Pearlite nose

[elementfe]

Today it happened again- I had three knives in my top loading kiln (52100) at 1500, took the top off, took a second to shake the first one off the jig that holds them up, so it was losing some color when I got it into the oil.
I don't have a tester here, but the file really skated on it- will check it out presently.

What I realized is, I don't really understand the pearlite nose.
Does this alloy need to be AT 1500 when you dunk it, or does it just need to still be above 12-1300 and pass that nose at a gallop? It seems like in the real world, you would have to move infinitely fast in order to get that blade from the oven, where it's 1500, into the quench still at 1500. I'm guessing there's some slack in this equation.

I'm not looking for excuses to be sloppy, just want to understand what's happening at that high end, between Aus. temperature and Ms.
Still Googling, but haven't found an explanation that really clarifies this for me.

Edit: The diagram (from Cashen's page) makes it look like you have just over two seconds to go all the way from 1500 to 1100 and falling. How important is rapid cooling between 15 and 1100?

Thanks for any links or comments!

 

[aarongough]

The blade doesn't have to be at 1500ºF when it hits the oil, as you pointed out that's impossible if your austenitizing temp is 1500ºF. What's important is that you austenitize at the right temperature, soak for the right time, then cool to 1000ºF in a maximum of two seconds after the blade starts losing temp, and then continue to cool the blade down to ~490ºF in 50 seconds from the time the blade was removed from heat. The slowest permissible timeline to avoid pearlite would be as follows:

0 secs: remove from kiln, blade is 1500ºF
2 secs: blade must be cooled to 1000ºF
50 secs: blade must be cooled to ~490ºF (MS)

After you get to Martensite Start (MS) as I understand it you will always get some mixture of austenite and martensite, as opposed to pearlite. The ratio of austenite to martensite is controlled to some extent by the rate of cooling, and also how low a temperature you go to with your quench. Austenite that remains in the microstructure after quenching is called 'retained austenite'. Note that above I said that the timeline I showed is the 'slowest permissible' times that will get you martensite.

To an extent faster is better, permitting that the steel can withstand the stress. The faster the quench the more hard martensite you will get, as opposed to the softer retained austenite. If you quench too fast the steel will likely crack from the stress.

For knifemaking purposes the 'faster is better' rule only works up to a point. From what I have seen some level of retained austenite can actually be beneficial as it's quite tough and will help the toughness of the blade overall.

As an anecdotal example of this I did some experiments with oil-quenched A2. It was also given a sub-zero treatment immediately after the quench in order to convert even more retained austenite... That blade was ~65HRC *AFTER* being tempered! It took an extremely fine and keen edge, but severely lacked toughness... By tweaking the quench rates within the permissible window you can vary how many steels will perform. quenching faster and getting more martensite will do wonders for edge retention in a light-duty kitchen knife, but may not do good things for the performance of a hard-use knife as it will reduce toughness.

I hope I explained that all in a way that made sense. Bear in mind too that I have no formal education in metallurgy, so I may be wrong-headed somewhere, and also that I have never heat-treated 52100... All the info from above was from my experiences with other steels and the TTT diagram on Kevin Cashen's site.

(Metallurgists of BF, please point out if I'm wrong-headed anywhere above!)

 

[quint]

Good info. Kevins site is a great little bit of knowledge to have.

I agree with the speed for certain steels. 52100 like you mentioned is a little different beast. I repeatedly get 66-67HRC with the canola. I also make sure I treat it as close to what Nick does with his which I have been recommended to do by several guys that seem to know what they are doing. Take it up high enough to get everything into solution (1620-1650F or so) cool to black, then 1500F, then 1400F or there abouts. Cooling each time. I have done it with quenches once black for each one and have done it without doing that. Not sure if there was a difference. Then bring back up to about 1500F and quench in warm canola. If I do the several quenches after each temp then it is usually warm enough. Like I said this gets about 66-67 which is about what you will get from Aldos 52100.

I think the big thing with this steel is getting it up to the higher temp and letting it sit there for a little while then cooling.

Wanted to add to watch Nicks video where he talks about his process. I could be off on my numbers a tad and dont want to include anyone else in my laps of specific memory.

 

[Stacy E. Apelt - Bladesmith]

Lets just describe it as it happens:

You place a blade in the kiln (Kiln is already per-heated to 1500°F)
The blade rises to 1500°F. On the way up, it crosses 1414°F and becomes non-magnetic. This happened because the structure of the steel changed to austenite. The time spent above this point is called "austenitization".
The steel needs to sit at the target temp for a while to let all the alloy ingredients dissolve. This time period is called the "soak".
When ready to quench, upon removal from the kiln the blade will start to cool in the air. As long as it stays above 1350°F, it will still remain austenite. Because air is a fairly slow heat exchanger, the blade usually has at least two seconds before it cools to 1350°F. The larger the blade, the more the mass, so the cooling in air is slower on larger and thicker blades. Thin blades will loose temperature much faster.Once it crosses that point ( called the Ac1) it wants to change its structure. You have a certain amount of time to get it below 900°F or it will automatically convert to pearlite. This time window is called the pearlite nose ( because it looks like a noise on a TTT graph). The amount of carbon and the alloy contents determine how fast or slow the time across the pearlite nose is. On some steels, like 1095, it is less than one second. On steels with high alloy content and lots of manganese it can be minutes. It is best to get past the pearlite nose in a quick and sudden drop. This is the "quench" part of HT. Selection of the proper quenchant is critical to get past the nose fast enough. If the steel does not "pass" the pearlite nose, it will become soft pearlite and the HT will have to be redone.
Once past this nose, the steel continues to cool down between 900°F and 400°F. In this range the cooling rate can be slower, but should be more or less continuous. The quenchant selected and methods use determines the time spent in this range. You have from 30 seconds to several minutes for most knife steels to make the drop between 900° and 400°F. During this time the steel is still austenite. That structure is very soft and easily bent by hand ( with proper gloves on, of course ). Straightening of minor warps and curves is easily done during this period of time.
The structure in the steel now is called "super-saturated austenite". That means the austenite should not still be there, but cooled in such a way that it didn't convert....yet. It wants to change structure, but was cooled too fast for that to happen. If it continues its cooling rate too slowly, it will start forming pearlite and bainite, but that takes very, very slow cooling in a programmed oven.
Once the steel reaches 400°F under normal cooling rates, the super-cooled austenite starts to convert to martensite. This point is called the martensitic start, or Ms. Below this point the steel continues to gradually change into martensite until all the austenite that can change is converted. Cooling in this range should be slower and gentle. This is where cracks form due to the stresses created as structures change. The different structures have different sizes. When a larger structure is created, it has to move the adjacent structures to get some room. This can cause warp and cracks if it does not happen evenly and smoothly. This conversion to martensite stops at what is called the martensitic finish point - Mf. Any austenite that was not converted is called "retained austenite" - RA. The Mf varies depending on the alloy content and type. It is around 200°F for simple carbon steels, and around -100°F for stainless and high alloy steels.
Once the austenite has converted, the new martensite is called "brittle martensite", and will break easily if not tempered because the structure is under great internal stress. Once tempering is done, it is called "tempered martensite". Tempering twice and reaching the full Mf point at the end of the quench will take care of most all the RA in a knife steel. If the tempering is delayed too long, the steel can spontaneously crack due to the internal stress, and the retained austenite can become stable, and will not convert in tempering. This is why the temper should be fairly soon after the quench. On the most crack prone steels, it should be tempered within 10 minutes or so.

 

[i4Marc]

Thanks Stacy. I love clear explanations in plain English.

 

[elementfe]

Thanks so much, this is great info.
I know it's been written again and again, but there's always one more little thing to learn about the process- I hope I never stop finding things to be curious about!

In trying to get the basics of making a good working blade, somehow I missed the part about the time/temp relationship between soak temp, the "safe" temp under the pearlite nose...so much more to learn...

I always try to get the blade into the quench medium in one smooth move, but sometimes you get a little hitch. I was actually surprised to get hardening without the blade being a fairly bright orange, and wondered how that could be...answer is Ac1, it seems.