What happens to the vacuum in the vacuum furnace? What are the disadvantages?
However, there are many disadvantages in heating the salt bath furnace: the cerium chloride raw material used is toxic, the generated exhaust gas and waste slag pollute the environment, the quenching operation and the deoxidation slag, the employees are subjected to high temperature radiation in front of the furnace, the working conditions are difficult, and the quenching oil is cooled. The black smoke rises directly into the roof, and the collection and disposal of the alkali bath and the salt bath waste are difficult. These are one of the reasons for the dirty, chaotic and poor heat treatment workshop. To this end, it is no longer possible to exchange immediate interests at the expense of the environment and employee health. The relevant government departments have strictly ordered the phase-out and prohibition of new furnace equipment.
The temperature difference between the surface of the part and the core, and the uneven heating and cooling are the main causes of quenching deformation and cracking. Quenching heating and subsequent holding are for the complete dissolution of the alloying elements and the homogenization of the austenite.
The quenching heating temperature should be set according to the data given in the relevant technical data. The upper limit is required for the case where the secondary hardening and tempering stability are high, and the lower limit is taken when the deformation and cracking are prevented. The holding time refers to the heart of the thickest part of the whole furnace. The residence time after reaching the quenching temperature is related to factors such as electric furnace power, furnace loading, furnace charging method and part size.
It is feasible to estimate the holding time H with H = 30 + thickness / 50 × 10 min. When the quenching heating temperature exceeds 1000 ° C, it is generally preheated twice: the first time 580 ° C, the second time 850 ° C. The first preheating holding time is twice the quenching holding time H, and the second preheating holding time is about H/3.
When the vacuum furnace is heated, the vacuum is too high, which will cause the alloying elements in the material to volatilize. The common physical quantities P a (Pascal), bar (bar 1 bar = 105 Pa) and Torr (Torr, 1 Torr = 133.322 Pa) are used to mark the degree of vacuum. There is a certain conversion relationship between them. The preheating stage uses a lower degree of vacuum, and the heating and holding stages have a higher degree of vacuum, generally between 10-1 bar, 10-2 bar and 10-3 bar. The cooling rate of high pressure gas quenching and the flow rate of heat exchanger cooling water; gas type, temperature and flow rate; gas pressure, gas flow form and distribution; workpiece size and furnace loading. Studies have shown that the cooling rate of 10 bar low temperature nitrogen is 30% to 40% higher than 6bar, and 20bar is more than 80% higher than 6bar.
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