JP4737512B2 - Creep damage estimation method for ferritic heat resistant steel - Google Patents

Description

  The present invention relates to a method for nondestructively estimating creep damage of a metal material, and in particular, a chromium (Cr) content that is frequently used in a high-temperature pressure resistant part of about 550 ° C. to 650 ° C. in boilers, chemical plants, etc. is nominally 9% or more. The present invention relates to a creep damage estimation method suitable for damage estimation of chromium-molybdenum (Cr—Mo) ferritic high-strength heat-resistant steel.

  In power generation boilers and various heat exchangers, many heat transfer tubes and pipes are used as heat-resistant and pressure-resistant members under high temperature and high pressure conditions. In recent years, power generation boilers, in particular, have been operated with higher steam temperature and pressure than before in order to improve power generation efficiency, and the Cr content that has been conventionally used as a heat transfer tube material for boilers A new high strength ferritic heat resistant steel is used instead of 1-2.25% Cr—Mo based low alloy steel. This ferritic heat-resisting steel is designed to add a niobium (Nb), vanadium (V), and / or tungsten (W), etc. to a nominal 9% Cr steel and to have a tempered martensite structure by normalizing-tempering heat treatment. (For example, 9Cr-1Mo-Nb, V steel or 9Cr-0.5Mo- containing 9% Cr, 1% Mo, and other elements that exhibit characteristic performance in a small amount of Nb and V) 1.8W, Nb, V steel, 11Cr-0.4Mo-0.2W, Nb, V, Cu steel, etc.), and has significantly superior high temperature strength compared to conventional materials.

In maintenance management of such heat-resistant steel, evaluation of creep damage that progresses with long-term use is one of the important issues. In the case of conventional base metal of 1-2.25% Cr-based low alloy steel, “grain deformation method” and “metal structure method” have been developed as non-destructive damage estimation methods, and are already established as actual techniques. It is applied to a plant (Patent Documents 1 and 2). On the other hand, the above-mentioned new high-strength ferritic heat-resistant steel has a fine tempered martensite structure, and since the structure change with long-term use is small, these conventional methods are difficult to apply. Therefore, as a result of various studies, in recent years, it is said that a so-called “hardness method” that utilizes a decrease in hardness accompanying the progress of creep damage is effective (Non-Patent Document 1).
JP-A-6-200701 Japanese Patent Laid-Open No. 6-222053 Proceedings of the General Meeting of the Japan Society of Mechanical Engineers NO. 1034 (April 1998)

  The basic idea of the hardness method is that a relationship between the hardness and the creep damage rate (master curve) is obtained in advance with a test piece, and the creep damage is estimated from the hardness measured on the surface of the actual machine member. In the case of ferritic heat-resistant steel with a tempered martensite structure, the change in thermal hardness when stress is not applied is very small, and the decrease in hardness can be regarded as a change due to damage. Can be.

  However, there have been problems with estimation accuracy in previous studies, and the damage estimation method based on hardness has not yet been established. As an example, the relationship between hardness and creep damage rate prepared for 9Cr-1Mo-Nb, V steel is shown in FIG.

  FIG. 5 shows a relationship between the damage rate (Φc = interruption time / rupture time) and hardness by performing a creep rupture test and an interruption test at different temperatures and pressures. There is a clear correlation between the damage rate and hardness under each load condition, but there is no direct relationship, and when data for different temperatures or load stresses are overlapped, a wide band is obtained. There was a problem with accuracy in estimating the damage rate. For example, in the case of Vickers hardness 200 in FIG. 5, the damage rate was estimated to be 33 to 74%, and practical evaluation was difficult.

  This problem also applies to the evaluation of welds. In the case of a ferritic heat resistant steel having a tempered martensite structure such as 9Cr-1Mo-Nb, V steel, the welded heat affected zone (HAZ) between the weld metal 1 and the base metal 2 as shown in FIG. 3) A softening region is generated in 3, and creep damage in this softening region proceeds most rapidly. Therefore, although the creep damage can be directly obtained from the hardness of this portion, there is a problem in estimation accuracy because it is greatly affected by temperature or load stress as shown in FIG.

  The object of the present invention is to solve the above-mentioned problems of the prior art, and a tempered martensitic structure ferritic steel (especially chromium-molybdenum (Cr-Mo) ferritic high strength heat-resisting steel with a chromium (Cr) content of 9% or more)). Another object is to provide a method for accurately estimating creep damage from the hardness of the steel.

The above-described problems of the present invention are solved by the following solution means.
The invention according to claim 1 is a method for estimating the creep damage of ferritic steel that non-destructively estimates the creep damage of ferritic heat-resistant steel having a tempered martensite structure. ferrite Sato creep strain of estimating the creep strain of the heat-resisting steel from the relationship, further characterized by determining separately calculated creep strain and creep strain songs line or we creep damage rate showing the relationship between the creep damage rate This is a creep damage estimation method for heat resistant steels.

According to a second aspect of the invention, as the hardness of the measurement to heat steel surfaces, by measuring the hardness of the weld heat affected zone which is between the weld metal and the base material, previously prepared hardness and creep strain of relations The creep damage estimation method for a ferritic heat resistant steel according to claim 1, wherein the creep strain rate of the heat resistant steel is measured, and the creep damage rate is determined by comparison with a separately obtained creep strain curve .

According to a third aspect of the present invention, when the relationship between the hardness and the creep strain amount is obtained in advance, the creep strain amount estimated by multiplying the strain amount of the tensile test by a correction coefficient is used. This is a creep damage estimation method for ferritic heat resistant steels.

(Function)
According to the first aspect of the invention, the change in hardness is not intended to correspond directly to creep damage rate, since the hardness corresponds to the creep strain amount, the member from the relationship previously created hardness and creep strain amount Creep damage can be estimated with higher accuracy than the conventional method by estimating the creep strain amount and obtaining the creep damage rate from comparison with a separately obtained creep strain curve.

  According to the invention described in claim 2, in addition to the action of the invention described in claim 1, the minimum hardness of the weld heat affected zone where creep damage proceeds most rapidly is the hardness of the surface of the member. Creep damage can be estimated with higher accuracy than the method.

  According to the invention described in claim 3, in addition to the action of the invention described in claim 1 or 2, there is a proportional relationship between the strain amount of the tensile test and the creep strain amount, so the strain amount of the high temperature tensile test is measured. Thus, the amount of creep strain can be estimated.

According to the invention of claim 1, since the creep damage rate of the ferritic high strength heat-resisting steel having a tempered martensite structure of 9% Cr or more can be accurately estimated from the hardness of the member surface, Maintenance management when using these new high-strength steels can be performed appropriately, increasing the reliability of equipment operation in the actual plant and expanding the applications of these high-strength heat-resistant steels. Great effect.

  According to the second aspect of the invention, in addition to the effect of the first aspect of the invention, the creep damage progresses the earliest in the weld heat affected zone of the welded portion. A good creep damage rate can be obtained.

  According to the invention described in claim 3, in addition to the effect of the invention described in claim 1 or 2, since there is a proportional relationship between the strain amount of the high temperature tensile test and the creep strain amount, the strain amount of the high temperature tensile test. Can be obtained, and the relationship between the hardness and the creep strain amount can be obtained in a short time.

  Examples of the present invention will be described below, and details of the creep damage estimation method according to the present invention will be described.

  FIG. 1 is a diagram showing a basic estimation procedure of the present invention. First, the hardness on the surface of an actual machine member made of 9Cr-1Mo-Nb, V steel (hardness in the vicinity of the welded part of the actual machine member) is measured, and then the member is determined from the relationship between hardness and strain obtained in advance. (In order to obtain this relationship, it is desirable to conduct a creep test with a HAZ (heat affected zone) reproduction material created by simulating the thermal history of welding). Further, the damage rate is finally obtained by comparison with a separately obtained creep strain curve.

Next, the technical basis of the above estimation procedure will be described.
As a result of examining the hardness under various temperature or load stress conditions, the present inventors have found that there is a good correlation between the hardness and strain of the member surface that is not greatly influenced by the load condition. I found it. An example is shown in FIG. This relationship is well reproduced in both the base metal and the weld heat affected zone. A linear relationship is substantially established between the two until the strain amount is close to 3%. If the strain exceeds about 5%, the hardness hardly decreases even if the strain further advances. As is apparent from this figure, the creep strain can be determined from the hardness if the strain amount is about 3 to 5% or less. In the example of FIG. 2, the relationship between the two can be expressed by the following equation (1) when approximated as a quadratic polynomial within a strain amount of 5% or less.
Hardness = 216.7-12.75 × (strain amount) + 1.276 × (strain amount) ² (1)

FIG. 3 shows an example of a creep strain curve. When the amount of strain is 5%, it is almost the end of life, and in practice, it is only necessary to accurately estimate a strain of 5% or less. The creep strain curve can be calculated from material data such as steady state creep rate and temperature and stress conditions. In recent years, formulas have been proposed that can calculate creep strain curves up to the middle of the accelerated creep region (for example, the modified θ method of formula (2); high-temperature strength material science (Uchida Otsuru)), and strain under various conditions. The curve can be calculated relatively easily.
ε = ε ₀ + A {1−exp (−αt)} + B {exp (αt) −1} (2)

In order to obtain the material data necessary for the calculation when the softening region of the weld heat affected zone is targeted, a creep test and a tensile test may be performed using the HAZ reproducing material.
According to the present embodiment, the creep damage can be accurately estimated from the hardness by a relatively simple procedure.

  In the first embodiment, an example in which the relationship between hardness and creep strain is obtained in advance has been shown. However, for that purpose, a large number of creep tests must be performed, which requires a lot of time. On the other hand, the decrease in hardness corresponds to not only creep strain but also strain in a high temperature tensile test.

  FIG. 4 shows a comparison of the high-temperature tensile strain and creep strain of 9Cr-1Mo-Nb, V steel, but there is a comparative relationship between them, and high-temperature tensile strain data up to a strain of 5% is shown as 0. When it was multiplied by .5, it was found that it substantially corresponds to creep strain data. Therefore, the relationship between the hardness and the amount of creep strain can also be obtained by obtaining a large number of data in a short time high temperature tensile test and multiplying by an experimentally obtained correction factor (0.5 in the case of FIG. 4). In this case, the relationship between the two can be obtained in a short time without reducing accuracy.

  A creep damage estimation method suitable for damage estimation of chromium-molybdenum (Cr-Mo) ferritic high-strength heat-resistant steels with a chromium (Cr) content of 9% or more frequently used in high-temperature pressure-resistant parts can be established. It can be used for safety measures such as.

It is explanatory drawing of the creep damage estimation method by this invention. It is a figure showing the relationship between the hardness shown in the Example by this invention, and a creep distortion. It is a figure showing the relationship between the creep distortion shown in the Example by this invention, and a creep damage rate. It is a figure showing the relationship between the hardness shown in the other Example by this invention, and high temperature tensile strain. It is a figure showing the relationship between the hardness shown as a prior art, and a creep damage rate. It is a figure showing the example of hardness distribution near a welding part.

Explanation of symbols

1 Weld metal 2 Base material 3 Weld heat affected zone (HAZ)

Claims (3)

In the method of estimating the creep damage of ferritic steel, non-destructively estimating the creep damage of ferritic heat-resistant steel with a tempered martensite structure,
The hardness of the heat-resistant steel surface is measured, the creep strain amount of the heat-resistant steel is estimated from the relationship between the hardness and the creep strain amount created in advance, and the creep strain curve showing the relationship between the creep strain and the creep damage rate obtained separately. Of creep damage of ferritic heat-resistant steel, characterized by obtaining creep damage rate from As the hardness of the measurement to the heat-resisting steel surface, the welding heat by measuring the hardness of the affected zone, creep strain amount of the heat-resisting steel from the relationship previously created hardness and creep strain amount which is between the weld metal and the base metal The creep damage estimation method for ferritic heat resistant steel according to claim 1, wherein the creep damage rate is calculated from a comparison with a separately obtained creep strain curve .   The creep damage of a ferritic heat-resistant steel according to claim 1 or 2, wherein the creep strain amount estimated by multiplying the strain amount of the tensile test by a correction factor is used when the relationship between the hardness and the creep strain amount is obtained in advance. Estimation method.

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