4<Ji> Mr. .F. Muir. 



letting down i.i -H-d which ]};i< IK-CM haidened by Mienchiiii; I'M mi ;i 

 red heat. This tempering t'rom a condition ] hardne-s induced by 

 overstrain, unlike ordinary tempering, is applicable not only to steel, 

 hut al-o to wrought iron, and jxissibly to other materials which < an l>e 

 hardened by overstrain and softened l>y annealing. 



The experiments described in the pa])er were all carried out on rods 

 of iron and steel about ;'ths of an inch in diameter and 11 inches long, 

 the elastic condition of the material l>eing in all cases determined by 

 means of tension tests in which the hardness of the material w.-is indi- 

 cated l>y the position of the yield-point. The straining wag performed 

 by means of the 50-ton testing machine of the Cambridge Engineering 

 Lalwratory, and the small strains of extension were measured by an 

 extensometer of Professor Ewing's design, which gave the extension on 

 a 4-inch length of the specimen to the 1 100,000th of an inch. 



For the purpose of tempering and annealing, a gas furnace was 

 employed 2 feet in length, the specimens being protected from direct 

 contact with the flame by inclosing them in a thick porcelain tube. 

 The temperature inside this tube was determined by means of a Cal- 

 lendar's direct-reading platinum-resistance pyrometer. 



The method of examining the materials employed is illustrated by 

 the following two diagrams, in which the material examined i.s a J,-inch 

 rod of semi-mild steel (0'35 per cent. C., 1 per cent. Mn).* Curve 

 No. 1 of the first diagram shows that this steel when in the condition ;is 

 supplied by the makers gave a well -defined yield-point at about 38 tons 

 per square inch, the material yielding at that stress by 0'13 of an inch 

 on a 4-inch length. 



Curve Xo. '2 illustrates the semi-plastic state of the material, pro- 

 duced by just passing this primary yield-point. The specimen was 

 laid aside for If days, then once more tested ; and Curve Xo. 3 shows 

 the progress made during this interval of rest towards recovery of 

 elasticity. Curve Xo. 4 shows the condition of the overstrained 

 material after it had been resting for two weeks. To insure peiiect 

 recovery of elasticity, the specimen was heated to 200 C., but a few 

 minutes at the temperature of boiling water would have leen nearly 

 as effective in restoring the elasticity lost by overstrain. t 



After cooling, the specimen was tested by reloading and carefully 

 increasing the load above its previous maximum amount till a well- 

 defined yield-point was obtained at 49 tons per square inch, as shown 

 by Curve Xo. 5, the yield-point having thus l>een raised by the large 

 step of 1 1 tons per square inch. The yielding which occurred at this 



* Details of the special method adopted in plotting these diagrams will be found 

 in the author's previous paper " On the Recovery of Iron from Overstrain,' ' Phil. 

 Trans.,' A, vol. 193, 1899, p. 12. 



t Ibid., p. 2-2. 



