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RKNichols

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Daryl, that is an interesting chart, I have never seen it.  I do have a question though.  The chart states that cementite has a specific volume of .1304, iron (ferrite) is.1271 and pearlite is .1286.  Since pearlite is comprised of two phases, ferrite and cementite, the specific volume must be product, sum or average of the two, right?  Since pearlite can have a WIDE variation in the spacing of the carbide lamellae, the specific volume should vary with the spacing, right?  So what is the spacing for the value they listed? At what temperature are the volumes listed?  Â I can only assume it is some kind of an average number.  The difference between austenite and pearlite is eighty five one hundreths of one per cent, .85%  By contrast, the volumetric expansion of martensite can be as much as 4% in high carbon steels.  I must admit to being skeptical that the pearlite expansion is causing the bends to go down in oil.  However, I think we are getting closer to the answer with all of this discussion.  Thanks for the chart!  I learned something today!

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Guest daryl meier

Don:  The first and second graphs that you kindly posted here show a comparison of cooling rates in the center and surface of a 1/2" cylinder quenched in "agitated" water and oil quenchants at various temperatures.  Note: the time scales are not the same for both oil and water.  The important things to notice are the times at which the center vs the surface reach 700C, and compare these times between water and oil quenches, keeping in mind that these quenches are faster than would be in still quenchants.

It becomes easy to see that the volume fractions of martensite vs pearlite will be quite different between a water quench and an oil quench.

The last graph shows the center cooling curves of "1095" steel cylinders (1/2" dia.) cooled in various "still" oils.  Please note that the time between the verticle lines is 4 seconds.

In the neighborhood of the AR1, the temperature stops falling and in some cased rises for a time of 2 or more seconds.  Therefore the center area will have "coarse" pearlite, with a gradient toward "fine" pearlite as one reads toward the surface.

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Guest daryl meier

Correction of order of graphs in previous post. Reading from top to bottom, "first and second" should read "second and third"  also "last" should read"first".  Sorry about that.

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Guest daryl meier

Mr Nichols:  I have trouble believing that coarse and fine pearlite have the same volume.  The text states that the volumes are at room temperature. ( wonder how they measured 1095 austenite at room temperature?)

It is my opinion that the up and or down curving in different severity of quenching mediums is a function of the "Timing Sequence" of the shrinking due temperature loss/ expansion due to transformations/ volume fraction of the different transformation products/size of the pearlite lamellae.

I believe that in your photomicrographs you will find evidence of the last two of these.

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This article:

http://www.asminternational.org/Templat....ID=9459

may offer some help. Table 1 on page 2 shows differences in centerline cooling speed between mediums, as well as breakage occurence.

The interesting part is that the rate of breakage is dramatically different between water and oil at 25 atm and vibrated while the centerline cooling speed is virtually identical. This tends to show maybe that the cooling speed is not a factor, but rather the interaction between the medium and the piece. The microscopic behavior seems to be at stake (bubbles or not). Maybe the regularity of cooling not disturbed by vapor bubbles makes a big difference. (it also seems intuitive)

 

I don't know if there is a correlation between breakage and bending up or down but it may be possible. That may explain why mediums like oil or polymer quench which do not boil (I presume) give a different result than water.

 

Is it known if the down bending in the blades in water and oil is similar? In other words is the blade bending down more at first in oil then not coming back up as much, where in water it bends down a littlle and then moves up. If so then maybe the first cooling down process is key (bubbles or not is important). If on the other hand they both bend down about the same and only the water one comes back up, then it's secondary cooling that is important which may mean either cooling speed difference or permeability of the clay to the medium may be the factors.

 

My 2 c :P

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:o   Don, this thread used to have 9 pages....... anyway,

 

 

I finished the analysis of the blades Don provided.  I could not explain the difference in curvature based only on the microstructure.  Both the oil quench and the water quench had very similar microstructures.  I am inclinded, in the absence of data to the contrary, to agree with Daryl Meier that this is probably an issue of timing.  In water, the martensite is probably forming and expanding just prior to the completion of the pearlite formation in the spine.  It can expand and in so doing, bend the soft, austenitic/pearlitic spine upward.  The oil quenched blade may see the pearlite form just before the martensite and the expansion of the martensite cannot bend the spine, so it stays bent down due to thermal contraction.  Or it could be just PFM!   WFT

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