SOME ASPECTS OF POWDER METALLURGY 447 



ing of parts from iron powder into three classes according to the type of 

 product and properties has been outUncd as follows^^-^: 



Type A Materials having mechanical properties similar to ordinary 

 cast iron suitable for applications where stresses are very low. 



Type B Materials similar to Type A but having improved tensile 

 strength, a definite yield point, and a noticeable elongation. 



Type C Materials having mechanical properties approaching ordinary 

 malleable iron, suitable for applications where stresses, including 

 impact, are moderate. 



Prior to 1941, the iron powder used commercially for pressed and sintered 

 parts was of Swedish origin because that was the only powder available 

 in quantity, quality, and at a price which allowed competition economically 

 with established methods of production. Domestic iron powders are now 

 available, however, that are superior to those formerly imported. 



Of the sintered iron products manufactured in this country, an interesting 

 example is a small gear for automobile oil pumps^^. This gear was formerly 

 made by machining cast iron blanks but was adapted for powder metal 

 production because of greater ease in fabrication at less cost and more 

 satisfactory operation. The gear teeth must be true involute curves with 

 surfaces such that noisy operation and binding are prevented. All of 

 these characteristics can be readily obtained by pressing and sintering, 

 while more difficulty is encountered with cast gears because of the intri- 

 cate machining work involved. The sintered gear avoids these expensive 

 machining operations, and the teeth have so much better surface finish, 

 and mesh so accurately, that noisy operation is avoided. In addition, 

 the associated porosity is helpful in that oil impregnation assists in smoother 

 and quieter operation. 



The pressed gears are lighter in weight than the cast gears, and while 

 the mechanical properties are not of high order, they are satisfactory for 

 the use. 



12. Cladding and Duplexing 



Powder methods are useful in cladding, duplexing, or any of the processes 

 whereby one metal or alloy may be coated with another for protective 

 purposes, to obtain special properties as in bimetal strip, to obtain hard 

 surface layers on strong, tough backing material, or to obtain a thin layer 

 of relatively high-cost metal of desirable properties on a suitable low-cost 

 backing strip. 



For fabrication of bimetal, layers of the respective component metal 

 powders may be placed in the die in the desired proportions and compacted. 

 Upon sintering, an alloy bond is formed between the layers, and the briquette 



