Research and Evaluation on Safety Performance of Low Hardness P91 Steel Main Steam Pipeline

In order to study the safety performance of low-hardness P91 steam pipes and headers in operation, anatomical tests were conducted on low-hardness P91 steel pipes that had been in service for 10,000 hours to analyze the microstructure and normal temperature mechanical properties of low-hardness pipes, 540℃, the relationship between high temperature mechanical properties and endurance test at 566℃ and 600℃, and the accuracy of the on-site Leeb hardness test method was compared and studied. The judgment standard of the P91 steel pipeline in service was obtained and the P91 steel pipeline in service was given. Safety evaluation results and technical countermeasures for supervision.

With the rapid development of large-capacity and high-parameter ultra (super) critical units in my country’s thermal power plants, P91 material with a durable strength of 100 MPa at 593 ℃/100,000 h is widely used in main steam with a temperature higher than 566 ℃ High temperature components such as pipelines, reheat steam pipelines and their bypasses, high temperature headers. In view of the good heat resistance of P91, even 540℃ subcritical units are also widely used. In the early stage of the construction of ultra (super) critical units, P91 mainly relied on imports. Due to the huge import volume, although my country has experienced the process of using fake P91, it has completed the localization goal of P91 in just a few years.

According to the requirements of ASME SA335-SA335M, the hardness of P91 steel pipes does not exceed 250HB. However, in the inspection process of P91 materials, whether it is imported or domestic materials, its low hardness is a very common problem. The hardness value can be divided into two grades: one is between 160-180HB, the other is between 140-160HB, and individual parts are even lower, and there are also unqualified tissues. Therefore, the newly promulgated DL/T438-2009 “Metal Technology Supervision Regulations for Thermal Power Plants” in the electric power industry clearly stated that the hardness of P91 material should not be lower than 180HB. However, there are still a lot of low hardness phenomena in the P91 pipelines and headers that have been in operation, which brings serious safety hazards to the long-term stable operation of the unit. It is undoubtedly extremely important to evaluate the safety performance of low-hardness P91 steam pipes and headers.

Summary of the low hardness of P91 steel main steam pipe

In summary, the common hardness problems of P91 can be divided into the following categories:

  • (1) The hardness of the outer surface of the straight pipe is evenly distributed, but the overall hardness is low;
  • (2) The hardness distribution on the outer surface of the straight pipe is uneven, and the local hardness is low;
  • (3) The hardness distribution of the pipe elbow is uneven, the hardness of the back arc surface is low, and the hardness of the inner arc and the straight pipe section on the elbow is normal;
  • (4) The hardness distribution of the pipeline from the outer wall to the inner wall is uneven, the hardness of the outer wall is low in a certain range, but the inner wall is normal;
  • (5) The overall hardness of the pipeline from the outer wall to the inner wall is low.
  • For example, the elevation of the 2MS-9 module elbow of a power plant is 23.6 meters, and the specification is Di368.3×41mm. The inspection plan and marking method are as follows: the hardness test is performed in the ring direction along the flow direction of the medium. Each ring is marked with the English letter A, and the distance between each ring is about 0.5m. Take eight points on the circumference of each ring, and the angular interval between the two points is about 45°. Observing the facing medium, they are marked with ① from the back arc of the elbow, and marked as ②~⑧ by analogy counterclockwise. The specific inspection positions are shown in Figure 1. Shown.

The elbow has a back arc length of about 2 meters and an included angle of about 90° (that is, the included angle between ② and ⑧). The hardness value is low, between 130HB and 160HB, the lowest area (near C①) ) Reaches 138HB, the outer surface hardness distribution is obviously uneven, and the low hardness is mainly distributed on the back arc surface. If the pipe is expanded in the radial direction, the specific hardness distribution is shown in Figure 2, and the microstructure of the low hardness area is shown in Figure 3.
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Figure 1 Schematic diagram of 2MS-9 assembly elbow inspection

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Figure 2 Hardness distribution of 2MS-9 component elbow

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Figure 3 Metallographic structure of abnormal hardness position

The same situation can be seen in many power plants. The 90° elbow of the main steam pipe of a certain power plant is made of P91. During the maintenance after service, it was found that the hardness distribution of the elbow was uneven, and the local hardness of the back arc surface was low, ranging from 140HB to 160HB. The metallographic structure of the low hardness position Abnormal, ferrite appears, and the martensite phase has disappeared obviously, and the hardness of the straight sections at both ends of the elbow and the inner arc surface are normal. Another power plant produced A335 P91 pipes in the United States with specification ID368×42mm. In the inspection of the pipe end surface, it was found that the hardness distribution of the inner and outer walls of the pipe was uneven. The hardness value within 20mm from the outer surface was low, between 160HB~170HB, and other parts the hardness value is 180HB~200HB.

Test methods and characteristics

In the research and on-site inspection of pipelines and headers, there are three commonly used test methods: one is to use a Leeb hardness tester to test the components on site, the other is to use a hammer hardness tester to test the components on site, and the third is to the parts were dissected and tested in the laboratory with bench-top hardness.
Brinell hardness is the ratio of the pressure to the indentation area of the sample under a certain detection force. It has high measurement accuracy and good repeatability. The Leeb hardness is used to specify the impact body of a certain quality to impact the surface of the sample at a certain speed. The hardness value is calculated by the ratio of the rebound speed of the impact head at a distance of 1 mm from the surface of the sample to the impact speed. It is characterized by high accuracy and can guarantee ±0.8%. The principle of hammer hardness is the same as the Brinell hardness test method, but its error is large, so it is also called “approximate hardness test method”.
In the field of hardness testing, desktop Brinell hardness measurement is not feasible, and portable Leeb hardness is often used for measurement. The test standard is GB/T17394-1998 “Metal Leeb hardness test method”. However, GB/T17394 clearly stipulates that the test objects are low-carbon steel, low-alloy steel and cast steel. If the range of these materials is exceeded, a comparative test should be carried out to find out the corresponding relationship, so that the true hardness of the material can be accurately recorded. Article 8 of GB/T17394 also stipulates that “should avoid converting Leeb hardness into other hardness as far as possible”, and “should be accompanied by the corresponding hardness symbol before the Leeb hardness symbol. For example, 400HVHLD means the value measured by the D-type impact device. The Vickers hardness converted from the hardness value is 400″.
Therefore, for P91 steel, when the hardness test is carried out on site, a laboratory simulation comparison test should be carried out to obtain the conversion relationship.

Test research and result analysis

The sample material was taken from a P91 steel pipe in service in a power plant, with a specification of Di368.3×41mm. The field hardness test is 130-160HBHLD.

Chemical composition analysis

As shown in Table 1, the chemical composition of the on-site sampling tube meets the requirements of A335 P91.
Table 1 Chemical composition analysis of P91 steel cut on site (%)

Element C Si Mn P S Cr Mo
Standard requirement 0.080.12 0.200.50 0.300.60 ≤0.020 ≤0.010 8.009.50 0.851.05
Measured content 0.076 0.3 0.44 0.0089 0.0014 8.33 0.88
Element Ni V Al Nb N O H
Standard requirement ≤0.40 0.180.25 ≤0.04 0.060.1 0.0300.07
Measured content 0.17 0.21 0.0042 0.072 0.05 0.0017 0.00007

Low hardness comparison test

A large number of field tests have shown that there is a certain deviation between the hardness value measured by the Leeb hardness tester and the measured value of the benchtop hardness tester in the laboratory. To get the actual hardness value, it is necessary to conduct a comparative test. The conversion curve between HLD and HHBLD drawn according to the data conversion table in GB/T17394 is shown in Figure 4.

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Figure 4 Conversion curve between HLD and HHBLD in GB/T17394

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Figure 5 The comparison curve of the direct-measured HB value of P91 by the desktop hardness tester and the direct-measured HLD value of the Leeb hardness tester

The low-hardness P91 specimens cut on site were tested, and the comparison relationship is shown in Figure 5.
According to the data conversion table of GB/T 17394 “Metal Leeb Hardness Test Method”, 472HLD is equivalent to 180HBHLD, and 448HLD is equivalent to 160HBHLD (Figure 4). However, in the anatomical comparison test of the actual pipe sample of P91, it can be seen that the measured value of the Leeb hardness tester is 450HLD and the test specimen of the desktop hardness tester is equivalent to 180HB, and the actual value of the Leeb hardness tester is 425HLD and the test specimen of the desktop hardness tester is 160HB. Figure 5).
It can be seen that the Leeb hardness value measured by the portable Leeb hardness tester is about 20HB lower than the HB value measured by the desktop Brinell hardness tester by the conversion table or the HBHLD value directly read by the instrument.
That is, the Brinell hardness of P91 steel about 160HBHLD (450HLD) should be 180HB, and the Brinell hardness of P91 steel about 140HBHLD (425HLD) should be 160HB.
In order to obtain different hardness values of P91 steel, the P91 steel sample was normalized at 1050℃, and then at 710℃, 730℃, 750℃, 770℃, 790℃, 810℃, 820℃, 830℃ and 840℃. After holding the fire for 1 hour, the metallographic structure was observed and the Brinell hardness value was tested. The colleagues sent the samples to the China Institute of Metrology for comparative tests on Richter and Brinell. The comparison results are shown in Figures 6 and 7. The metallographic structure is shown in Figure 8. It can be seen from the figure that after tempering at 730℃, it becomes lath martensite; after tempering at 750℃, 770℃, and 790℃, the metallographic structure becomes tempered martensite, but the lath becomes wider at 790℃; In the structure after tempering at ℃ and 830℃, in addition to the tempered martensite, there are also a small amount of small ferrite blocks and a small amount of narrow lath quenched martensite. The quenched martensite is composed of austenite. High temperature (above the A1 phase transition point) is transformed when cooling. The reason for the decrease in hardness is due to the high tempering temperature of P91.

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Figure 6 Comparison of Leeb hardness and Brinell hardness after tempering at different temperatures
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Fig. 7 Fitting curve of Leeb hardness and Brinell hardness of P91 steel

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Figure 8 Metallographic structure of P91 steel at different tempering temperatures

Mechanical properties

According to the hardness value of P91 steel tested on site, the hardness of 160HB/140HBHLD (425HLD) and 180HB/160HBHLD (450HLD) were tested for mechanical properties. The results are shown in Tables 2, 3 and 4, 5.
Table 2 Mechanical properties of P91 steel with hardness value of 160HB/140HBHLD (425HLD)

Serial number Temperature t/℃ Tensile strength Rm/MPa Yield strength RP0.2/MPa Break A/% Shrink Z/% Impact absorption KV2/J Remarks
1 Room temperature (average value) 560 282.5 33.8 70.5 126.3
ASME SA335 ≥585 ≥415 ≥20(Vertical) —— GB5310
Sumitomo Data 685 521 25.8 72.9
V&M Data 742 591 —— ——
2 540 313 185 42.3 81.8 253.3
GB5310 269.2
Information data (223HB) 485 423 17.8 74.2 546℃
3 566 275 161.5 39 83.8 249.5
GB5310 240
Data 412 376 20 85 167
4 600 245 152 50.3 89.3 242.7
GB5310 198
Data 290

It can be seen from Table 2 that the room temperature tensile strength and yield strength of P91 steel with a hardness of 160HB/140HBHLD (420HLD) have been greatly reduced, and the yield strength has been reduced the most. The results are shown in Table 3.
Compared with the normal P91 steel, the yield strength at room temperature is 32% lower than the requirements of ASME SA335, and the tensile strength is lowered by 4%. Compared with the data of Sumitomo Japan, the yield strength has dropped by 46% and the tensile strength has dropped by 18%. Compared with V&M, the yield strength has dropped by 52% and the tensile strength has dropped by 25%.
Table 3 Comparison of mechanical properties of P91 steel with hardness value of 160HB/140HBHLD (425HLD) and related data

Serial number Temperature t/ Tensile strength Rm/MPa Yield strength RP0.2/MPa Remarks
1 Room temperature Comparison with ASME SA335 room temperature data ↓4% ↓32%
Comparison with Sumitomo room temperature data ↓18% ↓46%
Comparison with V&M room temperature data ↓25% ↓52%
2 540 Compared with data (223HB) ↓36% ↓56%
Compared with GB5310 ↓31%
3 566 Compare with data ↓33% ↓57%
Compared with GB5310 ↓33%
4 600 Compare with data ↓16% —-
Compared with GB5310 ↓23%

Compared with relevant data, the yield strength and tensile strength at 540℃ decreased by 56% and 36%, respectively. The yield strength is reduced by 31% compared with the lower limit of GB5310.
The yield strength and tensile strength at 566℃ are reduced by 57% and 33% respectively compared with the relevant data. The yield strength is reduced by 33% compared to the lower limit of GB5310.
The tensile strength at 600°C decreased by 16%. The yield strength is reduced by 23% compared with the lower limit of GB5310.
It can be seen that the mechanical properties of P91 steel with a hardness value of 160HB/140HBHLD (425HLD) have dropped significantly.
Also analyze the mechanical properties of P91 steel with a hardness of 180HB/160HBHLD (450HLD). As shown in Table 4 and 5.
The tensile strength and yield strength at room temperature, 540°C, 566°C and 600°C are higher than the lower limit of ASME SA335 or GB5310.
But compared with the data provided by the relevant information, it is lower, between 14-25%. Especially compared with the mechanical properties of P91 steel with a hardness of 223HB at 546°C, the tensile strength and yield strength are both reduced by 25%.
It can be seen that the mechanical properties of P91 steel with a hardness of 180HB can meet the requirements of the specification, but it is at the lower limit. This is also the reason why the lower limit of the hardness of P91 steel is set to 180HB.
Table 4 Mechanical properties of P91 steel with hardness value of 180HB/160HBHLD (450HLD)

Serial number Temperature t/ Tensile strength Rm/MPa Yield strength RP0.2/MPa Break A/% Shrink Z/% Impact absorption KV2/J Remarks
1 Room temperature 637.5 445 24 65.3 94
ASMESA335 ≥585 ≥415 ≥20(纵向) —— GB5310
Sumitomo Data 685 521 25.8 72.9
V&M data 742 591 —— ——
2 540 365 317.5 25 79 180.8
GB5310 269.2
546 data of a factory (223HB) 485 423 17.8 74.2
3 566 342.5 300 28 81.5 176.2
GB5310 240
Data 412 376 20 85 167
4 600 310 277.5 31.5 85.5 201
GB5310 198
Data 290

Table 5 Comparison of mechanical properties of P91 steel with hardness value of 180HB/160HBHLD (450HLD) and related data

Serial number Temperature t/ Tensile strength Rm/MPa Yield strength RP0.2/MPa Remarks
1 Room temperature Compared with ASME SA335 data ↑9% ↑7%
Comparison with Sumitomo room temperature data ↓7% ↓15%
Comparison with V&M room temperature data ↓14% ↓25%
2 540 Compared with data (223HB) ↓25% ↓25%
Compared with GB5310 —- ↑18%
3 566 Compare with data ↓17% ↓20%
Compared with GB5310 —- ↑25%
4 600 Compare with data ↑7% —-
Compared with GB5310 —- ↑40%

High temperature durability

P91 steel with hardness of 160HB/140HBHLD (420HLD) and 180HB/160HBHLD (450HLD) was tested for endurance strength at 540℃ and 566℃ respectively. It can be seen from the short breaking time that the breaking time of 160HB/140HBHLD (420HLD) has a huge gap compared with P91 steel with a hardness of 223HB.
The breaking time of the pipe of 160HB/140HBHLD (420HLD) at 540℃ at 200MPa is only 135h, while the breaking time of P91 steel with hardness of 223HB is 9226h. The initial stress is reduced to 140MPa, and its breaking time is also very low, 1881h. The breaking time at 566°C and 140MPa is only 125h. It can be seen that although the short-term endurance test has a certain degree of dispersibility, the huge data drop is enough to show that the P91 steel with low hardness has poor high-temperature endurance strength, which will seriously affect the safety of operation.
The breaking time of the 180HB/160HBHLD (450HLD) pipe sample at 566°C at 200MPa is only 1692h, and there is a certain gap between the breaking time of the P91 steel with a hardness of 223HB, which is 9226h.

According to the endurance test data, the relationship between the fracture time and the initial stress is shown in Figure 9.

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Fig. 9 Curve of extrapolation trend of endurance strength of low hardness P91 steel at 540℃ and 566℃

After preliminary calculations, the extrapolated durability of 100,000 hours is shown in Table 7. It can be seen that when the Brinell hardness value is 180HB, its 100,000-hour endurance strength is equivalent to that of GB 5310-2008. This shows that the hardness of P91 steel requires the lower limit of 180HB to be an absolute lower limit. The 100,000-hour extrapolated strength of 160HB hardness value is half less than the recommended value of GB 5310-2008. It can be seen that in the field hardness test, the minimum requirements cannot be relaxed.
Based on the above experimental results, for the P91 steel with low hardness in the field test, according to the comparison curve, the P91 steel with the Leeb hardness of 420HLD can be directly read by the Leeb hardness tester or converted according to the Brinell hardness of GB/T17394. The Brinell hardness value is 140HBHLD, the test result of its desktop Brinell hardness tester is 160HB. For P91 steel with a Leeb hardness of 450HLD, read directly with a Leeb hardness tester or convert Brinell hardness according to GB/T17394. The Brinell hardness value is 160HBHLD, and the test result of the desktop Brinell hardness tester is 180HB.
The mechanical properties of P91 steel with a Leeb hardness of 425HLD are far lower than the lowest value of ASME and the experimental data of Sumitomo and V&M. The high temperature endurance data is also very low. Therefore, the P91 steel with a hardness of 425HLD, whether used for 540℃ The unit is still used for the supercritical unit of 566℃ and above, and it is difficult to guarantee the safety requirements of the unit. The mechanical properties of P91 steel with a Leeb hardness of 450HLD, although the room temperature performance can meet the requirements of ASME, are already at the lower limit, which is still low compared with the experimental data of Sumitomo and V&M of Japan. Compared with the P91 steel of 223HB, the high temperature performance also appears to be greatly reduced. The 100,000-hour endurance strength is equivalent to the recommended value of the standard. Although it can be used, it is necessary to strengthen supervision.

Conclusion

  • After the comparative test of P91 steel, it is obtained that the Leeb hardness value is 450HLD (160HBHLD) measured by the portable Leeb hardness tester, the corresponding Brinell hardness is 180HB; 425HLD (140HBHLD), the corresponding Brinell hardness is 160HB.
  • The mechanical properties of P91 steel with a Leeb hardness value of 160HB/140HBHLD (425HLD) have been greatly reduced, and the endurance strength has been reduced by half compared to the standard recommended value, and its safety cannot meet the requirements of safe operation.
  • P91 steel with a Leeb hardness of 180HB/160HBHLD (450HLD) has mechanical properties at room temperature that can meet the requirements of ASME, but it is at the lower limit, and its high temperature performance is lower than that of 223HB P91 steel. The 100,000-hour durability is equivalent to the standard recommended value.
  • The hardness of P91 steel controlled by DL/T438-2009 “Metal Technology Supervision Regulations for Thermal Power Plants” is 180HB, which is the minimum requirement for materials. Any P91 lower than 180HB will bring hidden dangers to the safety of unit operation.
  • For low-hardness P91 steel that uses 540℃ high-temperature pipelines, the strength should be checked according to the test results, so as to shorten the inspection period and strengthen the material aging identification work.

Author: Cai River, Li Wei Li, Zhao Weidong, Wang Zhichun

Source: Network Arrangement – China P91 Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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research and evaluation on safety performance of low hardness p91 steel main steam pipeline - Research and Evaluation on Safety Performance of Low Hardness P91 Steel Main Steam Pipeline
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Research and Evaluation on Safety Performance of Low Hardness P91 Steel Main Steam Pipeline
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The judgment standard of the P91 steel pipeline in service was obtained and the P91 steel pipeline in service was given. Safety evaluation results and technical countermeasures for supervision.
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