Research and development of HP2 13Cr seamless steel pipe

HP2 13Cr seamless steel pipe by hot extrusion + quenching and tempering heat treatment process is introduced in detail . The physical performance test results show that the HP2 13Cr seamless steel pipe produced by this process has good mechanical properties and metallographic control, fully meets the requirements of the ISO 13680: 2010 standard, and is mildly corroded in an acidic environment with high CO2 and H2S content. Corrosion is good.
Key words: seamless steel pipe; HP2 13Cr ; hot extrusion; quenching and tempering heat treatment; metallographic structure; mechanical properties; corrosion rate

During the Twelfth Five-Year Plan period, China's oil and gas equivalent will reach 300 million tons (excluding foreign parts) or above. As the number of oil and gas fields is increasing year by year, and gradually developing into areas with deep wells, subsea, and complex working conditions, while the demand for oil well pipelines is increasing in quantity, the quality requirements for varieties are also increasing. In order to solve the corrosion resistance problem of oil well pipes under the coexistence of CO2 , H2S and Cl - , oil well pipe manufacturers at home and abroad have successively developed 2Cr13 , HP2 13Cr , duplex stainless steel, iron nickel base and nickel base alloy oil well pipes to meet different well conditions. Among them, HP2 13Cr is mainly used in high CO2 partial pressure and low H2S partial pressure environment, usually H2S partial pressure below 0.003 MPa can be used safely [ 1-2 ] .
Compared with ordinary martensitic stainless steel, the carbon content of HP2 13Cr is greatly reduced, w ( 0 is up to 0.03% , and w ( N ) ( 4.5% ~ 5.5% ) and w ( Mo ) ( 1.5% ~ 2.5%) are increased , It improves the toughness while improving the strength and hardness. HP2 13Cr overcomes the shortcomings of traditional martensite such as stress crack sensitivity and poor weldability during welding, and greatly improves the corrosion resistance of CO2 and H2S . The research and development of HP2 13Cr seamless steel pipes in China started late, and the HP2 13Cr pipes demanded by domestic oil fields basically depend on imports, which not only cost a lot of money, but also restrict the development of China's petroleum industry and even threaten national energy security.
Technical requirements of HP2 13Cr seamless steel pipe
According to ISO 13680 : 2010 Delivery techniques for corrosion-resistant alloy seamless steel pipes for casings, pipes and couplings in the oil and gas industry

Technical Conditions and NACE RP0775-199 The chemical composition requirements of corroded HP2 13Cr seamless steel pipes in oilfield production are shown in Table 1 .
The preparation, installation and analysis of experimental data of coupons standard requirements, the requirements are shown in Table 2 .
Table 1 Chemical composition (mass fraction) requirements of HP2 13Cr seamless steel pipe %

C	Si	mn	P	S	Cr	Ni	Mo	Ti
W0.03	W0.50	W0.60	W 0.020	W 0.005	11.50 ~ 13.50	4.50 ~ 6.50	1.50 ~ 3.00	0.01 ~ 0.50

Table 2 Performance requirements of HP2 13Cr seamless steel pipe

mechanical properties					Microstructure	average corrosion Speed / ( mm·a - )
Rm /MPa	R p0.2 /MPa	A5 /%	Hardness HRC	Ak /J
M793	M758	M13	W 32	M27	8 Ferrite content below 5%	0.025

2 Process research of HP2 13Cr seamless steel pipe

1. process flow

with ordinary 13Cr seamless steel pipes, HP2 13Cr seamless steel pipes have more stringent requirements in terms of composition control, metallographic structure, and corrosion performance. Different from the common hot rolling process at home and abroad [ 7-8 ] , this project adopts hot extrusion process for production.
The most commonly used specification of oil pipe is ① 73.02 mmx5.51 mm . Combined with the finished product size, the production process of the whole line is designed as follows: smelting (electric furnace + VOD smelting) - blank casting + radial forging, and the blank is ① 230 mm round bar Tube extrusion + quenching and tempering - finished product inspection and evaluation).

1. key technology
2. The composition design is based on the Schaeffler organizational chart [ 9-10 ] , the main alloys and impurities of stainless steel

of high quality elements can be converted into the corresponding Cr and Ni equivalents, which determine the structure and morphology of the product. HP2 13Cr is an improved variety of typical martensitic stainless steel 2Cr13 , and the composition addition and content control are carried out on the basis of 2Cr13 .
Cr is ©2 to ensure the surface of the steel pipe . 3 The main elements of the passivation film form, in the composition design, the Cr content is on the upper line.
Mo can significantly improve the pitting corrosion resistance of stainless steel, and can be more dense with the Cr oxide film, further improving the corrosion resistance of the material, but Mo element is easy to cause segregation, so the w ( MO is about 2% added in the composition design .
Ni is an austenite forming element. On the one hand, it can ensure the austenite structure of the material at high temperature and promote the complete transformation of martensite; on the other hand, the presence of Ni can promote the formation of an appropriate amount of reversed austenite to ensure the The formation of a small amount of austenite in the tenite structure enhances the toughness of the material. In addition, Ni can also improve the corrosion resistance of the material. The Ni element with a mass fraction of about 5% is added to this material .
Ti preferentially forms TiC with C and avoids the formation of Cr 2 3 C 6 , which can effectively slow down the intergranular corrosion tendency of stainless steel. However, Ti element will affect the fluidity of molten steel, and it is easy to cause local segregation of components, so the excessive addition of Ti element will affect the uniformity of the structure. Considering that HP2 13Cr is a low-carbon martensitic stainless steel, w ( Ti can take the lower limit value required by the standard, that is, 0.01% ~ 0.02% .
In terms of impurity elements, in order to ensure sufficient corrosion resistance, the C , P and S elements that affect the corrosion resistance are strictly controlled, among which w ( C is required to be below 0.02% , w ( P is required to be below 0.02% , w ( S is required to be below 0.003% .

1. Smelting process

Since the w ( C of HP2 13Cr stainless steel is below 0.03% , it is an ultra-low carbon stainless steel; therefore, the two-step smelting process of electric furnace + VOD is used for steelmaking.
Electric furnace steel uses pure alloys or raw materials with less five harmful elements to ensure the purity of steel. To prevent oxidation of precious alloys, high-capacity transformers are used to accelerate melting. Immediately blow oxygen decarburization after the molten pool is melted, and the decarburization rate is controlled at 0.03% w ( C per minute . After the decarburization is completed, micro-carbon ferrochrome is added to the molten steel to cool the molten steel, and the temperature of the molten steel is controlled at 1700 Below 7. Use low-carbon ferrosilicon for chromium oxide reduction, and remove the slag when the steel slag is brownish gray, and transfer to VOD for refining after removing the slag.
VOD refining, the flow rate of bottom blowing argon gas is increased to 1 200 L/min to promote decarburization and denitrogenation reactions , and finally control w ( C below 0.02% . The die casting process is protected by Ar gas to prevent secondary oxidation of molten steel and Control the pouring time and the amount of mold slag to prevent subsequent defects in the slab.The actual chemical composition of HP2 13Cr seamless steel pipe is shown in Table 3 .

1. Diameter forging process

In the composition of HP2 13Cr seamless steel pipe, the forming elements of ferrite phase mainly include Cr , Mo , etc., and the forming elements of austenite phase mainly include Ni , C , etc. HP2 13Cr seamless steel pipe is heated and

Table 3 Actual chemical composition (mass fraction) of HP2 13Cr seamless steel pipe %

C	Si	mn	P	S	Cr	Ni	Mo	Ti
0.017	0.28	0.54	0.013	0.001	13.0	4.8	1.94	0.015

There is a phase transformation in the cooling process. The research in literature [ 11 ] shows that the austenitic transformation start temperature AC1 and end temperature Ac3 of TGOG13Cr-1 are 720 °C and 860 °C respectively , and the transformation ends when the temperature reaches 1150 ° C .
Literature [ 1 2 ] studies on the heating process and metallographic structure of 0Cr16Ni5Mo1 steel show that when the steel is heated above 1 220 C for a long time, the 8 -ferrite in the structure begins to increase. When the temperature reaches 1 270 C , 8 The ferrite content can reach 28% , and it is distributed in a network along the grain boundary.
Since the C element in 8 ferrite has a high solubility at high temperature, during the rapid cooling (normalizing or quenching) process, the C element will be supersaturated and dissolved in it. If it is cooled slowly, the ferrite will be decomposed into CT phase and y phase, and a Cr- poor area will be formed around phase a or y , and the preferential corrosion of the Cr -poor area will make the ferrite area preferentially corroded. Therefore, controlling the 8 ferrite content in HP2 13Cr steel is the main index of this steel. The ISO 13680:2010 standard has quantitative requirements for residual ferrite, which must be less than 5% .
In order to reduce the 8 ferrite content as much as possible and facilitate the performance control of the subsequent steel pipes, this project adopts 8 t die-cast ingots, which are rolled into square ingots of □ 400 mmx400 mm , and the rolling temperature is controlled at 1 100 ~ 1 200 C ; Then it is radially forged into ① 230 mm round ingot, and the radial forging temperature is the same as the rolling temperature; after radial forging, it is kept at 700 C for more than 8 hours , and then cooled to below 300 C with the furnace, and then air-cooled out of the furnace.

1. extrusion process

Usually the production process of HP2 13Cr seamless steel pipe is perforation +
+ hot rolling process, the hot extrusion process has a large amount of deformation, and the extrusion ratio can reach more than 20 , and the forming process is subjected to three-dimensional compressive stress. The structure is dense and the grains are finer. Usually, the grain size Above level 8 .
The hot extrusion process temperature of this project is set at 1 150~1 180
C , the specific process parameters are as follows:
Analysis of the microstructure of the extruded pipe shows that 8 there are many ferrite residues, and the shape is also very obvious. The ferrite content is about 6% , which has exceeded the requirements of the ISO 13680:2010 standard, and affects the service life and service safety of the material; therefore, it is necessary to adjust the structure through subsequent heat treatment to control the ferrite content within 1% .

1. heat treatment process

For 13Cr steel, the stress of the normalized martensite structure is large, and the 8 -ferrite is easy to decompose into a harmful phase, which is prone to serious failure behavior during service; therefore, all HP2 13Cr pipes after hot extrusion need to be processed Subsequent quenching and tempering treatment (normalizing + tempering). The normalizing temperature is above the Ac3 line, on the one hand, it can eliminate the structural stress caused by hot extrusion, and on the other hand, it can eliminate the harmful structure of 8 -ferrite; the effect of tempering is to precipitate the supersaturated C element in the martensite, It becomes a tempered martensite form, the hardness is obviously reduced after tempering, and the toughness is obviously enhanced.
HP2 13Cr seamless steel pipe cannot be selected too high. Excessively high normalizing temperature will cause a large amount of Cr , Ni , Mo and other elements to dissolve in the austenite matrix, and reduce the temperature at which martensite begins to form, resulting in low strength. ; On the other hand, as the normalizing temperature increases, the grains will grow and coarsen obviously, and 8 ferrite will be produced at the same time, which reduces the plasticity of the material.
Figure 2 shows the metallographic structure of HP2 13Cr seamless steel pipe after two different normalizing temperatures. The normalized structure is lath martensite, and clear original austenite grain boundaries can be seen. After normalizing at 1 000 C , the striped 8 -ferrite produced in the extruded state still exists without obvious signs of dissolution; when the normalizing temperature rises to 1 050 C , the 8 -ferrite changes from striped to discontinuous In the dot shape, obvious dissolution phenomenon can be seen. At the same time, with the increase of normalizing temperature, the grain size increases obviously, from the original 8th grade to about 5th grade. Considering the strengthening effect of fine grains and can improve the toughness, the temperature of normalizing can not continue to increase.

If the tempering temperature is too high, a large number of compounds will be precipitated, the solute atoms will be desolvated, the solid solution strengthening effect will be weakened, and as the compound aggregates and grows, the strength will decrease and the elongation rate will increase slowly; tempering The temperature is low, the solute atoms do not have a large amount of desolvation, the hardness and strength are high, and the elongation is low. This project adopts a 600 t tempering process for 2 hours , and precipitates carbides on the martensite grain boundaries and inside the grains. As shown in Figure 3 , by reducing the supersaturation of martensite, the microstructure presents a typical tempering martensitic form.

3 physical performance

1. mechanical properties

The mechanical properties of the produced HP2 13Cr seamless steel pipe are shown in Table 4 .

1. average corrosion rate

Under the condition of CO ? and H 2 S/CO 2 coexistence, the HP2 13Cr seamless steel pipe sample was tested for weight loss by using high temperature and autoclave.

Table 4 Mechanical properties of HP2 13Cr seamless steel pipe

project	tensile strength	Yield Strength	After breaking		Hardness HRC			Impact energy J
	Rm /MPa	R p0.2 /MPa	Elongation /%	1	2	3	4	head	tail
actual	989	888	25	27.2	27.6	27.5	27.3	97	94
ISO 13680 : 2010 requirements	$ 793	$ 758	$ 13		W 32			$ 27

The test conditions and results are shown in Table 5 . The average corrosion rate of the material is calculated by using the mass difference of the samples before and after the test, and each result given is the average value calculated with 3 parallel samples. According to the judgment of NACE RP0775-1991 on the degree of corrosion, the HP2 13Cr seamless steel pipe is slightly corroded under the three simulation test conditions . Figure 4 shows the microscopic corrosion morphology of the sample surface under the three test conditions . Under the conditions of Test 1 , the surface of the sample was even and flat, and no obvious corrosion pits were seen. Under the conditions of test 2 , there are tiny pitting pits on the surface of the sample, indicating that pitting corrosion occurs on the surface of the sample when the temperature rises to 130 t and the concentration of Cl - is 10 X 10 4 mg/L. The electron microscope is measured under the condition of magnification of 1000 times, and the maximum depth of the pitting pit is about 0.8 |±m . According to the calculation method of pitting corrosion and the regulations on the degree of pitting corrosion in the NACE RP0775-1991 standard , the pitting corrosion rate is about 0.04 mm/a , the degree of pitting corrosion is mild corrosion. Test 3 is a corrosion test carried out under the coexistence of H 2 S/CO 2 . The test was carried out at low temperature. After the test, the surface of the sample was even and smooth, with visible grinding traces, indicating that the corrosion product film was relatively thin . The energy spectrum analysis was carried out on the product film under different conditions respectively. Since the corrosion product film is thin, it basically shows the composition of matrix elements.

1. Surface quality and dimensional inspection

the HP2 13Cr seamless steel pipe produced by hot extrusion is smooth and clean, without visible defects; the ultrasonic flaw detection contrast depth is the thickness of the wall

Table 5 High temperature and high pressure corrosion test conditions and results

Test No.	temperature /t	Gas partial pressure /MPa	Ion concentration /fcg-L )	Test cycle /h	Average corrosion rate / fcm* a - )
1	90	PCO2 =5	10X10 4	168	0.004 5
2	130	PCO2 =5	10x10 4	168	0.018 3
3	60	P H2S =0.15 , P CO2=2	5x10 4	168	0.007 8

5% , and the passing rate of one inspection is over 95% . ① 73mmx _
6 for the finished dimensions of 5.51 mm HP2 13Cr seamless steel pipe .
Table 6 ① 73mmx5.51mm HP2 13Cr seamless steel pipe finished size

project	Outer diameter D/mm	Wall thicknessS /mm
measured value	73.2 - 73.5	5.5 - 5.7
ISO 13680 : 2010 Tolerance Requirements	-0.79%D ~ +0.79%D	-5%S - +5%S

4 Conclusion

1. The HP2 13Cr seamless steel pipe produced by the hot extrusion + quenching and tempering heat treatment process meets the requirements of the ISO 13680 : 2010 standard.
2. HP2 13Cr seamless steel pipe after hot extrusion has obvious banded 8- ferrite. After subsequent quenching and tempering heat treatment, 8 -ferrite can be dissolved, and the total amount can be controlled below 1% . Different normalizing temperatures have obvious effects on the morphology of 8 ferrite and grain size, and the normalizing process of 1 050 Yi is used in this project.
3. of HP2 13Cr seamless steel pipe sample was carried out under the condition of CO2 and H2S/CO2 coexistence in high temperature and autoclave, and it was slightly corroded under the condition of high CO2 and H2S .

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