Instruction manual

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ATMOSPHERE CONTROL OPTION
Control of the furnace atmosphere has become increasingly critical to successful heat treating with
precise metallurgical specifications. The prevention of surface oxidation or scaling when metals are exposed
to elevated temperatures remains the most important function of the furnace atmosphere.
The introduction of the inert gas into the furnace chamber must be carefully controlled to insure that
sufficient atmosphere gas is being admitted to the furnace to seal the mechanical leaks against air infiltration,
and to purge the chamber of air. In common practice, a chamber is considered purged after 5 volume
changes. The simple way to adjust atmosphere flow through a furnace is through a flow meter that can be
read directly in cubic feet per hour.
Gasses in an enclosed vessel or chamber are constantly moving about in the space in which they
are contained. Gasses are like liquids in that they flow and are constantly exerting pressure upon the
surfaces in which they are in contact with and contained by. Therefore, in operating a furnace that uses a
controlled atmosphere, it is important to keep the inside of the furnace and the door seals in good repair.
The inert gasses of argon and helium are frequently used during the heat processing of reactive
metals. Argon is about 1/2 the cost of helium and is therefore preferred over helium. All gasses put into the
furnace must be completely free from moisture because water will break down when heated into oxygen
and hydrogen. The oxygen will attack the metal, causing decarburization and scaling of the parts being
heat treated.
Nitrogen can also be used for an atmosphere. Nitrogen is passive to ferrite and is entirely satisfactory
for use in the annealing of low carbon steels. It also must be completely dry to be used as a protective
atmosphere for high carbon steels because, as with argon, the water vapor will cause decarburization. At
the higher heat treating temperatures, nitrogen is not a protective atmosphere because the H combines
with iron to form finely divided nitrides that import hardness to the surface. In the soaking periods for
hardening tool steels this is not a problem, however. Nitrogen is about 2/3 the cost of argon.
Most oil and air hardening tool steels require bringing the parts to be hardened up slowly to 1200°F
to 1400°F for pre-heating. They are then taken up to the austenitizing temperature, where they will be held
for a prescribed length of time. When the parts spend time in the furnace where temperatures are from
1000°F and up, it should be either in an atmosphere or wrapped up in a stainless bag to prevent scaling.
In a recent test at our factory, using cold rolled die steels, the DU-1020 equipped with an atmosphere
package, performed extremely well. A nitrogen atmosphere was introduced into the chamber at 500°F. At
the same time a purge valve automatically opened until the temperature reached 700°F. This takes 2 or 3
minutes and is enough time to purge the chamber of oxygen. The flow meter on the control panel is set at
approximately 20-25 CFH to maintain a positive pressure in the chamber. This is evidenced by an indicator
light on the panel marked “Low pressure”. This light will serve a dual function, indicating either positive
pressure in the chamber or, when it is lit, that the tank is running low. This can also be equipped with a
buzzer or bell to warn of dropping pressure, at the customer’s request.
This furnace will be programmed as it was in our testing for the gas entry and purge to start at 500°F
and the purge to end at 700°F. This, of course, can be adjusted by the customer to any range desired. The
furnace can also be equipped with programmable instruments with 12 segments, consisting of 6 ramps
and 6 soaks, as an additional option.