ABSTRACT

Quenched and tempered martensitic steels for Oil Country Tubular Goods can be subject to Sulfide Stress Cracking (SSC) when exposed to a sour environment. Basically, the failure mechanism of SSC includes an initiation step followed by a propagation step of a crack. Focusing on the latter, it is essential to model the conditions for crack propagation in order to discern the levers that enable to avoid propagation or to stop the crack. With this view, a hydrogen stress driven model was built that describes stress field and hydrogen activity at the direct vicinity of a crack tip. In complement, a second model based on the cohesive zone simulates the kinetic of a crack growth. In parallel, experimental works using hydrogen permeation under stress on flat un-notched and notched tensile specimens brought experimental data that were compared to simulation outputs. The respective influence of diffusible and trapped hydrogen on the cracking mechanism received a specific focus, based on fractographic analyses.

INTRODUCTION

Several works have already established that the diffusible and trapped hydrogen could have a strong influence on the mechanical properties of materials.1 However, this effect varies significantly with the material microstructure, chemical composition, and heat treatment. Due to their structure, quenched and tempered martensitic steels (developed for tubes suitable for sour service environments) have different types of traps such as dislocations, grain boundaries, precipitates, inclusions, vacancies and other interfaces that play an important role in the damage mechanisms. These high strength steels may break due to Sulfide Stress Cracking (SSC) if subjected to mechanical stress and an aggressive environment (which depends on the H2S partial pressure and solution pH). This phenomenon is a form of hydrogen embrittlement (HE) that includes a crack initiation followed by a propagation step leading to failure. However, the hydrogen contribution is still insufficiently understood. In addition to the impact of the microstructure on the hydrogen states, the stress and the deformation fields in the material also modify the effects induced by hydrogen. To investigate this event, electrochemical permeation tests under stress were used to perform mechanical tests under hydrogen flux until failure is reached. The results were compared to those mechanically loaded in air. This enabled the examination of the impact of the hydrogen flux and trapping on the mechanical behavior of martensitic steel. In this framework, flat specimens notched and un-notched were employed. With proper proportions, given that H2S gas is an efficient hydrogen absorption promoter in steel and enhances consequently its embrittlement,2 this experimental procedure is thus trying to reproduce the conditions of service of Oil Country Tubular Goods.

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