A Zoning Prediction Method for the Life of 9-12% Cr Heat-Resistant Steel

Abstract

The present invention provides a partition prediction method for the life of 9-12% Cr heat-resistant steel. a and T b A set of endurance fracture tests under adjacent stress levels were conducted under each condition; T was determined based on the endurance fracture test results. a The ductile-brittle transition stress σ at temperature a and T b The ductile-brittle transition stress σ at temperature b ; Establish the T‑σ rectangular coordinate system of test temperature T and loading stress σ, through (T a ,σ a ) and (T b ,σ b ) draw a straight line intersecting the two coordinate axes T and σ, and determine the target point (T x ,σ y ) is located in the first quadrant of the T-σ rectangular coordinate system, and the fracture time of the target point is extrapolated using an appropriate fitting function. This invention is applied to the life prediction and safety assessment of 9-12% Cr heat-resistant steel components. It can simply and quickly achieve more accurate zoned life prediction and predict whether the fracture mode will be plastic or brittle.

Description

Technical Field

[0001] The invention relates to the field of heat-resistant steel life prediction, and in particular to a partition prediction method for the life of 9-12% Cr heat-resistant steel. Background Art

[0002] 9-12% Cr heat-resistant steel is widely used in the high-parameter steam piping of supercritical and ultra-supercritical power generation units. Life prediction for these components, especially those with degraded microstructure and properties, is crucial. Furthermore, whether the pipeline will fail ductilely or brittlely at the end of its life is a key consideration in pipeline safety assessments, yet this aspect has received insufficient attention from practitioners.

[0003] In GB / T 30580-2014 "Technical Guidelines for Life Assessment of Main Pressure-bearing Components of Power Plant Boilers" and DL / T940, DL / T654 and other life assessment standards, the isothermal extrapolation method and LM parameter method are recommended. The isothermal extrapolation method assumes that at any temperature, the sample loading stress σ and its fracture time t r There exists σ=k(t r ) m The LM parameter is a function of time and temperature, expressed as P(σ), which is related to the Kelvin temperature T and the fracture time t. r There exists P(σ)=T(C+lgt r ), where C is the material constant, and the LM parameter P(σ) can be expressed as a polynomial of σ. However, extensive experimental data indicates that the C values ​​obtained from fitting for 9-12% Cr heat-resistant steel differ significantly between low-stress and high-stress regions (where stress is relative to the tensile strength at that temperature). Therefore, directly extrapolating the lifespan using these two methods is likely to overestimate or underestimate it.

[0004] Since different test loading stress-test temperature combinations may lead to changes in the fracture behavior of the endurance specimen, causing a turning point in the fitting curve and a change in the C value; in order to obtain accurate prediction results, the stress-temperature combination must be partitioned, and how to partition becomes the key to life prediction. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the present invention provides a zoning prediction method for the service life of 9-12% Cr heat-resistant steel. The method is simple, accurate and easy to implement.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A zoning prediction method for the service life of 9-12% Cr heat-resistant steel comprises the following steps:

[0008] Step 1: At two higher temperatures T a and T b A set of endurance fracture tests under adjacent stress levels were carried out under each of the two higher temperatures T a and T b It means 700℃≥T a >T b , and T a -T b ≥25℃;

[0009] Step 2: Determine T based on the endurance fracture test results a The ductile-brittle transition stress σ at temperature a and T b The ductile-brittle transition stress σ at temperature b ;

[0010] Step 3: Establish the T-σ rectangular coordinate system of the test temperature T and the loading stress σ, and mark the point (T a ,σ a ) and (T b ,σ b ), and draw a straight line through these two points that intersects the test temperature T and the loading stress σ. This straight line divides the first quadrant of the T-σ rectangular coordinate system into two areas: the inner area and the outer area. The inner area is the brittle fracture area and the low stress area, and the outer area is the plastic fracture area and the high stress area.

[0011] Step 4: Determine the target point (T x ,σ y ) is located in which area of ​​the first quadrant of the T-σ rectangular coordinate system, where T x To predict temperature, σ y To load stress, the fracture time of the target point is extrapolated by using a suitable fitting function using points in the region with known endurance fracture data.

[0012] To optimize the above technical solutions, specific measures taken also include:

[0013] In step 1, the sustained fracture test under adjacent stress levels means that the interval of decreasing loading stress in the test is not greater than 20 MPa, that is, σ n -σ n+1 ≤20,σ n is the loading stress during the nth test, σ n is the loading stress during the n+1th test.

[0014] Furthermore, when the area reduction rate decreases significantly, the short decreasing interval of the loading stress is shortened.

[0015] In step 2, during the sustained fracture test, as the loading stress decreases, when the cross-sectional shrinkage rate Z drops below 50% for the first time, the corresponding loading stress σ is the plastic-brittle transition stress, and there is no need to continue the sustained fracture test with a lower stress.

[0016] In step 3, by (T a ,σ a ) and (T b ,σ b )The equation of the line between the two points is T m , σ m are the temperature and ductile-brittle transition stress of a point on the straight line respectively.

[0017] In step 4, the predicted temperature T x Under the loading stress σ y If the break time Then (T x ,σ y ) point falls in the inner area, it is necessary to use the persistent fracture data of the inner area points to predict the life by using a suitable fitting function; if Then (T x ,σ y ) point falls in the outer area, and the life prediction needs to be carried out through the permanent fracture data of the outer area points through a suitable fitting function.

[0018] In step 4, the appropriate fitting function is to adopt the heat-resistant steel life prediction method in the prior art.

[0019] Furthermore, in step 4, the appropriate fitting function adopts the isotherm extrapolation method or the LM parameter method.

[0020] The beneficial effects of the present invention are:

[0021] The present invention provides a zoning prediction method for the service life of 9-12% Cr heat-resistant steel. The method can obtain the turning points of different temperature-stress fitting extrapolation function curves through short-term endurance tests. The model after the zoning method is open, and can use existing models (such as isothermal line extrapolation method and Lamy parameter method) or self-fitting.

[0022] The partition prediction method of the present invention can not only predict whether the fracture under a certain temperature-stress combination is plastic or brittle, but also avoid the problem that the life span is often overestimated or underestimated when using single-zone prediction.

[0023] The partitioning method of the present invention is simple, fast, accurate and reliable, and greatly improves the accuracy of life prediction. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the partition diagram in Example 1 of the present invention.

[0025] Figure 2 This is the plastic-brittle transition stress verification diagram of the present invention.

[0026] Figure 3 This is an example diagram of partition prediction of the present invention. DETAILED DESCRIPTION

[0027] The present invention will now be described in further detail with reference to the embodiments and accompanying drawings.

[0028] A zoning prediction method for the service life of 9-12% Cr heat-resistant steel comprises the following steps:

[0029] Step 1: At two higher temperatures T a and T b A set of endurance fracture tests under adjacent stress levels were carried out under each of the two higher temperatures T a and T b It means 700℃≥T a >T b , and T a -T b ≥25℃;

[0030] Step 2: Determine T based on the endurance fracture test results a The ductile-brittle transition stress σ at temperature a and T b The ductile-brittle transition stress σ at temperature b ;

[0031] Step 3: Establish the T-σ rectangular coordinate system of test temperature T and loading stress σ, and mark the point (T a ,σ a ) and (T b ,σ b ), and draw a straight line through these two points that intersects the temperature T and the plastic-brittle transition stress σ. This straight line divides the first quadrant of the T-σ rectangular coordinate system into two areas: the inner area and the outer area. The inner area is the brittle fracture area and the low stress area, and the outer area is the plastic fracture area and the high stress area.

[0032] Step 4: Determine the target point (T x ,σ y ) is located in which area of ​​the first quadrant of the T-σ rectangular coordinate system, where T x To predict temperature, σy To load stress, the fracture time of the target point is extrapolated by using a suitable fitting function using points in the region with known endurance fracture data.

[0033] To optimize the above technical solutions, specific measures taken also include:

[0034] In step 1, the sustained fracture test under adjacent stress levels means that the interval of decreasing loading stress in the test is not greater than 20 MPa, that is, σ n -σ n+1 ≤20,σ n is the loading stress during the nth test, σ n is the loading stress during the n+1th test.

[0035] Furthermore, when the area reduction rate decreases significantly, the short decreasing interval of the loading stress is shortened.

[0036] In step 2, during the sustained fracture test, as the loading stress decreases, when the cross-sectional shrinkage rate Z drops below 50% for the first time, the corresponding loading stress σ is the plastic-brittle transition stress, and there is no need to continue the sustained fracture test with a lower stress.

[0037] In step 3, by (T a ,σ a ) and (T b ,σ b )The equation of the line between the two points is T m , σ m are the temperature and ductile-brittle transition stress of a point on the straight line respectively.

[0038] In step 4, the temperature T is predicted x Under the loading stress σ y If the break time Then (T x ,σ y ) point falls in the inner area, it is necessary to use the persistent fracture data of the inner area points to predict the life by using a suitable fitting function; if Then (T x ,σ y ) point falls in the outer area, and the life prediction needs to be carried out through the persistent fracture data of the outer area points through a suitable fitting function.

[0039] In step 4, the appropriate fitting function is to adopt the heat-resistant steel life prediction method in the prior art.

[0040] Furthermore, in step 4, the appropriate fitting function adopts the isotherm extrapolation method or the LM parameter method.

[0041] Example 1:

[0042] A partition prediction method for improving the accuracy of life prediction of 9-12% Cr heat-resistant steel comprises the following steps:

[0043] Step 1: Get data

[0044] The test data in this example are all from the SA-335Grade T92 endurance fracture data in the NIMS CREEP DATA SHEET of the Japan Institute for Materials Research. First, obtain a set of endurance fracture data at 700℃ and 675℃, each of which is adjacent to the stress. The data includes the test temperature T, the loading stress σ, and the fracture time t. r And section shrinkage Z, see Table 1:

[0045] Table 1: NIMS data cited in Example 1

[0046]

[0047] Step 2: From the data in Table 1, it can be seen that the plastic-brittle transition stress at 700°C is 60 MPa, and the plastic-brittle transition stress at 675°C is 80 MPa.

[0048] Step 3: Mark the points (60, 700) and (80, 675) in the first quadrant of the T-σ rectangular coordinate system, and draw a straight line through these two points that intersects the two coordinate axes. This straight line divides the first quadrant into two areas, inner and outer. Figure 1 .

[0049] Figure 1 In the figure, the set of points in the inner area (including the straight line) conforms to the functional relationship of σ≤-0.8T+620, which is a low stress area; the set of points in the outer area conforms to the functional relationship of σ>-0.8T+620, which is a high stress area.

[0050] The plastic-brittle transition stress at other temperatures can be obtained by the equation σ = -0.8T + 620. For example, the plastic-brittle transition stress at 650 ° C is 100 MPa, and the plastic-brittle transition stress at 600 ° C is 140 MPa, which is consistent with the measured values. Figure 2 .

[0051] Step 4: If Figure 3 As shown, under the premise of knowing the sustained fracture time of points 1 to 7 (NIMS database), the fracture time (life) at 650°C and 70 MPa is predicted. Point (650, 70) is point numbered 8 in the low stress area.

[0052] Taking the isothermal extrapolation method recommended by GB / T 30580-2014 "Technical Guidelines for Life Assessment of Main Pressure-Bearing Components of Power Plant Boilers" as an example, the persistent fracture data of points 5, 6, and 7 in the low stress area should be used to predict point 8.

[0053] The extrapolation formula of isotherm extrapolation is σ=k(t r ) m , in σ and t r In the double logarithmic coordinates, the extrapolated curve is a straight line, so theoretically, two points in the low stress area can determine the equation of this straight line. The fracture time calculation formula fitted by the fracture data of points 5, 6, and 7 is:

[0054] t r =1×10 17 σ -6.669

[0055] The predicted fracture time at 70 MPa is 49,550 hours, which deviates less than 10% from the actual fracture time measured in NIMS data, as shown in Table 2.

[0056] In contrast, if the endurance data of points 1 to 7 are directly used to predict the fracture time of the 8th point without zoning, the prediction result is overestimated by 116%; in contrast, if the low-stress long-term endurance test is not carried out, and only the endurance data of points 1 to 4 in the high-stress area are used to predict the fracture time of the 8th point in the low-stress area, the prediction result is overestimated by 1393%.

[0057] Table 2: Comparison of the accuracy of the method in Example 1 and the traditional single-region prediction method

[0058]

[0059] Therefore, the partition prediction method of the present invention can not only predict whether the fracture under a certain temperature-stress combination is plastic or brittle, but also avoid the problem that the life span is often overestimated or underestimated when using single-zone prediction.

[0060] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions based on the principles of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A zoning prediction method for the service life of 9-12%Cr heat-resistant steel, characterized by: The following steps are involved: Step 1: At two higher temperatures T a and T b A set of endurance fracture tests under adjacent stress levels were carried out under each of the two higher temperatures T a and T b It means 700℃≥T a >T b , and T a -T b ≥25℃; Step 2: Determine T based on the endurance fracture test results a The ductile-brittle transition stress σ at temperature a and T b The ductile-brittle transition stress σ at temperature b ; Step 3: Establish the T-σ rectangular coordinate system of test temperature T and loading stress σ, and mark the point (T a ,σ a ) and (T b ,σ b ), and draw a straight line through these two points that intersects the test temperature T and the loading stress σ. This straight line divides the first quadrant of the T-σ rectangular coordinate system into two areas: the inner area and the outer area. The inner area is the brittle fracture area and the low stress area, and the outer area is the plastic fracture area and the high stress area. Step 4: Determine the target point for life prediction (T x ,σ y ) is located in the first quadrant of the T-σ rectangular coordinate system, where T x To predict temperature, σ y To load stress, the fracture time of the target point is extrapolated by a suitable fitting function using the points of known persistent fracture data in the area; In step 3, by (T a ,σ a ) and (T b ,σ b ) The equation of the line between the two points is , T m , σ m are the temperature and ductile-brittle transition stress of a point on the straight line respectively; In step 4, the predicted temperature T x Under the loading stress σ y If the break time , then (T x ,σ y ) point falls in the inner area, it is necessary to use the persistent fracture data of the inner area points to predict the life by using a suitable fitting function; if , then (T x ,σ y ) point falls in the outer area, and the life prediction needs to be carried out through the persistent fracture data of the outer area points through a suitable fitting function.

2. The method for predicting the service life of 9-12%Cr heat-resistant steel according to claim 1, characterized in that: In step 1, the sustained fracture test under adjacent stress levels means that the interval of decreasing loading stress in the test is not greater than 20 MPa, that is, σ n -σ n+1 ≤20,σ n is the loading stress during the nth test, σ n is the loading stress during the n+1th test.

3. The method for predicting the service life of 9-12%Cr heat-resistant steel by different zones according to claim 2, characterized in that: When the cross-sectional shrinkage rate decreases significantly, the decreasing interval of loading stress should be shortened.

4. The method for predicting the service life of 9-12%Cr heat-resistant steel by different zones according to claim 1, characterized in that: In step 2, during the sustained fracture test, as the loading stress decreases, when the cross-sectional shrinkage rate Z drops below 50% for the first time, the corresponding loading stress σ is the plastic-brittle transition stress. At this time, there is no need to continue the sustained fracture test with a lower stress.

5. The method for predicting the service life of 9-12%Cr heat-resistant steel by different zones according to claim 1, characterized in that: In step 4, the appropriate fitting function adopts the isotherm extrapolation method or the LM parameter method.

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