What are the actual white sparks that come off of steel

Fish30114

Fish30114

Joined
Jan 5, 2014
Messages
1,495
I have had some people ask 'what are the white sparks that come off of steel when you are grinding on a knife-and how hot are they. I didn't really have a complete answer for them--I was assuming it was bits of the used steel and their exposure to air that made them 'burn'...hopefully one of you knowledgeable folks can chime in and tell me the proper answer--I was also asked doesn't grinding after hardening a knife take the temper out--I told them not if you don't get it hot--and they asked how hot is one of those sparks?--So????
 
Yes, you are correct! The sparks are little bits of rapidly oxidizing (burning) steel. Not sure how hot they are though...
 
The sparks are probably 2100-2300F. While that is very hot, there is almost no mass, so it burns up quickly. If it touches something, it transfers the heat quickly, too. Skin will barely feel the heat, as it is very moist and dissipates the heat. Paper and dry leaves could easily catch fire .... and regularly do in the shop if you aren't careful.
 
Its tiny fragments of steel burning.
The action of grinding imparts enough energy (friction) into the fragment becomes heat to the point that fragment will burn in our 20% oxygen atmosphere.
If you ever seen oldschool flint & steel firemaking, or a sparks from chain dragging behind a car, its the same friction shaved off a fragment & burning process but at different scale.
 
Last edited:
Fish30114 said:
I have had some people ask 'what are the white sparks that come off of steel when you are grinding on a knife-and how hot are they. I didn't really have a complete answer for them--I was assuming it was bits of the used steel and their exposure to air that made them 'burn'...hopefully one of you knowledgeable folks can chime in and tell me the proper answer--I was also asked doesn't grinding after hardening a knife take the temper out--I told them not if you don't get it hot--and they asked how hot is one of those sparks?--So????
Click to expand...
I would ask that smart guys WHY the tip of the blade is most easy to overheat :D How hot is sparks .....................:eek: Or better ask them what will faster get 1500 degree in oven ...Piece 1x1 inch and thick 1mm or piece 5x5inch and thick 3 inch :p
 
Stacy E. Apelt - Bladesmith said:
The sparks are probably 2100-2300F. While that is very hot, there is almost no mass, so it burns up quickly. If it touches something, it transfers the heat quickly, too. Skin will barely feel the heat, as it is very moist and dissipates the heat. Paper and dry leaves could easily catch fire .... and regularly do in the shop if you aren't careful.
Click to expand...

Stacy! Those sparks may not have much mass but they still set a old cotton jacket I was wearing, on fire!:eek:
 
Rhinoknives1 said:
Stacy! Those sparks may not have much mass but they still set a old cotton jacket I was wearing, on fire!:eek:
Click to expand...

Yep, we have all set a vest, jacket, or tee shirt on fire.

One really cold winter I had my down vest on and was hogging blades. I had the respirator with the full hood on, which is fed by a hose delivering outside air. I noticed my neck feeling hot, and looked up to see that the shop was a bit smoky. I stopped the grinder, and when I pulled off then helmet realized my vest front was on fire.
 
Reading this post made me think about why some steel doesn't cut under a oxy acetylene torch. I'm going to take a guess and say it has something to do with the nickel addition changing how it interacts with O2?
 
Hmmmm that’s a good question!! Maybe Larrin, matte or one of the other science guys will pop in and comment on that...

my hypothesis is that since hardened steel is... well harder, it takes more energy to scrape it off, so more friction and heat. Thus a brighter burning spark!

Of course that’s just me talking, not any real scientific backing just pondering the mysteries and all that.
 
The hardness is most of the difference. I think the rest has to do with the placement of the carbon. The pieces shaved off are also smaller, which means they burn brighter and sparklier because the sort of explode into flames as they react with the oxygen.
 
Stacy, before I even read your reply I had a similar thought.
The steel is now hard and more RESISTANT to the grit. So rather than few large pieces being ripped out, many more tiny pieces are removed. That would seem to create a 'shower' of sparks instead of a stream. And being tiny they sort of burn faster and brighter.
 
Yep, imagine slow lathe turning, if the chip doesn't get cut easily, a lot of extra rubbing goes on (friction), rising the temperature up...burning your tools and stock.
In proper machining operations the most % of the generated heat leaves the workpiece through the chips, but when grinding there is sufficient friction (not all of the grit particles have the correct geometry/pressure to cut any time they meet the steel) to generate intense heat on the surface thus actually austenitizing a thin surface layer, if coolant isn't involved. That layer gets immediately "quenched" by the stock mass, but its new hardness generate more friction and heat and you are in a loop. Near the very edge this leads to edge holding issues, obviously.
Now, a few nanometers thick edge should be your focus, and in that range even honing on a dry benchstone can affect your temper badly. But you can still grind an hardened blade allowing a bit of skin to be removed with coolant without compromising your knife. Just be cautious, and don't let the blade to get perciebly hot while grinding, cool often or better continuosly if you can.

If you want to have fun looking at burning steel, try igniting a piece of fine steel wool!
 
Last edited:
I think the best way to explain to customers why the blade doesn't get hot is to compare it to a match and striker. The match burns ignites at 4-500° and burns at 11-1200. How hot does the striker get? Not very. Now how hot does the belt get? Also not very hot, even though a trail of sparks can often be seen going all the way around the circumference. Also make sure they know annealing temps are 350-400°.
 
I place welding magnets all over the grinding bench to catch the stray metal grit/dust. They get covered with the metal, which forms strings along the magnetic flux lines. They look beard-like and are similar to steel wool. Regularly, a spark will be attracted to the magnet and start a "fire" along these whiskers of steel dust. It is sort of cool to look down and see the little balls of fire running in and out along the beard of metal grit. When one gets too hot, I just dunk the magnet in the dip tank to extinguish the fire.
At the end of a grinding session, I turn off all the equipment, shut of the lights, and look for any glowing around the magnets. Most likely it will just burn out, but I have several black spots on the bench where it charred the wood before I dunked it. I go inside and wash up, then go back out after a while and look around in the dark again as well as do a "sniff" test. With the place closed up and nothing moving the air around, I can smell any smouldering easily.

For a fun experiment, set a piece of steel wool on the concrete floor. Light it on one end and watch the little fires run back and forth. Now, take your welding torch and tun on the oxygen only. Direct the oxygen flow at the burning wool. You will see how bright it can burn with more than the 20% atmosphering O2 level.
Don't lean over the wool while doing this unless you hate your eyebrows.
 
Karl B. Andersen said:
Why do hardened and unhardened steel spark differently?
Click to expand...

Because most of the energy doesnt come from friction, although that is part of it.

Steel is pyrophoric, which means it bursts into flames in air when in small particles with a small amount of heat. That is simply due to the reactivity of iron with oxygen. But the energy comes from the grinding itself. It takes a lot of energy to break the bonds holding steel together.

Think about working cold steel back and forth with hammer, it will heat up as the steel is hit. The energy is going to deform bonds and this generates a lot of friction between the molecules as the energy of the hammer is transformed into heat. Something similar happens with grinding.

The formation of a chip by each piece of abrasive requires an amount of energy to break the bonds that were attaching the chip to the material. In this process, a huge amount of localized work is done. The higher the hardness of the steel, the more energy is required to form the chip, and thus the hotter the resulting chip and the brighter it burns.

A harder steel would also mean more sparks, as some chips might not have enough heat to pyroforese in soft steel, but the increased energy when grinding hard steel means a greater percentage can glow.
 
Stacy E. Apelt - Bladesmith said:
I place welding magnets all over the grinding bench to catch the stray metal grit/dust. They get covered with the metal, which forms strings along the magnetic flux lines. They look beard-like and are similar to steel wool. Regularly, a spark will be attracted to the magnet and start a "fire" along these whiskers of steel dust. It is sort of cool to look down and see the little balls of fire running in and out along the beard of metal grit. When one gets too hot, I just dunk the magnet in the dip tank to extinguish the fire.
At the end of a grinding session, I turn off all the equipment, shut of the lights, and look for any glowing around the magnets. Most likely it will just burn out, but I have several black spots on the bench where it charred the wood before I dunked it. I go inside and wash up, then go back out after a while and look around in the dark again as well as do a "sniff" test. With the place closed up and nothing moving the air around, I can smell any smouldering easily.

For a fun experiment, set a piece of steel wool on the concrete floor. Light it on one end and watch the little fires run back and forth. Now, take your welding torch and tun on the oxygen only. Direct the oxygen flow at the burning wool. You will see how bright it can burn with more than the 20% atmosphering O2 level.
Don't lean over the wool while doing this unless you hate your eyebrows.
Click to expand...

You can also set steel wool on fire with a regular 9 volt battery. Just touch the connections to it!
 
You must log in or register to reply here.
Back
Top