Effect of microstructure and inclusions on hydrogen induced cracking susceptibility and hydrogen trapping efficiency of X120 pipeline steel

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Abstract

The susceptibility to hydrogen induced cracking (HIC) was evaluated for X120 steels containing different amounts of Mn and Al in a H2S environment. The hydrogen trapping efficiency was investigated by measuring the permeability (JL) and the apparent diffusivity (Dapp). The results demonstrated that larger amount of the inclusions, and larger area and volume fraction of the inclusions make steels more susceptible to HIC. The steel with a microstructure consisting of granular bainite and M/A (martensite/austenite) microconstituents is more susceptible to HIC. The ability of the microstructure and the inclusions to trap hydrogen was explained in terms of the apparent diffusivity (Dapp), permeability (JL), and solubility of hydrogen in steels (capp). The lower the values of Dapp and JL and the higher the value of capp are, the more the hydrogen trapping occurs in the steel, and the more the steel is susceptible to HIC.

Research highlights

▶ Increasing in the amount, the area and the volume fraction of the inclusion in steel makes the steel more susceptible to HIC. ▶ The steel with a microstructure consisting of granular bainite and M/A (Martensite/austenite) microconstituents is more susceptible to HIC. ▶ The lower the values of Dapp and J∞L and the higher the value of capp are the more the hydrogen trapping occurs in the steel, and the more the steel is susceptible to HIC.

Introduction

There are increasing tendencies to use pipelines in more severe environmental conditions and under in high gas pressures [1]. For example, high strength steel such as X100 and X120 is currently being considered for high pressure gas pipeline applications in the arctic. To operate in a high gas pressure, the pipeline steel should have not only high strength but also high corrosion resistance because with the increase of gas pressure, the material becomes more susceptible to stress corrosion caused by various gases, especially H2S [2], [3], [4]. The hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) are two major problems for pipeline steels to be applied in the acidic environment [5], [6]. When pipeline steel is exposed to a acidic environment, hydrogen atoms are produced by the 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 the interface between the non-metallic inclusions and steel matrix. Cracking occurs if the critical amount of hydrogen necessary for crack initiation is reached [2], [6]. Beidoknti et al. [1] studied the effects of alloying elements and microstructures on the susceptibility of the welded HSLA steel to HIC and SSC. Kim et al. [6], [7], [8] studied the effect of structure of pipeline steel on HIC with respect to the distribution of primary and secondary phases and diffusion of hydrogen. Lunarska et al. [9] found that hydrogen induced voids were formed at the grain boundaries when the pipeline steel with low sulfide content was subjected to hydrogen charging.

So far, the investigations on the HIC of steel have been mainly focused on the formation process of cracks [10], [11], [12], [13]. However, the combined effect of different microstructure and the presence of hydrogen on the cracking nature of HIC and SSC have not been clearly explained [6], [14]. The acicular ferrite structure is considered as the most desirable microstructure for high strength pipeline steels to in terms of providing high resistance against SSC [1], [6]. However, the role of AF (acicular ferrite) has not been properly explained in the enhancement of SSC resistance, which is known to be closely related to hydrogen diffusion [15]. Generally speaking, lower bainite can offer higher strength than acicular ferrite and it shows lower susceptibility to hydrogen embrittlement as compared with quenched and tempered martensite [16]. Nonetheless, no study has been made that compares hydrogen diffusion behaviour in acicular ferrite with that in bainite. Indeed, the relationship between microstructures and HIC susceptibility needs to be studied in order to understand the cracking problems induced by hydrogen. The goal of the present investigation is to evaluate HIC susceptibility of newly developed high strength low alloy pipeline steels of the X120 grade with different alloying elements in a H2S environment, and to measure its hydrogen permeation curves. The correlations of different microstructures to HIC susceptibility and to the hydrogen permeability, apparent diffusivity and solubility are described in detail.

Section snippets

Materials

The testing specimens were cut from the hot-rolled plate of three types of the X120 steels, each of which contains with different amounts of Mn and Al (marked as Mn1, Mn2 and Mn3 tested steel). The steel plates were produced using the thermomechanically controlled process (TMCP) that involves a controlled rolling and on-line accelerated cooling. The accelerated cooling was carried out in a temperature range of 750–850 °C by water spray. Chemical compositions and mechanical properties of the

Microstructures and inclusions

Microstructures and inclusions in the three steels tested were analyzed at t/2 (t is thickness of the plates) positions because HIC occurrence depends on the characteristics of microstructures and inclusions, and t/2 position represents the center region of steel plates. Fig. 4 shows the SEM images of the X120 steels. For Mn2 steel (Fig. 4, Mn2), the original austenite grain boundaries were visible and fully continuous. The major microstructure consisted of lath bainite and granular bainite,

Conclusions

  • (1)

    Inclusions and microstructure play significant roles in HIC susceptibility of X120 steels. Increasing in the amount, the area and the volume fraction of the inclusions in steel makes the steel more susceptible to HIC, as are microstructures containing granular bainite and M/A microconstituents.

  • (2)

    The ability of microstructures and inclusions to trap hydrogen was explained in terms of the apparent diffusivity (Dapp), permeability (JL), and solubility of hydrogen in the steels tested (capp). The

Acknowledgements

This work was supported by China National Natural Science Foundation (No: 50871077) and China Postdoctoral Science Foundation (No: 20090450293).

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