The Case-hardening of Steel: An Illustrated Exposition of the Changes in ... by Harry Brearley

Author:Harry Brearley
Language: eng
Format: epub
Publisher: Iliffe & sons limited
Published: 1914-03-25T05:00:00+00:00

• •

Toughness

OF BAR STEEL.

.• ;:..ic!2:;-**:Tl/^ JCASfl-HARDENlNG OF STEEL

The Brinell hardness numeral has been found to bear a fairly constant relationship to the maximum stress (hardness) of forged or forged and annealed carbon steels. The relationship is sufficiently constant for shop control purposes at any rate, and is expressed as follows, on the assumption that the ball impression is made transversely to the direction of rolling:

Hardness numeral x .26 = tons per square inch. i.e., if the hardness numeral, which can be determined with a Brinell machine in a few minutes, is multiplied by .26 the resulting figure will be the maximum stress in tons per square inch. The factor is reliable for material of approximately the same class so long as the yield point, or elastic limit, of the steel is not more than sixty to seventy per cent, of the maximum stress. But in quenched steels the ratio of the yield point to the maximum stress is higher and generally over seventy-five per cent., and in such cases the Brinell hardness numeral, being relatively greater, because a permanent indentation is not so easily formed, needs to be multiplied by the factor .25 in order to arrive at the approximate maximum stress of the material.

The suggested mode of testing can be used for workshop purposes by cutting off a few short lengths from various bars and brinelling them as received. Then submit a similar set of bars to all the forms of heat treatment, except actual cementation, to which the material will eventually be exposed, and again brinell them, in each case taking care before doing so to file or grind away the outer surface. These simple observations enable one to reject from a continuous supply of material those materials which are abnormal so far as hardness is concerned. The tests, on the quenched bars in particular, will detect unexpected variations, due either to differences in the amounts of carbon or manganese, or both, which the steel contains.

The toughness of case-hardening steels cannot be determined so readily. It is certainly not determined by the ordinary forms of tensile testing either from the elongation per cent, or the reduction of area per cent., of which statement the figures given on page 62 stand as illustration. If by toughness we may understand the ability to resist fracture or cracking when suddenly distorted, then toughness must be measured by some form of shock test, and on the subject and value of shock testing there is, unfortunately, no great

unanimity of opinion. It is, however, to some minds the test par excellence for structural steels, whether case-hardened or not, and merits, therefore, fair statement and careful consideration.

The impact shock test has for some years been discussed by testing associations and specially appointed committees notably in Germany and France, and also by international committees. Means of making the test have crystallised themselves into various forms of machines, of which the best

Fig. 64.—Iiod impact testing machine.

known are those designed by Charpy, Fremont, and Guillery. The Izod machine

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