In this paper, the stress distribution field in front of the crack tip was obtained by loading a modified WOL specimen using a bolt. Considering the relationship between microhardness and hydrogen content or internal stress in the metal, a model based on the change of microhardness increment is proposed to describe the trend of hydrogen concentration distribution in the stress environment. The agreement between theoretical model and experimental results is verified by the Vickers microhardness tester. Based on the model, there is a simple additive relationship between the hydrogen-induced microhardness increment and the stress-induced microhardness increment. Therefore, the microhardness tester can be employed to evaluate the hydrogen distribution in metals quantitatively. The experimental results demonstrated that the Vickers microhardness method has accurately revealed the hydrogen concentration behavior accurately in a known equibiaxial stress environment. The hydrogen distribution of specimens in the stress environment was analyzed by taking the change of the microhardness increment along the crack propagation direction of specimens as the indicator.
Hydrogen is regarded as a clean and green resource of energy. Hydrogen energy technology has attracted extensive attention with the industrialization of hydrogen fuel cells and hydrogen energy automobile. More and more production equipment is exposed to a hydrogen-containing environment. As a result of the working stress and external stress, the equipment is usually in a low-cycle fatigue working condition in the engineering environment. The stress and hydrogen in the metal can degrade its mechanical properties overtime. Therefore, in order to determine the mechanism for hydrogen corrosion in the stress environment and to ensure the safety of equipments exposed to hydrogen, the hydrogen distribution behavior in the stress environment has to be understood [
The determination of hydrogen distribution in metals has always been the focus and difficulty in the study of hydrogen corrosion. Currently, the proved effective methods include SIMS [
In the experimental evaluation of the hydrogen concentration behavior in the stress environment, it has been reported in previous publications that Yu et al. [
However, the above experimental methods are all laboratory methods that are difficult to apply in engineering for some reasons, including the experimental device structure is complicated, the procedure is cumbersome, and the experimental expenses are high. The present study attempts to find a method suitable for the engineering environment to evaluate the hydrogen behavior of metals under stress precisely.
It has been reported in previous publications that the microhardness method can be one of the most important methods to evaluate the equibiaxial stress field [
Hydrogen concentration behavior in the stress environment is investigated by the microhardness tester in this paper. The stress environment is created by loading the modified wedge-opening-loading (WOL) specimen with a bolt. The stress state along the crack propagation direction of the specimen is an equibiaxial stress field with a theoretical solution. The corrosion environment is a saturated hydrogen sulfide solution. The measured microhardness increments in front of the crack before and after corrosion were compared.
The following equation reveals that a substrate with a tensile stress develops a larger apparent contact area than the virgin material, when indented to the same load [
In the process of indentation experiment,
The stress field then can simply be written as
In this study, the diffused hydrogen atoms tend to accumulate around the high-stress field, interfaces, and grain boundaries, leading to the main hydrogen damage that can be obtained by means of stress-induced diffusion, which is given by Dean [
Since the increase in hydrogen concentration is proportional to the increase in hardness, the relative hydrogen concentration of
For a modified WOL specimen, the plane strain fields
In Equation (
In a stress-induced hydrogen corrosion environment, it is assumed that the microhardness increment along the crack propagation direction of the specimen can be generally divided into two parts: plane strain theoretical stress-induced microhardness increment and the hydrogen-induced microhardness increment. The relationship can be given by the following equation:
A modified WOL specimen of #45 steel was employed in this research, as shown in Figure
The modified WOL specimen (reproduced from Gu et al. [
Chemical composition of the tested steel (mass %) (reproduced from Gu
C | Si | Mn | S |
|
Cr | Ni |
---|---|---|---|---|---|---|
0.46 | 0.27 | 0.65 | 0.015 | 0.018 | 0.20 | 0.214 |
The stress intensity factor
The Zwick Roell Zhu 2.5 hardness tester produced in Germany was used to measure the hardness distribution in front of the crack before and after corrosion. Vickers hardness HV0.2 was chosen as the hardness index of the material. Starting from the crack tip, the microhardness values were obtained at the intervals of 80–100
The mean and standard deviation of the microhardness increment values of stress-free area with different corrosion periods are shown in Figure
Microhardness increment for each group. The
The microhardness increment distribution was calculated along the crack propagation direction of multiple groups of specimens after soaking for 0 h, 24 h, 48 h, and 72 h in saturated hydrogen sulfide solution, respectively, as shown in Figure
Microhardness increment distribution curves along the crack propagation direction of specimens before and after corrosion.
In Figure
The blue line in Figure
Calculated applied stress and microhardness increment distribution along the crack propagation direction of specimens.
The red line in Figure
In Figure
Linear relationship between applied stress and microhardness increment.
From Figure
This is in accord with the result of stress estimation by Suresh and Giannakopoulos [
Therefore, in the no corrosion environment, the stress can be described using the stress-induced microhardness increment linearly as follows:
Equation (
The blue line in Figure
Theoretical and experimental contrast curves of microhardness increment–distance from the crack tip (a) with 24 h corrosion period, (b) with 48 h corrosion period, and (c) with 72 h corrosion period.
The red line in Figure
Subtracting the red line (micro-hardness increment distribution curve under the no corrosion condition) from the blue line (microhardness increment distribution curve under the corrosion condition), a new micro-hardness increment distribution curve is obtained, shown as the green line in Figure
The orange line in Figure
The obtained green line is the experimental curve of hydrogen-induced microhardness increment (experimental hydrogen-induced microhardness increment) distribution along the crack propagation direction of specimens because the green line and the orange line are in good agreement with each other. Therefore, the hydrogen distribution behavior in the stress environment can be successfully measured using the microhardness method. In the actual project, as long as the stress environment is a known equibiaxial stress environment, the microhardness method can be used to accurately obtain the hydrogen concentration distribution in the equipment.
This also reveals that, under the combined effect of applied stress and hydrogen corrosion, the measured microhardness increment can be generally divided into two parts: plane strain theoretical stress-induced microhardness increment and the hydrogen-induced microhardness increment. In this study, the relationship can be given by the following equation:
In this paper, the hydrogen distribution of specimens in the stress environment was analyzed by taking the change of the microhardness increment along the crack propagation direction of specimens as the indicator. The conclusions are summarized as follows: In the corrosive environment, the hardness along the crack propagation direction of the specimen is significantly affected by stress and hydrogen concentration. The Vickers microhardness method can accurately reveal the hydrogen concentration behavior accurately in a known equibiaxial stress environment. The microhardness method can be employed to accurately evaluate the hydrogen concentration distribution in the engineering parts. In the Vickers microhardness measurement, there is a simple additive relationship between stress-induced microhardness increment and hydrogen-induced microhardness increment. The experimental results showed that the distribution of hydrogen corrosion concentration in the stress environment along the crack propagation direction of the specimen was consistent with the theoretical relationship between hydrostatic stress and hydrogen corrosion concentration. The experimental results showed that the applied stresses have good linear correlation with microhardness increment in the no corrosion environment, and the scale factor was –10. This showed that the closer the crack tip, the greater the stress and the smaller the microhardness.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This article is supported by the National Key Research and Development Program of China (Project no. 2016YFC0801905-16).