular Goods (OCTG) in sour environments, which is represented by the Kissc value. In this study DCB testing was performed on X52 SAWL line pipe steel and the results are reported with regard to the requirements of the test method.

INTRODUCTION

Pipeline steels exposed to hydrocarbon environments containing hydrogen sulfide (H2S), which are called sour environments, must be resistant to hydrogen assisted damage, such as Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC) or Stress Oriented Hydrogen Induced Cracking (SOHIC).1,2,3 Resistance tests on small specimens compared to the pipe dimensions are the usual way of qualifying a pipeline steel for sour service. The test method for HIC resistance testing is given in the NACE standard TM0284.4 In case of SSC testing a variety of test methods is available. The NACE standard TM0177 contains four different test methods, such as a tensile test (method A), a bent-beam test (method B), a C-ring test (method C) and a double cantilever beam (DCB) test (method D) and is intended for testing of all types of metals and alloys, regardless of their form or application, for service in H2S environments.5 In method A and C failure under an applied stress is checked, whereas method B and D are used to determine stress related factors. In addition, a four-point bend test method is given in NACE 1 standard TM0316.6 The reasonable choice of the test method has to be made by the end user. Tensile and DCB tests are typically required for testing high strengths or OCTG steels, whereas the four-point bend test is typically performed for low strength carbon and low-alloy steels.

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