Elsevier

Corrosion Science

Volume 50, Issue 7, July 2008, Pages 1865-1871
Corrosion Science

Effect of microstructure on the hydrogen trapping efficiency and hydrogen induced cracking of linepipe steel

https://doi.org/10.1016/j.corsci.2008.03.007Get rights and content

Abstract

The hydrogen trapping efficiency in different microstructures is compared, and the critical hydrogen flux for hydrogen induced cracking (HIC) is determined for API X65 grade linepipe steel. By controlling the start cooling temperature (SCT) and the finish cooling temperature (FCT) in thermomechanically controlled process (TMCP), three different kinds of microstructure such as ferrite/degenerated pearlite (F/DP), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) are obtained. A modified ISO17081(2004) standard method is used to evaluate the hydrogen trapping by measuring the permeability (JssL) and the apparent diffusivity (Dapp). Microstructures affecting both hydrogen trapping and hydrogen diffusion are found to be DP, AF, BF and martensite/austenite (M/A) constituents. The hydrogen trapping efficiency is increased in the order of DP, BF and AF, with AF being the most efficient. HIC is initiated at the local M/A concentrated region when the steel has such microstructures as F/AF or F/B. Although the trapping efficiency of bainite is lower than that of AF, bainite is more sensitive microstructure to HIC than to AF.

Introduction

For transportation of oil and gas which contain hydrogen sulfide (H2S), the application of linepipe steel is limited due to mechanical and corrosion problems induced by hydrogen. Especially, the hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) are the major problems of linepipe steels induced by hydrogen during service in the sour environment [1]. When the linepipe steel is exposed to a sour environment, hydrogen atom is produced by surface corrosion of the steel. In the presence of hydrogen sulfide, recombination reaction of hydrogen atoms to the molecular hydrogen is retarded, consequently allowing, hydrogen atoms to diffuse into the steel. Diffused hydrogen atoms are trapped at sensitive metallurgical defects such as at the interface between non-metallic inclusion and steel matrix. Cracking can occurs if the critical amount of hydrogen necessary for crack initiation is accumulated [2].

Although many investigators have studied on the HIC and SSC phenomena of the high strength linepipe steels, a great deal of attention has been focused on the effect of metallurgical parameters such as microstructure. However, many of those investigators had difficulty explaining the combined effect of microstructure and hydrogen on the cracking nature of HIC and SSC, exclusively [3], [4]. Few researchers have reported that acicular ferrite structure is most desirable microstructure for the high strength linepipe steels to assure high resistance to SSC, however, the role of AF has not been yet properly explained with respect to hydrogen diffusion [5]. In medium and high strength steels, lower bainite can generally offer higher strength than acicular ferrite and show lower susceptibility to hydrogen embrittlement as compared to quenched and tempered martensite. Nonetheless, the comparison between acicular ferrite and bainite in terms of hydrogen diffusion has not been exclusively examined [6]. Therefore, it is necessary to study the relationship between various microstructures and hydrogen diffusion for a clear understanding of cracking problems induced by hydrogen.

The aim of the present investigation is to understand the effect of microstructure on hydrogen diffusion in high strength linepipe steels. Using different types of heat treatment during TMCP, steels with three different microstructures such as F/DP, F/AF, and F/B were prepared, and standard hydrogen permeation measurements were carried out using a modified ISO17081(2004) method [7]. Precision analytical microscopy was carried out to characterize the microstructure; furthermore, HIC sensitivity was evaluated in reference to NACE TM0284-96 method and the crack initiation site was examined afterward by using an electron microscopy [8]. The correlation between the microstructural variations and the hydrogen permeability, apparent diffusivity, and solubility has been described in detail.

Section snippets

Test material preparation and microstructure analysis

Table 1 lists the chemical composition of the test steel used in this study. This steel is equivalent to the API X65 grade steel. All the steel samples were obtained from the same slab. By using the same slab for all the test steel samples, the level of inclusions in the steel can be fixed in same proximity since the inclusions significantly affect the hydrogen trapping property. The steel slab was divided into four equal parts and each part was reheated up to 1180 °C and hot-rolled to 20 mm

Microstructure

The effect of material factors on the hydrogen diffusion depends on two factors. One is the concentration of atomic hydrogen on the steel surface which forms due to the environmental factors such as the pH of solution, the partial pressure of H2S gas, i.e., potential amount of hydrogen which may diffuse through the steel. The other is the steel microstructure consisted of the primary and secondary phases including non-metallic inclusions and precipitates which affect both the hydrogen trapping

Conclusions

  • 1.

    The key microstructures of the high strength linepipe steel, affecting both hydrogen trapping and hydrogen diffusion, were the degenerated pearlite (DP), acicular ferrite (AF), bainite (B) and M/A constituents.

  • 2.

    The ability of microstructure to trap hydrogen was explained in terms of the apparent diffusivity (Dapp), permeability (JssL), and solubility of hydrogen in steel (Capp). The lower the values of Dapp and JssL are, the more the hydrogen trapping occurs in the steel.

  • 3.

    The results of this

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