1 - Mines Magazine - Colorado School of Mines

Sudden Release of EnergyThe sudden release of the energy of a high explosiontakes place in a microsecond or so, the pressure buildingup in thc same time; the explosion gases rapidlyexpand, dissipating energy, and the pressure dropssuddenly, coming to zero in a few microseconds. Thepressure generated by burning propellants takes severalmilliseconds to develop, but being confined will besustained for quite long times, decrease in pressurecoming about through cooling of the gaseous explosionproducts.The diverse applications of explosives to metalforming group into those situations in which the explosiveis detonated in intimate contact with the metalto be reworked or formed such as in the hardening ofsteels, the compaction of metal powders, the splittingof ingots, and cutting operations; and those in whichobjects such as cups, rocket nozzles, missile noses, andaircraft parts are sized or formed by drawing, usingpropellants or explosives detonated in air or in waterat some distance from the worked piece.When the explosive is placed in intimate contactwith a metal and detonated, the stresses just insidethe metal will instantaneously become exceedinglyhigh, and a transient stress disturbance is set up whichis transmitted through the metal, producing fracture,plastic flow, and other deformations, the exact natureof which will be strongly dependent upon the con--figuration of the metal-explosive system. Even underthese extreme pressure conditions, the metal is seldomconverted to a fluid, exhibiting in general the pi'opertiesof a quasi-elastic brittle material.Stratagem in Metal FormingThe stratagem in metal foi-ming is to turn to advantagespecific patterns of failures. Thus, ingots maybe split by simultaneously detonating explosive chargeson opposite sides of the ingot; or the strongly adheringinternal scales on heating pipes may be removed bydetonating an explosive charge on the exterior surfaceof the tube, thus dislodging the" scale by the samemechanism which causes a picture to be knocked froma wall by pounding on the reverse side of the wall.Explosive filled rivets have been used for many years.(See Figure 1).•V Rg. I. Explosive rivet as It looks before (left) and after (right)setting Into metal.The stress, being intense, work hardens the metal,in some cases to a depth of an inch or more, (See Figure2). Manganese or liadfield steel, used extensively inmining equipment and notorious both for its resistanceto abrasion and work hardening, is now being successfullywork hardened by the detonation of thin layersof sheet explosive placed on the surface, thereby greatlyincreasing its abrasive resistance.Explosive Separated from Work-PieceGenerally, in explosive forming the explosive is notplaced in intimate contact with the metal but is sep-Hardness Profile—Single Treatment- - —-1 IMPAC TL BETAL BHNSURFiCE i" {' I" r ij' IV" l4" 2"• Fig. 2. Hardness versus depth profile is shown for a possible explosiveImpact hardening treatment now being used for manganesesteel.parated from the w