T.O. 1-1A-9
-T7
Solution heat treated and stabilized
-TB
Solution heat treated, cold worked and then artificially aged
-T9
Solution heat treated, artificially aged and then cold worked
-T10
Artificially aged and then cold worked
-H1
Strain hardened only
-H2
Strain hardened and partially annealed
-H3
Strain hardened and stabilized
Added suffix digits 2, 4, 6, B, to the H1, H2, H3 symbols indicate the degree of strain hardening ,i.e., 2=1/4 hard, 4=1/2
hard, 6r 3/4 hard, and 8= full hard.
4-13. SAFETY REQUIREMENTS FOR HANDLING AND FABRICATION OF MAGNESIUM ALLOYS.
4-14. There are two special major areas of safety precautions to observe in processing of magnesium alloys other than
general shop safety practices. One is the fact some alloys contain thorium, a radioactive element (e.g., HK31A, HM21A,
HM31A) and the other is the low melting point/rapid oxidation (fire hazard) characteristics of the metal. Where the
application of heat is to be made to a thorium alloy, both of these areas must be considered.
WARNING
Magnesium thorium alloys shall be handled, stored and disposed of in accordance with T.O. 00-110N-4.
4-15. MAGNESIUM-THORIUM ALLOYS (HK31, HM21, HM31, HZ32, ZH42, ZH62) are mildly radioactive but are within
the safe limits set by the Atomic Energy Commission (AEC) and represent no hazard to personnel under normal
conditions. A standard of 0.1 milligram per cubic meter (mg/m3) of thorium in air is a safe limit for continuous
atmospheric exposure and is readily met in processing magnesium alloys cont2ining up to 10% thorium, For Example:
Stirring alloy melt of 5% thorium content resulted in 0.002 mg/m3 atmospheric contamination and grinding air alloy of 3%
thorium content gave thorium contamination in the breathing zone ranging from 0.008 to 0.035 mg/m3. Only long
exposure to fine dust or fumes need cause concern as to radioactive toxicity of magnesium thorium. Normal dust control
precautions, followed to avoid fire hazards, can be expected to control any health hazards that might result from fine dust
in grinding the low thorium content alloys. In welding these alloys without local exhaust, concentrations of thorium above
the tentative limitof0.1 mg/m3of air were found in the breathing zone. Use of local exhaust reduced thorium
concentrations to well within acceptable limits. If ventilation is such that the visible fumes flow away from the welder, it is
adequate, providing such fumes are not permitted to accumulate in the immediate vicinity. An alternate practice
involves use of a ventilated welder's hood, if there is not sufficient room ventilation to control contamination of the
general atmosphere. Thorium containing scrap and wet grinding sludge may be disposed of by burning providing an
ABC amendment is secured for the basic AEC license. If burned, the ashes which will then contain the thorium, must be
disposed of in accordance with AEC Standards for Protection Against Radiation 10 CFR Part 20. As an alternative the
ashes or scrap may be turned over to an AEC licensed scrap dealer, through applicable disposal procedures. See T.O.
00-110N-4.
4-16. For indoor storage of thorium alloy sheets and plates, the size of stacks should be limited to 1000 cubic feet with
an aisle width not less than one-half the stack's height. Such storage is within the normal recommendations for fire
safety.
4-17. Radiation surveys have shown that exposure of workers handling the referenced thorium alloys is well within the
safe limits set by the AEC. Assuming hand contact, the body one foot away from the alloy for an entire 40 hour work
week, the exposure would be 168 millirems (mr) to the hands and 72 mr to the whole body. These are maximum values
which probably would not be approached in actual practice. The corresponding AEC permissible safe limits are 1500
mr/week for the hands and 300 mr/week for the whole body.
4-18. Despite the relative safety present in the handling, storage and processing of thorium containing alloys, it is
mandatory that all such actions be made according to the requirements and restrictions of the 00-100 series technical
orders, as applicable, and ABC regulations. As previously stated, the normal precautions taken in the shop processing of
magnesium will suffice for safe handing of thorium alloys. These precautions are noted in the following paragraphs on
safety precautions.
4-19. SAFETY PRECAUTIONS FOR ALL ALLOYS (INCLUDING FIRE HAZARDS).
4-20. Since magnesium will ignite and burn fiercely when heated to a point near its melting temperature, certain
precautions should be taken during working of it.
4-21. Machining Safety Rules. During machining operations, observance of the following rules will control any potential
fire hazard:
a.
Keep all cutting tools sharp and ground with adequate relief and clearance angles.
b.
Use heavy feeds to produce thick chips.
4-2
