I had this thought with me for quite some time, that you can't use the usual formula for estimating carbon content on a Nital etched hypoeutectoid sample (DarkArea*0.77) if the steel obviously has more than around 0.6% carbon.

I am pretty sure I read that somewhere, but I can't find that, and I might have been operating in a false premise for some time now.

  • $\begingroup$ How ist it obvious that the steel has more than 0.6% carbon? As a hypoeutectoid steel, it could lie between 0.6% and around 0.8%, right? $\endgroup$ – Andrew May 23 '18 at 4:48
  • $\begingroup$ A steel with more than 0.6% carbon will have a very dark structure under nital etching after being annealed, because of the pearlite. If you're certainly dealing with a hypoeutectoid steel, and it looks very dark under the microscope, it is usually a very good bet you're dealing with more than 0.6%C. And yes, that is the issue you have to deal with, not knowing where in the 0.6%-0.8% scale it is, as I believe the usual formula won't give you the precise carbon content. But I'm not sure why I believe that, even though I think I read it somewhere. I wanted someone to maybe point me to a source. $\endgroup$ – Walther Wennholz Johnson May 23 '18 at 5:55

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