Metal Heat Treatment
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared to other processing techniques, heat treatment generally does not alter the shape or overall chemical composition of steel products. Instead, it changes the internal microstructure or modifies the surface chemical composition, thereby endowing or improving the performance of the products. The purpose of metal heat treatment is to enhance the intrinsic quality of metals, ensuring they possess the required mechanical, physical, and chemical properties. In addition to reasonable material selection and various forming processes, heat treatment is often indispensable. Steel is the most widely used material in the machinery industry, and its complex microstructure can be controlled through heat treatment, making the heat treatment of steel the main focus of metal heat treatment.
Heat Treatment Process
The heat treatment process generally includes three stages: heating, holding, and cooling. Sometimes it may only involve heating and cooling. These stages are interconnected and cannot be interrupted. Heating is one of the crucial steps in heat treatment. There are many methods for heating metals; originally, wood and coal were used as heat sources, while more recent practices now include liquid and gas fuels. The application of electricity has made heating easier to control and has reduced environmental pollution. These heat sources can be used for direct heating or can be indirectly applied through molten salts, metals, or even floating particles.
The heating temperature is one of the essential parameters in the heat treatment process. The selection and control of the heating temperature are critical for ensuring heat treatment quality. The heating temperature varies depending on the metal material being processed and the purpose of the heat treatment, but it generally needs to be above the phase transformation temperature to achieve high-temperature microstructure. Additionally, phase transformation requires a certain amount of time; therefore, once the metal surface reaches the required heating temperature, it must be held at that temperature for a specified time to ensure uniform internal and external temperatures, allowing microstructural changes to complete. This duration is referred to as holding time. In cases of high energy density heating and surface heat treatment, the heating speed is fast, often with no holding time, while chemical heat treatment usually requires a longer holding time.
Cooling is also an indispensable step in the heat treatment process. The cooling method varies according to the process, primarily focusing on controlling the cooling rate. Generally, the cooling rate for annealing is the slowest, while normalizing is faster, and quenching is faster than normalizing, although different steel grades have different requirements.
Process Classification
Metal heat treatment processes can be broadly classified into three categories: bulk heat treatment, surface heat treatment, and chemical heat treatment. Each category can be further divided into various heat treatment processes based on heating medium, heating temperature, and cooling methods. Different heat treatment processes applied to the same metal can yield different microstructures and, consequently, different properties.
Bulk heat treatment involves heating the entire metal piece and then cooling it at an appropriate rate to achieve the desired metallographic structure, thereby altering its overall mechanical properties. The bulk heat treatment of steel generally encompasses four basic processes: annealing, normalizing, quenching, and tempering.
- Annealing involves heating the metal to an appropriate temperature, holding it for a specified time based on material and metal size, and then slowly cooling it. The goal is to bring the internal structure of the metal to a near-equilibrium state, achieving good processing and usability.
- Normalizing entails heating the metal to a specific temperature and then cooling it in air. The effect of normalizing is similar to that of annealing, but it results in a finer microstructure. It is commonly used to improve cutting performance and sometimes serves as the final heat treatment for less demanding parts.
- Quenching involves heating the metal, holding it, and then rapidly cooling it in a quenching medium such as water, oil, or other inorganic salts or organic solutions. After quenching, the steel hardens but also becomes more brittle.
- Tempering is a process to reduce the brittleness of the quenched steel. It involves holding the steel at a temperature above room temperature but below 650°C for an extended period before cooling.
Surface heat treatment focuses on heating only the surface layer of the metal to modify the mechanical properties of that layer. To heat only the metal surface without transferring excessive heat to the interior, the heat source must have a high energy density, providing significant thermal energy to the metal in a short time to achieve high surface temperatures. Main methods of surface heat treatment include flame hardening and induction heating. Common heat sources include oxyacetylene, oxypropane, induction currents, lasers, and electron beams.
Chemical heat treatment is a process that alters the chemical composition, structure, and properties of the metal surface. This involves placing the metal in a medium containing carbon, salts, or other alloying elements (gases, liquids, or solids) and heating it for an extended time to allow elements to penetrate the surface. Sometimes, additional processes such as quenching or tempering are performed after element penetration. The main methods of chemical heat treatment include carburizing, nitriding, and carbonitriding.
Overall, metal heat treatment is a critical step in the manufacturing process of mechanical parts and molds. It not only ensures and enhances various metal properties, such as wear resistance and corrosion resistance, but also improves the microstructure and stress state, facilitating various cold and hot processing methods.
