Blade finish before heat treat affecting hardness...

[Macchina]

In material science classes and applications at my work, I have found that the surface finish greatly affects heat transfer in metals. A mirror finish on a bar of steel provides exactly the surface area of that bar for heat to dissipate from. If that bar is sand, shot, or bead blasted, the surface area can increase by as much as 100% due to the micro vertical surfaces that are now present on the surface. When heat treating, I have found that the time the bar is removed from the heat source until the quench is extremely critical. Also, the time it takes the steel to cool in the quench is also critical (less time means greater hardness). I wonder if a sandblasted blade would heat treat better than a semi-finished or finished blade. What are your thoughts on this?

 

[Chuck Gedraitis Knives]

I think that you would develop a lot of scale in all the the micro vertical surfaces that are now present on the surface of a sand blasted blade, thus having to sand blast it again.

I don't think that surface prep makes a difference, as long as the blade is at a uniform temperature when quenched.

 

[Macchina]

The scale forms (I believe) during the actual quench, not during the heating. The surface finish remains the same until the knife is put in the quench...

 

[mete]

The heating is by radiant, convection and conduction. Surface finish should effect radiant heating only ....Scale is an oxide layer formed above the critical temperature.So the surface finish effects are sort term, until the oxide forms. Oxidation ceases as soon as the steel hits the quenchant.