T.O. 1-1A-9-T7Solution heat treated and stabilized-TBSolution heat treated, cold worked and then artificially aged-T9Solution heat treated, artificially aged and then cold worked-T10Artificially aged and then cold worked-H1Strain hardened only-H2Strain hardened and partially annealed-H3Strain hardened and stabilizedAdded 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=1/2hard, 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 thangeneral 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 theapplication of heat is to be made to a thorium alloy, both of these areas must be considered.WARNINGMagnesium 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 withinthe safe limits set by the Atomic Energy Commission (AEC) and represent no hazard to personnel under normalconditions. A standard of 0.1 milligram per cubic meter (mg/m3) of thorium in air is a safe limit for continuousatmospheric 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 longexposure to fine dust or fumes need cause concern as to radioactive toxicity of magnesium thorium. Normal dust controlprecautions, followed to avoid fire hazards, can be expected to control any health hazards that might result from fine dustin grinding the low thorium content alloys. In welding these alloys without local exhaust, concentrations of thorium abovethe tentative limitof0.1 mg/m3of air were found in the breathing zone. Use of local exhaust reduced thoriumconcentrations to well within acceptable limits. If ventilation is such that the visible fumes flow away from the welder, it isadequate, providing such fumes are not permitted to accumulate in the immediate vicinity. An alternate practiceinvolves use of a ventilated welder's hood, if there is not sufficient room ventilation to control contamination of thegeneral atmosphere. Thorium containing scrap and wet grinding sludge may be disposed of by burning providing anABC amendment is secured for the basic AEC license. If burned, the ashes which will then contain the thorium, must bedisposed of in accordance with AEC Standards for Protection Against Radiation 10 CFR Part 20. As an alternative theashes 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 withan aisle width not less than one-half the stack's height. Such storage is within the normal recommendations for firesafety.4-17. Radiation surveys have shown that exposure of workers handling the referenced thorium alloys is well within thesafe limits set by the AEC. Assuming hand contact, the body one foot away from the alloy for an entire 40 hour workweek, the exposure would be 168 millirems (mr) to the hands and 72 mr to the whole body. These are maximum valueswhich probably would not be approached in actual practice. The corresponding AEC permissible safe limits are 1500mr/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 ismandatory that all such actions be made according to the requirements and restrictions of the 00-100 series technicalorders, as applicable, and ABC regulations. As previously stated, the normal precautions taken in the shop processing ofmagnesium will suffice for safe handing of thorium alloys. These precautions are noted in the following paragraphs onsafety 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, certainprecautions should be taken during working of it.4-21.Machining Safety Rules. During machining operations, observance of the following rules will control any potentialfire 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
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business