Hydrogen trapping sites and hydrogen-induced cracking in high strength quenching & partitioning (Q&P) treated steel
Introduction
In the concept of designing modern steels with excellent strength and ductility, retained austenite has been subjected to extensive attention due to the high work-hardening rate induced by phase transformation during deformation [1]. Quenching and partitioning (Q&P) treatment is one promising approach to produce a microstructure of martensite and carbon-enriched retained austenite [2], [3], [4]. The high strength is attributed to the presence of martensite, while the good elongation is due to the considerable amount of retained austenite.
Hydrogen embrittlement (HE) is a crucial issue for the application of advanced high strength steels (AHSS) in various industrial field [5], [6], [7], [8]. Generally, HE susceptibility increases with the increasing steel strength level [6], [9]. The presence of hydrogen has a detrimental influence on the mechanical properties, giving rise to unpredictable catastrophic failure [8], [10]. In modern AHSS containing certain amount of retained austenite, the role of metastable austenite on HE susceptibility remains controversial. Reviewing the literature, austenite is considered as an effective hydrogen trapping site and beneficial for the alleviation of HE susceptibility due to the low diffusivity and high solubility of hydrogen [11]. In contrary, retained austenite was reported detrimental because the metastable austenite may transform to martensite which is susceptible to hydrogen embrittlement [12]. Preliminarily study showed that Q&P treated steel has higher hydrogen embrittlement susceptibility than conventional quenching and tempering (QT) treated one [13]. However, the mechanism for the role of metastable austenite in hydrogen susceptibility of these Q&P treated steels has not yet revealed.
Investigations have been carried out on the initiation of hydrogen-induced cracking (HIC) cracks in AHSS; nevertheless, the relationship between HIC and multiphase microstructure is still unclear. In martensite containing steels, such as dual phase (DP) steels, it is postulated that HIC cracks initiated at the martensite/ferrite interfaces [14], [15]. In transformation-induced plasticity (TRIP) steels, which consist of ferrite, bainite and retained austenite, cracks were related to the regions of retained austenite which transformed to martensite during deformation [7], [16]. While in the fully austenite twinning-induced plasticity (TWIP) steel, grain boundaries and twin boundaries were proved to be possible crack initiation sites [17], [18]. Thus, it is of great interest to investigate the potential crack initiation sites in the Q&P treated steel.
The present work aims at revealing the HIC mechanism with direct observation of hydrogen distribution in the recently developed Q&P treated steel consisting of ferrite, martensite and retained austenite phases.
Section snippets
Materials
A commercial Q&P 980 steel with ultimate tensile stress of 980 MPa was supplied in the form of 1.6 mm thin sheets. The nominal chemical composition of the present steel is Fe–0.22C–1.40Si–1.80Mn (in wt.%). The steel was austenitized at 1133 K for 5 min and cooled at a rate of 5 K s−1 to ∼1000 K, subsequently rapidly quenched to 553 K and then reheated and held at 623 K for 10 s, and finally quenched to room temperature.
Microstructure was characterized by scanning electron microscopy (SEM),
Microstructure characterization
The microstructure of the as-received Q&P steel is shown in Fig. 2. Fig. 2(a) illustrates the typical SEM micrograph, which consists of martensite, ferrite and retained austenite. Fig. 2(b) shows a corresponding EBSD micrograph by combining band contrast (BC) map and phase map, in which white corresponds to bcc lattice (ferrite and martensite) and red corresponds to fcc lattice (austenite). The gray regions correspond to a very low BC, most probably indicating martensite based on the high
Discussion
The results presented confirm that this Q&P treated steel suffer from HE in degradation of tensile properties, which can be explained by hydrogen-enhanced localized plasticity (HELP) mechanism based on the fractography. The existence of ductile markings shown in Fig. 4(c and e) demonstrates that hydrogen assisted fracture is a ductile fracture rather than a cleavage fracture. According to HELP mechanism [27], dissolved hydrogen enhances dislocation mobility and deformation can become localized
Conclusion
In the present work, the hydrogen effects on the tensile properties and fracture characteristics of quenching & partitioning (Q&P) treated steel with a multiphase microstructure consisting of ferrite, martensite and 9% retained austenite were investigated by slow strain rate tensile tests and fractographic analysis. With the aid of 3DAPT, the hydrogen trapping sites were identified. The main results are summarized as follows:
- 1.
Hydrogen dramatically causes degradation in the Q&P treated steel. By
Acknowledgments
This research was supported by the National Basic Research Program of China (973 Programs Grant No. 2011CB706604 and No. 2010CB630803), National Natural Science Foundation of China (No. 51174251 and No. 51201105) and State Key Lab of Development and Application Technology of Automotive Steels (No. Y12ECEQ07Y). The authors are grateful for the EBSD measurements in Oxford Instruments. The authors would also thank Prof. W.Q. Liu in Key Laboratory for Microstructures at Shanghai University for the
References (41)
- et al.
Morphology effect on the stability of retained austenite in a quenched and partitioned steel
Scr Mater
(2013) - et al.
Carbon partitioning into austenite after martensite transformation
Acta Mater
(2003) - et al.
Quenching and partitioning martensite – a novel steel heat treatment
Mater Sci Eng A
(2006) - et al.
Effect of hydrogen charging on the mechanical properties of advanced high strength steels
Int J Hydrogen Energy
(2014) - et al.
Microstructural aspects upon hydrogen environment embrittlement of various bcc steels
Int J Hydrogen Energy
(2010) Influence of martensite content on the hydrogen embrittlement of dual-phase steels
Scr Metall
(1983)- et al.
Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging
Acta Mater
(2013) - et al.
Hydrogen-induced cracking at grain and twin boundaries in an Fe–Mn–C austenitic steel
Scr Mater
(2012) - et al.
Effects of cryogenic and tempered treatment on the hydrogen embrittlement susceptibility of TRIP-780 steels
Int J Hydrogen Energy
(2013) - et al.
Direct observation of hydrogen-trapping sites in newly developed high-strength mooring chain steel by atom probe tomography
Prog Nat Sci Mater Int
(2013)
Interpreting hydrogen-induced fracture surfaces in terms of deformation processes: a new approach
Acta Mater
On the formation and nature of quasi-cleavage fracture surfaces in hydrogen embrittled steels
Acta Mater
Carbon partitioning to austenite from martensite or bainite during the quench and partition (Q&P) process: a critical assessment
Acta Mater
Hydrogen-enhanced localized plasticity – a mechanism for hydrogen-related fracture
Mater Sci Eng A
Hydrogen induced shear localization of the plastic flow in metals and alloys
Eur J Mech A Solids
The role of hydrogen in hydrogen embrittlement fracture of lath martensitic steel
Acta Mater
The solubility and diffusivity of hydrogen in well-annealed and deformed iron
Acta Metall
The first direct observation of hydrogen trapping sites in TiC precipitation-hardening steel through atom probe tomography
Scr Mater
Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy
Acta Mater
Atom-probe study of hydrogen chemisorption on Fe and Ni
Surf Sci
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