Vacuum protection atmosphere heat treatment method

Vacuum protection atmosphere heat treatment method–recharge gas selection：

When using the partial pressure of heating vacuum heat treatment back filling gas should choose the high-purity neutral or inert gas, hydrogen, helium, nitrogen, argon and other common gases. Four kinds of gas cooling speed from fast to slow is hydrogen, helium, nitrogen and argon. If the cooling rate of air is 1, the ratio of the cooling rates of hydrogen, helium, nitrogen, argon, and air is 7, 6, 0.99, and 0.7, respectively.

Hydrogen has the highest thermal conductivity and maximum cooling rate at any pressure and can be used as a vacuum furnace for graphite as a heating and thermal insulation element. However, steel grades with high carbon content may cause slight decarburization during the high temperature stage of cooling (above 1050°C); for high-strength steels, there is a risk of hydrogen embrittlement. The gas supply system using hydrogen as the cooling medium should be closed and reliable. After the cooling operation is completed, hydrogen should be discharged in a timely manner and filled with nitrogen to open the furnace door. Otherwise, there is a danger of explosion.

Helium is the most expensive. Since it also has a certain cooling capacity at low pressure, it is generally used under pressure of 1 x 104Pa.

Nitrogen is the most commonly used gas, and it is also the cheapest. Forced circulation cooling is performed at a pressure slightly lower than atmospheric pressure, and the cooling intensity value can be increased by about 20 times. In the range of 200-1200°C, nitrogen is neutral to general steels and exhibits certain activity for titanium alloys, stainless steels, and high-temperature alloys. If vacuum heating and cooling are performed under nitrogen partial pressure, Cr2N is formed on the surface of the parts which results in deterioration of the surface properties, that is, an increase in the tensile strength, a decrease in the reduction of the area, and a certain influence on the corrosion resistance. Therefore, it is not appropriate to use nitrogen.

Argon is more common and its price is lower than that of helium, but it is more expensive than nitrogen. It is necessary to use argon in vacuum heating and cooling of stainless steel, high-temperature alloys and titanium alloys where nitrogen is not suitable.