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2019
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Heating Defects and Their Control in Metal Heat Treatment
I. Overheating Phenomenon
We know that overheating during the heat treatment process most easily leads to the coarsening of austenite grains, thereby reducing the mechanical properties of the parts.
1. General overheating
Overheating occurs when the heating temperature is too high or the holding time at high temperature is excessively long, leading to coarsening of the austenite grains. Coarse austenite grains can reduce the strength and toughness of steel, raise the ductile-to-brittle transition temperature, and increase the tendency for deformation and cracking during quenching. The causes of overheating typically include uncontrolled furnace temperature readings or improper material mixing—often resulting from a lack of process knowledge. Overheated microstructures can be refined by annealing, normalizing, or multiple high-temperature tempering treatments, which allow the austenite to re-austenitize under normal conditions, thereby refining the grain size.
2. Fracture inheritance
For steels with overheated microstructures, although reheating and quenching can refine the austenite grains, coarse, particle-like fracture surfaces sometimes still appear. There is considerable theoretical debate regarding the origin of this fracture-heritage phenomenon. Generally, it is believed that overheating during prior heating causes inclusions such as MnS to dissolve into the austenite and become concentrated at grain boundaries. During cooling, these inclusions tend to precipitate along the grain boundaries, making the steel prone to fracture along the coarse austenite grain boundaries when subjected to impact.
3. Inheritance of coarse tissues
When steel parts with coarse martensite, bainite, or Widmanstätten structures are re-austenitized, even if heated slowly to the conventional quenching temperature—or even slightly lower— their austenitic grains still remain coarse. This phenomenon is known as structural heredity. To eliminate the hereditary nature of coarse microstructures, intermediate annealing or multiple high-temperature tempering treatments can be employed.
II. Overburning Phenomenon
If the heating temperature is too high, not only will the austenite grains become coarse, but localized oxidation or melting may also occur at the grain boundaries, leading to a weakening of these boundaries—a phenomenon known as "overburning." After steel has been overburned, its properties are severely degraded, and cracking will develop during quenching. Overburned microstructures cannot be restored and must simply be scrapped. Therefore, it is crucial to avoid overburning during work processes.
III. Decarbonization and Oxidation
When steel is heated, the carbon in its surface layer reacts with oxygen, hydrogen, carbon dioxide, and water vapor present in the medium (or atmosphere), thereby reducing the carbon concentration in the surface layer—a process known as decarburization. After quenching, decarburized steel exhibits lower surface hardness, reduced fatigue strength, and diminished wear resistance. Moreover, the residual tensile stresses formed on the surface can easily lead to the development of reticular cracks on the surface.
The phenomenon in which the iron and alloy elements on the surface of steel react with oxygen, carbon dioxide, water vapor, and other substances present in the medium (or atmosphere) during heating, forming an oxide film, is known as oxidation. At high temperatures (generally above 570°C), oxidation of workpieces leads to a deterioration in dimensional accuracy and surface finish. Steel parts with oxide films tend to exhibit poor hardenability and are prone to developing soft spots after quenching.
To prevent oxidation and reduce decarburization, the following measures can be taken: coating the surface of the workpiece, sealing and heating it in packaging made of stainless steel foil, using a salt-bath furnace for heating, employing protective-atmosphere heating (such as purified inert gases or controlled carbon potential within the furnace), and using flame-fired furnaces (to ensure the furnace atmosphere remains reducing).
IV. Hydrogen Embrittlement Phenomenon
The phenomenon in which high-strength steels exhibit reduced plasticity and toughness when heated in a hydrogen-rich atmosphere is known as hydrogen embrittlement. Workpieces that have undergone hydrogen embrittlement can have the embrittlement eliminated through dehydrogenation treatments such as tempering or aging. Hydrogen embrittlement can be avoided by heating in a vacuum, a low-hydrogen atmosphere, or an inert atmosphere.
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