Stress or Permanent Strain to a Bright-red Heat. 267 



The Working of Iron at High Temperatures. 



There can be little doubt that such experiments as these are 

 important from a technical point of view. It is known, I am 

 not aware whether generally or not, that it is dangerous to 

 work iron within certain ranges of temperature. This one 

 can easily understand from what has gone before ; for it is 

 manifest that if the temperature of a mass of iron is not the 

 same throughout, and if the mean temperature of the mass be 

 near the higher of the two critical temperatures, or if it is 

 cooling from a bright red heat near either of the two critical 

 temperatures, the effect of a blow or of any stress will be very 

 different on different parts of the mass, and will be fatal to 

 that uniformity of structure and strength which it is so 

 desirable to procure. 



Summary. 



(1) There are two distinctly marked critical temperatures 

 for iron ; the lower somewhere about dull red, and the higher 

 somewhere about bright red*. 



(2) At the former of these critical temperatures the iron 

 begins suddenly to lose its magnetic properties if it is being 

 heated, and to gain them if it is being cooled. 



(3) At or near\ the latter critical temperature the iron, on 

 being heated, if under the slightest torsional, longitudinal, 

 or flexural stress or strain, begins to exhibit a remarkably 

 sudden change. If the metal be under stress, the sudden 

 change resembles that which would result from a sudden 

 increase of elasticity. If the metal be permanently strained, 

 there is a sudden increase in the amount of the strain. 



(4) When the iron is cooling, a sudden change, but oppo- 

 site in direction to that which occurs on heating, takes place. 

 The temperature at which the sudden change takes place is, 

 for very well annealed iron, nearly the same as that at which 

 the change takes place on heating. If, however, the wire be 

 not annealed, the change may be delayed by the so-called coer- 

 citive force of the metal until the lower critical temperature 

 is reached. When the change is so delayed, the phenomenon 

 of recalescence occurs. 



* I hope at some future time to be able to fix these critical tempera- 

 tures more exactly. 



f It is not unlikely that the so-called coercitive force may cause the 

 change to take place at a slightly higher temperature than the critical 

 temperature. 



