JPH05223726A - Method for detecting secular embrittlement - Google Patents

Abstract

PURPOSE:To clearly form an intergranular corrosion groove in a short time and improve the reliability of measured data and evaluation of degree of embrittlement by corroding the grain boundary of a part to be measured by reversely sweeping the potential at the part after a passive film is formed at the part. CONSTITUTION:An electrolytic cell 1 is mounted on the surface of a part to be measured after polishing the surface and an electrolyte 7 is poured in the cell 1. Then, while a potential is applied across an object 6 to be measured from a potentiostat 14 and the potential value is swept with a scanner 15 so that anode polarization can be obtained, a polarization curve indicating the relation between the potential and current density is recorded on a recorder 16 and, when the current density becomes the smallest value, a passive film is formed at the part to be measured by once stopping the sweeping. Then the smallest value of the current density is confirmed by reversely sweeping the potential and, at the same time, the grain boundary at the part to be measured is corroded by maintaining the potential which gives the smallest value for a fixed period of time. When the depth to grains suffering from intergranular corrosion is measured after the part to be measured is washed and dried and measured data are plotted on a fracture transient temperature diagram, the reliability of embrittlement evaluation is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of detecting aged embrittlement of an object to be measured of a low alloy steel, and particularly to aged embrittlement for evaluating aged embrittlement based on a change in grain boundary corrosion of low alloy steel. Regarding detection method.

[0002]

2. Description of the Related Art Cr-Mo, which has excellent heat resistance, is used as a component for handling high-temperature fluids such as power plants and chemical plants.
Low alloy steels such as steel and Cr-Mo-V steel are used.

However, when it has been used for many years, it may handle a high temperature fluid, which causes a structural change in the steel and causes deterioration of the material. This material deterioration is largely due to so-called temper embrittlement. It is considered that this temper embrittlement is due to segregation of P, which is an impurity element, among P, Sn, As, and Sb contained in the low-alloy steel, especially at grain boundaries and weakens the grain boundary strength. ing.

On the other hand, when tempered embrittlement occurs in a low alloy steel, the toughness is lowered and it causes a damage accident. Therefore, from the viewpoint of quality evaluation of low alloy steel and prevention of accidents, various techniques have been developed for measuring aged temper embrittlement of low alloy steel.

Conventionally, the Sharpi impact test method has been widely known as a method for detecting this kind of brittleness. This detection method is based on the degree of decrease in the absorbed energy value when the test piece is destroyed by impact, or the movement of the impact value / test temperature curve, that is, the transition temperature rise amount of the impact value, and the ductile fracture surface of the test piece that is impacted Measure the ratio of the area to the total fracture surface area,
The ductile fracture surface ratio is obtained from the measured value, and the increase amount (ΔFATT) of the fracture surface transition temperature defined by the temperature for the ductile fracture surface ratio of 50% is detected from this temperature dependence. However, this method must be cut out of the test piece from the defining device, and cannot be applied in practice at all, even if it is an actual device before assembly.

In response to such problems of the prior art, a method for nondestructively detecting the degree of embrittlement of a metal member has recently been developed by measuring the grain boundary segregation caused by impurities and measuring the corrosion groove depth of the crystal grain boundary. The disclosure of the dimensions is found in, for example, Japanese Patent Application Laid-Open No. 1-110259.

This technique is generally called the intergranular corrosion method, and as shown in FIG. 4, the corrosive liquid 3 in the cell 2 is brought into contact with the object to be measured 1 to corrode the inspection site. ing. The corrosion conditions are a temperature of 25 ° C. and a time of 2 to 3 hours. As shown in FIGS. 5 and 6, the inspection portion after corrosion appears as a thin line as shown in FIG. 5 when the degree of embrittlement of the crystal grain boundary 4 is small, and the degree of embrittlement of the crystal grain boundary 4 is small. When it is large, it appears as a thick line as shown in FIG.

The intergranular corrosion groove depth thus produced is measured by indentation by Vickers, corrosion surface roughness, etc., and the measurement result is prepared in advance. Fracture transition temperature diagram (FATT) And the degree of embrittlement is evaluated.

[0009]

However, the above-mentioned detection method has a problem that it takes a long time of 2 to 3 hours because the sweeping operation is trial and error to corrode the crystal grain boundaries. In addition, at the rate of spending a long time in corroding the crystal grain boundaries, the intergranular corrosion groove depth, which is the measurement result, can be obtained only up to about 1.5 μm. It was not always satisfactory in terms of accuracy.

In view of the above points, the present invention provides a method for detecting aged embrittlement that can corrode crystal grain boundaries in a relatively short time, has a large measurement grain boundary groove depth, and has few measurement errors. The purpose is to

[0011]

According to the present invention, an electric potential is applied to a portion to be measured in contact with an electrolytic solution, and a current density flowing between the portion to be measured and a counter electrode reaches a minimum value while sweeping the potential. Stop the electric potential sweeping once to form a passivation film on the measured part.After forming the passivation film, reverse the potential and check the minimum value of the current density of the electrolyte. Corrosion of the grain boundary of the measured part by holding the potential for applying for a certain period of time, after which the depth of the intergranular corrosion groove is measured, and the measurement result is plotted on the fracture surface transition temperature diagram obtained in advance. Is characterized by.

[0012]

With this structure, the potential applied to the measured portion becomes a passivation region other than the grain boundaries where impurities are segregated by the electrolytic solution, so that corrosion is hardly caused by the passivation film. Will disappear. Therefore, in the measured portion, the grain boundary is selectively divided into a corroded portion and a non-corroded portion, so that the intergranular corrosion groove can be measured in a short time. In this way, since the data with high measurement accuracy is plotted on the fracture surface transition temperature diagram (FATT) which has been obtained in advance, the reliability of the embrittlement measurement is remarkably increased as compared with the conventional case.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of the aged embrittlement detection method according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of a conventionally well-known device applied to the method for detecting aged embrittlement according to the present invention.

Reference numeral 1 denotes an electrolytic cell 1 which is in close contact with the object to be measured 6 via a packing 5. The electrolytic cell 1 is filled with an electrolytic solution 7 and is exposed through an opening 8. Measured object 6
Touches. The electrolyte solution 7 is highly corrosive and sensitive, for example, a solution in which a small amount of sodium trimethylbenzenesulfonate as a surfactant is added to saturated picric acid. The electrolytic solution 7 is supplied from the inlet 9 via the valve 11a, and its discharge is discharged through the outlet 10 provided with the valve 11b.

The electrolysis cell 1 has a reference electrode 12 and a counter electrode 13, and these electrodes 12, 13 are connected to a potentiostat 14 together with the object 6 to be measured. The potentiostat 14 has one end connected to the scanner 15 and the other end connected to the recorder 16.

Therefore, the potential applied to the DUT 6 is swept by the scanner 15, and the current value flowing between the DUT 6 and the counter electrode 13 corresponds to the potential change value applied by the sweep. It is recorded with recorder 16.

A specific example of the aged embrittlement detection method according to the present invention using the above apparatus will be described. As the test piece used in this example, three kinds of Cr-Mo were used.
-V steel is used and each one has a different degree of embrittlement. Further, the fracture surface transition temperature FATT of each of these types is FATT = 110 ° C, 150 ° C, 180 ° C from the Sharpi impact test.
Is getting

First, the surface of a test piece of Cr-Mo-V forged steel is buffed, the electrolytic cell 1 is mounted, and the electrolytic solution 7 is injected from the injection port 9. As the electrolytic solution 7, a mixed solution of the above-mentioned saturated picric acid and sodium trimethylbenzenesulfonate is used, but the electrolytic solution 7 is not limited to this mixed solution, and as a surfactant added to the mixed solution, a sodium alkylbenzene sulfonate-based compound such as triethylbenzenesulfonate is used. Even if sodium decylbenzene sulfonate or the like is used, if the crystal grain boundaries have impurities, the electrolytic solution 7 having a high corrosive property has a liquid temperature of 25 ° C., and the room temperature at that time is also 25 ° C.

Then, a potentiostat is attached to the object 6 to be measured.
An electric potential is applied from 14, and during this period, the electric potential value is swept by the scanner 15 so as to obtain the anodic polarization, and the relationship between the electric potential and the current, that is, the polarization curve is recorded in the recorder 16 from this operation.

FIG. 2 shows Cr-Mo-at FATT = 150 ° C.
It is an example of the polarization curve obtained from V forged steel. In this figure, when the potential is increased from the natural potential E point at a sweep rate of 5 mV / sec, the current density relatively increases and reaches the maximum value A point. After the point A, the crystal plane of the Cr-Mo-V forged steel enters the passive region by the electrolytic solution 7 and the current density gradually decreases.

When the current density reaches the minimum value B, the sweep is stopped once, and during this period, it is kept for 2 minutes under the potential of about 400 mV so that the passivation film is sufficiently formed.
After forming the passivation film, reverse sweep is performed at a potential of 5 mV / sec in the direction of decreasing the potential, thus confirming the above-mentioned minimum value point B.

The reverse sweep is stopped at the point c where the potential reaches about 300 mV, and the potential is kept at about 300 mV for 20 minutes to perform intergranular corrosion. During the intergranular corrosion, there is almost no dissolution in the grain where the concentration of the impurity element such as P is low so that the passivation film is sufficiently formed, but at the grain boundary where the impurity element is segregated,
Since the passivation film is not formed, the grain boundary corrosion proceeds. After the intergranular corrosion, the electrolytic cell 1 is removed from the measured portion 6, the surface to be measured is washed with water and dried to measure the intergranular corrosion grain depth. The Vickers indentation method is used to measure the intergranular corrosion groove depth. In this method, first, an indenter is applied to the surface of a sample that has undergone intergranular corrosion using a Vickers hardness tester with a quadrangular pyramid, and the dimension is measured. Next, the sample surface is buffed and scraped until the intergranular corrosion grooves almost disappear. In this state, if the Vickers indentation dimension is measured again, the depth of scraping can be calculated from the difference from the initial indentation dimension, and this becomes the measured value of the intergranular corrosion groove depth.
When the intergranular corrosion groove depth D was measured for samples with three types of embrittlement, D = 0.35 μm, 3.25 μm, and 5.12 μm corresponding to materials with FATT = 115 ° C, 150 ° C, and 180 ° C, respectively.
Met.

On the other hand, the same test piece was used to measure the grain boundary groove depth by the conventional method. As the electrolytic solution, the same solution as in the above example was used, the test temperature was the same, immersion corrosion was performed for 2.5 hours, and then the intergranular corrosion groove depth was measured. As a result,
D = 0.31 μm, 1.23 μm, 1.79 μm were obtained.

FIG. 7 is a graph comparing the data measured by the method for detecting aged embrittlement according to the present invention with the data obtained by the conventional method. From this figure, it is easily understood that the method for detecting aged embrittlement according to the present invention shortens the time required for corrosion to about ⅕ and also has a large grain boundary corrosion groove depth. Let's do it. Then, if the data thus obtained is plotted in a fracture surface transition temperature diagram (FATT) as shown in FIG. 6, which has been obtained in advance, the evaluation reliability of the degree of embrittlement becomes high.

In the above-mentioned embodiment, the minimum value of the current density is confirmed by reverse sweeping and preparation for intergranular corrosion is started from this point. However, the present invention is not limited to this embodiment and the anode activity peak Preparation for intergranular corrosion may be started with reference to the potential or the passivation holding potential.

[0026]

As described above, in the method of detecting aged embrittlement according to the present invention, in corroding the crystal grain boundaries segregated by impurity elements, the crystal grain boundaries are anodically polarized to a potential at which the crystal grain boundaries are selectively corroded. Therefore, it is possible to form the intergranular corrosion groove clearly in a short time. Therefore, the reliability of the measurement data is high, and it can be said that the evaluation of the degree of embrittlement is extremely excellent even when plotted on the fracture surface transition temperature diagram obtained in advance. Therefore, in the maintenance and management work of this kind of inspection, the reliability can be greatly improved as compared with the conventional one, together with the improvement of the efficiency.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a grain boundary corrosion apparatus applied to a method for detecting aged embrittlement according to the present invention.

FIG. 2 is a graph showing a measurement example of a polarization curve.

FIG. 3 shows the intergranular corrosion groove depth--measurement data obtained by the aged embrittlement detection method according to the present invention and measurement data obtained by the conventional method.
The graph which compared and was plotted on the fracture transition temperature FATT relationship diagram.

FIG. 4 is a conceptual diagram showing a conventional embodiment.

FIG. 5 is a schematic view of a grain boundary showing a case where the degree of embrittlement is small.

FIG. 6 is a schematic view of crystal grain boundaries showing a case where the degree of embrittlement is high.

FIG. 7 is a diagram showing a fracture surface transition temperature FATT obtained in advance.

[Explanation of symbols]

 1 ... Electrolytic cell 6 ... Object to be measured 12 ... Reference electrode 13 ... Counter electrode 14 ... Potentiostat 15 ... Scanner 16 ... Recorder

Claims (1)

[Claims] 1. A potential is applied to a portion to be measured in contact with the electrolytic solution,
While sweeping the potential, when the current density flowing between the measured part and the counter electrode reaches a minimum value, the potential sweep is temporarily stopped and a passive film is formed on the measured part. By reverse sweeping the electric potential to check the minimum value of the current density of the electrolyte, and by holding the potential that gives this minimum value of the current density for a certain period of time, the grain boundaries of the measured part are corroded, and then the intergranular corrosion. A method for detecting aged embrittlement, which comprises measuring a groove depth and plotting the measurement result on a fracture surface transition temperature diagram obtained in advance.