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Why does metal harden? Explore the mysterious process of heat treatment!
The hardness of a metal is often closely related to the scenario in which it is used. Whether it is construction, manufacturing or daily practical tools, hardness is one of the key physical properties. But did you know that the hardness of metal can be adjusted through a series of heat treatment processes? Heat treatment not only hardens metals, it also affects their other important properties such as strength, toughness, ductility, etc. In this article, we will explore the basic principles of heat treatment and its important role in the process of hardening metals.
Basic concepts of heat treatment
Heat treating is a group of industrial, thermal and metalworking processes designed to change the physical and, in some cases, chemical properties of a material. The main applications of heat treatment are concentrated in the field of metallurgy. Common treatment methods include tempering, surface hardening, precipitation strengthening, etc.
Physical processes: changes in microstructure
The microstructure of metallic materials is made up of small particles called "grains" or crystals. The properties of grains, such as grain size and composition, are one of the effective factors determining the mechanical behavior of metals. Heat treatment can effectively control the diffusion rate and cooling rate of the grains, thereby manipulating the properties of the metal.
Heat treatment can change the mechanical properties of metal alloys, such as hardness, strength, toughness, ductility and elasticity.
Mechanisms of heat treatment
During heat treatment, metals are heated or cooled, often to extreme temperatures, to achieve the desired change in properties. These temperature changes cause the metal's crystal structure to change, resulting in an increase or decrease in hardness. Two main mechanisms involve martensite formation and diffusion mechanisms, both of which can significantly change the properties of the alloy.
When a metal is cooled rapidly, there may not be time for insoluble atoms to migrate out of solution, resulting in a structural change known as a "diffusionless transformation."
Effect of alloy composition
The specific composition of the alloy system can have a significant impact on the results of heat treatment. When the percentage of each component in the alloy is within the appropriate range, the alloy will form a single continuous microstructure; if the components are unbalanced, multiple different microstructures may be formed simultaneously. For example, alloys below 0.77% carbon are called subeutectic alloys, while those above this value are called supereutectic alloys.
Heat treatment methods
There are a variety of heat treatment techniques available, the most common methods include:
- Annealing: Heating to a specific temperature and slowly cooling to produce a fine microstructure.
- Normalizing: Aims to make the grain size and composition uniform, usually applicable to ferrous alloys.
- Stress relieving: Reducing or removing internal stress in metal.
- Aging: Affecting the mechanical properties of metal components by pre-treating them.
- Quenching: Rapid cooling of a metal to promote martensitic transformation, usually in ferrous alloys.
Common heat treatment processes
For example, in steel alloys, varying amounts of alloy components are heated to the austenite stage and then rapidly transformed to the martensitic structure through quenching. This process may increase the hardness of the alloy, but its plasticity will decrease. For example, steel containing a certain proportion of carbon will produce a different microstructure within the right temperature range.
End: The importance of thinking
The heat treatment process is not only about physical changes, but also involves the grains, alloy composition and different microstructures formed. Mastering this knowledge will enable engineers to make more effective decisions in metal processing. In your daily life, have you ever realized that there are so many scientific mysteries hidden behind seemingly ordinary metal objects?
