TECHNICAL FIELD

The present invention relates to a technique for estimating penetration behavior of hydrogen penetrating high-strength steel.

BACKGROUND ART

High-strength steel suffers ductility loss and significant strength reduction due to hydrogen penetrating into the steel. This phenomenon is called hydrogen embrittlement (Non-Patent Literature 1). The steel fractures due to hydrogen embrittlement and fracture time of the steel decreases with increases in hydrogen content in the steel (Non-Patent Literature 2).

Therefore, in predicting occurrence of hydrogen embrittlement, it is important to examine penetration behavior (saturated hydrogen content, variation of hydrogen content with time) of hydrogen penetrating the steel. For example, hydrogen penetration behavior can be examined by measuring hydrogen content in steel using thermal desorption spectroscopy (TDS) or the like.

CITATION LIST

Non-Patent Literature

Non-Patent Literature 1: Shiraga, “Hydrogen Embrittlement of Steel” Zairyo-to-Kankyo. 2011, p. 236-p. 240

Non-Patent Literature 2: Suzuki. et al., “Estimation of Delayed Fracture Property of Steels” Tetsu-to-Hagane. 1992, Vol. 79, No. 2, p. 227-p. 232

Non-Patent Literature 3: Doshida. et al., “Enhanced Lattice Defect Formation Associated with Hydrogen and Hydrogen Embrittlement under Elastic Stress of a Tempered Martensitic Steel” Tetsu-to-Hagane. 2012, Vol. 98, No. 5, p. 197-p. 206.

SUMMARY OF THE INVENTION

Technical Problem

Hydrogen penetration behavior in steel changes with the magnitude of load stress on the steel (Non-Patent Literature 3). If load stress on steel is equal to or lower than threshold stress below which the steel is not fractured by hydrogen embrittlement, by measuring hydrogen content in the steel at regular time intervals using TDS, it is possible to know penetration behavior of hydrogen up until hydrogen content reaches saturation.

However, if load stress on steel is equal to or higher than the threshold stress, the steel fractures while absorbing hydrogen and the load stress on the steel becomes 0 (zero) at the time of fracture, making it impossible to know penetration behavior of hydrogen up until hydrogen content in the steel reaches saturation.

The present invention has been made in view of the above circumstances and has an object to estimate penetration behavior of hydrogen up until hydrogen content in steel reaches saturation even if load stress on the steel is equal to or higher than the threshold stress (i.e., the stress at or above which steel is fractured by hydrogen embrittlement).

Means for Solving the Problem

To solve the above problem, according to embodiments of the invention, there is provided a hydrogen penetration behavior estimation method, for estimating penetration behavior of hydrogen that penetrates steel, the method making a computer carry out: a first step of inputting first measurement data concerning variation of hydrogen content with time up until hydrogen content in steel-to-be-estimated reaches saturation when measurements are taken by applying such stress to the steel that no fracture due to hydrogen embrittlement occurs; a second step of inputting second measurement data concerning variation of hydrogen content in the steel-to-be-estimated with time up until the steel fractures when measurements are taken by applying such stress to the steel that fracture due to hydrogen embrittlement occurs; a third step of reading the first measurement data and the second measurement data out of a storage unit, calculating increases in hydrogen contents in an initial stage of measurement using the first measurement data and the second measurement data, respectively, and calculating a ratio of the increase in the hydrogen content according to the second measurement data to the increase in the hydrogen content according to the first measurement data; and a fourth step of multiplying saturated hydrogen content based on the first measurement data by the ratio and designating a resulting product as saturated hydrogen content of the steel-to-be-estimated, the saturated hydrogen content being reached upon application of such stress that fracture due to hydrogen embrittlement occurs.

According to an example embodiment, in the hydrogen penetration behavior estimation method, the computer is further made to carry out a fifth step of calculating a hydrogen diffusion coefficient of the steel-to-be-estimated based on the variation, with time, of the hydrogen content in the steel on assumption that hydrogen diffuses in the steel, and calculating penetration behavior data concerning the hydrogen content in the steel-to-be-estimated upon application of such stress that fracture due to hydrogen embrittlement occurs, using a diffusion equation based on the hydrogen diffusion coefficient.

According to an example embodiment, in the hydrogen penetration behavior estimation method, the third step includes plotting the first measurement data and the second measurement an example embodiment data on a graph whose ordinate represents hydrogen content and whose abscissa represents one-half power of hydrogen charging time used for measurement, and calculating the ratio using slopes of respective straight lines passing through plot points in the initial stage of measurement.

According to an example embodiment, there is provided a hydrogen penetration behavior estimation device that estimates penetration behavior of hydrogen that penetrates steel, the hydrogen penetration behavior estimation device comprising: a first measurement data input unit adapted to input first measurement data concerning variation of hydrogen content with time up until hydrogen content in steel-to-be-estimated reaches saturation when measurements are taken by applying such stress to the steel that no fracture due to hydrogen embrittlement occurs; a second measurement data input unit adapted to input second measurement data concerning variation of hydrogen content in the steel-to-be-estimated with time up until the steel fractures when measurements are taken by applying such stress to the steel that fracture due to hydrogen embrittlement occurs; a ratio calculation unit adapted to read the first measurement data and the second measurement data out of a storage unit, calculate increases in hydrogen contents in an initial stage of measurement using the first measurement data and the second measurement data, respectively, and calculate a ratio of the increase in the hydrogen content according to the second measurement data to the increase in the hydrogen content according to the first measurement data; and a saturated hydrogen content calculation unit adapted to multiply saturated hydrogen content based on the first measurement data by the ratio and designate a resulting product as saturated hydrogen content of the steel-to-be-estimated, the saturated hydrogen content being reached upon application of such stress that fracture due to hydrogen embrittlement occurs.

According to an example embodiment, the hydrogen penetration behavior estimation device further comprises a hydrogen penetration behavior estimation unit adapted to calculate a hydrogen diffusion coefficient of the steel-to-be-estimated based on the variation, with time, of hydrogen content in the steel on assumption that hydrogen diffuses in the steel, and calculate penetration behavior data concerning the hydrogen content in the steel-to-be-estimated upon application of such stress that fracture due to hydrogen embrittlement occurs, using a diffusion equation based on the hydrogen diffusion coefficient.

According to an example embodiment, in the hydrogen penetration behavior estimation device, the ratio calculation unit plots the first measurement data and the second measurement data on a graph whose ordinate represents hydrogen content and whose abscissa represents one-half power of hydrogen charging time used for measurement, and calculates the ratio using slopes of respective straight lines passing through plot points in the initial stage of measurement.

According to an example embodiment, there is provided a hydrogen penetration behavior estimation program configured to make a computer function as the hydrogen penetration behavior estimation device.

Effects of the Invention

The present invention makes it possible to estimate penetration behavior of hydrogen up until hydrogen content in steel reaches saturation even if load stress on the steel is equal to or higher than the threshold stress (i.e., the stress at or above which steel is fractured by hydrogen embrittlement).

