Low pressure gas protection reduces metal element evaporation

To reduce or avoid the evaporation of alloying elements, pressure can be increased by filling the vacuum heating chamber with high-purity neutral or inert gas (such as nitrogen, argon, helium, etc.). The pressure inside the vacuum chamber is adjusted within a range of 0.1 to 650 Pa. This vacuum protection atmosphere heat treatment method (also referred to as low-pressure gas protection method) can prevent evaporation of the alloy elements, and can obtain a bright surface. Another advantage of this method is that it increases convective heat transfer, which is more conducive to uniform heating of parts.

The purity of the gas used in the vacuum protection atmosphere heat treatment method should generally be greater than 99.99%. If the purity of the recharged neutral or inert gas is not sufficient, the evaporation rate of the alloying element will not only be reduced, but will be increased, resulting in depletion of the alloy elements on the surface of the part and the corrosion resistance.

Stainless steel contains a large amount of alloy elements such as Cr, Ni, Al, Mn, and Ti, and its vapor pressure is relatively high. Vacuum solution treatment requires higher vacuum (1.33×10-2～1.33×10-3Pa); in order to prevent the evaporation of alloying elements, vacuum partial pressure treatment should be adopted, that is, the vacuum of the heating chamber is first pumped to 1.33×10-2～1.33×10-3Pa, and then refill with high-purity neutral or inert gas, keeping the inflation pressure at 133~13.3 Pa or higher. The high-purity neutral or inert gas must always be recharged into the heating chamber.

The first is to make the gas fully protect and increase the pressure to reduce the evaporation of metal elements; the second is to increase the convective heat transfer of the gas, which is more conducive to the uniform heating of the parts. Vacuum aging can use a higher degree of vacuum (1.33 × 10-2 ~ 1.33 × 10-3Pa). Since the aging time is longer, the pressure rise rate of the vacuum furnace is preferably less than or equal to 0.67 Pa/h. Otherwise, it is not easy to ensure the surface brightness of the parts.