Heat treatment is a process that changes the arrangement of atoms within a material's crystal structure. This manipulation can lead to significant changes in the mechanical properties of the material.
How Does Heat Treatment Affect Microstructure? 7 Key Insights
1. Formation of Martensite from Austenite
One of the most common effects of heat treatment is the formation of martensite from austenite. Martensite is a hard and brittle phase that forms when austenite is rapidly cooled. This process can result in an increase in the hardness and strength of the material.
2. Impact of Tempering on Hardness
Tempering after heat treatment often leads to a reduction in hardness. The reduction in surface hardness is most significant in normalizing, while it is least in annealing.
3. Role of Microstructure in Mechanical Behavior
The microstructure of metallic materials consists of small crystals called grains or crystallites. The nature of these grains, including their size and composition, plays a significant role in determining the overall mechanical behavior of the material. Heat treatment provides a way to manipulate these properties by controlling the rate of diffusion and the rate of cooling within the microstructure.
4. Changes in Metal Properties
Heat treating can improve or change various properties in metal, including strength, hardness, ductility, toughness, wear resistance, elasticity, and magnetism. The specific changes depend on the desired outcome and the specific heat treatment process used.
5. Importance in Metal Additive Manufacturing
Heat treatment is crucial in metal additive manufacturing, where it is used to enhance the mechanical properties and microstructure of metal parts produced through processes like SLM/DMLS/L-PBF printing. During the printing process, the layers of the model undergo different stresses due to thermal expansion and cooling. If these residual stresses are not removed through heat treatment, they can lead to build failure and plastic deformation.
6. Mechanisms of Property Change
There are two main mechanisms that may change an alloy's properties during heat treatment. The formation of martensite can cause the crystals to deform intrinsically. The diffusion mechanism can cause changes in the homogeneity of the alloy.
7. Benefits of High Vacuum Heat Treating
High vacuum heat treating is especially beneficial for hardening tool steels and for reactive metals like titanium. It helps improve the material's microstructure and increase its tensile strength. Vacuum heat treating is necessary for reactive metals like titanium because they tend to oxidize at elevated temperatures, leading to surface deterioration.
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