Cause Analysis of Deviation in Brinell Hardness Conversion Value of Low Hardness P91 Steel Pipe Fittings

Taking P91 steel pipe fittings and low-alloy steel pipe fittings with different hardnesses as the research objects, after eliminating the errors caused by external factors (sample temperature, rigidity, quality and surface condition, etc.), the hardness comparison test found that compared with the real Brinell The hardness is good, the Brinell hardness conversion value (HBLD) of low hardness P9 copper pipe fittings still has a large negative deviation. According to the mechanical performance test results, combined with the Leeb hardness detection principle and strengthening theory, the internal reasons for the deviation of the HBHLD value are analyzed. The results show that the decrease in the hardness of P91 steel pipe fittings is accompanied by the weakening of the solid solution strengthening and dislocation strengthening effects, resulting in yield strength The decrease of the ratio is the main reason for the negative deviation of the HHLLD value; the slight change of the elastic modulus has little effect on it.

The Leeb hardness test is a kind of rebound hardness test method. The portable Leeb hardness tester solves the test problems of large workpieces that are difficult to move and large parts and components that are difficult to disassemble, which are difficult to be handled by other methods. Non-destructive and other advantages, it has been widely used in hardness testing of alloy steel pipelines in power stations.
Because P91 steel has good oxidation resistance and structural stability at high temperatures, it has been widely used in boiler high temperature headers, main steam pipes and reheat steam hot section pipes of subcritical and supercritical units. However, in recent years, there have been frequent problems of low hardness and abnormal organization of P91 steel pipe fittings, which seriously affects its service life and safety. Existing domestic and foreign standards stipulate the range of press-in hardness value of P91 steel pipe fittings. For example, Appendix C of DL/T438-2009 “Metal Technical Supervision Regulations for Thermal Power Plants” recommends the hardness value of P91 steel pipe fittings to be 180~250HB. In order to facilitate the quality acceptance, a portable Leeb hardness tester is usually used for hardness testing, and then the conversion function of the instrument is used to convert it into a press-in hardness value. For example, the Leeb hardness value of the D-type probe is expressed in HLD, and the Brinell hardness value converted from it is expressed in HHLLD.
The test error of the Leeb hardness tester is related to the surface roughness, dirt, radius of curvature, surface temperature, vibration and other factors of the test part [l]. After eliminating these influencing factors, the HHLLD of low hardness P91 steel pipe fittings is found through comparative tests. The value is still generally lower than the HBW value measured by a thousand-bench Brinell hardness tester. Through the hardness comparison test and mechanical performance test of different types of pipe fittings, based on the principle of Leeb hardness test, the author analyzes the internal reasons for the deviation of P91 steel low hardness pipe fittings HHLLD from a new perspective, so as to determine the Leeb hardness value. A correct understanding, find out the way to correct.

Test methods and results

Sample

Low hardness P91 steel pipe fittings and normal hardness P91, P22, 12CrlMoV steel pipe fittings from different power plants were used as test objects. The pipe fitting parameters and hardness ranges are shown in Table 1.
Tab.1 Parameters of specimens

Sample name Codename Material Specification/mm Running time/X10h) Hardness/HBHLD
Superheater three-stage desuperheater straight section JW P91 Ø559 X 88 3.5 130~180 
90° elbow of reheat steam hot section pipe RW P91 Ø732 X 23 5. 7 140~155 
Main steam pipe 135° elbow zw P91 Ø33 X 30 5. 7 130~140 
Main steam pipe reducer DXT P91 Ø474/333 X 53/30 6.2 155~195 
Straight section of reheat steam hot section pipeline RZA P91 Ø732 X 23 5. 7 220~235 
Straight section of reheat steam hot section pipeline RZB P22 Ø6l0 X 25 3.4 110~120 
New pipeline RZC  P22 Ø6lO X 25 0 155~160 
Straight section of main steam pipeline zz 12CrlMoVG Ø273 X 40 6.1 150~155 

Hardness comparison test

Cut strip samples with a mass of more than 5kg in different hardness areas on different pipes with a thickness of more than 20mm. According to 7.9 of GB/T17394-1998 “Metal Leeb Hardness Test Method”, the Brinell hardness and Leeb hardness test are carried out on three areas on the same sample, as shown in Figure 1.
After the test, the average value of 15 HBHLD values and the average value of 3 Brinell hardnesses are regarded as a set of corresponding values. Repeat the above test, and the test results are summarized in Table 2.
Tab.2 The results of hardness test

Pipe fitting code Hardness Pipe fitting code Hardness
/HBHLD /HBW /HBHLD /HBW
JW 132 144 DXT 156 168
135 145 159 171
136 146 164 175
137 148 175 182
143 153 193 198
151 162 RW 141 163
154 164 146 165
159 168 152 168
170 174 RZA 225 212
172 179 230 214
178 184 235 218
ZW 132 143 RZC 156 169
139 153 158 172
RZB 110 128 zz 151 152
114 128
119 131 155 154

Mechanical performance test

In order to analyze the internal reasons for the deviation between the values of HBHLD and HBW, a strip specimen with uniform hardness was selected for mechanical performance test to test the elastic limit, elastic modulus and tensile strength of the specimen. The tensile specimen is ¢, a 10 mm standard 5 times circular cross-section specimen. The elastic limit is expressed by the stress when 0.01% plastic strain is generated, which represents the strength when a slight yield is generated. The test results are shown in Table 3. The table also lists the deviations of the HHLLD value and the yield ratio data calculated based on the test results.
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Fig.1 Schematic diagram of hardness test positions
Tab.3 The results of mechanical property test

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Analysis and discussion

Principle of Leeb hardness test

The Leeb hardness test is a dynamic load test method, as shown in Figure 2. The principle is to eject a carbonized pigeon ball of a certain mass and diameter through a spring loaded and hit the surface of the workpiece, and measure the ball at a distance of 1 mm from the surface of the sample by an electric sensor. The ratio of rebound speed to impact speed is used to characterize the hardness. The hardness value of the rebound method mainly represents the size of the metal elastic deformation work [2]. Part of the impact energy (G0) of the carbonized pigeon ball is converted into plastic deformation work (G2) and absorbed by the specimen, and the other part is converted into elastic deformation work (Gl) and stored in the specimen. When the deformation is restored, the energy is released to make the carbonized inscription ball rebound [3]. The Leeb hardness value is proportional to the rebound speed, so the Leeb hardness value depends on the elastic deformation work.
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Fig.2 Leeb hardness test principle
Elastic deformation work, also known as elastic specific work (Gl), is represented by the area of the elastic deformation stage on the stress-strain curve (Figure 3), and its magnitude is determined by the elastic modulus and elastic limit.

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Fig.3 Elastic strain energy

Deviation from HLD value to HB value conversion

The HHBLD value in Table 2 is the Brinell hardness value after the HLD value is automatically converted by the hardness tester. The measurement error caused by the surface condition of the sample and the environment has been basically eliminated during the test, but the HHBLD value and the test value of the desktop Brinell hardness tester are still There is a big deviation. As shown in Table 3, the maximum deviation of RW pipe fittings HHLLD value reaches -13%, and the deviation of other low hardness P91 pipe fittings also exceeds -6%.
In summary, the deviation of HHBLD value is caused by the inaccurate conversion relationship of the hardness tester. The imported Leeb hardness tester uses the Swiss version of the conversion relationship. The instrument has only two conversion curves for steel materials. The inspectors usually use the conversion curve of “steel and cast steel”, so the hardness of the low hardness P91 steel is a big deviation occurred during conversion.

The influence of elastic modulus on the deviation of HHBLD value

The Leeb hardness value depends on the elastic modulus and elastic limit. The elastic limit represents the resistance of the material to small amounts of plastic deformation [2], which is a strength index. Therefore, the Leeb hardness value can only be tested with materials with the same elastic modulus. Compare the sizes; in the same way, when converting it to the Brinell hardness value, materials with different elastic modulus have different conversion relationships. The data shown in Table 4 comes from GB/T17394-1998, which lists the corresponding relationship between the HLD value and HB value of several different elastic modulus metal materials. It can be seen that the larger the elastic modulus, the higher the HB value converted by the same HLD value. Big.
Tab.4 Effect of elastic modulus on hardness transformation

Leeb hardness/HLD Material Brinell hardness conversion value/HBHLD Elastic modulus/GPa
450 Carbon steel, low alloy steel and cast steel 162 210
Cast aluminum alloy 110 65~85 
Copper zinc alloy 126 85-130 
Copper aluminum alloy and copper tin alloy 133 94-130 
Pure copper and low copper alloy 145 110~ 135 

For steel materials, alloying, heat treatment (microstructure), and cold plastic deformation have little effect on the elastic modulus. The elastic modulus of P91 steel pipe fittings shown in Table 4 is slightly higher than that of P22,12Cr1MoVG pipe fittings. This difference It is not enough to cause such a large deviation in the HHLLD value; in addition, the elastic modulus of RW pipe fittings is close to that of ZZ pipe fittings, but the deviation of HHLLD is large (the HHLLD of ZZ pipe fittings has no obvious deviation). In summary, the modulus of elasticity of P91 steel pipe fittings with high alloy content is slightly higher than that of low-alloy steel is not the main reason for the deviation of HHBLD value.

The influence of the yield ratio on the deviation of the HHLLD value

Under the premise that the modulus of elasticity is constant, the rebound hardness value represents the resistance of the material to a small amount of plastic deformation, while the indentation hardness value represents the material’s resistance to large amounts of plastic deformation and strain hardening [2]. They are in true stress-true The corresponding position on the strain curve is shown in Figure 4. Therefore, when converting Leeb hardness to Brinell hardness, it is also necessary to consider the gap between Rp0.01 and Rm. The size of this gap can be expressed by the ratio of the two, that is, the yield ratio. It is not difficult to understand that when the yield ratio is lower than a certain range (the range corresponding to the hardness tester conversion relationship), the gap between Rp0.01 and Rm is too large, and the actual HBW value will be larger than the converted HHLLD value, that is The HBHLD value has a negative deviation; similarly, when the value is higher than this range, the HBHLD value has a positive deviation.

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Fig.4 The relationgship between hardness and stress-stain curve
The deviation of HHBLD value and yield ratio data in Table 3 can verify the above viewpoint. As shown in Figure 5, for P91 steel pipe fittings, as the yield ratio increases, the negative deviation of the HBHLD value becomes smaller and smaller; when the yield ratio is higher than 0.6 and 0.7, the HBHLD value becomes a positive deviation; Alloy steel pipe fittings have similar laws.

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Fig.5 The relationgship between yield ratio and the deviation of HBHLD

Reasons for deviation of HBHLD value

There is no obvious correspondence between the yield ratio and the parameters such as plasticity, deformation hardening index and static toughness [4]. It can be seen from Table 3 that for P91 steel pipe fittings, the higher the yield strength Rp0.01, the higher the yield ratio. The smaller the negative deviation of the HHBLD value, so the reason for the negative deviation of the HHBLD value can be analyzed from the strengthening theory of P91 steel.
The main microscopic factor that affects the yield ratio of low carbon steel is the strengthening mechanism, among which dislocation strengthening and interstitial solid solution strengthening (ie solid solution strengthening of carbon and nitrogen) have the greatest influence on the yield ratio, which can improve the tensile strength In the case of improving the yield strength, the effects of fine-grain strengthening and precipitation strengthening are followed [5]. The normal microstructure of T/P91 steel should be fine tempered lath martensite. The martensite lath does not undergo lath disintegration and recrystallization during high temperature tempering, but only high temperature recovery occurs; its main Strong
The mechanism is dislocation strengthening and solid solution strengthening [6]. The decrease in hardness of P91 steel pipe fittings is usually accompanied by the disappearance of martensite laths and the precipitation and accumulation of carbides. The yield ratio decreases due to the weakening of dislocation strengthening and solid solution strengthening. When it is lower than a certain value, HHBLD The value will have a negative deviation.

Conclusion

 (1) When testing the Leeb hardness of low hardness P9 steel pipe fittings, after eliminating the measurement error caused by external factors, the Brinell hardness conversion HBHLD will still have a large negative deviation compared with the measured Brinell hardness.
 (2) The elastic modulus of P9 steel pipe fittings with high alloy content is slightly higher than that of low-alloy steel, which is not the main reason for the deviation of its HHLLD value.
 (3) Due to the disappearance of martensite laths and the precipitation of carbides, the low-hardness P9 steel pipe fittings cause dislocation strengthening and agglomeration strengthening effects to weaken, which in turn reduces its yield ratio. This is the main reason for the negative deviation of the HHLLD value.
 (4) When the material yield ratio is within a certain range, the HHBLD value is relatively accurate, and deviation from this range will cause a large deviation. You can consider how to correct the HHBLD value from the microscopic factors that affect the yield ratio.
 Author: Yang Chao, Tang Chun slope, Sun Hung

Source: Network Arrangement – China 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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References:

  • [1] Liu Chisen, Zhang Yi The influence of site factors on the measured value of Leeb hardness [JJ. Physical Testing and Chemical Testing-Physics Section, 2005,41(10):513-515.
  • [2] Shu Delin. Mechanical properties of engineering materials [M]. Beijing: Mechanical Industry Press, 2003.
  • [3] Hou Xuming Metal Mechanical Properties [M]. Beijing: Machinery Industry Press, 1999.
  • [4] Ouyang Zhiying, Shu Zhiqiang, Yuan Pengbin. The effect of yield-strength ratio on the performance of high-strength and high-plasticity drill pipes [JJ]. Physical Testing and Chemical Analysis-Physics Section, 2013,490):17-21.
  • [5] Wei Heng, Zheng Shaobo, Wang Sen, etc. Microscopic factors affecting the yield-strength ratio of low-carbon steel [J]. Shanghai Metal, 2012, 34(2): 19-22.
  • [6] Hu Guangli, Xie Xiwen. Heat treatment of steel [M]. Xi’an: Northwestern Polytechnical University Press, 1993.

Summary
cause analysis of deviation in brinell hardness conversion value of low hardness p91 steel pipe fittings - Cause Analysis of Deviation in Brinell Hardness Conversion Value of Low Hardness P91 Steel Pipe Fittings
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Cause Analysis of Deviation in Brinell Hardness Conversion Value of Low Hardness P91 Steel Pipe Fittings
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The decrease in the hardness of P91 steel pipe fittings is accompanied by the weakening of the solid solution strengthening and dislocation strengthening effects, resulting in yield strength The decrease of the ratio is the main reason for the negative deviation of the HHLLD value; the slight change of the elastic modulus has little effect on it.
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