Quenching defects on the surface can include cracks, cracking, deformation (such as bending, size expansion or shrinkage beyond tolerance), surface corrosion, and other microstructural issues like excessive hardness (either too high, too low, or uneven), thickened structure, oxidation (including surface and intergranular oxidation), decarburization, and abnormal microstructures.
1. **Quenching Distortion and Quench Cracks**
Quenching distortion is a common issue that occurs during the cooling process. While it's not entirely avoidable, it only becomes a problem when it exceeds allowable tolerances or cannot be corrected later. Proper material selection, improved design with sufficient structural integrity, and careful choice of quenching and tempering methods can significantly reduce distortion. Distorted parts can often be straightened using hot or cold working, spot heating, or re-tempering.
Cracks, however, are irreversible defects caused by excessive thermal or mechanical stress during quenching. Preventive measures such as controlling stress distribution, ensuring proper raw material quality, and optimizing part design are essential to avoid them.
2. **Oxidation, Decarburization, and Overheating**
If proper surface protection is not used during quenching, oxidation and decarburization can occur. These defects lower the hardenability of the surface, leading to failure to meet technical specifications, surface mesh cracks, or even out-of-tolerance dimensions. To prevent this, finished parts should be quenched in a protective atmosphere or salt bath furnace. Small batches may be coated with an oxidation-resistant layer.
Overheating during quenching results in coarse martensite formation, which increases the risk of quench cracks and reduces impact toughness. It can also lead to microcracks along grain boundaries. The correct quenching temperature, appropriate holding time, and strict furnace control are crucial for preventing overheating. If there is enough machining allowance, overheated parts can be re-annealed and re-quenched to refine the grain structure.
Overburning is a severe form of overheating, commonly seen in high-speed steels, where fishbone-like eutectic structures form. This makes the steel extremely brittle and often results in scrap.
3. **Insufficient Hardness**
After quenching and tempering, insufficient hardness can result from inadequate heating, surface decarburization, excessive retained austenite in high-carbon alloy steels, or improper tempering. In some cases, like with CR bearing steel, surface quenching may produce lower hardness than the core due to prolonged vapor film formation, causing what is known as reverse quenching. This happens when the surface cools more slowly than the interior, leading to a softer outer layer.
4. **Soft Spots**
Soft spots refer to localized areas of uneven hardness on quenched parts. These spots often show a distinct contrast in hardness and are typically caused by coarse or uneven original microstructure (such as severe segregation, large carbide clusters, or free ferrite), contaminated quenching media, oxide scale on the part surface, or improper movement in the quenching solution. These issues can lead to vapor film formation in certain areas, affecting the cooling rate and resulting in soft regions. A detailed analysis of the microstructure and process parameters is usually needed to identify the root cause.
5. **Other Microstructural Defects**
Qualified parts must not only meet hardness requirements but also have a microstructure that conforms to specified standards—such as the amount of martensite, retained austenite, undissolved ferrite, and the distribution and morphology of carbides. When these specifications are exceeded, even if the hardness test passes, the part may still be rejected. Common structural defects include coarse martensite from overheating, network carbides in carburized or tool steels, large carbides, excess free ferrite in quenched and tempered steels, and excessive retained austenite in tool steels.
By understanding and addressing these defects through proper material selection, process control, and post-treatment techniques, manufacturers can significantly improve the quality and reliability of quenched components.
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PPF (luminous flux density) is an indicator used to measure the light energy output of plant lamps, indicating the flow of light energy through a unit area in a unit time. The higher the PPF, the following effects on plant lights:
1. Provide more adequate light energy: high PPF means that plant lamps can output more light energy, which is very important for plant photosynthesis. Photosynthesis is a key process for plant nutrient synthesis and growth, and high PPF can provide more adequate light energy and promote the efficiency of plant photosynthesis, thus promoting plant growth and development.
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