Fastener heat treatment, in addition to the general quality inspection and control, there are some special quality inspection and control, now we say the heat treatment of several control points
01 Decarburization and carburization
In order to timely determine the furnace carbon control, you can use spark detection and Rockwell hardness test for decarburization and carburization for preliminary judgment.
Spark test.
Is the quenched parts, in the grinder from the surface and inside gently grinding spark judgment surface and the heart of the carbon amount is consistent. But this requires the operator to have skillful techniques and sparks to identify the ability.
Rockwell hardness test.
Is carried out on a side of the hexagonal bolt. First the hardened parts of a hexagonal plane with sandpaper gently polished, measured the first Rockwell hardness. Then this surface in the sander to grind away about 0.5mm, and then measure the Rockwell hardness.
If the hardness value of the two times is basically the same, that neither decarburization, nor carburization.
When the former hardness is lower than the latter hardness, it means that the surface is decarburized.
The former hardness is higher than the latter hardness, that the surface carburization.
In general, the two hardness difference of 5HRC or less, with the metallographic method or microhardness method, the parts of the decarburization or carburization is basically within the scope of qualification.
02 Hardness and strength
In the threaded fastener test, can not be simply based on the hardness value of the relevant manual, converted into strength value. There is a hardenability factor in the middle.
In general, the hardenability of the material is good, the hardness of the cross-section of the screw section can be uniformly distributed, as long as the hardness is qualified, the strength and ensure that the stress can also meet the requirements;
When the hardenability of the material is poor, although according to the prescribed part of the check, the hardness is qualified, but the strength and guarantee stress often do not meet the requirements. Especially when the surface hardness tends to the lower limit, in order to control the strength and guarantee stress in the qualified range, often improve the lower limit value of hardness.
03Retempering test
Retempering test can check the quenching hardness is not enough, with too low temperature tempering to barely reach the specified hardness range of incorrect operation, to ensure that the comprehensive mechanical properties of the parts.
Especially low carbon martensitic steel manufacturing threaded fasteners, low temperature tempering, although other mechanical properties can meet the requirements, but the measurement of the guaranteed stress, the residual elongation fluctuation is very large, far greater than 12.5um, and in some conditions of use will be a sudden fracture phenomenon, in some automobiles and construction of bolts, has appeared in the phenomenon of sudden fracture.
When the lowest tempering temperature tempering, can reduce the above phenomenon, but with low carbon martensitic steel manufacturing 10.9 grade bolts, should be particularly careful.
04 Inspection of hydrogen embrittlement
The susceptibility to hydrogen embrittlement increases with the strength of the fastener. External threaded fasteners of grade 10.9 and above, surface hardened self-tapping screws, combination screws with hardened steel washers, etc. should be dehydrogenated after plating.
Dehydrogenation treatment is generally in the oven or tempering furnace, holding at 190~230 ℃ for more than 4h, so that hydrogen diffusion out.
"Iron still needs its own hardness!" No matter how the market situation changes, refining the manufacturing process is also one of the effective means to resist risks.
In the fastener heat treatment process, it is undoubtedly very important to do a good job in the key control points, which is also one of the things that every good fastener heat treatment enterprise should do well.
Post time: Jan-17-2024