Looking at the posts on metal hardening I was wondering what are the dynamics of oil versus water hardening.What is happening to the metal using one or the other?I assume water brings down the temperature quicker but have no empirical knowledge on the subject.
snipped-for-privacy@webtv.net (Time Traveler) wrote in news:17373-42729A16-424 @storefull-3135.bay.webtv.net:
The quenching determines, to a large degree, what the hardness of a particular material will be. The biggest factor is delta T, or the change in temperature over time. The higher the delta T, the harder the material, within it's hardening range. Since water has the (AFAIK) most efficient transfer of heat, using water will make Delta T large. Oil doesn't wick away heat as fast, so Delta T will be smaller over the same time period. Water will also cause the surface to cool much faster than the inside, resulting in a somewhat stratified hardening of the material. Oil quenching narrows the temperature gap between the inside and outside of the material, allowing for a more even hardening as you go deeper into the material.
Water offers the most "severe" quench. It would normally extract the heat from the steel the quickest. The metal will vaporize the water and form a steam layer. Heat transfer is not so fast through the steam layer. On the other hand the very act of transforming the water into steam means the water has to take in enormous amounts of energy to transform the water from liquid state to gaseous state (steam). Moving the part and recirculating the water aids in getting the best quench.
With oil there is much less vapor formation of the oil. There might be a little bubbling seen but nowhere as violent as with water. Therefore the heat removal rate is slower.
There is another quench in between these two, which is polymer quenchants. These offer a compromise.
The choice for quench mdeia depends on the steel grade being quenched, the part geometry and some other factors like whether quenching is being done in an atmospheric controlled integral-quench furnace, for example.
Mark
As others have said, you're right about that, and, depending on the alloy, thickness, and temperature of the quenching bath, the water quench may well give you a little more hardness.
In the case of plain-carbon steels, water will give you more hardness on all but the thinnest pieces. But slower-quenching alloys, especially those that collectively are called "oil hardening," may not harden any more with a water quench. In fact, if the piece is more than 1/4" thick or so, all that water will do is increase the risk that the piece will crack. Water may, however, result in a *deeper*, if not a harder, hardened zone in that case.
As for what is happening to the metal, you'd do well to find a description online or in a book. It involves a couple of phase-changes in the steel. The phase changes are generally the same for carbon steel and common alloys, but it's better absorbed from a description written by experts who have had the time to re-write, and an editor.
-- Ed Huntress
Gentlemen,thanks for the advice.Will try both ways in the backyard over the next few days.
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