ICF100490OR ANALYSIS OF THE PHYSICO-CHEMICAL MECHANISMS RESPONSIBLE FOR HYDROGEN ATTACK S. M. Schlo¨gl and E. Van der Giessen Koiter Institute Delft, Delft University of Technology, The Netherlands ABSTRACT The failure mode known as ‘hydrogen attack’ involves several processes which are active when steel components are exposed to high hydrogen pressures at elevated temperatures. In this paper, a microstructural model is presented which covers these processes through a combination of continuum mechanics with solid solution thermodynamics, kinetics and chemistry. The model is applied to study the response of 2.25Cr-1Mo steel consisting of a ferritic matrix and alloy carbides (e.g., M7 C 3 ,M 23 C 6 ,M 6 C,M2 C) to an exposure of 18MPa hydrogen pressure at a temperature of 530 C. The investigated microstructures vary in their carbide types and/or their carbide volume fractions. The numerical simulations show that the microstructure that does not contain M 7 C 3 carbides is most resistant to hydrogen attack. KEYWORDS hydrogen attack, 2.25Cr-1Mo steel, diffusion, dissolution, methane pressure, creep, driving force INTRODUCTION 2.25Cr-1Mo steel is a standard material for reactors used in the petro-chemical industries. In case of hydrocracking or hydrotreating applications, the material of the reactor is exposed to a high hydrogen pressure and to elevated temperatures. It is this combination that is responsible for the material degradation process called hydrogen attack (HA). During HA, carbon, present in the steel, and hydrogen, originating from the gas atmosphere inside the reactor, form methane molecules. These molecules are captured in cavities which have nucleated at the grain boundaries. Due to the presence of methane and hydrogen molecules, the cavities are internally pressurized. Consequently, the cavities grow and coalesce which finally results in intergranular fracture. Carbon is present in the steel in two forms, namely interstitially dissolved in the ferritic matrix ( -Fe) and bonded in carbides. A standard 2.25Cr-1Mo steel contains various types of carbides, such as M7 C 3 , M 23 C 6 , M 6 C andM2 C [1]. The microstructure, characterized by the composition of the various alloy carbides and their volume fraction, depends on the heat treatment. Generally, the steel that is exposed to the high hydrogen pressure, does not possess a microstructure which is in equilibrium. So, the question arises whether this influences the susceptibility to hydrogen attack. Our recently developed microstructural model [2] enables us to shed some light on this issue. In this paper, we numerically study the response of non-equilibrium microstructures and compare their resulting methane pressures and void growth to the ones of the equilibrium microstructure.
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