Overview
12Cr2MoG is a chromium-molybdenum (Cr-Mo) alloy steel specifically designed for manufacturing seamless steel pipes and tubes for boilers and pressure vessels. The "G" in its designation stands for "Gao Ya Guo Lu Yong Gang" (高压锅炉用钢), which is Chinese for "High-Pressure Boiler Steel."
It is the Chinese standard (GB 5310) equivalent to international grades like ASTM A335 P22 and DIN 1.7380. Its primary characteristic is excellent creep strength and oxidation resistance at elevated temperatures.
Chemical Composition (Typical % per GB 5310)
The chemical composition is precisely controlled to achieve the desired high-temperature properties.
| Element | Content (%) | Role of the Element |
|---|---|---|
| Carbon (C) | 0.08 - 0.15 | Provides base strength and hardness. |
| Silicon (Si) | 0.50 Max | Deoxidizer during steelmaking, improves strength. |
| Manganese (Mn) | 0.40 - 0.70 | Increases strength and hardenability. |
| Chromium (Cr) | 2.00 - 2.50 | Key element: Improves high-temperature strength, oxidation, and corrosion resistance. |
| Molybdenum (Mo) | 0.90 - 1.10 | Key element: Significantly enhances creep strength and stability at high temperatures. |
| Phosphorus (P) | ≤ 0.025 | Impurity; kept low to prevent embrittlement. |
| Sulfur (S) | ≤ 0.025 | Impurity; kept low to improve hot workability and toughness. |
Mechanical Properties (Per GB 5310)
The properties are guaranteed after a specific heat treatment (usually normalization and tempering).
| Property | Value |
|---|---|
| Tensile Strength (Rm) | 450 - 600 MPa |
| Yield Strength (Rp0.2) | ≥ 280 MPa |
| Elongation (A) | ≥ 20% |
| Impact Energy (KV2) | ≥ 35 J (at room temperature) |
| Hardness | ≤ 165 HB |
Key Characteristics & Advantages
Excellent Creep Strength: This is its most critical property. Creep is the slow, permanent deformation of a material under constant stress at high temperatures. 12Cr2MoG is formulated to resist this deformation over long periods (e.g., 100,000 hours), which is essential for boiler components that operate for decades.
Good High-Temperature Oxidation Resistance: The chromium content forms a stable, protective chromium oxide layer (Cr₂O₃) on the surface, preventing scaling and degradation from flue gases and steam.
Good Thermal Stability: The steel maintains its microstructure and mechanical properties without significant deterioration when exposed to long-term high-temperature service.
Good Weldability: While it requires pre-heating and post-weld heat treatment (PWHT) due to its alloy content, it is generally considered to have good weldability for a Cr-Mo steel. Proper procedures must be strictly followed.
Common Applications
12Cr2MoG pipes are used in the sections of boilers and power plants that experience the highest temperatures and pressures:
Superheater Tubes: Where saturated steam is heated into superheated steam.
Reheater Tubes: Where steam is cycled back from the turbine to be reheated for efficiency.
High-Temperature Headers and Main Steam Pipes: The large-diameter pipes that collect and transport superheated steam to the turbine.
High-Temperature Pressure Vessels in petrochemical and power industries.
Manufacturing Standard: GB 5310
The primary standard governing this material is GB 5310 "Seamless Steel Tubes for High-Pressure Boilers". This standard specifies the technical delivery conditions, including chemical composition, mechanical properties, non-destructive testing (ultrasonic, hydrostatic), and heat treatment requirements.
Heat Treatment
Pipes made of 12Cr2MoG are always supplied in the heat-treated condition. The standard heat treatment is:
Normalizing + Tempering (N+T)
Normalizing: Heated to ~900-960°C, then air-cooled. This creates a fine, uniform microstructure.
Tempering: Reheated to ~680-750°C, then air-cooled. This relieves internal stresses from normalizing and improves toughness and ductility.
Comparison with International Grades
| Chinese Standard (GB) | U.S. Standard (ASTM) | European Standard (DIN/EN) | Common Name |
|---|---|---|---|
| 12Cr2MoG | A335 P22 | 1.7380 / 10CrMo9-10 | 2.25Cr-1Mo |
Note: While these grades are broadly equivalent, there can be slight variations in chemical composition limits and mechanical property requirements. They are generally considered interchangeable for most applications, but specific project specifications must be consulted.
Welding Considerations
Welding 12Cr2MoG requires careful procedure:
Pre-heating: Typically to 150-200°C to prevent cracking.
Welding Consumables: Must match the base metal's composition (e.g., ER90S-B3 for TIG, E9015-B3 for SMAW).
Post-Weld Heat Treatment (PWHT): Mandatory. Performed at ~680-700°C to temper the weld zone, reduce residual stresses, and restore toughness.
Summary
12Cr2MoG is a critical, high-performance alloy steel used for its superior strength and durability at temperatures typically ranging from 520°C to 580°C. It is the backbone material for high-efficiency thermal power plants and critical pressure vessels, ensuring safe and reliable operation under extreme conditions for decades.
