What is P92 steel?
What is P92 steel?
P92 steel is American steel grade ASTM A335 P92, which is a martensitic heat-resistant steel. In recent years, it has been used more in thermal power plants, mainly for main steam pipes and reheat steam pipes. The main alloy composition is Cr: 9%, Mo: 1%, etc. It can be used in ultra-supercritical units, above 600 degrees Celsius.
SA-335 P92 steel is based on SA-335 P91 steel, appropriately reducing the content of Mo element, while adding a certain amount of W, increasing the molybdenum equivalent (Mo+0.5W) of the material from 1% of P91 steel to about 1.5%, the steel also contains a small amount of boron. After the above alloying and improvement, compared with other commonly used heat-resistant steels of the 9% Cr series, its high temperature corrosion and oxidation resistance are similar, but the high temperature strength and creep properties Has been improved.
Executive standard: ASTMA335/A335M ASTMA213/213M
The Development of ASTM A335 P92
In order to improve the heat efficiency of reheating steam pipeline in power plant, the reheating-temperature can be designed as high as 621°C, which is known as the super-critical temperature. The Unite States firstly developed ASTM A335 P92 alloy steel in late 1980’s to meet the heat-resisting requirements of such a high-temperature power unit. The alloy steel P92 is an improved version of ASTM A335 P91 alloy which has deliberate addition of tungsten, vanadium, niobium, decreased amount of molybdenum to fine-tune to ferrite/austenite ratio, as well as carefully controlled amount of boron and nitrogen.
Designated as UNS K92460, ASTM A335 P92 is also known as 9Cr-2W alloy steel. Its equivalent includes ASTM A213 T92, ASME SA335 P92, ASME SA213 T92, as well as EN standard X10CrWMoVNb9-2. The Grade P92 are generally furnished as seamless pipes, tubes, or fittings.
Chemical Requirements of ASTM A335 P92
*V: 0.15–0.25; N: 0.03–0.07; Ni: ≤0.40; Al: ≤0.040; Nb: 0.04-0.09; W: 1.50-2.00; B:0.001-0.006.
Mechanical Properties of ASTM A335 P92
|Tensile Strength, min.||Yield Strength, min.||Elongation %, min.|
*The tensile tests shall be conducted at room temperature.
The Grade P92 of ASTM A335 shall be normalized at 1900°F [1040°C] minimum and tempered at 1350°F [730°C] minimum as a final heat treatment. After heat treatment, the hardness shall not exceed 250 HB/ 265 HV [25 HRC].
Postweld Heat Treatment Requirements for P92/T92 Steels
Seamless steel pipes or tubes manufactured to ASME SA335 P92 or ASME SA213 T92 are widely used in supercritical and ultra-supercritical power plant units. Postweld heat treatment for this material is mandatory. The P92/ T92 steel (UNS K92460, 9Cr-2W) shall be considered P-No. 15E Group 1 according to ASME BPVC Code Case 2179-8.
The postweld heat treatment(PWHT) should be conducted in conformance with ASME BPVC Section I Table PW-39 or ASME BPVC SECTION VIII UCS-56-11.
- Generally, the minimum holding temperature is 1350°C[730°F]. When the nominal weld thickness is ≤ 5 inch[125 mm], the minimum holding time at normal temperature shall be calculated as per 1 hour per inch[2 minutes per mm] with a 30-minutes minimum limit. When the nominal weld thickness is over 5 inch[125 mm], the minimum holding time at normal temperature shall be 5 hours plus 15 min for each additional inch[25 mm] over 5 inch[125 mm].
- If the nominal weld thickness is ≤ 0.5 inch[13 mm], the minimum holding temperature is 1325°F[720°C]. Moreover, for dissimilar metal welds (welds made between P92/ T92 and another alloy steel), if filler metal chromium content is less than 3.0% or if the filler metal is nickel based or austenitic, the minimum holding temperature shall be 1300°F[705°F].
- If the actual chemical composition of the matching filler metal used when making the weld is unknown, the maximum holding temperature shall be 1425°F[775°C]. If the chemical composition of the filler metal is known, the maximum holding temperature can be increased as follows: (a) If Ni+Mn<1.50% but ≥1.0%, the maximum PWHT temperature is 1,450°F (790°C); (b) If Ni+Mn<1.0%, the maximum PWHT temperature is 1,470°F (800°C). Since the lower transformation temperature for matching filler metal is affected by alloy content, primarily the total of Ni+Mn, the maximum holding temperature has been set to avoid heat treatment in the intercritical zone.
- During PWHT process, if a portion of the P92/ T92 steel pipe is heated above the heat-treatment temperature allowed above, the base metal (pipes) must be entirely re-normalized and tempered. The weld metal shall be removed and replaced.
Typical Physical Properties of P92/ T92 Steel
The P92/ T92 seamless pipes or tubes manufactured to either ASME SA335 or ASME SA213 are widely applied in modern ultra-supercritical power plant units to improve heat efficiency. According to Metals-Piping’s engineering practice, it is most applicable to the steam-temperature of 580°C~600°C[1080°F~1110°F] and metal-temperature of 600°C~620°C[1110°F~1150°F]. The density of P92/ T92 steel is 7850 kgs/m3. The other main physical properties are listed below:
|Temperature||Elastic Modulus||Ave. Expansion Coefficient||Specific Heat|
Allowable Stress, Tensile, Yield – Temperature Chart
The P92 steel, also known as 9Cr-2W, is designated as UNS K92460. It may be used for the construction of ASME BPVC Section I and Section III, Division 1 according Code Case 2179-8. The P92 steel can be furnished in different piping product forms in conformance with relative standards which are listed in below table:
|Product Form||Standard/ Grade|
|Flanges, Fittings, Valves||ASTM A182 F92, ASME SA182 F92|
|Forged & Bored Pipes||ASTM A369 FP92, ASME SA369 FP92|
|Forgings||ASTM A336 F92|
|Plates||ASTM A1017 Gr.92|
|Seamless Pipes||ASTM A335 P92, ASME SA335 P92|
|Seamless Tubes||ASTM A213 T92, ASME SA213 T92|
*The material shall conform to one of the standard specifications listed above.
Allowable Stress – Temperature Chart for P92 Steel
|Temperature °F *A||Max. Allowable Stress, ksi||Temperature °C *A||Max. Allowable Stress, MPa|
- *A: for metal temperature not exceeding.
- *B: These stress values are obtained from time-dependent properties.
- *C: The maximum use temperature of P92 steel is 649°C. The value at 650°C is provided for interpolation purposes only.
Tensile Strength – Temperature Chart for P92 Steel
|Temperature °F *A||Min. Tensile Strength, ksi||Temperature °C *A||Min. Tensile Strength, MPa|
- *A: for metal temperature not exceeding.
Yield Strength – Temperature Chart for P92 Steel
|Temperature °F *A||Min. Yield Strength, ksi||Temperature °C *A||Min. Yield Strength, MPa|
- *A: for metal temperature not exceeding.
Tensile & Bending Process Analysis of P92 Alloy Steel
The Tensile Test Results for ASTM A335 P92 Pipe Bends
The mechanical properties of the four pieces of ASTM A335 P92 pipe bends shall be tested and reviewed. It includes tensile strength, yield strength, elongation, as well as the impact test value. Tensile tests were conducted at room temperature and elevated temperature on both longitudinal(“L”) and transverse directions(“T”). The specimens were taken respectively from the ASTM A335 P92 raw pipe, bend-starting area, bend-ending area, outer-back-arc area of the finished pipe bends. In below 4 tables, “Y.S” stands for yield strength with the unit “MPa”, “T.S” stands for tensile strength with the unit “MPa”, “EL.” stands for elongation with the unit “%”, “Akv” stands for the average energy absorption in impact test with an unit of “J”. Besides, “RM”: raw material, “BS”: bend-starting area, “BE”: bend-ending area, “BA”: bending-back-arc area, “R.T”: room temperature, “E.T”: elevated temperature.
- *The tensile test results for item B1 of P92 pipe bend.
- *The tensile test results for item B2 of P92 pipe bend.
- *The tensile test results for item B3 of P92 pipe bend.
- *The tensile test results for item B4 of P92 pipe bends.
Analysis & Conclusion: Optimum Working Conditions for P92/ T92 Bending
- All the finished ASTM A335 P92 pipe bends conform to chemical composition requirements, mechanical properties requirements at both room & elevated temperatures.
- Corresponding to its lower yield & tensile strength, the raw-pipe hardness of item B4 is also lower than the other 3 pieces of raw pipe. This verifies that in non-destructive test hardness is an important parameter to evaluate the strength.
- The wall thickness deduction rate after bending is generally okay for all 4 pipes which is negatively correlated to the ratio of bend-centerline-radius to outside diameter of the pipe. The hardness values of finished products in different areas conform to the standard. The microstructure of P92 raw material and finished products are in conformance. No intergranular cracking exists.
- Among the 4 different portfolios of working-temperature and bending rate(velocity), item B4 has the most uniform strength distribution in different area of finished pipe bends.
- Due to the relatively faster bending velocity, the yield strength at room temperature increased while its yield ration decreased. It means faster bending velocity may result in negative effect on the P92/ T92 steel.
- For the manufacture of ASTM A335 P92 or ASTM A213 T92 pipe bends, the optimum working temperature should be 920°C±10°C while the best bending rate(velocity) shall be 10~15 mm per minute.
The Production Process of ASTM A335 P92 Pipe Bends
For 600 MW and 1000 MW power plant electric-unit, the P92 alloy steel is widely applied in super-critical main steam line or reheating steam line. It can be furnished in accordance with ASTM A335 P92, ASME SA335 P92 or its EN equivalent X10CrWMoVNb9-2. Many piping structures of the high-temperature steam line require 3D mid-frequency hot-induction pipe bends made of ASTM A335 P92 materials. The bending quality, microstructure, as well as the reliability of the finished pipe bends mainly depend on the manufacture process control. Metals-Piping has supplied 4 pieces of R=3D ASTM A335 P92 bend to a power plant project in South China. All the four pipe bends are manufactured by mid-frequency hot-induction bending machine. Although all the fittings are tested okay, due to some fine adjustment of the manufacturing processes & conditions, the finished P92 pipe bends have distinctive difference in mechanical properties.
The Specification of ASTM A335 P92 Pipe Bends Required
|Item No.||I.D x W.T||Center-Line-Radius||Angle||CLR/O.D Ratio|
- *All values are in mm unit unless otherwise specified.
The Bending Process of P92 Pipe Bends
Generally, the hot-forming temperature for ASTM A335 P92 or ASTM A213 T92 should be 850~1100°C [1560~2010°F]. Its forging temperature should be 950~1100°C [1740~2010°C] while the bending or extruding process should be conducted at 850°C~950°C [1560~1740°C].
|Item No.||Heating-Temperature, °C||Bending Rate, mm/min||Cooling Method|
- *The allowable heating-temperature variation for B3 can be ±10°C in practice.
Heat Treatment after Hot-Bending
|B1||1050~1070°C, holding for 90 minutes.||750~770°C, holding for 150 minutes.|
|B2||1050~1070°C, holding for 90 minutes.||750~770°C, holding for 150 minutes.|
|B3||1050~1070°C, holding for 90 minutes.||750~770°C, holding for 150 minutes.|
|B4||1050~1070°C, holding for 90 minutes.||750~770°C, holding for 150 minutes.|
- *All the P92 pipe bends take the same heat treatment after its bending process(N+T).
Actual Composition, Hardness & Microstructure of Raw Pipes
Before the bending process, PMI has been conducted to determine the actual chemical composition & microstructure of the raw materials: 4 pieces of P92 seamless pipe. According to ASTM A335(ASME SA335), the maximum hardness of grade P92 is 250 HB. The hardness of the raw pipes are respectively 201 HB for B1, 201 HB for B2, 200~206 HB for B3, 180~185 HB for B4. The typical microstructure is tempered martensite with small amount of δ-Fe(≤5%). Besides the chemical composition are listed below:
- *All values are actually tested.
Inspection and Test for Finished Products
|W.T Reduction Rate, %||/||≤10.7||≤10.9||≤9.1||≤8.3|
|Corrugation Rate, %||≤2.0||0||0||0.6||0|
|Hardness, HB||E.A*||180 HB ~ 250 HB||210~229||215~234||187~202||213~220|
- *The above table shows the test results for the four finished A335 P92 pipe bends.
- *MT:Magnetic Particles Test; *E.A/ I.A: the hardness value at the external/internal arc of the bend;
- *M.L: the hardness value at the mid-layer of the pipe bend;
- *STD: the standard value specified by relative Code.
Source: China P92 Pipe Bend Manufacturer – Yaang Pipe Industry (www.epowermetals.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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