Analysis of Low-Temperature Toughness Control Techniques for 06Ni9DR Steel Plates Used in Cryogenic Storage Tanks

06Ni9DR steel is a nickel-based alloy widely used in the fabrication of low-temperature storage tanks for liquefied gases, such as LNG and liquid nitrogen. Its high nickel content provides excellent toughness at cryogenic temperatures. Controlling the low-temperature impact toughness of 06Ni9DR steel plates is critical to prevent brittle fracture and ensure the long-term safety and reliability of storage tanks.

1. Factors Affecting Low-Temperature Toughness

Chemical Composition:
The nickel content (around 9%) improves austenite stability, reducing the ductile-to-brittle transition temperature (DBTT). Controlled levels of carbon, manganese, and trace elements (such as sulfur and phosphorus) are essential to minimize impurity segregation and enhance toughness.

Grain Size:
Fine-grained microstructure enhances toughness by reducing crack propagation. Grain size can be refined through controlled rolling and heat treatment.

Microstructure Control:
A uniform ferritic-austenitic microstructure is desirable. Avoiding coarse carbides, martensite-austenite (M-A) constituents, or banding improves impact performance at cryogenic temperatures.

2. Control Techniques

Thermomechanical Controlled Processing (TMCP):
TMCP combines controlled rolling and accelerated cooling to refine grain size and produce a homogeneous microstructure, enhancing both strength and low-temperature toughness.

Heat Treatment:

Normalizing: Refines grain structure and reduces residual stresses.

Quenching and Tempering: Optimizes hardness and toughness. Proper tempering temperature is selected to balance strength and low-temperature impact properties.

Inclusion and Impurity Control:
Reducing non-metallic inclusions and segregated elements (S, P) through vacuum degassing, ladle refining, and secondary metallurgy ensures consistent toughness across the plate thickness.

Thickness and Cooling Rate Control:
During plate production, uniform cooling avoids thermal gradients that could lead to coarse grains or martensitic patches, which reduce low-temperature toughness.

3. Testing and Verification

Impact Testing: Charpy V-notch tests at cryogenic temperatures (-196°C for LNG applications) verify toughness.

Microstructural Analysis: Optical microscopy and scanning electron microscopy (SEM) assess grain size, phase distribution, and inclusion content.

Fracture Toughness Testing: Ensures plates can resist crack initiation and propagation under low-temperature service conditions.

Conclusion
Controlling the low-temperature toughness of 06Ni9DR steel plates involves careful management of chemical composition, microstructure, thermomechanical processing, and heat treatment. These measures ensure cryogenic storage tanks maintain structural integrity under extreme low-temperature conditions, preventing brittle fracture and enhancing operational safety.

References

GB/T 3274-2016 – Low-Temperature Steel Plates for Cryogenic Storage Tanks (06Ni9DR).

Totten, G.E. (2006). Steel Heat Treatment: Metallurgy and Technologies. CRC Press.

Lippold, J.C., & Kotecki, D.J. (2005). Welding Metallurgy and Weldability of Stainless Steels. Wiley.

Kou, S. (2003). Welding Metallurgy, 2nd Edition, Wiley.

Barron, R.F. (1999). Cryogenic Systems, 2nd Edition, CRC Press.