THE MEASUREMENT OF GRAIN SIZE. 



By Zay Jeffries, Cleveland, Ohio. 



Just as the telescope has given us certain information in astronomy 

 which we know no other way of obtaining, so the microscope has 

 permitted us to obtain direct knowledge concerning many things 

 unresolvable with the naked eye. Much of the knowledge gained with 

 microscopes would not be obtainable in any other way. For example, 

 the quantitative determination of grain size of fine grained metal is 

 only possible because of the microscope. The purpose of this brief 

 note is to point out a case in which chemical analysis varies but little, 

 and success or failure depends on the grain size which can be deter- 

 mined only with a microscope. 



In the mechanical working of tungsten it was found that some lots 

 of metal would work well, and some only with great dif&culty. Some- 

 times the metal would be so hard that it could not be drawn to the 

 smaller sizes ; it would either break too frequently or the die wear would 

 be so great that it could not be tolerated. A careful study of these 

 materials was made from both chemical and physical standpoints. The 

 chemical analysis was found to be so nearly constant that errors of 

 analysis would mask any differences wdiich might actually be present. 

 It is not maintained that slight differences in analysis did not exist, 

 but only that the determination of the impurities which, aside from 

 thoria, probably did not exceed 0.05%, gave no definite clue to the 

 difficulty. 



It was found that the variation in grain size was greater than the 

 variation of any of the chemical or physical properties, and that the 

 working properties varied with the grain size. The larger grains had 

 more ability to stand extreme deformation than the smaller ones. On 

 the other hand the tendency to break in the early stages of w^orking was 

 greater in the coarse grained material. If the grains were too small 

 in the tungsten metal containing 0.75, per cent. ThOa the wires broke 

 frequently in the smaller sizes and the die wear was excessive. In this 

 metal the danger line is reached if the number of grains per square 

 millimetre exceeds about 6,000. On the other hand, it is desirable 

 that the tungsten metal have an inherent high resistance to grain growth 

 to insure a long life in the lamps. This factor is usually satisfied if 

 the number of grains per square millimetre exceeds 1,500. It is, 

 therefore, desirable to control the grain size between 1,500 and 6,000 

 grains per square millimetre in the ingot. 



In the early days of working tungsten no such control was exercised 

 and lots of metal were encountered which were unworkable, and no one 

 knew the reason. The inference now is that the ingots were too fine 

 grained since it is possible to reproduce these results with fme grained 

 metal to-day. A contributing, and sometimes the major cause of 

 trouble was the failure to eliminate the oxide of tungsten, but even this 

 is more readily detectable with the microscope than by chemical analysis. 



Every lot of tungsten metal made at the Cleveland Wire Division 

 of the General Electric Company is now tested for grain size ; in fact, 

 treated to give the proper grain size in many cases. The lots not falling 



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