continuous cooling transformation diagram

[c.baum]

Hi folks, 

got a question concerning the cooling times in CCT. If you have a look at the diagram there are several curves showing the composition after different cooling speeds. Everything ok for me but how do i know how fast a cooling medium does the cooling? Are there any rough rules of thumb (e.g. cooling in oil does xxx K/s, cooling in water yyy K/s,...)? Many thanks in advance!!! 

[John B]

I don;'t know if this helps, but it is a section from the handout and general notes I work to when taking a toolmaking course

HARDENING To harden steel, the metal must be brought from hot to cold quickly , and this rapid rate of cooling is done by quenching

The more rapid the rate of cooling, the harder the steel will become, but care must be taken in choosing the appropriate quenching medium, because certain steels will crack if quenched too quickly, or if the item being quenched is of an intricate shape

The quenching medium is chosen according to the rate at which it is desired to cool the steel

For most steels we use oil or water,

Water should be clean and fresh from a tap, warm water will give a much slower rate of cooling but will be somewhat more rapid than oil,

Warm oil is more rapid than cold oil, Mineral oils are more rapid than Animal oils, and Animal oils are quicker than Vegetable oils.

Air is also used and results in a very slow rate of cooling and is referred to as Air Hardening.

If extreme hardness is required, 5% caustic soda solution,  or 5—20% brine solution can be used.

 

[c.baum]

On 14.9.2016 at 5:57 PM, John B said:

Warm oil is more rapid than cold oil, Mineral oils are more rapid than Animal oils, and Animal oils are quicker than Vegetable oils.

Nice info, never heard of that. But it still is just a relation. Many thanks anyway! 

[John B]

All steels react differently, and if you are buying new stock, advice is available on heat treatment possible and advis available, This may help  (or confuse) the issue further. http://www.msm.cam.ac.uk/phase-trans/2012/Manna/Part3.pdf

I would suggest trying a sample in various ways to see what works best for you.

[c.baum]

I actually got the following information: water does 200 K/s, oil 100 K/s. The "source" doesn't say which kind of oil. But after thinking about the whole thing i don't know if it is that simple. Quenching a thing with a high mass will take longer cause there's more heat saved in the core beeing conducted to the outside, so in my eyes you can't say "quenching in water at 800°C takes 4 sec". Seems to be a bit more difficult...

[ThomasPowers]

yup factoring in the leidenfrost effect can make a big difference.

[c.baum]

Of course Leidenfrost is a additional factor. But in my eyes even moving the tool in the quenching medium will show different cooling speeds at different diameters, not to mention the warming of the medium resulting a alteration of the cooling speed...

[Steve Sells]

saw a funny video where a guy was attempting to compensate for this effect by rapid motion,  which would have been fine if he were not swishing it around like a child attempting to get nasty sticky goo stuff off his hand. and he wondered why it came out bent and warped?  forgetting a red hot knife is about as solid as wet pasta, and bends just as easily.

[JHCC]

31 minutes ago, Steve Sells said:

saw a funny video where a guy was attempting to compensate for this effect by rapid motion,  which would have been fine if he were not swishing it around like a child attempting to get nasty sticky thing off his hand. and he wondered why it came out bent and warped?  forgetting a red hot knife is about as solid as wet pasta, and bends just as easily.

Kind of like the video of the guy trying to harden a sword by heating it in a lava flow?

[ThomasPowers]

Or the old urban legend about quenching sword blades in people.  If it had any validity you'd have folks still doing it using farm animals.  (Now blood works as a weak brine IF you use it before it clots; urine works better and has such an amusing smell...)  When I was researching the old UL I would run into a reference that basically said "While we NEVER did it our terrible enemies would do it" and sometimes I could track down the exact same thing in the other culture about the first one.  Never found one saying "yup that's the way *we* do it"  but then who likes to admit they are stupid as a red hot sword is about as solid as wet pasta and people are NOT a uniform density quench!

[anvil]

A few things to note 

Cooling in air is actually "normalizing" unless it's a tool steel  that is designed to air harden.

 

Also the speed of the quench (oils vs water) are less important knowledge than what you find in the spec sheet concerning correct quench medium.

And, a bit old school but W1 and most of the 10xx series steels are as close as we can come to "traditional" steels of the past.

These can be quenched in either water or oil depending on usage

A fast quench produces a hard edge suitable for cutting tools, while a slower oil quench will produce a tough steel best used for hammers etc.

[ThomasPowers]

I've had a 100+ year old buggy spring that one would swear would be water quenching shatter in water.  Using scrap you spin the wheel and takes your chances...

[Steve Sells]

Anvil, I disagree abut oil vs water, for usage.  It is either reaching martinsetic transformation speed or it isnt, nonsense about oil for hammer and water for a knife,  the thickness dictates more than intended use,  it is the  TEMPERING that will be adjusted for hammer vs kniife.

[anvil]

Tempering certainly makes the difference.

 

The other is pretty old school, without a doubt.

 

I'm pretty sure that Verhoeven supports this as well. He speaks a little on the differences between a slow quench vs fast quench and it's effects on steel. 

I found a few things in his book that support old school ways. Ways that have been considered myth by modern black/knife smiths.

 "Packing" is one of these.

the problem is he hits a few of these topics so briefly it's easy to miss.

such as normalizing  more than once is not needed other than for a few specialized steels

very critical! All the predicted data from the phase change diagrams is derived from a fully annealed parent stock and small variations in your anneal process "may" cause asymptotic differences from the predicted charts.

On the other hand he leaves out or barely touches on such things as the difference between "hard" and "tough".

Also, he says nothing about differential tempering.

I think his focus is on contemporary knife makers who primarily seek a single temper done in an oven.