Thread: Another dumb question- Cryo Treatment

In my mind, Ice hardening means cryo tempering.

Because if you quench a steel in ice, it will probably crack.

As for cryo temp itself, you normally give it a snap tempering (quick and not too hot IIRC), then cryo, then "normal" tempering.

To resume: Don't do it on an already Heat Treated blade.

Main differences between cryogenics and subcooling
Cryogenic Treatment


I think most of the razors like HENCKELS friodur, divers candle, the NOVODUR are supercooled, those are cryogenically treated Zowada


Cryogenic treatment is performed at the boiling point of liquid nitrogen or -196 ° C through the fog of 'nitrogen.
The cryogenic has a 'greater effectiveness in increasing wear resistance and hardness of steel with respect to subcooling. This is also where a proper subcooling allows the complete transformation dell'austenite residual martensite.

The reason is still under study, but it seems that there is a precipitation of carbides during the cryogenic treatment purposes.

Now debating the
Subcooling
Introduction


The subcooling is performed at temperatures up to between -40 ° C to -120 ° C.
Generally a temperature of -80 ° C is enough for most tool steels, to transform all the residual austenite into martensite TCC, if the subcooling is done immediately after shutdown.
Otherwise, the residual austenite tends to stabilize too.


Of course immediately after treatment or after treatment crogenico undercooling, when the steel is slowly returned to room temperature, it should make for detensionare tempering steel.

However, the undercooling can be run in the traditional way, in the freezer, for example, although the efficacy is not comparable to that of a subcooling industrial performed with specific equipment.

Practical application of the treatment of undercooling during the forging of a cutting tool.

Take for example the steel 52100.

Usually it applies on the break at 204 degrees for one hour to homogenize the temperature that is between heart and superficie.In general, steels are using these stops at these temperatures and when it was still 'under the formation of Martensite Start.

MS stands for Martensite start and the temperature at which martensitic transformation begins. The martensite structure of steel is hardened state. Or rather, the tempering is done to obtain a martensitic structure.

At a temperature of quenching the steel takes on a structure called austenite. If cooled fast enough (Vs, higher critical speed) the austenite, the martensite get (transformation adiffusionale).
Now, if I stopped off at a temperature slightly higher than Ms and keep the steel at that temperature for a long time, the austenite will transform into bainite .*
If I stopped off at a temperature between ** Ms and Mf (martensite finish) I will stabilize and then residual dell'austenite been transformed martensite start later.

In my opinion, if you want to take a break to homogenize the temperature, or make a thermal quenching, it is good to just above Ms, for a time sufficient to ensure improved [b] ** equalization of temperature, but not enough to Start pouring bainitic transformation. [/ b]
Time and temperature, of course, depend on the type of steel.
Also, please note that while the pearlitic transformation, and bainitic ferrite advance, at a given temperature, with the advance of time, the martensitic transformation progresses only with decreasing temperature (obviously in the range between Ms and Mf) .


Triple Hardening Technique.

This technique, described in the book of John Verhoeven, can refine the austenitic grain and therefore the martensitic, according to the principle that affirms "the finer will be the starting structure, the finer will be the final structure (which forms Departure from the hotel).
Explanation of Triple Hardening.
In practice, this technique is nell'austenizzare steel at a temperature as close as possible to Ac1 (obviously above Ac1) or Ac3 for steels ipoeutettoidi, while maintaining the shortest possible time (just enough to ensure austenitising heart piece).
Without this you do a shutdown, usually high in oil drastic.
Repeat the cycle three times. (Preferably consecutive).
In this way, with the first austeinizzazione you get a fine austenitic grain. line that will form martensite from austenite that will be fine (the crystals of martensite split the previous austenitic grains).

Similarly, the austenite to martensite will be formed from the fine will also be very fine and then the next shutdown will result in even finer martensite and so on.

Subcooling, if it takes place immediately after shutdown, or after the final quenching, the real and propane, helps transform dell'austeite remaining part (or all of the residual austenite) in martensite. If you do it after three discoveries, 24 hours away, is of little value to me, or because the residual is austere and 'already' transformed into martensite (or intermediate structures such as pearlite or bainite) or you and 'stabilized so as to render ineffective the subcooling.
The rate of "subcooling" must be low, because the thermal shock may cause the appearance of cracks. Keep in mind that both the thermal shock, the subsequent martensitic transformation due to subcooling create internal tensions that are being combined with the shutdown.

The duration of subcooling may be even a few hours, but over time has the temperature raggiunta.Dopo subcooling you return the steel to room temperature and then gets the discovery.

Closing remarks


Both these methods (cryogenics and subcooling) increase internal tensions, due to the martensitic transformation and stresstermico dell'austenite residual (voltages that are additive to those of hardening).

In some cases the subcooling or cryogenic treatment can be performed between the first and second discovery, but as I wrote there is a risk of an excessive stabilization of martensite.

The cryogenic not be done in a home (soak the hardened steel in liquid nitrogen provocorebbe heat stress too high).

The amount of residual austenite of a steel depends on the content of alloying elements. All in solution in the iron range, increase the amount of Aust remaining after shutdown (with the exception of Co and Al).

Bainite *: The lower bainite structure with a hardness of not very high (about 50 to 55 HRC) but with a 'high impact resistance and flexibility (in general).

Off-Equalization **: Do not confuse the stops equalizers to heating with those turned off.
The heat can make one or two stops equalizers (for low alloy steels practically-most), one at 400-450 ° C and to 600-650 ° C (otherwise only at 600 ° C.). The residence time depends on the thickness of the pieces. Let's say 120 seconds for 4-5 mm thick.

The rest of equalization, temperature higher than Ms, to be implemented for a period not long enough, to enter the field of bainitic transformation (CCT must see diagrams and TTT, also of temperature austeizzazione, at least for steels ipereutettoidi) .
Further Reading:
Personally, if you want to Start pouring the study of metallurgy, steel, and heat treatments on the board to begin to study the diagram Fe-C (and their principal characteristics of the various phases and various constituents of steel) and CCT and TTT diagrams.
Then the effects of tempering. Then you can deepen the topics (in particular the influence of various alloying elements on various characteristics of the steel, especially in the structure and function of heat treatment).
Regarding the advice to start with reading books in Italian. "Metallurgy, general principles" of Walter Nicodemi published by Zanichelli. "The Fe-C diagram and TTT curves" of Leno Matteoli, published by AIM.

Then you can deepen the topics with readings in English. First of all, "Metallurgy of Steel for bladesmith and other who heat treat and forge steel" John Verhoeven.