Key Control Points in Fastener Heat Treatment: Ensuring Quality and Performance through Comprehensive Control
In modern manufacturing, heat treatment is a crucial process for enhancing the performance, extending the service life, and ensuring the safety and reliability of fasteners. Through heat treatment, fasteners can achieve the desired strength, good ductility, and toughness, thereby better meeting usage requirements and reducing the occurrence of loosening. However, in addition to routine quality inspection and control, heat treatment of fasteners requires special attention to several key control points to ensure the precision of the process and the high quality of the products.
I. Precise Control of Decarburization and Carburization
Decarburization and carburization are common issues in the heat treatment process, directly affecting the surface hardness and wear resistance of fasteners. To promptly assess the carbon control in the furnace, the following two detection methods can be used:
1. Spark Testing
Spark testing is a rapid method to determine whether the surface carbon content is uniform. By lightly grinding a quenched part on a grinding wheel and observing the color, length, and shape of the sparks, one can judge whether the carbon content of the surface and core is consistent. This method requires operators to have proficient skills and the ability to identify sparks.
2. Rockwell Hardness Testing
Rockwell hardness testing is another commonly used detection method. The specific procedure is as follows:
First, lightly polish one hexagonal face of a quenched part with sandpaper and measure the Rockwell hardness.
Then, grind off about 0.5mm of the same face on the grinding wheel and measure the Rockwell hardness again.
Result Interpretation:
If the hardness values of the two measurements are essentially the same, it indicates that there is neither decarburization nor carburization.
If the first measurement is lower than the second, it indicates surface decarburization.
If the first measurement is higher than the second, it indicates surface carburization.
Generally, when the difference between the two hardness values is within 5HRC, parts are considered to be within the acceptable range of decarburization or carburization when checked by metallographic or microhardness methods.
II. The Deep Connection Between Hardness and Strength
In the inspection of threaded fasteners, it is not advisable to simply convert hardness values to strength values by consulting reference tables, as the hardenability of the material significantly affects the results. Hardenability refers to the ability of a material to achieve a martensitic structure during quenching, which directly impacts the hardness distribution and mechanical properties of the part.
Materials with Good Hardenability
When the material has good hardenability, the hardness distribution across the cross-section of the screw is uniform. As long as the hardness is qualified, the strength and proof stress can also meet the requirements.
Materials with Poor Hardenability
When the material has poor hardenability, even if the hardness is qualified at the specified locations, the strength and proof stress may not meet the requirements. Especially when the surface hardness is close to the lower limit, to ensure that the strength and proof stress are within the acceptable range, the lower limit of hardness is often increased.
III. The Importance of Re-tempering Tests
Re-tempering tests are an essential means of checking whether the hardness is insufficient due to quenching and whether a low temperature is used for tempering. This test ensures that the comprehensive mechanical properties of the parts meet the requirements, especially in threaded fasteners made of low-carbon martensitic steel. Low-temperature tempering can lead to significant fluctuations in residual elongation, and even sudden fractures under certain operating conditions.
Using the minimum tempering temperature can reduce the occurrence of the above phenomena. However, when manufacturing Grade 10.9 bolts with low-carbon martensitic steel, extra caution is required.
IV. The Critical Nature of Hydrogen Embrittlement Inspection
The sensitivity to hydrogen embrittlement increases with the strength of the fastener. For Grade 10.9 and above external threaded fasteners, surface-hardened self-tapping screws, and combination screws with hardened steel washers, hydrogen removal treatment should be performed after electroplating. Hydrogen removal treatment is usually carried out in an oven or tempering furnace at a temperature of 190-230°C for at least 4 hours to ensure that hydrogen diffuses out.
V. Summary: Comprehensive Control of Key Heat Treatment Points
The precise control of key points in the heat treatment of fasteners is crucial. It not only affects the performance and quality of the products but also directly impacts their reliability and safety in actual use. Every excellent fastener heat treatment enterprise should pay attention to these key control points to ensure that each batch of products meets high-quality standards. Through strict inspection and control, it is possible to effectively reduce product defects caused by improper heat treatment, thereby enhancing the overall performance and service life of fasteners.
In actual operations, it is recommended that enterprises combine advanced detection equipment with professional technicians to regularly evaluate and optimize the heat treatment process to meet the increasingly complex market demands and high-quality requirements.
