Hydrogen embrittlement behavior of high strength low carbon medium manganese steel under different heat treatments
Introduction
In recent years, low carbon medium manganese steel has emerged as a competitive candidate of third-generation green vehicle steel, attracting significant attention because it has a good tradeoff between material cost and mechanical properties [1,2]. Meanwhile, the idea of low carbon medium manganese was increasingly employed in design and manufacture of high strength and toughness heavy steel plates [[3], [4], [5]]. The intercritical annealing treatment is applied to improve the toughness and elongation of water-quenched steel plates. From the point of view of microstructure, the main purpose of this intercritical annealing treatment is to control the fraction of retained austenite (RA) and its stability because RA is a critical phase for transformation induced plasticity (TRIP) effect. According to the literature, the fraction of RA in low carbon medium Mn steels can reach 40 vol%, which guarantees a good combination of high yield strength, high elongation, and excellent toughness [[2], [3], [4], [5], [6], [7], [8], [9]].
The high strength offshore steel plates, developed by using the idea above has been industrially produced by Nansteel and Ansteel in China [3,5]. However, it faces another challenge, i.e. hydrogen embrittlement (HE), during service in practical environmental. Although hydrogen-related energy is regarded as one of the future sources of new clean energy in the world and hydrogen storage, transportation as well as utility are widely reported in the literature [[10], [11], [12], [13]], it is also well known that hydrogen atoms in high strength structural steels may cause HE behavior, especially the corrosion and cathodic protection of steels in the marine environment [14,15]. Hydrogen is a ubiquitous element that can enter the steel interior anytime and anywhere, such as during the manufacture and service. The HE susceptibility of different microstructures is mainly dependent on the diffusible hydrogen atoms [[16], [17], [18], [19], [20]]. Martensite is generally recognized as one of the most sensitive phases to HE on account of its high brittleness and hardness [21]. The solubility and diffusivity of hydrogen in martensitic lattice is influenced by grain size (and/or lath width), carbide precipitates, and dislocation density [[22], [23], [24]]. Tsay et al. [24] showed that the over-aged martensite is more resistant to stress corrosion and hydrogen induced cracking than solution-annealed condition because in the over-aged specimen, uniform distribution of RA together with numerous precipitates impedes the transport of hydrogen. Nagao et al. [25] concluded that HE fracture of lath martensitic steel is driven by hydrogen-enhanced and plasticity-mediated decohesion mechanism.
It is known that austenite (fcc crystal structure) has a lower hydrogen diffusivity and higher hydrogen solubility compared with martensite or ferrite (bct or bcc crystal structure) [24,26,27]. Although RA usually acts as a strong hydrogen trap to improve the tolerance of hydrogen uptake, the effect of RA on HE susceptibility is unclear mainly because its metastable nature. Sojka et al. [28] investigated the HE susceptibility of TRIP 800 steels and found that the HE susceptibility resistance of TRIP 800 steels is poor in comparison with other advanced high strength steels since the RA in TRIP 800 steel absorbs more hydrogen. Lovicu et al. [29] considered that RA in Q&P steel transforms into martensite during deformation (i.e., TRIP effect) and the high hydrogen concentration inherited from RA is likely to induce microcracking in the newly formed martensite regions. Zhu et al. [30] observed that the hydrogen concentration in RA is three times greater than in martensite and these hydrogen atoms may promote hydrogen-induced cracking at martensite/austenite interface. In contrast, Wang et al. [31] found that introducing inter-lath austenite nano-film in the martensitic matrix can reduce HE susceptibility when austenite nano-film has high and dispersed stability. Many other studies [24,[32], [33], [34]] concluded that the presence of a small amount of RA is somewhat in favorable in suppressing HE susceptibility.
As stated above, the low carbon medium Mn steel normally has complex microstructure composed of tempered martensite (probably including various carbides) and RA, but only a few of studies focused on HE susceptibility [30,35,36]. On account of the complex microstructures, it is very hard to quantitatively characterize the relationship between microstructure and HE susceptibility. In this study, different intercritical annealing temperatures were employed to adjust the volume fraction of RA. The interaction between hydrogen and microstructure obtained by different heat treatments, is elucidated using electrochemical hydrogen permeation test and melt extraction tests. The focuses here is on the effect of RA on HE susceptibility through SSRT tests before and after hydrogen charging.
Section snippets
Materials and intercritical annealing treatment
The studied steel of composition Fe-0.065C-0.2Si-5.45 Mn (in wt.%) was obtained from 30 mm thick low carbon medium Mn plate produced by Ansteel. The continuous casting slab was homogenized at 1200 °C, hot-rolled to ~30 mm thick plate at temperature in the range of ~980 °C–900 °C, and then on-line water quenched directly to room temperature. The on-line quenched steel was annealed at 630 °C for 30 min to obtain a certain amount of RA, and the obtained steel is called quenched and
Microstructure characterization
Fig. 2 shows the representative SEM microstructures for QHA, DQ, and QLA specimens. The microstructures of QHA and QLA specimens are similar, mainly consisting of tempered martensite with a number of tempered carbides, while the microstructure of DQ specimen only contains quenched martensite. It seems that increasing intercritical annealing temperature made the lath/packet boundaries of martensite become more obscure from the point of view of morphology (comparing Fig. 2a and d). Fig. 1b shows
The effect of microstructures on hydrogen diffusion and trapping
It is known that the interaction of different microstructure with hydrogen atoms determines hydrogen uptake, diffusion, trapping, and effusion [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. The microstructural characteristics, such as lattice interstitial sites, dislocations, grain boundaries, phase boundaries, precipitates, and inclusions, can act as different reservoirs
Conclusions
In this work, three different heat treatments were performed in a low carbon medium Mn steel to introduce RA with different stability and fraction. We obtained the following results mainly based on the electrochemical hydrogen permeation test and slow strain-rate test:
- (1)
The retained austenite increased (from 10.2 pct. to 22.4 pct.) with intercritical annealing temperature (from 610 °C to 630 °C). The total hydrogen concentration after hydrogen charging increased and the effective H diffusion
Acknowledgement
The authors gratefully appreciate the financial support by the National Natural Science Foundation of China (No. 51605084) and the National High-tech R&D Program (863 Program) [NO. 2015AA03A501]. R.D.K Misra gratefully acknowledges continued collaboration with northeastern University as an Honorary Professor providing guidance.
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