Serious blade damage, advice?

[matthewhoffman]

I sell at a weekend market in a small town in Mexico. In the last few months, I've had two kitchen knives returned with serious damage: dime- or quarter-size chunks that broke off the blades (along the edge, not at the tip), apparently during normal use. Discouraging, to say the least.

Both blades were AEB-L, heat-treated according to Larrin's protocols. No cryo, just a freezer for a few hours. I do all my grinding post-HT, working evenly on both sides. The steel is thin at the edge, but not crazy: usually about .010-.012. Tempering was on the low side: 300 degrees for two, 1-hour cycles.

Any ideas what might be going on? Appreciate your thoughts.

- matthew

 

[not2sharp]

Perhaps too much edge against a ceramic or glass cutting surface.

n2s

 

[FredyCro]

It's probably not good to guess, but i say it's a bone or similar. Did you try the knives beforehand? Did you warn them that they are hard and will not chop hard and frozen food?

 

[imill3567]

Yeah definitely some guessing going on but it might be worthwhile to ask them what normal use is in their minds. I stick with two hour tempers but still that heat treat seems reasonable and in my mind at least should work fine for kitchen use. I run tempers a bit higher (325 for kitchen) but also use dry ice so they're a bit harder out of the quench.

 

[DevinT]

Customers usually lie about “normal use” and you need to temper at ~350’f.

Hoss

 

[Bigfattyt]

People are idiots.

AEBL is tough steel!!

Even thin, and higher hardness...


I'm reminded of my father in law. He had a Cut-Co knife set.

I heard a heavy thunk from the kitchen.... then an "uh oh".

He tried to chop through a ham bone with a chef's knife.


The edge was mangled. Big dent. Too big to sharpen out without a grinder.

His response was "well, their add said world's sharpest knives"

I advised him, they did not say toughest!!

 

[Londinium Armoury]

I'm always hesitant giving out hard thin knives to people, make some thicker cleaver blades tempered a bit softer and try explaining that the thin hard knives are for slicing, and the cleavers are for chopping. Maybe print out some little cards, explaining what each style of blade should be used for.
I've had all sorts of people doing terrible things to knives I've given them. I had somebody take a high carbon steel kitchen knife off me, single bevel. They went and put it in the kitchen sink and left it on the draining board with soapy water all over it. Orange rust all over the blade by the morning. I told them dry the knife off and hand wash it seperately and dont let it bang up against a sink full of cutlery. They didn't listen and thought they knew best, tons of little chips along the fine single bevel edge.
I still repaired it and polished it for free, but it was quite annoying.

 

[matthewhoffman]

Appreciate all the thoughts! I'd love to blame it on user error, but man! These are thumb-sized chunks out of the blade, not just edge chipping. And the fact that it's happened twice makes me worry. I'll add some time/temperature to the temper. If anyone feels that something's off with the HT or grinding, please chime in. I don't exactly have a huge reputation to protect, but I'd like to salvage what little I have.

 

[daizee]

Ooh yeah, 300F sounds positively chilly to me. I do 325F for 62Rc with sub-zero and that's as brave as I'll get. What's your soak temp? 350F as per Devin is making me consider bumping my protocol a smidge too. Without a deeper freeze cycle you might have some retained austenite that converts to untempered martensite on impact and then the next shock takes it apart?

 

[Cushing H.]

Appreciate all the thoughts! I'd love to blame it on user error, but man! These are thumb-sized chunks out of the blade, not just edge chipping. And the fact that it's happened twice makes me worry. I'll add some time/temperature to the temper. If anyone feels that something's off with the HT or grinding, please chime in. I don't exactly have a huge reputation to protect, but I'd like to salvage what little I have.

Do you know how accurate your tempering oven is? Have you calibrated it or checked it against another thermometer? (Ie, your tempering temperature might not be what you think it is?)

 

[Hubert S.]

@Larrin has a comprehensive page about AEB-L here. From the page

There is no big drop off in toughness by using 300°F, so it is probably safe to got that low for tempering temperature with this steel, if the extra hardness is desired

The page has a graph that shows hardness and toughness vs tempering temperature. Between 325 and 350F, there is not much change in hardness or toughness. At 300F, you gain over a point in hardness, but the toughness drops by about 30%. The toughness of AEB-L is still higher than many other steels at lower hardness (there is a graph in the same article with a comparison). I just did a bunch at 325F, maybe that's pushing it, but I think it will be ok. Unfortunately, I don't have access to a hardness tester anymore due to the pandemic. Maybe I need to buy one.

My guess is "normal use" included some light bone chopping, maybe chicken wings, especially if the knife was a nakiri or chuka bocho. Easy to mistake those for a meat cleaver...

 

[matthewhoffman]

Reallly appreciate your help, guys. I'll bump the tempering temperature, and double-check that the reading is accurate ... and be a little more conscientious about advising folks to treat these things a little more gently. I wish I had access to dry ice/liquid nitrogen, but the town's too small for that. Thanks!
- matthew

 

[FredyCro]

I think that one point in hardness/or the gain in toughness will not make a lot of difference if they were smashing a thin hard blade against bones. "People" tend to think that if they buy an expensive Japanese or custom knife that it is made of special steel that can cut through bones like in the good old movies. Big chunks missing is a pointer in that direction imho. Small chips could happen accidently, for example hitting a fish bone while fileting or similar. Explain to your customer that your knives will stay longer sharp and take an better edge but are not tougher (and are actually not as tough) then most industry knives, which are tempered far down (ca 56 hrc) so they can take abuse of inexperience or misguided user.

 

[Londinium Armoury]

There's one way to be certain if it was user error or your heat treat was the problem. Replicate those blades and do exactly what you did before, wrap some paracord around the tangs and use them as test blades. Do everything you consider normal use with them, then step it up and start chopping into some bone or frozen food until you get the same size chips as the ones that come back to you. That way you can guage the failure point for yourself and judge if it was abuse or HT failure.
Also make a chopper and follow the same HT as on the thinner knives, if you make a thicker chopper that has a more robust bevel, that should be able to handle the bone chopping, if the chopper chips out you might have a HT problem, if the chopper holds up you know it's the thin knives being used to chop things they shouldn't. You'd be surprized what people try to chop, don't think they wont hack into a frozen leg of lamb, some will.

 

[Hubert S.]

Reallly appreciate your help, guys. I'll bump the tempering temperature, and double-check that the reading is accurate ... and be a little more conscientious about advising folks to treat these things a little more gently. I wish I had access to dry ice/liquid nitrogen, but the town's too small for that. Thanks!
- matthew

One other thing that may be worth mentioning to customers is not to use steel honing rods or knife sharpening gadgets.

 

[matthewhoffman]

Glad you mentioned sharpening gadgets. I figured that out some time back when I noticed ripples at the edges of blades sharpened (so-called sharpened) with those things. I now tell people I won't guarantee a knife if they use them. I think an information sheet is a great idea. Should have done it before.

 

[Richard338]

Warning against dishwashers and sharpening gadgets is a good idea, but if someone told me that their knives won't stand up to a honing rod, then I'd buy a different knife...

 

[Hubert S.]

Warning against dishwashers and sharpening gadgets is a good idea, but if someone told me that their knives won't stand up to a honing rod, then I'd buy a different knife...

I don't know, maybe a ceramic honing rod. I would not want some home chef taking one of my knifes and going at it with a steel rod at a hundred miles an hour because they saw a TV chef do it...

I don't really know what steel rods do for hard knives. I thought they were good to straighten bent edges on softer knives, but don't really do anything for hard knives other than present a chipping hazard. Is there any benefit to using a steel on hard knives?

 

[Richard338]

I don't know, maybe a ceramic honing rod. I would not want some home chef taking one of my knifes and going at it with a steel rod at a hundred miles an hour because they saw a TV chef do it...

I don't really know what steel rods do for hard knives. I thought they were good to straighten bent edges on softer knives, but don't really do anything for hard knives other than present a chipping hazard. Is there any benefit to using a steel on hard knives?

I agree that they work best on softer knives (I have an old henckel probably around 56 RHC that touches up great on the steel).
I use them fairly precisely, holding a consistent angle etc, not flashing them together like some think should be done.
I guess part of the question is about your business philosophy. Places like CPK will tell their customers to go ahead and use the knives hard, they will fix them for free if necessary.
I don't know how many would come in for repair, but it instills confidence even in the more careful user, and no doubt increases sales.

 

[fitzo]

There's no saying a replaced knife can't now have a thicker, more idiot proof edge. Say, maybe 50thou before sharpening.

On another note: I sharpen all my kitchen knives with a big ceramic rod. Pity people can't learn how to use those. Great tool.