Fire Making How bad really is using knife edge to scrape firesteel from microstructural and heat treatment perspective of steel?

[damib]

Hello,
I have been questioning this for a few months now. I had to scrape firesteel with the edge of the knife in september because I didn't have any knife with 90 degree spine with me. I am aware that this is certainly bad for the edge and that you can destroy it. However, my friend told me that I will not be ever able to properly repair the damage on the edge because of the high temperatures during the firesteel scraping. I have read somewhere that the temperatures generated can reach as high as 3000 degrees Celsius which is very high. But are the sparks and the friction from firesteel long enough in contact with the steel to cause any significant damage to the steel in microstructural or heat treatment context? If so, why would it be able to not damage the steel when being scraped by 90 degree spine? I apologise if this is either too sciency or too easy of a question, but this is something that doesn't give me enough sleep.

 

[Mistwalker]

Ferrocerium burns at 5,800 F. If a piece of the ignited ferrocerium sticks to the edge of the knife and burns there, it will ruin the heat treat in that area. Doing it for survival is one thing, but doing it for practice is another.

I prefer a dedicated striker

 

[gadgetgeek]

Yes but. To be fair, I'm not going to disagree with Mist on anything important, he's got me beat for experience. However... Even if a spot gets heated enough to be soft, we are likely talking a spot that is a few thou of an inch wide and maybe even a few ten thou deep.
What we are looking at is how hot can a spot get before the thermal transfer pulls that heat out and dissipates it. with a 90 degree edge, those spots will occur, but they are far from the cutting edge and likely never make a difference. At the cutting edge, sure it will damage some of that edge, and the heat will impact the heat treatment, but I do not know by how much. It would really matter on the steel, it's original hardness, overall thickness and use.
For what it's worth, way back in the old days of straight razors, if a razor was too hard to be honed by a barber-stone (manufactured abrasive stone, very very mild) it was recommended by some to cut into a fresh from the oven loaf of bread, and then let the razor cool in the loaf. From talking to metallurgists and guys with a lot of razor experience, the most plausible explanation is that (keep in mind these are very simple carbon steels, no carbide) the temp shift would be enough to drop the temper of the outer few microns of the edge, allowing the razor to be honed. So a very thin edge, at baking product temps, and all the hope was to just change the edge itself by maybe less than a single point of RC. So could a spark from a ferro rod damage an edge? I'll say plausible. Will it ruin it to the point where a whole section will never take an edge. I doubt it. Once it was fully resharpened, I doubt you'd ever know. That said, I'm not going to go spending time chewing up my knives. ESEE said their knives can do it, and said that if you did it enough that it ruined your knife, they'd replace it, and last I heard they still stood by their warranty, but also those dudes are kinda nuts, and they sold tools expecting them to get used up.

 

[damib]

Yes but. To be fair, I'm not going to disagree with Mist on anything important, he's got me beat for experience. However... Even if a spot gets heated enough to be soft, we are likely talking a spot that is a few thou of an inch wide and maybe even a few ten thou deep.

So could a spark from a ferro rod damage an edge? I'll say plausible. Will it ruin it to the point where a whole section will never take an edge. I doubt it.

I totally agree with you while I also have to say that my experience and use of the ferrorod is negligible compared to some of the other outdoorsmen (mainly because I am in Europe where making a fire is a big no no in many places other than your garden).

It's a pity that I don't have access to electron microscopy and all of the other fancy scientific stuff to be able to study it anymore. It may have either been made into interesting case study.

Maybe I will be able to make some calculations by myself...

 

[Drop bear]

Pick a bit of the blade you don't really use?

 

[Mistwalker]

For what it's worth, way back in the old days of straight razors, if a razor was too hard to be honed by a barber-stone (manufactured abrasive stone, very very mild) it was recommended by some to cut into a fresh from the oven loaf of bread, and then let the razor cool in the loaf. From talking to metallurgists and guys with a lot of razor experience, the most plausible explanation is that (keep in mind these are very simple carbon steels, no carbide) the temp shift would be enough to drop the temper of the outer few microns of the edge, allowing the razor to be honed. So a very thin edge, at baking product temps, and all the hope was to just change the edge itself by maybe less than a single point of RC.

So could a spark from a ferro rod damage an edge?

I think most baking ovens will only get up to about 550F and the magic number I'm told for ruining heat treats in forged steels is around 800-1000F IIRC. I don't think 500F would even bring the straw color some knife makers use a propane torch to achieve, to tempewr back spines. but I could be wrong on that and that may be exactly what it will do.

Yet, I think it really mostly comes down to two factors. Which type of ferro rod, and ones fortunes being good or bad at the time.

The harder rods like the Light My Fire Swedish Firesteels use, or used 20 years ago at least It's been a while since I bought one, and like the ESEE Fire Starter, and the current Gerber and Strike Force fire starters use, the spark sticking to the blade isn't so much an issue. We cannot cut slivers off of this type of rod so they just throw sparks.

It is with the softer mischmmetal rods that the biggest chance lies. With the softer rods the knife edge can scrape off, and roll up a sliver of the material nearly the length of the rod before it ignites, and then it can ignite while stuck to the edge of the knife and just sit there and burn out for 2 seconds at 5800 F.

It in was sitting across from a fire pit watching someone else light a fire that I got to see this first hand from that side of the knife. But to me it explained what happened with one of my own knives earlier. When in the process of trying to ignite wet tinder material with my first michmetal rod, from Firesteels dot com I think...it has been almost 20 years lol, It was dark and later that evening I notced my edge had 3 shallow scallops in it and the steel around the area was the purple and black color of overheated steel.

 

[Mistwalker]

Pick a bit of the blade you don't really use?

This can halp if you have no choice, or pick an area easier to fix

 

[Danke42]

The good news is that if you keep this up you will soon need to purchase a new knife with a 90 degree spine. Huzzah!

 

[damib]

I think most baking ovens will only get up to about 550F and the magic number I'm told for ruining heat treats in forged steels is around 800-1000F IIRC.

I think that some "primitive" ovens that run on firewood can reach even up to 1000 degrees Celsius. However, I doubt that the baked goods would reach this temperature. Maybe some expert in baking will tell us.

The harder rods like the Light My Fire Swedish Firesteels use, or used 20 years ago at least It's been a while since I bought one, and like the ESEE Fire Starter, and the current Gerber and Strike Force fire starters use, the spark sticking to the blade isn't so much an issue. We cannot cut slivers off of this type of rod so they just throw sparks.

Thanks, that makes sense.

 

[gadgetgeek]

I think most baking ovens will only get up to about 550F and the magic number I'm told for ruining heat treats in forged steels is around 800-1000F IIRC. I don't think 500F would even bring the straw color some knife makers use a propane torch to achieve, to tempewr back spines. but I could be wrong on that and that may be exactly what it will do.

Yet, I think it really mostly comes down to two factors. Which type of ferro rod, and ones fortunes being good or bad at the time.

The harder rods like the Light My Fire Swedish Firesteels use, or used 20 years ago at least It's been a while since I bought one, and like the ESEE Fire Starter, and the current Gerber and Strike Force fire starters use, the spark sticking to the blade isn't so much an issue. We cannot cut slivers off of this type of rod so they just throw sparks.

It is with the softer mischmmetal rods that the biggest chance lies. With the softer rods the knife edge can scrape off, and roll up a sliver of the material nearly the length of the rod before it ignites, and then it can ignite while stuck to the edge of the knife and just sit there and burn out for 2 seconds at 5800 F.

It in was sitting across from a fire pit watching someone else light a fire that I got to see this first hand from that side of the knife. But to me it explained what happened with one of my own knives earlier. When in the process of trying to ignite wet tinder material with my first michmetal rod, from Firesteels dot com I think...it has been almost 20 years lol, It was dark and later that evening I notced my edge had 3 shallow scallops in it and the steel around the area was the purple and black color of overheated steel.

It's one of those odd things, and I agree, it may be an old wives tale, but when it comes to thin edges, we already know they don't follow the rules quite how we expect. I guess it also matters on what we think of the steel in question. Is a millimeter of edge worth caring about? We've all seen re-profiles that remove several mm of edge to bring a bevel to where the user wants it. Is it a safe queen, or is it something I'm hoping to rely on for decades? I honestly cannot answer those questions for myself, let alone anyone else. I think though the bigger thing would be understanding that we are killing at most a portion of the edge, not say the whole temper like an ill-advised trip to the bench grinder.