A335 P22 large-caliber nuclear power seamless steel pipe

As the world pays increasing attention to issues such as the environment and climate warming , carbon dioxide emissions have become a bottleneck restricting global economic and social development. Nuclear power as a clean and safe
One of the low-carbon energy sources with comprehensive technology, mature technology and strong supply capacity , It will play an active and important role in the future energy structure adjustment [ 1 ] [ 2 ] . While vigorously developing nuclear power plants , our country has put forward the requirement of localization of nuclear power plants. In recent years, Tianjin Steel Pipe Group Co., Ltd. ( hereinafter referred to as TP-CO) has actively promoted the localization of nuclear power pipes , and jointly developed A335 P22 nuclear power pipes used to replace imported nuclear power plant pipes with China Guangdong Nuclear Power Engineering Corporation. Although the operating parameters of nuclear power plants are lower than those of thermal power plants , due to the need to work under high temperature, strong radiation and corrosion conditions , the materials should have good mechanical properties, radiation stability, compatibility with nuclear fuel, high thermal conductivity, etc. [ 3 ] . Container damage and leakage are not allowed under any circumstances, and the safety level is high . Therefore, ensuring good performance and stable quality is the primary task of the research group for the research and development of A335P22 nuclear power tubes.
A335 P22 belongs to 21/4CZM . Type ferritic heat-strength steel , whose basic performance is mainly based on the standards and specifications of ASTM A335M/A335M and GB 5310-2008 , is used in high-pressure cylinder exhaust pipes of conventional islands of nuclear power pipes, etc. 4 . This article focuses on the technical process and product performance of the A335P22 nuclear power tube produced by TPCO . The nuclear power technology agreement mentioned in this article is a stricter nuclear power standard based on ASTM A335M/ A335M and GB 5310 -2008 standards.

2 Test materials and methods
The test material is TPCO killed steel smelted by electric furnace + refined outside the furnace . Seamless steel pipes produced by hot rolling, heat treatment, and finishing to finished pipes , The delivery state is normalizing + tempering , Since the A335P22 nuclear power tube usually contains relatively high Cr and Mo, if the tube blank is produced by continuous casting, the surface quality and internal quality of the tube blank will be poor . In severe cases, cracks will occur throughout the body ( especially large-section tube blanks ) , Therefore, it needs to be produced by die casting and forging. According to the nuclear power technology agreement and combined with the actual situation of the company , TPCO formulates the following pipe-making process flow: EAF t LF t VD t die casting t forging t annealing t tube billet heating t piercing t PQF continuous rolling t sizing t flaw detection t normalizing t tempering t Straightening t grinding t flaw detection t hydraulic test t derusting t length measurement, weighing, spraying t external painting, internal oiling t wearing caps t storage.
2.1 Chemical composition design
on the low temperature impact toughness of A335P22 nuclear power tubes found that , With the increase of Cr and Mn content in the steel and the decrease of proeutectoid ferrite in the structure , The low temperature impact toughness of the steel is obviously improved, so the content of Cr and Mn in the steel should be increased appropriately . Since the steel contains more chromium , Therefore, its resistance to high temperature oxidation , The high-temperature corrosion resistance is better than that of ordinary low-alloy pearlitic heat-resistant steel [ 5 ] . Therefore, attention should be paid to controlling the Cr content during production to meet the requirements of "FAC (Flow Accelerated Corrosion ) " resistance . In addition, too low C content will reduce the high temperature strength , but too high C content , The solderability is reduced , At the same time reduce high temperature plasticity. The C content of our company's final finished pipe is about 0.11% .
In addition , Phosphorus has a grain boundary segregation behavior in A335P22 steel , and the segregation behavior of phosphorus at the grain boundary can lead to non-hardened brittleness , resulting in intergranular fractures , reduce performance , It will also increase the ductile-brittle transition temperature of the material , The impact on material properties is quite large , so the content of P must be strictly controlled [ 6 ] . The element contents of our company's A335P22 nuclear power tube finished products are: C : 0.10 % - 0.13 % ; Si : 0. 27% - 0. 37% ; Mn :
0. 45% - 0. 52% ; Cr : 2. 15% - 2. 40. % ; Mo :
0. 94% - 1. 03% ; P : 0.006 % - 0.010 % ; S : 0. 001% ~0. 006% .
2.2 Heat treatment process system
2. 2. 1 Technological exploration
The nuclear power technology agreement stipulates that the heat treatment system of A335P22 nuclear power tubes is: normalizing ( 900 ~ 960°C ) + tempering ( 3 675°C ) . The heat treatment temperature range given in the specification is relatively large . Combined with the furnace type and equipment working conditions of our company , the influence of different heat treatment systems on the properties of the steel is studied . It is of practical significance to explore the best heat treatment process. The A335P22 nuclear power tube with a specification of 406.40mm x 12.70mm was subjected to heat treatment process tests with different process parameters . Figures 1 and 2 are the relationship curves between different heat treatment systems and mechanical properties.
2.2.2 Result analysis
The results show that , When the tempering temperature is 740C and the holding time is 60 minutes , when the normalizing temperature W is 940°C , the strength of the steel pipe has little effect with the increase of the normalizing temperature
, and the elongation increases, but when the normalizing temperature reaches 960°C Above, at this time, the alloying elements are fully dissolved , and the strength increases , but the grain grows , Elongation is significantly reduced. Therefore , The normalizing temperature is more suitable to choose 940C , After normalizing at this temperature , The material increases with tempering temperature , decrease in strength , But there is still a lot of room to spare , Increased elongation. In industrial production, the tempering temperature can be appropriately increased to ensure that it is about 740C . At the same time , it can be seen from the experimental data that With the prolongation of tempering holding time , Little change in intensity , But the toughness slightly increased.

According to the test analysis results , And combined with industrial furnace conditions , To formulate the optimal heat treatment process system is , Normalizing temperature
930 ~950°C , the holding time is 1.5min/mm, not less than 30min , Tempering temperature 740C , The holding time should be no less than 1h for the heat treatment process to organize mass production.
2.3 Comparative analysis of the overall performance of A335 P22 large-diameter nuclear power pipes
Select the A335P22 nuclear power tube produced by TPCO with a specification of 406.40mm X 12.70mm for geometric dimension analysis , And with the P22 produced by Sumitomo, Japan , 470. 00mm X 27. 50mm and ! 679. 00mm X 39. 00mm finished steel pipe for performance anatomical comparative analysis .
2. 3. 1 Size detection
Inspection method : measure a wall thickness value at every 45° position , A total of 8 points ; outer diameter measurement : measure a value every 90° apart , There are four in total. Since the Japanese Sumitomo steel pipe is a sheet sample , Therefore, the indicators such as the deviation of the outer diameter of the steel pipe and the unevenness of the wall thickness cannot be measured . Therefore, only the size data of our company's products are shown in Table 1 .
From the measurement results in Table 1 , it can be seen that TPCO products meet the standard requirements regardless of the wall thickness or outer diameter of the steel pipe . And the uniformity of steel pipe size is good , Meet the requirements of the Nuclear Power Technology Agreement.
2. 3. 2 Chemical composition of A335P22 nuclear power tube
of the above TPCO products and Sumitomo products were analyzed and compared with Sumitomo products , The analysis results are shown in Table 2 . The results show that , Compared with Sumitomo, the content of various elements in TPCO products is well controlled and at the same level . Meet the requirements of the Nuclear Power Technology Agreement.
2.3.3 Microstructure, grain size and non-metallic inclusions Check the grain size, non-metallic inclusions and microstructure of the finished tube , And compared with Sumitomo products. The non-metallic inclusions are rated according to the A method in GB/T10561 ; the grain size is tested according to the GB/T6394 standard ; the microstructure is evaluated according to the GB/T13298 standard. The test results are shown in Table 3 . The photo of metallographic structure is shown in Figure 3 a ~ c .
Table 3 and Figure 3a ~ c can be seen , Compared with Sumitomo, the content of metal inclusions in our products is lower , The purity of steel is high , evenly organized , The grain size is smaller than that of Sumitomo , Therefore , The metallographic inspection results of A335P22 large-diameter nuclear power pipes fully meet the requirements of the standard.

2. 3. 4 Mechanical performance testing The results of the mechanical performance testing are shown in Table 4 .
Longitudinal force is applied to the A335P22 steel pipe from the above origin
Table 1 Dimensional measurement and deviation

project	Specifications ( mm)	Measured value ( mm)	deviation		Ovality / Unevenness
standard	--	--	Outer Diameter: -0.75%D ~ +1%D Wall Thickness: -5 ~ +17.5%		Ovality 1.6% Unevenness 18%
outer diameter	406. 40	406. 00, 406. 75, 406. 35, 407. 15	-0. 10% -	+ 0. 18%	0. 28%
wall thickness	12. 70	12.64, 12.37, 12.89, 13.18 13.06, 12.79, 12.98, 12.59	-2. 60% -	+ 3. 78%	6. 3%

Table 2 Analysis and comparison of chemical composition and element content of steel pipes ( wt%)

place of origin	steel number	Specifications ( mm)	C		Si		mn	P	S	Ni	Cr	Mo	Cu	Al
	Nuclear Power Technology Agreement		0. 05 ~ 0.	15	W 0.50 0.30 -0. 60			W 0.025 :	W 0.015 _	- 1.	90 -2. 60	0. 87 - 1. 13	-	-
Sumitomo	P22	! 470. 00 X 27. 50	0. 13		0. 25		0.47	0. 017	0.001	0. 15	2. 17	0.94	0.07	0.009
Sumitomo	P22	! 679. 00 X 39. 00	0. 12		0. 27		0.46	0.015	0.001	0. 14	2. 07	0.93	0.06	0.010
TPCO	A335 P22	! 406.40 X 12.70	0. 11		0. 28		0.50	0.010	0.002	0.05	2. 25	0.98	0.07	0.009
TPCO	A335 P22	! 406.40 X 12.70	0. 11		0. 28		0.49	0.009	0.002	0.04	2. 24	0.96	0.07	0.009
						Table 3	Metallographic examination results
place of origin	Specification	A rough	A fine	B thick		B thin	C thick	C thin	D thick	D thin	organize	grain size	Remark
Sumitomo	! 470. 00 X 27. 50 0		0	0		1. 0	0	0	0	0.5	F+B	6.0	Figure 3a
Sumitomo	! 679. 00 X39. 00 0		0	0		1. 0	0	0	0	0.5	F+B	5.5	Figure 3b
TPCO	! 406. 40 X12. 70 0		0.5	0		0	0	0	0	0.5	F+B	8.0	Figure 3c
TPCO	! 406. 40 X12. 70 0		0.5	0		0	0	0	0	0.5	F+B	8.0

Table 4 mechanical properties results
	place of origin	Specimen direction	Yield strength ( MPa)	Tensile strength ( MPa) Elongation after break A (%)		AK1	AK2	AK3	Average AK
ASME SA335M		portrait	$ 205	$ 410	$ 22	$ 39	$ 39	$ 39	$ 39
TPCO	! 406. 40 X12. 70	portrait	345	580	40	226	268	252	249
mm	! 406. 40 X12. 70	portrait	350	575	37	242	302	235	260
Sumitomo	! 679. 00 X39. 00	portrait	305	520	35	238	240	244	241
mm	! 679. 00 X39. 00	portrait	330	525	34	280	226	248	251
	! 470. 00 X27. 50	portrait	340	565	29	300	302	288	297
	! 470. 00 X27. 50	portrait	340	555	32	306	278	300	295

As can be seen from Table 4 , The batch of A335P22 nuclear power tubes produced by TPCO has good mechanical properties at room temperature . And under the premise that the strength has a large margin , still plastic enough , Meet the requirements of nuclear power standards. Compared with Sumitomo products , The mechanical properties of the A335P22 nuclear power tube produced by TPCO are good , The elongation rate is higher than that of Sumitomo products , Impact performance is comparable.
2. 3. Comparison of 5 series of impact tests
The brittle transition temperature is an important index to evaluate the performance of a steel grade. transition temperature too high , brittle fracture may occur during the application process , cause an accident. A series of impact toughness tests were carried out on this batch of A335P22 nuclear power tubes by a series of impact methods and compared with Japanese Sumitomo products . To study its ductile-brittle transition temperature , Boltzmann function fitting analysis was performed on the series of impact values and section shear ratios , and the results are shown in Fig. 4a ~ c . The test results show that the ductile-brittle transition temperatures of Sumitomo products in Japan are - 4C and - 3C , The ductile-brittle transition temperature of TPCO A335P22 nuclear power tube is - 25C , It can be seen that the brittle transition temperature of TPCO products is much lower than that of Sumitomo products . This is due to the low P content control (0.010%) of the A335P22 nuclear power tube in this furnace , And the grain is finer , Therefore, the brittle transition temperature is low. The experimental results fully meet the requirements of the A335P22 nuclear power tube technology agreement.

---