- (Stoughton.) Meshes of pearlite in a netv.-ork of ferrite, from hypo-eutectoid steel.
FIC. r3.--(Stoughton.) Meshes of pearlite in a network of cementite from hyper-eutectoid steel.
When of eutectoid composition, it is called " hardenite."
Eutectic here freezes 7 Austenite+Cementite Pro-eutectoid Cementite forms progressiuely 882930 1200 0 U a 5 6 ?
13) sufficient in quantity to represent their excess of carbon over the eutectoid ratio; they are called " hyper-eutectoid," and are represented by region 8 of fig.
12) sufficient in quantity to represent their excess of iron over this eutectoid ratio; is called " hypo-eutectoid "; and is represented by region 6 of fig.
A " eutectoid " is to such a transformation in solid metal what a eutectic is to freezing proper.
The fact that this decrease of strength begins shortly after the carboncontent rises above the eutectoid or pearlite ratio of o 90% is natural, because the brittleness of the cementite which, in hypereutectoid steels, forms a more or less continuous skeleton (Alloys, Pl., fig.
The large massive plates of cementite which form the network or skeleton in hyper-eutectoid steels should, under distortion, naturally tend to cut, in the softer pearlite, chasms too serious to be healed by the inflowing of the plastic ferrite, though this ferrite flows around and Steel White Cast Iron 100 75 K 0 ?
Immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels.
This formation of cementite continues as the temperature falls,- till at about 690° C., (U, called Ar 2 _ 1) so much of the carbon (in this case about 0.10%) and of the iron have united in the form of cementite, that the composition of the remaining solid-solution or " mothermetal " of austenite has reached that of the eutectoid, hardenite; i.e.
The cementite which has thus far been forming may be called " pro-eutectoid " cementite, because it forms before the remaining austenite reacnes the eutectoid composition.
To take a second case, molten hypo-eutectoid steel of 0.20% of carbon on freezing from K to x passes in the like manner to the state of solid austenite, -y-iron with this 0.20% of carbon dissolved in it.
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.
The reason for its birth, of course, is that the solubility of carbon in austenite progressively decreases as the temperature falls, from about 2.2% at 1130° (a), to 0.90% at 690° (Ar 1), as shown by the line aS, with the consequence that the austenite keeps rejecting in the form of this pro-eutectoid cementite all carbon in excess of its saturation-point for the existing temperature.
This formation of cementite through the rejection of carbon by both the primary and the eutectic austenite continues quite as in the case of 1.00% carbon steel, with impoverishment of the austenite to the hardenite or eutectoid ratio, and the splitting up of that hardenite into pearlite at Ari, so that the mass when cold finally consists of (1) 1 Note the distinction between the " eutectic " or alloy of lowest freezing-point, 1130°, B, with 4.30% of carbon, and the " eutectoid," hardenite and pearlite, or alloy of lowest transformation-point, 690° S, with 0.90% of carbon.
In the black-and-white ground mass the white is the eutectic cementite, and the black the eutectic austenite, now split up into pearlite and pro-eutectoid cementite, which cannot here be distinguished from each other.
Both the primary and eutectic austenite have changed in cooling into a mixture of pearlite and pro-eutectoid cementite, too fine to be distinguished here.
In it the normal constituents are, for region II., molten metal+primary austenite; for region III., molten metal+primary graphite; for region IV., primary austenite; for region VII., eutectic austenite, eutectic graphite, and a quantity of pro-eutectoid graphite which increases as we pass from the upper to the lower part of the region, together with primary austenite at the left of the eutectic point B' and primary graphite at the right of that point.
Finer; while the pro-eutectoid and eutectoid graphite, if they exist, are probably in very fine particles.