Minimum time to harden a thin blade

[HennieL]

Hi everyone,

I'm a total beginner with regards to heat treating of steel, and am doing my research before attempting to harden my first blade

I'm planning to harden a 3mm blade, which is made from a German steel similar to N690.

The spec sheet states that the steel should be treated at 1030 °C to 1080 °C for a period of 1 hour per 30mm of thickness.

My question is, do I reduce this time by one tenth, given that my blade is only one tenth the thickness for which the treatment time is given, or is there a minimum treatment time for a steel, regardless of the thickness of the material being treated? I will be doing the heating in a digitally controlled electric kiln, which should keep the temperature pretty close to what is required.

Hennie

[MattBower]

According to the manufacturer, Bohler-Uddeholm, you need a 20 minute soak at austenitizing temp. (Presumably that's for blade thicknesses, since N690 is designated as a knife steel). They also call for ramping up the temp in stages, with 15 minute soaks in the ranges of 650-705 deg C, and 815-870 deg C. Also note that although the data sheet says it's an oil quench steel, the heat treat summary says a plate quench with forced air is typical. That's more in line with what little I know of heat treating stainless steels. They tend to be very deep hardening, so it surprised me when you said (and I verified via the data sheet) that N690 is listed as an oil quench steel. The oil quench recommendation may be for much thicker cross-sections than a typical knife. It is common to use more severe quenchants as the size of the cross-section increases, because large pieces hold so much heat that it can present problems in achieving an acceptable cooling rate. But what works well for a very large piece may destroy a thinner cross-section of the same steel

All of the above is based on the manufacturer recommendations and my own limited knowledge. (I use much simpler steels than you're dealing with, because I lack the tools to properly heat treat fancy steels.) If you can find a knifemaker who has experience heat treating your steel -- I read this morning that N690 is a common blade steel in South Africa -- you should certainly listen to what he has to say.

[ThomasPowers]

There are two aspects to heating alloy steels: one is to be sure the entire piece is at the proper temperature for quenching. The other is to alloy carbides to go into solution. In knifemaking the proper temp doesn't take long as it's so thin; however it may take the same amount of time to solution the carbides as that's not so much dependent on thickness but on time and temperature.

Have you asked the manufacturer what they suggest for heat treating thin sections?

[HennieL]

Thanks, Matt

The steel I'm using is actually called Panzer 36, but as I said above, it is similar to N690, so I suppose the Bohler data should be pretty relevant.

The actual spec sheet for the Panzer 36 steel talks about 1 hour per 30mm thickness - hence my question.

I don't feel comfortable with just extrapolating the 60 minutes for 30mm to only 6 minutes for 3mm, and unless someone else advises differently, I will use your recommended 20 minutes.

Can you (or someone else...) please elaborate on the plate quench method - it's a new one for me

Hennie

[HennieL]

Have you asked the manufacturer what they suggest for heat treating thin sections?

That's one of the problems - the supplier won't disclose who the manufacturer is...

Here's a link to the data sheet on the suppliers' website: Panzer 36

Hennie

[MattBower]

Sorry, you did say that; it slipped my mind somehow. Still and all, the composition of N690 is right in the same range as that of Panzer36 with respect to the most important alloying elements. I would treat them as the same, at least as a starting point for experimentation.

As Thomas explained, with stainless and high alloy tool steels, an important reason for soaking is to get the carbides into solution. Alloying elements like chromium, vanadium and molybdenum get in the way of the carbon dissolving out of the carbides and going into solution in the iron, which is why those steels need longer soaks at higher temperatures compared to simple carbon steels. I think you are absolutely correct not to extrapolate to 6 minutes for a 3mm thick blade. I doubt that's long enough to get everything into solution. I'd try 20 min., quench in oil as recommended by the manufacturer, and see what happens. Be prepared to temper immediately after the quench, and also be prepared for the possibility that you'll lose the blade in the quench. It happens. That's just part of bladesmithing. If it happens to you with this steel, consider trying plates for the quench on the next blade.

Plate quenching simply involves pressing the blade between two thick plates of aluminum, which will suck the heat out of it quite rapidly compared to air, but not as fast as a liquid quenchant. At least that's the theory. I've never done it, so I can't give you any practical tips. I bet some here can, though. Scroll down on this page for a visual, and a description of hardening 154CM (another stainless knife steel) with plates: http://mickleyknives...ml/page_21.html

[Rich Hale]

Please allow me to throw my old school two cents in: With you being new to heat treat, it seems to me like you are starting as if you are tuning race car engines without learning by starting with small engines and working up. If you have already purchased a lot of that steel I suggest you set it aside and purchase some simple steels. Some great ones are 1084 and 1095 They are high carbon oil quench steels that you can make knives that are great to work with and use. They can be forged or done with stock removal. After you get all of your skills developed that are needed for knife making then reach out for steels that are more demanding. Heat treat is just one small highly important facet of knife making. If you get stuck there you will delay the rest. You seem to have the attitude that will allow you to become real good at this. I will watch as your work grows and improves.

[MattBower]

That's good advice, Rich. in his other thread I mentioned that he isn't exactly starting slow, which was probably a little too subtle. But I gather that he's already got the steel and the kiln, and in his place I probably wouldn't be able to resist trying them out, either.

Personally, I'm stickin' with my 1075-1084 for now. :)

[HennieL]

Thanks for all the advice, guys

Yes, I know that I'm aiming high for a first heat treatment, but my choice in steel is rather limited to good knife steels, and I enjoy a challenge - what can I say...

Although I AM new at knife making and heat treating, I do have many years of DIY experience in both wood and metal work, seeing that I've been tinkering since around 1965 or thereabout (I'm 56 years old, and started quite young )

The knife in question (a little skinner that I'm building to use in our next hunting season) has already been shaped, ground (by hand), finished to 1200 grit, brass bolsters made, and African Lebombo Ironwood scales shaped in the rough, so if the heat treatment goes OK I should have a pretty nice knife in a week's time... I just need to learn as much as possible about that subject as I can before I hang the knife in the kiln. I will have a few practise runs with a mild steel copy of this knife, that I made especially for this purpose (I know it won't harden, but I will still gain a lot of experience playing with it before tackling the real knife).

Well, off to bed now... have some steel to heat tomorrow

Hennie

[MattBower]

I noticed that your supplier does carry O1 and 5160, both of which are much more common blade steels for beginners or near-beginners. I'm just saying. . . .

Good luck with your heat treat.

[Steve Sells]

He isnt listening Good luck mate let us know how it turns out.

[MattBower]

It may turn out OK. And if it doesn't, it'll be a learning experience.

[Ric Furrer]

Do keep in mind that poor heat treatment or blade profile can negate those lovely colored charts.
I am surprised at the toughness rating in that chart..usually the 1% carbon steels do not out-perform the .6-.7 carbon steels in toughness...such as the Sandvik ones listed.

As to alloy solubility.....easy enough to test....do 5,10,15 and 20 minutes soaks on several bars and see what you get.

Do keep in mind that poor heat treatment or blade profile can negate those lovely colored charts.

Ric

[HennieL]

He isnt listening

Yes, stubborn as XXXXXX

In reality, my supplier is about 450km away from me, and everything must be sent by post or courier - not good if one is impatient...

Hennie

mod note: foul language removed due to violation of IFI rules

[pkrankow]

Finished to 1200 grit? A little extreme in my opinion since you will have to grind it again after heat treat, and you needed to leave material at the edge to grind away or the edge will be too thin for the quench (warp or crack). At the very least I hope you invested in some stainless foil (as sheet or bags) and have some carbonaceous material to enclose in the package before it goes in the oven (Paper? Charcoal?)

Pre-heat treat finish of about 60 grit is adequate in my opinion, prior to heat treat. (Opinions vary, and I would want a finer finish if engraving prior to heat treat, but I don't engrave.)

I do agree if I had the fancy oven I would probably try using fancy materials too.

Phil

[Golden_eagle]

[snip]I do agree if I had the fancy oven I would probably try using fancy materials too. Phil

Bah, I'll take a 10mm thick block of 5160 over a wafer think piece of fancyshmancy steel alloy with .6% unobtainium and .2% adamantium any day. nothing like a knife that weighs as much as a hammer.

[HennieL]

Finished to 1200 grit? A little extreme in my opinion since you will have to grind it again after heat treat,

Well, having done my first "heat treatment" of a mild steel clone of the knife today, I must agree - it certainly will need a lot of work to finish it off after being baked at 1050 °C for 20 minutes

The reason for going all the way down to 1200 grit was my fear that a scratch close to the edge could cause cracking. I don't know if this is a valid point, but would rather put in a bit more elbow grease to prevent possible problems.

you needed to leave material at the edge to grind away or the edge will be too thin for the quench (warp or crack).

I'm aware of that. What I don't know is how thick/thin can the edge be? Would 0.5mm be sufficient?

At the very least I hope you invested in some stainless foil (as sheet or bags) and have some carbonaceous material to enclose in the package before it goes in the oven (Paper? Charcoal?)

Yes, I have some stainless foil, and have used it on the mild steel "knife" as well, just to make sure that I can properly remove it from the knife before quenching. On tro "treatments2 of the mild steel clone, I once used a piece of cardboard, and the second time some toilet paper. In my opinion, the toilet paper resulted in less black scale forming on the knife blade. When I do the real blade tomorrow, I will wrap it in toilet paper, and then in the stainless steel foil.

I do agree if I had the fancy oven I would probably try using fancy materials too.

Phil, I'm glad that you think that my kiln is "fancy" - it certainly impressed me today... heated up from about 22°C to 1050°C in less than 15 minutes, and then kept the temperature within 3°C for 20 minutes whilst soaking the blade.

So, tomorrow is D-day for the Panzer 36 blade (if it does not rain...)

Hennie

[pkrankow]

Some say the thickness of a dime, others a nickel, I have left a whole lot more on one of my very few blades. I am not a really knife maker but heat treat is something to me because I had some epic fails in class (even handing the finished part to the lab aid to heat treat, still broke), just glad that redoing the projects later regained all the lost points for the failure.

Phil

[HennieL]

OK, report back time

This morning, I "stress relieved" the blade at 625°C (within a tolerance of 3°C, thanks to the electric kiln) for an hour, then allowed it to cool down to 150°C inside the kiln (took about 5 hours), and then allowed it to cool further in still air to about 25°C.

I then wrapped it in a layer of kitchen paper towel, then wrapped it in stainless foil, and heated it up to 1050°C, kept it at that temperature for 20 minutes, and then quenched in oil heated to 50°C until it had cooled down to around 70-80°C, after which I dunked it into cold running water.

I'm ecstatic to report that everything appears to have gone OK, with no visible cracks or warping

The knife has since been moved to the wife's freezer (around -27°C) where it is being "cryo-treated". If it survives the cold treatment, I will temper it twice, at around 120°C and 140°C, and then give it a final sanding to remove the darkened "skin" (there is only one very small patch of black "crud" on the back of the tang, the rest has just discoloured).

Thanks to everyone for the good advise - I'll be mack with many new questions soon :D

Hennie

[Steve Sells]

that is not cryo treatment that is only cold. If you would take the time to read up as suggested many times you would know that Cryo is done from -170 F, to -270F

[HennieL]

that is not cryo treatment that is only cold.

Sure, that's your definition of it... but if you want to nitpick, cryo is not even a proper word (perhaps rather use cryogenic...)

If you would take the time to read up as suggested many times you would know that Cryo is done from -170 F, to -270F

Again, that's what YOU say... I have rather more faith in what the suppliers' data sheet say... Note that the "sub-zero quenching" on the graph below (which is the term they use, and which is in fact more accurate) only drops a little teeny bit below the "0" line (oh, and that's °C...)



Steve, I really don't know why you come across so aggressive - I have certainly not given you any reason to be offended, but you have been making snide remarks from your first post on this thread... hey man, ease up

Hennie

[Steve Sells]

You started by stating you are new to heat treating, and then many of us with many years experience tell you things that you ignore, it seems a waste of our time to answer when you are determined to do things your own way, as there is no law against it, you are free do as you choose.

I was informing you of my experience with high alloy steels in blades and why we use cryo, but you have your data sheet which typically is for much thicker cross sections than blades, but what do I know, Have fun.

[Sask Mark]

Steve, when will you be asking Thomas if you can borrow his 'Curmudgeon' title?

[MattBower]

OK, report back time
snip
Thanks to everyone for the good advise - I'll be mack with many new questions soon :D

Hennie

First, congratulations. I look forward to seeing pics! (Hint, hint.)

Second, I had no idea residential freezers get that cold. I've never actually measured the temperature in mine. (I believe you. I did a little googling, and -27 C/-16 F is well within reason. It was just a surprise to me.)

Third, although Steve is being perhaps a little more curmudgeonly than is absolutely necessary here , he's right that within the bladesmithing world there's commonly a distinction made between "cold" treatment" and "cryo" treatment. The exact dividing line isn't clear -- hey, we love our poorly-defined jargon as much as anyone else, and we also love to argue over the definitions -- but many bladesmiths would say that a home freezer treatment isn't "cryo." Liquid nitrogen clearly is. A dry ice and acetone bath? Hmm. That one could probably start an argument.

Ultimately, though, that's a question of semantics. If your manufacturer says holding at -15 C for time "X" will eliminate retained austenite (and that is what the data sheet suggests), then that's a pretty good place to start. -15 C is not cold enough to have any significant effect on retained austenite in most steels that tend to retain austenite, but perhaps yours is right on the edge of full hardness at room temperature. (If that's true, then the cold treatment will make very little difference in final hardness. But that's OK. You're running the freezer anyway.)

Again, we'll be waiting for pictures.

[HennieL]

Thanks, Matt

Regarding the semantics: I do apologise for not getting it right , but at least I can put up the defence that English is not my native language - and although you will be exasperated by the exuberance of my verbosity, I do get it wrong from time to time

You're probably correct that there is not much to gain by the relatively low (or should that be high...) temperature of the low-temperature treatment (is that better, Steve ), every bit does help - and as you quite rightly state, I do have a freezer...

You started by stating you are new to heat treating, and then many of us with many years experience tell you things that you ignore, it seems a waste of our time to answer when you are determined to do things your own way

Steve, I do understand that it's frustrating when some youngster asks for advice, and then just ignores everyone who does not give the advice that he wants to hear. As an "old man" myself (I beat you by about 10 years...), I don't suffer fools lightly, but I also don't put up with people being rude to me. Now, let's look at me "ignoring" the good advice:

• In my first post, I asked advice on keeping oxygen away from the blade being treated, and on the suitability of using paper to use up the O2 during this process - nothing more, and nothing less. I received some advice in this regard, and to remove the foil before quenching, and I accepted this advice with thanks - so no ignoring of good advice there... except for your "advice" not to use the steel that I'm using because it is apparently too complicated for a beginner.
• In this thread, I asked for advice on the time needed to "soak" a thin blade vs the time specified by the suppliers for much thicker steel. Again, I received positive advice from most people, and again, I accepted the advice regarding the question that I asked... I still don't understand why you want me to use a different steel from what I have already purchased (and spent a long time on shaping...), just because I'm new to heat treating. As an engineer, I solve difficult problems almost every day, and if I do not have the knowledge, I will do research until I'm satisfied that I can solve the problem. Dealing with something like heat treating, I aim to do exactly the same - with your help and experience guiding me, but with me making the final decisions, based on MY evaluation of the problem or challenge... that's what I do
  

Anyway, enough of this.

Matt - just for you:

Before heat treatment:

...And after:

There are no visible cracks, and only one tiny spot of scaling on the back of the tang, where the foil was not properly wrapped - the rest of the blade is just discoloured. I tested it with a file, and even with hard pressure the file just slid over the blade without "biting" at all.

I gave it a 4 hour "low temperature" (is that better, Steve ) treatment, then it's first temper at 125°C for 1.5 hours. After cooling, it went back into the deep freezer for a 12 hour "soak", and I will do a second temper at 150°C for two hours tomorrow evening... then it's back to the bolsters and handle for another week or two (remember, I'm doing this all by hand).

Hennie