Spark testing for hardness

[R.C.Reichert]

I don't know how useful this is to anyone else or not, but I found it is actually kinda useful on my carbon steel knives to check for hardness. What I did was after quenching, I took a piece of sharp flint and glanced the edge of the blade down against the flint edge. If it sparks really well, then I know it got really hard. If it does not throw good sparks then it didn't get hard enough. I found this little tidbit out by making flint and steel fire strikers. Some would throw sparks better than others depending on how hard they got. I use this in combination with the old file test. I would just recommend using caution and wearing thick leather gloves to hold the flint.

So if it's useful to anyone great, if not the perhaps just something to keep in mind if you wanna make some flint and steel fire strikers.

 

[Stacy E. Apelt - Bladesmith]

Not a reliable way of testing hardness. The strike can vary in so many ways ( angle speed, distance, etc.), and the skill at striking a spark will also vary. The best way is to put the blade to the grinder and observe the sparks. If it is high carbon, the sparks will fly out in abundance as bright white streaks and then explode as tiny fireworks. Yellower sparks and low amount of sparks indicate low carbon and low hardness.

 

[R.C.Reichert]

Not a reliable way of testing hardness. The strike can vary in so many ways ( angle speed, distance, etc.), and the skill at striking a spark will also vary. The best way is to put the blade to the grinder and observe the sparks. If it is high carbon, the sparks will fly out in abundance as bright white streaks and then explode as tiny fireworks. Yellower sparks and low amount of sparks indicate low carbon and low hardness.

I wasn't referring to finding the carbon content in a steel, but rather if the blade got hard after quenching. There is undoubtedly variables, but there are variables with the file test too. Was it a new file, or was it worn out? Was there scale or decarb on the blade? These could effect a file test. A carbon steel blade that did not get hard will not spark with flint. A very hard blade out of the quench will throw lots of sparks with a flint

 

[DevinT]

Steel in the annealed condition is ferrite plus carbide. Ferrite does not hold carbon so the carbon goes into the carbides. Annealed steel has twice the carbide volume as hardened steel.

Above Ac1 carbides start to dissolve and above Acm all carbides are dissolved. (carbon and low alloy steels) Austenitizing temperature is somewhere between Ac1 and Acm dissolving some or most of the carbides putting carbon and sometimes alloy into solution. The carbon in the matrix allows the steel to get hard during the quench. Some carbides remain giving the steel more wear resistance. Carbon in solution is what burns giving off the sparks.

Hardened and tempered steel gives off more and brighter sparks than annealed steel does.

Hoss

 

[R.C.Reichert]

Hardened and tempered steel gives off more and brighter sparks than annealed steel does.

Hoss

Yes, that is the case when putting the steel to a grinder, but try striking a sharp piece of flint rock to an annealed piece of high carbon steel and you will likely not get any sparks at all. A hardened, but not not fully hardened, or poorly hardened piece will throw some sparks, but not very well. A fully hardened piece of high carbon steel struck against a sharp piece of flint will throw a shower of very bright sparks with very little effort. A 1095 blade quenched in oil that was too slow will not throw very good sparks against a piece of flint. I find that I basically have to quench 1095 in water to get a really good sparking fire steel out of it. Found that out with 80crv2 as well. This is useful to me because it tells me if my blades got really hard in the quench or not depending on how well or how easily they throw sparks. The blades can still seem hard when you run a file across them...making the typical glassy, skating sound..., but they might not be as hard as possible and I can tell that by doing this spark test.

Btw , it does not cause damage to a blade. It would only be done to a blade that would still need finish grinding/sanding anyways.

 

[Mac an sealgair]

Found that out with 80crv2 as well.

R C Reichert, sorry, I know this is an old thread but are you saying you water quench your 80crv2?
I’ve been using that recently but have never been happy with the hardness I’ve been getting. I thought a water quench would have been too harsh, isn’t 80crv2 a medium quench steel?
I’ve been using sunflower oil as quench and have been blaming that and recently bought a UK equivalent to Parks AAA but haven’t used it yet. I’d be interested what oil you use and how you water quench eg straight from the forge/oven into the water? Is it warm water or cold?