Computational study of the electronic structure and the chemical bonding of hydrogen embrittlent in Fe55Cr25Ni20 industrial alloy

S. SIMONETTI; C. LANZ; G. BRIZUELA; A. JUAN

Congreso; Condensed Matter in Paris; 2014

1. Introduction The petrochemical industry has been using cast steel of the 25% Cr?20% Ni type (HK type steel) since the early 1960s in reformer and pyrolysis furnaces. This class of steel had replaced the traditional superalloys with a reduction in costs and has similar properties under conditions of creep, which is one of the principal degradation mechanisms leading to failure in service at elevated temperature. A number of studies were conducted subsequently to evaluate of phase changes during service and the influence of these changes on the mechanisms of failure by creep. In the 80s, HP (25Cr/35Ni) modified alloys were developed by using certain metals such as molybdenum, niobium, tungsten, titanium. The problem that was encountered with these materials is severe in-service embrittlement. One factor which can influence the serviceability of reformer tubes is the deficiency in the reformer tube material in terms of mechanical properties, inherent defects. It is well known that defects such as vacancies, act as traps for the hydrogen, however, very little is known about the mechanisms by which embrittlement occurs. In order to study the problem in detail, it is necessary to use computer-simulation techniques which can investigate events on a much finer space and time scale than can be reached with presently available experimental techniques. Analysis of the Fe-Cr-Ni ternary cell by computational study can provide a fundamental level of understanding for stress corrosion cracking initiation. The purpose of this work is to computationally study the changes in the electronic structure and the chemical bonding after the H adsorption to better understanding the H embrittlent phenomenon by decohesion mechanism of Fe55Cr25Ni20 industrial alloys. 2. Computational calculations Approximate solutions to Schrodinger's equation are applied to determine the distribution of electron density throughout the model and the resulting distribution is used to analyze the bonding characteristics. A cell was modeled to simulated a based Fe alloy of 25% Cr?20% Ni type, with solid gamma structure, containing 99 atoms of iron, 45 atoms of Cr and 36 atoms of Ni distributed in five layers stacking (111) planes. The reference plane is the central one, which contains a vacancy. The calculations were performed using the Vienna ab initio simulation package VASP. The overlap population was computed using the YAeHMOP code. 3. Results and discussion The H atoms are trapped in the zone of the γ- Fe55Cr25Ni20 vacancy. The impurities are located aligned both along [1-10] direction and with the vacancy, in the (111) plane. This behavior can be related with the easiness of hydrogen trapping and forming a lineal hydrogen-vacancy clusterization, as a precursor to crack initiation. An electron transfer to the H atoms from the Fe, Cr and Ni nearest neighbor atoms is observed. The electron transfer process helps to form the new H-metal bonds. The atomic orbital occupations of the metallic bonds close to the H atoms are affected. The mainly changes are presented in Cr 4py and Fe 4px orbitals. The 3dx2-y2, 3dz2 and 3dxz metallic orbitals have also participation in the hydrogens-metal interactions. The strengths of the metallic bonds nearest neighbors to the H atoms decrease. The Cr atoms have an important role in the embrittlent process, the H-Cr overlap population is the highest and the Cr-metals bonds strengths are the most affected after H adsorption. Hydrogen embrittlement effects can take the form of ease of crack initiation and/or propagation or the development of hydrogen-induced damage, such as surface blisters and cracks or internal voids. It appears that any material can become embrittled by a pressure effect if hydrogen bubbles are introduced and this state remains unchanged until hydrogen atoms escape from the bubbles. In the present study, we found same H-H interaction that could be associated with the precursor of hydrogen bubble but it is far away to a typical H2 chemical bond. The embrittlement mechanism helps to form the bonds between metals and hydrogen atoms and finally break the metal?metal bonds. These processes play a key role in subsequent localized corrosion nucleation such as the initiation of stress corrosion cracking.