JPH0713639B2 - Evaluation method for residual life of ferritic heat resistant steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for evaluating the remaining life of a ferrite heat-resistant steel used at high temperature.

[Conventional technology]

For example, since the boiler superheater for thermal power generation is used in a high temperature and high pressure environment, the material deteriorates with long-term use,
There is a risk of causing an accident such as a blast. Conventionally, as a residual life evaluation method for heat-resistant steel parts for the purpose of preventing such accidents, attention was paid to the method of evaluating the residual life by the cleave rupture test of the members used and the change in mechanical properties such as hardness. The method is known.

[Problems to be solved by the invention]

However, the method based on the creep rupture test requires cutting of the member in use and a long-term test, so that the remaining life cannot be evaluated nondestructively and quickly. In addition, the method that focuses on changes in mechanical properties such as hardness is because the changes occur at the end of the life and there are large variations in the changes. It was not possible to accurately evaluate the remaining life.

On the other hand, it is known that the mechanical properties of the ferrite heat-resistant steel have a close relationship with the precipitated carbide, and that the carbide gradually grows and coarsens with the passage of time when used at high temperatures, but the carbide Above all, there is no residual life evaluation method focusing on grain boundary carbides that have a great influence on the deterioration state of the heat resistant steel used for a long time.

The present invention intends to provide a residual life evaluation method focusing on this grain boundary carbide.

[Means for solving problems]

The present inventors have studied a ferrite heat-resistant steel that has been used for a long time at a high temperature, and when the heat-resistant steel is used for a long time at a high temperature, the grain boundary precipitates become spherical with the use time, It was confirmed that the degree of spheroidization depends on the operating temperature and the operating stress.

Therefore, as a measure showing the degree of spheroidization of the grain boundary carbides of the ferrite heat-resistant steel used at high temperature for a long time, the ratio of the maximum value to the minimum value of the diameter of the carbide was measured as the spheroidization rate. It was found that the spheroidization rate of various factors has a good correlation with the creep rupture life consumption rate or the life consumption rate based on stress analysis, and the remaining life can be easily and accurately evaluated from the spheroidization rate of grain boundary carbides. And completed the present invention.

That is, the present invention is based on the above findings, the spheroidization rate of the carbide precipitated in the grain boundary of the ferrite heat-resistant steel used for a long time under high temperature and high pressure, and the life consumption rate by the fracture test of the material used in advance or The remaining life of the heat-resistant steel is evaluated using a remaining life evaluation reference diagram showing the relationship between the life consumption rate by stress analysis and the spheroidization rate of the carbide.

According to the present invention described above, it is possible to easily and accurately evaluate the remaining life of a ferrite heat-resistant steel that has been used for a long time, non-destructively without subjecting it to a destructive test or the like.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

First, an electron microscope was used to observe ferritic heat-resistant steels such as the 2 1 / 4Cr-1Mo steel that had been used for a long time as a high temperature pressure resistant member of a commercial boiler.

Fig. 1 is a schematic diagram of the electron microscopic structure. For various materials used, the maximum value la and the minimum value lb of the grain boundary carbide 2 on the grain boundary 1 were measured, and the spheroidization rate was measured. Was calculated.

Next, the creep rupture test of the pressure resistant member is performed to calculate the life consumption rate from the ratio of the creep rupture time with the unused material,
As shown in FIG. 2, a residual life evaluation standard diagram showing the relationship between the spheroidization rate of grain boundary carbides and the creep rupture life consumption rate was prepared.

Next, extract replica samples from the surface of the overheated organ that has been used for a long time in a commercial boiler, measure the spheroidization rate of the grain boundary carbides based on the above method, and estimate the creep rupture life consumption rate from the residual life evaluation reference diagram. Then, a value of 50% was obtained.

On the other hand, 600 for the overheated organ and its unused material
When the creep rupture test was carried out at a temperature of 10 ° C and a stress of 10 kg / mm ² , the rupture time was 1100 hours for the used material and 21 for the unused material.
The service life consumption rate of the used material was 51.4% after 40 hours, which is in good agreement with the service life consumption rate obtained by the method of the present invention. It was possible to accurately determine the remaining life of the diameter heat resistant steel.

〔The invention's effect〕

As described in detail above, according to the present invention, it is possible to rapidly and non-destructively estimate the remaining life of a ferrite heat-resistant steel without using a destructive test, and to provide a remaining life evaluation method with sufficiently high estimation accuracy. .

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an electron microscopic structure for measuring the spheroidization rate of grain boundary carbides, and FIG. 2 is a graph showing the relationship between the spheroidization rate of grain boundary precipitates and the creep rupture life consumption rate. It is a life evaluation reference line diagram. 1 ... Grain boundary, 2 ... Grain boundary precipitate.

Claims (1)

[Claims] 1. A method for evaluating the remaining life of a ferrite heat-resistant steel used in a high temperature environment, comprising a first step of determining a spheroidization rate of carbides precipitated at grain boundaries of the heat-resistant steel as defined below. Second step of determining the remaining life of the heat-resistant steel from a remaining life evaluation reference diagram showing the relationship between the life consumption rate obtained by a fracture test of the material used in advance or the life consumption rate based on stress analysis and the spheroidization rate of the carbide A method for evaluating the remaining life of a ferritic heat-resistant steel, comprising: