Hydrogen trapping sites and hydrogen-induced cracking in high strength quenching & partitioning (Q&P) treated steel

https://doi.org/10.1016/j.ijhydene.2014.06.079Get rights and content

Highlights

  • Hydrogen dramatically causes deterioration in Q&P treated steel.

  • Hydrogen trapping sites are directly observed by atom probe tomography.

  • Hydrogen is three times more soluble in austenite than that in martensite.

  • Hydrogen-induced cracks initiate mainly at martensite/austenite interfaces.

Abstract

The effect of hydrogen on the tensile properties and fracture characteristics was investigated in the quenching & partitioning (Q&P) treated high strength steel with a considerable amount of retained austenite. Slow strain-rate tensile (SSRT) tests and fractographic analysis on cathodically charged specimens were performed to evaluate the hydrogen embrittlement (HE) susceptibility. Total elongation was dramatically deteriorated from 19.5% to 2.5% by introducing 1.5 ppmw hydrogen. Meanwhile, hydrogen caused a transition from ductile microvoid coalescence to a mixed morphology of dimples, “quasi-cleavage” regions and intergranular facets. Moreover, hydrogen trapping sites were directly observed by means of three-dimensional atom probe tomography (3DAPT). Results have shown that hydrogen in austenite (33.9 ppmw) is 3 times more soluble than that in martensite (10.7 ppmw). By using DENT specimen, hydrogen-induced cracking (HIC) cracks were found to initiate at martensite/austenite interfaces and then propagate through retained austenite and martensite. No crack was observed to be initiating from ferrite phase.

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)

Cited by (158)

View all citing articles on Scopus
View full text