R & D of P92 Seamless Steel Pipe

Described here are the research and development achievements of the P92 seamless steel pipe and the characteristics of its production process. The results of the tests of hi -temperature tensile property and hi-temperature endurance property of the P92 seamless steel pipe indicate that the seamless steel pipe of this grade has homogeneous metallographic structure， and its mechanical properties and technological properties can fully satisfy the requirements of ASME SA 335/SA 335M and the application requirements of the high temperature boiler pipeline.
Key words： P92 seamless steel pipe； ultra supercritical power unit； high temperature boiler pipe；

Since the advent of USC units in the 1980s , ultra -supercritical technology has been put into production and use in Japan , Germany , Denmark and other countries and regions . China is a latecomer to USC unit technology, and it has only begun to introduce design technology and build its own USC unit in the 21st century . By 2010 , China had more than 97 1 000 MW USC units (steam pressure 25.0-26.5 MPa , steam temperature 600 ° C ) in operation and under construction [ 5 ] , becoming the country with the largest number of USC units in the world . Harsh steam of USC units The parameters require ferritic heat-resistant steel to have good mechanical properties and oxidation resistance, which makes the research and development of new high- Cr steel a key link in the development of ultra-supercritical units .
China's USC unit technology research has just started, and a complete set of technology has not yet been formed . The fundamental reason is that high-end boiler steel materials and product technology are backward; To a certain extent, it restricts the speed of China's economic development and also threatens the country's energy security .

Table 1 Relationship between steam parameters, thermal efficiency and coal consumption of thermal power units

Unit type	steam pressure / MPa	steam temperature / Late	thermal efficiency / %	coal consumption / g (kw・h -1 ]
medium voltage	3.5	435	27	460
high pressure	9.0	510	33	390
ultra high pressure	13.0	535	35	360
subcritical	17.0	540	38	324
Supercritical ( sC	24.1	566	41	300
	25.0	600	44	278
Ultra-supercritical ( USC	30.0	700	57	215
	-	A 700	60	205

1 Process research
P92 steel is a new type of heat-strength steel with "multi-element composite strengthening". It has high alloy content, high metallurgical quality requirements, difficult smelting control technology, narrow temperature range of thermoplastic deformation, and complicated pipe making process . First of all, in the composition design, the combined effects of various strengthening methods such as solid solution , precipitation , dislocation and substructure should be considered in a balanced manner, and the influence of each element on the durable strength should be fully considered to determine the optimal chemical composition control range; secondly, heat treatment Process is a key process to ensure the performance of boiler tubes, and the best heat treatment process should be studied and optimized during process design . The P92 seamless steel pipe production process developed by Pancheng Steel has the following characteristics:

1. "electric furnace + refining LF + vacuum degassing VD " or electroslag remelting can ensure the high purity and low gas content of P92 steel, and meet the smelting quality requirements and chemical composition control of P92 steel used in USC units request .
2. lays the foundation for obtaining the optimal performance and structure of P92 seamless steel pipes. At the same time, by controlling the Cr and Ni equivalents, the problem of 8 -ferrite content control is solved , and it is also helpful to prevent excessive 8 -ferrite. The internal bending defect of the steel pipe is conducive to improving the durability of the steel pipe .
3. In view of the particularity of P92 steel as a ferritic martensitic heat-resistant steel in terms of hot-piercing performance, the appropriate piercing temperature and piercing process, including the use of hollow ingot forging billet rolling process, solved the problem of hot-rolled pipe of this steel type. The key process technology [ 6 ] .
4. According to the different outer diameters and wall thicknesses of P92 seamless steel pipes, the heat treatment process solves the problem of obtaining uniform tempered slabs for P92 seamless steel pipes by taking different process control measures for parameters such as heating temperature, holding time, cooling medium and cooling speed. It is a technical problem to ensure the stable performance of the martensitic structure .

2 Product quality inspection results
According to ASME SA 335/SA 335M " Seamless ferritic alloy steel pipe for high temperature " standard [ 7 ] and related technical agreements, the chemical composition , tensile properties , impact properties , gold Various tests such as phase structure , grain size and high-magnification inclusion rating .

1. chemical composition

P92 steel in ASME SA 335/SA 335M standard is shown in Table 2 . It can be seen from Table 2 that the limits of each element in the standard are relatively wide, but the key is how to determine the optimal ratio of each alloy element .
Table 2 P92 steel in ASME SA 335/SA 335M standard
Chemical composition (mass fraction) %

C	Si	mn	S		P
0.07 ~ 0.13	W 0.50	0.30 ~ 0.60	W 0.01		W 0.02
Cr	Know	W	V		Nb
8.50 ~ 9.50	0.30 ~ 0.60	1.50 ~ 2.00	0.15 ~ 0.25		0.04 ~ 0.09
N	B	Zr	Of	In	Al
0.030 ~ 0.070	0.001 ~ 0.006	W 0.01	W 0.01	W 0.40	W 0.02

the developed P92 seamless steel pipe show that its chemical composition fully meets the requirements of technical standards, and the control of harmful element content is better than the standard, which is w ( S =0.003% ~ 0.006% , w ( P =0.012% ~ 0.013 % ; Cr and Ni equivalent control is appropriate, which can fully guarantee the performance of P92 products and meet the control requirements of 8 ferrite content .

1. Normal temperature mechanical properties

The mechanical properties of the developed P92 seamless steel pipe at room temperature are shown in Table 3 . It can be seen from Table 3 that the mechanical properties of the developed P92 seamless steel pipe are stable at room temperature, and the indicators are better than those specified in the standard .

Table 3 The mechanical properties of the developed P92 seamless steel pipe at room temperature

Specifications /mm	Tensile strength / MP)	Yield strength / MP)	elongation / %	Normal temperature impact energy J				hardness HBW
				1	2	3	average value
① 299x30	705	515	23.5	116	116	113	115	209
① 325x75	700	535	26.0	132	131	133	132	212
① 508x80 ①	765	615	24.0	73	76	92	81	219
① 508x80 ②	725	555	24.5	125	121	114	120	217
ASME SA 335/SA 335M	M 620	M 440	M20 _			-		-

Note: ①The first batch of @ 508 mmx80mm products has coarse martensite due to abnormal cooling after quenching, and the impact performance is poor; ②The second batch is the mechanical properties after the optimized process .

3 P92 seamless steel pipe performance evaluation
in the electric power and iron and steel industries have successively conducted a series of performance evaluations on the P92 seamless steel pipe developed by Pancheng Steel . Strength , creep test , welding process performance and long-term aging performance , etc.

• Partial assessment test results

1. high temperature tensile properties

For ① 299 mm x 30 mm P92 seamless steel pipe, take vertical, horizontal , inner and outer layer samples, and carry out high-temperature tensile tests at 400 °C , 500 °C , 550 °C , 600 °C , 610 °C , 620 °C and 630 °C In the tensile test, the tensile properties of the P92 sample pipe were compared with the relevant standards . The test results show that the strength uniformity of the vertical and horizontal and inner and outer layers of the steel pipe is good, and its high-temperature strength is above the epitaxial curve required by GB 5310-2008 standard , the high-temperature strength data is near the average data of Mannesmann test, and the high- temperature R po .2 is also better than the data value in the EN 10216-2007 standard .

• High temperature durability

The endurance test temperature is 610 ° C and 630° C . The extrapolated endurance strength of 100,000 hours by isothermal method is 120.1 MPa and 98.5 MPa respectively , which is higher than the requirement of extrapolated endurance strength of 100,000 hours stipulated in GB 5310-2008 ( 510 °C is not Less than 106 MPa , not less than 82 MPa at 630 °C , and higher than the endurance strength requirements specified in EN 10216 ( not less than 100 MPa at 610 °C , not less than 75 MPa at 630 °C ) .
According to DL/T 5366-2006 " Technical Regulations for Stress Calculation of Steam and Water Pipelines in Thermal Power Plants", the allowable stresses of P92 seamless steel pipes at 610 C and 630 C are calculated to be 80.1 MPa and 65.7 MPa respectively , which is better than GB 5310 — 2008 and EN 10216 — 2007 recommended allowable stress and European Creep Cooperation Committee ( European Creep Cooperation Committee , ECCC 2005 evaluation and calculation of allowable stress . The developed P92 seamless steel pipe high temperature endurance strength and allowable stress are shown in Table 4 .

Table 4 High temperature durable strength and allowable stress of P92 seamless steel pipe

Endurance test	development of steel pipes	Actual permission	GB 5310 — 2008 regulation	According to GB 5310 — 2008	EN 10216 — 2007 regulation	According to EN 10216-2007 _
temperature / C	Durable strength /MPa	Stress /MPa	Durable Strength /MPa	Calculated allowable stress /MPa	Specified enduring strength /MPa	Calculated allowable stress /MPa
610	120.1	80.1	106	70.7	100	66.7
630	98.5	65.7	82	54.7	75	50.0

① The test results of 600 °C high temperature durability strength of 508 mm x 80 mm P92 seamless steel pipe are shown in Fig. 6 . It can be seen from Figure 6 that the high-temperature durability strength performance of the developed P92 seamless steel pipe meets the recommended value of ECCC and Japan International Institute for Materials Science ( National Institute for Materials Science , NIMS) , and meets the requirements of ASME SA 335/SA 335M and other related standards.

1. 1. Product Evaluation Results
2. The chemical composition meets the technical requirements of ASME SA 335/SA 335M and GB 5310-2008 and other relevant standards for P92 steel composition; the room temperature tensile strength , yield strength , elongation , hardness and other indicators of the steel pipe meet the requirements of the corresponding standards; the steel pipe The uniformity of vertical and horizontal , inner and outer layer strength, and hardness is good .
3. P92 steel pipe is tempered lath martensite, there is no obvious difference in vertical and horizontal directions , and the content of ferrite is 1% ~ 2 % , which meets the DL/T 438-2009 standard [ 12 ] For ferrite content not greater than 5% requirement .
4. durable strength of P92 sample pipe at 610 °C and 630 °C for 100,000 hours is 120.1 MPa and 98.5 MPa respectively , and the allowable stress is 80.1 MPa and 65.7 MPa respectively , which are better than those recommended by GB 5310-2008 and EN 10216-2007 Use stress.

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