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Four points need to master high-quality heat treatment of fasteners

fastenerHeat treatment, in addition to the general quality inspection and control, there are some special quality inspection and control, to master the following four points to complete high-quality fasteners heat treatment.

1. carburization and decarbonization

In high-volume production of the heat treatment process, microstructure law or, Microhardness good, only the timing of sampling. Check for a long time because of its high cost.

In order to timely determine the furnace carbon control the situation, you can use a spark detection and Rockwell hardness testing for decarbonization and carburizing preliminary judgment. Spark detection is to fire quenched parts in the grinder outside to the inside lightly grinding sparks determined the amount of carbon surface and center are the same. Of course, this requires the operator to have a skill is the ability to identify and sparks.

Rockwell hardness testing is carried out on one side of the hexagon bolt. First fire quenched part of a hexagonal plane with sandpaper lightly polished, Rockwell hardness test for the first time. Then put the face on a Grinder rubbed off About 0.5mm, Rockwell hardness test once again. If the hardness is substantially the same value twice, indicating neither decarburization nor carburization. When the hardness is lower than the previous post-secondary hardness, indicating surface decarburization. Higher than at the previous hardness after time, saying Ming carburizing. In general, twice the hardness difference is within 5HRC, or by optical method Microhardness inspection, replacement of decarburization or carburization substantially within the acceptable range.

2. The hardness and strength

inThreaded fastenersDetection, not simply based on the hardness value, check the manual, converted into intensity values. This is the middle affect a hardenability factors. Because national standards and national standards GB3098.1 GB3098.3 stipulated arbitration hardness is a cross-sectional 1/2 radius measured in parts. Tensile specimen is intercepted from 1/2 radius. Because they do not rule out the central portion of the part has a low hardness and low strength portion memory in.

Under normal circumstances, the material good hardenability, hardness screw section cross section can be evenly distributed. As long as qualified hardness, strength and proof stress can also meet the requirements. But when the material hardenability is poor, although according to the provisions of the inspection site, the hardness is qualified, but the strength and proof stress often fail to meet requirements. In particular, the surface hardness tends to be the lower limit.

In order to ensure the strength and stress control within the acceptable range, the lower limit tends to increase the hardness. Hardness control range such as 8.8: The following specifications of the M16 is 26 ~ 31HRC, M16 containing the above specifications for 28 ~ 34HRC appropriate; 10.9 control in 36 ~ 39HRC appropriate. Above 10.9 is another matter.

3. To return to the fire test

8.8 to 12.9 bolts, screws and studs should be low for 30min in 10 ℃ return fire tested in accordance with the actual production of the minimum tempering temperature. On the same sample, the difference between before and after the test shall not exceed the average of three hardness 20HV.

Return to the fire test to check due to insufficient hardness after quenching, tempering by low temperature barely reaches the improper operation of the specified hardness range, to ensure the mechanical properties of the part. Especially low carbon martensitic steel spiral Wen fasteners, low-temperature tempering, although other mechanical properties can meet the requirement, but the measure guaranteed stress, residual elongation volatile, far greater than 12.5um. And under certain conditions can occur suddenly The breakage. In some automotive and construction bolt, it has appeared suddenly rupture phenomenon. When using the minimum tempering temperature tempering, can reduce this phenomenon. However, manufacturing 10.9 bolts with low carbon martensitic steel, the It should be especially careful.

4. Check the hydrogen embrittlement

Hydrogen embrittlement sensitivity with strength fasteners increases. In addition to hydrogen should be treated for 10.9 and above the externally threaded fastener or surface hardened self-tapping screws in combination with hardened steel washers and screws, such as electroplating.

In addition to hydrogen treatment is generally in an oven or furnace, under 190 ~ 230 ℃ heat 4h above, the hydrogen diffusion out.

Threaded fastenersAvailable tighten approach, on a dedicated fixture, is screwed to the screw to withstand stress under considerable tension to ensure and maintain 48h, after releasing the threaded fastener does not have a fracture. This approach as a screening method of hydrogen embrittlement.

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