2.25Cr-1Mo Steel: Properties and Key Applications

2.25Cr-1Mo Steel (F22/P22 type) is a low-alloy steel primarily classified as a medium-carbon alloy steel. It contains chromium (Cr) and molybdenum (Mo) as its principal alloying elements, which significantly enhance its mechanical properties and resistance to high-temperature environments. This steel grade is particularly known for its excellent strength and toughness, making it suitable for applications in power generation and petrochemical industries.

The primary characteristics of 2.25Cr-1Mo steel include:

• High Strength and Toughness: The alloying elements contribute to improved tensile strength and yield strength, allowing it to withstand high-stress conditions.
• Good Weldability: This steel can be welded using various methods, although preheating and post-weld heat treatment are often recommended to minimize the risk of cracking.
• Resistance to High Temperatures: It maintains its mechanical properties at elevated temperatures, making it suitable for use in pressure vessels and piping systems.

Advantages and Limitations

Advantages:
- Excellent mechanical properties at elevated temperatures.
- Good resistance to oxidation and creep.
- Versatile in fabrication and welding processes.

Limitations:
- Susceptible to hydrogen embrittlement if not properly handled during welding.
- Requires careful heat treatment to achieve desired properties.
- Limited corrosion resistance compared to stainless steels.

Historically, 2.25Cr-1Mo steel has been widely used in the construction of pressure vessels, piping, and other components in the oil and gas industry, reflecting its importance in critical applications.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	K21590	USA	Closest equivalent to ASTM A335 P22
ASTM	A335 P22	USA	Used for high-temperature service
ASTM	A387 Gr. 22	USA	Similar properties, but for pressure vessels
EN	1.7380	Europe	Equivalent in European standards
JIS	G3461 STBA22	Japan	Minor compositional differences
DIN	13CrMo44	Germany	Similar, but with different alloying elements

The table above highlights various standards and equivalents for 2.25Cr-1Mo steel. Notably, while ASTM A335 P22 is a direct equivalent, differences in heat treatment and mechanical properties can affect performance in specific applications.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.10 - 0.20
Cr (Chromium)	1.90 - 2.50
Mo (Molybdenum)	0.87 - 1.13
Mn (Manganese)	0.30 - 0.60
Si (Silicon)	0.10 - 0.40
P (Phosphorus)	≤ 0.025
S (Sulfur)	≤ 0.010

The key alloying elements in 2.25Cr-1Mo steel play crucial roles:
- Chromium: Enhances oxidation resistance and strength at high temperatures.
- Molybdenum: Improves hardenability and creep resistance.
- Carbon: Increases strength and hardness, but must be controlled to avoid brittleness.

Mechanical Properties

Room Temperature Properties

Property	Condition/Temper	Typical Value/Range (Metric)	Typical Value/Range (Imperial)	Reference Standard for Test Method
Tensile Strength	Annealed	415 - 550 MPa	60 - 80 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	250 - 350 MPa	36 - 51 ksi	ASTM E8
Elongation	Annealed	20 - 30%	20 - 30%	ASTM E8
Hardness (HB)	Annealed	150 - 200	150 - 200	ASTM E10

Elevated Temperature Properties

Property	Condition/Temper	Test Temperature (°C)	Typical Value/Range (Metric)	Typical Value/Range (Imperial)	Reference Standard for Test Method
Creep Strength	Quenched & Tempered	600	150 - 200 MPa	22 - 29 ksi	ASTM E139
Impact Strength (Charpy)	Quenched & Tempered	-40	27 J	20 ft-lbf	ASTM E23

The mechanical properties of 2.25Cr-1Mo steel make it suitable for high-stress applications, particularly in environments where elevated temperatures are a concern. Its strength and toughness ensure structural integrity under mechanical loading.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
Density	-	7.85 g/cm³	0.284 lb/in³
Melting Point	-	1425 - 1540 °C	2600 - 2800 °F
Thermal Conductivity	20 °C	45 W/m·K	31 BTU·in/h·ft²·°F
Specific Heat Capacity	-	460 J/kg·K	0.11 BTU/lb·°F

The density and melting point of 2.25Cr-1Mo steel indicate its robustness, while its thermal conductivity suggests good heat dissipation properties, which are critical in high-temperature applications.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C)	Resistance Rating	Notes
Chlorides	3-5	25	Fair	Risk of pitting corrosion
Sulfuric Acid	10	60	Poor	Not recommended
Hydrochloric Acid	5	25	Poor	Not recommended

2.25Cr-1Mo steel exhibits moderate resistance to corrosion, particularly in environments with chlorides. However, it is not suitable for highly acidic conditions, as it can suffer from pitting and stress corrosion cracking (SCC). Compared to stainless steels, its corrosion resistance is limited, making it less ideal for applications where exposure to aggressive environments is expected.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400	752	Suitable for long-term exposure
Max Intermittent Service Temp	500	932	Short-term exposure only
Scaling Temperature	600	1112	Begins to oxidize at this temperature
Creep Strength considerations	600	1112	Creep resistance begins to decline

2.25Cr-1Mo steel maintains good mechanical properties at elevated temperatures, making it suitable for applications in power plants and refineries. However, care must be taken to avoid prolonged exposure to temperatures above its maximum continuous service limit, as this can lead to degradation of material properties.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
SMAW	E8018-C3	Argon/CO2	Preheat recommended
GTAW	ER70S-6	Argon	Post-weld heat treatment advised

2.25Cr-1Mo steel is generally weldable using standard techniques, but preheating is often necessary to prevent cracking. Post-weld heat treatment can further enhance the properties of the weld.

Machinability

Machining Parameter	2.25Cr-1Mo Steel	AISI 1212 Steel	Notes/Tips
Relative Machinability Index	60	100	Requires slower cutting speeds
Typical Cutting Speed	20 m/min	40 m/min	Use carbide tools for best results

Machining 2.25Cr-1Mo steel can be challenging due to its toughness. Optimal conditions and tooling are essential to achieve desired surface finishes.

Formability

2.25Cr-1Mo steel exhibits moderate formability. It can be cold and hot formed, but care must be taken to avoid work hardening. Recommended bend radii should be adhered to in order to prevent cracking.

Heat Treatment

Treatment Process	Temperature Range (°C)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	700 - 800	1 - 2 hours	Air	Softening, improved ductility
Quenching	900 - 950	30 minutes	Oil	Hardening, increased strength
Tempering	600 - 700	1 hour	Air	Reducing brittleness, improving toughness

Heat treatment processes significantly affect the microstructure and mechanical properties of 2.25Cr-1Mo steel. Proper heat treatment can enhance toughness and strength, making it suitable for high-stress applications.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Oil & Gas	Pressure vessels	High strength, toughness, and heat resistance	Essential for high-pressure environments
Power Generation	Boiler tubes	Creep resistance and oxidation resistance	Critical for steam generation systems
Chemical Processing	Piping systems	Corrosion resistance and mechanical integrity	Necessary for transporting fluids safely

• Other applications include:
• Heat exchangers
• Turbine components
• Reactor vessels

2.25Cr-1Mo steel is chosen for applications where high strength and resistance to high temperatures are critical. Its properties make it ideal for environments where mechanical integrity is paramount.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	2.25Cr-1Mo Steel	AISI 316 Stainless Steel	AISI 4140 Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength	Excellent corrosion resistance	Good toughness	Trade-off between corrosion resistance and strength
Key Corrosion Aspect	Fair resistance	Excellent resistance	Poor resistance	2.25Cr-1Mo is less resistant than stainless steel
Weldability	Good	Excellent	Fair	2.25Cr-1Mo requires pre/post heat treatment
Machinability	Moderate	Good	Fair	2.25Cr-1Mo is tougher to machine
Approx. Relative Cost	Moderate	Higher	Lower	Cost considerations may influence selection
Typical Availability	Common	Common	Common	Availability is generally good for all grades

When selecting 2.25Cr-1Mo steel, considerations include its mechanical properties, cost-effectiveness, and availability. While it offers excellent performance in high-temperature applications, its limitations in corrosion resistance compared to stainless steels must be acknowledged. Understanding the specific requirements of the application will guide the selection process, ensuring optimal performance and safety.