﻿Molecular Structure of Metal s. 267 



until the stress is considerably in excess of that which would 

 .suffice to make it yield if it stood alone, or had neighbours 

 equally favourably inclined. 



Apply this consideration to the case of steel, where there 

 arc two classes of grains; the ferrite, which is simply iron, 

 and the pearlite, which is a harder structure. Slip on any 

 ferrite grain is resisted partly by the strength of the surface 

 itself, and partly by the impossibility of it> yielding without 

 forcing slip to take place on neighbouring (harder) grain-. 

 Now suppose the structure is a very gross one, such as 

 Mr. Stead has shown may lie found in steel that i- seriously 

 overheated. On the large grains of ferrite in overheated steel 

 the resistance to slip will lie but little greater than it would be 

 in iron, and. consequently, under an alternating stress fatigue 

 of strength. leading to rupture, may be produced by a very 

 moderate amount of load. Mr. Stead* has shown how the 

 effects of OYerheating can be removed by the simple expedient 

 of raising the steel to a temperature sufficient to cause recry- 

 staliization — a homoeopathic remedy that transforms the gross 

 structure of the overheated metal into an ordinarily tine 

 structure, where no ferrite grain can yield without compelling 

 the yielding of many pearlite grains. Hence we find, as 

 Rogers thus demonstrated by experiment, that steel cured by 

 reheating from the grossness of structure previously produced 

 by overheating, has an immensely increased power to re>i>t 

 the deteriorating effects of often repeated stress. 



I trust you will not feel I have abused the license of the 

 Chair in presenting contributions to molecular theory that are 

 for the most part in the nature of speculative suggestion^ 

 thrown out in the hope that they may some time lead to fuller 

 and more definite knowledge, llemote as they may seem to 

 be from the concerns of the workaday engineer, they relate to 

 the matter which it is his business to handle, and to the 

 rationale of properties without which that matter would be 

 useless to serve him. TVe have attempted to peuetrate into 

 its very heart and substance in order the better to comprehend 

 the qualities and functions on which the practical work of 

 enoineerino- relies. The man whose daily business lends him 

 through familiar tracks in a forest does well to stray from 

 time to time into the shady depths that lie on either hand. 

 The eyes of his imagination will be opened. He will at least 

 learn his own limitations, and. if he is fortunate, he mar gain 

 some clearing on a hilltop which commands a wider view than 

 he has ever had before. 



* See especially a paper by J. E. Stead and A. W. Richards on 

 " The Restoration of Dangerously Crystalline Steel by Heat Treatment.** 

 Jo urn. of the Iron and Steel Inst. No. 2, 1903. 



t F. Rogers. " Heat Treatment and Fatigue of Steel," Journ. of the 

 Iron and Steel Inst., No. 1, 1905. 



