TREATMENT OP ALLOY STEEL 67 



which may be summarized as follows: 1. By changing the 

 critical ranges and recalescent temperatures. 2. By modi- 

 fying the condition in which the carbon exists. 3. By 

 removing harmful occluded gaseous impurities. 4. By 

 combining chemically with iron or carbon or both. 5. 

 Either combined or free, forming isomorphous solutions 

 with iron or separating into distinct microscopic particles. 

 By these means steel is improved or injured, hardened or 

 strengthened, toughened, or made more brittle. 



Notwithstanding its complexity, it is reasonable to expect, 

 and much evidence has been brought forward to show that 

 steel and its alloys obey the laws of physical chemistry 

 which hold good for simpler and purer alloys, namely, the 

 laws of solution. The problems presented by steel alloys 

 have engaged the attention of the world's leading chemists 

 and physicists, and they have made rapid strides within a 

 generation in elucidating the molecular relations of the ele- 

 ments of steel. They have isolated by ingenious methods 

 many well defined chemical compounds, such as the car- 

 bides, phosphides, and sulphides of iron and manganese. 



The complexity of steel from a chemical standpoint is 

 further increased by the allotropic character of iron, and 

 by the fact that carbon and probably sulphur and phos- 

 phorus may exist in several conditions or combinations. 

 As iron is cooled from a molten condition, it has been dis- 

 covered that at from one to three temperatures below its 

 freezing point cooling momentarily stops. For carbonless 

 iron these temperatures are designated as Ar 3 and Ar 2 and 

 occur at about 895 C. and 765 C. When carbon is also 

 present a third well marked arrest in cooling is noted at 

 about 690 C., known as Ar x the ordinary ''recalescent'' 

 point. It is believed by many that the Ar 3 and Ar 2 tem- 

 peratures indicate transition or critical changes in the 

 nature of iron itself. In other words, that just as phos- 

 phorus may exist in two distinct forms, one yellow and one 

 red, so the element iron is supposed to be capable of exist- 

 ing in different physical conditions at different temr>era^ 



