Evaluation of crack resistance of carbon and low-alloy structural steels under conditions of hydrogen charging

Abstract

Fracture of materials caused by hydrogen embrittlement is a very serious problem in the operation of pipelines subjected to the effect of hydrogen charging (hydrosulfide-bearing media, external corrosion cracking under stress, contact with active sulfate-reducing bacteria, etc.). As a rule, this damage occurs in the final stage of the process of crack propagation initiated by hydrogen. In order to predict the transition from a subcritical stage to a critical stage of unstable fracture and to calculate the strength characteristics of pipes with cracks of a metallurgical or operational origin we must know the permissible margin of safety with allowance for hydrogen absorption in the operating period.The initial data for determination of the permissible size of defects are the values of the critical coefficients of stress intensity Klc, K c, the J-integral, and other parameters characterizing the disruptive stresses at a known length of the crack. In the present work we studied the effect of hydrogen absorption on the characteristics of crack resistance under various loading conditions for steels used for pipelines to obtain initial data for determining the permissible sizes of crack-like defects.We studied the effect of hydrogen absorption on the mechanical properties and the parameters of crack resistance of steels 16G2SAF, 20YuCh, and an imported analog of steel 20u denoted as T8. The chemical composition and the mechanical properties of the studied steels after normalizing are presented in Tables 1 and 2.The effect of hydrogen absorption on the properties of the steels was studied by testing standard cylindrical tensile specimens and prismatic specimens with a side notch and a previously deposited fatigue crack. The tests were conducted for noncentral extension and slow fracture under noncentral extension (GOST 25.506). The tested specimens were of two types differing by the distance b between the center of the application of the load and the edge of the specimen, namely, compact specimens with b = 54 mm and rod specimens with b= 70 mm.The specimens were fabricated so that the crack propagated in the plane with the least fracture toughness. For this purpose, sheet specimens of steel 16G2SAF were cut in the transverse direction so that the crack propagated along the fibers. Specimens of steel T8 were cut from a pipe normally to the direction of the axis so that the crack propagated along the generatrix of the pipe. The preforms for the specimens were not straightened, in order to eliminate straining with an inverse sign.Specimens of steel T8 were notched on the side of the plane of rolling so that the crack propagated over the thickness of the sheet.Compact and rod specimens were cut from a sheet 70 mm thick of steel 20YuCh so that the crack propagated over the middle plane of the sheet along P Cu the direction of rolling. 0.013 0.