Development of 07Cr25Ni21NbN austenitic heat-resistant seamless steel pipe

Abstract : In order to meet the manufacturing requirements of ultra-supercritical power plant boiler superheaters and reheaters, BBN Iron and Steel Co., Ltd. developed 07Cr25Ni21NbN (TP310HCbN/s31042) austenitic heat-resistant seamless steel pipes . The composition optimization design, production process and performance evaluation of this steel grade are discussed. The data show that the technical indicators and performance of the produced products meet the requirements of GB 5310-2008 and ASME SA-213 standards .
Key words : boiler ; supercritical pressure ; austenitic heat-resistant steel pipe ; performance

Ultra-supercritical power plant boiler high-temperature superheaters and reheaters work under harsh environmental conditions for a long time . If the inner wall is oxidized by high-temperature steam above 600 °C , the outer wall will suffer from flue gas corrosion and fly ash erosion . Therefore, it is required The material not only has good thermal strength , but also has good corrosion resistance . In the 1980s , Japan 's Sumitomo Metal developed a steel pipe with good thermal strength and corrosion resistance on the basis of TP310 austenitic stainless steel , and the brand was HR3C . It was included in ASME code case 2115 in 1991 , and later included in AS TM A213 and AS ME S A-213 standards , The grade is TP310HCbN , and the UNS number is S31042 . In accordance with the needs of the rapid development of ultra-supercritical units in China , this steel type was included in 2008
GB 5310-2008 Standard for Seamless Steel Tubes for High Pressure Boilers , The grade is 07Cr25Ni21NbN .
before , All the products needed in China depend on imports . expensive , Therefore, the boiler manufacturing and electric power industries urgently hope that China's metallurgical industry can develop and produce such steel pipes . to replace imported products . BBN Steel Co., Ltd. began to develop 07Cr25Ni21NbN steel pipe in 2006. This article will focus on the development , production process , product performance and evaluation results of this product .

1 Alloying principle
Since the traditional 188 type austenitic heat-resistant steel is resistant to water vapor

Oxidation performance and flue gas corrosion resistance are low, and the final parts of the boiler superheater and reheater require good resistance to water vapor and flue gas oxidation, so 25-20 type austenitic heat-resistant steel has been developed , its Cr mass fraction reaches 25 % . During the long-term service, the Cr element will diffuse to the surface and combine with oxygen to form a dense oxide layer , which has a good anti-oxidation effect . But with the increase of Cr content, the brittleness . The tendency of phase precipitation is more obvious . A high Ni content can stabilize the austenite structure and at the same time inhibit it . phase precipitation tendency . Therefore , the 25-20 type austenitic heat-resistant steel not only has significantly improved oxidation resistance , And has a stable austenite structure , It can adapt to the harsh working conditions of the steam parameter of 600 °C ultra-supercritical pressure boiler superheater .
Cr and Ni content will not improve the creep rupture strength . In order to improve the creep rupture strength of the material, 07Cr25Ni21NbN adopts the principle of multi-element alloy strengthening . That is, on the basis of TP310 , by limiting the C content , and compounding the addition of a strong carbonitride forming element Nb with a mass fraction of 0.20 % to 0.60 % and a mass fraction of 0.15 % to 0.35 % of N , using the fine NbCrN phase and Nb- rich carbonitride and M 23 C 6 to strengthen .

2 Production process
The production process flow charted for 07Cr25Ni21NbN steel pipe is shown in Figure 2. Its main feature is that the steelmaking adopts AC electric arc furnace (AC EAF) smelting + argon oxygen furnace (AOD) and refining furnace (LF) for decarburization, degassing and deinclusion. and refined to obtain fine steel quality. The pipe making adopts hot rolling + cold rolling (drawing) process, which can make the steel pipe obtain high dimensional accuracy. Heat treatment adopts high temperature solid solution, which can almost completely dissolve the primary niobium in the steel into the matrix. The chemical composition of the developed 07C r25Ni21Nb N steel pipe is shown in Table 1 [ 2-3 ] .

Table 1 Chemical composition ( mass fraction ) of 07Cr25Ni21NbN steel
Standard w(C) w(Si) w(Mn) w(P) w(S) w(Cr) w(Ni) w(Nb) w(N)

ASME SA-213TP310HCbN				0.04	~0. 10		C0.75		C2.00
GB5310—200807Cr25Ni21NbN				0.04	~0. 0		C0.75		C2.00
Furnace number.873D1083				0	.06		0.35		1.18
C0.03		24.00 yen	17.00 yen			0.20 _		0.15 _
	C0.030	26.00	23.00			0.60		0.35
C0.03		24.00 yen	19.00 yen			0.20 _		0.15 _
	C0.015	26.00	22.00			0.60		0.35
0.02	0.002	24.84	20.54			0.41		0.23

3 Performance and organization evaluation test
3 . 1 Tensile properties at room temperature
According to GB/t 228-2002 "Metal Materials Tensile Test Method at Room Temperature" and GB/T 230.1-2004 "Metal Rockwell Hardness Test ", respectively on the SHT5106 universal material testing machine and the HR- 150a Rockwell hardness tester, the 07Cr25Ni21NbN steel pipes were tested for tensile properties and hardness at room temperature , and the test results are shown in Table 2 . The room temperature mechanical properties and hardness test results of steel pipes show that the room temperature mechanical properties of 07Cr25Ni21NbN steel pipes used in the test are required by the standard requirements .
table 2 Test results of mechanical properties of 07Cr25Ni21NbN steel pipe

Sample ( diameter X wall thickness )/mm	Yield strength Tensile strength Elongation			Hardness ( HRB)
	R el /MP a	R m /MP a	A/%
57X10	364	739	47.3	88.5	87.0	89.5
	373	740	49.8	89.0	89.0	88.0
38X7.5	380	775	44.0	88.0	88.5	88.0
	370	780	46.5	88.0	89.0	89.0
ASME SA-213	>295	>655	>30.0		C100.0
GB5310—2008	>295	>655	>30.0		C100.0
3 . 2 microstructure

The microstructure of the 07Cr25Ni21NbN steel pipe after solution treatment is austenite, and the grain size is 5 to 7 grades . It can be seen under the scanning electron microscope that the 07Cr25Ni21NbN steel pipe has very few primary niobium particles on the matrix, and almost all of them are solid-dissolved into the matrix . The transmission electron microscope observation shows that the second phase in the crystal

3 . 3 high temperature tensile properties of steel pipe
a cylindrical sample with a working section diameter of 8 mm along the longitudinal direction of a steel pipe with a diameter of 57 mm and a wall thickness of 10 mm ; take a parallel section with a width of 10 mm along the longitudinal direction of a steel pipe with a diameter of 38 mm and a wall thickness of 7.5 mm . For pipe arc samples with a gauge length of 50 mm , According to GB/T 4338-2006 " High Temperature Tensile Test Method for Metal Materials " , the high temperature tensile test was carried out on the SHT5106 electro-hydraulic servo universal material testing machine , and the test results are shown in Figure 4 . The test results show that the high-temperature tensile properties of steel pipes meet the requirements of GB 5310-2008 .

 

3 . 4 Long-term aging performance
07Cr25Ni21NbN steel pipe samples were subjected to 650 C high temperature aging treatment . After aging for 1 000h , 3 000h , 5 000h , 8 000h and 0 000h, they were taken out respectively, and tested on the WE-100 hydraulic universal material testing machine according to GB/T 228-2002 " Metallic Materials Tensile Test Method at Room Temperature " . After the sample was subjected to a tensile test, the test results are shown in Figure 5(a) . According to GB/T 229-1994 " Metal Charpy Notched Impact Test Method ", the sample is processed into a standard Charpy V -notch 10 mm X 10 mm X 55° impact sample, and the test is carried out on the ZBCZ452N1 pendulum impact testing machine; XHB.-3000 closed-loop control digital display Brinell hardness tester is used for hardness testing, and the test results are shown in Figure 5(b) . It can be seen from Figure 5(a) that with the prolongation of the aging time, the yield strength fluctuates slightly, the tensile strength increases, and the elongation shows a downward trend; it can also be seen from Figure 5(b ) that the high temperature aging time of 1000h Afterwards, the impact energy decreases significantly, and then remains stable until 10 000 h , while the hardness increases after aging at high temperature for 1 000 h , and then increases slowly until 10 000 h .

3 . 5 lasting strength
According to the GB/t 2039-1997 standard, a cylindrical standard sample with a diameter of 5 mm is taken along the longitudinal direction of the 07Cr25Ni21NbN steel pipe , The 650C , 700C , 750C durable strength tests were carried out on the RD2-3 creep durability tester, and the durable strength test curves are shown in Figure 6 . Fitting calculations were performed using the least squares method , The test results of 07Cr25Ni21NbN steel pipe at 650 C , 00 C and 50 C were obtained respectively , With the existing data , The enduring strength extrapolation equation is as follows :
650 C durable strength extrapolation equation lg o=3.034 5—0.201 5 lgt
700 C Endurance Strength Extrapolation Equation

750 C Endurance Strength Extrapolation Equationlgo=2. 821 0—0.258 9 lgt
Where : is the durable strength of the material ; is the time corresponding to the durable strength of the material .
According to this calculation, it can be concluded that the durability strength of 07Cr25Ni21NbN steel pipe at 650 C , 700 C and 750 C for 100 000 h is 106.4 MPa , 62.5 MPa and 33.6 MPa respectively , which meet the requirements of AS ME standard and GB5310-2008 standard . 3 . 6 Anti-steam oxidation performance
07Cr25Ni21NbN steel was tested in a supercritical water corrosion test device at 650 C and a pressure of 25 MPa to simulate the actual operating conditions of a power plant . The test time reaches 1000h . Comparison of test results of x20CrMoV121 steel ( the mass fraction of Cr is 12.3 % , 11.2 % and 10.2 % ) and Super304H anti-steam oxidation test is shown in Figure 7 . The test results show that , in evil

07Cr25Ni21NbN steel still exhibits excellent steam corrosion resistance under harsh working conditions . The oxidation weight gain is only 1/5 of that of Super304H , and 1/92 of that of x20CrMoV 121 steel ( the mass fraction of Cr is 12.3 % ) .

epilogue
It can be seen from the above results that :

1. 07Cr25Ni21NbN austenitic heat-resistant steel pipes produced meet the requirements of the AS ME SA-213 standard and the GB5310-2008 standard . Indicates that the selected process is correct .
2. The performance of the 07Cr25Ni21CbN steel pipe produced ( including special properties such as high temperature durable strength and steam corrosion resistance ) is excellent , It can fully meet the requirements of super ( super ) critical power plant boiler manufacturing .
3. 7Cr25N21CbN austenitic heat-resistant steel pipe was successfully developed , Shorten the gap between China and foreign countries in the field of heat-resistant steel research , At the same time, it has greatly improved the manufacturing capacity and level of China's power plant boilers .

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