Defects in Forgings Due to Improper Heating Processes

The heating process is a crucial stage in the production of forgings, directly influencing their quality and performance. Improper heating can lead to various defects that not only affect the structure and strength of the forgings but may also have serious consequences for subsequent processing and the final product's performance. Based on the causes of these defects, they can be categorized as follows.

Defects Caused by Medium Influence

The medium affects the blank primarily through changes in its chemical composition, including oxidation, decarburization, carburization, and sulfidation or copper infiltration.

1. Decarburization

 
Decarburization occurs when the carbon in the metal surface is oxidized during high-temperature heating, significantly reducing the carbon content in the surface layer. The depth of the decarburized layer is closely related to the material composition, furnace gas composition, heating temperature, and holding time. This phenomenon is particularly pronounced when heating high-carbon and high-silicon steels in oxidizing atmospheres. Decarburization not only reduces the strength and fatigue resistance of the components but can also lead to local defects, making them susceptible to fatigue failure during use.

2. Carburization

 
During heating in oil furnaces, carburization can occur on the surface of the forgings. This is especially true near the oil furnace nozzles or in areas where two nozzles intersect, leading to a reducing atmosphere due to incomplete combustion and the formation of a carburizing layer. The carburization layer can be as thick as 1.5 to 1.6 mm, with a carbon content reaching 1% (by weight) and locally exceeding 2%. This not only impacts the machining performance of the forgings, leading to tool wear during cutting, but may also cause instability in subsequent processing.

Abnormal Changes in Internal Microstructure

 
During heating, excessively high temperatures or prolonged holding times can result in abnormal changes in the metal's microstructure.

1. Overheating

 
Overheating refers to heating the metal blank beyond the required temperature range or maintaining it in the forging and heat treatment temperature range for too long, leading to coarse grain formation. Overheated carbon steels often exhibit Widmanstätten structures, while martensitic steels may develop intra-granular textures. In cases of severe overheating of alloy steels, the fracture surface may show stone-like or banded features. Coarse grains significantly decrease mechanical properties, particularly impacting impact toughness and the material's performance under high-stress conditions.

2. Stability Issues

 
For overheated structural steels, normal heat treatment (such as normalizing or quenching) can improve the microstructure and restore performance, a phenomenon known as unstable overheating. However, severe overheating of alloy structural steels may result in a microstructure that cannot be fully eliminated, referred to as stable overheating, which often affects the material's performance in high-temperature applications.

Defects Due to Uneven Temperature Distribution

 
Controlling the heating rate of the blank is critical during the forging process, especially for large ingots and high-alloy steels with poor thermal conductivity.

Thermal Stress

 
If the temperature distribution is uneven during the heating phase, leading to significant temperature differences between the interior and exterior, considerable thermal stress can develop in the blank. When thermal stress exceeds the blank's strength limit, cracks may form, typically radiating from the center outward. This situation is common in large cross-section forgings or high-alloy steel blanks, potentially resulting in complete cross-sectional cracks, severely compromising the overall quality and performance of the forgings.

Burning Phenomenon

 
Burning refers to the metal blank being subjected to excessively high temperatures or prolonged exposure in high-temperature areas, allowing oxygen and other oxidizing gases to penetrate between the metal grains.

Oxidation and Melting

 
This phenomenon can react with elements like iron, sulfur, and carbon, forming easily meltable oxide eutectics that disrupt the connections between grains, leading to a dramatic reduction in plasticity. Forgings that have been burned often exhibit brittleness during processing, with minor impacts during rough machining potentially causing cracks. For carbon steels, melting at grain boundaries and severe oxidation during burning can render the forgings unprocessable and unusable.

Copper Embrittlement

 
Copper embrittlement is typically characterized by the appearance of surface cracking on the forgings, where light yellow copper is distributed along the grain boundaries under high magnification.
This phenomenon primarily arises from residual copper oxide debris reacting at high temperatures or when the copper content in steel exceeds 2% during heating in oxidizing atmospheres, forming a copper-rich layer. This not only affects the mechanical properties of the forgings but may also lead to brittle fractures during use, compromising product reliability and safety.

Conclusion

 
Defects in forgings resulting from improper heating processes significantly impact product quality and performance. By effectively controlling heating temperature, holding time, heating medium, and its distribution, these defects can be substantially reduced. A comprehensive understanding of the various defects that may arise during heating and their causes is essential for implementing preventive measures in actual production, ensuring the quality and performance of forgings, enhancing production efficiency, and minimizing losses.