marthinus, thanks for the insightful remarks and informative links . (Also, it's good to see the member from the other forum here as well.)
I don't mean to hijack the thread, and maybe we should start another thread but here goes.
What exactly do people mean when they say the edge has lost or ruined temper.
I'm not a metalsmith, but my basic understanding is that metal goes through several steps. Forging, hardening, and tempering.
Followed by griding/sharpening, that hopefully, leaves the metal well enough alone.
There is only one step that makes a knife hard, the hardening step.
The metal is heated to high temp, where it favours BCC formation, then quenched quickly, forming martensites.
These high-strain, high-dislocation crystalline organization is what gives steel their hardness.
The high-strain state is prone to chipping and brittleness upon impact.
So to mitigate that, they introduce some controlled amount of heat, tempering.
This relaxes some of the structures in the martensite organization, introducing more toughness, at the cost of hardness.
The full cycle is: Forging, annealing/stress relieving, hardening, quenching, tempering (might include quenching as well).
Lets keep to the datasheet of S30V for this as you linked. But here is one from crucible:
http://faq.customtacticals.com/datasheets/s30v.pdf
Forging:
2100°F (1150°C) Do not forge below 1750°F (950°C).
Annealing:
Heat to 1650°F (900°C), hold 2 hours, slow
cool no faster than 25°F (15°C) per hour to 1100°F (595°C),
then furnace cool or cool in still air to room temperature.
Now this is normally how the maker buys the steel. In its annealed state.
Then the maker has the choice to either work on the blade in its annealed state, grinding etc before heat treatment. Some makers prefer to go the other route by cutting out the basic shape, have it heat treated and before they start grinding away.
So, the hardening stage is described as:
1) Austenitization step: Heat the steel to form austenite (usually 100 % austenite, but not always).
The recommended austenitization temperatures vary with the carbon composition of the steel
2) Quenching step: Rapidly cool the steel by immersion in water, oil liquid nitrogen.
3) Tempering step: Heat the steel to a low temperature to remove brittleness.
(Verhoeven, J.D. 2005. Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel) A very good book to read.
Hardening
Preheat:
Heat to 1550-1600°F (845-870°C) Equalize.
Austenitize:
1900-2000°F (1035-1095°C), hold time at temper-
ature 15-30 minutes.
Quench:
Air or positive pressure quench (2 bar minimum) to
below 125°F (50°C), or salt or interrupted oil quench to about
1000°F (540°C), then air cool to below 125°F (50°C).
Quite complicated without the right tools. So obviously care needs to be taken.
Now, the tempering:
Temper:
Double temper at 400-750°F (200-400°C). Hold for
2 hours minimum each time. (See Table) A freezing treatment
may be used between the first and second tempers. Freezing
treatments help to attain maximum hardenability and must
always be followed by at least one temper.
NOTE: For optimum stress relieving, CPM S30V may be tem-
pered at 1000-1025°F (540-550°C). Tempering in this range
may result in a slight decrease in corrosion resistance.
If you look at the S30V pdf there are 3 different Tempering tempretures, 200°C, 315°C, 540°C, each one has to be applied twice. Now the graph just below shows the influence on the tempering on the steel. It is a non linear graph for many high alloy steels. This can be seen in M390 datasheet nicely:
http://www.bohler-edelstahl.com/files/M390DE.pdf
Now, when a steel loses its temper the steel was subjected to heat greater then its lowest tempering temperature, in this case 200°C.
Okay, so that's the limit of my metallurgy.
It seems to me introduction of heat through grinder, or any other means, should add (not lose) tempering - again, softening the metal and preventing chipping even further.
Am I wrong about this?
Tempering cannot be added to. Once a blade is hardened and tempered it is done.
Well, normally at least some people can temper a blade again, but they really have to know what they are doing and can damage a blade severely. Normally not worth the risk.
Now remember, tempering is done for 2hours, if one creates to much heat while grinding and it goes above 200°C phase transition will start to take place. Even if just for 1 second.
From physics of it, overheating should have a very characteristic consequence, softening of metal.
Therefore, ruined temper due to belt grind overheat should have one symptom only - a softer edge that doesn't get keen easily and loses edge quickly.
Further, s30V has annealing temp of 900 Celsius and austenitizing temp of 1100 Celsius, and is recommended to hold those temps for 2 hours.
No its not recommended that way.
Austenitize:
1900 - 2000 degrees F ( 1065-1092 degrees C ). Hold time at temperature 15-30 minutes.
The Tempering is held for 2 hours at either 200°C, 315°C, 540°C.
Now, you mentioned "lose its edge quickly". How does an edge get lost?
There are generally 3 ways:
-Edge Chipping
-Edge rolling
-Pitting due to corrosion
In my previous post I stated: "The problem with S30V is that when you lose the temper even without suddenly putting it in water, one can experience "carbide fallout". This looks similar to chipping and the second aspect.
To illustrate this have a look here:
http://www.hypefreeblades.com/files/schneiden.pdf
See the white carbides in S90V? Those are kept in place by the matrix, fine grain structure. If the steel is over heated in sharpening the matrix is influenced and they wont be able to keep the carbides in place, thereby the carbides will simply fall out, looking like a chip."
So if the temper is affected and the steel is softer then one can experience either significant edge rolling or the carbides just falling out or tearing out while you use it because the matrix of the steel has been affected.
http://www.trugrit.com/heat-treatment-cpm.htm
S30V, I suppose being a super steel of a sort, seems quite different from some other carbon steels in that respect.
I have my doubt that even a tired and distracted factory grinder will sustain that kind of heat for that period of time (or anything remotely close).
So, since the OP is specifically worried about chipping, I think I will stick to my gun and recommend no removal from the edge.
You see it is not necessary for it to be sustained for 2 hours or not. Think of it this way. When ice goes above its lowest solid form 0 degrees Celsius it starts going into liquid form. If you subject ice to something that is 1 degree Celsius for 1 second some of the ice would have changed to water. Might not even be visible to the naked eye but phase transition is there.
In my previous post I gave examples of Elmax and people seeing brown marks on their edges.
Now look at the Tempering graph of Elmax:
http://www.uddeholm.com/files/PB_Uddeholm_elmax_english.pdf
Non linear as well, but also one can chose what tempering temperature to use between 200-600 degrees Celsius. So the lowest temperature where even Elmax will start to undergo phase transition is at 200 degrees celsius.
Those brown marks on the edges of Elmax are created by heat between 249-260 degrees Celsius. Here is a clolour indicator used for judging heat:
Now, the last thing to consider. The edge is thinner then rest of the knife, the thinner the area, the easier it can be influenced by temperatures, so if it goes anywhere near 200 degrees Celsius the temper will be influenced/lost.
Now, when people are sharpening on power equipment and due to the demand for high wear resistant steels it is easy to start pushing a bit harder with your hands into the belt if you are not use to S30V, M390. The belt the person is using can be an older belt and not cutting as it should thereby creating friction, not using fluid for cooling the blade. It can take one slip, one push or a split second to long. We are human, mistakes happen. It is not a train smash and with normal sharpening you can remove the damaged metal.
My apologies for Squashfan derailing his thread. I hope you get that burr off.
