What is P22 Steel?

What is P22 Steel?

T22/P22 steel, UNS 21950, is a 2.25Cr-1Mo low alloy chromium molybdenum ferrite heat-resistant steel; based on controlling C, P, and S elements, P22 steel adds Cr, Mo, and other alloying elements, which greatly improves its comprehensive mechanical properties, and has good high-temperature thermal strength and welding performance.

A335 P22 is a grade in the ASTM A335/A335M
standard, 2.25Cr-1Mo chromium molybdenum high-temperature ferrite for boilers
and superheaters.

• A182 F22 is the material standard for flanges;
• A335 P22 is the standard for seamless pipe materials.

They are all UNS 21950, the same metal material with different processing techniques.

Equivalent Grades of T22/P22 (UNS 21950)

Grade	JIS	Mat. No.	DIN	UNS	GB
P22/T22/F22	STBA24	1.7262	15CrMo5	21950	12Cr2MoG

Chemical Composition of T22/ P22 Steel

Component Elements Properties	P22/T22/F22 (UNS 21950)
Boron, B	0.0010 – 0.0030 %
Carbon, C	0.10 – 0.15 %
Chromium, Cr	2.75 – 3.25 %
Copper, Cu	≤ 0.25 %
Iron, Fe	93.882 – 95.734 %
Manganese, Mn	0.30 – 0.60 %
Molybdenum, Mo	0.90 – 1.10 %
Nickel, Ni	≤ 0.25 %
Phosphorous, P	≤ 0.025 %
Silicon, Si	≤ 0.13 %
Sulfur, S	≤ 0.025 %
Titanium, Ti	0.015 – 0.035 %
Vanadium, V	0.20 – 0.30 %

Mechanical Properties of T22/ P22 Steel

Mechanical Properties	Metric	English
Tensile Strength, Ultimate	415 MPa	60200 psi
Tensile Strength, Yield	205 MPa	29700 psi
Elongation at Break	30%	30%
Rupture Strength	25.0 – 31.0 MPa @Temperature 600 °C, Time 3.60e+8 sec	3630 – 4500 psi @Temperature 1110 °F, Time 100000 hour
	49.0 – 56.0 MPa @Temperature 575 °C, Time 3.60e+8 sec	7110 – 8120 psi @Temperature 1070 °F, Time 100000 hour
	74.0 – 81.0 MPa @Temperature 550 °C, Time 3.60e+8 sec	10700 – 11700 psi @Temperature 1020 °F, Time 100000 hour
	95.0 – 104 MPa @Temperature 525 °C, Time 3.60e+8 sec	13800 – 15100 psi @Temperature 977 °F, Time 100000 hour
	118 – 134 MPa @Temperature 500 °C, Time 3.60e+8 sec	17100 – 19400 psi @Temperature 932 °F, Time 100000 hour

Physical Properties of P22/ T22 Steel

Physical Properties	Metric	English	Comments
Density	7.80 g/cm3	0.282 lb/in³	Based on UNS number

Characteristics of P22/ T22 Steel

P22 steel is a 2.25Cr-1Mo low alloy steel with high thermal strength, good oxidation resistance, hydrogen corrosion resistance, and welding performance. It is widely used in high-temperature equipment such as petrochemical, nuclear power, and power plant boilers. In the actual operating environment of power plant boilers, excessive steam temperature, and pressure pose higher requirements for heat-resistant steel.
Therefore, evaluating the safety performance of P22 steel is of great significance. Research on P22 steel at home and abroad mainly focuses on welding performance, life assessment, and the impact of heat treatment processes on its performance. Hardness testing is a commonly used evaluation method in the safety inspection of power plant boilers. The microstructure, residual stress, and processing technology of heat-resistant steel are closely related to its hardness. Improving the accuracy and reliability of hardness testing plays an important role in the safety assessment of power plant boilers.

Heat treatment of T22/P22 (UNS 21950)

The normalizing temperature of the P22 steel grade is controlled between 930 – 960 ℃, and the tempering temperature is controlled under a heat treatment system of 760 ℃, which can meet the mechanical performance requirements of the product.

Soft Annealing of T22/P22 (UNS 21950)

Heat to 680 – 700 ℃, cool slowly. This will produce a maximum Brinell hardness of 207.

Intermediate Annealing of T22/P22 (UNS 21950)

Temperature: 650 – 680 ℃.

Carburizing of T22/P22 (UNS 21950)

Temperature of 840 – 800 ℃, cooling media: salt bath 580-680oC, air, case hardening box, water, oil, hot quenching 160 – 250℃.

Core Hardening of T22/P22 (UNS 21950)

Temperature: 820 – 850 ℃.

Surface Hardening of T22/P22 (UNS 21950)

Temperature: 810 – 830 ℃, air or hot quenching.

Tempering of T22/P22 (UNS 21950)

Tempering temperature: 150 – 180 ℃.

Forging of T22/P22 (UNS 21950)

Hot forming temperature: 1050 – 850 ℃.

Applications of P22/ T22 Steel

P22 steel is a chromium molybdenum
alloy steel with good oxidation and corrosion resistance. Therefore, it
performs well in high-temperature and high-pressure working environments. The
following are some applications of P22 steel:

• Petrochemical industry: used for
  transporting high-temperature and high-pressure oil and natural gas.
• Electricity: Used for boilers and
  supercritical pressure steam pipelines in thermal power plants.
• Aerospace: Used to manufacture engine
  components for aircraft and rocket engines.
• Nuclear industry: cooling systems and other
  critical components used in nuclear reactors.
• Other: In various high-temperature and
  high-pressure industrial equipment, such as heat exchangers, furnace pipes,
  etc.

Research on optimization of heat treatment process of P22 high pressure boiler pipe

After normalizing and tempering heat treatment of P22 high-pressure boiler pipe with specifications of Φ325 mm×10 mm, the hardness value exceeds the upper limit of the standard due to the thin wall thickness and fast cooling speed. Through experimental research, a new heat treatment process of “slow cooling after normalizing + air cooling + tempering” is proposed. Control the cooling rate after normalizing, P22 steel pipe heat treatment to get ferrite + pearlite-based microstructure, is to ensure that it has good overall mechanical properties of the key.
China in 1985, P22 alloy for the first time into the GB 5310, and named 12Cr2MoG, the current GB/T 5310-2017 increased hardness index requirements (HBW: 125-180), so the heat treatment process put forward higher requirements. Production practice has proved that: P22 high-pressure boiler pipe, especially thin-walled steel pipe by “normalizing + tempering” treatment, easy to appear high hardness value or even unqualified phenomenon. In this paper, the heat treatment process of P22 steel pipe is studied to provide theoretical reference for the production of the steel pipe.

1. Industrial production problems and analysis

ASME SA-335/SA-335M specifies the minimum tempering temperature of P22, GB/T 5310-2017 provides P22 high-pressure boiler pipe (wall thickness of not more than 30 mm) for normalizing + tempering, normalizing temperature 900-960 ℃; tempering temperature 680- 730 ℃, can be adjusted within a narrower range. A steel pipe factory production Φ325 mm × 10 mm P22 high-pressure boiler pipe, its chemical composition (see Table 1) In line with the ASME SA-335/SA-335M and GB/T 5310-2017 standard requirements, the normalizing + tempering heat treatment, the hardness value exceeded the standard Upper limit, the product is unqualified, the inspection data is shown in Table 2.
Table.1 Φ325 mm × 10 mm P22 high-pressure boiler pipe chemical composition (mass fraction)

C	Si	Mn	P	S	Cr	Mo
0.14	0.25	0.43	0.017	0.006	2.08	0.97

Table.2 Industrial production of Φ325 mm × 10 mm P22 high-pressure boiler pipe performance unqualified situation

Heat treatment process	Yield strength/MPa	Tensile strength/MPa	Elongation/%	Hardness HBW
Inspection results	511	637	22	193	191	192
GB/T 5310-2017	≥205	≥415		125-180

Take the above P22 pipe samples processed as blank specimens for laboratory heat treatment process research. According to the relevant standards on the heat treatment process requirements for the design of heat treatment process, process system and specimen mechanical properties are shown in Table 3, Table 4.
Table.3 small specimen laboratory heat treatment process

Number	Normalizing	Temper
1	940 ℃+60 min	680 ℃+80 min
2	940 ℃+60 min	710 ℃+80 min
3	940 ℃+60 min	740 ℃+80 min
4	940 ℃+60 min	760 ℃+80 min

Table.4 Mechanical properties of small specimens after heat treatment

Number	Yield strength/MPa	Tensile strength/MPa	Elongation rate/%	Hardness HBW
1	596,595	710,710	21.5,22.5	218,227,226	226,225,225
2	564,550	686,668	23.5,23.0	211,211,210	212,213,213
3	510,511	626,630	24.5,25.0	198,198,198	196,195,194
4	467,460	584,578	29.5,28.0	183,183,185	184,183,182

From the above heat treatment process and properties can be seen, the material tensile properties to meet the requirements of P22 tensile properties, but it can be seen when the tensile strength reaches 600 MPa or more, the hardness of the material are more than the standard requirements, through the use of normalizing + tempering heat treatment process, even if the tempering temperature is set to the standard requirements of the highest temperature, the hardness of the material is still not able to meet the requirements of the use of the highest tempering temperature are not able to Reduce the tensile strength of the material to below 600 MPa. Specimen microstructure for the bainite, see Figure 1. P22 steel CCT continuous cooling transformation curve shown in Figure 2, the analysis that the material normalized air cooling, cold faster, will not enter the ferrite + pearlite zone, but directly into the bainite zone. Bainite phase transformation in steel is a medium temperature transformation that occurs between the pearlite transformation and martensite phase transformation temperature range. It is neither pearlitic as the diffusion type phase transition, nor martensite as the non-diffusion type phase transition, but “semi-diffusion type phase transition”, both only carbon atoms can diffuse, and iron atoms and replace the alloying elements of the atoms have been difficult to diffuse. Bainite in steel is essentially a bainitic ferrite as the substrate, on which the distribution of θ^– carburized (or ε^– carburized) or residual austenite and other phases of the organic structure of the combination. It is a complex integrated organization composed of bainitic ferrite (BF), carbides, residual austenite, martensite and other phases. Fig. 2 CCT continuous cooling transformation curve of P22 steel The strength of tempered material does not decrease much in the subsequent narrow interval temperature range. If the microstructure of the material can be made to become ferrite + pearlite or ferrite + pearlite + bainite, then it will significantly reduce the strength and hardness of the material, and the subsequent tempering to make the material properties to meet the standard requirements, to get the original organization of ferrite + pearlite or ferrite + pearlite + bainite, you can normalize the heating and then reduce the cooling rate, so that it is in the ferrite + pearlite or ferrite + pearlite + pearlite and bainite zones to transform, the heat treatment process needs to be readjusted.

Fig.1 Photograph of microstructure of P22 after heat treatment of small sample

Figure.2 CCT Continuous Cooling Transformation Curve

2. Process optimization

From the CCT curve, it can be seen that the P22 steel Ac₁ is 721 ℃, Ac₃ is 750 ℃. If you control the cooling rate of normalization, slow cooling after normalization, by reducing the cooling rate, so that the steel pipe in the medium temperature zone of the organization transformation, so that the organization to maintain a certain amount of ferrite, to get ferrite + pearlite organization, so that the tempering of the material strength and hardness is not easy to exceed the upper limit of the standard. In the original industrial production of unqualified batches of steel pipe on the interception of pipe samples, processing for tensile, hardness test blank specimens, the design of the following two heat treatment process (see table 5), for laboratory heat treatment test of small specimens, the corresponding specimen group numbered 1, 2. After inspection, process 1 treatment of specimen hardness value is in the standard requirements of the middle limit, the overall performance of the process 2 treatment of specimens with the value of the hardness of the standard exceeds the upper limit, see table 6.
Table.5 P22 small specimen heat treatment test

Grade	Serial number	Heat treatment system	Actual control
P22	1	Normalizing and holding at (930 ± 10) ℃ for 40 minutes, furnace cooling to 770 ℃, air cooling after discharge, tempering at (750 ± 10) ℃, holding for 150 minutes	It took a total of 85 minutes to reduce the furnace cooling from 930 ℃ to 770 ℃.
	2	Normalizing and holding at (930 ± 10) ℃ for 40 minutes, furnace cooling to 850 ℃, air cooling after discharge, tempering at (750 ± 10) ℃, holding for 150 minutes	It took a total of 40 minutes to reduce from 930 ℃ to 850 ℃.

Table.6 P22 mechanical properties of small specimens after heat treatment

Number	Yield strength/MPa	Tensile strength/MPa	Elongation rate/%	Hardness HBW
1	385	525	38	155	156	155
2	454	572	40	185	184	182

No. 1, 2 of the hardness specimen processing metallographic specimens, microstructure inspection, see Figure 3, Figure 4. Table 6 results show that the use of normalizing after slow cooling process, the strength of the material have a certain magnitude of reduction, P22 steel by 930 ℃ normalizing insulation 40 min furnace cooling to 770 ℃ air-cooled to room temperature, 750 ℃ tempering insulation after 150 min of process processing, the strength of the material to meet the standard requirements, especially the Hardness (HBW) in 155 or so, in the standard requirements of the middle limit, more ideal. In 930 ℃ normalizing insulation 40 min furnace cooling to 850 ℃ air cooling to room temperature, 750 ℃ tempering insulation 150 min heat treatment process, the material strength to meet the requirements, but the hardness exceeds the upper limit of the standard requirements. Figure 4 microstructure shows that the furnace cooling to 750 ℃, the organization of ferrite + pearlite, furnace cooling to 850 ℃, the organization of ferrite + pearlite + bainite. Bainite is a hard phase, resulting in high hardness.

Figure.3 Microstructure photo of sample 1

Figure.4 Microstructure photo of sample 2

3. Industrial test

The heat treatment equipment of the existing production line has no other method of controlling the cooling rate except air cooling, air cooling and complete quenching. After laboratory tests and several industrial debugging, innovative use of the existing protective atmosphere roller-hearth furnace beat mode (a similar stepper furnace operation mode), the 1-3 zone is set as the heating zone, 4-6 zone is set as the holding zone, 7-10 zone is set as the cooling zone, to achieve the purpose of slow cooling process after normalizing. Using the process shown in Table 7 to re-treat the above batches of steel pipe, the test found that the product has a good overall performance (see Table 8), in line with the standard requirements. Using the same process, treatment of other specifications of P22 products, test results in line with expectations, see Table 9.
Table.7 P22 industrial heat treatment process

Material	Specifications	Heat treatment system
P22	Φ325 mm×10 mm	Normalizing temperature 930 ℃, holding for 50 minutes, then cooling to 770 ℃ and cooling out of the furnace (roller hearth furnace temperature setting: 930 ℃ in zone 1-3, 930 ℃ in zone 4-6, 770 ℃ in zone 7-10), tempering temperature 750 ℃, holding for 150 minutes.

Table.8 P22 performance after heat treatment by normalizing and then slow cooling + air cooling + tempering heat treatment

Specifications	Yield strength/MPa	Tensile strength/MPa	Elongation rate/%	Hardness HBW
Φ325 mm×10 mm	364	525	31.5	157	158	160
	366	528	31.5	154	156	154

Table.9 Other specifications of P22 industrial heat treatment performance

Specifications	Yield strength/MPa	Tensile strength/MPa	Elongation rate/%	Hardness HBW
Φ457.2 mm×13.5 mm	362	550	33	158	159	159
	362	547	33.5	160	160	159
	361	542	32	161	161	161
	356	525	32.5	159	159	157

4. Conclusion

• (1) Normalizing after controlling a reasonable cooling rate, to obtain the microstructure of ferrite + pearlite is the main microstructure, is to ensure that the P22 high-pressure boiler pipe has good comprehensive mechanical properties of the key.
• (2) Adopt “slow cooling after normalizing + air cooling + tempering” heat treatment process, the use of existing heat treatment equipment, can be produced to meet the ASME SA-335 / SA-335M and GB / T 5310- 2017 standard P22 high pressure boiler pipe products.

Author: Guo Zhiwen