Forging is the plastic working of metal by mean of localized compressive force exerted by manual or power hammers ,presses ,or special ofrging machines .It may be done either hot or cold .However ,when it is done cold ,special name usually are given to the processes . Consequently ,the term forging usually implies hot forging done above the recrystallization temperature .
Modern forging is a development from the ancient art practiced by the armor makers and the immortalized village blacksmith.High-powered hammers and mechanical presses have replaced the strong arm .the hammer and the anvil and modern metallurgical knowledge supplements the art skill of the craftsman in controlling the heating and handling of metal .
Various types of forging have been developed to provide great flexibility, making it economically possible to forege a single piece or to mass-produce thousands of identical parts .The metal maybe drawn out ,increasing its length and decreasing its cross section.upset ,increasing the cross sectiong and decreasing the length,or squeezed in closed impression dies to produce multidirectional flow .
For small press forgings colsed impression dies are used ,and only on stroke of the ram is normally required to perform the forging operation .the maximum pressure is built up at the end of the stroke which force the metal into shape .Dies may be mounted as seperate units ,or all the cavities maybe put into a single block.For small forgings individual dies units are more convenient. there is some difference in the desing of dies for different metals .copper-alloy forgings can be made with less draft than steel ,consequently more complicated shapes can be produced .These alloys flow well in the die and are rapidly extruded.
In the forging press a greater proportion of the total work put into the machine is transmitted to the metal than in a drop hammer.the machine and foundation absorbmuch of the impact of the drop hammer. press reduction of the metal is faster and the cost of operation is consequently lower. Most press forgings are symmetrical in shape ,haveing surfaces ,which are quite smooth,and provide a closer tolerance than is obtained by a drop hammer. However .drop forging can forge many prats of irregular and complicated shapes more economically .forging presses are oftern used for sizing operations on parts made by other forging processes .
In drop forging, a piece of metal ,roughly or approximately of the desired shapes,is placed between die faces having the exact form of the finished piece,and ofrced to take this form by drawing the dies together .Theis method is widely used for the manufacture of the parts bot of steel and brzss .Large ingots are now almost always forged with hydraulic presses insteads of with steem hameers ,since the work done by a press goes deeper.
Further,the press can take a cooler ingot and can work to closer dimensions .The forging should be done at about the same temperature as rolling .the process improves the physical properties of the steeel just as rolling does .In the final forging it is important not to have the steel too hot ,for overheated steel will have poor mechanical properties when cooled .In heating for forging the temperature is usually judged by the eye, but where large number of the same pattern are made .the piece to be forged are heated in furnaces in which the temperature is indicated by pyrometers and otheni automatically controlled .
