Steels for low temperature service refer to steels specially designed for operation in environments below 0°C (typically -20°C, -40°C or even lower such as -196°C). Their core design principle is to maintain sufficient strength while possessing extremely high toughness to prevent cold brittle fracture. The main mechanical properties of low temperature steels are as follows:
1. Extremely High Low-Temperature Toughness (Core Index)
This is the essential difference between low temperature steels and ordinary steels.
- Characteristics: As temperature decreases, the impact toughness of ordinary steels drops sharply (cold brittleness occurs). However, through special chemical composition design (such as nickel addition) and heat treatment processes, low temperature steels can still maintain high impact absorbed energy (e.g., Akv value) at extremely low temperatures.
- Key index: Such steels usually require Charpy V-notch impact toughness values at service temperature to be much higher than ordinary structural steels, ensuring the material will not fracture brittlely like glass under impact or sudden loads.
2. Good Matching of Strength and Plasticity
- Strength: Low temperature steels generally have medium to high strength (yield strength usually between 240 MPa and 690 MPa), capable of withstanding high pressure or heavy loads in low-temperature environments.
- Plasticity: Even at low temperatures, the material must retain certain elongation and reduction of area, allowing the structure to undergo minor deformation under stress without fracture.
3. No Ductile-Brittle Transition Temperature (or Extremely Low Transition Temperature)
- Characteristics: The curve of toughness versus temperature is relatively gentle. Ordinary carbon steel has an obvious ductile-brittle transition temperature (DBTT), below which it becomes brittle. For low temperature steels, the transition temperature is lowered below service temperature, or grain refinement is applied to keep the material in the ductile region even at cryogenic temperatures.
4. Excellent Weldability
- Characteristics: Low-temperature equipment (such as LNG storage tanks and cryogenic pipelines) usually requires extensive welding. After welding, the weld and heat-affected zone (HAZ) of low temperature steels can still maintain low-temperature toughness similar to the base metal, without local embrittlement caused by welding thermal cycles.
5. Good Fatigue Resistance
- Characteristics: In low-temperature environments, stress concentration inside the material is more likely to initiate fatigue cracks. By strictly controlling inclusion content and grain size, low temperature steels have good resistance to fatigue crack growth, ensuring equipment safety under long-term alternating loads.
Summary
The mechanical properties of low temperature steels can be summarized as: strong but not brittle.They require not only sufficient strength to bear loads at low temperatures, but also extremely high toughness to resist sudden impacts. To achieve such performance, low temperature steels usually adopt low carbon content (to ensure toughness and weldability) and are alloyed with elements such as nickel (Ni) to lower the ductile-brittle transition temperature.
