1. Normalizing

Normalizing is a heat treatment process in which steel parts are heated to 30–50 °C above Ac₃ (or Acₘ), held at temperature for an appropriate time, and then cooled in still air.

The purposes of normalizing are to refine grains, homogenize the microstructure, adjust hardness, and thereby improve the strength and toughness of Seamless Steel Tubes.

The holding time for normalizing is the same as for full annealing, determined by ensuring thorough heating of the tube (i.e., the core reaches the required temperature), while also considering the steel grade, initial microstructure, furnace loading, and heating equipment. The most common cooling method for normalizing is to remove the tubes from the furnace and cool them naturally in air. For relatively large-dimension tubes, cooling rate can be controlled by air blowing, water spraying, or adjusting stacking spacing to achieve the desired microstructure and properties.

Normalizing is particularly suitable for low-carbon and low-alloy seamless steel tubes. It can eliminate banded structures, reduce inhomogeneity in mechanical properties, and provide better overall performance. Seamless steel tubes treated by normalizing are widely used in machinery manufacturing, automotive components, and other fields.

2. Annealing

Annealing is a heat treatment process in which tubes are heated to an appropriate temperature (above or below Ac₁), held for a certain time, and then cooled slowly to obtain a near-equilibrium microstructure.

Annealing is a very important heat treatment process in seamless steel tube production. Its purposes are to improve the microstructure of the tube, eliminate internal stress, reduce hardness, and enhance ductility and toughness, making the tube easier for subsequent processing (such as cold drawing and cold rolling) or meeting final service performance requirements.

Annealing standards for seamless steel tubes are generally not standalone documents, but are specified in product standards for specific applications. Annealing parameters (temperature, holding time, cooling method) and required mechanical properties vary with steel grade and application. After annealing, seamless steel tubes have low hardness and good ductility, making them ideal for subsequent processes such as cold bending and flaring.

Based on heating temperature, annealing can be classified into full annealing, spheroidizing annealing, diffusion annealing, incomplete annealing, and stress-relief annealing.

Stainless steel seamless tubes usually require solution annealing, a special type of annealing designed to dissolve carbides, eliminate work hardening, and restore corrosion resistance. Relevant standards specify the exact solution temperature and cooling method.

Main Differences Between Annealing and Normalizing

Aspect	Annealing	Normalizing
Cooling rate	Slower (furnace cool)	Faster (still air)
Undercooling	Lower	Greater
Microstructure	Coarser, near-equilibrium	Finer, higher strength/hardness

Selection Between Annealing and Normalizing

• Low-carbon tubes (carbon < 0.25%) are usually normalized instead of annealed. The faster cooling prevents precipitation of free tertiary cementite at grain boundaries, improving cold formability for stamping parts. Normalizing also increases hardness for better machinability and refines grains to enhance strength when no other heat treatment is applied.
• Medium-carbon tubes (0.25%–0.50% carbon) can also be normalized instead of annealed. Although those near the upper carbon limit may have higher hardness after normalizing, they are still machinable, and normalizing offers lower cost and higher productivity.
• Steels with 0.50%–0.75% carbon exhibit significantly higher hardness after normalizing than after annealing, making machining difficult. Full annealing is therefore generally used to reduce hardness and improve machinability.
• High-carbon or tool steels (carbon content > 0.75%) are usually subjected to spheroidizing annealing as a preliminary heat treatment. If a network of secondary cementite is present, preliminary normalizing is required to eliminate it.

3. Quenching

Quenching involves heating tubes to the quenching temperature, holding for a period, then rapidly cooling in water, salt water, oil, or (for certain materials) air.

The purpose is to achieve high hardness and wear resistance. Additionally, quenching, when followed by tempering, provides high strength, elasticity, and toughness.

Quenching methods for seamless steel tubes include direct quenching (single medium), interrupted quenching (dual medium), flame surface hardening, and induction surface hardening.

4. Tempering

Tempering is a process in which quenched seamless steel tubes are reheated to a specified temperature and then cooled in a controlled manner.

Its purposes are to relieve internal stresses caused by quenching, reduce hardness and brittleness, and achieve the desired mechanical properties. Tempering is divided into three categories: high-temperature tempering, medium-temperature tempering, and low-temperature tempering. It is often used in combination with quenching or normalizing.

Seamless steel tubes treated by quenching and tempering exhibit excellent overall mechanical properties, making them especially suitable for critical components subjected to complex stresses, such as hydraulic cylinder barrels, drive shafts, and similar parts.

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