SOME ASPECTS OF POWDER METALLURGY 433 



(luced, however, are pressed at room temperature. The presses-''--^ which 

 now arc designed primarily for this type of work may be of the mechanical 

 or hydraulic types for high production rates with modifications for rapid 

 plunger strokes as required. 



The dies are generally of hardened steel having the inner surfaces highly 

 polished by lapping with polishing rouge in the direction of the plunger 

 stroke so that any fme scratches that remain are in the direction of ejection 

 of the pressed part'-^ In some instances where parts are made from highly 

 abrasive particles, the dies are made of or lined with hard carbide materials. 

 Die depth depends upon the apparent density of the powder being pressed, 

 but the usual ratio of depth to part thickness is approximately 3 to 1. The 

 greater die depth required for powders of lower density introduces the 

 complications of friction at the die sides, unevenness of pressure distribution, 

 and internal friction of the powder itself. There is almost no lateral flow in 

 the powder mass, a condition which limits the shapes that can be pressed. 



Pressure used varies from 5 to over 100 tons per square inch, in general, 

 and is an important factor in limiting the size of parts that can be made 

 by the powder process. 



Following pressing, a powder compact may sometimes be given a pre- 

 sintering treatment below the normal sintering temperature in order to 

 increase its strength to facilitate handhng, or to remove lubricants or 

 binders which might cause difficulties later. 



Sintering is the fundamental process in powder metallurgy whereby solid 

 bodies are bonded by atomic forces. 



Theoretically it is possible to obtain bonding by bringing the powder 

 particles into so close contact with one another that the atomic forces 

 of cohesion may become operative. But this occurs only when the respec- 

 tive atoms of such adjacent particles are distant in the order of magnitude 

 of the crystal interatomic spacings; this is a condition against which there 

 are many obstructions. Visually and even microscopically smooth particles 

 have surfaces which are extremely jagged with respect to interatomic spac- 

 ings and crystal planes. Then not large, flat areas representing large 

 numbers .of atoms, but only successive points representing relatively very 

 small groups of atoms can be brought into sufficiently intimate contact. 

 IVIoreover, even this small contact may be reduced by the presence of 

 oxide films. 



An increase of pressure will improve the bonding of such powders since 

 the particles are deformed and pressed against one another to give in- 

 creased surface contact. At the same time, rupture of the oxide films may 

 occur with subsequent closer contact of the metal particles. This is the 



