JPS62245155A - Residual life evaluation for ferrite heat-resisting steel - Google Patents

Abstract

PURPOSE:To nondestructively and rapidly measure the residual life of a ferrite heat-resisting steel with a high accuracy by measuring the spheroidizing rate of carbide precipitated on a grain boundary and evaluating the residual life by using a residual life evaluation criterion chart indicative of the relationship between a life consumption rate and the spheroidizing rate. CONSTITUTION:A variety of ferrite heat-resisting steels which have been used in such environment at a high temperature and a high pressure as in a boiler superheater tube for a thermal power generation or the like are observed under an electron microscope and the maximum and minimum diameters la and lb, respectively, of an intergranular carbide 2 on a grain boundary 1 are measured to calculate the spheroidizing rate la/lb. Then, a creep rupture test is conducted on a used material and a life consumption rate is calculated from the ratio of the creep rupture time of the used material to that of an unused material to make a residual life evaluation criterion chart indicative of the relationship between the spheroidizing rate of the intergranular carbide and a creep rupture life consumption rate. Thus, the residual life of the heat-resisting steel can be accurately evaluated without a destructive test.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for evaluating the remaining life of ferritic heat-resistant steel used at high temperatures.

[Conventional technology]

For example, boiler superheater tubes for thermal power generation are used in high-temperature, high-pressure environments, so the material deteriorates over long periods of use.
There is a risk of causing an accident such as a blowout. Conventional methods for evaluating the remaining life of heat-resistant steel parts aimed at preventing such accidents have focused on evaluating the remaining life through creep rupture tests of the used parts and on changes in mechanical properties such as hardness. Men's laws are known.

[Problem that the invention seeks to solve]

However, the creep rupture test method requires cutting of the member during use and long-term testing, and cannot quickly and non-destructively evaluate the remaining life. The method that focuses on changes in mechanical properties such as hardness is used from a relatively early stage of life to the end of life, since these changes occur at the end of life and there is large variation in these changes. It was not possible to accurately evaluate the remaining life of the material.

On the other hand, it has been known that the mechanical properties of ferritic heat-resistant steel are closely related to precipitated carbides, and when used at high temperatures, the carbides gradually grow and become coarser over time. Among these, there is a method for evaluating remaining life that focuses on grain boundary carbides, which have a large effect on the deterioration state of heat-resistant steel that has been used for a long time.

The present invention aims to provide a remaining life evaluation method that focuses on these grain boundary carbides.

[Ninth method to solve all problems]

The present inventors have investigated ferritic heat-resistant steels that are used at high temperatures for long periods of time, and have found that when these heat-resistant steels are used at high temperatures for long periods of time, grain boundary precipitates become spheroidal with the use time. Please confirm that the degree of spheroidization depends on the operating temperature and operating stress.

Therefore, as a measure of the degree of spheroidization of grain boundary carbides in ferritic heat-resistant steel that has been used at high temperatures for long periods of time, the ratio of the maximum and minimum diameters of the carbides was measured as the spheroidization rate. We found that the new factor spheroidization rate has a good correlation with the creep rupture life consumption rate and the life consumption rate based on stress analysis, making it easy and accurate to evaluate the remaining life from the spheroidization rate of grain boundary carbides. He discovered that there is a silver lining and completed the present invention.

That is, based on the above knowledge, the present invention is based on the measurement of the spheroidization rate of friend carbides that precipitate at the grain boundaries of nine-ferritic heat-resistant steel that has been used for a long time under high temperature and high pressure, and the life consumption rate through a destructive test of the materials used in advance. The method is characterized in that 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 determined by stress analysis and the spheroidization rate of the carbide.

As described above, according to the present invention, the remaining life of ferritic heat-resistant steel that has been used for a long time can be evaluated non-destructively, easily and accurately 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, it was observed using a heat-resistant fAf electron microscope of a ferrite type such as 92% Or -I Mo@f, which was used for a long time as a high-temperature and pressure-resistant member of a commercial boiler.

FIG. 1 is a schematic diagram of the structure under an electron microscope, and shows the maximum value l! of the diameter of grain boundary carbide 2 on grain boundary 1 for various materials used! ,
and the minimum value lb'fI: Measure and spheroidize 1 = 5) □, eh.

Next, a creep rupture test is performed on the pressure-resistant member, and the life consumption rate is calculated from the ratio of the creep rupture time to that of the unused material.
A remaining life evaluation reference diagram showing the relationship between the spheroidization rate of grain boundary carbides and the creep rupture life consumption rate as shown in FIG. 2 was created.

Next, an extracted replica sample was taken from the surface of a superheater tube that had been used for a long time in a commercial boiler, and the spheroidization rate of grain boundary carbides was measured based on the method described above, and the creep rupture life consumption rate was calculated using the remaining life evaluation standard diagram. Estimated hole is 50% fill
was gotten.

On the other hand, 00 on the superheater tube and its unused material
When a creep rupture test was conducted at C1 stress 'Oky / m1I2, the rupture time was 1100 for the material used.
The life consumption rate of the used material is 51.4X when the virgin material is 2140 hours, which is in good agreement with the life consumption rate determined by the method of the present invention. It is possible to accurately determine the remaining life of the ferritic heat-resistant steel used.

〔Effect of the invention〕

As detailed above, according to the present invention, the remaining life of ferritic heat-resistant steel can be estimated non-destructively and quickly without using a destructive test, and a remaining life evaluation method with sufficiently high estimation accuracy can be provided. .

[Brief explanation of drawings]

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

Claims (1)

[Claims] In a method for evaluating the remaining life of ferritic heat-resistant steel used in a high-temperature environment, a first step of determining the spheroidization rate of carbides precipitated at grain boundaries of the heat-resistant steel as defined below. and spheroidization rate = minimum value of grain boundary carbide diameter / maximum value of grain boundary carbide diameter A method for evaluating the remaining life of a ferritic heat-resistant steel, comprising: a second step of determining the remaining life of the heat-resistant steel from a remaining life evaluation reference diagram showing the relationship between the following: