Lattice Dilation in a Hydrogen Charged Steel

The effect of hydrogen charging on the lattice dilation in an ASTM A710-HSLA steel specimen was studied using neutron diffraction. First, spatially-resolved strain measurements were made on a pre-cracked steel double cantilever beam specimen under a constant crack opening displacement (COD). Measurements were made at the crack tip region for in-plane and throughthickness lattice strains. Then, the specimen, under the same applied COD, was electrochemically charged with hydrogen using dilute sulfuric acid and arsenic trioxide as a promoter to study the effect of hydrogen on the lattice dilation. A significant increase in the lattice strain was observed in the hydrogen charged specimen compared to the mechanical-loadonly specimen. INTRODUCTION Recent observations of a compliance drop during fatigue tests when the environment was changed from hydrogen charging to air in pre-cracked fatigue samples imply that a relaxation of the elastic modulus occurs [1]. Compliance is influenced by the effective modulus of material as observed during dynamic compliance measurements [2]. The effect has been successfully modeled by a finite element model (FEM) by assuming a modulus relaxation of about 25% in a region up to 3 mm ahead of the crack tip [3]. The modulus relaxation is postulated to occur due to the stress gradient assisted flux of hydrogen to the crack tip under loading. This phenomenon is usually referred to as the Gorsky effect [4]. The hydrogen concentration needed to produce the lattice dilation necessary for the large drop in modulus is several orders of magnitude larger than the equilibrium solubility typically achieved in charging experiments. The increase in hydrogen content at the crack tip causes an anelastic lattice dilation, which consequently results in a reduced elastic modulus. Such anelastic behavior is characterized by a relaxation time. In this case the relaxation time is of the order of 0.1 seconds and the phenomenon is therefore of importance in low frequency fatigue of structural materials used in ships, chemical plants and off-shore oil wells. This proposed explanation for the hydrogen embrittlement effect in steels provides an elegant basis for the prediction of fatigue crack growth rates in such structures. The objective of this work is to experimentally study the effect of hydrogen charging on the lattice dilation near and far from the crack tip in an ASTM A710-HSLA steel specimen using Copyright©JCPDS International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 238