Knowledge of ASTM A213 T91 seamless steel

▶Introduction of steel types
T91 seamless steel pipe is a ferritic heat-resistant steel. It was researched and developed by the American Oak Ridge National Laboratory in the 1970s and was listed in ASME in 1983. In 1995, my country included the steel grade in the GB 5310 standard, and the grade was set to 10Cr9Mo1VNb.
 
T91 seamless steel tube steel is a new generation of medium alloy heat-resistant steel developed on the basis of 9Cr1Mo (T9) steel, using purification, fine-grained metallurgy technology, micro-alloying and controlled rolling, controlled cooling and other processes. The steel has good comprehensive mechanical properties, the structure is stable after high temperature aging, and the welding performance and process performance are good, which is suitable for the material of high temperature compression parts.
 
 
ASTM A213 T91 seamless steel tube
T91 steel can be used for superheater and reheater tubes with wall temperature below 600℃, as well as headers and steam pipes with wall temperature below 600℃
ASTM A213 T91 Ferritic Alloy-Steel Boiler Tubes are made by the seamless process and shall be either hot finished or cold finished, as specified.
 
 
 
A213 T91 Equivalent Grade

ASTM A213 Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater, and Heat-Exchanger Tubes	T91
ASTM A335 Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service	P91
JIS G3462 Alloy steel tubes for boiler and heat exchanger	STBA 26
GB 5310 Seamless steel tubes and pipes for high pressure boiler	10Cr9Mo1VNbN (10Cr9Mo1VNb)

 
 
 
▶Chemical composition
 

Grade	T91	STBA 26	10Cr9Mo1VNbN
C	0.08~0.12	≤0.15	0.08~0.12
Si	020-0.50	0.25~1.00	0.20~0.50
Mn	0.30-0.60	0.30~0.60	0.30~0.60
P	≤0.020	≤0.030	≤0.020
S	≤0.010	≤0.030	≤0.010
Cr	8.00~9.50	8.00~10.00	8.00~9.50
Mo	0.85~1.05	0.90~1.10	0.85~1.05
V	0.18~0.25	—	0.18~0.25
Ni	≤0.40	—	≤0.40
Al	≤0.04	—	≤0.02
Nb	0.06~0.10	—	0.06~0.10
N	0.030~0.070	—	0.030~0.070

 
 
 
▶The historical development of T91 steel
In order to improve thermal efficiency, reduce fuel consumption and carbon dioxide emissions, the most effective means of thermal power plants is to increase the temperature and pressure of steam, that is, ultra-high temperature and high cycle. Higher steam parameters require higher strength and durability materials at high temperatures.
 
Before the successful development of T91 steel, in the 1950s, power plant boiler tubes mostly used ferritic alloy steel (such as 12Cr1MoV) with a maximum use temperature of 580℃ and austenitic stainless steel (TP304, TP347, etc.) with a use temperature of up to 700℃ . The problems with these materials are their high price, low thermal conductivity, large thermal expansion coefficient, and stress corrosion cracking tendency.
 
In the 1960s, France began to use EM12 grades that can withstand temperatures of 620°C to replace stainless steel pipes used in the past. EM12 as a superheater tube has good performance and can work for a long time, but due to the binary structure, the impact toughness is poor.
 
In the late 1960s, Germany developed 12% chromium steel X20CrMoV12.1 (X20 steel), which was officially included in the DIN17175 standard in 1979. X20 steel has higher creep strength, but its creep rupture strength is not as good as EM12 when the temperature is higher than 520℃, and because of the high carbon content, the welding performance is poor.
 
In 1974, the United States began to study improved 9Cr-1Mo steel grades. The endurance strength of this kind of steel at 593℃/100000h reaches 100MPa, and the toughness is also relatively good. With this type of steel, under the same operating parameters, a smaller wall thickness can be used. No matter from a technical or economic point of view, it is superior to EM12 and X20. In the 1980s, T91 steel was widely used in various countries.
 

 
 ▶T91 steel advantages
T91 steel is widely used in high-parameter thermal power generating units, because the performance of this steel has the following advantages:
❶ Compared with stainless steel, the steel has a low coefficient of thermal expansion and good thermal conductivity.
❷ The steel has high tensile strength at room temperature, δb is up to 770MPa, and the plasticity is also good.
❸ The impact toughness and brittleness transition temperature of this steel are significantly better than similar X20 and EM12 steels.
❹The steel has higher high-temperature endurance strength and allowable stress. The high-temperature endurance strength after 105h operation at 550℃ is twice that of T22 steel. The allowable stress in 540～610℃ is significantly higher than T22 and TP304H And X20 steel.
❺ The steel has good bending performance of the whole pipe.
❻ The high temperature fatigue performance of this steel is better than T22 and TP304H steel, and its high temperature oxidation resistance is also much higher than that of T22 steel.
 
 

 ▶T91 steel application
After testing, T/P91 can be used for superheater and reheater tubes with wall temperature below 600℃, as well as headers and steam pipes with wall temperature below 600℃.
After the successful development of T/P91, it fills the gap between T/P22 and TP304H, and has been widely used in super (super) critical boilers to replace TP321H at a temperature below 600℃.
Although the upper temperature limit of T/P91 is only 600℃, its successful R&D has laid the foundation for the research and development of ultra-supercritical boiler steel for better performance. It is a series of steel grades such as T/P92, E911, T/P122 The research and development of such provide a basic idea. It can be said that the successful development of T/P91 has made the technology of thermal power units mature from supercritical to super (super)critical.
 
 

 ▶T91 steel applied in China
Most of the 200MW and 300MW old unit boilers in China use T22, 12Cr1MoV and 102 (12CrMoWVTiB) steel and other materials. The tube explosion phenomenon caused by ultra-high temperature is more common. Some power plants are replaced with TP347H and TP304H stainless steel tubes during boiler transformation Not only is the cost high (TP347H is twice as expensive as T91 steel pipe), but this type of unstabilized austenitic steel will still be prone to burst due to stress corrosion.
 
In the domestic 600MW unit project, the main steam pipeline and the reheated steam pipeline hot section are all selected A335 P22 pipe. Compared with this material, under the same pressure, temperature and inner diameter, the main steam pipeline and the reheating hot section pipeline adopt P91 alloy steel pipe, the wall thickness can be reduced by about half, and the amount of pipe fittings such as right-angle tee can be reduced by about 65 %.
From the perspective of performance and economy, T/P91 steel is an ideal boiler replacement material and has a broad application space in my country.
 

 ▶Market reference
The domestic well-known enterprises producing boiler tubes include Baosteel and Chengde Steel Tube.
Foreign companies producing boiler tubes are highly concentrated, and are well-known as German V&M, Japanese Sumitomo Metal, Italian Dalmin, and Wiman Gordon-the world's first P91 high-alloy high-quality seamless steel tube was born in Wiman Gordon's factory in Houston, USA.