TLDR: What is happening with this galvanized metal strap that I heated and quenched? After quenching, the metal showed a change that abruptly stops at this pronounced line. I think I scored the metal before tempering, but I'm not positive. Is there some kind of temperature threshold, where cooling happens at a different rate? I took a grinder and cut some width off of one end of a 1/8" x 1-3/8" x 12" piece of hot dipped galvanized metal strapping. I bent the narrower piece 90 degrees. I used butane torch to get the steel red hot and quenched it with water. It came out with this pronounced line that ran across both sides of the strap. You can see the metal is very different on either side of this line. The line itself is even shiny. I kind of remember scoring the metal prior to tempering it, but I'm not positive I did. Is this just my bad memory, where the rate of cooling changed abruptly at my score mark? Does the quenching process have some sort of temperature threshold, where the rate of cooling changes drastically? I'm sure it might be silly to some, but I was fascinated. I've never done metal work though.