Laser engraving causing a severe loss of strength in 8670 @ 59 RC

[P.Brewster]

Prior to laser engraving, the steel will survive any destructive test I can think of (e.g. https://www.instagram.com/p/CIRg4H-jXbs/).

After engraving, one lazy hammer blow will snap the knives in two. The engraving is approx 0.003" deep, and a simple engraved 3/8"-long line is enough to create the weakness. The engraved area is warm to the touch. Again, to be clear, prior to laser engraving, I can pound away on these knives with a hammer.

Steel was purchased from the mill, HT'd at Peters (tempered at 400F). A post-engraving 400F temper will not fix the problem. Tempering to 700F does fix the problem at the expense of 8-ish hardness points (I'm assuming there's a TME loss of toughness in the 500-650F tempering range).

Obviously engraving is adding a stress riser, but I'm astounded by the difference in apparent toughness between the pre and post-engraved knives.

Some questions I have for the experts:

1. Toughness is the energy absorbed when the steel is broken, which is distinct from the force/energy required to break the steel. Is the former (energy absorbed when broken) directly translatable to the latter (force required to break)? Obviously my crude hammer test is measuring the apparent force required to break the knife; I am not necessarily measuring nor concerned with energy absorbed.
2. On one hand, we have the well-established concept of TME, and plenty of hardness/tempering graphs that show a toughness peak around 400F, a toughness dip in the TME zone, and increasing toughness above the TME zone. On the other hand, we have the notion of "bluebacking" (drawing down the hardness of the spine), we have blue springs, and in my testing on the engraved knives, an apparent increase in toughness at "blue" tempers (e.g. 500, 550F). How do we reconcile these two? Why are springs tempered to 40s or 50s if optimal toughness is closer to 60RC for many steels? Why do saw manufacturers heat-treat 8670 to 52RC?

 

[Stacy E. Apelt - Bladesmith]

Show us some photos, please.

 

[P.Brewster]

Pic 1: bottom knife shows laser-engraved line used for testing purposes; this knife (in a different steel) survived the hammer test. Broken knives are 8670 that were tempered at 450F and 500F. Thickness of broken knives is 0.225".

Pic 2: close-up of broken star, through text "1Ni". Text height is 0.09".

 

[Stacy E. Apelt - Bladesmith]

The straight test line certainly is a stress riser.

I suspect that there may be some HAZ caused by the laser, but without a lot more info on the laser type, time of cut, etc. It would be hard to say. Normally, I wouldn't think that would create a weakened blade.

I don't know what "The Hammer Test" is, but it sounds pretty severe. Pounding on a blade isn't a valid test as far as I know. If you hammered hard on the blades initially, the stresses and micro-fractures created could well be why it broke the second time after the laser engraving

 

[Hubert S.]

It looks like the engraved text may not have had anything to do with the failure of the star and the crack more likely grew out of the blunt notch in the center. Did you hit the star with the hammer as well?

On the bar, I would expect the same failure if the notch had been machined vs laser engraved. The questions is whether the laser engraving weakens it more than a machined or chemically etched notch of similar geometry.

 

[Bill DeShivs]

Laser engraving steel does produce a hot zone. After all- it's burning away the steel.
I don't have a laser engraver, but when we do micro laser welding-if we use high carbon steel filler it will air harden much harder than a file. I have to use diamond abrasives to smooth it out.

 

[AVigil]

You could have written it with a Sharpie and it would still break at that stress riser in that apex.

 

[Joshua Fisher]

On those samples maybe it’s just me or the pictures but it looks like the grain is a bit coarse, have you tried heat treating some samples in house and replicating the tests to see what happens? It seems like you’ve been using 8670 for a while and marking blades with a fiber laser for a while as well so I’m guess this is a relatively new issue. Just makes me wonder if the heat treat has anything to do with the issue

 

[P.Brewster]

You could have written it with a Sharpie and it would still break at that stress riser in that apex.

No, as per the very first sentence of my post: "Prior to laser engraving, the steel will survive any destructive test I can think of"

 

[AVigil]

No, as per the very first sentence of my post: "Prior to laser engraving, the steel will survive any destructive test I can think of"

with stress risers it will survive until it does not.
There is a reason the crack is at that apex.

On the same piece of steel lazer engrave it in another spot, just above the first one, and do the destructive test again and see if it cracks again.

 

[Mitchell Knives]

Increase the diameter of the inside curve to at least .250" and see what happens.

 

[Bill DeShivs]

The steel could be hardness tested at the site of the engraving.

 

[Kraken Tactical Edc]

I do a lot of laser engraving and extremely complex designs sometimes running the laser for upto a hour each side. The laser actually shouldn't leave the steel hot to touch at all provided it is set up correctly hight, speed, power and focal strength it shouldn't even create any smoke or charring on the edges of the marking. Each hatch pass needed to be tuned- higher power slower speed one pass then alternate lower power faster speeds. You can't just run it at high power as once it's vapourised first layer and it forms that black carbon layer the if left at the same setting its just going to keep generating heat and not actually remove much lmaterial from the steel as the its just creating another layer of carbon. By the time you got to the visible depth on the steel you have shown where its broken the laser has actually affected the steel probably all completely through to its thickness like a perforated line invisible to the eye. If you were to surface grind that steel to the depth of 0.003 as you mentioned was the depth of the etch it will still be visible past that even if you continue to surface grind till it was no longer visible the line would show once you dipped it in some ferric chloride. Laser etching won't affect strength if done correctly also not sure what type of laser you are using but to mark a line like the images you posted should only take a fraction of a second even at a slow speed speed of 100mm/s it will be done faster than you can blink.

 

[Hubert S.]

Each hatch pass needed to be tuned- higher power slower speed one pass then alternate lower power faster speeds.

Can you explain in some more detail and put some rough numbers on it? What is the power of the laser and what does slower/faster mean?

I have access to a laser and have done a couple of engravings that turned out pretty nice, but getting good marking parameters is pretty much a guessing game for me. The laser has around 25W I believe.

 

[Kraken Tactical Edc]

Hi Hubert faster/ slower is marking speed mm/s millimetres per second it depends on the machine I've seen some being advertised recently that have a maximum of 700mm/s they are around $2500usd. I have had my machine up to 4000-5000mm/s on occasion I have a 30w fibre laser that cost me nearly $11k.

Getting good results is a guessing game especially if it's not something you are using on a regular basis I had no idea how to operate one when I purchased mine and I tell you what the amount of knives and other things I've destroyed or had to start again when I was almost completed I've lost count of.
To get the same results one day on something like a knife then get them again on another the later on especially if its something complex is extremely hard as the hardness of each blade could be slightly different fraction of a millimetre in the thickness of the material will give a different effect on each. It's extremely complex drives me insane at times because if someone is having success with their machine and suggest you change your settings it doesn't mean it will work when you do it.

 

[Hubert S.]

Hi Hubert faster/ slower is marking speed mm/s millimetres per second it depends on the machine I've seen some being advertised recently that have a maximum of 700mm/s they are around $2500usd. I have had my machine up to 4000-5000mm/s on occasion I have a 30w fibre laser that cost me nearly $11k.

Getting good results is a guessing game especially if it's not something you are using on a regular basis I had no idea how to operate one when I purchased mine and I tell you what the amount of knives and other things I've destroyed or had to start again when I was almost completed I've lost count of.
To get the same results one day on something like a knife then get them again on another the later on especially if its something complex is extremely hard as the hardness of each blade could be slightly different fraction of a millimetre in the thickness of the material will give a different effect on each. It's extremely complex drives me insane at times because if someone is having success with their machine and suggest you change your settings it doesn't mean it will work when you do it.

That sounds a bit discouraging... I have settings for AEB-L now that give decent results, but I wish I could get it darker. I can get dark on the surface, but cannot engrave deeply and get it to be dark, even if I run the same settings that work on the surface after the engraving. The good news is that at least for AEB-L, hardness does not seem to make an appreciable difference in how the engraving comes out.

 

[Hubert S.]

I had an old 1mm thick AEB-L blade that warped badly during heat treating, so I engraved a line across it at 20W power and 20mm/s velocity. The blade snapped with an extremely lazy hammer blow. This wasn't really a great test, though, as I can fracture the blade without engraving a line, it just takes a harder hit. I am curious if Kraken Tactical Edc 's theory is right and the laser settings are responsible for what caused the failures. I will heat treat some coupons for testing with my next batch of knives, hopefully next weekend or the week after. P.Brewster , can you tell me what laser settings you used for the engravings?

 

[CallumRD1]

Also remember that if you're carving a shallow groove in the steel you're creating a very clean and sharp stress riser regardless of any chemical changes to the steel. That alone would cause the steel to fail there instead of elsewhere in the blade.

 

[Natlek]

Also remember that if you're carving a shallow groove in the steel you're creating a very clean and sharp stress riser regardless of any chemical changes to the steel. That alone would cause the steel to fail there instead of elsewhere in the blade.

Just like we cut glass............actually we don t cut glass , we just make scratch where we want glass to break

 

[Kraken Tactical Edc]

I had an old 1mm thick AEB-L blade that warped badly during heat treating, so I engraved a line across it at 20W power and 20mm/s velocity. The blade snapped with an extremely lazy hammer blow. This wasn't really a great test, though, as I can fracture the blade without engraving a line, it just takes a harder hit. I am curious if Kraken Tactical Edc 's theory is right and the laser settings are responsible for what caused the failures. I will heat treat some coupons for testing with my next batch of knives, hopefully next weekend or the week after. P.Brewster , can you tell me what laser settings you used for the engravings?

What type of laser are you using and what power?