Industry insiders are aware that the heat treatment of fasteners can impart certain strength, good plasticity, toughness, etc., allowing for better utilization while reducing the occurrence of loosening, thereby ensuring the quality and reliability of fasteners.
However, in addition to general quality inspections and control, there are some special quality inspections and controls for the heat treatment of fasteners. Now let's discuss several control points of heat treatment.
I. Decarburization and Carburization
In order to timely judge the carbon control situation of the furnace, spark testing and Rockwell hardness testing can be used to make a preliminary assessment of decarburization and carburization.
Spark Testing
This involves taking a quenched part and gently grinding it with a grinding wheel from the surface towards the core to discern whether the carbon content of the surface layer and the core is consistent. This requires the operator to have proficient skills and the ability to distinguish sparks.
Rockwell Hardness Testing
This is conducted on one lateral side of a hexagonal bolt. First, gently polish one of the hexagonal planes of the quenched part with sandpaper and measure the Rockwell hardness for the first time. Then grind off about 0.5mm of this face on the grinding wheel and measure the Rockwell hardness again.
1. If the hardness values from the two tests are basically the same, it indicates neither decarburization nor carburization.
2. If the first hardness value is lower than the second, it indicates surface decarburization.
3. If the first hardness value is higher than the second, it indicates surface carburization.
In general, when the hardness difference is within 5HRC, the decarburization or carburization of the part is basically within the acceptable range when inspected by metallographic method or microhardness method.
II. Hardness and Strength
In the inspection of threaded fasteners, it is not simply a matter of converting hardness values to strength values by referring to relevant manuals, as there is a factor of hardenability involved.
1. Normally, if the material has good hardenability, the hardness across the cross-section of the screw can be evenly distributed, and as long as the hardness is acceptable, the strength and guaranteed stress can also meet the requirements;
2. When the material has poor hardenability, even if the hardness at the specified location is acceptable, the strength and guaranteed stress often do not meet the requirements. Especially when the surface hardness tends to be at the lower limit, the lower limit of hardness is often increased to control the strength and guaranteed stress within the acceptable range.
III. Re-tempering Test
The re-tempering test can check for incorrect operations where the hardness from quenching is insufficient and an excessively low temperature tempering is used to barely reach the specified hardness range, thereby ensuring the comprehensive mechanical properties of the part.
Especially for low-carbon martensitic steel fasteners made with low-temperature tempering, although other mechanical properties can meet the requirements, the residual elongation measured during the assurance stress test fluctuates greatly, far exceeding 12.5μm, and sudden fracture phenomena may occur under certain usage conditions. Such sudden fracture phenomena have been observed in some automotive and construction bolts.
This can be reduced by using the minimum tempering temperature, but special care should be taken when manufacturing 10.9 grade bolts with low-carbon martensitic steel.
IV. Hydrogen Embrittlement Inspection
The sensitivity to hydrogen embrittlement increases with the strength of the fasteners. Fasteners of grade 10.9 and above, surface hardened self-tapping screws, and combination screws with hardened steel washers that have been electroplated should undergo dehydrogenation treatment. This generally involves keeping the parts in an oven or tempering furnace at 190~230℃ for more than 4 hours to allow the hydrogen to diffuse out.
During the heat treatment process of fasteners, doing a good job at the key control points is undoubtedly very important, and this is something every excellent fastener heat treatment enterprise should accomplish.
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