Brief analysis of forming and heat treatment process of P91 tee

With the massive application of P91 steel in domestic thermal power generating units, the demand for P91 pipe fittings (elbows, tees, reducers, etc.) used for pipeline connections is also increasing day by day, due to the special characteristics of P91 material, P91 pipe fittings (especially tees) forming and heat treatment process has been plagued by many pipe manufacturing enterprises. This paper discusses and summarizes the forming and heat treatment of P91 tee from the performance of P91 material, combined with the manufacturing practice of P91 tee of this enterprise for many years.

20220221010224 13437 - Brief analysis of forming and heat treatment process of P91 tee

1. Properties and uses of P91 material

P91 material is a medium-alloy heat-resistant steel, based on 9Cr-1Mo steel after the following improvements and development: the addition of vanadium and niobium elements, while controlling the content of nitrogen elements, its creep strength increased significantly. Subsequently, the carbon content was consciously reduced in consideration of processability. By comparing the physical properties required by design requirements, the thermal conductivity and linear expansion coefficient of P91 with P22 and austenitic stainless steel TP304H, once again proves that P91 is significantly better than TP304H and P22, P91 is better than stainless steel because it has better thermal conductivity and lower linear mean expansion coefficient.
P91 steel room temperature yield limit is two times higher than P22 steel, tensile strength is 41% higher than P22 steel, its advantages: increased flexibility of the piping system, reducing the expansion force, the load of the support hanger is reduced, the end-point thrust and torque is reduced, allowing faster changes in unit load, shorter start-up time, lower investment costs. In thermal power plants, in order to ensure the safe operation of the main steam piping, creep monitoring is performed on each main steam piping with a medium temperature of 500°C and above. The main factors affecting creep include temperature, stress and the steel itself, the higher the temperature, the greater the stress and the faster the creep rate. According to the manufacturer’s test data, the creep strength of P91 steel is almost twice that of P22 steel at 105H and 550°C. At 540 ℃, the permissible stress of P91 steel is about 90% higher than P22 high permissible stress makes the pipe design wall thickness is greatly reduced, the weight of the corresponding pipe design wall thickness is also greatly reduced, the weight is reduced, due to the weight is reduced, making the pipe system support hanger stress level is also greatly reduced, in addition, due to the smaller coefficient of linear expansion, the pipe system thermal stress is also greatly reduced.
Since 1984, P91 steel has been included in ASTM A335 and ASME SA335 standards in the United States, respectively, as the ideal high-pressure boiler steel pipe material for use in the temperature range of 600-650, and is increasingly used in the main steam piping and high-temperature reheat steam piping of subcritical and supercritical thermal power generating units, showing excellent overall performance.
Because of its good performance, the most ideal material for high temperature and high pressure heat-resistant steel used in power stations at present, filling the gap between low-alloy pearlite heat-resistant steel and high-alloy austenitic stainless steel.

2. Material selection requirements of P91 tee

Hot pressing tee raw material steel pipe outside diameter selection requirements: pipe diameter selection than the tee pipe diameter P91 steel pipe, for example, the tee main pipe diameter φ114mm, then use φ168mm seamless steel pipe processing.

Hot pressing tee length material selection requirements.

(1) general-purpose (mainly in the power pipe fittings), the length of the selected material is the diameter of the main pipe + branch height + 30-50mm (processing margin).
(2) special type (oil and gas pipeline), the length of the selected material is:

  • Equal tee main pipe diameter * 1.5.
  • Reducing tee (large reducer) main pipe diameter * 1.25.
  • Reducing tee (small reducer) main pipe diameter * 1.0.

3. Forming process of P91 tee

P91 steel has good hot pressing properties, China’s current production of high-temperature and high-pressure P91 tee, basically using the hot extrusion molding process. The following molding method, to give you an introduction, I hope to give you help and enlightenment.

20220221010734 47759 - Brief analysis of forming and heat treatment process of P91 tee

P91 undercutting using plasma cutting process, to achieve the width of the cut, the cut surface quality, heat-affected zone control, etc., as intended, in order to reduce the hardness of P91, a slower cooling rate was adopted.
P91 critical transformation temperature, depending on the material composition, AC1 temperature between 800 ℃ and 830 ℃, AC3 temperature between 890 ℃ and 940 ℃; according to the provisions of the relevant ASME standards, hot forming temperature shall not be carried out between the two-phase zone (between AC1-AC3), so the hot extrusion temperature shall not be lower than 950 ℃, taking into account the impact of grain size growth at high temperatures, forming Considering the influence of grain growth at high temperature, the molding temperature is generally controlled between 950-1100℃. After years of exploration and production practice, the tee formed under this temperature is tested and verified to be more ideal.
The software to control the flow of metal is the reasonable choice of process parameters, and the hardware is how to design suitable tooling and molds, hot press molding in the solid state of metal is different from metal casting molding in the liquid state of metal, because the fluidity of the two has a great difference, if the shape of the tee mold and the shape of the finished product are exactly the same, it is impossible to make full use of the flow capacity of solid metal, and the result is that the shape does not meet the intended Requirements, especially the transition and branch height, using radial compensation as the main axis of compensation as a supplement to the joint compensation mode molding, in the molding process or after molding not to pull the branch pipe to avoid thinning the branch pipe wall thickness and transition wall thickness, after molding by punching and expanding the branch pipe port way, so that the inner diameter of the branch pipe to meet the requirements, by back pressure branch pipe to thicken the transition zone to meet the design requirements. Key process: hot pressing forming process, the key process parameters are mold form, forming temperature, deformation rate.
The heating of the billet can be done by using an oil-fired heating furnace.
Before the billet is heated, first clean up the chip angle, oil, rust and low melting point metals such as copper and aluminum on the surface of the billet tube with tools such as hammer and grinding wheel, and check whether the marking of the billet meets the design requirements.

4. Post-forming heat treatment of P91 tee

According to the iron-carbon phase diagram curve of P91 material, AC1 temperature between 800℃ and 830℃, AC3 temperature between 890℃ and 940℃; Ms temperature (martensite transformation start temperature) is quite high, about 400℃. mf temperature (martensite transformation end temperature) is above 100℃; P91 material in a relatively wide range of cooling rate, austenite tissue Completely transformed into martensite organization, and this speed range is usually normalized to meet, so the heat treatment process of P91 material is generally normalized + tempering.
Tests show that the smaller the grain size, the better the toughness of P91 steel, so the heat treatment process must be strictly controlled normalizing temperature, a large amount of data to prove that P91 material at 1080 ℃ does not occur in the obvious grain growth, if the normalizing temperature is too high, more than the temperature, the austenite grains will grow, cooling and tempering to get the tempered martensite will also be coarse, so accurate control of the normalizing temperature is to obtain Fine martensite is the key. Therefore, the best normalizing temperature of P91 is in the range of 1040-1080℃, and the normalizing temperature selected by our company is 1060±15℃.
P91 material normalized to obtain the martensite organization, by controlling the normalizing temperature to obtain fine and uniform martensite organization can improve the strength of P91 steel, and according to previous studies and references show that the tempering process within the martensite lath the formation of fine subcrystalline blocks and the formation of sub-stable dislocation network is also the strong toughening effect of P91 steel is not the same, the high temperature tempering process of P91 steel will not cause the organization of The high-temperature tempering process of P91 steel does not cause changes in grain size, but only has an effect on the thickness of the martensite laths within the grain. Moderate tempering temperature and time, the martensite lath does not occur coarsening, the formation of small subcrystalline lath fragmentation, the emergence of dense sub-stable dislocation network, subcrystalline lath and sub-stable dislocation network toughening effect is obvious, the comprehensive mechanical properties of steel excellent. Therefore, tempering temperature and tempering time are the important influencing factors for P91 steel to obtain excellent comprehensive mechanical properties, and controlling the high-temperature tempering process is an effective measure to improve the toughness of P91 steel.
According to our previous product supply experience, P91 tempering temperature of 730-760 ℃ range is the best, holding time of at least 60min, cooling: cooling in still air.
P91 steel should be cooled to below 180℃ after normalizing to ensure complete martensitic transformation. In order to avoid intergranular stress corrosion cracks under hardening conditions, tempering should be carried out immediately after the completion of martensitic transformation process. The heat treatment process can fully meet the performance requirements of the product, high creep fracture strength; limited hardness; good toughness. Tempering temperature is low, the tempering effect is not obvious, the weld metal is easy to aging and embrittlement, tempering temperature is too high (more than AC1 line), the joint can again austenite, and in the subsequent cooling process re-hardening. 1060 ± 15 ℃ temperature normalizing most of the carbide melting and does not occur obvious grain growth. 750 ± 15 ℃ temperature tempering martensite organization in the carbide can be uniform precipitation This improves the creep performance of the material.
P91 thick-walled pipe heat treatment hourly heating rate of 6250 ÷ wall thickness (℃ / h) is appropriate, tempering constant temperature of 8min/mm.

5. Microstructure inspection of P91 tee after heat treatment

Non-metallic inclusions test method according to GB/T10516, piece by piece metallographic inspection. Grain rating method in accordance with GB/T6394, grain size is not coarse than 4, the same furnace grade difference is not more than 3, no microcracking, overburning organization and hardened martensite organization.

6. Precautions for P91 material processing

P91 steel cold cracking tendency, under certain conditions, prone to delayed cracking, so the welded joints must be tempered within 24h after welding.
The best hot press forming temperature of P91 material is 950-1100℃, which prevents the temperature from growing too fast due to high grain size and prevents the product from cracking due to low forming temperature.
P91 post-welding heat treatment before, must be cooled to 100-150 ℃, insulation 1h, tempering temperature 730-780 ℃, insulation time of not less than 1h.

7. Conclusion

In this paper, according to the characteristics of P91 material, combined with the company’s long-term experience in manufacturing P91 pipe fittings, to explore and summarize the data and parameters of the forming and heat treatment process of P91 tee, with a certain degree of applicability. P91 tee is only a small part of the entire P91 pipeline project, in order to give full play to the important role of P91 pipe in the project, you need to support the corresponding links such as P91 welding and post-welding heat treatment to ensure the entire P91 pipeline project.

Source: China P91 Tee Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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