Analysis of Reasons for Low Hardness Test Value of P91 Steel Pipe

In response to the low hardness of the P91 steel pipe found during the overhaul of a thermal power plant’s supercritical unit, tests such as chemical composition analysis, mechanical properties and hardness testing, and metallographic testing were carried out, combined with many cases of low hardness of P91 pipes in the field test. , It is concluded that the phenomenon of low hardness test value of P91 steel pipe caused by surface decarburization is widespread. In view of this phenomenon, a suggestion to comprehensively inspect the P91 steel pipe with low hardness in the test is proposed to avoid misjudgment.

As the boiler steel pipe used in thermal power plant, P91 steel pipe has good high temperature durability, creep resistance and impact toughness, excellent welding performance and process performance, strong oxidation resistance and high temperature steam corrosion resistance, low coefficient of thermal expansion, good thermal conductivity and structural stability, and excellent comprehensive performance [1]. If the hardness of P91 steel pipe is low, it will lead to insufficient durable strength, which will directly affect the service life of the pipe and the safety of unit operation. In the process of on-site inspection, it is found that there is decarburization layer on the outer surface of P91 steel pipe, which leads to the low hardness test value. If the correct comprehensive inspection is not carried out and the waste is judged directly, it is easy to lead to misjudgment and great economic loss. Combined with the test results of P91 steel pipe with low hardness in the past two years, this paper analyzes the phenomenon of low surface hardness of P91 steel pipe found in the maintenance process of supercritical unit in a power plant, so as to prevent the occurrence of misjudgment and unnecessary economic loss.

On-site inspection

During the overhaul of a supercritical unit, during the hardness test of the main steam pipeline, it was found that the hardness of a straight P91 pipe with an inner diameter of 273 mm and a wall thickness of 30 mm was lower than “DL/T 438—2009 Metal Technical Supervision for Thermal Power Plants” Regulations”[2] require values. The inspection was carried out in accordance with “GB/T17394-1998 Metal Leeb Hardness Test Method” [3]. Three different types of Leeb hardness testers were used to test the three sections of the steel pipe in different directions. The test values are shown in Table 1. The data in Table 1 shows that the data obtained by different hardness testers on different cross-sections and different directions of the steel pipe have uniform values and small dispersion. According to DL/T 438-2009, the Brinell hardness value of P91 steel pipe should range from 180 to 250 [2], so it is preliminarily determined that the hardness value of this steel pipe is lower than the standard value.

Table 1 Brinell hardness values (average value) of 3 sections of P91 main steam straight pipe in different directions

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Laboratory testing and analysis

Chemical composition analysis

The chemical composition analysis of the above-mentioned pipes with low hardness values is carried out. The analysis data is shown in Table 2. It can be seen from the data in the table that the mass fractions of the chemical elements in the chemical composition of the steel pipe meet the standard requirements of “ASTM 335/335M—2010 Standard Specification for Seamless Ferritic Alloy-Steel Pipe For High-Temperature Service” [4], and there is no abnormality.

Table 2 Test data of various chemical element mass fractions of steel pipe

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Mechanical performance testing

Three tensile and impact specimens were taken from the pipe cut pipe longitudinally and transversely, and subjected to room temperature mechanical tests (the results are shown in Table 3). The room temperature mechanical properties of the pipe meet the standard [4], and the impact toughness is good.
Table 3 Room temperature mechanical properties (average value)

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Hardness testing

The steel pipe was sampled along the cross section, polished and corroded, and tested radially from the outer surface inward using the Wilson TUKON2500 automatic Vickers hardness tester. The test data is shown in Figure 1.
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Figure 1 Vickers hardness distribution diagram of sample cross section

Figure 1 shows that the Vickers hardness of the outer surface of the sample is about 150. With the increase of the depth from the surface to the inside, the hardness value increases. When the depth is greater than 1.72 mm, the hardness value tends to be 255. According to the requirements of ASTM A335/335M, the Vickers hardness value of P91 steel pipe should range from 196 to 265 [4]. Combined with the field inspection data, it can be determined that the hardness of the steel pipe matrix meets the standard requirements, but the outer surface hardness is low.

Metallographic inspection

Test results

The metallographic analysis of the pipe cross-section and the outer surface is shown in Figure 2. The structure near the outer surface is ferrite plus tempered martensite, which is a decarburization phenomenon on the outer surface.

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Figure 2 Pipe cross section and metallographic structure of the outer surface

Cause Analysis

(1) In the process of steel heating or heat preservation, due to the effect of the surrounding oxidizing atmosphere, the surface of the steel will be decarburized, causing the surface carbon mass fraction to decrease. The chemical reaction formula is as follows:

  • 2Fe3C+O2⇌6Fe+2CO
  • Fe3C+2H2⇌3Fe+CH4
  • Fe3C+H2O⇌3Fe+CO+H2
  • Fe3C+CO2⇌3Fe+2CO

(2) The factors that affect steel decarburization include the chemical composition of the steel material, heating temperature, holding time and gas composition. Decarburization of the steel surface will greatly reduce the surface hardness of the material [5, 6].
(3) During the production and processing of steel pipes, the decarburized layer will be used as a surface defect to remove the machining allowance. However, if the design of the machining allowance is insufficient, the surface decarburization layer may remain and affect the surface mechanical properties. Combined with the Vickers hardness test data, it is determined that the depth of the decarburized layer of the steel pipe is about 1.7 mm.
In summary, the hardness of the P91 steel pipe matrix meets the standard, but due to the presence of a decarburized layer on the surface, the test result is lower than the standard value.

Case of low hardness of P91 steel pipe

During the nearly 2 years of inspection, there are 19 P91 steel pipes with hardness values lower than the standard value, as shown in Table 4. Metallographic inspection was done on each steel pipe, and it was found that there was a ferrite structure. Re-grind each steel pipe. According to different steel pipe wall thickness, the thickness is 1.5~2.2mm, and the thickness is polished in several times, each time is about 0.5 mm; each time it is polished, a hardness test is performed. The result is that the hardness value of the pipeline increases with the increase of the polishing depth. After polishing to a certain depth, the hardness value meets the standard requirements, and the ferrite structure disappears.
Table 4 Test results of P91 steel pipe with hardness value lower than standard value

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From Table 4, it can be inferred that the low hardness of P91 steel pipe due to surface decarburization is more common. If the surface decarburization layer is polished, the hardness value will generally meet the standard requirements.

Conclusion and suggestion

It is common for P91 steel pipes to have a decarburized layer on the surface to reduce the hardness. If the hardness of the steel pipe is found to be lower than the standard value during the on-site inspection, you should not rush to conclusions. Metallographic inspection should be performed first to determine whether there is decarburization on the surface. Ferrite to prevent misjudgment and cause unnecessary economic losses. If there is no decarburization phenomenon in the steel pipe during testing, it can be determined that the test result is the true test value of the pipe matrix. If there is ferrite, it should be ground deep without affecting the service life of the steel pipe: if the hardness value increases with the increase in depth and the ferrite area is reduced under the microscope, the steel pipe has a surface decarburization layer. After removing the decarburized layer, the hardness may meet the standard requirements; if the hardness value of the steel pipe and the ferrite area change little with the increase of depth, the steel pipe may have low hardness due to improper heat treatment.

Author: Dong Peng

Source: China P91 Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (

(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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  • [1] Yan Zesheng, Liu Yongchang, Ning Baoqun. Phase transformation process and strengthening of high Cr ferrite heat-resistant steel [M]. Beijing: Science Press, 2009: 11-18.
  • [2] Technical Committee for Standardization of Metallic Materials for Power Plants in the Electric Power Industry. DL/T 438-2009 Supervision Regulations for Metal Technology in Thermal Power Plants [S]. Beijing: China Electric Power Press, 2010.
  • [3] National Steel Standardization Technical Committee. GB/T 17394—1998 Metal Leeb Hardness Test Method [S]. Beijing: China Standards Press, 2000.
  • [4] American Society for Testing and Materials.ASTM 335/335M—2010 Standard Specification for Seamless Ferritic Alloy-Steel Pipe For High-Temperature Service[EB/OL].(2010)[2015-04-20].
  • [5] Wang Guangsheng. Case analysis of metal heat treatment defects[M]. Beijing: China Machinery Industry Press, 2007: 100-102.
  • [6] Shanghai Institute of Machinery Manufacturing Technology. Metallographic Analysis Technology [M]. Shanghai: Shanghai Science and Technology Literature Publishing House, 1987: 325-326.

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