542 



MECHANICAL ENGINEERING, THE PROGRESS OF. 



the main results of recent progress in mechan- 

 ical engineering. The improvements in the 

 materials used in oonstmoobn and mechanical 

 applications form one of the features of this 

 progress which are most likely to escape their 

 proper estimation. The principal changes in 

 the use of materials are the substitution of iron 

 or steel for wood, which has been going on for 

 a long time, and the progressive displacement 

 of iron by steel, which is a more recent develop- 

 ment. The expensive and perishable material 

 wood and the weak and brittle minerals are 

 everywhere giving place to iron and steel, 

 while iron is slowly but steadily and inevita- 

 bly being displaced by steel. Cast-iron in small 

 parts is less and less used as steel castings 

 become more and more reliable, and especially 

 as the art of making drop-forgings of larger 

 size and in more intricate forms is perfected. 

 Sheet-steel, very low in carbon and other hard- 

 ening elements, is becoming, year by year, more 

 generally adopted in boiler-making, not because 

 of its greater strength, for the stronger grades 

 are always rejected by the experienced boiler- 

 maker, but because of the greater uniformity, 

 ease of working, freedom from cinder, and the 

 durability of those grades which are well suited 

 to such use. A tenacity of less than 65,000 

 pounds per square inch and great ductility are 

 demanded for this work. In rods and bars, and 

 for sheets to be used where mechanical forces 

 only are present, steel is produced which, with 

 a tenacity of 80,000 pounds per square inch, 

 stretches 25 per cent before breaking. A grade 

 is sometimes obtained very low in carbon, but 

 high in manganese, which has 10 per cent 

 higher tenacity and equal ductility ; and the 

 progress seems to be in the direction of a 

 manganese steel as the metal for use in general 

 construction. 



In alloys the important service has been ren- 

 dered by R. H. Thurston of discovering an alloy 

 of copper, zinc, and tin of maximum possible 

 strength, and of pointing out approximately its 

 composition. Other investigators have inde- 

 pendently hit upon alloys closely related to 

 this "maximum metal," and possessing prop- 

 erties of hardly less value. It is now known 

 that by carefully proportioning the constitu- 

 ents, by properly fluxing the alloy, and by 

 special mechanical treatment, brasses and 

 bronzes may be obtained having strengths un- 

 dreamed of by earlier engineers. Tenacities of 

 from 75,000 to over 100,000 pounds per square 

 inch have already been attained. 



The introduction of special alloys having 

 extraordinary strength and uniformity of com- 

 position, as the phosphor bronzes, manganese 

 bronzes, and sterro-metal, indicate that work- 

 ers in metal are beginning to enter upon the 

 path long since opened to them by scientific 

 research. 



Dr. Fleitman's discovery of a method of 

 making nickel malleable and capable of weld- 

 ing, and his similar improvement of commercial 

 cobalt by. the use of magnesium, is in itself 



important, and promises to lead the way to 

 further progress. 



The effect of variation of temperature in the 

 annealing of iron and steel, and in the hard- 

 ening and tempering of steel, has long been 

 known. That annealed and unannealed wire 

 differ widely in tenacity and in ductility, that 

 very "mild" steel and good iron are softened 

 by the very process which gives hardness to 

 steel, are familiar facts, and it has been known 

 that there exists a critical temperature, prob- 

 ably definite and fixed for each grade, at which 

 the hardening of steel occurs. Passing this 

 point in cooling the metal takes on its temper, 

 but variations of temperature on either side 

 that point produce no observable effect on its 

 condition, however rapidly they may take place. 

 This critical temperature has now been identi- 

 fied in certain cases, and may prove to be 

 nearly the same for all steels. 



The process of cold-rolling has long been 

 known as a method of enormously increasing 

 the strength and elasticity of iron. It is now 

 found that it is applicable to the soft steels; 

 and it seems probable that its full effects may 

 be obtained at any temperature below that 

 critical point which defines the limit of molec- 

 ular stability in steel. Lauth's process has been 

 applied with equal success to certain alloys of 

 copper and tin, by Sears, in the United States, 

 and later by Rosetti, in Italy, and very exten- 

 sively and successfully by Uchatius, in Austria. 

 Tobin has cold-rolled bronzes approaching the 

 " maximum " alloy in composition, and has at- 

 tained tenacities exceeding 100,000 pounds per 

 inch. 



A radical distinction has been detected in 

 the behavior of metals under stress, which 

 leads to their division into the iron class and 

 the tin class. The difference is shown in the 

 variation of the normal line of elastic limits by 

 intermitted stress. Engineers are beginning to 

 perceive that that exaltation of the normal 

 elastic limit which is observable in the former 

 class is a valuable quality, the existence of 

 which may justify the use of smaller factors of 

 safety than have hitherto been thought allow- 

 able ; and this leads to less expense in station- 

 ary structures, and to the elimination, to some 

 extent, of stresses due to the inertia of moving 

 parts in machinery. The opinion of many 

 engineers, that moderate static loads may be 

 sustained indefinitely by iron and steel, are also 

 to this extent sustained. The same conclu- 

 sions lead to the observance of more than usual 

 caution in the use of metals of the tin class, 

 including most of the brasses and bronzes, and 

 to the use of higher factors of safety than are 

 demanded in constructions of iron and steel. 



Preliminary straining to secure an elevated 

 initial elastic limit with relief of internal stress 

 is likely to be of service in the applications 

 of iron and steel, as by cold-rolling, by " frigo- 

 tension" and " thermo-tension," and by wire- 

 drawing, while it proves to be probably less 

 effective with other metals. 



