1. What is Heat Treatment?
Heat treatment is to heat the metal material to a certain temperature. After a certain period of time, it is cooled to room temperature or lower at a certain speed. Thereby improving the material structure and obtaining materials with excellent performance. Generally, it refers to the treatment of metal materials, especially steel. The technique involves heating or cooling. Often extreme temperatures, to get the desired result. Such as hardening or softening of metals.
Metal heat treatment is one of the important processes in machinery manufacturing. Compared with other processes, heat treatment generally does not change the shape and chemical composition of the workpiece. Instead, it changes the microstructure inside the workpiece or changes the chemical composition of the workpiece surface. In order to give or improve the performance of the workpiece. Its characteristic is to improve the intrinsic quality of the workpiece. And the process is not visible to the naked eye.
Common heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. In fact, the term heat treatment applies only to the process of heating and cooling. The goal is to change properties. But, heating and cooling sometimes occur as incidental stages to other manufacturing methods. Such as thermoforming or welding.
Heat treatment is primarily associated with increasing material strength. It can also be used to alter certain manufacturing goals. For example, improving machining, improving formability, restoring ductility after cold working operations. So, it is a very favorable manufacturing process. It can also improve product performance by increasing strength or other properties.
2. Heat Treatment Crafting Process
The heat treatment process generally includes three processes heating, heat preservation , cooling. Sometimes there are only two processes of heating and cooling. These processes are interconnected and uninterrupted.
When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of the steel part is reduced). This has a very adverse effect on the surface properties of the parts after heat treatment. So, the metal should usually be heated in a controlled atmosphere or protective atmosphere, in molten salt and in a vacuum. Protective heating can also be carried out by coating or packaging methods.
Heating temperature is one of the important process parameters of heat treatment process. Selection and control of heating temperature are the key points to ensure the quality of heat treatment.
The heating temperature varies with the metal material to be treated and the purpose of the heat treatment. But, it is generally heated above the phase transition temperature to get a high temperature structure. Besides, the transformation takes a certain amount of time. So, when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain period of time. Make the temperature inside and outside the same. Make the microstructural transformation complete. This period of time is called the holding time.
When high-energy density heating and surface heat treatment are used, the heating speed is extremely fast. Generally, there is no holding time. The holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the heat treatment process. Cooling methods vary from process to process. The main thing is to control the cooling rate.
3.Heat Treatment Process Classification
The metal heat treatment process can be divided into three categories. overall heat treatment, surface heat treatment and chemical heat treatment.
Depending on the heating media, temperature and cooling method, each category is divided into different heat treatment processes. Different structures can be obtained by using different heat treatment processes for the same metal. thus have different performances. Steel is the most widely used metal in the industry. And the microstructure of steel is also the most complex. So, there are most types of steel heat treatment processes.
The overall heat treatment is to heat the workpiece as a whole. Then cool at an appropriate rate. Obtain the required metallographic structure. Heat treatment of metals to change their overall mechanical properties. The overall heat treatment of steel generally has four basic processes: annealing, normalizing, quenching and tempering. That is the so-called “four fires”.
Four Types of Heat Treatment Processes
Hardening (including quenching)
Metal hardening treatment constitutes of heating the steel to a definite temperature and then cooling it quickly by plunging it into oil, brine, or water. Majority of steels require rapid cooling (quenching) for hardening, however, a few can be air-cooled with the same results. Although hardening enhances the hardness and strength of the steel, it makes it less ductile. Usually, the harder the steel, the more brittle it becomes. To remove some of the brittleness, the steel should be tempered after hardening.
Many nonferrous metals can be hardened and their strength can be increased by controlled heating and rapid cooling. In this case, the process is called heat treatment, instead of hardening.
Generally, annealing is the opposite of hardening. Metals are annealed to relieve internal stresses, to soften them, to make them more ductile, and to refine their grain structures. Annealing comprises of heating a metal to a particular temperature, holding it at that temperature for a definite time, and then cooling the metal to room temperature. The cooling process depends on the metal and the characteristics desired. Some metals are furnace-cooled, while others can be cooled by burying them in ashes, lime, or other insulating materials.
Welding produces areas, which have molten metal next to other areas, which are at room temperature. As the weld cools, internal stress occurs along with brittleness and hard spots. Welding can in reality weaken the metal. Annealing is just one of the techniques used for correcting these problems.
Normalizing a metal means bringing it up to recrystallization temperatures by soaking, and then pulling it from the furnace and allowing it to cool in the atmosphere. Many of the properties of annealed metals are evident in normalized metals, but because there is not quite the same evenness of cooling, the grains tend to be a little less regular. Still, a much smaller temperature differential than is found in freezing metal means a normalized product is less brittle.
The cooling rate found in normalizing creates smaller grains in the metal than annealing does, which means that in general, it will be stronger or harder than annealed metal.
Finding the right mixture of hardness-and-ductility can also be achieved through a process called tempering. Tempering is often done with quenched steel to make it less brittle while preserving some of the hardness. In tempering, a metal is reheated yet again, but now to a lower temperature than in annealing, normalizing, or quenching.
Martensite is not a stable molecule in heat—it is achieved under shock—so tempering steel means destabilizing the martensite to let it start to convert to cementite and ferrite. A range of temperatures and length of times in the temper oven will influence how much of the martensite is converted and therefore how soft the metal becomes. For example, metal springs may be tempered at higher temperatures for increased elasticity compared to tools tempered at lower temperatures to maintain hardness.
Tempering is often used to relieve the internal stresses in a quenched material. A metal that’s undergone other heat stress like welding or blacksmithing can be tempered to allow the molecules within to relax a little into one another.