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Defenition of prior austenite grain boundary

by krautkramer
Tags: austenite, boundary, defenition, grain
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Dec7-12, 08:48 AM
P: 25
What is prior austenite grain boundary?Grain boundary and prior austenite grain boundary are same terminology or different?I am confused

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Dec7-12, 10:08 PM
Astronuc's Avatar
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Quote Quote by krautkramer View Post
What is prior austenite grain boundary?Grain boundary and prior austenite grain boundary are same terminology or different?I am confused

The prior austenite grain boundary would be the grain boundary of the parent austenite grain.

Perhaps this will help:
In heat-treated steels, it is recognized that the grain size of the product of the heat treatment, usually martensite, is not measured or cannot be measured. For low-carbon steel, the martensite forms in packets within the parent austenite grains. In high-carbon martensites, we do not observe any convenient structural shape that can be measured. In most cases, we try to measure the size of the parent austenite grains that were formed during the high temperature hold during the heat treatment. This is usually referred to as the "prior-austenite grain size" and it has been widely correlated to the properties of heat treated steels. The most difficult process here is the etching procedure needed to reveal these prior boundaries. Sometimes they cannot be revealed, particularly in low-carbon steels. In this case, it may be possible to measure the low-carbon lath martensite packet size, which is a function of the prior-austenite grain size.

. . . .

From the previous paragraph in the cited page -
Grain size measurement is also complicated by the different types of grains that can be present in metals, although their fundamental shapes are the same. For example, in body-centered cubic metals, such as Fe, Mo, and Cr, we have ferrite grains; in face-centered cubic metals, such as Al, Ni, Cu, and certain stainless steels, we have austenite grains. The grains exhibit the same shapes and are measured in the same way, but we must be careful in describing what kind of grains we are measuring. In the face-centered cubic metals, we may observe so-called twin boundaries within the grains (see sidebar on grain types). . . .
See also
Once an alloy steel part is heat treated, only etching can be used to try to reveal the prior-austenite grain boundaries, that is, the austenite boundaries present when the part was soaked at the austenitizing temperature. While many etchants have been developed for this purpose, such work is fraught with difficulty. One of the most successful prior-austenite grain boundary etchants is a saturated aqueous solution of picric acid containing a wetting agent, several of which have been used. This etch is sensitive to phosphorus segregated to the prior-austenite grain boundaries and will not work otherwise. Figure H illustrates a fairly successful effort with a quenched and tempered experimental alloy steel. This type of etch rarely, if ever, yields an etch quality adequate for image analysis and is usually accompanied by substantial pitting.

Some significance and relevance -
Jan2-13, 11:40 PM
P: 25
Thanks astronuc for the reply.Is it possible to visible prior austenite grain boundary in a ferrite+pearlite microstructure?I read somewhere that during austenite decomposition, further nucleation is happening in austenite grain boundary.

PS:Prior austenite grain boundary term is used in martensite microstructure.

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