High vacuum will cause the volatile of alloy elements in the material
Because there are many shortcomings salt bath furnace heating: barium chloride used toxic raw materials, the environmental pollution by waste gas, waste residue, quenching homework and deoxidization in dregs, employees in front of the furnace under high temperature radiation, hard working conditions, quenching oil, black smoke billowed from the roof, salt bath, nitrate bath waste collection and treatment of difficult, these are one of the reasons for the heat treatment workshop, disorder and poor. As a result, immediate benefits can no longer be obtained at the expense of the environment and the health of employees.
The temperature difference between the surface and the core of the parts and the unevenness of heating and cooling are the main causes of quenching deformation and cracking. The quenching heating and subsequent heat preservation are for the full dissolution of alloy elements and the homogenization of austenite.
Should be set according to relevant technical data are given to quenching heating temperature, requiring high secondary hardening and tempering stability limit, lower limit to prevent cracking and deformation of the occasion, refers to whole furnace heat preservation time the thickest part of heart to stay time after quenching temperature, with the amount of electric power, furnace charging, furnace charging mode and size and other factors.
It is feasible to estimate the insulation time H by H=30+ thickness /50*10min. Quenching heating temperature exceeds 1000 ℃, typically after two preheating: first 580 ℃, 850 ℃ for the second time. The first preheating time is twice as long as the quenching time H. The second preheat insulation time is about H/3.
Vacuum furnace heating, vacuum degree is too high will cause the volatilization of alloying elements in materials, commonly used physical quantities Pa (PASCAL), bar (, 1 bar = 100 kpa) and Torr (, 1 Torr = 133.322 Pa) to indicate vacuum degree, there is a certain conversion relationship between the three. Low vacuum is used in the preheating stage and high vacuum is used in the heating and heat preservation stage. The cooling rate of high pressure gas quenching is related to the flow rate of cooling water, types of gas, temperature and flow rate of heat exchanger, the pressure of gas, the form and distribution of air flow, the size of workpiece and the amount of furnace charge. Studies have shown that the cooling rate of nitrogen at low temperature of 10bar increases by 30%~40% compared with 6bar, and that the cooling rate of 20bar increases by more than 80% compared with 6bar.