This is an effective gaseous means of extinguishing magnesium fires in heat treating furnaces. The gas is introduced
into the furnace from a storage cylinder through an entry port preferably located near floor level. Connect the gas feed
line to this port, open the feed line valve to provide about 2 lbs/minute (depending on furnace size and number of gas
cylinders) and maintain gas flow until furnace temperature drops to 700F indicating the fire is out. The furnace door
should be kept closed during this action and until a definite temperature drop below 700°F is evident. Running the
furnace circulating fans for about 1 minute after the gas is first introduced will assist in gas dispersal, then but the fan off.
The gas cylinder used should be fitted with a Monel needle valve and a "tee" for attaching a 0-160 psi pressure gauge. A
suitable gas transfer system uses a 5/16" flexible bronze hose to carry the gas to the furnace where it enters through a
1/4" steel pipe entry port. Using 10 feet of hose and feed of pipe, a gauge pressure of 15-30 psi will deliver 1-2 lbs of
BF3 per minute. The cylinders may be permanently connected or brought to the furnace, when needed, on a suitable
dolly. This gas does not require heating in order to flow. The cylinders should be weight checked for contents every 6
Boron Trichloride (BCL3) Gaseous Method. This material has been successfully used to extinguish
magnesium heat treat furnace fires. However, there are several factors involved with its use which makes It less
preferred than boron trifluoride, These include: ten times more concentration than the 0.04% of boron trifluoride, the gas
must be heated to flow freely; it is more expensive than trifluoride; the liquid is corrosive and the fumes irritating with a
health hazard similar to hydrochloric acid fumes. Workmen should not occupy areas where noticeable vapors are
present unless wearing a gas mask with an acid gas canister containing a dust filter. if this agent must be used, the
liquid containing cylinders should be heated with infrared lights to provide the heat necessary to insure adequate gas
flow. The cylinder outlet should be fitted with a special valve and gauge to control gas flow. Flexible 5/8" ID neoprene
hose may be used to connect the cylinder to a steel pipe for insertion into the furnace port. Otherwise its use in
extinguishing a furnace fire is similar to the procedures for boron trifluoride.
4-27. IDENTIFICATION OF ALLOY.
4-28. Positive identification of an alloy, from a constituency standpoint, can only be determined by laboratory analysis.
However, whether a light metal is magnesium or not can be generally determined by a simple test consisting of placing
the test metal in contact with an 0.5% solution of silver nitrate, and observing the reaction for 1 minute. The solution is
made by dissolving 0.5g. of silver nitrate in 100 ml. of water. Formation of a black deposit of metallic silver on the
metal indicates magnesium or high-magnesium alloy. Then immerse the metal in a chrome pickle chemical solution,
Type I Specification MIL-M-3171 (Commercially known as DOW No. 1). The solution should be freshly prepared and
the test operator familiar with the colors of chemical treatment. If the metal assumes a very bright brassy coating, it
indicates it is aluminum free alloy. If a greyish iridescent coating forms the alloy contains aluminum. The solution is
made in the proportions of 24 ounces sodium dichromate and 24 fluid ounces concentrated nitric acid to enough water to
make one gallon. Prior to the test the metal should be thoroughly cleaned down to the base metal, if necessary, by
grinding or filing a clean area on the surface.
4-29. HEAT TREATING MAGNESIUM ALLOYS - GENERAL.
MIL-M-6857, Heat Treatment of Magnesium Alloy Castings, will be the control for heat
treatment of magnesium alloy castings used on aerospace equipment. For complete
description of magnesium alloy castings heat treat requirements, refer to latest issue of MIL-
4-30. PRECAUTIONS DURING HEATING . Of first importance in the heat processing of these alloys is a clear
understanding of the characteristics of the metal relative to heat. Pure magnesium will melt at approximately 1202F.
The alloys melting points range from 830°F to 1204°F, approximately, according to their element constituency.
Therefore, during any heating of alloy items, specified temperature maximums must be closely adhered to, particularly
during solution heat treating. The metal is easily burned and overheating will also cause formation of molten pools within
it, either condition resulting in ruining of the metal. Certain alloys such as AZ63A Type 1, or AZ92A Type 1, are subject
to eutectic melting of some of its elements if heated too rapidly. They must be brought up to heat treating temperature
slowly enough to prevent this. In the case of these two examples, no less than two hour should be consumed in bringing
them from 640F to treating temperature.
4-31. An additional and no less important characteristic of the metal relative to heat treatment, is that it is subject to
excessive surface oxidation at 750°F and higher temperatures. In an oxidizing atmosphere, this characteristic can result
in ignition and fierce burning. To prevent such occurrences, a protective atmosphere containing sufficient sulphur
dioxide, carbon dioxide or other satisfactory oxidation inhibitor shall be used when heating to 750°F and over. When
oxidation inhibitors are used, their concentration percentages in the furnace atmosphere should be periodically checked
for correct amounts. The particular requirements for various alloys are detailed in paragraph 4-46 in this section. These
requirements and those of other pertinent specifications and instructions should be consulted and strictly adhered to in
processing the metal. The safety measures defined in paragraph 4-1 must be rigidly practiced.
4-32. HEAT TREATING EQUIPMENT.
4-33. Furnaces used for solution beat treatment shall be of the air chamber type with forced air circulation. Heating
provisions can be gas, electricity or oil. Their design must be such as to make impossible, direct heating element
radiation or flame impingement on the articles being treated. The furnaces shall be installed with the necessary control,
temperature measuring and recording instrument equipment to assure complete and accurate control.