S. WHYTE, B.Sc. 175 



and waste it, and much time in the machine shop, through bad harden- 

 ing. For machine parts, pyrometers and testing machines are neces- 

 sary in standardising methods after the temperatures have been 

 estabhshed, but with high-speed steel, where the hardening tem- 

 peratures are usually high, the recording of these temperatures is not 

 so rehable and calls for all the more precaution in testing the con- 

 ditions by micro-examination of the steel after hardening. Micros. 

 Nos. II. and III. show the structure of an 18 per cent, tungsten steel 

 heated to a satisfactory temperature, and overheated, respectively. 

 The overheated, or burnt structure of No. III., shows the large crystals 

 of austenite with oxide beginning to form round their boundaries. On 

 the other hand, the best cutting properties of the steel are not brought 

 out unless the steel is heated to a temperature high enough to diffuse 

 all the free iron tungstide, which is present in the annealed condition. 

 Micro. No. IV. shows the same high speed steel where the hardening 

 temperature has not been high enough or the time of soaking not long 

 enough, and too much free tungstide is still present. 



Thirdly, and most important for the engineer, is the use of the 

 microscope in helping to locate the causes of failures, and in working 

 out the processes by which these fractures develop. The causes of 

 failure are numerous, and apart from those due to inherent defects in 

 "the steel as mentioned above, the principal one is that of " fatigue." 

 In " fatigue " fractures, the origin is usually found in a weakness of 

 design or in using steel of too low an elastic limit. Sometimes a piece 

 of non-elastic slag, occuring at a point of maximum stress, sets up 

 local stresses high enough to start a fracture. In designing machines, 

 a radius replaces a sharp corner whenever possible, when working 

 stresses are set up at these points, so mat the stresses shall be dis- 

 tributed as evenly as possible. Sometimes one finds an accidental 

 notch, such as a file mark, in a radius, which sets up a " fatigue " 

 fracture. An example of this may be given, as it brings out points 

 in connection with the microscope objectives, which appear to be 

 ■worthy of consideration. Micro. No. V. shows such a V-notch, 

 accidentally made by a file in the radius at the foot of a stop in a 

 machine gun lock mechanism, which received rapidly repeated blows. 

 The notch has concentrated the stresses to such an extent that over- 

 straining of the material has taken place, and a crack is seen originat- 

 ing at the apex of the notch. The crack, as it develops, is seen to be 

 deflected through a slag inclusion, Micro. No. VI., and in other places 

 in the same specimen it was noticed that " strain picture " was 

 highly developed round these slag inclusions, although fracture had 

 not commenced. 



In microphotograph No. VII. this strain structure is also seen 

 around the end of the crack which had penetrated about 1-16 in. 



It is in cases such as the above that good objectives are necessary, 

 and more so when alloy steels are being examined. In non-ferrous 

 metals the crystal grains are usually much larger, and strain structure 

 is easily resolved with comparatively low magnifications. Microphoto- 

 graph No, VIII. shows a brass which had been strained during 

 machining. 



It is in photographing the fine-grained steels that the differences 

 in the microscope objectives show up. In photographing Micro. No. 

 VII. the secondary spectrum of the achromats would give bad defini- 

 tion but, with the elimination of this in the apochromats, by the union 



