<Stand back, I am a metallurgist> On 3 August 2016 at 01:19, Gene Heskett <ghesk...@shentel.net> wrote: > I forget which is which, but I believe the point at which the ferrous > alloy becomes austenitic, eg non magnetic, is the "curie" temperature
It's partly a coincidence. Austenite is non-magnetic, but the actual Curie temperature is a separate phenomenon. The Curie temp of Iron is 770C and is a property of iron, but if you look at the iron/carbon phase diagram http://www.calphad.com/graphs/Metastable%20Fe-C%20Phase%20Diagram.gif Most compositions of steel will have transformed to Austenite by then. > Weller, back in the '50's brought out a soldering iron whose temperature > was regulated by a magnet on the end of a wire tha I have one. Very clever idea, and super-reliable. A very early example of a "smart material". I think that the Curie-point is a nickel alloy. > Steel in the austenitic state can be quenched quickly enough to remain in > the austenitic state. This is true of nearly all stainless steels, Nickel is an austenite stabiliser, and high-nickel stainless steelsexist as austenite at room temperature. Which is why some stainless is not attracted to a magnet. Other compositions can be martenisitic or ferritic, and those grades of stainless are magnetic. > Technically, when in that state, its a supercooled liquid, and has > dimensional instability to match No, it is a metastable crystalline solid. Ferritic stainless steels have an expansion coefficient of about 10 ppm/K, Austenitic stainless is about 17 ppm/K. This may explain the larger valve clearances. The property that makes steel heat-treatment so interesting that it is an academic discipline all of its own is that the high-temperature phase has a much higher solubility for carbon than the low-carbon phase, and by controlling how long the carbon has to re-arrange itself as you lower the temperature you can create a wide range of physical properties. The phase-diagram above is an equilibrium diagram and at room temperature some alloys won't reach equlibrium even on geological timescales. A more useful diagram is the TTT diagram, but that is specific to each individual alloy. There is one here, along with an explanation of what it means. http://sparkyswordscience.blogspot.de/2013/12/alloys-microstructures-and-phase.html Basically you start at the left at zero-time and the normalising temperature, then plot the temperature-time history. If you quench fast enough to miss the "Bainite Nose" you will get pure Martensite at the Ms Temperature. Otherwise you will get Bainite and/or Pearlite. For plain-carbon eutectic spring steels the ideal is a very rapid quench to about 400C and then a hold at that temperature to form fine pearlite. The challenge is getting a fast-enough quench to miss the nose, which is hard with thicker sections. Properties such as the infra-red transparency of your quench medium start to matter at that point, and it turns out that molten sodium hydroxide works well. And I have the scars to prove it. Alloying elements can push the "Nose" to the right. Erbium works well. It's a strange coincidence that I had two consecutive research projects, both using Erbium, the first was related to its optical properties when used in glass optical fibres, the second concerned with using it to make better spring steels. -- atp "A motorcycle is a bicycle with a pandemonium attachment and is designed for the especial use of mechanical geniuses, daredevils and lunatics." — George Fitch, Atlanta Constitution Newspaper, 1916 ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
