How Razor Blades Dull

[RickKrung]

An interesting line of discussion came up recently in a maillist on the making of bamboo fly rods, in regard to dulling of edges.  This may be old news to knife sharpeners, but as it pertains to razors, something we use as references or standards, I found it interesting and confirming. 

Relevant discussion from the original thread is further below, immediately below are links to the source information referenced in the discussion. 

Rick

I posted this to the BESS Exchange also.

Article URL
https://www.sciencemag.org/news/2020/08/your-hair-can-crack-steel-when-it-hits-right-spot?utm_campaign=news_daily_2020-08-13&et_rid=17102816&et_cid=3445815

Video URL
https://www.youtube.com/watch?v=6kgw6j9n_2o#action=share

On 8/17/2020 5:32 AM, H...V... via Rodmakers wrote:
>
>
> On 17 Aug 2020, at 14:27, T...S... via Rodmakers <rodmakers@lists.canerodmakers.com> wrote:
>
>> What they don't talk about, and the razor companies don't tell you is that what causes most early failure in a blade is actually corrosion. Stainless is not naturally corrosion resistant, what makes it so is a microscopically thin layer of oxide that forms on it's surface. When you shave, that oxide wears away allowing corrosion to start. As long as the metal is exposed to oxygen in the air, the oxide layer will restore itself fairly quickly, and the steel is again protected.
>
>
> I assume that is a chromium oxide? As far as I know it is primarily the chromium content that makes stainless steel 'stainless' - and iron oxides are, for the most part, and that includes all non-esoteric oxides, non-passivating, i.e. an iron oxide layer (aka rust) does not prevent further oxidation underneath - unlike e.g. an aluminium oxide skin on aluminium.


On 8/17/2020 5:07 AM, H...V... via Rodmakers wrote:
> Note that there is nothing special about the 'process' they describe - see also their introduction:
>
>> Steels for sharp edges or tools typically have martensitic microstructures, high carbide contents, and various coatings to exhibit high hardness and wear resistance. Yet they become practically unusable upon cutting much softer materials such as human hair, cheese, or potatoes. Despite this being an everyday observation, the underlying physical micromechanisms are poorly understood because of the structural complexity of the interacting materials and the complex boundary conditions of their co-deformation. To unravel this complexity, we carried out interrupted tests and in situ electron microscopy cutting experiments with two micromechanical testing setups. We investigated the findings analytically and numerically, revealing that the spatial variation of lath martensite structure plays the key role leading to a mixed-mode II-III cracking phenomenon before appreciable wear.
>
> It's only their detailed look at what's happening at the 'between nano and micro' scale, after each individual use that is (somewhat) new.
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> So yes, this is definitely also what's happening to our plane blades on bamboo, and to plane blades on 'normal' wood, and to kitchen knives while cutting onions on a chopping board.
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> Way I understand it is the extent of this process is primarily dependent on the alloy type and heat treatment of the steel. My guess is that razor blade steel is harder, more crystalline, than our plane blade iron.
>
>>
>>> On 17 Aug 2020, at 13:56, T...S... via Rodmakers <rodmakers@lists.canerodmakers.com> wrote:
>>>
>>> Dear All,
>>> here is an interesting study and especially a nice video how razer blades become dull.
>>> Perhaps same is happening to our plane blades due to the hard cane fibres.
>>> https://www.sciencemag.org/news/2020/08/your-hair-can-crack-steel-when-it-hits-right-spot?utm_campaign=news_daily_2020-08-13&et_rid=17102816&et_cid=3445815
>>>
>>> br
>>> Tapani
>>>
>>> Rodmakers List - https://rodmakers.canerodmakers.com

[sharpening_weasel]

Very interesting!

I wonder how a solid carbide blade would hold up. I've also read about monocrystaline steel- apparently there's a special way of cooling the molten steel so that instead of tons of little bits and bobs, one crystal structure forms.

[John_B]

I also remember an article from some time ago where Vadim discussed how stainless knife edges lost their initial level of sharpness for the same reason.

Perhaps another good reason to steel a blade before use each day.