JPH0649538A - Method for testing hydrogen embrittlement sensitivity of metallic material - Google Patents

Abstract

PURPOSE:To accurately evaluate and judge the hydrogen embrittlement sensitivity of austenitic stainless steel with high sensitivity by repeating the charging and discharging of hydrogen in the austenitic stainless steel by an electrochemical method and thereafter observing its fractured face. CONSTITUTION:The austenitic stainless steel 3 to be judged with hydrogen embrittlement sensitivity is immersed, as the cathode, into a normal soln. 1 of sulfuric acid added with a small amt. of NaAsO2 as a surfactant. As the counter electrode, the anode 4 made of Pt is used, and a constant current is allowed to flow and hydrogen charging is executed. After that, by a tension test, the ratio of the intercrystalline cracking area in the fractured face is recorded. Next, it is held in a vacuum heat treating furnace of 100 deg.C, and hydrogen discharging is executed. After that, by a tension test, the ratio of the intercrystalline cracking area in the fractured face is measured and is compared with the intercrystalline cracking area ratio in the fractured face after the hydrogen charging, by which the hydrogen embrittlement sensitivity of the austenitic stainless steel is evaluated and judged with high sensitivity.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention provides a highly sensitive hydrogen embrittlement susceptibility test method.

[0002]

2. Description of the Related Art Hydrogen embrittlement of austenitic stainless steel has hitherto been caused by hydrogen cracking or cold working in a weld overlay which is the lining of a container used in an atmosphere containing hydrogen in the petrochemical industry. It has been reported that hydrogen cracking occurred in the weld heat affected zone of SUS304 steel, which has undergone the heat treatment, and hydrogen embrittlement has become an important issue. Many studies have been conducted on this, and in particular, a method for confirming hydrogen embrittlement susceptibility is important.

As such, the 46th journal of the Japan Institute of Metals
Vol. 9, No. 7, pp. 877-886 (1982), it is known that cathode hydrogen charging of a stainless steel sensitizer reduces the ductility, but at room temperature the amount is very small. Is.

[0004]

However, in the above method, the decrease in ductility is small and no grain boundary fracture surface is observed.
It was found that the cathode hydrogen charge alone required a long time hydrogen charge to obtain a grain boundary fracture surface in a tensile test at room temperature, as described later. That is, it was found that the conventional hydrogen charging method cannot give sufficient embrittlement to detect hydrogen embrittlement susceptibility.

An object of the present invention is to provide a hydrogen embrittlement susceptibility test method which has a large effect of hydrogen embrittlement of a metal sample to be observed at room temperature and can detect the sensitivity with high sensitivity.

[0006]

Means for Solving the Problems The gist of the present invention to achieve the above object is to charge a metal test piece with hydrogen by an electrochemical method, and to heat the metal test piece charged with hydrogen by hydrogen treatment. A hydrogen embrittlement susceptibility test method for a metal material is characterized in that a step of discharging is repeated to observe a ratio of a grain boundary crack surface of a fracture surface of the metal test piece.

Grain boundary embrittlement of a metal sample is promoted by repeating charging and discharging of hydrogen as described above, and then the metal material is pulled at a low speed to break, and the fracture surface is observed to determine the ratio of intergranular crack surface. Is measured.

The discharge can achieve the object by, for example, heat treatment at about 100 ° C. in a vacuum heating furnace.

[0009]

Function: Austenitic stainless steel, eg SU
When the S304 steel is sensitized, chromium carbide is precipitated at the grain boundaries, and along with that, a chromium deficient layer is formed near the grain boundaries. On the other hand, in the austenitic steel, it is known that the influence of the amount of chromium on the diffusion rate of hydrogen makes it difficult for the diffusion of hydrogen as the amount of chromium increases.
Therefore, it is considered that the diffusion of hydrogen is facilitated in the chromium-deficient layer having a small amount of chromium.

When the sensitizer is charged with hydrogen, hydrogen preferentially penetrates into the chromium deficient layer in the vicinity of the grain boundary to cause hydrogen-induced martensite transformation and embrittle the grain boundary. However, the hydrogen diffusion rate of austenitic stainless steel is smaller than that of ferritic steel, and the depth of penetration into the interior is also small.

Next, when the invading hydrogen is discharged, the hydrogen is released through the invasion route in reverse, at which time hydrogen-induced martensitic transformation is further promoted. Two
At the time of the second hydrogen charge, in the region where the hydrogen-induced martensite transformation is caused by the first charge → discharge, hydrogen easily enters due to the large diffusion rate of hydrogen, and further into the chromium-deficient layer in the back. Upon infiltration, new hydrogen-induced martensitic transformation occurs, causing embrittlement to proceed inside. Therefore, embrittlement can be promoted as the number of times of hydrogen charge ⇔ discharge is repeated.

As mentioned above, hydrogen is more
Grain boundary embrittlement progresses as charge ⇔ discharge is repeated. Therefore, even in a metal material having the same hydrogen embrittlement susceptibility, the ratio of grain boundary embrittlement increases.

A metal material whose crystal grain boundaries are brittle causes grain boundary fracture when pulled at a low speed. A material having a higher rate of grain boundary embrittlement has a higher rate of causing grain boundary fracture, and as a result, hydrogen embrittlement susceptibility can be detected with high sensitivity.

[0014]

EXAMPLES The present invention will be described based on examples. FIG. 1 is an explanatory diagram showing the principle of the cathode hydrogen charging method based on the constant current method.

Sodium arsenite (Na
A 1 N sulfuric acid solution 1 containing a small amount of ASO ₂ ) was kept at 50 ° C. by a mantle heater 2, and a sensitized stainless steel (650 mm) was formed into a tensile test piece having a thickness of 1 mm therein.
The sample electrode (cathode) 3 is used as the sample electrode (cathode) and the platinum electrode (anode) 4 is used as the counter electrode, and a constant current (4.2 mA / cm ² ) is supplied by the potentiostat 5 to charge hydrogen. The potential change at that time is measured using the reference electrode 6 and the potentiostat 5 and recorded in the recorder 7.

First, the sample electrode 3, which was charged with hydrogen only for a predetermined time, was subjected to a tensile test at a low speed and the fracture surface was observed by SEM. In FIG. 2, the values obtained by calculating the ratio (%) of the intergranular crack area to the area of the entire fracture surface are shown by ◯ marks 8, 8 ′ and 8 ″.

It can be seen that the ratio of intergranular cracked surfaces increases as the hydrogen charging time increases. However, the sail ratio of the intergranular cracked surface tends to be saturated with time.

On the other hand, in this embodiment, after hydrogen charging for 24 hours under the above conditions, hydrogen discharge is carried out by holding in a vacuum heat treatment furnace at 100 ° C. for 24 hours. This 4
After repeating the number of times, a low speed tensile test was conducted in the same manner as above. The result is shown in FIG.

The above total time of hydrogen charging is 96 hours, the ratio of the intergranular crack surface is higher than that of the case where only hydrogen charging is performed for 96 hours, and the total time required (hydrogen charging time + discharge time) is compared. Even 19
At 2 h, the ratio of intergranular cracked surfaces is about 1.5 times higher than the result of 8 ″ when only hydrogen charge is 192 h. From this, according to the method of the present embodiment, the hydrogen embrittlement susceptibility of the metal material can be detected efficiently and with high sensitivity.

[0020]

EFFECTS OF THE INVENTION According to the present invention, hydrogen embrittlement can be efficiently generated even for a material having low susceptibility to hydrogen embrittlement.
It is possible to evaluate with high sensitivity the susceptibility to hydrogen embrittlement of such metal materials.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the principle of a cathode hydrogen charging method using a constant current according to this embodiment.

FIG. 2 is a graph showing a relationship between a hydrogen charging time and a ratio of an intergranular crack surface to a total fracture surface after a tensile test.

[Explanation of symbols]

1 ... Sulfuric acid solution, 2 ... Mantle heater, 3 ... Sample electrode (cathode), 4 ... Platinum electrode (anode), 5 ... Potentiostat, 6 ... Reference electrode, 7 ... Recorder, 8 ... Particles for hydrogen charging only Ratio of intergranular crack surfaces, 9 ... (Hydrogen charge and discharge) x ratio of intergranular crack surfaces when performed 4 times.

Claims (2)

[Claims] 1. A step of charging a metal test piece with hydrogen by an electrochemical method and a step of discharging hydrogen by heat-treating the hydrogen-charged metal test piece are repeated to obtain a fracture surface of the metal test piece. A hydrogen embrittlement susceptibility test method for metallic materials, characterized by observing the ratio of intergranular crack surfaces. 2. The hydrogen embrittlement susceptibility test method for a metal material according to claim 1, wherein the metal test piece is austenitic stainless steel.