Tempering Steel

Steel has been defined as any kind of iron which v.hen heated to rodness and suddenly plunged into cold water becomes hard; and every kind of malleable or flexible iron that can be hardened by that procesa is steel. Hut this definition is not applicable to the steel of mild quality now made for many mechanical uses, says the Mechanical News. One of the requisites for this mild steel is thp.tit will not harden after being heated to a cherry red and plung'ed into water. To include all the kinds of steel now used in the agri cultural and mechanical industries, the botter definitioD of steel will be, a malleable iron combined with a percentage of carbon, Chemically considered,, steel oceupics a half-way position betweeB wroujrht and cast iron, wrought iron being siraply iron almost entiiefly free froin carbon, wliile steel that is to be bempered éontains one to one and one-lialf percent, of carbon. The reason why this very slight chango in the chemical constrnction should produce suiflj wonderful resulta in the properties of bon and steel is as yet an unsolved mystery. We know that a bar of Iron converted into steel becomes more granular or open, and while it loses to some ex tent its toughness, it gains, instead, elaeticity, greater strength and closeness of fiber. Blister steel is made by heating bars of iron packed in charcoal in a furnace for a period of from six to ten days. When the metal is withdrawn the bars are found to be of crystalline texture, and have a blisterod snrfaee - henee the name. (.'ast steel was formerly made of blister steel broken into fragrnents, melted in crucibles and cast into infjots; Imt the modern practice is to charge the crucibles with pieces of good Swedish or American bar iron, addlüg charcoal and black oxide of mang-anese. The heat of the furnace soon seals the lid of the crucible, and the iron absorbs carbon irom the fumes of the charcoal, thus shortening the tedious process of makinji "blister" bar. The cast steel is rolled or hammered from the ingot to any desired bar, sheet or plate. The chief characteristic of steel consists in its capability of hardened and tempered, and when exposed to heat it takes on in suc jession the following colors: 1. A faint yellow. whi 'i indieates a pnoper temper for 1 :icct ■ r small cutters that refluiré ti. í!i l edge, with but little strength ,f mi 2. A palé yellow, whit h indieates the temper for razors and Burgical instrximents. ?. Full yellow, for penkr.ives, etc., with increased toughness. 4. Iirown. with purple spots - that being1 for axes and carpenters' tools. 5. liright blr.j, for swords and watchsprin.Ts. 6. Full blue, for fias saws, dasrgers, et-.-. 7. Dark line. for urge saws er üistruments that laav be ïharpened with a file. The abore colora are b;:-;-.'cl on ste'. i suitable for the requirements. A .ie-.-e of steel suitable for razors, U:ncots. etc. would not take tha color indicated íor large saivs, as that quality oí st-cl is but little above the "blister" quality. The íiner steel is the less heat it wül temper at, requiringf a lower color cf temper. Itecently there have been soiae valuable discoveries iu tempering', welding and restoring steel, both fromburnt or a low grade.