Effect of strain rate on hydrogen embrittlement in low-carbon martensitic steel
Graphical abstract
Introduction
High strength martensitic steels have been used in various industrial applications, such as automobiles, constructions, tools etc. However, it is well known that high strength martensitic steels are highly sensitive to hydrogen embrittlement [1], [2], [3], [4], [5]. Because a certain amount of hydrogen is inevitably introduced to the steels during fabrication processes, it is desired to overcome hydrogen embrittlement of high strength martensitic steels. In addition, this is important for application of high strength martensitic steels in hydrogen-energy society as infrastructures. Low and medium carbon high strength steels used usually have lath martensite structures. The lath martensite structure consists of several structural units with different length scales, i.e., lath, block, packet, and prior austenite grain [6], [7], [8], [9]. The lath is a single crystal of martensite with a thickness of about 0.2 μm. The block consists of many laths having nearly identical crystal orientation. The packet consists of several blocks with the same habit plane. It is thus important to characterize hydrogen-related fracture in high strength martensitic steels from a viewpoint of lath martensitic microstructure, in order to improve hydrogen embrittlement properties.
Takai et al. [10] investigated local hydrogen distribution in lath martensitic steels by the use of secondary ion mass spectroscopy, and reported that prior austenite grain boundaries were main hydrogen trapping site. Shibata et al. [11], [12] characterized hydrogen-related crack propagation behavior in martensitic steels by electron backscattering diffraction (EBSD) analysis. They found that the micro-cracks formed on or in the vicinity of prior austenite grain boundaries and propagated along certain {011}M crystallographic planes. Kim et al. [13] and Nagao et al. [14] observed microstructures just beneath the quasi-cleavage fracture surface in high strength steels by transmission electron microscopy. Their results suggested that quasi-cleavage fracture surfaces were parallel to lath boundaries. It is well known that hydrogen also induces intergranular fracture. In the case of martensitic steel, hydrogen-related intergranular fracture occurs at prior austenite grain boundaries [15], [16], [17], [18], [19].
On the other hand, it is well known that hydrogen embrittlement behavior depends on not only microstructure but also deformation conditions [20], [21], [22], [23], [24]. Previous studies reported that the susceptibility to hydrogen embrittlement increased with decreasing the strain rate at any temperatures. Although several attempts have been conducted, the reason why hydrogen embrittlement behavior changes depending on the strain rate is still unclear. We considered that hydrogen accumulation behavior has a large influence on the strain rate sensitivity of hydrogen embrittlement. Then, the present study conducted uniaxial tensile tests at various strain rates in a low-carbon martensitic steel and discussed effect of strain rate on hydrogen embrittlement from the viewpoint of hydrogen accumulation behavior.
Section snippets
Experimental procedure
An Fe-0.2C (wt.%) binary alloy was used in the present study. This is a simple model alloy for obtaining high strength martensitic structure. The chemical composition of the 0.2C steel is shown in Table 1. A cast ingot of the steel was cold-rolled to about 1.5 mm thickness and then austenitized at 1323 K for 1.8 ks in vacuum, followed by iced-brine quenching and sub-zero cooling in liquid nitrogen. After the heat treatment, both sides of the specimens were mechanically ground until the final
Results & discussion
Fig. 2 shows (a) an optical microscopy image and (b) a corresponding EBSD orientation map of the as-quenched specimen. In the EBSD orientation map, block boundaries, packet boundaries, and prior austenite grain boundaries identified by an orientation analysis are represented as black lines, yellow lines, and black broken lines, respectively. The as-quenched specimen showed fully lath martensite structures consisting of blocks and packets inside each prior austenite grain. The mean block,
Summary
In the present study, tensile tests of the hydrogen-charged and uncharged specimens of 0.2C martensitic steel were conducted at various strain rates, and the effect of strain rate on hydrogen embrittlement behavior was studied. The main results obtained are as follows:
- 1.
The total elongation of the hydrogen-charged specimens decreased significantly with decreasing the strain rate in the uniaxial tensile test at room temperature. Three types of characteristic fracture surfaces, i.e., intergranular
Acknowledgements
This study was financially supported by the Grant-in-Aid for Scientific Research (B) (No. 15H04158), and the Elements Strategy Initiative for Structural Materials (ESISM), all through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. YM was supported by the JSPS Fellowship (No. 26∙2927). NT and AS were also supported by the ISIJ Research Promotion Grant. The authors greatly appreciate all the supports.
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