Due to the many disadvantages of the heated salt bath furnace, the cesium chloride used is toxic, the exhaust gas and waste residue pollute the environment, the quenching operation and deoxidation slag, the employees are exposed to high temperature radiation in front of the furnace, the working conditions are difficult, and the quenching oil is cooled, the black Smoke rises straight into the roof, and the salt bath and salt bath waste collection and disposal are difficult. These are one of the reasons for the expansion, chaos and poorness of the heat treatment plant. For this reason, the immediate interests can no longer be exchanged for the cost of sacrificing the environment and the health of employees. The government departments have strictly ordered the phase-out and prohibition of newly-built bath 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 quench deformation and cracking. Quenching heating and subsequent insulation are for the full dissolution of alloying elements and the homogenization of austenite.
The quenching heating temperature shall be set according to the data given in the relevant technical data, and the upper limit shall be taken for occasions with high secondary hardening and tempering stability to prevent the cracking and deformation occasions from taking the lower limit. 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 the power of the furnace, the amount of the furnace, the charging method and the size of the parts.
It is feasible to estimate the holding time H with H=30+thickness/50*10 min. When the quenching heating temperature exceeds 1000°C, two preheatings are generally performed: 580°C for the first time and 850°C for the second time. The first preheating holding time is twice the quenching holding time H; the second preheating holding time is about H/3.
Vacuum furnace heating, the vacuum degree is too high will cause the volatilization of the alloy elements in the material, commonly used physical quantity Pa (Pascal), bar (bar, 1bar = 100kpa) and Torr (Torr, 1torr = 133.322pa) to mark the degree of vacuum, three There is a certain conversion relationship between. The preheating stage uses a lower degree of vacuum, and the degree of vacuum in the heating and holding stages is relatively high. The cooling rate of high pressure gas quenching is related to the flow of heat exchanger cooling water, the type of gas, temperature and flow rate, the pressure of the gas, the form and distribution of the gas flow, and the size of the work piece and the amount of charge. Studies have shown that the cooling rate of nitrogen at 10 bar is 30%~40% higher than 6bar, and 20bar is more than 80% higher than 6bar.