What is P91 steel

What is P91 steel?

P91 steel is a 9% Cr-1% Mo low-carbon martensitic steel micro-alloyed with vanadium and niobium elements and controlled nitrogen content, which has excellent high-temperature strength and creep properties in the temperature range of 560℃~610℃, as well as good corrosion resistance, oxidation resistance and high-pressure hydrogen resistance, and is widely used in the superheater and reheater pipes in power plant boilers, and distillation, cracking units and refineries in the construction of high-temperature, high-pressure, hydrogen-proof equipment.

20220221025413 96488 - What is P91 steel
P91 steel is equivalent to the national standard 10Cr9Mo1VNbN, which not only has high oxidation resistance and high temperature steam corrosion resistance, but also has good impact toughness and high and stable lasting plasticity and thermal strength properties. In the use of temperature below 620 ℃, the allowable stress is higher than austenitic stainless steel. Above 550 ℃, the recommended design allowable stress is about twice as much as T9 and 2.25 times Cr-1Mo steel.
P91 is a martensitic heat-resistant steel, its organization is at the boundary of M-F, he has higher thermal strength, thermal stability and corrosion resistance than SA335-P22. P91 not only has high oxidation resistance and high temperature steam corrosion resistance, but also has good impact toughness and high and stable lasting plasticity and thermal strength properties. In the use of temperature below 620 ℃. P91 can be used as subcritical, supercritical boiler wall temperature ≤ 625 ℃ high-temperature superheater, reheater with steel pipe, as well as wall temperature ≤ 600 ℃ high-temperature collector and steam piping, but also as a nuclear power heat exchanger and oil cracking device furnace pipe.
P91 steel has high temperature enduring strength and creep fracture resistance, and T22 (10r910) steel compared to the same temperature and pressure conditions, the pipe wall thickness reduced by 50%; compared with austenitic steel, the coefficient of expansion is smaller, good thermal conductivity, thermal cracking tendency is small, the price is relatively cheap, making P91 steel become a high-temperature superheater joint box, the main steam pipe and other high temperature, high-pressure pipeline of choice and Alternative steel. Whether the use of performance, or economic, P91 has shown its superiority, and its use is increasing.

Chemical composition of SA335-P91 steel

SA335-P91 steel chemical composition, as can be seen from Table 1, the alloy content is high, Cr is resistant to high-temperature oxidation corrosion and improve the thermal strength of the alloying elements; control in the content at ≥ 8%, while adding the elements of vanadium and niobium, and control the content of nitrogen elements. Carbon elements in the main formation of carbide or carbon nitride, the use of dispersion strengthening and martensite strengthening to improve strength, consider the processing process performance and consciously reduce the content of carbon elements.
Table 1 P91 steel’s main chemical composition (Wt%)

Chemical composition C Mn P S Si Cr Mo V Nb N Ni
Min 0.08 0.3 0.2 8 0.85 0.18 0.06 0.03
Max 0.12 0.6 0.02 0.01 0.5 9.5 1.05 0.25 0.1 0.07 0.4

P91 steel has been included in the ASEM , JIS and GB standards. P91 steel, although weldability is better than F11, F12 steel, but its hardening tendency is still large, prone to reheat cracking and cold cracking.
Cold cracking is generated by three factors: First, P91 steel is air-cooled martensitic steel, which has a greater susceptibility to cold cracking in the organization; second, in the martensitic transformation after welding, hydrogen remains in the martensite in a supersaturated state, prompting further embrittlement in the region; third, due to the martensitic phase transformation after welding, so that the tissue stress at the joint increases. The combined effect of these three factors makes P91 steel more sensitive to cold cracking. Therefore, P91 steel should be welded with appropriate measures.
SA335-P91 pipe and SA335-P91 pipe fittings should be retested upon arrival as follows:

  • 1. According to Table 1 for 100% spectral re-inspection, the content of alloying elements should meet the composition requirements.
  • 2. Review the material certificate of the corresponding batch of pipes and fittings, mechanical properties indicators should be in line with the standard. Review the heat treatment process in the supply state according to the ASME II volume A iron-based materials in SA-335/SA-335M, with a positive goods temperature of not less than 1040°C and a tempering temperature of not less than 730°C, and check the metallographic organization photos in the material certificate to see if the tempered martensite organization.
  • 3. The pipe and fittings for hardness recheck, the hardness value should be about 220HB, and ≤ 250HB, strict control of Brinell hardness of not less than 180HB.
  • 4. 100% visual inspection of pipes and fittings, including diameter, wall thickness review, appearance quality, whether there are scars, blemishes, scratches and other problems.

The main properties of P91 steel

P91 steel is an improved 9r-1 steel, which is based on 9r-1 steel by adding v, nb, n and other alloying elements, using purification, fine crystallization metallurgical technology. As well as micro-alloying and controlled rolling, controlled cooling and other processes. Developed a new generation of medium alloy heat-resistant martensitic steel. Its yield strength at room temperature σs ≥ 415pa, tensile strength σb ≥ 585pa, δ ≥ 20 shrinkage at break, hardness hb ≤ 250. t91/P91 steel in normalized and tempered by 730 ~ 760 ℃ after heat treatment, the metallographic organization is typical of the martensite skeleton structure, resulting in 236 chromium carbide precipitation in the edge of the martensite skeleton, and the formation of x-shaped v/nb carbide. After the coarser 236 carbides and the internal finer precipitates are converted into fine foils, a larger dislocation density within the sub-particles is found, and this fine sub-grain structure with high dislocation density is the determining factor for the high-temperature creep strength of t91/P91. Fang Juan, Zhou Aijun through the butt weld, using argon arc welding + submerged welding (gta + sa), by 732 ~ 766 ℃, holding time 240in heat treatment to obtain σb ≥ 660pa, weld Vickers hardness hv10 ≥ 183.

Welding performance of T91 / P91 steel

With the rapid development of the power industry, high parameters, high-capacity units continue to emerge, the high-temperature creep properties of steel materials and stress corrosion resistance and other properties: higher requirements. To this end, the world’s major industrial countries have conducted a lot of research, has developed a series of new ferrosome type heat-resistant steel, and successfully used in large-capacity thermal power generating units, including high CT-type 9Cr1MoVNbN heat-resistant steel is T91/P91 steel.
T91/P91 steel is widely used in high-parameter thermal power generating units because of the following advantages of the steel’s performance.

  • ① Compared with stainless steel, the steel has a low coefficient of thermal expansion and good thermal conductivity.
  • ② The steel has a high room temperature tensile strength, δb up to 770MPa, and also good plasticity.
  • ③ The impact toughness and material brittle transition temperature of this steel are significantly better than those of similar X20 and EMl2 steels
  • ④ The steel has higher high temperature enduring strength and allowable stress, the high temperature enduring strength after 105h operation at 550℃ is twice that of T22 steel, and the allowable stress within 540-610℃ is significantly higher than that of T22, TP304H and X20 steel.
  • ⑤ The steel has good bending processing performance of the whole tube.
  • ⑥ The steel’s high temperature fatigue performance is better than T22 and TP304H steel, high temperature oxidation resistance is also much higher than T22 steel.

Welding process points of T91 / P91 steel

Power station boiler superheater and reheater with T91 steel pipe commonly used specifications such as ψ54mm and ψ57mm, wall thickness of 4mm, 8mm and 9mm and several other. Large diameter main steam P91 steel pipe with ψ457mm × 45mm, ψ347mm × 46mm and many other specifications. Small diameter pipe site construction is mostly horizontal butt fixed position, requiring single-sided welding double-sided forming, and the reverse weld is required not to be oxidized. In order to make the joint with satisfactory performance, the early welding process mainly uses two methods: one is full argon arc welding (TIG priming TIG filler); the second is argon arc welding (TIG) priming welding rod arc welding filler. The main points of the welding process are as follows.

  • (1) The use of the correct bevel shape and size.
  • (2) The use of reasonable welding materials (TIG wire and filler electrode).
  • (3) Select the correct welding specification (including wire and electrode grade and diameter, tungsten electrode diameter, welding current, argon gas flow, power supply polarity, the number of layers of the weld and the number of passes, etc.).
  • (4) Select the correct preheating temperature of the workpiece, the interlayer temperature of the weld.
  • (5) The choice of reasonable post-weld heat treatment specifications (including the rate of temperature rise and fall, tempering temperature and holding time, etc.).
  • (6) Adhere to the correct joint assembly positioning welding and skilled, highly skilled manual operation skills, etc.

After the welding method and welding materials to determine the key process measures to obtain high-quality joints are: preheating before welding, control layer temperature, and “timely and effective” post-weld heat treatment. The essence of preheating before welding is to control the behavior of hydrogen in the weld area, reduce the hardness and stress peak in the weld area and improve toughness. Control the interlayer temperature on the one hand to prevent the weld from overheating; on the other hand, to maintain slow cooling. Timely and effective post-weld heat treatment contains two layers of meaning: First, the process can indeed play a role in improving the weld and its HAZ organization, improve the toughness and high-temperature lasting strength of the joint and eliminate internal stresses in the weld; Second, in the implementation of the process, the minimum temperature of the weld must be strictly controlled after the weld cooling, as well as the time interval to the post-weld heat treatment. The latter is essential for obtaining excellent joint performance.
(A) The construction process flow is shown below.

Pre-alignment inspection → Set argon filling device → Alignment inspection → Preheating before spotting → Preheating before spotting → Argon filling → Argon arc priming → Interlayer welding → Cover welding → Post-welding self-inspection, special inspection → Heat treatment → Metal laboratory testing.

(B) pre-welding preparation and construction operation points.
1. SA335-P91 steel welding process using: GTAW + SMAW.
2. Welding equipment: ZX7-400ST inverter welding machine.
3. Welding materials: welding wire using WCrMO91 (model ER90S-B9 specifications Ф2.4mm), welding rod using CROMOCORD9M (model E9015-B9 specifications Ф3.2mm, Ф4.0 mm).
4. Check before mating: bevel size and mating gap should be in line with the “Technical Specification for Welding of Thermal Power Plants” (DL/T869-2004).
5. Bevel processing: bevel using mechanical processing, bevel should be smooth and uniform, the size of the requirements, the need to carefully check before the counterpart, found unqualified must be repaired with tools such as grinders to qualified. Clean the bevel inside and outside the wall of 15-20mm range of rust, oil and other dirt, until revealing the metal luster.
6. Preheating before welding: the use of computer temperature control equipment, the weld port to track preheating, thermocouple symmetrical arrangement, thermocouple and pipe fittings should be in good contact, argon arc welding preheating temperature of 100-150 ℃, welding rod arc welding preheating 200-300 ℃, preheating width from the center of the bevel on each side of not less than 3 times the wall thickness of the pipe calculation.
7. Welding in accordance with the welding process parameters; bottoming layer welding, when the preheating temperature reaches the specified temperature and uniformity, the wire selection ER90S-B9 specifications Ф2.4mm using DC positive connection by two people symmetrical welding, the first and second layer of the weld are used argon arc welding. When priming layer welding, using the internal wire feeding method, pay attention to the root fusion is good, the thickness control in the range of 2.8-3.2mm.
8. Argon arc welding bottoming process, with a spotlight flashlight to carefully check the root weld, to ensure that there is no with unseeable defects, bottoming is completed and visually inspected and qualified, immediately after the next layer of welding. To prevent the root weld metal oxidation, should be filled with argon protection in the main steam pipe.
9. Argon-filled protection can be carried out with reference to the following requirements.

  • (1) Argon-filled device tools as shown in Figure 1. Can be first before the counterpart, in the weld mouth on each side of the use of sticky two layers of water-soluble paper, weld mouth gap with high-temperature tape glued firmly, argon can be used to fill φ6mm × 1.5mm copper tube, one end of the copper tube processed into a width of 8mm, the thickness of 3mm flat body, and then φ1mm drill bit in the above 4-6 small holes to ensure that the gas flow is uniform when filling argon. Then it will be inserted into the weld bevel to fill argon.
  • (2) Root layer and near root layer welding, the tube must be filled with argon protection, generally should last more than 2 layers.
  • (3) With a lighted lighter or match placed near the gap to the weld, when the flame goes out, indicating that the internal air has been emptied, can be bottomed argon arc welding.

(C) Argon arc welding priming is completed, the preheating temperature to 200-300 ℃, the beginning of interlayer and cover welding using electrode arc welding (SMAW) method; electrode selection E9015-B9 φ3.2mm, φ4.0mm.
1. Using the DC reverse method, two people symmetrical welding. As P91 steel welding, the molten pool of iron viscosity, poor mobility, welding specifications and small, and therefore prone to slag, interlayer defects such as failure to fuse, so to avoid large defects and to ensure the comprehensive mechanical properties of the welded joint, must be multi-layer multi-channel.
2. The welding process should be staggered 10-15mm for each layer of welded joints, while paying attention to welding as smooth as possible to facilitate interlayer slag cleaning and avoid dead ends.
3. After each layer of welding, the slag must be cleaned up, especially the slag in the groove on both sides, the application of polishers or wire brushes, etc. will be cleaned up slag, spatter and other debris, after passing the self-inspection, before welding the next layer.
4. The welding process, carefully observe the melting state, special attention should be paid to the quality of the welded joint and the closing arc, the closing arc should be filled with molten pool to avoid to avoid the emergence of arc pit cracks.
5. Welding seam appearance welding is completed, the welders are required to immediately clean up self-inspection, found that the appearance of poorly formed immediately fill welding, is strictly prohibited in the weld after cooling and then directly fill welding.
6. In order to control the interlayer temperature, each layer can be intermittent welding, if necessary, can increase the residence time of each layer, welding mouth should be equipped with monitoring temperature device.

  • (1) Self-test after welding: ①. After the weld is completed, the weld surface should be promptly cleaned of slag, spatter, etc., to exceed the standard appearance defects polished, patch welding, patch welding process requirements and welding the same, and in the same position in the number of digging and patching generally shall not exceed twice.
  • (2) After passing the self-inspection should fill out the welding self-inspection record sheet in a timely manner to facilitate the next process.

(D) Post-weld heat treatment.

  • 1. Post-weld heat treatment and post-heat treatment heating methods, heating range, insulation requirements and temperature measurement requirements in accordance with the “Thermal Power Plant Welding Heat Treatment Technical Regulations” (DL/T819-2002) relevant provisions.
  • 2. For SA335-P91 steel, in the welding process was forced to stop or after welding failed to timely heat treatment, should be in the temperature range of 200-300 ℃ for 10min before self-cooling in insulation. Re-welding, in accordance with the original process requirements for welding.
  • 3. P91 steel preheating temperature: argon arc welding priming preheating temperature of 100-150 ℃, electric welding cover preheating temperature of 200 ℃ -300 ℃. Interlayer temperature: 200 ℃ -250 ℃.
  • 4. Post-welding heat treatment temperature: 760±10℃, constant temperature time of 3-4 hours, lifting temperature speed of 140℃/h.
  • 5. Heating width according to DL/T-819-2002 regulation requirements.
  • 6. Insulation materials, such as aluminum silicate, asbestos cloth, mineral sand cotton (or asbestos rope), etc. by the heat treatment personnel according to the actual situation selection (remember not to affect the work).

(E) SA335-P91 steel post-weld inspection; weld quality inspection weld quality according to the acceptance standards for Class I welds, inspection content and requirements are.

  • 1. Appearance inspection according to DL/T869-2004 “Thermal Power Plant Welding Technology Regulations”, the quality standard acceptance of the appearance of Class I welds.
  • 2. Nondestructive inspection according to JB4730 on the weld 100% ultrasonic (UT), magnetic particle (MT) flaw detection, Class I qualified.
  • 3. Hardness, after the completion of heat treatment, do 100% hardness determination, hardness value less than 350HB.
  • 4. Spectrum, the weld metal alloy composition for 100% spectral analysis review.

Welding, heat treatment process selection and control of SA335-P91 steel

1. Determine the preheating temperature and interlayer temperature

P91 steel alloy content is high, there is a strong tendency to harden when welding, cold cracking sensitivity. Plus P91 steel for large diameter, thick-walled tubes, structural rigidity and constraint stress, and after welding cooling at the joint tissue stress also increases the tendency to cold cracking, so the preheating temperature should be strictly controlled before welding.
According to GB9446-88 pin cold cracking test to determine the preheating temperature, a total of four groups of 16 specimens, number 1, 2 for the full argon arc welding, specimen number 3, 4 for the full manual stick welding; specimen load to maintain 36 h. Test results are shown in Table 3.
The test data can be determined by the preheating temperature should be more than 200 degrees Celsius, due to the higher heat input of argon arc welding, and low hydrogen content of the wire, argon arc welding can be controlled at 180 degrees Celsius or more. At the same time preheating can extend the cooling time, reduce the martensite transformation of the tissue stress, help the hydrogen overflow. Martensite phase transformation temperature between 380-400 ℃, martensite transformation end temperature at about 100 ℃, the interlayer temperature control between 200-300 ℃, so control the interlayer temperature between 200-300 ℃, the purpose of the welding process is conducive to part of the martensite transformation began.

2. Control the line energy, the development of welding process parameters

Welding should be multilayer row channel welding, strict control of line energy to avoid the occurrence of cracking (type IV cracking) in the HAZ fine grain area adjacent to the base material, through the line energy formula, it can be explained that the control of welding heat input to take the welding current and welding speed into account, the general P91 steel welding heat input control at 19-22KJ/cm, refer to the recommended welding current voltage of the welding material manufactured by Manster, the development of P91 steel welding process as shown:

Q = 60UI/V

In Formula:

  • Q – welding line energy, (J/cm).
  • I-welding current, (A).
  • U-welding voltage, (V).
  • V – welding speed, (cm/min).

3. The choice of welding materials

Determination of alloying elements, analysis of the impact of each alloying element on the weld. Due to the different design concepts of welding material manufacturers welding material, the same type of welding material and different manufacturers of the alloy element content of the ratio of deviations. Therefore, the choice of welding materials, in order to make the weld and the base material has equivalent high temperature creep resistance and lasting strength, try to ensure that the alloying elements close to the base material composition, but also to take into account the T / P91 alloy steel Mn, Ni, N, Nb and other elements of the ratio is reasonable, so as to choose a more appropriate welding material.
(1) Mn and Ni element content

Mn and Ni elements of the content of the appropriate more than the upper limit specified in the parent material, can promote the weld deoxidation, improve the impact toughness of the weld, to ensure the quality of the weld. However, the combination of Mn and Ni tends to lower the low-temperature transformation temperature Ac1 of the weld metal, therefore, according to ASME II Volume C requirements Mn + Ni content should be controlled less than 1.5% to avoid post-weld heat treatment temperature close to the Ac1 temperature, which may cause incomplete transformation of the microstructure and re-transformation to austenite. For example, Manchette (METRODE) P91 welding material E9015-B9, the content of Mn + Ni is indicated and less than 1.5%. See Table 2.

20220221025826 48348 - What is P91 steel

Table.2 E9015-B9 alloying elements

C Mn P S Si Cr Mo V Nb N Ni Mn+Ni
0.09 0.49 0.003 0.009 0.24 9.03 0.99 0.21 0.05 0.04 0.06 1.09

(2) Chromium equivalent control
By controlling the amount of chromium equivalent, you can ensure that the P91 steel weld metal to get a relatively single tempered martensite organization, chromium equivalent formula can be referred to the U.S. CE company in Newhuose et al. in the work based on the improvement proposed.
Creq = Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb + 9Ti + 12Al-40C-30N-4Ni-2Mn-1Cu
When Creq≥12, δ-Fe will appear and the content increases with the increase of Creq value.
The δ-Fe organization in the weld will significantly reduce the high-temperature creep strength and impact toughness of the metallic material.
When 10<Creq<12, the steel organization cannot be simply predicted, but its chemical composition can be used as a reference.
When Creq≤10, δ-Fe does not appear in the organization.
(3) The influence of other alloying elements
P91 pipes are mainly used in boiler steam pipes and heating furnace pipes, etc., so we should strictly control the C content, which can be slightly lower than the base material. nb, B content has a negative impact on toughness, but can improve creep strength, so it can be controlled at the lower limit of the base material elements. n element improves yield strength and tensile strength by forming nitride, but it will reduce plasticity and toughness, control nitrogen element about 0.04% N is the lower limit of the specified value of the base material element. For the element Si, the appropriate reduction is conducive to the improvement of the weld metal toughness, so the selection in the welding material, to consider its content slightly lower than the parent material specified value, not higher than 0.3%.

4. The control of the welding process

  • (1) P91 welding construction, should be selected to a higher level of technical skills, strong work responsibility of the welders to serve, welding inspection of the radiographic film level requirements to improve the passing grade is also appropriate to reduce the defects of welding, to avoid stress concentration.
  • (2) The welding process, argon arc welding to be filled with argon protection, root protection argon flow is generally controlled at 10L/min-15L/min, in the actual process, can be adjusted according to the needs of the welder, priming the last sealing welding, argon flow should be adjusted appropriately low, to be welded at least two layers (including priming layer) above, and then close the root protection argon gas. For large pipe diameter fixed port welding, root argon protection is more difficult. Available small pipe head at the bevel to fill argon. To avoid preheating and interlayer temperature caused by water-soluble paper rupture or fall off, you can add a layer of water-soluble paper at a distance of 200-300mm from the heating pad and seal it with two layers of water-soluble paper. During the welding process, test the size of the argon gas flow at the weld mouth by hand in order to adjust the argon filling measures.
  • (3) Each weld port as far as possible to complete continuous welding, such as forced to interrupt, must be cooled to about 100 ℃ constant temperature, the martensite for complete transformation. Continue welding, should be reheated to the preheating temperature. The welding interval should not be too long, otherwise the completed part of the weld will have to be heat treated.

5. The heat treatment process to determine

(1) The timing of heat treatment.
P91 steel welding should be completed at 100 ℃ (martensite transformation end temperature Mf) stay about 1h, try to make the complete transformation of martensite, heat treatment immediately after completion. For the construction site applied thick-walled forged or cast flanges, pipe fittings and valves, etc., after the completion of welding, should be controlled at 90-100 ℃, and not less than 90 ℃, insulation for about 1h, directly after the heat treatment to avoid cracking.
(2) The determination of the heat treatment temperature.
Martensite low-temperature transformation temperature of 800-830 ℃. Tempering temperature on P91 tensile strength and yield, strength is not significant, only in close to its AC1 point, tensile strength and yield strength occurred significantly decreased. Therefore the heat treatment temperature should be lower than the low-temperature phase transition temperature Ac1. In the actual application of the field system, the temperature of heat treatment differs slightly due to the different manufacturing philosophies of the selected welding consumables manufacturers. Therefore the heat treatment temperature should be verified by referring to the material certificate of the welding rod and by welding process evaluation. Petrochemical cracking furnace project, the selected welding material manufacturer for the Manster manufacturing E9015-B9, the material certificate recommended heat treatment temperature of 755 ℃, constant temperature of 3h.
For Cr-Mo steel, the tempering parameters [P] range of about 19.5-22, strictly speaking, each Cr-Mo alloy steel has an optimal range of tempering parameters, while the relatively reasonable tempering parameters of P91 steel [P] = 21. According to the tempering parameters [P] formula, the heat treatment temperature and time in the manual of E9015-B9 made by Manster It is verified that [P] is in the optimal range of tempering parameters.


In formula:

  • [P] – tempering parameters.
  • T-heating temperature, K.
  • t – holding time, h.

(3) Reasonable monitoring of heat treatment temperature and control of temperature uniformity.
To ensure that the heat treatment process, the heat treatment temperature of the pipe is uniform, the heating band and thermocouple should be reasonably arranged. Due to the influence of convection, the temperature on the upper half of the vertical pipe heating belt is higher than the lower half, and the temperature on the horizontal pipe is highest at 12:00 and lowest at 6:00. Therefore, thermocouples must be installed on the upper half side of the vertical pipe heating and on the 12:00 position of the horizontal pipe to monitor the temperature at the highest point, which exceeds the allowable temperature. At the same time, the temperature uniformity can be improved by moving the insulation wool or heating tape down, increasing internal insulation, increasing the thickness of insulation at weak locations, and controlling the location of the heating tape tying port. Note that when the thermocouple arrangement, a small piece of insulation cotton should be used to isolate the thermocouple from the heating piece, the purpose is to monitor the temperature of the area to be monitored to avoid the temperature of the heater.
(4) The hardness check after the completion of heat treatment.
P91 steel heat treatment is completed, in order to check the quality of heat treatment, to carry out 100% hardness testing. Its hardness depends on the one hand on the actual carbon content of the base material and alloy composition, on the other hand depends on the temperature specification and cooling conditions of the welding and post-welding heat treatment. Attention should be paid to the heat treatment process at the highest temperature and lower hardness value inspection and comparison, the hardness value of the weld can not be lower than the base material, controlled at about 220-230, more ideal. However, the hardness test is one-sided, for example, the temperature of heat treatment exceeds the low temperature transition temperature, the hardness will be high; heat treatment temperature is not to heat treatment requirements temperature hardness value will also be high. Therefore, the heat treatment curve should be combined with the hardness test value to determine the quality of heat treatment, such as hardness value exceeds the standard, according to the actual situation to take a cut to re-weld or re-treatment.
For the above measures, the process evaluation of the verification, pipe for the German Mannesmann Steel SA335-P91, welding materials for the Manster manufacturing 9MV-N / 9CRMOV-N. The results are tensile strength and yield strength in line with ASME requirements, the average room temperature impact work reached 73J, metallographic analysis of tempered martensite.

What is the difference between T91 steel and P91 steel?

P91 alloy pipe and T91 alloy steel pipe are not a kind of steel pipe. The composition structure and production process of the two kinds of steel pipes are different.
1. Refers to different

  • P91 steel: in order to fill the pearlite heat-resistant steel and austenitic heat-resistant steel between 600-650 ℃ temperature area used in the new steam pipe steel.
  • T91 steel: is a new martensitic heat-resistant steel developed in cooperation with the U.S. National Laboratory and American Combustion Engineering Corporation Metallurgical Materials Laboratory.

2. The characteristics of different

  • P91 steel: P91 steel in 650 ℃ below, the allowable stress of all temperatures are higher than P22 steel. In the design of the pipe, the size of the permissible stress directly affects the choice of pipe wall thickness.
  • T91 steel: is based on 9Cr1MoV steel to reduce the carbon content, strictly limit the content of sulfur, phosphorus, add a small amount of vanadium, niobium elements for alloying.

3. The use of different

  • P91 steel: better chemical composition, mechanical properties, high temperature performance, processing performance, good welding performance. Due to these advantages of P91 steel, in the sub-critical unit, the main steam pipe using P91 steel instead of P22 steel, can save engineering investment, but also to the design, installation convenience.
  • T91 steel: 91 steel in each alloying element to play a solid solution strengthening, dispersion strengthening and improve the steel resistance to oxidation, corrosion resistance.

Application of P91 steel

In previous projects, the main steam pipeline of 125MW and 200MW units of ultra-high pressure were selected from 10CrMo910 steel; from the late 1970s to the 1980s, sub-critical 300MW and 600MW units were introduced, whose main steam pipeline was selected from A335 P22 steel, both of which have comparable performance, they are pearlitic heat-resistant steel, with a maximum working temperature of 580- 590, the temperature is higher only austenitic heat-resistant steel, austenitic steel maximum working temperature of 700, but the thermal expansion of this steel is high, sensitive to stress corrosion, short life of dissimilar steel joints, not suitable for high temperature, high pressure steam pipeline.
P91 steel with its good high temperature lasting strength, thermal stability and high temperature creep resistance and other comprehensive performance in the power station boiler superheater, reheater and main steam piping to obtain more and more extensive applications.

Source: China P91 Steel Pipe 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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