Overview
15CrMoG is a chromium-molybdenum (Cr-Mo) pearlitic heat-resistant steel. The "G" in its designation stands for "Gang" (Chinese for pot or vessel), indicating it is a steel specifically for high-pressure boiler and pressure vessel applications.
It is a low-alloy steel that offers a good combination of strength, weldability, and, most importantly, creep strength at elevated temperatures, making it a classic choice for power plant and boiler components.
Chemical Composition (Typical %)
The composition is precisely controlled to achieve the desired properties. Key elements are:
| Element | Content (%) | Role and Effect |
|---|---|---|
| Carbon (C) | 0.12 - 0.18 | Provides base strength and hardness. |
| Silicon (Si) | 0.17 - 0.37 | Deoxidizer during steelmaking, improves strength. |
| Manganese (Mn) | 0.40 - 0.70 | Improves strength and toughness. |
| Chromium (Cr) | 0.80 - 1.10 | Enhances oxidation (scaling) resistance and creep strength. |
| Molybdenum (Mo) | 0.45 - 0.65 | Significantly increases high-temperature strength and creep resistance. |
| Phosphorus (P) | ≤ 0.025 | Impurity; kept low to prevent cold brittleness. |
| Sulfur (S) | ≤ 0.015 | Impurity; kept low to prevent hot shortness. |
Note: Exact limits may vary slightly depending on the manufacturing standard (e.g., GB, ASTM).
Mechanical Properties
The properties are typically specified for the normalized and tempered condition.
| Property | Value |
|---|---|
| Tensile Strength (Rm) | 440 - 640 MPa |
| Yield Strength (Rp0.2) | ≥ 295 MPa |
| Elongation (A) | ≥ 21% |
| Impact Energy (AKV) | ≥ 34 J (at room temperature) |
Key Characteristics & Advantages
Excellent High-Temperature Strength: Its primary advantage is its ability to withstand high mechanical loads for long periods at temperatures typically up to 550-580°C. It has much better resistance to "creep" (slow, continuous deformation under stress) than plain carbon steels.
Good Oxidation Resistance: The chromium content forms a stable, protective oxide layer (Cr₂O₃) on the surface, reducing the rate of scaling (surface degradation) in steam and flue gas environments.
Good Thermal Stability: Maintains its microstructure and properties under long-term exposure to heat.
Good Weldability: It is readily weldable using appropriate procedures and suitable filler metals (often matching Cr-Mo composition like E8018-B2).
Main Applications
15CrMoG pipes are exclusively used in high-temperature and high-pressure environments, such as:
Power Station Boilers:
Superheater Tubes
Reheater Tubes
Main Steam Pipes
High-Temperature Header Pipes
Petrochemical Plants:
Heat Exchanger Tubes
Piping for high-temperature process fluids
High-Pressure Pressure Vessels where elevated temperature service is required.
Standards and Specifications
This steel grade is primarily governed by Chinese standards, as it is a very common material in China's power generation industry.
GB 5310 "Seamless Steel Tubes for High Pressure Boiler": This is the primary standard for 15CrMoG in China.
Equivalent Grades in Other Standards:
ASTM A335 P12: The American equivalent is very similar in composition and application.
DIN 17175 13CrMo4-5: The German equivalent.
EN 10216-2 13CrMo4-5: The European standard equivalent.
It is crucial to note that while these grades are similar, they are not always chemically and mechanically identical. Direct substitution requires careful verification of the specific requirements.
Manufacturing Process
15CrMoG boiler pipes are manufactured as seamless pipes to ensure integrity under high pressure. The process typically involves:
Hot Rolling or Extrusion: A solid billet is pierced and rolled/extruded into a hollow pipe.
Heat Treatment: The pipes are almost always supplied in the Normalized & Tempered condition.
Normalizing: Heating to ~900-960°C, then air cooling to refine the grain structure.
Tempering: Reheating to ~650-730°C, then cooling to relieve stresses and achieve the desired toughness.
Welding and Fabrication
Welding 15CrMoG requires careful procedure:
Preheating: Mandatory. Typical preheat temperature is 150-250°C to prevent cold cracking.
Post-Weld Heat Treatment (PWHT): Essential. Performed at ~650-680°C to temper the weld zone, reduce residual stresses, and restore the microstructure to match the base metal's properties.
Filler Metal: Commonly used electrodes are E8018-B2 (for SMAW) or ER80S-B2 (for GTAW/GMAW).
Summary
15CrMoG is a workhorse material in the thermal power and boiler industry. It provides an excellent balance of high-temperature strength, oxidation resistance, and fabricability at a cost that is lower than higher-alloy steels like 12Cr1MoVG or T/P91. Its use is critical for the safe and efficient operation of high-pressure steam systems.
