What is P91 Steel

What is P91 Steel?

T91/P91 steel, UNS K91560, is a 9Cr-1Mo ferritic alloy steel; P91 steel is equivalent to the national standard 10Cr9Mo1VNb. It is a kind of improved tempered martensitic heat-resistant steel with chromium, molybdenum, vanadium, niobium, and other high melting point metals as the main components. Its maximum use temperature is 650 ℃. Newly added vanadium and niobium are strong carbide-forming elements; carbide formation will impede nail dislocation movement and has a good dispersion-strengthening effect so that it can improve the creep resistance of the metal material. P91 steel is improved based on 9Cr-1Mo heat-resistant steel, making its good creep resistance more prominent. It is widely used in super (super) critical thermal power units in various high-temperature, high-pressure service components.
P91 steel has good toughness and strength, strong oxidation resistance, and superior weldability.

Chemical Composition of T91/ P91 Steel

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

T91 steel alloying elements play a solid solution strengthening, dispersion strengthening, and improve the resistance to oxidation and corrosion resistance of steel; specific analysis is as follows.

• ① Carbon is the most obvious element in the steel solution strengthening; with the increase in carbon content, the short-term strength of steel, plasticity, and toughness decline; for T91, such as martensitic steel, the rise in carbon content will accelerate the speed of carbide spheroidization and aggregation, accelerate the redistribution of alloying elements, reduce the weldability of steel, corrosion resistance, and oxidation resistance, so heat-resistant steel generally want to reduce the carbon content. Still, the carbon content is too low, and the strength of steel will be T91 steel compared with 12Cr1MoV steel; the carbon content is reduced by 20%, which is a combination of the above factors to consider the impact of the decision.

• ② T91 steel contains traces of nitrogen; the role of nitrogen is reflected in two aspects. On the one hand, with the solid solution strengthening effect, the solubility of nitrogen in steel at room temperature is very small; T91 steel heat-affected zone after welding heating and post-weld heat treatment process, there will be a solid solution of VN and precipitation process: welding heating heat-affected zone has been formed in the austenite tissue due to the dissolution of VN, nitrogen content increased, after which the degree of supersaturation in the organization at room temperature increased, in the subsequent post-weld heat treatment has a fine VN precipitation, which increases the organizational stability and improves the heat-affected zone of the lasting strength value. On the other hand, T91 steel also contains a small amount of A1; nitrogen can form A1N with it, A1N in 1100 ℃ or more before a large number of dissolved into the matrix at a lower temperature and re-precipitation, can play a better dispersion strengthening effect.
  

• ③ Adding chromium is mainly to improve the resistance to oxidation and corrosion resistance of heat-resistant steel; chromium content of less than 5%, 600 ℃ began to oxidize violently, while chromium content of 5% has good oxidation resistance. 12Cr1MoV steel has good oxidation resistance below 580 ℃, corrosion depth of 0.05 mm/a, 600 ℃ performance began to deteriorate, corrosion depth of 0.13mm / a. T91 chromium content increased to about 9%, the use of temperature can reach 650 ℃, the main measure is to make more chromium dissolved in the matrix.
  

• ④ Vanadium and niobium are strong carbide-forming elements, added to form a fine and stable alloy carbide with carbon, with a strong dispersion strengthening effect.
  ⑤ Molybdenum is added mainly to improve steel’s thermal strength and play a solid solution-strengthening effect.

• ⑤ Molybdenum is added mainly to improve steel’s thermal strength and play a solid solution-strengthening effect.

Mechanical Properties of T91/ P91 Steel

Standard	Grade	Yield Strength min.	Tensile Strength min.	Elongation min.	Hardness Max.
		MPa[ksi]	MPa[ksi]	%	HB
ASTM A213/ A335	T91/ P91	415[60]	585[85]	20	250

• *The tensile test shall be conducted at room temperature.
• *It is 2″ specimen for the test; the elongation shall be longitudinally tested.

Physical Properties of P91/ T91 Steel

Temperature		Modulus of Elasticity		Thermal Conductivity	Linear Expansivity		Specific Heat	Density
°C	°F	GPa	103ksi	W/mK	10-6/°C	10-6/°F	J/kg*K	kg/m3
20	68	218	31.6	26	0	0	440	7770
50	122	216	31.3	26	10.6	5.9	460
100	212	213	30.9	27	10.9	6.1	480
150	302	210	30.5	27	11.1	6.2	490
200	392	207	30	28	11.3	6.3	510
250	482	203	29.5	28	11.5	6.4	530
300	572	199	28.9	28	11.7	6.5	550
350	662	195	28.3	29	11.8	6.6	570
400	752	190	27	29	12.1	6.7	630
450	842	186	27	29	12.1	6.7	630
500	932	181	26.3	30	12.3	6.8	660
550	1022	175	25.4	30	12.4	6.9	710
600	1112	168	24.4	30	12.6	7	770
650	1202	162	23.5	30	12.7	7.1	860

• *The typical physical properties at different temperatures.

Characteristics of P91/ T91 Steel

In previous projects, 10 CrMo910 steel was used as the main steam pipeline for ultra-high pressure 125 MW and 200 MW units; From the late 1970s to the 1980s, subcritical 300 MW and 600 MW units were introduced A335 P22 steel was used as the main steam pipeline. These two types of steel have similar properties, both of which belong to pearlite heat-resistant steel with a maximum working temperature of 580-590 ℃. Only austenitic heat-resistant steel can be used when the temperature is high, with a maximum working temperature of 700 ℃. However, this type of steel has a high thermal expansion, sensitivity to stress corrosion, and short service life of dissimilar steel joints, not suitable for use in high-temperature and high-pressure steam water pipelines.

The Allowable Stress – Temperature Chart for ASTM A335 P91

The maximum allowable stress in different temperature ranges shall be determined by ASME B31.1 and ASME B31.3 respectively.

1. ASME B31.1: Allowable Stress – Temperature of ASTM A335 P91

*Temp. °F	-120	200	300	400	500	600	650	700	750
*AS-I, ksi	24.3	24.3	24.3	24.2	24.1	23.7	23.4	22.9	22.2
*AS-II, ksi	24.3	24.3	24.3	24.2	24.1	23.7	23.4	22.9	22.2
*Temp. °F	800	850	900	950	1000	1050	1100	1150	1200
*AS-I, ksi	21.3	20.3	19.1	17.8	16.3	14	10.3	7	4.3
*AS-II, ksi	21.3	20.3	19.1	17.8	16.3	12.9	9.6	7	4.3

• *Temp.: the metal temperature shall be interpreted as “not exceeding”.
• *AS-I: the maximum allowable stress for thickness less than 3 inches.
• *AS-II: the maximum allowable stress for thickness 3 inches or greater.
• *The tabulated values are applicable to ASTM A335 P91 pipes used for power piping construction as per ASME B31.1.

2. ASME B31.3: Allowable Stress – Temperature of ASTM A335 P91

*Temp. °F	-20~100	200	300	400	500	600	650	700	750
*BS, ksi	28.3	28.3	28.3	28.2	28.1	27.7	27.3	26.7	25.9
*Temp. °F	800	850	900	950	1000	1050	1100	1150	1200
*BS, ksi	24.9	23.7	22.3	20.7	18	14	10.3	7	4.3

• *Temp.: the metal temperature shall be interpreted as “at the temperature and not exceeding”.
• *BS: the basic allowable stress for thickness no greater than 3 inches.
• *The tabulated values are applicable to both A335 P91 & A691 P91 pipes used for process piping construction as per ASME B31.3.

Heat Treatment of P91/T91 Steel

The final heat treatment of T91 is normalizing + high – temperature tempering. The normalizing temperature is 1040 ℃, the holding time is not less than 10 min, the tempering temperature is 730 – 780 ℃, and the holding time is not less than 1 h. The microstructure after the final heat treatment is tempered martensite.

Welding properties of P91 steel

P91 steel can be arc welded according to existing methods, including the available argon-arc welding (TIG) method. The electrode and flux should be selected so that the chemical composition of the weld and the base material are identical or as close as possible so that the weld metal has the same or better creep and tensile strength than the base material. Because the steel is not sensitive to thermal cracking, preheating to 150-200 ℃ before welding will not crack, and can be welded with a variety of steels, such as P22 steel (pearlite heat-resistant steel), X20CrMoV121 (martensitic heat-resistant steel) and TP304H (austenitic heat-resistant steel) and other steels, the following describes the welding of P91 steel.

• a) P91 steel and P91 steel welding, you can choose 9Cr-1Mo (T9) or improved 9Cr-1Mo (T91) welding rod, wall thickness greater than 25mm pipe using the latter, preheating temperature of 200 ℃, after welding slowly cooled to room temperature, and then tempered at a temperature above 730 ℃;
  b) P91 steel and 10CrMo910 welding, welding materials to match 10CrMo910, stress relief at 730

• b) P91 steel and 10CrMo910 welding, welding materials to match 10CrMo910, stress relief at 730 ℃ should be cooled in the air for 2h, due to the two materials welded parts have a decarburization zone, if 10CrMo910 welding rod is used, to ensure that the carbon content of the weld metal is high enough to meet the requirements of lasting strength;
  c) P91 steel and P22 steel welding, welding electrode can be selected 2.25Cr-1Mo welding rod, the preheating temperature of 200

• c) P91 steel and P22 steel welding, welding electrode can be selected 2.25Cr-1Mo welding rod, the preheating temperature of 200 ℃, after welding slowly cooled to room temperature, and then tempered at 700-725 ℃, can also be first in the P91 steel side overlay 5Cr1-1Mo welding rod, and then welded with P22 steel;
  d) P91 steel and X20CrMoV121 welding, to use the two matching welding materials, such as P91 steel welding rod, the preheating temperature of 250

• d) P91 steel and X20CrMoV121 welding, to use the two matching welding materials, such as P91 steel welding rod, the preheating temperature of 250 ℃, after welding slow cooling to 80-100 ℃, tempering at a temperature of 750 ℃ or more;
  e) P91 steel and TP304H welding with Inconel 182Ni-based alloy welding rod, the preheating temperature of 200

• e) P91 steel and TP304H welding with Inconel 182Ni-based alloy welding rod, the preheating temperature of 200 ℃, after welding cooling to room temperature, tempering at 700-730 ℃.
  The above heat treatment temperature after welding depends on the wall thickness of the pipe, small diameter pipe processing 0.5h, large diameter pipe to the wall thickness of every 25mm processing 1h. Due to the high creep strength of P91, under the same conditions, the pipe wall thickness is thinner than using P22 steel, and the amount of weld filler metal is relatively less, but P91 steel is sensitive to weld type IV cracking; therefore, to minimize the system stress in P91 steel, the wall thickness of Greater than 12.5mm of the pipeline, required after welding cold to more than 100

• The above heat treatment temperature after welding depends on the wall thickness of the pipe, small diameter pipe processing 0.5h, large diameter pipe to the wall thickness of every 25mm processing 1h. Due to the high creep strength of P91, under the same conditions, the pipe wall thickness is thinner than using P22 steel, and the amount of weld filler metal is relatively less, but P91 steel is sensitive to weld type IV cracking; therefore, to minimize the system stress in P91 steel, the wall thickness of Greater than 12.5mm of the pipeline, required after welding cold to more than 100 ℃ that tempering.

Post weld Heat Treatment Requirements for P92/ T92 Steels

After the welding is completed, the welding heat treatment is also performed. Welding heat treatment must have two conditions before it can be carried out.

• ① In 80 ℃ -100 ℃ temperature conditions, due to its last layer of the welding channel without the subsequent tempering effect of welding.
  ② After 2h of holding in the welding treatment to achieve a complete martensite transformation.
• ② After 2h of holding in the welding treatment to achieve a complete martensite transformation.

However, welding to welding heat treatment of the whole process is not to move the weld, not to mention the destruction of the weld, which needs to be fixed to ensure its successful welding.
T91 / P91 steel welding heat treatment requires three steps.

• ① Preheating, mainly to control the temperature, for different parts of the preheating temperature required different.

• ② Interlayer temperature control, the temperature required for different layers varies according to the material.

• ③ Post-weld heat treatment in the process of post-weld heat treatment, we must first pay attention to the type of welded parts before welding; the temperature required for different welded parts is different.

T91 steel in the post-weld heat treatment before the cooling temperature is generally not less than 100 ℃; if the cooling is at room temperature and the environment is relatively humid, prone to stress corrosion cracking. German regulations: before the post-weld heat treatment must be cooled to below 150 ℃. In the case of thicker workpieces, fillet welds, and bad geometry, the cooling temperature is not less than 100 ℃. If cooled at room temperature, humidity is strictly prohibited. Otherwise, stress corrosion cracks are likely to occur.

Applications of P91/ T91 Steel

The remarkable properties of T91/P91 steel make it suitable for various applications. Here are some of the most common industries and applications where this alloy steel is used:

• Power Generation: T91/P91 steel is widely utilized in power generation plants for components such as superheater tubes, steam pipes, and turbine blades due to its high-temperature strength and corrosion resistance.
• Petrochemical Industry: The excellent corrosion resistance and high strength of T91/P91 steel make it a popular choice for components in the petrochemical industry, including pressure vessels, reactors, heat exchangers, and piping systems.
• Oil and Gas Industry: T91/P91 steel is used in the oil and gas industry for applications like downhole tools, drilling equipment, and flow lines. Its strength and corrosion resistance are ideal for handling corrosive fluids and gases at high pressures and temperatures.
• Chemical Processing: In the chemical processing industry, T91/P91 steel is utilized for manufacturing equipment like reactors, heat exchangers, and piping systems due to its resistance to various corrosive chemicals and high-temperature strength.
• Automotive and Aerospace: The high strength-to-weight ratio and excellent mechanical properties of T91/P91 steel make it suitable for automotive and aerospace applications, such as engine components, exhaust systems, and structural parts.
• Marine Engineering: T91/P91 steel is used in marine engineering applications, including shipbuilding and offshore structures, where it’s the corrosion resistance and high strength provide long-lasting performance in harsh environments.
• Nuclear Industry: T91/P91 steel is employed in the nuclear industry for components like heat exchangers, pressure vessels, and piping systems that require high strength and resistance to radiation-induced corrosion.

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