Super Quench Question

[glilley]

When quenching mild stell in super quench, assume I quech when it is a cherry red - much hotter and I won't gain the benifit of using super quench. Am I correct? Thanks!

[glilley]

Oops - meant "mild steel" vs "mild stell". I don't quench mild stell -ha ha!

[pezking7p]

You will want to be as close to the magnetic point of your steel as possible before quenching to get the maximum hardness. So if your magnetic point is at red, I think you'll want to about one color hotter than that before you quench. Your results will vary depending on the actual composition of your steel.

[MattBower]

You will want to be as close to the magnetic point of your steel as possible before quenching to get the maximum hardness.

? I haven't played with quenching mild steel much, since if I want a hard tool I tend to use a steel with enough carbon to harden properly. But for most medium and high carbon steels you actually want well above (100+ degrees F) non-magnetic. And recommended austenitizing temperatures generally climb as carbon content falls. So if it were me, I'd probably quench mild steel from around 1600 F, maybe even a little more.

much hotter and I won't gain the benifit of using super quench

Why would that be?

[glilley]

Matt/pezking7p - I'm making some light-duty punches/drifts from 1040 but I probably shouldn't have started this hare since if I had searched more I would have found answer!! pezking7p answers the question correctly (which I forgot) - I just remember in a class I took that 1040 tends to go nonmagnetic around cherry red/dull red but it ain't consistant. I purchased a round magnet for this purpose several months ago and forgot about it so I'll take the time and hang it over the forge!
Re. the part about any hotter and effects of super quench lessen, this is something I seem to have read several places, but under torture I would be hard pressed to tell you where.

[Jack Evers]

Re. the part about any hotter and effects of super quench lessen, this is something I seem to have read several places, but under torture I would be hard pressed to tell you where.

I have a horseshoeing client who used to want shoes hardened. When using SQ, I just made sure it was plenty hot - often at yellow - and never had problems with it not getting hard. On a very hot shoe, it doesn't seem to really breakdown the vapor layer until it cools somewhat, but they still get hard.

[glilley]

I have a horseshoeing client who used to want shoes hardened. When using SQ, I just made sure it was plenty hot - often at yellow - and never had problems with it not getting hard. On a very hot shoe, it doesn't seem to really breakdown the vapor layer until it cools somewhat, but they still get hard.

Now its coming back to me (although I still don't know where I read it). Something to do with the hotter the metal the more difficult it is/longer it takes for the surfacant in SQ to break down the surface tension between the metal and the quench.

[MattBower]

All steels go non-magnetic at essentially the same temperature, the Curie point, which is right around 1415 to 1420 F, depending on your reference. (That's on the way up. On the way down, magnetism doesn't entirely return until the steel reaches a considerably lower temperature.) Non-magnetic is nothing more or less than a convenient way of estimating temperature if you don't have anything more precise; it really has nothing to do with whether the steel is ready to harden.

For most medium/high carbon steels the recommended austenitizing temperature (the temp you quench from) is a good 100 degrees F hotter than non-magnetic, sometimes more. So no, it really isn't right to say that you "want to be as close to the magnetic point of your steel as possible before quenching." You generally want to be hotter than that.

The instructions I've seen for Super Quench say to heat the steel to 1550, nearly 150 degrees hotter than non-magnetic, and quench from there. I still suspect you might get slightly more hardness from 1600, if you're using something like 1018.

[glilley]

All steels go non-magnetic at essentially the same temperature, the Curie point, which is right around 1415 to 1420 F, depending on your reference. (That's on the way up. On the way down, magnetism doesn't entirely return until the steel reaches a considerably lower temperature.) Non-magnetic is nothing more or less than a convenient way of estimating temperature if you don't have anything more precise; it really has nothing to do with whether the steel is ready to harden.

For most medium/high carbon steels the recommended austenitizing temperature (the temp you quench from) is a good 100 degrees F hotter than non-magnetic, sometimes more. So no, it really isn't right to say that you "want to be as close to the magnetic point of your steel as possible before quenching." You generally want to be hotter than that.

The instructions I've seen for Super Quench say to heat the steel to 1550, nearly 150 degrees hotter than non-magnetic, and quench from there. I still suspect you might get slightly more hardness from 1600, if you're using something like 1018.

Matt - thanks a ton for the insight. I believe this thread answers my concern and now I'd best print it!

[pezking7p]

Matt: Shouldn't demagnetization indicate a transformation to austenite, and thus that the steel is, basically, ready for hardening? Please note that I recommended to quench from one color hotter than the magnetic temperature.

glilley: The reason you want to quench from the coldest possible temperature is because the transformation of steel to the harder martensite phase is time-dependent. You want the surface of your steel to cool below the transformation temperature as quickly as possible to get the highest possible % of martensite. I think this is much less important when working with 1040 steel than, say, a 1018 steel.

[thingmaker3]

All steels go non-magnetic at essentially the same temperature, the Curie point, which is right around 1415 to 1420 F, depending on your reference.

I've seen a chart in one of the ASM books showing non-magnetic (which is indeed the "Curie point") decreasing as carbon content increases to eutectoid, and then holding steady thereafter. The nonmagnetic Curie point also depends on composition - for instance: enough nickel will push "nonmagnetic" down below room temperature. (This is why magnets don't stick to 300 series stainless.)

I would not use superquench for 1040. Too much carbon for that harsh a quench.

[glilley]

I've seen a chart in one of the ASM books showing non-magnetic (which is indeed the "Curie point") decreasing as carbon content increases to eutectoid, and then holding steady thereafter. The nonmagnetic Curie point also depends on composition - for instance: enough nickel will push "nonmagnetic" down below room temperature. (This is why magnets don't stick to 300 series stainless.)

I would not use superquench for 1040. Too much carbon for that harsh a quench.

The metal I was talking about in this thread was incorrectly ID - I got a few lengths from an acquaintance who SAID it was 1040 but when I called yard he bought it from all their hot rolled stock is A 36. Spark test seems to confirm this.

[MattBower]

"I've seen a chart in one of the ASM books showing non-magnetic (which is indeed the "Curie point") decreasing as carbon content increases to eutectoid, and then holding steady thereafter."

I've heard some rumors to that effect before, but since practically every iron-carbon phase diagram I've ever seen shows A2 as a straight line at 1414 F (or 1418 F), I didn't follow up on them. But you got me researching, and I did find a table in a book called the Handbook of Induction Heating that shows the Curie temp of 1008 as 1414 F, and the Curie temp of 1060 as 1350 F. Very interesting. I'll have to ponder that a little; it could affect how we use the magnet to heat treat steels in a certain carbon range. Nevertheless....

Shouldn't demagnetization indicate a transformation to austenite, and thus that the steel is, basically, ready for hardening?

No! That's not necessarily true! That's exactly what too many folks fail to understand about the magnet! Please look at the iron-carbon phase diagram at this link. Alloying elements may move some of the lines around a little bit -- up or down, left or right -- or may affect the precise shapes of their curves, but I believe the basics of this diagram apply to any common steel.

Please note the line, A3, that begins on the Y axis and slopes downward to 0.83% carbon, the eutectoid point. (Other references will show the eutectoid at 0.77% or 0.84% C. And we're talking metallurgy textbooks! Where is it actually located? I'm not sure. I've even seen metallurgists admit they're not sure. For now let's stick with 0.83%.) At that point it changes names -- from A3 to Acm -- and slopes back upward as it moves to the right. Note the flat line, A1, at 1333 F -- that's the minimum temperature at which some austenite will form. And while we're at it, note the flat line, A2, at roughly 1415 F; that's your Curie point (but see above). Steel in the area above the curve constituted by A3 and Acm has formed all the austenite it can possibly form. Steel below Acm, but above A1, is a mixture of austenite and cementite (iron carbide). Steel below A3, but above A1, is a mixture of alpha-iron (basically pure iron) and austenite. In order to produce as much martensite as possible, we need to quench from a condition in which we have as much austenite as possible; that means quenching from above A3 or Acm, as applicable. Also note that while the Fe-C diagram shows you the necessary temperatures to achieve particular structures, there's also a certain time factor, especially when there are lots of alloying elements tied up in carbides along the grain boundaries of the steel, and we want to put them into solution. By exceeding the minimum temperature somewhat (not too much!) we can shorten the time for the conversion/alloy dissolution to take place, which is a good thing when we lack the precise equipment to allow us to soak the steel at a fixed temperature for 5, 10 or more minutes.

Now, yes, the magnet can be very useful for helping us to judge A3/Acm within a certain carbon range: roughly (as Bob Nichols says in his heat treating blueprint) 50 to 95 points carbon. But note that the farther you get to the right or left of the eutectoid point, the higher Acm/A3 is, and the less useful the magnet becomes. Since this thread was about mild steel, note that down toward the bottom end of the curve the A3 line approaches -- and at very low carbon levels even exceeds -- 1600 degrees F. If you use the magnet to judge A3 for something like 1018, you're likely going to shortchange yourself on austenite and therefore martensite -- which you can't afford to do with a steel that won't make much austenite in the first place That's why I said to go hotter with the low carbon stuff.

Also note an interesting fact about the eutectoid point: A1 and A3/Acm actually touch. There's no open area under the curve, in which you have mixed structures. Eutectoid steel kicks over to 100% austenite more or less immediately upon reaching A1/A3 -- and a magnet is a pretty darned good way to judge whether you're there. This is why 1084 has become very popular among bladesmiths with simple heat treating capabilities; you can heat treat it with simple methods and still get very good results. (And it's why the Steel Fairly delivered me some 1084 on Monday morning. )

"The nonmagnetic Curie point also depends on composition - for instance: enough nickel will push "nonmagnetic" down below room temperature. (This is why magnets don't stick to 300 series stainless.)"

That's certainly true; I should have specified that I was talking about relatively simple steels. But the highly alloyed stuff is totally unsuitable for primitive heat treating anyway.

[pezking7p]

Matt, Thanks. I did some reading and found I was confused about the curie transition. I had assumed it was due to a phase transformation. But, hey, you know what they say about assuming...:)

[Francis Trez Cole]

The way it was explained to me for making punches was. Heat your metal to a orange and quench it. Then re heat to 500 F using a thermal marker to anneal and quench it again. Make my own punches and drifts out of mild steel all the time works for me.

[lbender]

(Other references will show the eutectoid at 0.77% or 0.84% C. And we're talking metallurgy textbooks! Where is it actually located? I'm not sure. I've even seen metallurgists admit they're not sure. For now let's stick with 0.83%.)

@MattBower
There are two different phase diagrams for steel. One is the carbon-iron equilibrium phase diagram and the other is the iron-iron carbide metastable phase diagram. The equilibrium diagram is of interest to metallurgical scientists, where the eutectiod is exactly 0.68% carbon at 738

[MattBower]

Ah, somebody who actually understands this stuff! Great! We need more of those. (I'm certainly not one. I get by with what I've picked up from smarter people on the Internet.)

If you wouldn't mind, can you explain what's meant by an "equilibrium phase" as opposed to a "metastable phase"?

[pezking7p]

Matt: An equilibrium phase (or equilibrium state) is one in which all transformation has taken place, and no matter how long you wait it's structure will never change, it is at equilibrium. A metastable phase (or state) is one in which the structure will change if you wait, eventually reaching the equilibrium state if you wait long enough.

In the iron-carbon system, graphite is the equilibrium phase of carbon below A1. However, the speed at which the carbon can diffuse and form graphite is very limited, especially at low temperatures, so unless you hold the sample at a high temperature (but below A1) for a long time you won't see graphite...at least not in most steel structures. But that carbon has to go somewhere, so you see cementite formation because it takes less time to form than graphite. In the case of cementite, it is essentially stable at room temperature. What that all means is that while you are heating and cooling steel, unless you go slow enough for the carbon atoms to have plenty of time to move around, you are more likely following the iron-iron carbide phase diagram.

The extension of this is what happens when we harden steels. If you cool the steel fast enough, the carbon doesn't have enough time to even form cementite, and it is trapped within the lattice of the iron atoms and forms martensite. All we are doing when we temper steel is giving the carbon atoms enough energy to shuffle around a bit and form more stable (and softer) carbon phases, such as cementite.

[MattBower]

Thanks. That's been bugging me for a long time.