Thus, first, for the brittle glass-hard cementite there is gradually substituted the relatively harmless temper graphite; and, second, even this is in part removed by surface oxidation.
Immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels.
In short, from Ar 3 to Ar t the excess substance ferrite or cementite, in hypoand hyper-eutectoid steels respectively, progressively crystallizes out as a network or skeleton within the austenite mothermetal, which thus progressively approaches the composition of hardenite, reaching it at Ar t, and there splitting up into ferrite and cementite interstratified as pearlite.
I) solidifies, its carbon may form cementite following the cementite-austenite diagram so that white, i.e.
The fact that graphite may dissolve in the iron as austenite, and that when this latter again breaks up it is more likely to yield cementite than graphite, is only an apparent and not a real exception to this law of the greater stability of graphite than of cementite.
Again, as the temperature in turn falls past Ar l this hardenite mother-metal splits up into cementite and ferrite grouped together as pearlite, with the resulting recalescence, and the mass, as shown in Alloys, Pl., fig.
Here the mass consists of (1) primary austenite, (2) eutectic austenite and cementite interstratified and (3) pro-eutectoid cementite.
Measures the percentage of the excess of ferrite or cementite for hypoand hyper-eutectic steel and white cast iron respectively.
Hastening its cooling by casting it in a cool mould, favours the formation of cementite rather than of graphite in the freezing of the eutectic at aBc, and also, in case of hyper-eutectic iron, in the passage through region 3.
Second, though the brittleness should be lessened somewhat by the decrease in the extent to which the continuity of the strong matrix is broken up by the graphite skeleton, yet this effect is outweighed greatly by that of the rapid substitution in the matrix of the brittle cementite for the' very ductile copper-like ferrite, so that the brittleness increases continuously (RS), from that of the very grey graphitic cast irons, which, like that of soapstone, is so slight that the metal can endure severe shock and even indentation without breaking, to that of the pure white cast iron which is about as brittle as porcelain.