Compared to 12CrMoV steel, 12Cr1MoV alloy steel pipe offers superior oxidation resistance and thermal strength, making it ideal for high-temperature applications. One of its key features is that its creep limit closely matches its permanent strength, allowing it to maintain good plasticity under sustained stress. The material also exhibits excellent workability and weldability, though it requires preheating to 300°C before welding, followed by stress relief to ensure structural integrity.
This alloy steel is widely used in high-pressure, ultra-high-pressure, and subcritical power plant boiler systems, particularly in superheaters, headers, and main steam pipelines. It retains high thermal strength and oxidation resistance even at 580°C, along with significant permanent plasticity. Its production process is relatively simple, and it has favorable welding properties. However, it is sensitive to the normalizing cooling rate, and prolonged exposure at 580°C can lead to pearlite spheroidization. A major advantage of 12Cr1MoV is its 100% recyclability, aligning with national environmental protection and energy-saving policies. Domestic regulations are encouraging broader use of this material across various industries.
Currently, China's consumption of 12Cr1MoV alloy steel pipes accounts for only about half of what is seen in developed countries. Expanding its application fields presents a great opportunity for industry growth. According to research from the 12Cr1MoV Alloy Steel Pipe Branch of the China Special Steel Association, the demand for high-pressure long products made from this alloy is expected to grow by an average of 10–12% in the coming years.
**Process Overview**
Depending on the manufacturing method, 12Cr1MoV alloy steel pipes are categorized into hot-rolled (extruded) seamless pipes and cold-drawn (rolled) seamless pipes. Cold-drawn tubes can further be divided into round and special-shaped types.
The hot-rolling process involves: round tube billet → heating → piercing → three-roll skew rolling, continuous rolling, or extrusion → tube removal → sizing (or diameter reduction) → cooling → straightening → hydrostatic testing (or flaw detection) → marking → storage.
For cold-drawn seamless pipes: round tube billet → heating → piercing → heading → annealing → pickling → oiling (or copper plating) → multi-pass cold drawing (or rolling) → heat treatment → straightening → hydraulic test (or flaw detection) → marking → warehousing.
**Three Characteristics of Microstructure Strengthening in 12Cr1MoV Alloy Steel Pipes**
The microstructure of the alloy significantly influences the mechanical properties of the steel pipe. However, microstructure strengthening plays a crucial role in determining the final structure. After cold rolling, changes in the cooling environment cause transformations in bainite, martensite, and other structures, leading to variations in the mechanical properties of the pipe. This allows for the production of pipes with different strength levels to meet various performance requirements.
First, the parent phase must be present in the alloy for effective microstructure strengthening. Second, microstructure strengthening involves both deformation and diffusion processes. In low-temperature environments, non-diffusion mechanisms dominate, while in high-temperature conditions, diffusion becomes the primary factor. Third, two key factors—microstructure strain and environmental cooling—greatly affect the strengthening process. As temperature changes, the internal energy state of the material shifts, influencing its structural properties. Additionally, the presence of fine precipitates in the alloy highlights the importance of monitoring microstructural changes when adjusting the properties of the steel pipe.
Hardenability is primarily influenced by the critical cooling rate, which depends on the stability of the supercooled austenite in 12Cr1MoV alloy steel. Several factors affect this stability:
1. **Chemical Composition**: Carbon content plays a significant role. When carbon content is below 1.2%, increasing C% reduces the critical cooling rate and shifts the C curve to the right, improving hardenability. Beyond 1.2% C, the cooling rate increases, and the C curve shifts left, reducing hardenability. Most alloying elements, except cobalt, enhance hardenability by shifting the C curve to the right.
2. **Austenite Grain Size**: Larger grain sizes shift the C curve to the right, lowering the critical cooling rate but increasing the risk of deformation and cracking, and reducing toughness.
3. **Uniformity of Austenite Composition**: A more uniform composition delays pearlite nucleation, extending the transformation incubation period and shifting the C curve to the right, thereby increasing hardenability.
4. **Original Structure of the Steel**: The distribution and thickness of the original microstructure influence the overall behavior of the alloy.
5. **Alloying Elements**: Elements like Mn and Si improve hardenability but may introduce other negative effects on the material’s properties.
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