1919] on The Hardening of Steel 491 



ordinary temperature the form of iron stable between A and G. 

 The rate of change is too rapid. By means of either manganese or 

 nickel, however, elements whicli lower the temperature of this trans- 

 formation point below the ordinary temperature, it is possible to 

 make alloys containing nothing but y iron, and these possess a 

 characteristic crystalline form. They consist mainly of polyhedral 

 crystals which exhibit a decided tendency to twin — that is, there are 

 certain planes in each crystal about which the crystal elements tend 

 to rotate through an angle of 180", thus giving rise to the charac- 

 teristic twin formation easily recognizable under the microscope. 

 As already mentioned, between /? and a iron no difference in 

 crystalline symmetry has been detected. These irons also crystallize 

 in polyhedral crystals, which, however, exhibit no tendency to twin. 



According to the equilibrium diagram, the solution of carbon in 

 y iron known as austenite must crystallize in the form of polyhedral 

 crystals, since it is a solid solution, and these have the same general 

 character as those of a pure metal. The structure of austenite, 

 however, as obtained by quenching within the area of A G- S E, 

 does not correspond to this theoretical deduction. Taking as an 

 example the tool steel containing 0'9 per cent carbon, if this is 

 quenched, say at 750' C, a faintly marked acicular structure is 

 obtained when the specimen is examined. If the temperature of 

 quenching be raised this structure becomes more strongly marked 

 and the needles are decidedly larger, and even at the highest 

 temperatures polyhedral crystals are never obtained. This acicular 

 structure is known as martensite, and it is universally agreed that it 

 is the characteristic structure of properly quenched steel. All the 

 theories of the hardening of steel which have been put forward 

 during the last forty years centre round this one question : "VYhat is 

 martensite ? If and when this question can be correctly answered 

 it will furnish a complete answer to the question : Why is steel 

 hardened by quenching ? 



Before considering these theories two other facts must be briefly 

 mentioned. Firstly, the carbide of iron which exists structurally 

 free in an annealed tool steel is a very hard substance, but as it is 

 mixed with soft a iron in the proportion of 13 '5 to 86 '5, the tool is 

 comparatively soft. This carbide can be separated from the steel by 

 appropriate solvents, which remove the iron. On the other hand, it 

 is not possible to separate any carbide of iron from a properly 

 quenched tool steel. When the latter is treated with dilute acids, the 

 steel dissolves without residue and liberates a complicated mixture of 

 hydrocarbons both liquid and gaseous. There is therefore a funda- 

 mental difference in the form of the carbide in a quenched as 

 compared with an annealed steel. In the former it is w^holly dis- 

 solved, in the latter wholly segregated. When a hardened tool steel 

 is softened by anneahng, the carbide of iron, FcgC, is gradually 

 precipitated. If the annealing is carried out at low temperatures it 



