blade materials

im confused. after 0.8 % iron carbides are formed to increase wear. but what does the 0.8 do? does it not form carbides?also, when you add another element does it always form carbides? and if carbides help wear resistence they have to be hard, therefore increasing the overall hardness of the steel.also if there is under 0.8 percent, and if the o.8 does not form carbides, where does the other element go to if there is no carbon to bond with? and one last thing, would carbides of a tough metal increase toughness as well? thanks
 
At .8% carbon the martesite is saturated with carbon and that will be as strong as the martensite can get.Other elements not combined with carbon also strengthen the martensite and effect other properties .The .8% carbon will give you max wear resistance of the martensite further carbon will form carbides which will further increase wear resistance of the steel. The problem of using hardness tests to determine wear resistance is that the test measures strength of the martensite not the carbides ( the perfect example of this problem is Talonite which has low hardness { soft matrix} but exceptional wear resistance due to the carbides). Carbides make the steel more brittle not tougher.
 
Originally posted by blademan312

after 0.8 % iron carbides are formed to increase wear. but what does the 0.8 do? does it not form carbides?
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You do get some carbides, although the amount and type of carbides can be controlled to an extent during the steel making and heat treating process. Steels intended to be both hardenable and very tough typically have .4-.6% carbon, this is enough carbon for nearly full hardenability but not enough to form a lot of carbides.



also, when you add another element does it always form carbides? and if carbides help wear resistence they have to be hard, therefore increasing the overall hardness of the steel.also if there is under 0.8 percent, and if the o.8 does not form carbides, where does the other element go to if there is no carbon to bond with?
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Not always. Some elements don’t form carbides and it is also possible to keep carbide forming elements in solution, without forming carbides. With stainless steels, for instance, it is usually desirable to keep much of the chrome in the matrix without forming carbides, for rust resistance. With high chromium tool steels like D2, maximum wear resistance is usually the goal rather than stainlessness, so much of the chrome is intended for carbides.

The carbides in steel have little effect on the Rockwell hardness since, as Mete pointed out, is basically a measure of the matrix hardness not the carbide hardness. Wear resistance tests will generally show the effects of the carbides.


and one last thing, would carbides of a tough metal increase toughness as well?
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As far as I know, all of the carbides normally found in steel are more brittle than the steel matrix and therefore decrease toughness. In other words, increasing the carbide content of a steel increases wear resistance but reduces toughness.




- Frank
 
so 0.8 carbon is infused in the matrix. when you add alloying elements, there properties only matter for the amount that goes into the matrix. taht is, if alloy is vey tough and strong, it will only be that for as much of the matrix as it can saturate, after that , it will make hard, brittle carbides that decrease toughness. are all carbides of any metal the same tougness? and when they say a metal is hard, do they mean at its maximum tougness, because you can make any metal hard? is that crrect? also, how do yuo know when the matrix is saturate with a certain alloy/ how do you know when it will start to make carbides?also, the tougess of a metal controlls everything, because no matter what you can harde it, so its all about the toughness you want, correct? thanks
 
Originally posted by blademan312
so 0.8 carbon is infused in the matrix. when you add alloying elements, there properties only matter for the amount that goes into the matrix. taht is, if alloy is vey tough and strong, it will only be that for as much of the matrix as it can saturate, after that , it will make hard, brittle carbides that decrease toughness.
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That’s basically correct. Carbon content above .8 % in steels tends to get used up forming carbides and carbides reduce toughness.


are all carbides of any metal the same tougness?
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No, but they are more brittle than the steel matrix.


and when they say a metal is hard, do they mean at its maximum tougness, because you can make any metal hard?
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No.


how do yuo know when the matrix is saturate with a certain alloy/ how do you know when it will start to make carbides?
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You look at it under a microscope.


also, the tougess of a metal controlls everything, because no matter what you can harde it, so its all about the toughness you want, correct?
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No toughness is just one of many important properties.





- Frank
 
with more, harder carbides, too reach maximum toughness would the hardness increase or decrease? also, what stops a steel from hardening? will it crack if its hardened to far?
 
Originally posted by blademan312
with more, harder carbides, too reach maximum toughness would the hardness increase or decrease?
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I am not sure, I would guess that increasing the carbides wouldn't have a whole lot of effect on the toughness of the matrix on way or the other.


also, what stops a steel from hardening? will it crack if its hardened to far?
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Hardness of the steel matrix is limited to about 67 HRC, to get this the steel must be all Martensite or all Martensite plus carbides. Adding alloys, for the most part, will not increase the maximum hardness of the matrix.

When steel is quenched to form Martensite the grains tend to be jumbled from the rapid freezing and are distorted and pushing against each other. This internal stress can cause warping or shattering (brittle failure), for this reason hardened steel is tempered. Tempering invoves heating the steel enough that the grains can move and settle in, relieving much of the internal stress.




- Frank
 
Frank, Tempering - the grains don't move but there is realinement of atoms which reduces micro stresses, and there is formation of carbides which also changes the stresses.
 
i was reading in joes faq that molybendum is a carbide former. does that mean none of the molybendum goes into the matrix? also, will the matrix only hold a certain amount of any metal before it starts to form carbides?
 
Carbide formation can also increase the hardness of a steel slightly (secondary hardening), and some of the alloys like Cr and V can increase grain refinement by preventing grain growth, though this is usually when the alloys are in small amounts (like in 52100), not in the highly alloyed states like CPM-10V.

Tempering martensite induces a change from one crystal structure to another (BCT to BCC) . Martensite forms when the change in temperature is so rapid that the carbon can not precipitate out when the matrix goes from austenite to ferrite and thus you end up with a metastable state of supersaturated carbon which will degrade rapidly if not tempered.

-Cliff
 
Originally posted by mete
Frank, Tempering - the grains don't move but there is realinement of atoms which reduces micro stresses, and there is formation of carbides which also changes the stresses.
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Thanks for the clarification Mete. I stand corrected. :)




- Frank
 
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