Improved Reliability of Small Punch Testing by Correction of Testing System Stiffness and Measurement of Hydrogen Sensitivity

Abstract

Conventionally embrittlement of components for petrochemical equipment has been determined by small punch (SP) testing, which involves carrying out mechanical testing of specimens sampled from practical structures to estimate the integrity and residual life of the equipment. The application range of SP testing has been expanded to cryogenic environments, and attempts have recently been made to expand it further to high-pressure hydrogen environments. To examine the effect of hydrogen on pipeline materials before SP testing, the amount of hydrogen permeation was measured under several testing conditions. The test specimens were prepared as follow. First, after obtaining a section (10 mm in width, 10 mm in height, and 0.7 mm in thickness) from a steel pipe made of X70, X65, or X42 as the base metal by wire cut electric discharge processing. Subsequently, grinding both surfaces, the test specimen was finished to maximum surface roughness of less than 6 μm, with a final thickness of 0.5 ± 0.005 mm. The aim of this study was to determine whether sensitivity to hydrogen could be measured by SP testing; hence, the test was conducted at a punch speed of 4 × 10−3 mm/min, which is much slower than the 0.2 ∼ 2 mm/min adopted for conventional SP testing, like CWA 15627. The maximum load measured for all three steels under 10 MPa was not influenced by machine type (servo hydrodynamic or electromechanical), holding time after charging hydrogen, or bolt fastening torque. Different means for correcting SP curves depending on the stiffness of the testing machines were suggested, and their effectiveness throughout testing was demonstrated.