INSIDE S OF METALS — ZAPFFE 257 



tlie pattern in plate 2, figure 2, which gives visual evidence for the 

 fact that fracture has traversed the grains in this steel without the 

 consistent interruption experienced in the tougher steel. 



Wliile it is too early to point to useful application of this dis- 

 covery with respect to the ship-plate problem, the contribution still 

 being in the research stage, its promise is indicated by the fact that 

 the contrast between plate 2, figure 1, and plate 2, figure 2, is outstand- 

 ing, whereas previous microscopic methods have revealed no detectable 

 changes. In addition, the application of mechanical testing to this 

 problem has involved the construction of huge testing machines at 

 great cost, and much of the steel is destroyed in its testing. Fr^c- 

 tography requires only a fractured chip and a microscope, and there is 

 good reason to believe that the information obtained from the chip 

 serves as well for the entire heat of perhaps 100 tons of steel. 



METALS FOR SERVICE AT HIGH TEMPERATURES 



In the new and important field of metals for service at very high 

 temperatures — gas turbines, rockets — there is an application of frac- 

 tography that can already be described. 



A pattern appears in plate 3, figure 1, which has some aspects of a 

 good detective story, and has proved of great importance in the 

 production of molybdenum metal. Molybdenum has one of the high- 

 est known melting points for any metal in the periodic system. At 

 temperatures of white heat, where the strongest steel has not only 

 melted, but begins to boil, molybdenum scarcely begins to melt. This 

 fact simultaneously makes the metal a very attractive one for special 

 services at high temperature, but one difficult to produce. A special 

 furnace was finally designed a few years ago which melted molyb- 

 denum in vacuum by means of an electric arc. Castings of promising 

 size and solidity resulted, but when they were subjected to the diffi- 

 cult forging operations they would often fracture. 



To shorten a long research story, the metallurgists at the Climax 

 Molybdenum Corp. in Detroit found that fractographic examination 

 of a small chip broken from the casting with a hammer always re- 

 flected one of two characteristic patterns. The first was feathery 

 in its appearance and connoted forgeable metal. The second was 

 pearly and granular and always meant nonforgeable metal. De- 

 pending upon the presence of one or the other of these patterns, de- 

 termined by a brief and simple fractographic examination, the proc- 

 essing of the ingots was directed either toward forging or remelting. 



In the upper portion of the field in plate 3, figure 1, the "feathery" 

 constituent is clearly visible. These small markings, resembling oat- 

 heads, are now known to be molybdenum carbide. In tlie lower half 

 of this same field is a weedy-looking growth of the fine granular 



