The purpose of final heat treatment is to enhance mechanical properties such as hardness, wear resistance, and strength.
(1) Quenching
Quenching can be categorized into surface quenching and full quenching. Surface quenching is widely applied due to its minimal deformation, oxidation, and decarburization, and it offers advantages like high external strength, good wear resistance, while maintaining good internal toughness and impact resistance. To improve the mechanical properties of surface-quenched parts, preparatory heat treatments like tempering or normalizing are often performed. The general process flow is as follows: blanking → forging → normalizing (or annealing) → roughing → tempering → semi-finishing → surface quenching → finishing.
(2) Carburizing and Quenching
Carburizing and quenching is suitable for low-carbon and low-alloy steels. It first increases the carbon content of the part's surface, allowing for high hardness after quenching, while the core retains a certain strength, high toughness, and plasticity. Carburizing can be either full or partial. For partial carburizing, anti-carburizing measures (such as copper plating or coating with anti-carburizing materials) must be taken for the non-carburized areas. Due to significant deformation during carburizing and quenching, and typical carburizing depths ranging from 0.5 to 2mm, the carburizing process is generally scheduled between semi-finishing and finishing.
The typical process flow is: blanking → forging → normalizing → roughing and semi-finishing → carburizing and quenching → finishing.
When the non-carburized part of a partially carburized part adopts a process plan of increasing the allowance and then removing the excess carburized layer, the step of removing the excess layer should be arranged after carburizing but before quenching.
(3) Nitriding Treatment
Nitriding is a treatment process where nitrogen atoms are infused into the metal surface to form a layer of nitrogen-containing compounds. The nitrided layer enhances the part's surface hardness, wear resistance, fatigue strength, and corrosion resistance. Since nitriding is performed at relatively low temperatures, resulting in minimal deformation, and the nitrided layer is thin (typically not exceeding 0.6-0.7mm), the nitriding process should be scheduled as late as possible. To minimize deformation during nitriding, a high-temperature stress-relieving tempering is generally required after cutting.
