On Wednesday 03 August 2016 04:37:58 andy pugh wrote: > <Stand back, I am a metallurgist> > > On 3 August 2016 at 01:19, Gene Heskett <[email protected]> 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.
Probably. I have noted that even the 800F tip, which has of course led to more oxidation damage to the other materials that make up the heater and tip, that button has not been noticeably discolored. But with modern eutectic solders incorporating up to 3% silver and/or similar copper to the alloy, its far more comfy with a 650F or 700F tip. I have much more modern soldering stations of course, including hot air rework, but they are also 4x the weight, and 20x the volume of that original little black brick. For a 1 to 10 joint project, its the goto iron. > > 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. ISTR that was what Ford said in their propaganda touting the longer life of such valves at the time they were new. Entirely possible that was sales speak for we don't know but it works. :) > 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. Precisely why when I have to assemble something by "welding", I am partial to the gas torch. By adjusting the color of the flame and the length of its central "feather" one can make a weld have almost any property you want, from dead soft with an excess oxygen flame, a hard tip IOW, to quite hard & hi tensile strength just by setting a 1/2" long central feather from oxygen starvation, all with nothing but a pile of straightened out coat hangers for fill metal. > 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-a >nd-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. And I assume that Erbium is another of those rare earths in vanishingly short supply & priced accordingly? ISTR reading that it was used in trace amounts in the super magnets too. Thanks Andy. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
