The effect of undissolved and temper-induced (Ti,Mo)C precipitates on hydrogen embrittlement of quenched and tempered Cr-Mo steel

doi:10.1016/j.corsci.2019.108421

Corrosion Science

Volume 166, 15 April 2020, 108421

https://doi.org/10.1016/j.corsci.2019.108421 Get rights and content

Highlights

•
Undissolved (Ti,Mo)C could not trap hydrogen through electrochemical charging.
•
Tempering (Ti,Mo)C has a lower hydrogen desorption activation energy than pure TiC.
•
Partial replacement of Ti by Mo reduced the hydrogen desorption activation energy.
•
Fine, temper-induced (Ti,Mo)C is deleterious hydrogen trapping site.

Abstract

The effect of undissolved and temper-induced (Ti,Mo)C precipitates on hydrogen embrittlement of quenched and tempered Cr-Mo steel is investigated by thermal desorption spectroscopy and slow strain rate tensile tests. The results indicate that spherical undissolved (Ti,Mo)C with an activation energy of 142 kJ/mol could not trap hydrogen through electrochemical charging. However, fine, temper-induced (Ti,Mo)C with an activation energy of 17.0–21.1 kJ/mol, which is far lower than that of pure (semi-)coherent TiC and similar to that of dislocations and grain boundaries, is deleterious hydrogen trap. Partial replacement of Ti by Mo reduces the hydrogen desorption activation energy at the (Ti,Mo)C/matrix interface.

Graphical abstract

Introduction

Hydrogen-induced delayed fracture or hydrogen embrittlement (HE) is one of the most serious failure modes of high strength bolts, which are usually fabricated using medium carbon Cr-Mo steels after quenching and tempering [1]. To lower the HE susceptibility, precipitates such as titanium carbides (TiC) have been introduced into high strength steels because these precipitates act as strong hydrogen trap sites to prevent hydrogen diffusion toward stress concentration regions or crack tips [2]. The effect of precipitates on HE susceptibility, however, is controversial due to the complexity of the interaction between the precipitates and hydrogen. It has been found that the hydrogen trapping capacity depends not only on the chemistry and structure of the precipitates but also on the coherence of the interface between the precipitates and the matrix.

Pressouyre and Bernstein [3] conducted electrochemical permeation experiments and demonstrated that incoherent TiC particles with a size of 0.12–2 μm were irreversible traps with large occupancy and Hui et al. [4] also reported that undissolved TiC increases the resistance to HE. However, Turnbull [5] reported that the activation energy for hydrogen atoms to jump into the trap is very high; therefore, incoherent TiC is not an effective hydrogen trap to lower HE susceptibility. It is also controversial whether (semi-)coherent TiC can improve HE resistance. Depover et al. [6] reported that quenched and tempered 0.313C-1.34Ti showed lower HE resistance than the as-quenched samples because TiC introduced during tempering weakly trapped a large amount of diffusible hydrogen, which is harmful to the material. Subsequently, Depover and Verbeken [7] reported that when a similar amount of hydrogen was charged into the as-quenched and tempered samples, the tempered sample showed lower hydrogen-induced ductility loss because hydrogen was first trapped in the deeper trapping sites of the temper-induced TiC and the hydrogen had not yet been filled in the reversible trapping sites such as the lath boundaries. Therefore, fine, temper-induced TiC reduces the HE susceptibility. In addition, it was reported that mixed carbides exhibited better ability to capture hydrogen than pure carbides since their coherence can be manipulated by adding other alloy elements [8]. Nagao et al. [9,10] demonstrated that the addition of nanosized (Ti,Mo)C precipitates to high-strength tempered lath martensitic steel increased the resistance to HE; the authors conducted a detailed study on the hydrogen-induced fracture mode and the deformation structure immediately beneath the fracture surfaces with and without (Ti,Mo)C precipitates. It was demonstrated that the failure mechanism consisted of hydrogen-enhanced plasticity-mediated decohesion. Song et al. [11] found that diffusible hydrogen was significantly reduced by the addition of Mo to Ti-bearing steels to form (Ti,Mo)C precipitates.

Although the influences of nanosized (Ti,Mo)C precipitates on HE have been investigated, systematic investigations of the effect of nanosized (Ti,Mo)C precipitates on hydrogen trapping and desorption behaviour have rarely been reported. Therefore, the objective of this study is to examine the effect of undissolved and temper-induced (Ti,Mo)C precipitates on hydrogen trapping and desorption and on HE of quenched and tempered martensitic steels using thermal desorption spectroscopy (TDS) analysis and slow strain rate tensile (SSRT) tests. The investigated steels were Ti-free and Ti-added medium carbon Cr-Mo steels for high strength bolts.

Section snippets

Materials and heat treatment

Two types of commercial medium carbon Cr-Mo steels were selected for the study; they are referred to as Steel A and Steel B. The experimental steels were hot-rolled wire rod with a diameter of 12 mm; the chemical composition of the materials as determined by chemical analysis is listed in Table 1. The main difference between the two steels is that Steel B is Ti-bearing whereas Steel A is not and the carbon content is higher in Steel A than in Steel B. The samples were first austenitized at

Microstructural characterization

The SEM images of the experimental steels are shown in Fig. 3. All samples exhibited a typical tempered martensitic microstructure. Due to higher austenitizing temperature, the microstructure of B-1350-500 and B-1350-550 (Fig. 3c and Fig. 3d) was clearly coarser than that of the other samples. The prior austenite grain (PAG) sizes of the 880 °C and 1350 °C austenitized samples were 10 μm and 150 μm, respectively. The high-magnification SEM images show that the carbides were distributed in a

Effect of undissolved (Ti,Mo)C precipitates on HE

It has been demonstrated that undissolved (Ti,Mo)C particles in the B-880-450 steel could not trap hydrogen during electrochemical hydrogen charging at room temperature. Based on a TDS analysis, Wei and Tsuzaki [29] reported that octahedral carbon vacancies in the incoherent TiC particles were the hydrogen trap sites rather than the particle/matrix interface; this indicates that incoherent TiC particles are not able to trap hydrogen during cathodic charging at room temperature due to the high

Conclusions

The effect of undissolved and temper-induced (Ti,Mo)C precipitates on hydrogen trapping and HE susceptibility of high-strength martensitic steels has been investigated using TDS analysis and SSRT tests. The following conclusions can be drawn,

1)
Spherical undissolved (Ti,Mo)C precipitates in the B-880-450 steel had a high hydrogen desorption activation energy of 142.6 kJ/mol but could not absorb hydrogen through electrochemical charging at room temperature and had no effect on HE.
2)
The fine,

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank Dr. Xiaoyuan Li and Mr. Kuo Yang for their help with the TDS analysis. This work was supported financially by the National Key Research and Development Program under the subject No. 2017YFB0304802, No. 2016YFB0300102, and No. 2016YFB0300104.

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The effect of undissolved and temper-induced (Ti,Mo)C precipitates on hydrogen embrittlement of quenched and tempered Cr-Mo steel

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and heat treatment

Microstructural characterization

Effect of undissolved (Ti,Mo)C precipitates on HE

Conclusions

Data availability statement

Declaration of Competing Interest

Acknowledgements

Mater. Sci. Eng. A

Scr. Mater.

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Corros. Sci.

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J. Mech. Phys. Solids

Corros. Sci.

Acta Metall.

Mater. Sci. Eng. A

Acta Metall.

Mater. Sci. Eng. A

Acta Mater.

NDT E Int.

Mater. Sci. Eng. A

Mater. Charact.

Mater. Sci. Eng. A

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