The core objective of heat treatment for low-temperature service steels is clear: to maximize low-temperature toughness and prevent brittle fracture on the premise of ensuring sufficient strength. To achieve this goal, heat treatment of low-temperature steels usually focuses on grain refinement and obtaining uniform low-carbon bainite or tempered martensite microstructures. The following are the most commonly used heat treatment methods for low-temperature steels:
1. Quenching and Tempering (QT)
This is the most essential and widely used heat treatment method for low-temperature steels, especially nickel steels and high-strength low-alloy steels. Process: Heat the steel to austenitizing temperature, perform quenching (usually water quenching or oil quenching), followed by high-temperature tempering at a relatively high temperature.Purposes:
- Grain refinement: Quenching produces an extremely fine grain structure.
- Optimal toughness: High-temperature tempering eliminates quenching stresses and transforms the microstructure into tempered sorbite or tempered bainite, which exhibits excellent impact toughness at low temperatures (low Fracture Appearance Transition Temperature, FATT).
- Increased strength: Compared with normalizing, quenching and tempering significantly improves yield strength.Applications:
- 3.5% nickel steels (e.g., A420 WPL6, 09MnNiD): This is the standard heat treatment for such materials, ensuring good toughness at approximately -100°C.
- 9% nickel steels (e.g., A353, 06Ni9) for LNG storage tanks, typically requiring double quenching and tempering to achieve outstanding low-temperature properties.
- Low-temperature pressure vessel steel plates (e.g., A517, A533 Grade B).
2. Normalizing
Normalizing is the primary heat treatment method for some medium‑ and low‑strength low-temperature steels. Process: Heat above the critical temperature, hold, then cool in still air.Purposes:
- Grain refinement: Eliminates coarse columnar grains formed during casting or forging; grain refinement is critical for improving low-temperature toughness.
- Microstructure homogenization: Uniformizes chemical composition and microstructure.Applications:
- Carbon‑manganese steels (e.g., A350 LF2, 16MnDR): For thin plates or forgings, normalizing is usually sufficient to meet low-temperature impact requirements at -46°C or -50°C.
- Large-diameter pipe fittings: Used as a pre-heat treatment before quenching and tempering, or for low-temperature pipelines with moderate requirements.
3. Normalizing + Tempering
This combined process is adopted when normalizing alone cannot meet toughness requirements or to further reduce internal stresses. Process: Normalizing followed by high-temperature tempering.Purposes:Based on the grain refinement effect of normalizing, it further eliminates internal stresses and improves ductility and toughness.Applications:
- Thick-walled low-temperature vessels and thick-walled pipe fittings: Due to slow cooling rates in the core during normalizing, thick sections tend to develop coarse or non-uniform microstructures, which can be improved by tempering.
- Thick plates of pressure vessel steels such as A516 Gr. 70.
4. Post Weld Heat Treatment (PWHT)
Although low-temperature steels generally have good weldability, PWHT is still necessary in the fabrication of welded fittings or on-site installation. Process: Heat welded joints to a temperature below Ac1 (typically 550°C – 650°C), hold, then cool slowly.Purposes:
- Relieve welding residual stresses: Prevent stress corrosion cracking or brittle fracture under low-temperature, high-stress conditions.
- Improve the heat-affected zone (HAZ): The welding thermal cycle may cause grain coarsening or hard, brittle microstructures near the weld; tempering softens this zone and restores toughness.Notes:For some high-nickel steels, tempering temperatures should not be excessively high to avoid carbide precipitation that reduces corrosion resistance or toughness.
5. Thermo-Mechanical Control Process (TMCP)
This is an advanced technology combining plastic deformation and heat treatment, mainly used in steel plate production. Process: During hot rolling, strictly control rolling temperature and reduction (controlled rolling), followed by accelerated cooling immediately after rolling (controlled cooling).Purposes:Utilize deformation-induced phase transformation to obtain an ultra-fine grain structure.Steels produced by this process exhibit excellent low-temperature toughness and high strength without subsequent quenching-tempering or normalizing.Applications:
- Modern low-temperature storage tank steel plates (e.g., part of the production process for 9Ni steel).
- Heavy-thickness low-temperature pipelines.
