Question about carbon content, why the tendency towards high content in knives?

oic0

oic0

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Jun 16, 2012
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Lets say you have 1050, 1070, 1080, and 1095. Pretend heat treat process is done perfectly by a professional service, same price for any type of steel. You want to create a knife with 55HRC. With all of them at the same hardness what is the advantage or disadvantage to the extra carbon. I've wondered this because in knives people always tend towards the higher carbon stuff. Even in large choppers that might as well be called short swords. For some reason though as soon as something is called a sword its suddenly acceptable to use a lower carbon content steel that knife folks would scoff at.
 
Well let me take a shot. First off 55 Rc is too soft. 65 Rc is more like it. whats going on inside the steel with increasing carbon conttent? that is a good question. To consider it lets look at the phase diagram for steel, and consider for now only simple steels (carbon, iron, and minor impurities)

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look at just the left side up to 2% carbon

steel can exist in many different phases depending on the temperature, and carbon content. These phases are ferrite, cementite (Iron carbide), pearlite, martensite, and some others but they are not directly relevant.

Ferrite is soft and relatively weak. Cementite is very hard and very brittle. Pearlite is a combination of cementite 12% and Ferrite 88% arranged in a layered structure.
When you heat treat your blade, Ideally all the pearlite becomes martensite which is tougher, and hard (~66 Rc). Martensite is what we want in our knives.

depending on how much carbon there is, the steel will be made up of one or more of these phases. If the steel has .8% carbon then after heat treatment the knife will contain only martensite, the good stuff. if the steel has less than .8% then you end up with martensite and ferrite which is weak (bad). if the steel has more than .8% C then you end up with martensite (good) and cementite. Extra Cementite is not bad because the cementite is supper hard, and contributes greatly to wear resistance.

so for knives and I would even say for most swords at least .7-.8% carbon is neccasary for a good end product. Swords can get away with a lower carbon content because nobody really cares about the wear resistance of swords. If you call it a sword then you indicate it will not be used. If you call it a large chopper then you expect it to be used, and so wear resistance is important.

I more complicated alloys the extra carbon will form carbides with tungsten, vanadium, chromium, molybdenum, and just about whatever else is present. This is usually very desirable because these carbides, like the iron carbide (cementite) are very hard and thus vastly increase the wear resistance.
 
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So do I have this right?
Tempering turns some of the martensite back in to ferrite right? after which a steel with a higher percentage of carbon than is needed for the percentage of martensite required for its current hardness has the rest of the carbon contributing to wear resistance but making the steel more brittle than it would be if the steel had only enough for its current hardness? So theoretically, if your goal was a certain toughness and the maximum hardness you could achieve at that toughness level, then a lower carbon steel would win out at the expense of wear resistance? IE swords?
 
zaph1 said:
There used to be a sticky which is several pages long that you might want to peruse, written by KevinCashen. It was called "Working the three steel types" and explains the difference of 1060, 1080, and 1095(for example). It will have far more detail than you probably want, but it is a good read.

http://www.bladeforums.com/forums/showthread.php/673173-Working-the-three-steel-types
Click to expand...

Thank you, that was a good read. Here was what he had to say on the matter:

Fortunately there is another factor that naturally gives these steels an edge on toughness; lathe martensite. There are two common morphologies of martensite referred to as “lathe” and “plate”. The form created is actually dependant upon Ms temperature, however since Ms is determined by chemistry we can look at it from a carbon content perspective. Below .6% carbon (notice reoccurring patterns here?) a steel will most likely have all lathe martensite, above 1% carbon the steel will have primarily plate martensite. From .6% to 1% (the range we knifemakers love) there will be a mixture of both. Plate martensite is rather chaotic and its units tend to impinge on each other at high angles resulting in strain issues that render it more brittle, sometimes to the point of micro-fracturing. Lathe martensite forms in orderly little packets that nestle next to each other more comfortably, making it naturally tougher.
Click to expand...
 
It is a pretty complicated subject, but a simple answer is that the extra carbon makes for a blade with more wear resistance.
 
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