heat risers

[Mark Williams]

I have observed that one is suppossed to countersink pin holes before heat treat to avoid heat risers. If this is to avoid having a 90 degree angle then should you not round of the edge of your blade also?
Also on some level would it not be better to polish the blade before heat treat to give a flatter surface without as many (micro heat risers) caused by the surface scratches at 220 grit or whatever you stop at?
Sorry for the long sentences and hopefully not stupid question.

Mark

 

[peter nap]

There are no stupid questions and this one is a very good question. I hope you get a lot of answers because I would like to know more on the subject. As to the pre HT polish I have noticed that when I do that I get blistering of the hotter parts on the blade. They require more than a finish grind and I've just decided to stop the grind at 220 prior to the heat treat. I don't understand it, I just know it happens.

 

[tmickley]

I've had one blade shatter at the pin holes when dunked in cryo. (This was my fault as I mislabled the steel) The pin holes on one blade was not countersunk. On a couple others (also mislabled in the same batch - bad day) the pin holes were counter sunk and they did not shatter. Could just be coincidence or not, I don't know. I do counter sink, slightly, all pin holes now as a matter of practice before sending them off to heat treat any more though. All superstitions aren't bad are they?

 

[Eric M Ericson]

I've had blades rough ground with 80g go through the hardening cycle, and crack on a grind line, but have not had the same trouble with blades ground to 220g

I think at some point the heat riser effect become miniscule enough to be ignored.

I do ease my spine corners (not round, just a small champfer) also although this has nothing tro do with heat, they are just easier to work before they are hard. Eric

 

[Ed Fowler]

When it comes to 5160 and 52100, I have seen the effects of scratches go deeper than the bottom of the scratch, into the blade. This does not seem to happen when I use a 220 grit finish. I always leave a little extra steel on the blade through the hardening and tempering operations to provide what I call a 'thermal cushion' that reduces potential stress risers. Just a little insurance, doesn't cost the knifemaker anything other than a little time and well worth the effort with these steels. I only drill into soft steel and these areas would never be hardened so don't have any experience with these events in hardened steel.