W I T H the introduction of ingot metal, or mild steel, as a constructional material, the necessity arose for careful specification of the quality required, and for systematic testing of the material supplied. The properties of steel, either in plates or in forgings, vary a good deal, with comparatively small differences of composition or treatment in manufacture; and the engineer has to rely on tests of samples for assurance that he is obtaining material suitable for any given purpose. As experience has been gained, manufaoturers produce material of very uniform and trustworthy character, and the quantity rejected for failure to pass tests is very small. But the time has not arrived, and is not likely to arrive, when, for this material, regular testing can be dispensed with. It is important, therefore, that the tests should be such as to afford definite information, and should be so conducted that the results of different tests are strictly comparable. I n order to secure this, something more is required than accuracy in the testing-machine, and care and honesty in the observer who makes the tests. Circumstances which modify the results of tests can be known only by a study of the behaviour of plastic materials when strained, and the results of tests require interpretation. F o r the determination of the quality of steel recourse is had to : (1) chemical analysis; (2) mechanical tests, in which the material is subjected to straining action and definite measurements are made; (3) workshop tests, such as cold and temper bend tests; and (4) for certain purposes, microscopical examination of structure. Chemical analysis is of great value to the steel-manufacturer, but
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