MECHANICS AND USEFUL ARTS. 69 



fit when hardened, and the workman has therefore to resort to the 

 process of grinding or lapping to make the work fit. 



The amount of the expansion (or the amount of the contraction) 

 of steel cannot be exactly stated, as it varies according to the 

 size of the steel operated upon, and the depth to which the steel 

 hardens ; also in the different kinds, according to the amount of 

 carbon combined, and even in the same steel operated upon at 

 different degrees of heat. Steel which is the most liable to injury 

 by excess of heat is the most liable to these expansions, and steel 

 which is less liable to injury by heat is most liable to contractions. 

 As, for example, the more carbon the steel contains, the greater 

 will be its expansion ; and the nearer the steel approaches to the 

 state of iron, the less will be this increase of bulk. 



Although the steel expands in hardening, it is not universal for 

 pieces of all sizes to increase in dimensions ; for sometimes it is 

 smaller in dimensions after hardening. 



Steel, like all other substances composed of particles, varies in 

 its dimensions with a change in temperature. It follows that 

 when the steel is at a red heat the natural positions of its particles 

 are in a measure displaced, and it is expanded to a greater bulk ; and 

 when immersed in water and suddenly cooled such a change of its 

 particles takes place as to make it hard and brittle. It also con- 

 tracts to a smaller bulk by the loss of heat ; but this cannot so rap- 

 idly occur at the central part, because it is protected by the sur- 

 face steel. Consequently, large pieces of steel do not harden all 

 through, or, in other words, do not harden properly to their cen- 

 tres, but toward the centre the parts are gradually less hard, and 

 will sometimes admit of being readily filed ; and as it is only the 

 outer parts of the steel which harden properly, consequently it is 

 only those parts of the steel which harden that increase in bulk. 

 When the steel is immersed in the water, the water begins first of 

 all to act upon the outer crust of the steel, and then cooling it 

 gradually toward the centre. The outer crust being the first to 

 part with its heat, it is of course the first to contract and become 

 smaller. The outer crust in contracting is held in a state of great 

 tension, by having to compress the central steel, the central 

 steel at the time being expanded by the heat. While the surface 

 steel is in this state of tension, and the central steel in this state 

 of compression, the particles of the surface steel, by the strain, 

 are displaced to a greater distance from each other, and the 

 particles of the central steel, by the compression, are compressed 

 into a denser state. The particles of the central steel being 

 compressed into a denser state, it causes the central steel, after 

 it has become quite cool, to occupy less space than what it did 

 previous to hardening. The particles of the surface steel become 

 hard while in this state of tension ; consequently the hardened part 

 of the steel becomes fixed, and cannot return to its original bulk ; 

 consequently, the hardened part of the steel occupies more space 

 than what it did previous to hardening. 



If the displacement of the particles of the outer steel predomi- 

 nates over the compression of the particles of the central steel, the 

 piece of steel under operation will then be larger in dimensions. 



