1084 after the quench. A few Question.

S

shurik6

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Sep 18, 2013
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Doing some basic heat treatment of Aldo's 1084. Realized one thing that was never very clear to me.
What temp does the blade need to cool off to before tempering? Does it have to come to room temp? or is cold enough to hold with my hands enough?

What is the general guide for 1084 when it comes to tempering? I have worked with O1 before and it's a couple rounds at 400 for an hour in the oven. I am shooting for somewhere between 58-61 rockwell. Please advise.

Alex
 
I usually take mine to room temp between the quench and temper. Gives me a chance to wash off the quench oil before I put it in my home oven and stink up the place with oil smoke :)
 
I was going to say what jason said. just give it a quick scrub in warm soapy water before before temper cycles. you could always turn the water cooler when done washing to bring it more towards room temp.
 
You need to let the steel to air cool to at least ROOM TEMPERATURE before tempering. This will allow it to reach Mf. Any interruption could lead to problems, including retained austenite.

You don't want to speed this process up, either. Just let the steel do it's thing in still air until Mf.
 
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I've read the Mf for 1084 is below room temperature so you are going to have some RA. I've also read that cooling slower from Ms to Mf will increase the temperature for Mf. So taking the blade from quench at 200 F and letting it air cool slowly to room temperature will probably increase the % martensite in the blade. You can also increase the martensite by cooling the blade below room temperature. (from quench)
 
For a simple carbon steel like 1084:
The steel fully converts to austenite as it nears the 1500F austenitization temp. The drop in quench must be very fast to miss the pearlite nose. This requires a fast quenchant. Once dropped to 900F the cooling rate should slow a bit. When it reaches 400F the structure begins to convert to martensite. This point is called the Ms. Once it reaches 200F, most all the austenite has converted. This is called the Mf. Once dropped to room temp (70F), the conversion is considered complete. The structure is now called brittle martensite. The first temper converts that to tempered martensite and also converts any RA into new martensite. The cooling after the first temper should be direct, and dunking by water or running the tap over the blades. The second temper converts and stabilizes the new martensite. Two one hour tempers with a water cooling between them is all that is needed to temper the blades. RA is not a concern to knifemakers in carbon steels.

Some higher alloy carbon steels, like 52100, have a Mf below room temp. It can be anywhere between 50F and -50F, depending on the alloy. Some people put the blade in the freezer to finish the Mf. Done for an hour it won't do any harm, but don't keep it there overnight, as this delays the temper. Others don't feel this is needed. If you just cool to room temperature, the RA will be converted more than enough in the tempers. As said above, RA is not really a concern in carbon steels.

High alloy tools steels and all stainless steels have a Mf of -30 to -100F. A dry ice bath or cryo are how these steels reach the MF. There is no advantage to repeating the sub-zero treatment between the tempers, but a water cooling to room temp is good here, too.
 
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"RA is not really a concern in carbon steels". That threw a wrench into my cog!!! I thought RA was indeed a concern, at least in the hypereutectiods. Would you mind elaborating?
 
On a lot of mine I've used water to cool the steel to touchable temperatures after oil quench. Is this bad practice? I'm usually doing so to go ahead and make sure it hardened edge to spine before I shut down the fire.
 
From my reading (Verhoeven), 1095 properly quenched (minimal pearlite) to room temperature will have @ 10% RA. That RA will be converted to eta carbides and ferrite upon tempering. This results in an increase in durability (carbides) and perhaps toughness (ferrite, metastable). My own guess - the 90% martensite will provide the hardness necessary for most blades tempered to a suitable toughness. 1084 will have even less RA before tempering.
 
That is what I mean when I say, "RA is not a concern in most knives made with carbon steel."
 
Ok, certainly not meaning to derail the thread here, and with all respect to Alex, the RA thing still has me confused. 52100 for example....we have such a huge range of hardening temperatures suggested. Some say 1550, but others say 1475. Why 1475? I've heard.....to reduce RA. I had started a thread, maybe on another forum, asking about the difference in the suggested austentizing temperatures, why there was such a huge range. IIRC, the consensus was to use the lower temps, and utilize soaks, to reduce RA. Which, in a hypereutectoid, would increase overall brittleness, without any sort of wear resistance gain. I can see how, with 1084, RA wouldn't be much of an issue, because there isn't the extra carbon to deal with. What you're saying here is basically, RA isn't a concern, which is contrary to what I thought. Ah....found it. For example, here is a quote from Cashen in response to a question I had asked about CruForge V and austenitizing temperatures.

"For this question what you have to remember is that carbon content for austenite solution, and subsequent other phases that result, is a matter of available carbon. Carbide forming elements must be considered when measuring available carbon. Vanadium is the third strongest carbide former there is but W2 only has .25% so all it does is stabilize grain boundaries, at .75% Vanadium Cruforge has triple the amount of this extreme carbide former, thus potentially locking up that much more carbon, and requiring that much more heat to break it free. I very often do tests with O-1 that involve soaks that will not unlock its carbon from the Cr, W or V carbides, with the result of having it behave pretty much like a much lower carbon content steel.

Less aggressive carbide formers such as chrome need to be given slightly different consideration and that is why industry may say 1650F for 52100, but if you want to avoid retained austenite 1475F is where you actually want to be. The higher temp will quickly break the chromium carbide bonds and result in retained austenite by putting over .85% carbon into solution, this is not as much the case with vanadium which can require as much as 1925F to get it to let go completely.

The real concern here is almost always retained austenite rather than grain growth. Remember that grain growth cannot occur until all the grain boundary carbide is gone, retained austenite will be a problem long before this will happen."
 
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