Introduction to Heat Treatment Process for Precision Casting

Introduction to Heat Treatment Process for Precision Casting

Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing techniques, heat treatment generally does not change the shape and overall chemical composition of the workpiece. Instead, by changing the microstructure inside the workpiece or the chemical composition on the surface of the workpiece, it endows or improves the mechanical, physical, and chemical properties required for the metal workpiece. Its characteristic is to improve the internal quality of the workpiece, which is generally not visible to the naked eye. Today, precision casting manufacturer Tianyao Metal will discuss in detail what precision casting heat treatment process is.

Steel is the most widely used material in the mechanical industry, with a complex microstructure that can be controlled through heat treatment. Therefore, the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, and their alloys can also be modified by heat treatment to achieve different performance in terms of mechanical, physical, and chemical properties.

1、 Heat treatment process type:

1. Annealing: refers to the heat treatment process in which a metal material is heated to an appropriate temperature, maintained for a certain period of time, and then slowly cooled. Common annealing processes include recrystallization annealing, stress relief annealing, spheroidization annealing, complete annealing, etc. The purpose of annealing is mainly to reduce the hardness of metal materials, improve plasticity, facilitate cutting or pressure processing, reduce residual stress, improve the uniformity of microstructure and composition, or prepare microstructure for subsequent heat treatment.

2. Normalization: refers to the heat treatment process of heating steel or steel components to a temperature above or (the upper critical point temperature of steel), holding them at 30-50 ℃ for an appropriate period of time, and cooling them in still air. The purpose of normalizing is mainly to improve the mechanical properties of low-carbon steel, improve machinability, refine grain size, eliminate structural defects, and prepare the microstructure for subsequent heat treatment.

3. Quenching: refers to the heat treatment process of heating a steel component to a temperature above Ac3 or Ac1 (the lower critical point temperature of steel), holding it for a certain period of time, and then obtaining a martensitic (or bainitic) structure at an appropriate cooling rate. Common quenching processes include salt bath quenching, martensitic graded quenching, bainitic isothermal quenching, surface quenching, and localized quenching. The purpose of quenching is to obtain the required martensitic structure in steel parts, improve the hardness, strength, and wear resistance of the workpiece, and prepare the structure for subsequent heat treatment.

4. Tempering: refers to the heat treatment process in which steel parts are hardened, heated to a certain temperature below, held for a certain period of time, and then cooled to room temperature. Common tempering processes include low temperature tempering, medium temperature tempering, high temperature tempering, and multiple tempering. The purpose of tempering is mainly to eliminate the stress generated during quenching of steel parts, so that the steel parts have high hardness and wear resistance, as well as the required plasticity and toughness.

5. Quenching and tempering: refers to the composite heat treatment process of quenching and high-temperature tempering steel or steel components. Steel used for quenching and tempering treatment is called quenched and tempered steel. It generally refers to medium carbon structural steel and medium carbon alloy structural steel.

6. Carburization: Carburization refers to the process of allowing carbon atoms to penetrate into the surface layer of steel. It also gives the surface layer of low-carbon steel to the workpiece, and after quenching and low-temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the central part of the workpiece still maintains the toughness and plasticity of low-carbon steel.

7. Solid solution: Heat the alloy to a high-temperature single-phase zone and maintain a constant temperature, allowing the excess phase to fully dissolve into the solid solution, then rapidly cool to strengthen the solid solution and improve toughness and corrosion resistance, eliminate stress and softening, and continue processing and forming.

8. Aging: After solid solution heat treatment or cold plastic deformation, the alloy is placed at room temperature or slightly above room temperature to precipitate the strengthening phase, which can harden and improve strength.

2、 Metal heat treatment processes can generally be divided into three categories: overall heat treatment, surface heat treatment, and chemical heat treatment. According to the different heating media, heating temperature, and cooling methods, each major category can be divided into several different heat treatment processes. Using different heat treatment processes for the same metal can result in different microstructures and thus different properties.

1. Integrated heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to change its overall mechanical properties.

2. Surface heat treatment is a metal heat treatment process that only heats the surface of a workpiece to change its mechanical properties. In order to only heat the surface of the workpiece without excessive heat transfer into the interior of the workpiece, the heat source used must have a high energy density, that is, to provide a large amount of heat energy per unit area of the workpiece, so that the surface or local area of the workpiece can reach high temperature in a short time or instantaneously. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. Commonly used heat sources include flames such as oxyacetylene or oxypropane, induced currents, lasers, and electron beams.

3. Chemical heat treatment is a metal heat treatment process that involves changing the surface chemical composition, structure, and properties of a workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface layer of the workpiece. Chemical heat treatment is the process of heating a workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen, or other alloying elements, and holding it for a long time, so as to allow the surface of the workpiece to infiltrate elements such as carbon, nitrogen, boron, and chromium. The main methods of chemical heat treatment include carburization, nitriding, and metalizing.

Heat treatment is one of the important processes in the manufacturing of mechanical parts and molds. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the microstructure and stress state of the blank (as shown in the figure below), which is conducive to various cold and hot processing.

The above is an introduction to the heat treatment process of precision casting, hoping to be helpful to everyone.

magnesium alloy