182 APPLIED MECHANICS *€ 
muffle were kept at that temperature for the time required (half-an-hour 
in these tests), the gas was then turned off, and the whole allowed to 
. cool slowly. 
It will be observed that the general results of annealing are, reduction 
of strength and increase of elongation of the steel when tested. 
Soaking.—The difference between annealing and soaking is, that in 
the latter operation the bars are heated for a much longer time. 
Hardening. —“ By heating steels to the hardening temperatures, some 
or all of the iron carbide is dissolved in the iron, and the latter is re- 
strained from reverting to its soft condition by sudden cooling. Within 
certain limits, the more rapidly the heat is abstracted from the bar the 
more effective will be the hardening.” 
Referring to Fig. 254, and comparing the D, d lines with the A, a 
lines, the effect of quenching the steels at 800° C. in water is to increase 
the tenacity of all the steels considerably, but at the same time the 
elongation is diminished, except in the case of the 0°13 and 0°18 carbon 
steels. . 
Tempering.—Steel which has been hardened by quenching in water 
or oil may be tempered, that is, its hardness may be reduced to any 
required extent, by subsequent annealing at a temperature depending on 
the degree of hardness required. 
Oil hardening.—“ When steel is quenched in oil at 80° C., the offect 
is to increase the tensile strength, but to a somewhat less extent than by 
quenching in water at 20° C., and also at the same time to increase the 
elastic limit and rather diminish the ductility.” 
“The most suitable temperature for quenching steel, in order to 
obtain the best combined results as regards tensile strength, elastic limit, 
and elongation, is about 900° C., and the most suitable temperature for — 
reheating, when elongation, and consequently resistance to shock, is not 
of paramount importance, is about 350°C. If, however, the steel be 
required to withstand violent percussive action, as in a gun tube, then 
reheating at a higher temperature, say 600° C., will be found to be 
necessary, as such steel, when thermally treated ‘in this way, although 
possessing a relatively high tensile strength and elastic limit, nevertheless 
has also a high percentage of elongation.” 
The student is recommended to study carefully the results given in 
the table on p. 181 and in Fig. 254, and to plot the results as directed in 
Exercise 12, p. 190. 
172. Tests of Copper-Zinc Alloys (Brasses).—The curves in Fig. 255 
show the tenacity and extensibility of alloys containing different pro- 
portions of copper and zinc. These curves have been reproduced, with 
modifications as to scales, from the fourth report to the alloys research 
committee of the Institution of Mechanical Hagieg by Professor W. C. 
Roberts-Austen.* =i 
* Proceedings of the Institution of Mechanical Engineers, 1897. 
