CruForge V

me2

Well-Known Member
https://www.youtube.com/watch?v=lOJHrkh__SQ


See if my explanations in that video make sense to anyone other than me.

Basically, too much carbon is possible, just as too little is possible. The region around 1450 has cementite and austenite mixed together. The cementite is the stuff we want in hypereutectoid steel. It adds wear resistance, and helps control grain growth. You want to dissolve enough cementite to put 0.6 to 0.8 % C in the austenite. Above that, and hardness gains are slim to none, RA goes up, and there is a greater risk of cracking both in the quench and in use.
 
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samuraistuart

Well-Known Member
I will watch your video as soon as I get more time. I hear you, just not quite fully understanding why we want cementite. You touched on something I've been curious about....that .6-.8% of carbon in austenite is going to give max hardness. I guess phase diagrams are confusing me. They show what temps pearlite forms to austentite, and ferrite/cementite, etc. But does the phase diagram directly relate to carbon dissolving in austenite? In other words, I can see that a 1% carbon steel will transform to austenite at 1508F (820c), but does this also tell me all carbon will be disolved at 1508? I really think that is a stupid question, but hey, what do I know! :)

Tried the youtube link, but it takes me to a youtube home page of sorts.
 

me2

Well-Known Member
Ok, I think the link is fixed. Try again and let me know if it (still) doesn't work.

Cementite is the carbide in simple 10xx steels. It is what gives 1095 at 58 HRc greater wear resistance than 1060 at the same hardness. (1095 at 58 isn't a hardness I'd recommend, just for the record.) This is quickly getting into the heat treating forum, where I usually hang out here. If it needs to be continued there, someone throw up a hand.

We want cementite for the same reason we want vanadium carbide in Cruforge V. (see what I did there, full circle). It adds abrasive wear resistance.

0.8 %C gives maximum hardness in as quenched 10xx steels. The rest of the carbon in 1095 forms the cementite. It's not really that we want cementite, it's just there, whether we want it or not, so we might as well use it to our advantage.

Dissolving all the cementite and putting all 0.95 %C into austenite will cause problems. First, retained austenite becomes a bigger problem than before. There can be so much that the hardness peaks at 0.8 %C and then goes back down as %C continues to go up. Second, quenching austenite with 0.95 %C gives us martensite with the same amount of C. That much C starts to form plate martensite. Plate martensite tends to have quench cracks, microscopic though they might be. Plate martensite starts to show up at about 0.6 %C and increases as C goes up. So, by dissolving all the C into austenite in hypereutectoid steels, we increase quench cracking, decrease toughness, decrease wear resistance, increase hardening temperature, decrease hardness, and increase retained austenite. Not a desirable combination, IMHO.

Yes, the phase diagram directly relates to carbon dissolving in austenite. You can use the diagrams to predict the amount of austenite, the amount of cementite, and the amount of carbon in each one. A 1% carbon steel will completely transform to austenite at 1508 F. No carbides will be left, thus all the carbon is dissolved into the austenite. Not good for hardening, but excellent for normalizing and/or refining carbide size.
 

samuraistuart

Well-Known Member
Now I follow you, Scott. What I normally do is sand to 220 grit pre-heat treat. If needed after heat treat, I go back to 120, but usually can pick up right at 220 and go up to 400 or what have you. I think you were recommending that I try really hard to get the scratches out before heat treat, to minimize work afterwards. I try!

me2....thank you so much for that explanation. I'm digesting that fairly well! I have some 1.2519 on the way. Also called 110WCrV5. 1.1% carbon tool steel with a touch of Cr, V, and a good bit of W. I am making a couple of kitchen knives out of it, japanese petty style. From what I gather, this steel (the tungsten) will allow very low edge angles, great for kitchen slicers. I have heard a fellow getting great results hardening at 1525. According to the phase diagram, this will not dissolve all the carbon, but leave some of it as cementite, which is supposed to be a good thing, from what you've mentioned. Good to go.....I think I got it! Thanks again!!!!!
 

me2

Well-Known Member
Those diagrams are handy, but remember the limitations/conditions. They are binary (Fe, C) equilibrium diagrams. Even the trace elements will move things a bit. W and V will take considerably more heat to dissolve, which is why we use them. Also, normalizing and quenching are definitely not equilibrium processes. As the maker, you'll have to pick your temperatures to do what you want. I just wanted to put out there what any maker will be facing if they start thinking 100% austenite is the way to go for hypereutectoid steels. I personally would like to try some undersoaked 1095 (1400-1420 F), but that's more curiosity than anything.
 
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