Austenite sentence example
These freezing-point curves and transformation curves thus divide the diagram into 8 distinct regions, each with its own specific state or constitution of the metal, the molten state for region 1, a mixture of molten metal and of solid austenite for region 2, austenite alone for region 4 and so on.
Austenite, gamma ('y) iron.-Austenite is the name of the solid solution of an iron carbide in allotropic y-iron of which the metal normally consists when in region 4.
On cooling into region 6 or 8 austenite should normally split up into ferrite and cementite, after passing through the successive stages of martensite, troostite and sorbite, Fe 0 C= Fe 3 C +Fe(i 3).
But this change may be prevented so as, to preserve the austenite in the cold, either very incompletely, as when high-carbon steel is " hardened," i.e.
The important manganese steels of commerce and certain nickel steels are manganiferous and niccoliferous austenite, unmagnetic and hard but ductile.Advertisement
Austenite may contain carbon in any proportion up to about 2.2 It is non-magnetic, and, when preserved in the cold either by quenching or by the presence of manganese, nickel, &c., it has a very remarkable combination of great malleability with very marked hardness, though it is less hard than common carbon steel is when hardened, and probably less hard than martensite.
Only in the presence of much manganese, nickel, or their equivalent can the true austenite be preserved in the cold so completely that the steel remains non-magnetic.
Beta (13) iron, an unmagnetic, intensely hard and brittle allotropic form of iron, though normal and stable only in the little triangle GHM, is yet a state through which the metal seems always to pass when the austenite of region 4 changes into the ferrite and cementite of regions 6 and 8.
Martensite, Troostite and Sorbite are the successive stages through which the metal passes in changing from austenite into ferrite and cementite.
Ferrite and cementite, already described in § 10, are the final products of the transformation of austenite in slow-cooling.Advertisement
Clearly the mushy mixture of solid austenite and molten iron of which the metal in region 2 consists cannot cohere under either the blows or the pressure by means of which welding must be done.
The first particles of austenite to freeze contain about o 33% of carbon (p).
As freezing progresses, at each successive temperature reached the frozen austenite has the carbon-content of the point on Aa which that temperature abscissa cuts, and the still molten part or " mother-metal " has the carbon-content horizontally opposite this on the line AB.
This, of course, brings the final composition of the frozen austenite when freezing is complete exactly to that which the molten mass had before freezing began.
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.Advertisement
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.
At this point selection ceases; the remaining molten metal freezes as a whole, and in freezing splits up into a conglomerate eutectic of (1) austenite of about 2.2% of carbon, and therefore saturated with that element, and (2) cementite; and with this eutectic is mixed the " primary " austenite which froze out as the temperature sank from v to v'.
Here the mass consists of (1) primary austenite, (2) eutectic austenite and cementite interstratified and (3) pro-eutectoid cementite.
Here the black bat-like patches are the masses of pearlite plus proeutectoid cementite resulting from the splitting up of the primary austenite.Advertisement
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.
As we pass to cases with higher and higher carbon-content, the primary austenite which freezes in cooling across region 2 forms a FIG.
The black bat-like areas are the primary austenite, the zebra-marked ground mass the eutectic, composed of white stripes of cementite and black stripes of austenite.
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.
Looking at the matter in a broad way, in all these carbon-iron alloys, both steel and cast irons, part of the carbon may be dissolved in the iron, usually as austenite, e.g.Advertisement
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.
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.
The hardening of steel consists in first transforming it into austenite by heating it up into region 4 of fig.
Like the hardening of steel, it hinders the transformation of the austenite, whether primary or eutectic, into pearlite+cementite, and thus catches part of the iron in transit in the hard a state.
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.Advertisement
It can be caused by the transformation of retained austenite to martensite or by the precipitation of alloy carbides.
The material retains the nodular graphite distribution of ductile iron, but the matrix is acicular ferrite in a high carbon austenite.
Also the temperature range within which austenite decomposes to form ferrite and carbide on cooling.
Elements which stabilize austenite include manganese, nickel, cobalt and copper.
As the temperature sinks still farther, pro-eutectoid cementite (see § 22) forms progressively in the austenite both primary and eutectic, and this pro-eutectoid cementite as it comes into existence tends to assemble in the form of a network enveloping the kernels or grains of the austenite from which it springs.Advertisement