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Application of Vacuum Heat Treatment in Fastener Manufacturing
The vacuum heat treatment of fasteners is a complicated process, and there are many details to master.
Why fasteners need vacuum heat treatment
We can divide the reasons into three categories:
1.Improve the strength of parts;
2.Improve surface hardness and strength;
3.Improve machinability and refine microstructure.
Vacuum quenching, vacuum tempering and austempering belong to the first category, surface hardening belongs to the second category, and process annealing belongs to the third category.
Vacuum heat treatment of fasteners
The vacuum heat treatment mechanism of different materials is often different. For example, aluminum alloy bolts and alloy steel fasteners have completely different strengthening mechanisms. In addition, fasteners using similar materials will also undergo different vacuum heat treatments according to different needs. For example, self-tapping threaded bolts are likely to be case hardened or vacuum quenched, and ordinary steel bolts are vacuum quenched and vacuum tempered.
The Importance of Vacuum Tempering
When the steel is vacuum quenched, the austenite crystal rearranges to form a center body tetragonal martensite crystal form. When this happens, the new lattice is under high stress and the steel is harder and stronger than it was before the vacuum heat treatment, but also more brittle and too brittle to serve under certain types of loads that would otherwise be very brittle. May be easily damaged.
So, we have to change this situation by vacuum tempering. The part is reintroduced into a vacuum furnace whose temperature is set below the critical austenite transformation temperature for the specified time. This way the part becomes less brittle, but also loses some hardness.
Proper tempering temperature and tempering time are two important parameters that must be controlled during vacuum heat treatment. If the tempering temperature isn’t high enough, or if the part isn’t heated at that temperature long enough, the parts won’t be tempered enough, which means they’ll still be somewhat brittle.
Quenching and tempering refers to the vacuum heat treatment process of vacuum quenching and vacuum tempering, and it is also the vacuum heat treatment process of most medium-strength (grade 8.8) and high-strength (grade 10.9, 12.9) fasteners.
There are three key and important steps:
The fastener is heated long enough to fully reach the critical austenitizing temperature (this is the austenite at which the steel will rearrange into a cubic crystal structure in the center of the surface); when fully transformed to 100% austenitic, the part Vacuum quenching a substance will rapidly lower its temperature. Transform it into a center body tetragonal martensite organization. Martensite is very hard, but it is also brittle; parts are too brittle to be put to use after vacuum quenching. To remedy this, the third step is to reheat the part to a temperature just below the critical austenite transformation temperature for a period of time. This has the effect of relieving the lattice stress caused by the quenching shock, making the part less brittle.
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