Post-Heat-Treat grinding Q&A - for Kevin, mete, et al

Daniel Koster

Daniel Koster

www.kosterknives.com
Knifemaker / Craftsman / Service Provider
Joined
Oct 18, 2001
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I have a question for somebody smarter than me (metalurgically speaking :D)


What exactly takes place inside the steel when you overheat it and discolor it when grinding AFTER heat-treat?


I am curious, that's all.


I have searched and have read many accounts of "hearsay" and "pass-along" quips...but have yet to see a metalurgical description of it....and why it is bad.


I know it's bad :(....but can't rationalize it in my mind why.


If you temper a steel to straw color....and then "over grind" it and it turns straw...how is the temper affected? and in what way?

Let's say I even over-grind it further to purple/brown/blue/etc....the equivalent in some steels of only 50-100 degrees hotter in the temper oven. Have I softened it? by how much? In what way?

And then, to complicate things further....what about those super-steels that temper at 950 degrees? I'm pretty sure that I could if I wanted, grind the tip till it's purple, and yet it will not reach 950 degrees and "spoil" the temper.

It's been said that a knife recovered from a fire can be put back into service...re-heat-treated, and so on. How does this fit in?


Lots of questions this morning....though this has been plaguing my mind for a couple years. I have just never felt fully satisfied with the answers.


Any comments?

Dan
 
Remember that the color you see is an oxide layer and influenced by temperature , alloy and surface contaminants such as oil.
Grinding creates heat and there's not much you can do about that. However excessive heat you can do something about. Grinding wheel in good condition , coolant [throw away old belts !!] .The color you see is heat which in turn tempers the steel .If you really overdo it you can get transformation to austenite and on cooling reharden the steel. Keep the temperature below the intial tempering temperature .For most applications if the steel is too hot to hold dunk it into the water !
We do have a problem with knives.While grinding the spine is not a big deal, grinding the edge and especially the tip is a very different story.The heat created in grinding the edge has no where to go so the edge really heats up. Perhaps you have had experiences as I have had. A small bit of tip breaks off even with minor forces. An edge even though sharp is a poor performer yet when resharpened it performs very well. To these problems you can look to grinding !! Treat edges and tips gently and cool often !
 
Like Mete said, when they get too hot to hold, dunk them in the bucket of water! This way, you don't have to be concerned with ruining the temper.
Just don't wear gloves, and the problem will take care of itself.:thumbup:
 
I'm far from knowing a lot about how the whole thing works but I imagine if you are heating the blade to purple you are in effect running another drawing cycle. If the blade is heated to straw brown, and at that color it is in the 58-59 range, then when you grind it to purple, you are running the drawing cycle to a higher temperature therebye lowering the hardness. Another words, whether you temper it in the oven to purple or grind it hot until purple the effect is the same...running it into a lower hardness range. The only difference being that in the oven, the whole blade would be that color....on the grinder, only the part that is being ground is getting that hot. So, my guess is that you are changing the internal structures of the steel in different spots to an unpredictable hardness by over-grinding. Of course I really have no idea but it seems logical that this may be why you don't want to overheat treated blades.
 
thanks for the reply, mete.

Can you elaborate more on this?



What's the threshold for transforming back to austenite? Do you have to go above non-magnetic?

What's goes on with the steel "along the way"? I imagine hardness lowers as you get higher and higher...but then what? Is it back to the same ole thermal-cycling?


Dan
 
The hotter you get the steel the more you temper the martensite ,precipitating more carbides.The higher the temperature the softer the steel gets. When you get to about 1400 F you convert the martensite to austenite .When this cools it can transform to UNtempered martensite on cooling .Depending of course on the variables of time, temperature , alloy. The UNtempered martensite can cause cracking.There are etchants available to show grinding damage.Perhaps Scott Ickes can get some photos of grinding damage on bearings when etched. The damaged areas can be rather thin as heating and cooling can be very fast.Therefore ,as I mentioned before , a simple sharpening may remove the damaged layer.
 
How can you tell how hot it gets? (other than the oxidation colors)



mete said:
The damaged areas can be rather thin as heating and cooling can be very fast.Therefore ,as I mentioned before , a simple sharpening may remove the damaged layer.
Click to expand...

So, really, it's more about cooling than the heating?



Dan
 
http://www.uddeholm.com/files/grinding-english.pdf
Here's a primer on grinding ! Note the photo of the gear. The white areas are untempered martensite which means those areas have seen 1400 F + temperatures ! Dark areas are tempered back due to the heat.

What I meant was that if the heating and cooling is quick the damaged layer may be thin enough to be removed by simple sharpening.
 
I have a question on this subject too. Since the oxide layer color of the steel is dependent on both temperature and time, can you really use the color to estimate how hot the edge got? For example, if the steel turns a golden straw color after half an hour in the tempering oven, how hot is it actually getting to form that color in half a second when grinding an edge? Don't the temperatures actually have to be much higher than we normally think of, to form those colors so quickly?
 
i think it takes a long time to change color in the oven because air is such a poor conductor of heat. Things seem to happen instantly on the grinder, esp with thin edges and single speed 5000+fpm machines like my piece of **** because the heat is generated on the steel itself
 
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