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

Element	Carbon	Manganese	Phosphorus	Sulfur	Silicon	Chromium	Molybdenum	Others
Content, %	0.07-0.13	0.30–0.60	≤0.020	≤0.010	≤0.50	8.50-9.50	0.30-0.60	*

*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.
ksi	MPa	ksi	MPa	Longitudinal	Transverse
90	620	64	440	–	20

*The tensile tests shall be conducted at room temperature.

Heat Treatment

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.

1. 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].
2. 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].
3. 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.
4. 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
°C	°F	GPa	103ksi	10-6/°C	10-6/°F	J/kg*K
20	70	191	27.7	–	–	420
50	120	–	–	11.2	6.2	420
100	210	184	26.6	11.4	6.3	430
150	300	–	–	11.6	6.4	450
200	390	184	26.6	11.8	6.5	460
250	480	–	–	12.0	6.7	470
300	570	173	25.1	12.1	6.7	480
350	660	–	–	12.3	6.8	500
400	750	–	–	12.6	7.0	510
450	840	–	–	12.8	7.1	530
500	920	152	22.0	12.9	7.2	580
550	1020	–	–	13.0	7.2	600
600	1110	98	18.1	13.1	7.3	630
650	1200	–	–	13.1	7.3	640

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
-20-100	25.7	-30-40	177
200	25.7	85	177
300	25.3	100	177
400	24.5	125	177
500	23.8	150	174
600	23.2	200	169
650	22.8	250	165
700	22.4	300	161
750	21.9	325	159
800	21.4	350	156
850	20.8	375	154
900	20.1	400	151
950	19.2	425	148
1000	18.3	450	144
1050	15.7*B	475	140
1100	12.0*B	500	135
1150	19.2*B	525	129
1200	18.3*B	550	123
–	–	575	99.5*B
–	–	600	77.0*B
–	–	625	56.5*B
–	–	650*C	38.3*B

• *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
-20-100	90.0	-30-40	621
200	84.0	85	621
300	80.6	100	621
400	77.9	125	621
500	75.8	150	610
600	73.7	200	592
650	72.5	250	577
700	71.2	300	563
750	69.8	325	556
800	68.0	350	547
850	66.1	375	539
900	63.8	400	528
950	61.2	425	517
1000	58.1	450	504
1050	54.6	475	489
1100	50.6	500	472
1150	46.1	525	452
1200	40.9	550	429
–	–	575	404
–	–	600	376
–	–	625	344
–	–	650	309

• *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
-20-100	64.0	-30-40	441
200	61.0	85	428
300	59.7	100	419
400	58.9	125	415
500	58.2	150	411
600	57.3	200	406
650	56.6	250	402
700	55.7	300	397
750	54.7	325	393
800	53.4	350	389
850	51.7	375	383
900	49.7	400	377
950	47.4	425	368
1000	44.5	450	359
1050	41.2	475	347
1100	37.4	500	333
1150	33.0	525	316
1200	28.0	550	297
–	–	575	276
–	–	600	251
–	–	625	223
–	–	650	191

• *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 “%”, “A_kv” 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.

Tensile Test		RM		BS		BE		BA
Direction		L	T	L	T	L	T	L	T
R.T	Y.S	565	575	565	575	590	590	580	575
	T.S	705	720	715	720	735	730	725	735
	EL.	23	21	20	22	24	27	22	23
	Akv	87	95	87	95	139	97	125	86
E.T	Y.S	308	339	319	322	332	332	335	328
	T.S	341	359	340	340	349	349	336	345
	EL.	28	30	32	32	38	38	30	29

• *The tensile test results for item B1 of P92 pipe bend.

Tensile Test		RM		BS		BE		BA
Direction		L	T	L	T	L	T	L	T
R.T	Y.S	565	575	620	625	655	655	625	630
	T.S	705	720	745	755	775	780	755	760
	EL.	23	21	25	23	20	22	27	22
	Akv	87	95	138	84	56	65	94	55
E.T	Y.S	308	339	339	351	337	384	351	347
	T.S	341	359	359	371	360	390	366	364
	EL.	28	30	28	34	33	34	26	30

• *The tensile test results for item B2 of P92 pipe bend.

Tensile Test		RM		BS		BE		BA
Direction		L	T	L	T	L	T	L	T
R.T	Y.S	555	560	540	545	565	570	560	565
	T.S	705	710	685	690	705	715	705	705
	EL.	23.5	22.5	23	24	22	21	22.5	22
	Akv	130	90	134	93	60	51	73	61
E.T	Y.S	300	310	310	310	325	320	325	325
	T.S	345	360	355	360	370	365	370	365
	EL.	23.5	22.5	30.5	30.5	30	30	29	25

• *The tensile test results for item B3 of P92 pipe bend.

Tensile Test		RM		BS		BE		BA
Direction		L	T	L	T	L	T	L	T
R.T	Y.S	490	495	535	530	540	550	565	540
	T.S	660	665	695	690	700	705	715	700
	EL.	26.5	24.5	23.5	23.5	24.5	24.5	23	22
	Akv	115	88	118	78	74	49	107	50
E.T	Y.S	300	305	320	310	325	340	340	335
	T.S	325	325	335	330	340	355	360	365
	EL.	28	31	30	26	30.5	27	26	25

• *The tensile test results for item B4 of P92 pipe bends.

Analysis & Conclusion: Optimum Working Conditions for P92/ T92 Bending

1. All the finished ASTM A335 P92 pipe bends conform to chemical composition requirements, mechanical properties requirements at both room & elevated temperatures.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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
B1	248×53	1100	45°	3.1
B2	248×53	1100	45°	3.1
B3	254×62	1200	55°	3.2
B4	254×53	1200	62°	3.3

• *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
B1	900	10~15	Air Cooling
B2	940	15~20	Air Cooling
B3	920	10~15	Air Cooling
B4	980~1040	10~15	Air Cooling

• *The allowable heating-temperature variation for B3 can be ±10°C in practice.

Heat Treatment after Hot-Bending

Item No.	Normalizing	Tempering
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:

Element	B1	B2	B3	B4
C	0.11	0.11	0.10	0.09
Si	0.20	0.20	0.20	0.29
Mn	0.43	0.43	0.47	0.44
P	0.007	0.007	0.013	0.018
S	0.002	0.002	0.005	0.008
Cr	8.56	8.56	8.88	8.61
W	1.52	1.52	1.70	1.84
Mo	0.37	0.37	0.48	0.41
Ni	0.39	0.39	0.19	0.30
Nb	0.06	0.06	0.075	0.06
V	0.15	0.15	0.21	0.23
Al	0.02	0.02	0.003	0.02
N	0.05	0.05	0.039	0.040
B	0.003	0.003	0.0021	0.0013

• *All values are actually tested.

Inspection and Test for Finished Products

Inspecting Item		STD*	B1	B2	B3	B4
Roundness, %		≤5.0	/	/	≤1.1	≤0.8
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
	I.A*		189~218	221~234	196~202	213~220
	M.L*		192~242	215~243	181~228	210~218
MT*		Class I	OK	OK	OK	OK

• *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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