Hardening cooling method for screw fasteners
Screw fasteners seem to be simple and small iron, but in the production and manufacturing, it is a test of the technical strength of a factory, screw fasteners in production, a few hours may cause screw tolerances are too large, resulting in the failure of assembly, or in the surface treatment of screw fasteners, the surface treatment temperature and solution is not well controlled, it is easy to lead to screw fasteners Now the question. Today, let’s briefly introduce the hardening cooling method of screw fastener surface treatment.
The quenching cooling method of screw fastener surface treatment includes the type of quenching cooling medium used. Cooling time. The final cooling temperature of screw fastener surface is a very important and difficult technological problem in the tempering heat treatment of large size bolt fasteners
- Segregation of carbon alloying elements leads to different cooling rates in different regions of the warm upsetting parts, resulting in different martensitic transition temperatures. Therefore, component segregation must be considered when determining the final cooling temperature of the surface of bolt fasteners. If the final cooling temperature of the surface is to be controlled, time-limited quenching should be used. When the cooling is close to the risk temperature of quenching crack, the bolt fasteners should be removed from the quenching medium for slow cooling to prevent the advanced tissue transformation in some areas.
- The quenching effect of screw fasteners determines the cooling mode and surface final cooling temperature in the cooling medium. At present, all aqueous solutions use PAG polymer, which has a wide cooling rate range and easy concentration control. For medium carbon alloy structural steel, 9%~ is generally used 11% concentration ratio, mesh belt furnace discharge into the cooling tank in the form of free fall, this cooling mode has a great impact on the screw and the cooling rate of each section, in addition to the warm upsetting component itself caused by component segregation caused by martensitic transition temperature difference, and the cooling speed of different local areas caused by non-martensitic phase transition, resulting in the specific tolerance of tissue along the section, several stress overlap It is easy to crack bolt fasteners.
The final cooling temperature of the surface of the screw fastener is also one of the important reasons for the crack of the screw fastener. Practice has proved that the general quenching crack occurs in the low temperature stage of the late cooling period. At this time, the screw fastener is not plastic, resulting in the stress can not be relaxed due to the plastic deformation When the chemical composition of steel is constant, the strongest cooling medium is used, and it is difficult to maintain the stability of austenite in a certain section without precipitation of iron particles and pearlite transformation. That is to say, reducing the final cooling temperature can not improve the comprehensive mechanical properties of screw fasteners, but increase the tendency of quenching cracking. For the medium carbon alloy steel, the final cooling is all The temperature should not be controlled below 250℃, and the cooling time should not be too long.