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 Post subject: Alloy question
PostPosted: Wed Jun 18, 2014 2:06 am 

Joined: Thu May 29, 2014 8:38 pm
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It seems that the biggest difference between some of the higher carbon stainless/PM steels and carbon steels is the presence of significant chromium. In various knife reviews, here and elsewhere, there is always an offhand comment about percent of chromium and stainless properties, but I haven't heard much about why the inclusion of chromium results in a degradation of performance vs. carbon knives. Based on the general platitudes about steel, it seems most stainless steels are a bit tougher, more ductile and harder to sharpen than carbon counterparts. Assuming the main difference is high levels of chromium, I would assume that chromium introduces those traits to the steels.
Side note: I've noticed some PM steels have higher carbon content than any carbon steel. Given, alloy composition is a balancing act given desired outcome, but I wonder what could be achieved in terms of hardness, edge retention and ease of sharpening if the chromium/stainless/toughness properties were dialed back in PMs. Probably not commercially viable as most PM steels seem to be developed with the goal of monetizing in industrial settings or overcoming the deficiencies of stainless vs. carbon in a cutlery application.



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 Post subject: Re: Alloy question
PostPosted: Wed Jun 18, 2014 3:02 am 
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I'll try and be as thorough, yet simple as I can be here (based on my limited knowledge of metallurgy).

Carbon or Stainless, if you are below .8 percent carbon, then you will not form carbides, the iron will simply use up all the carbon. When you go above .8 carbon, you have excess carbon and can form carbides as well as different structures within the steel. This is why AEB-L is so fine grained. It has 13% chromium, but only .68 carbon, much below the .8 threshold, thus it does not form chromium carbides. The term "high-carbon" steel is "supposed" to refer to a knife with greater than .8 carbon, but I've seen makers often use it for steels with .6 carbon.

It is when we combine high-carbon with high-alloy (in this case 13% + chromium) that you will get big grain boundaries if the steel is not powdered or of utmost purity. If you stay below the .8 threshold you really don't need powdered metal if you have even decent temperature control. It's up to the heat treater to properly normalize and anneal the steel prior to heat treat. Powdered metals and inert atmospheres are 2 things we use today to produce fine grained high alloy steel. Some of these alloys allow for higher hardness and some of them even control grain growth to a degree, so yes, it is very much a balancing act, especially given the specific set of properties we are looking for to make knives with. The best that we can offer for better toughness and lower chromium are semi-stainless steels like the Kikuichi TKC and Konosuke HD steel. Semi-stain steel is some of the best all around steel for kitchen knives, many swear by it.

These high carbon alloys start to produce different types of grains when you start adding lots of alloys. It's the combination of alloys coupled with being powdered metal that makes a really great powdered metal. Adding Chromium in powdered metals greatly increases wear resistance and stain resistance. At low temps Chromium can actually control grain boundaries to a degree (one reason why 52100 is so great) but at higher levels and higher temps it will grow grain boundaries. Vanadium can combat grain growth problems but greatly increases wear resistance and temperature requirements. From what I gather, 2 of the best steels to balance most attributes on an exotic steel are s35vn and ELMAX. Both were actually developed specifically for cutlery, nearly an industry first. Like you stated before, very few steels are actually developed specifically for knives. These are supposed to have all the wear resistance but still be able to take to stropping well. Generally, the higher the alloy the harder to strop or put a very fine edge on, but pretty much all of the 3rd gen powdered metals will take a fine edge without much fuss. Some of the first gen powdered steels still suffered from losing their fine edge fast but that problem has pretty much gone away with newer alloys and finer grains.

About chromium and degradation: This is only partly true. Chromium can increase stiffness, making an edge less likely to roll meaning less honing throughout the day. However, this extra stiffness leads to brittleness, meaning the edge will break off faster than a carbon steel knife, leading to a knife that can't be honed back as many times or as fast as a carbon steel knife. It's the ease of sharpening, ease of honing, and extra toughness that makes carbon so attractive. If you don't have time to hone your knife, or are working with a lot of high-acidic foods, you will generally want a stainless knife over a carbon.

Did this answer your question? I apologize for any redundant information or stuff you may have already known, I always assume there's some first time reader's here. It's getting late and I'm a bit tired lol. :)



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 Post subject: Re: Alloy question
PostPosted: Wed Jun 18, 2014 3:24 am 

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Shaun, that's about as good as it gets. Too many cocktails to process fully but, in fairly short order was able to see where my abbreviated understanding of steel composition is completely lacking. Will def study with a more sober eye in the morning. Sorry if some of my questions seem noobish but, after all, I am a noob just trying to better understand the products I'm obsessing over.



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 Post subject: Re: Alloy question
PostPosted: Wed Jun 18, 2014 3:39 am 
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Your questions are not noobish at all! There are knife makers who for years can't fully understand what is going on with the carbides in their knives. This is why I am so thankful for the internet and the countless hours that others have spent sharing their experiments with the world. I'm pretty sure that I still don't understand steel at all, but the more I fool with it the more I think I understand it. ;)



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 Post subject: Re: Alloy question
PostPosted: Wed Jun 18, 2014 5:53 pm 
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Shaun, one of these days your going to have to start giving yourself a little credit for your talent and knowledge. :ugeek: Your far to modest! ;)



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 Post subject: Re: Alloy question
PostPosted: Thu Jun 19, 2014 1:58 am 
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Shaun, dude, you've been doing some studying! :) It shows! Nicely written.

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 Post subject: Re: Alloy question
PostPosted: Thu Jun 19, 2014 3:09 am 
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Thanks :)

I started reading up on a lot of this stuff when first starting to heat treat 1095. I kept wondering why some guy kept saying "if you don't have the means to soak 1095 precisely for at least 5 minutes then you are better off saving money and just using 1080." Finally it was explained; When heat treating 1080, you only need to get it to critical temp and then quench right away, and you will get the optimum results given you did everything right up to that step. But when you go above .8 carbon, if you don't have the ability to soak it at temp then no more carbon will get used up than if you had used .8 carbon.

Whoa, it's been another long day and I'm losing it, time to call it a night. :D



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 Post subject: Re: Alloy question
PostPosted: Thu Jun 19, 2014 12:56 pm 
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Here's a cool chart about alloys that I found somewhere a little bit ago, can't remember where though.


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 Post subject: Re: Alloy question
PostPosted: Thu Jun 19, 2014 1:43 pm 

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Great, chart and even better explanation of blade composition and characteristics. Thanks Shaun!



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