Development of SA -335 P91 Seamless Steel Tube for Power Plant Boiler

Abstract: SA-335 P91 is the key material used in supercritical and ultra-supercritical power station boiler headers and main steam pipes. The domestic boiler and power industries are eager to develop this material on the basis of SA - 213 T91 developed by BBN STEEL . product. With the commissioning of the 460PQF continuous rolling tube unit of BBN Steel Tube Co., Ltd. , it has the ability to develop SA -335 P91 large-diameter tubes . Mainly introduces the development process of BBN STEEL SA -335 P91 , as well as the evaluation and test of material properties. The test data shows that the performance of BBN STEEL SA -335 P91 meets the requirements of ASME and GB 5310-2008 standards , and can replace imported products for supercritical , Manufacture of ultra-supercritical power plant boilers.
Key words: SA-335 P91 ; power plant boiler; supercritical; ultra supercritical; seamless steel pipe

From the perspective of the primary energy structure, China's energy is still dominated by coal at present, and according to China's actual situation, the energy structure dominated by coal will not change for a long time. Therefore, improving the efficiency of coal-to-electricity conversion and developing clean coal Power generation technology and the construction of large-capacity, high-parameter ultra-supercritical units are the key to achieving the goal of energy conservation and emission reduction, and are also an inevitable choice for China's sustainable economic and social development. According to statistics, since November 2006 , when two USC units in Yuhuan and Zouxian were put into operation, by the end of 2011 , China had 41 USC units in operation , becoming the country with the largest number of USC units in the world . Undoubtedly, in order to ensure the construction and safe and stable operation of USC units in China, as well as the development of
USC units in the future, it is very necessary to develop key materials for use units. Based on this situation, BBN Iron and Steel Co., Ltd. has been committed to the research and development of ultra-supercritical power plant boiler steel since 1999 , and has successively researched and developed T91 , T23 , T92 , S30432 , S31042 and other steels used in the manufacture of ultra-supercritical power plant in China's boiler industry. The materials urgently needed by the boiler have the ability to supply the water wall, superheater and reheater, the key components of the heating surface of the main steam 600* ultra-supercritical power plant boiler . The gap between foreign countries supports the development of China's ultra-supercritical units. At the same time, with the construction of BBN STEEL 460PQF continuous rolling tube unit, the development of large-diameter high-pressure boiler tube SA-335 P91 has been carried out. The following mainly introduces the development process of BBN STEEL SA -335 P91 , as well as the evaluation and test work of material properties.
1 P91 steel chemical composition and alloying principle
According to the requirements of AsME sA -335 P91 and GB 5310-2008 high - pressure boiler tube standards, the chemical composition of P91 high-pressure boiler tubes is the same as the ASME SA-213 T91 composition system, and is basically the same as the 10Cr9MolVNbN composition system in the GB 5310-2008 standard. Its main chemical composition is shown in Table 1 ( ASME SA-335M-04 ) .

Table 1 Chemical compositions of T /P91 and 10Cr9Mo1VNbN%

	we	wSi	WMn	wS	Wp	WCr	wMo	Wy	wNb	wN	wNi	wAl	wTi	WZr
ASME	0.08 〜	0.20 〜	0.30 ~	W	w	8.00 -	0.85 ~	0.18 ~	0.06 -	0.030 ~	w	w	w	w
SA-335 P91	0.12	0.50	0.60	0.010	0.020	9.50	1.05	0.25	0.10	0.070	0.40	0.020	0.01	0.01
ASME	0.08〜 _	0.20〜 _	0.30〜 _	IN	In	8.00 〜	0.85〜 _	0.18〜 _	0.06 -	0.030〜 _	In	In	In	In
SA-213 T91	0.12	0.50	0.60	0.010	0.020	9.50	1.05	0.25	0.10	0.070	0.40	0.020	0.01	0.01
GB 5310	0.08〜 _	0.20〜 _	0.30〜 _	IN	In	8.00 〜	0.85〜 _	0.18〜 _	0.06 -	0.030〜 _	In	In
10Cr9Mo1VNbN	0.12	0.50	0.60	0.010	0.020	9.50	1.05	0.25	0.10	0.070	0.40	0.020

The chemical composition determines the mechanical properties of steel, and the standard only gives a general composition range. In order to obtain the best performance coordination, the composition needs to be optimized and controlled [ 2 ] . The research on the production and use of relevant domestic manufacturers and boiler factories found that the dispersion strengthening of V , Nb , and N carbonitrides in T/P91 steel depends on the concentration of solid solution N in the steel . Therefore , T/P91 steel The content of Al , Ti and other elements in the steel should be strictly controlled to avoid the uneven distribution of dispersed phase Nb , V ( C , N ) in T/P91 steel and the decrease of volume fraction, which will cause the decrease of creep rupture strength. In addition, sulfur easily causes the grain boundary weakening of T/P91 steel, which is the main cause of creep embrittlement of T/P91 steel.

2 P91 seamless steel pipe production technology
BBN STEEL produces the SA -335 P91 product production technological process which formulates as shown in Figure 2 . Its main feature is that desulfurized molten iron is used for steelmaking, and it uses AC electric arc furnace ( AC EAF ) smelting + refining furnace ( LF ) and vacuum furnace ( VD/VOD ) for decarburization, degassing, deinclusion and refining to obtain excellent Steel. The completion of the 460PQF continuous rolling pipe unit can make the steel pipe obtain high dimensional accuracy. The main production process is shown in Figure 3 . The typical specification is 0 273 x 28. 6 mm at an interval of 1.15 m along the longitudinal direction , and the outer diameters of 10 cross-sections are actually measured . The test results are shown in Table 2 .

According to the outer diameter inspection standard, the outer diameter is required to be controlled at ±0.75% , and the actual measurement is +0.15 -0.29% , which meets the requirements and has high precision. For the wall thickness interval of 2.3m along the length direction , the wall thickness of 5 cross-sections was actually measured , and the test results are shown in Table 3 . According to the inspection standard, the wall thickness is required to be controlled at -10%-+12.5% , and the actual measurement is +0.98% -3.2% , which meets the requirements and has high precision.

Table 2 Outside diameter measurement results of 273 x28.6 mm steel pipe mm

section	1	2	3	4	maximum value	minimum value	extremely bad	average
1	273.9	273.7	273.8	273.8	273.9	273.7	0.2	273.80
2	273.6	273.8	273.7	273.8	273.8	273.6	0.2	273.73
3	273.8	273.7	273.8	273.7	273.8	273.7	0.1	273.75
4	273.5	273.5	273.7	273.7	273.7	273.5	0.2	273.60
5	273.5	273.2	273.4	273.5	273.5	273.2	0.3	273.40
6	273.7	273.4	273.5	273.3	273.7	273.3	0.3	273.48
7	273.8	273.6	273.7	273.7	273.8	273.6	0.2	273.70
8	273.8	273.5	273.7	273.7	273.8	273.5	0.3	273.68
9	273.8	273.7	273.7	273.8	273.8	273.7	0.1	273.75
10	273.8	273.7	273.8	273.7	273.8	273.7	0.1	273.75

Table 3 Wall thickness measurement results of 273 x28.6 mm steel pipe mm

section	1	2	3	4	5	6	7	8	maximum value	minimum value	extremely bad	average
1	29.12	27.98	28.26	28.21	28.27	28.62	30.04	30. 19	30. 19	27.98	2.21	28.80
2	29.87	28.90	28.76	27.64	29.14	29.56	29.02	30.31	30.31	27.64	2.67	29.15
3	29.54	29.18	29.34	29.82	29.25	28.41	29.01	28.75	29.82	28.41	1.41	29.16
4	29.84	29.01	29.29	29.08	29.45	29.21	29.31	28.77	29.84	28.77	1.07	29.24
5	30.98	29.19	29.89	30.05	28.88	29.07	29.18	28.78	30.98	28.78	2.20	29.50

The heat treatment adopts bright annealing furnace, and the heat treatment process refers to BBN STEEL T91 heat treatment process, so as to ensure stable product performance and good internal and external surface quality.
3 P91 seamless steel pipe performance evaluation and test
3.1 Conventional performance
The 0 273 x 28.6 mm P91 seamless steel pipe was subjected to conventional tensile, conventional impact and hardness tests, and the results are listed in Table 4 . The test results show that the developed 0 273 x 28.6 mm P91 seamless steel pipe meets the requirements of ASME and GB 5310-2008 standards in terms of conventional tension, room temperature impact and hardness .
3.2 Flattening process performance
a flattened sample at the end of the 0 273 x28. 6 mm steel pipe , and perform the flattening test on the WHY-6000 microcomputer-controlled steel pipe according to the ASME SA - 999M " Specification for General Requirements for Alloy and Stainless Steel Pipe " test method The flattening test was carried out on the machine, and the results are shown in Table 5 and Figures 4 and 5 .
The flattening test results show that after flattening the 0 273 x 28. 6 mm steel pipe, there are no visible cracks on the inner and outer surfaces and end faces of the steel pipe, and the flattening process performance of the steel pipe meets the requirements of ASME SA-999M .

Table 4 General properties of 273 x28. 6 mm P91 steel pipe

Data Sources	Yield strength /MPa	Tensile strength /MPa	Elongation /%	HB	Impact energy /J
No.1	579	731	21.2	220	212 , 211 , 221
No.2	561	713	22.6	226	197 , 192 , 189
No.3	589	743	21.2	225	192 , 180 , 193
ASME	$415	$585	$20	W250	Horizontal
SA - 335 P91					$ 27
GB5310	$415	$585	$20	W250	horizontal
10Cr9Mo1VNbN					$ 27

Table 5 Flatting property test results of the evaluated steel pipe

Data Sources	Flattening process		test result
0 273 x 28 . 6mm	Step 1: Ductility Test	h = 167mm	qualified
	Step 2: Integrity Test	h = 0	qualified
ASME SA-999M	Press to the specified height and then press to fold		There are no cracks or cracks on the inner and outer surfaces and end faces of the steel pipe

3.3 Instantaneous high temperature tensile properties
Take 30 high-temperature tensile samples at the end of the 0 273 x 28.6 mm steel pipe near the 1/4 wall thickness of the outer wall of the steel pipe. A standard round bar sample with a diameter of 10 mm was tested on the SHT5106 electro-hydraulic servo universal material testing machine. The test results are shown in Figures 6 and 7 , which meet the requirements of the GB 5310-2008 standard.
3.4 High temperature durable creep performance
The specification of the test steel pipe is 0 273 x 28.6 mm . Cylindrical samples with diameters of 5 and 10 mm in the working section are taken , and subjected to 600T high temperature and durable creep according to GB/T 2039-1997 "Metal Tensile Creep and Durability Test Method" test. According to the test data ( n = 12 ), the least square method is used to obtain the equation: lg b = 2. 533 47 -0. 111 15 lg T , and the enduring strength of 100,000 h is extrapolated according to the equation to be 95.0 MPa , meeting
GB 5310 — 2008 standard and ASME standard requirements.
Similarly, at 600T , the creep test stresses are 160 , 140, and 130 MPa respectively. According to the deformation data of the 600T steady-state creep stage, the creep rates of the steady-state creep stage under the stresses of 160 , 140 and 130 MPa are respectively calculated as: 3.53 x10 -3 , 5.72 x 10 -4 , 4.32x10 -4 mm/h .

3.5 Microstructure Morphology
a transverse sample at the 1/4 wall thickness of the end of the 0 273 x 28.6 mm P91 steel pipe. According to GB / T 13298-1991 " Metal Microstructure Inspection Method ", use a metallographic microscope to analyze the microstructure and morphology. The organizational morphology is shown in Figure 10 . The GB5310 standard stipulates that the microstructure of the finished 10Ci9Mo1VNbN high-pressure boiler tube steel pipe should be tempered martensite or tempered sorbite, and no overheating structure is allowed; the grain size grade of the finished steel pipe should be M 4 , and two test The difference between the maximum level and the minimum level of grain size on the chip is not more than

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