Thread: Ruining the temper: How hot is too hot?

The title speaks for itself. I'm trying to polish up my razor here with a dremel tool and some compound, and I noticed that the blade got pretty hot when I left the buffing wheel on it in the same spot too long. It was at this point when I wondered: how hot is too hot before I ruin the temper? The metal hasn't changed color or turned blue or anything, just got hot to the touch.

I'm not too experienced with it i hope someone else helps out but i know that heat-treatment to get the right hardness in the carbon steel could range from 300 to like 450 degrees F. So to be safe i wouldn't let it get close to that, maybe 150 degrees....many people rinse with really hot water, boiling point of water is 212 degrees... so maybe a little higher is ok but the main thing is if you keep it cool you wont have to worry.

*whew* dodged a bullet then...I mean the thing got hot, but nowhere near the temperature of boiling water...I think I'll just dip it in a glass of water in between

Originally Posted by strudel

The title speaks for itself. I'm trying to polish up my razor here with a dremel tool and some compound, and I noticed that the blade got pretty hot when I left the buffing wheel on it in the same spot too long. It was at this point when I wondered: how hot is too hot before I ruin the temper? The metal hasn't changed color or turned blue or anything, just got hot to the touch.

There are some tempering tables for steel that will answer your question
if you search on the net.

In general if you keep the blade cooler than hot tap water you are OK.
But, this can be harder to do than it sounds.... So cool in cold tap water...

This is a time and temperature problem. For example double edge
razor blades are coated and the coating sintered to the surface. The
sintering temp at about 300C must be kept very short or the steel will
be too soft to hold the edge.

Since the working edge of a str8 is thin the risk of killing the temper
is very real. The thin working bit of the razor can overheat yet not
feel too hot because it is thin and by the time you test it, it has cooled.

Power tools are problematic for a couple of reasons overheating being
one of them.

I recall somebody posting once that too hot is too hot to touch. That is the measure I have been going by since.

Too hot to touch is almost too hot. The problem is that heat has a lag time and can still increase before you can move the blade from a hot touch to the cold water. The best thing is to work barehanded touching the steel near where you are working and cool frequently even (and especially) when you don't think you need to.

If you did not change the color of the blade, you are probably still okay.

One smallish correction: tempering is the process of drawing back the hardness of a piece of steel. That is often done for carbon steels at or near 400 Fahrenheit. Stainless tempering can occur at various temperatures even up to about 1000 F. But each specific steel has specific numbers for optimum performance. That's the one good reason to really know what steel you're working with.

Sintering is a welding process and needs at least better than 2350 Fahrenheit to work efficiently. But this can also vary based on the particular industrial process and tooling versus home shop and available tools.

So, "better safe than sorry" and "don't be a fool, dip your tool"(into cold water) is what I'm hearing. Advice taken, thank you Btw, you guys seem to really know your shit, thanks!

Mike is a walking encyclopedia & can apply everything he states. The advice on too hot to hold/touch is right on. If you are working with a dremel and a buffing pad, keep a bare finger on the opposite side of the blade where you are working the buff. Never let the tool stay in one spot...ever. Localized heat can really hurt the blade. Keep it moving, use a light touch, & always pay attention to the direction of rotation & you will be ok.

+1 what Mike Blue said - he knows just about everything there is to know about heat treating. If you want more information, download "Metalurgy of Steel for Bladesmiths and Others who Heat Treat and Forge Steel" by John D. Verhoeven. Around 200 pages of PDF, and EVERYTHING about heat treating.

Whenever I am grinding or polishing, I use bare hands.
I don't linger on one spot, and if it is too hot to comfortably hold, I dunk it in water or let it air-cool.