The purpose of heat treatment is to enhance the mechanical properties of steel pipes and precision steel pipes, eliminate residual stresses, and improve the machinability of steel metals. Depending on the specific objectives, heat treatment processes can be broadly classified into two categories: preparatory heat treatment and final heat treatment.
Preparatory Heat Treatment
The goal of preparatory heat treatment is to improve workability, eliminate internal stresses, and prepare a favorable metallurgical structure for final heat treatment. The processes involved include annealing, normalizing, aging, and quenching and tempering.
(1) Annealing and Normalizing
Annealing and normalizing are applied to hot-worked blanks. Carbon steels and alloy steels with a carbon content greater than 0.5% are often annealed to reduce their hardness and facilitate cutting. Conversely, those with a carbon content below 0.5% undergo normalizing to avoid excessive softness that can lead to tool sticking during cutting. Annealing and normalizing also refine grain structures, homogenize microstructures, and prepare the material for subsequent heat treatments. These processes are typically performed after blank manufacturing and before rough machining.
(2) Aging Treatment
Aging treatment is primarily used to eliminate internal stresses generated during blank manufacturing and machining. For parts requiring general precision, a single aging treatment before finishing is sufficient to avoid excessive transportation. However, for parts with higher precision requirements (such as the box of a coordinate boring machine), two or more aging treatments may be necessary. Simple parts generally do not require aging treatment.
Apart from castings, precision parts with poor rigidity (e.g., precision lead screws) often undergo multiple aging treatments between rough and semi-finish machining to eliminate internal stresses and stabilize processing accuracy. Some axial parts also require aging treatment after straightening.
(3) Quenching and Tempering
Quenching and tempering involves quenching followed by high-temperature tempering. This process yields a uniform and fine-grained tempered sorbite structure, preparing the material for reduced deformation during subsequent surface quenching and nitriding. Thus, quenching and tempering can also serve as preparatory heat treatment.
Due to its excellent comprehensive mechanical properties, quenching and tempering can also be used as the final heat treatment for parts with moderate requirements for hardness and wear resistance.
Final Heat Treatment
The objective of final heat treatment is to improve mechanical properties such as hardness, wear resistance, and strength.
(1) Quenching
Quenching can be surface quenching or through quenching. Surface quenching is widely used due to minimal deformation, oxidation, and decarburization. It offers high external strength, good wear resistance, and maintains good internal toughness and impact resistance. To enhance the mechanical properties of surface-quenched parts, preparatory heat treatments like quenching and tempering or normalizing are often performed beforehand. The typical process flow is: cutting → forging → normalizing (or annealing) → rough machining → quenching and tempering → semi-finish machining → surface quenching → finish machining.
(2) Carburizing and Quenching
Carburizing and quenching are suitable for low-carbon and low-alloy steels. This process increases the carbon content of the part's surface, resulting in high surface hardness after quenching, while the core retains moderate strength, high toughness, and plasticity. Carburizing can be either full or partial, with the latter requiring anti-carburizing measures (e.g., copper plating or anti-carburizing coatings) for non-carburized areas. Due to significant deformation and a carburizing depth typically between 0.5 and 2mm, the carburizing process is generally scheduled between semi-finish and finish machining.
The typical process flow is: cutting → forging → normalizing → rough and semi-finish machining → carburizing and quenching → finish machining.
When the non-carburized portion of a partially carburized part is enlarged to allow for the removal of excess carburized layers, this removal step should occur after carburizing but before quenching.
(3) Nitriding
Nitriding involves infiltrating nitrogen atoms into the metal surface to form a layer of nitrogen compounds. The nitrided layer enhances the part's surface hardness, wear resistance, fatigue strength, and corrosion resistance. Since nitriding operates at low temperatures with minimal deformation and produces a thin layer (typically no more than 0.6-0.7mm), the nitriding process should be scheduled as late as possible. To minimize deformation during nitriding, a stress-relieving high-temperature tempering is typically performed after cutting.
