- (Osmond.) Pearlite, steel (carbon about r%) forged and annealed at 800° C. Magnified 1000 diameters.
- (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.
This alloy, if allowed to solidify completely before chilling, turns into a uniform solid solution, and at still lower temperatures the solid solution breaks up into a pearlite complex.
The pearlite when highly magnified somewhat resembles the lead-tin eutectic of fig.
I) consists of pearlite alone.
The percentage of pearlite and of free ferrite or cementite in these products is shown in fig.
2, in which the ordinates of the line ABC represent the percentage of pearlite corresponding to each percentage of carbon, and the intercept ED, MN or KF, of any point H, P or L, FIG.
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.
13) should be much more effective in starting cracks under distortion than that of the far more minute particles of cementite which lie embedded, indeed drowned, in the sixfold greater mass of ferrite with which they are associated in the pearlite itself.
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 ?
- Physical properties and assumed microscopic constitution of the pearlite series, graphiteless steel slowly cooled and white cast iron.
By " total ferrite " is meant both that which forms part of the pearlite and that which is in excess of the pearlite, taken jointly.
Immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels.
Carbon-Content of Hardened Steels.-Turning from these cases in which the steel is used in the slowly cooled state, so that it is a mixture of pearlite with ferrite or cementite, i.e.
As the temperature now falls past 690°, this hardenite mother-metal in turn splits up, after the fashion of eutectics, into alternate layers of ferrite and cementite grouped together as pearlite, so that the mass as a whole now becomes a mixture of pearlite with cementite.
The iron thus liberated, as the ferrite of this pearlite, changes simultaneously to a-ferrite.
13, the slowly cooled steel now consists of kernels of pearlite surrounded by envelopes of the cementite which was born of the austenite in cooling from T to U.
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.
12, then consists of kernels of pearlite surrounded by envelopes of ferrite.
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.
Here the black bat-like patches are the masses of pearlite plus proeutectoid cementite resulting from the splitting up of the primary austenite.
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 the former case, the objects are heated only to the neighbourhood of Aci, say to 730° C., so that the 0-iron may slip into the a state, and the transformation of the austenite into pearlite and cementite may complete itself.
Would have consisted chiefly of graphite with pearlite and ferrite (which are all relatively soft bodies), if thus chilled and annealed consists of cementite and pearlite.
The molecular freedom which this high temperature gives enables the cementite to change gradually into a mixture of graphite and austenite with the result that, after the castings have been cooled and their austenite has in cooling past Aci changed into pearlite and ferrite, the mixture of cementite and pearlite of which they originally consisted has now given place to one of fine or " temper " graphite and ferrite, with more or less pearlite according to the completeness of the transfer of the carbon to the state of graphite.
Thus, prior exposure to a temperature materially above Ac 3 coarsens the structure of most steel, in the sense of giving it, when cold a coarse fracture, and enlarging the grains of pearlite, &c., later found in the slowly cooled metal.
Steel differs in many ways from iron in respect of atmospheric corrosion; the heterogeneous nature of steel gives occasion to a selective rusting, ferrite is much more readily attacked than the cementite and pearlite; moreover, the introduction of other elements may retard rusting; this is particularly the case with the nickel-steels.